Post on 05-Dec-2023
transcript
1. Introduction: In recent years, it has become evident that development of new drugs alone is not
sufficient for optimal therapy. After drug discovery, drug candidates are usually evaluated for
their efficacy in vitro; a step which usually shows promising results. However, when tested in
vivo, the drug may fail to show any activity because of the following reasons:
• Poor drug solubility.
• Poor absorption, rapid metabolism and/or elimination.
• Widespread of drug distribution to the non-target tissues.
• Fluctuation of drug level in plasma due to erratic absorption after oral administration.
A drug's therapeutic efficacy depends on its pharmacokinetic parameters. These include
fundamental pathways of drug absorption form the administration site into the plasma,
distribution to the tissues where metabolism takes place and elimination from the body. Unless
the drug is given via the intravenous route, it first undergoes absorption, which depends on many
physicochemical parameters related to the drug such as its hydrophobicity, particle size, and
crystallinity. Many leading drugs, that have beneficial roles in treatment of serious chronic
diseases, are lipophilic or poorly-water soluble. For example, anticancer drugs such as etoposide,
camptothecin, and paclitaxel; the leading antifungal drugs such as amphotericin B, fluconazole
and itraconazole; antioxidants such as vitamin A, vitamin E, retinol, lycopene, and P-carotene
are lipophilic (Baskin and Salem, 1997; Basu et al., 1999; Blomhoff, 1994; Garewal, 1997;
Kumpulainen and Salonen, 1996; Prasad et al., 1995; Rosales, 2002; Salonen and Kumpulainen,
1999; Sies and Krinsky, 1994). The delivery of lipophilic drugs is challenging due to their
instability in aqueous biological environments, food interactions, reduced bioavailability, non-
specific targeting, and toxicity. Lipophilic, or poorly-water soluble drugs, must be formulated
and delivered in a safe, efficacious, and cost effective manner. Therefore advanced drug delivery
research, with emphasis on nanotechnology, has surged during the past decade. Nanotechnology
has provided scientists with new techniques for creating novel and advanced drug delivery
technologies. The specific goals of advanced drug deliverysystems are to maximize drug
bioavailability, to enable tissue targeting, and to controldrug release kinetics meanwhile eliciting
minimal immune response.
Dept. of Pharmaceutical Technology CPS, JNTUH
2
Advanced drugdelivery systems can be classified according to their size into the following
categories:
• Colloidal drug carriers.
• Microparticles.
• Implants.
Of most importance are the colloidal drug carriers (CDCs), which have been used to
improve the pharmacokinetic and pharmacodynamic properties of various types of drug
molecules. For this reason, there is growing interest in CDCs, which can be categorized into
polymeric nanoparticles, liposomes, nanosuspension, lipid-basedformulations (such as self-
emulsifying drug delivery systems (SEDDS) and self- microemulsifying drug delivery systems
(SMEDDS)), solid lipid nanoparticles (SLNs);and nanostructure lipid carriers (NLCs).
1.1. Solid Lipid Nanopartilces (SLNs):
Solid lipid nanoparticles (SLNs) were first introduced by Muller et al. in 1991.Since then
SLNs have attracted increasing interest as a carrier system for therapeutic and cosmetic
applications (Almeida et al., 1997; Muller et al., 2002a; Schwarz et al., 1994; Wissing et al.,
2004). SLNs are considered emerging alternative carriers to colloidal systems for controlled and
targeted drug delivery. They have the colloidal particles of a lipid matrix that remain in solid
state at body temperature. SLNs are aqueous colloidal dispersions with a size in the range of 50-
1000 nm (Castelli et al., 2005), the matrix of which is comprised of biodegradable and
biocompatible solid lipids. SLNs combine the following advantages (Mehnert and Mader, 2001):
• Provision of controlled drug release.
• Protection of incorporated drugs against chemical degradation.
• Biosafety of the carrier.
• Feasibility of large-scale production.
• Physical stability and lack of drug leakage because of the reduced mobility of the incorporated
drugs (Freitas and Muller, 1998).
Dept. of Pharmaceutical Technology CPS, JNTUH
3
• Improved bioavailability (Fundaro et al., 2000; Zara et al., 2002).
• Enhanced cytotoxicity against multidrug resistant cancer cells (Wong et al., 2006a; Wong et al.,
2006b).
• SLNs particularly those in the range of 120-200 nm are not taken up by the cells of the Reticulo
Endothelial System (RES) and thus bypass liver and spleen filtration (Chen et al., 2004).
• Possibility of coating or attaching some ligands to SLNs, thereby increasing the scope of drug
targeting (Lockman et al., 2003).
• The feasibility of incorporating both hydrophilic and hydrophobic drugs (Fundaro et al., 2000).
Fig 1: Structure of solid lipid nanopartcile
Dept. of Pharmaceutical Technology CPS, JNTUH
4
Table 1: Lipids and surfactants used in SLN production (Adapted from Mehnert and Mader 2001)
Lipids Surfactants/cosurfactants
Triglycerides Acylglycerol
Polyoxyethylene-polyoxypropyleneco-
polymer
Polyoxyethylene sorbitan monofatty acid
esters
TricaprinTrilaurin
TrimyristinTripalmitinTristearin
Softisan® 142
Glycerol monostearateGlycerol monolaurateGlycerol palmitostearate
Poloxamer 188Poloxamer 182Poloxamer 407
Tween® 20Tween®40Tween® 60
Hard fats Fatty acids Phospholipids Bile saltsWitepsol®
W35Witepsol®
H35Witepsol®
H42Witepsol®
E85
Stearic acidPalmitic acidDecanoic acid
Behenic & Butyric acids
Phosphatidylcholine (Epikuron®
170, Epikuron® 200), Egg lecithin
(Lipoid® E 80), Soybean lecithin(Lipoid® S 75, Lipoid® S 100)
Sodium cholateSodium glycocholateSodium taurocholate
Sodium taurodeoxycholate
1.1.1. Factors affecting quality of SLN dispersions
The overall qualities of SLNs are a function of selection of lipid cores, selection of
surfactant(s) and/or co-surfactant(s) used to cover the lipid cores, and the drug solubility in
lipids. Selection of lipid cores Lipid cores used for the production of SLNs for IV administration
should have the following properties (Miiller et al., 2000):
• Ability to produce small particles within nanometer size range.
• Possess sufficient loading capacity for all types of drugs (lipophilic or hydrophilic).
• Suitability for sterilization by autoclaving or by any other means i.e. filtration.
Dept. of Pharmaceutical Technology CPS, JNTUH
5
• Must exhibit long-term storage stability in aqueous dispersions with respect toparticle size and drug entrapment.
• Suitability of freeze-drying and/or spray drying for increasing shelf life.
• Toxicologically acceptable with minimum (if any) toxic residues, such as organicsolvents.
• Biodegradable.
1.1.1.1. The choice of lipids for SLNs preparation is critical for the following reasons :
( Wong et al., 2007a):
• To achieve efficient drug loading capacity.
• To achieve stability and, in some cases, sustained or controlled drug release.
• Polymorphism of lipids has an influence on drug payload.
Choice of the lipid cores for SLN preparation is dependent on many factors like their
degree of crystallinity, fatty acid chain length, and drug loading capacity in the lipids. The
loading capacity depends upon solubility of drug in lipid melt, physical and chemical structure of
the lipid matrix and polymorphic state of lipids (Manjunath et al., 2005). Lipids that form highly
crystalline particles with a perfect crystalline lattice, such as triglycerides, cause drug expulsion;
however, lipid mixtures of mono-, di-, and triglycerides containing fatty acids of different chain
length form crystals with many imperfections, which provide more space to accommodate the
drugs. For example, lipids, such as glyceryl monostearate and glyceryl behenate, are known to
possess less ordered crystal lattices; a property that favor successful drug inclusion. Other lipids,
such as beeswax, cetyl palmitate, tripalmitate and solid paraffin, however, have more ordered
crystal packing lipids with limited distance between fatty acid chains, a that cause expelling of
drugs outside the lipid core on storage or immediately after SLNs preparation. High drug loading
capacity into SLNs can be accomplished by disturbing the crystal order structure. This can be
performed by mixing low melting lipids, such as medium chain glyceride oils, with solid lipids.
The long-term stability of the lipid cores is dependent on their composition. For example,
tribehenin has higher physical stability if compared to tripalmitin. This is usually attributed to the
presence of 15% monoglycerides in tribehenin that possess the surfactant properties (Manjunath
et al., 2005). Presence of these monoglycerides may prevent lipid precipitation and/or crystal
Dept. of Pharmaceutical Technology CPS, JNTUH
6
growth. On the contrary, other monoglycerides such as glyceryl monostearate is extremely
unstable and considerable particle growth takes place few days after preparation (Manjunath et
al., 2005) This may be attributed to the presence of 50% of monoglycerides in glyceryl
monostearate, which cause physical destabilization (Jenning and Gohla, 2000; Jenning and
Gohla, 2001). Undoughtedly, the lipid loading capacity and its intended use play a crucial role in
its selection for SLN preparation. For instance, hard fats are not suitable for controlled release
applications because they melt at body temperature (Jenning and Gohla, 2000). Moreover, it has
been shown that the lipid core has a great influence on SLNs' particle size. For instance, the
average particle size of SLN dispersions increased with higher melting point lipids (Siekmann
and Westesen, 1992). The influence of lipid composition on SLNs particle size was also
confirmed. For instance, the average particle size of Witepsol® W35 SLNs was found to be
significantly smaller than the size of Dynasan®! 118 SLNs (Ahlin et al., 1998). These results
may be attributed to the shorter fatty acid chains and the presence of mono- and diglycerides, in
Witepsol® W35, which possess surface-active properties (Ahlin et al., 1998; Mehnert and
Mader, 2001). With regard to the lipid content, concentrations above 5-10% usually results in the
formation of SLNs with microparticles and broader particle size distributions (Siekmann and
Westesen, 1994a). However, other parameters, such as the presence of impurities that vary from
different suppliers, may impact the quality of SLN dispersions, such as particle size, zeta
potential, crystallization tendency; all of which will affect the storage stability (Mehnert and
Mader, 2001).
1.1.1.3. Selection of surfactant(s) and/or co-surfactant(s)
Surfactants in SLNs are used to disperse the molten lipid into aqueous phase and then to
stabilize the lipid/aqueous interface by covering nanoparticles' surfaces after cooling (Wong et
al., 2007a). High surfactant concentrations reduce the lipid/water interfacial tension, resulting in
a decrease in particle size, with a subsequent increase in surface area. Surfactants used in SLN
preparation process should possess the following properties (Manjunath et al., 2005):
• Must be nontoxic, non-irritant, and compatible with other excipients used in SLN preparation
(such as other surfactants, drugs and lipids).
Dept. of Pharmaceutical Technology CPS, JNTUH
7
• Capable of producing low particle size.
• Provide stability to SLNs by covering lipid surfaces.
• Effective to produce SLNs at low concentrations.
Several factors should be considered when choosing surfactants for SLN preparation:
For instance, the concentration of surfactant should be optimum in order to decrease the
interfacial tension at the lipid/water interface and cover the surface of the nanoparticles. It was
evident from literature that low surfactant concentrations result in particle aggregation with a
consequent increase in particle size. Nonetheless, excess amount of surfactant should be avoided
to prevent the decrease in entrapment efficiency, burst drug release, and toxicity (Miiller et al.,
2000). The effect of surfactant concentration on the particle size of SLNs has been extensively
studied and report elsewhere (Miiller et al., 1995; Zur Mtihlen 1996). For example, it was
reported that concentrations down to 5% (w/w) of either sodium cholate or poloxamer 188 were
able to produce good quality Compritol® SLN dispersions (Zur Miihlen 1996). SLNs prepared
using lower surfactant concentrations; however, produce dispersions with broader particle
distribution contained higher amounts of microparticles (Siekmann and Westesen 1994b).
Nonionic surfactants (such as poloxamers and Tween®) stabilize SLNs dispersions, producing
products with particles larger than those obtained with ionic surfactants (Manjunath et al., 2005).
The combination of nonionic surfactants wit lecithin has been shown to produce dispersions of
large particle size because of the formation of mixed surfactant films at the interface (Cavalli et
al., 1998). These mixed surfactants have dual role in SLNs dispersions. First, they are capable of
covering the surface efficiently; second, they can produce enough viscosity to promote SLN
stability (Cavalli et al., 1998). The effect of combination of surfactants on SLNs' particle size
was extensively evaluated. It was found that SLNs stabilized with mixtures of Lipoid
S75/poloxamer 188 or tyloxapol/lecithin has lower particle size and higher storage stability
(Siekmann and Westesen 1994b). It was also demonstrated that using ionic
surfactant/cosurfactant blends of Epikuron® 100, taurodeoxycholate and monooctylphosphate to
prepare stearic acid based SLNs produced considerably smaller particles compared to a nonionic
system composed of Tween® 80 and butanol (Cavalli et al., 1998). Some times surfactants are
not capable of covering the newly created interfaces during lipid recrystallization, leading to
Dept. of Pharmaceutical Technology CPS, JNTUH
8
particle aggregation and increase in the particle size of SLNs. In this case it is very important to
employ a secondary surfactant, or so called co-surfactant, such as glycocholate (ionic) as well as
tyloxapol (nonionic polymer), in order to cover the interfaces in a much shorter time than
phospholipids do. These types of water-soluble polymer surfactants are able to form micelles
having higher diffusion/mobility capability and may serve as reservoirs too (Siekmann and
Westesen, 1997).
1.1.2. Preparation methods of SLNs:
1.1.2.1. High-pressure homogenization (hot and cold)
High-pressure homogenization (HPH), using a high pressure homogenizer, has emerged
as a powerful technique for the preparation of SLNs (Miiller et al., 1995; Zur Miihlen and
Mehnert, 1998; Zur Miihlen et al., 1998). This method was first used for production of
nanoemulsions for total parenteral nutrition. For SLN production by HPH, dispersions are forced
under high pressure, within 100-2000 bar range, through a narrow gap of very small size. This
produces shear stress and cavitational forces to the moving dispersions with a consequent
decrease in their particle sizes. HPH could be performed under hot or cold conditions as depicted
in Figure 6. In both techniques, the lipid is first molten, and then the drug is either dissolved or
dispersed in the drug lipid melt. In the hot homogenization technique, lipids are molten at
temperature 5-10°C above their melting points. Drugs are then either dissolved or dispersed in
the molten lipid. Afterwards, the drug-loaded lipid is dispersed by high shear homogenizer at
high temperature in a hot aqueous surfactant solution to form pre-emulsion. The high
temperature decreases the lipid phase viscosity in the hot surfactant solution, yielding small
particle sizes (Lander et al., 2000). The formed emulsion is subsequently homogenized, by high-
pressure homogenizer, at a temperature above the melting point of the lipid, and the
homogenization process is usually repeated at number of cycles until the desired particle size is
obtained. However, it is recommended not to use more than 3-5 cycles at 500- 1500 bar because
it may initiate particle coalescence, resulting in particle size increase (Siekmann and Westesen
1994b). Eventually, SLNs are formed by cooling the nanoemulsion to room temperature. In the
cold homogenization technique, the drug is first dissolved or dispersed in the lipid melt.
However, in contrast to the hot homogenization procedure, the drug-lipid blend is rapidly cooled
Dept. of Pharmaceutical Technology CPS, JNTUH
9
using either liquid nitrogen or dry ice, producing a solid solution of the drug in the lipid matrix
(Manjunath et al., 2005). The formed powder is then ground by means of ball or mortar milling
to produce 50-100 (im particles. Eventually, the microparticles are dispersed in a previously
chilled aqueous surfactant solution forming pre-suspension that is subsequently homogenized at
room temperature. The main disadvantages of the cold HPH method when compared to hot
homogenization include the production of SLNs with larger particle sizes with broader size
distribution (Zur Muhlen et al., 1996) and the specific need to regulate the temperature during
the homogenization process. Temperature control is mandatory to avoid lipid melting and
subsequent drug degradation. In contrast, the main disadvantages of the hot HPH method include
temperature-induced drug degradation, drug distribution into the aqueous surfactant solution
during homogenization, and the formation of nanoemulsion and/or supercooled melts, instead of
SLNs, that can persist for weeks or even months (Mehnert and Mader, 2001).
1.1.2.2. Microemulsion technique
Microemulsion is defined as clear, thermodynamically stable and heterogeneous system
that is composed of inner lipid phase dispersed in an aqueous solution of surfactant and co-
surfactant blend. This method was first developed by Gasco and coworkers (Gasco, 1993). The
procedure of SLNs preparation starts with melting of lipid and subsequent dispersion of drug in
the molten lipid matrix. Then, the surfactant, and co-surfactant blend is dissolved in water, and
the aqueous solution is heated to the same temperature as the lipid phase containing the drug.
Thereafter, the aqueous surfactant solution is added to the lipid melt while stirring until a
transparent microemulsion is formed. Eventually, the produced microemulsion is dispersed in
cold water (2-10 °C) under mild mechanical stirring. The SLNs are subsequently formed by
rapid recrystallization of oil droplets in the cold aqueous solution. Surfactants and co-surfactants
used in this method include lecithin, bile salts, sodium taurodeoxycholate and butanol. High
shear homogenization followed by sonication High shear homogenization and sonication are
dispersing techniques, which were used to prepare SLNs (Domb, 1993). In this method, the drug
is dissolved or dispersed in lipid melt, which may be enhanced by the use of organic solvent
followed by evaporation of the solvent. The drug loaded lipid melt is then added to a hot aqueous
surfactant solution while homogenized by high shear homogenizer to produce coarse emulsion.
Thereafter, the coarse emulsion is further homogenized using ultrasonic homogenizer to obtain a
Dept. of Pharmaceutical Technology CPS, JNTUH
10
10
nanoemulsion, which is subsequently cooled to produce SLNs. Clozapine SLNs were prepared
by this method (Venkateswarlu and Manjunath, 2004). The effect of different process
parameters, including emulsification time, stirring rate and cooling conditions on the particle size
and the zeta potential of SLNs were previously reported (Ahlin et al., 1998). The lipids used in
this study were trimyristin, tripalmitin, tristearin, a mixture of mono-, di- and triglycerides
(Witepsol® W35, Witepsol® H35) and glyceryl behenate. Poloxamer 188 was used as a
surfactant at 0.5% (w/v). The average particle size of the SLNs was in the range from 100-200
nm. Higher stirring rates did not significantly change particle size, but slightly improved particle
size distribution as determined by polydispersity index. The main disadvantages of this method
include presence of microparticulates due to the inhomogeneous power developed by the
sonicator and the possibility of metal contamination by the probe, which may compromise the
SLNs dispersion quality.
1.1.2.3. Solvent emulsification/evaporation
A preparation of SLNs by this method has been reported in literature (Siekmann and
Westesen, 1996a; Sjostrom and Bergenstahl, 1992). The procedure, given in, begins by
dissolving the lipid in water immiscible organic solvent, such as chloroform, toluene, or
cyclohexane with subsequent emulsification in a surfactant aqueous solution to form an o/w
emulsion. SLN dispersion is subsequently produced upon solvent evaporation and precipitation
of the lipid in the aqueous medium. The average particle size produced by this method depends
on the lipid concentration. For instance, smaller particles would be obtained with lipid
concentrations up to 5% (w/v). Increasing lipid content above this level would produce
dispersions of higher viscosity, and the homogenization efficiency would diminish.
The advantage of this technique is the avoidance of the thermal degradation of drugs.
However, complete removal of solvent is hardly possible. The organic solvent residues might be
problematic due to their toxicity. Additionally, the limited solubility of lipids (e.g. tripalmitin) in
organic solvents necessitates the use of relatively diluted SLNs dispersions (e.g. 0.5-2.5%
(w/w)).
1.1.2.4. Solvent injection method
Dept. of Pharmaceutical Technology CPS, JNTUH
11
The first step in this method is to dissolve the lipid in water miscible solvents, such as
acetone, ethanol, isopropanol, and methanol. The resulting solution is then injected into water,
resulting in lipid crystallization. SLNs will be then obtained by centrifugation. SLNs having
particles of 80-300 nm were prepared by this method (Schubert and Muller-Goymann, 2003).
1.1.2.5. W/O/W double emulsion method
This method is designated for the formulation of hydrophilic drugs in the form of
liposheres (Cortesi et al., 2002). Briefly, the aqueous drug solution is dispersed, under vigorous
stirring, into a molten lipid phase containing lipophilic stabilizers (hydrophobic surfactants) to
form a primary w/o emulsion. This step is followed by dispersing the w/o emulsion into a large
volume of aqueous solution containing hydrophilic surfactant to produce w/o/w double emulsion.
SLNs are subsequently formed by cooling the emulsion, and the product can be separated by
centrifugation or ultrafiltration.
1.1.2.6. Choice of preparation method is dependent on the following factors:
• The desirable particle size of SLNs.
• The surfactants/co-surfactants added in the formulation.
• The intended route of administration.
• The drug stability (if it is thermolabile or not).
Effect of preparation procedure on particle size of SLNs As reported, it is possible to
obtain SLNs with particle size in the range from 30 to 180 nm by ultrasonication (using probe
sonicator) (Siekmann and Westesen, 1996). However, it is difficult to disperse higher lipid
concentrations by probe sonication, and therefore HPH is applied for effectively dispersing SLNs
with high lipid content. By using HPH, a reduction in the average particle size from 474 to 155
nm was obtained after the first homogenization cycle (800 bars). Increasing homogenization
cycles produced SLNs with lower particle sizes, which explain the dependence of the particle
size on the homogenization pressure and the number of cycles (Muller et al., 1995; Schwarz,
1995). For example, for poloxamer 188 stabilized systems, the optimal dispersion was obtained
Dept. of Pharmaceutical Technology CPS, JNTUH
12
12
with 500 bars at three cycles (Schwarz, 1995). Solvent-emulsification is sometimes considered
superior to melt-homogenization with respect to its ability to produce SLNs with small particle
sizes. This might be explained by the lower homogenization efficiency required for a lipid that is
dissolved in an organic solvent compared to a lipid melt. Additionally, the mobility of surfactant
molecules would be higher in organic solvents than in lipid melt, which promote immediate
coverage of the lipid molecules upon dispersion. In literature, the solvent
emulsification/evaporation process was compared to the melt-homogenization method
(Siekmann and Westesen, 1996). It was found that solvent emulsification method yielded
significantly smaller particles than melt-homogenization at the same production conditions when
lecithin/sodium glycocholate was used to stabilize tripalmitin dispersions. Effect of
surfactants/co-surfactants on particle size during preparation of SLNs It is important to consider
the surfactant composition for production of SLNs with small particle sizes. For instance, it was
found that SLNs stabilized by phospholipids and nonionic surfactants and prepared by melt-
homogenization procedure produced smaller particles than those prepared by solvent-
emulsification method (Siekmann and Westesen, 1996). This was attributed to the formulation
composition rather than preparation procedure. The influence of the surfactant concentration on
the SLNs particle size prepared by high shear homogenizer was investigated (Ahlin et al., 1998).
In this study, it was found that the mean particle size decreased with increasing surfactant
concentration up to 2-3% (w/w). Further increase produced large particles. On the contrary,
when using the same ingredients to produce SLNs by HPH resulted in a continuous decrease in
particle size with an increase in lecithin concentrations (Ahlin et al., 1998). These results showed
the difference between the dispersing capacity of HPH and high shear homogenizer.
Inhomogeneous power distribution is observed in high-shear homogenizers. On the other hand,
high pressure homogenizers attain the highest power densities and the most homogenous power
distribution due to the small size of the homogenizing gap (25-30|im) (Mehnert and Mader
2001). Additionally, the increase in the surface area during HPH occurs very rapidly.
1.1.3. Secondary SLNs production steps
Dept. of Pharmaceutical Technology CPS, JNTUH
13
After the preparation of SLNs, they may be further processed to improve their quality.
These steps include sterilization and/or lyophilization.
1.1.3.1. Sterilization
The next step after the production of SLNs includes the sterilization of the prepared
dispersions. Sterilization is essential especially if SLNs are taken by parenteral or ophthalmic
routes. The common sterilization methods for pharmaceutical dosage forms are autoclaving
(steam sterilization), filtration, and y-irradiation. Some times, sterilization by filtration is not
practical because the possibility of membrane clogging if the particles are greater than 0.2 nm.
Sterilization by autoclaving at 121 °C for 15-20 minutes is the most popular and convenient
method; however, it always creates the following concerns:
• Temperature-induced drug degradation.
• Formation of supercooled lipid melts with uncontrolled recrystallization of molten lipid,
resulting in the loss of controlled release properties (Manjunath et al., 2005).
• Possibility of particle size aggregation
1.1.3.2. Lyophilization
SLNs are aqueous dispersions. The presence of water may create undesirable storage
stability issues resulting in drug degradation or particle size agglomeration due to Ostwald
ripening (Mehnert and Mader, 2001). Therefore, it may be necessary to convert SLN aqueous
dispersions into dry product by freeze-drying or called lyophilization. The basic principle of this
technique is to freeze the sample at approximately -60 to -80 °C, and then subliming the ice into
water vapors under lower pressure. However, lyophilization usually is accompanied by changes
in the properties of the surfactant layer around the lipid particles, and an increase in the particle
concentration, which favor particle aggregation (Mehnert and Mader, 2001). Therefore,
lyohilization may result in close packing of lipid nanoparticles resulting in poor reconstitution in
water. To overcome this problem, cryoprotectants (e.g. trehalose, glucose, mannose, mannitol,
sorbitol, sucrose, lactose), or so-called lyoprotectants, are used during the lyophilization process.
The mechanism of these cryoprotection include the formation of a hydrophilic sheath interacting
Dept. of Pharmaceutical Technology CPS, JNTUH
14
with the polar head groups of surfactants around lipid particles (Mobley ans Schreier 1994),
which is easily reconstituted by simple shaking with water. The freezing process should be
optimized because it critically affects the quality of lyophilizate, such as the crystal structure. For
instance, rapid cooling leads to production of small and amorphous lyophilizates; whereas, slow
freezing leads to the formation of large crystals (Mehnert and Mader, 2001). Generally, in order
to obtain a good quality lyophilizate, samples should be of low lipid content, up to 5% (w/v), and
mixed with the cryoprotector trehalose (Cavalli et al., 1997; Heiati et al., 1998). Slow freezing in
ultrafreezer (at -70 to -80 °C) is superior to rapid cooling using liquid nitrogen; furthermore,
some modifications in thermal treatment to SLN dispersions (e.g. 2 hours at -22°C followed by a
2-hour temperature decrease to -40°C) was found to improve the lyophilizate quality
(Zimmermann et al., 2000).
1.1.3.3. Spray cooling :( Jannin et al., 2008)
Spray cooling also referred to as spray congealing is a process whereby the molten
formula is sprayed into a cooling chamber. Upon contact with the cooling air, the molten
droplets congeal and re-crystallize into spherical solid particles that fall to the bottom of the
chamber and subsequently collected as fine powder. The fine powder may then be used for
development of solid dosage forms — tablets or direct filling into hard shell capsules.
1.1.3.4. Spray drying: (K.Manjunath, J.Suresh Reddy and V.Venkateswarlu).
This an alteranative method to lyophilization to convert aqueous dispersion of SLN’s in
to dry product.Spray drying is defined as a process by which a liquid solution is sprayed into a
hot air chamber to evaporate the volatile fraction, i.e. the organic solvent or the water contained
in an emulsion. The process yields solid microparticles. The same equipment described for spray
cooling can be used for spray drying, the main difference relating to the temperature of the air
circulating in the atomizer chamber.
1.1.4. Types and proposed structures of SLNs
Classical SLNs
Dept. of Pharmaceutical Technology CPS, JNTUH
15
SLN structure was postulated based on the difference in melting points between the drug
and the lipid matrix (Zur Mtihlen et al., 1998). The three hypothetical models for classical SLN:
drug-enriched core, drugenriched shell, and solid solution are shown in Figure 2 (Müller, R. H.
Mäder, K., and Gohla, S., 2000). In core-enriched model, the drug concentration in the molten
lipid is near its solubility limit (Zur Miihlen and Mehnert, 1998). The drug in this case will
precipitate before lipid crystallization during cooling step because of drug supersaturation, and
subsequently surrounded by a lipid shell (Figure 2) (Müller, R. H. Mäder, K., and Gohla, S.,
2000). .An example of this model is prednisolone-loaded SLNs, which have inner crystalline
drug core and an outside amorphous shell (Zur Miihlen et al., 1996). In contrast, in the drug-
enriched shell model, the lipid crystallization precedes drug precipitation. The drug
concentration is below its saturation solubility; thus, upon cooling, the lipid crystallizes, and the
drug partitions into the lipid phase forming drug-rich layer covering the lipid core (Figure 2). If
drug precipitation and lipid crystallization occur simultaneously, the drug will be molecularly
dispersed in the lipid matrix
Fig: 2. Proposed structural models for drug loading profiles in lipid nanoparticles (Müller, R. H. Mäder, K., and Gohla, S., 2000).
1.1.5. Drug loading in SLNs
Drug loading implies drug localization in the solid lipid matrix. During the preparation of
classical SLNs and NLCs, heating and subsequent cooling processes affect distribution of the
Dept. of Pharmaceutical Technology CPS, JNTUH
16
drug between the lipid and the aqueous phases. Drug molecules may be accommodated in the
interspaces of the fatty acids chains of the lipid crystal lattice, resulting in biphasic release
kinetics. Factors determining the loading capacity of a drug in the solid lipid are (Kaur et al.,
2008a):
• Drug solubility or miscibility in the molten lipid.
• Chemical and physical structure of solid lipid matrix.
• Polymorphic state of the lipid material.
Typically, drug solubility is higher in the molten lipid than in the solid lipid (Kaur et al.,
2008a): Solubilizers can be added to enhance drug solubility in the lipid melt. Alternatively,
lipids that contain mono- and diglycerides may be used to promote drug solubilization. High
loading capacity can be achieved by adding liquid oils in SLNs to produce NLCs. Loading
capacity can also be affected by the chemical nature of the lipid. For instance, lipids with highly
crystalline particles form perfect lattice leading to drug expulsion (Westesen et al., 1993). Lipids
that are mixtures of mono-, di- and triglycerides and/or containing fatty acids of different chain
lengths form crystals with many imperfections offering more space to accommodate the drugs.
Polymorphic transformations of lipid have a crucial role in drug loading. Generally, lipid
nanoparticles recrystallize preferentially in the P'-modification, which transforms to the stable P-
form (Westesen et al., 1993). The formation of the more stable modification leads to a decrease
in the number of imperfections in the lattice and thereby promoting drug expulsion. Triglyceride
lipids with long chain fatty acids undergo transformation more rapidly than those with short
chain fatty acids (Westesen et al., 1993). Therefore, with a gradual transformation from P'- to P-
forms, a controlled drug delivery would be maintained (Jenning and Gohla 2001). In PLN, the
physicochemical compatibility between the drug-polymer complex and the solid lipid phase is
the primary factor that predicts drug loading (Wong et al., 2007a).
1.1.6. Drug release from SLNs
Drug release rate from is determined by the physicochemical properties of the lipid
material (Scholer et al., 2002), choice of surfactant composition and ratio (Kabanov and
Dept. of Pharmaceutical Technology CPS, JNTUH
17
Alakhov, 2002; Scholer et al., 2001), particle size and the inner structure of SLNs. In most cases,
drug release kinetics exhibits a biphasic feature: a burst release followed by a sustained or
prolonged release (Wong et al., 2007a). Drug release is also influenced by its localization in the
solid lipid. For example, in drug-enriched core model, sustained release profiles are usually
obtained. On the other hand, in both drug-enriched shell and solid solution models fast drug
release kinetics are expected.
During Homogenization During Cooling
Fig: 3. Proposed redistribution of drug from molecularly dispersed state to enriched shell state, postulated as a cause of drug burst release phenomena observed in lipid nanoparticles (Müller, R. H. Mäder, K., and Gohla, S., 2000).
1.1.7. Characterization of SLNs
SLNs and NLCs may be characterized with respect to particle size and zeta potential,
particles morphology and/or shape, crystallinity and lipid modification, mobility of molecules
within nanoparticles, drug entrapment efficiency (EE), and in vitro drug release for the
assessment of drug release from SLNs.
1.1.7.1. Measurement of particle size and zeta potential:
Photon Correlation Spectroscopy (PCS) and Laser Diffraction (LD)
Dept. of Pharmaceutical Technology CPS, JNTUH
18
PCS, also called dynamic light scattering, measures the fluctuation of the scattered light
intensity caused by particle movement (Mehnert and Mader, 2001). Therefore, it estimates the
particle hydrodynamic radius. PCS, however, can not detect large microparticles more than 3 ^m.
LD method, on the other hand, is used to measure particle size from few nanometer up to several
millimeter. This method is based on measuring the degree of light diffraction from the surface of
particles. Zeta potential is defined as the difference in potential between the actual particle
surface and the dispersion medium (bulk medium). The zeta potential value primarily depends on
two parameters: Surface charge of the particle and the presence of adsorbed layers at the
interface. For example, presence of ionic surfactants, either anionic or cationic, greatly
influences the zeta potential and hence overall SLNs stability by providing electric repulsion
between particles. Improved stability is expected if zeta potential is greater than +30 mV (for
cationic surfactants) or lower than -30 mV (for anionic surfactants) (Lai et al., 2006). Non ionic
surfactants, such as polyethylenepolypropylene block co-polymers (poloxamer 188), stabilize the
particles by serving as steric stabilizers, thereby preventing particle flocculation and coalescence
(Porter, 1994).
1.1.7.2. Observation of particle morphology and/or shape in SLNs Scanning Electron
Microscopy (SEM) and Transmission Electron Microscopy (TEM):
They provide information about the morphology of lipid nanoparticles and can be utilized
for the estimation of particle size. While the same instrument provides SEM and TEM, they
utilize different principles for particle observation. In SEM, backscattered or secondary electrons
transmitted from the specimen surface are observed; whereas, in TEM the electron beam
transmitted through the sample is detected. In TEM, it is possible to use visualization
enhancement tools, such as staining with phosphotungestic acid. In either method, however,
SLNs might be coated with gold for improved visualization and particle size determination.
Alternatively, cryo-imaging (such as cryo-SEM or cryo-TEM) in which specimen is quickly
frozen in order to reduce the morphological distortion important for structural observations, may
be used for the visualization of SLNs. It was reported that SLNs made from well-defined lipids
of high purity (e.g. pure triglycerides) might have cubical or platelet-like patterns, and the
chemically homogenous lipids tend to form more perfect crystals with the typical platelet-like
Dept. of Pharmaceutical Technology CPS, JNTUH
19
19
pattern (Siekmann and Westesen, 1992; Westesen and Wehler, 1993). Measurement of
crsytallinity, lipid modification, and other colloidal structure that might coexist with SLNs After
crystallization, solid lipids might undergo polymorphic changes or modifications resulting in
instability associated with drug incorporation and release. Alternatively, lipids might not
crystallize at room temperature, thereby producing supercooled melts, which may aggregate to
form liposomes or some other colloidal structures. For instance, Dynasan® 112 SLNs could
remain as a supercooled melt for several months, which generally happens to SLNs due to the
small size of the particles and the presence of surfactants that retard lipid crystallization
(Westesen and Bunjes, 1995). Lipid crystallinity, lipid modification, and co-existence of other
colloidal structures can be detected by Differential Scanning Calorimetry (DSC), and X-ray.
Diffraction (XRD), Infrared and Raman spectroscopy, and Proton Nuclear Magnetic Resonance
('H-NMR) spectroscopy.
1.1.7.3. Differential Scanning Calorimetry (DSC):
This technique can be widely applied to investigate the crystalline status of the lipid.
Different lipid polymorphs possess different melting points and enthalpies. Therefore, the
presence of drugs inside the lipid core as amorphous or crystalline lattice, the presence of
supercooled lipids, and the interaction of lipid components with other SLNs ingredients could be
evaluated by DSC.
1.1.7.4. X-ray diffraction (XRD):
Detecting the crystalline state of lipids and/or the presence of drug either in amorphous or
crystalline state is a challenge. Therefore, complementary to DSC, XRD was used to collect
information about formulation parameters (such as crystallization temperature) and to detect
phase separation that might occur during SLN preparation. Time resolved XRD studies, for
example, was used to study the kinetic phenomena associated with the polymorphic transition of
lipids (Bunjes and Koch, 2005).
1.1.7.5. Determination of entrapment efficiency:
Dept. of Pharmaceutical Technology CPS, JNTUH
20
In the cooling step, during the production of SLNs, drug expulsion might occur.
Therefore, after SLNs preparation, it is important to measure the amount of drug incorporated,
which is a measurement of the solid lipid efficiency to encapsulate the drug. The entrapment
efficiency can be calculated from the following equation:
Amount of drug entrapped in SLNs
Entrapment efficiency (%) = × 100
Theoretical total amount of drug added to SLNs
The amount of drug incorporated is calculated by subtracting the free drug from the total
amount of drug added to the SLNs. In order to determine the free drug, it must be separated from
the drug loaded into SLNs by ultracentrifugation, centrifugation filtration, or gel permeation
chromatography. In centrifugation filtration, filter assembly is used. This assembly consists of
sample chamber and a recovery chamber. Both chambers are separated by a filter with specific
molecular weight cutoff (e.g. molecular weight cutoff (MWCO) is 100,000 Da). After
centrifugation, the supernatant aqueous solution containing the free drug is analyzed by HPLC,
spectrophotometry, or spectrofluorophotometry. The quantity of the entrapped drug can be then
estimated using the equation given above. In gel permeation chromatography, Sephadex® and
Sepharose® gels are used to remove the free drug from the SLNs.
1.1.7.6. In vitro drug release from SLNs:
1.1.7.6.1. Dialysis tubing to from a bag
In this method, SLNs nanodispersion is placed in dialysis tubing, which can be
hermetically sealed to from a dialysis bag having a molecular weight limit of 12,000-14,000. The
bag allows the transport of free drug while hindering the passage of SLNs with the encapsulated
drug. The bag is then placed in a continuously stirred and suitable dissolution medium, usually a
buffer, at 37 °C. In addition to stirring, sink conditions can be maintained by the addition of
0.5% polysorbate 80, sodium dodecyl sulfate, or 30% ethanol. Aliquots are withdrawn from the
receiving compartment at different time intervals, centrifuged and analyzed for drug content.
1.1.7.6.2. Reverse dialysis
Dept. of Pharmaceutical Technology CPS, JNTUH
21
In this technique, the small dialysis bag is filled with one mL of the dissolution medium
and then placed in SLN dispersion. The sample inside the bag is analyzed for drug content. This
technique is usually applied for potent compounds.
1.1.7.6.3. Franz diffusion cell
There are two types of Franz diffusion cells: side-by-side and vertical diffusion cells.
Both are utilized for the assessment of drug release from SLNs. Franz diffusion cell consists of a
donor compartment in which SLN sample is placed, and a receptor compartment containing the
dissolution medium, usually buffer, to which the drug will diffuse. The two compartments are
separated by a cellophane membrane of suitable molecular weight cutoff size. The temperature in
both compartments is maintained at 37 °C. Sink conditions are attained by the addition of 0.5%
polysorbate 80, sodium dodecyl sulfate, or the addition of 30% ethanol. Aliquots are withdrawn
from the receptor compartment at different time intervals, centrifuged and analyzed for drug
content.
1.1.7.6.4. In vitro Dissolution Testing
A number of biorelevant dissolution test media and experimental methodologies have
found application in assessing drug release from both lipid-based and conventional oral
formulations (Dressman et al., 2005 & 2007). Unlike conventional dosage forms, from which the
drug substance simply dissolves in the aqueous dissolution test media, lipid-based formulations
release the drug from an oily solution which is often immiscible with water.
1.1.8. Problems encountered during SLN preparation:
The following are problems that might appear during or after SLNs preparation:
• Existence of supercooled melts.
• Presence of some lipid modifications.
• Change in the shape of lipid nanodispersions with a possibility of gelation.
• Coexistence of several colloidal species.
Dept. of Pharmaceutical Technology CPS, JNTUH
22
1.1.8. 1. Existence of supercooled melts
Supercooling is defined as the temperature difference between the melting and
crystallization points. It describes a phenomenon whereby the lipid fails to crystallize although
the sample is stored at a temperature below the melting point of the lipid. This occurs most often
when SLNs are prepared by the application of heat such as melthomogenization. Supercooling
range can reach 30-40°C. In this case, lipid dispersions are considered nanoemulsions rather than
SLNs.
1.1.8. 2. Coexistence of several colloidal species
During the preparation of SLNs, other colloidal species might be formed. These include
micelles, liposomes, and surfactant monomers. For example, when sodium dodecyl sulfate (SDS)
is used, it might produce micelles or mixed micelles. Lecithin, on the other hand, could form
liposomes (Siekmann and Westesen, 1998). The problem of these colloidal species is that they
are considered as alternative sites for drug incorporation. Lipophilic drug molecules might be
relocated into the micelles rather than into the solid lipid matrix of SLNs, resulting in the
hydrolysis of unstable drugs in the aqueous environment, or it might result in burst drug release.
1.1.8. 3. Gelation phenomena
Gelation can be defined as the transformation of SLNs from low-viscosity dispersions
into a viscous gel, which involves the loss of the colloidal particle size (Mehnert and Mader,
2001). The possible mechanism for gelation is the increase in the surface area of the particles due
to platelets formation, which could not be covered by sufficient surfactant molecules (Siekmann
and Westesen, 1994a). Gelation may occur when the lipid phase undergoes structural changes
resulting in a decrease in the zeta potential with a consequent particle growth (Freitas and
Miiller, 1998). There are different factors that potentiate gelation, such as high lipid
concentrations and high ionic strengths (Bunjes et al., 1996; Freitas and Miiller, 1999a). Rapid
crystallization of the lipid may promote gelation (Bunjes et al., 1996). One of the stability
indicators of SLNs is their zeta potential. Stable samples have a zeta potential greater than -25
mV, while SLNs having -15 mV zeta potential would produce gel (Freitas and Miiller, 1999a).
Other environmental factors may also contribute to SLNs stability and gelation. For instance, it
Dept. of Pharmaceutical Technology CPS, JNTUH
23
23
was found that high temperatures and the exposure to light and mechanical stress, which increase
the kinetic energy and particles' collision, promote SLN gelation (Freitas and Miiller, 1998).
Oxygen may facilitate fatty acids oxidation with subsequent gelation. Gelation can be retarded or
prevented by the addition of surfactants or coemulsifying surfactants, such as glycocholate
(Westesen and Siekmann, 1997). Storage in dark place at 8°C may prevent particle growth
(Freitas and Muller, 1998). Samples stored under nitrogen were shown to be more stable than
samples exposed to air due to the inhibition in the lipid degradation (Freitas, 1998).
1.1.9. General stability aspects and storage stability of SLNS:
Typically, the shelf life of SLNs formulation should be at least one year (Wong et al.,
2007a). The criteria for the assessment of long-term physical stability of SLN include particle
size and size distribution, zeta potential, drug content, drug entrapment efficiency, and drug
expulsion outside the lipid matrix during storage. All of these parameters are influenced by the
lipid type, sterilization process, and lyophilization. Temperature and light are considered as the
most important factors affecting SLN stability, and therefore for optimum long-term stability of
SLNs, vials containing SLNs should be stored at controlled temperature and protected from light.
1.1.9.1. Zeta potential:
Optimum zeta potential for physically stable dispersions should be, in general, higher
than -60 mV. A decrease in zeta potential may lead to particle agglomeration and rapid growth in
particle size, which usually occurs when SLNs are stored at 50 °C (Freitas and Miiller, 1998).
Although in some cases long term storage at 20 °C does not result in SLN aggregation, storage
of SLNs at 4 °C is generally more favorable (Freitas and Miiller, 1998).
1.1.9.2. Particle size and size distribution:
Particle size is a critical safety factor for SLNs parenteral administration. In addition, it
greatly affects SLNs biodistribution and clearance by the reticuloendothelial system (RES). The
degree of polydispersity can affect particle size growth and the overall drug release. The factors
that influence change in particle size and/or zeta potential include type of lipid, sterilization
process, lyophilization, storage temperature, light, and packing materials.
Dept. of Pharmaceutical Technology CPS, JNTUH
24
1.1.9.3. Drug expulsion outside lipid matrices:
Lipid polymorphism greatly affects SLNs stability. For illustration, during storage of
SLNs, or even in the cooling step during SLNs preparation by melt emulsification, the lipid may
crystallize or produce supercooled melts. If the lipid favors crystallization into a specific
polymorph that has low intermolecular distance within its matrices (i.e. with perfect crystalline
structure), subsequent drug expulsion leakage may occur.
1.1.9.3.1. Factors affecting drug expulsion outside lipid matrices
1.1.9.3.1.1. Type of lipid:
Waxes usually lead to slower particle growth and aggregation than glycerides (Jenning
and Gohla, 2000). Monoglycerides-containing lipids, such as Dynasan® 116 and Compritol®
888 ATO, are usually more stable than those do not contain monoglycerides. Ionic surfactants
stabilize SLNs better than non-ionic (Wong et al., 2007a).
1.1.9.3.1.2. Sterilization process:
Sterilization, which is required for SLNs administration by the parenteral route, may
affect the physical stability of SLNs, particularly particle size and zeta potential. Steam
sterilization (autoclaving) may affect the physical stability, depending on the lipids, surfactants,
and drugs used in SLNs preparation. In one study, autoclaving was found to have insignificant
effect on the particle size, polydispersity index (PI), and zeta potential of SLNs (Cavalli et al.,
1997). On the contrary, and in another study, a 2-3 fold increase in SLNs particle size, a shift of
zeta potential from positive to negative, and a slight drop in drug loading was reported after
sterilization of SLNs (Muller et al., 2006; Penkler et al., 2003).
1.1.9.3.1.3. Lyophilization (freeze drying):
Lyophilization is usually utilized to prevent SLN aggregation or drug hydrolysis due to
the aqueous medium. It is carried out using cryoprotectants, such as trehalose, glucose, lactose,
mannose, mannitol, or sucrose, to stabilize SLNs and to prevent lipid adhesion after freeze
drying by forming a hydrophilic protective sheath (Shahgaldian et al., 2003). Optimization of the
Dept. of Pharmaceutical Technology CPS, JNTUH
25
25
lyophilization process parameters, such as freezing velocity and application of different
lyophilization cycles, is essential to produce reconstitutable SLNs that are suitable for
intravenous injection (Zimmermann et al., 2000). However, a few studies have shown particle
size increase by freeze drying (Schwarz and Mehnert, 1997). In these studies, the reconstituted
SLNs attained sufficiently small and stable particle sizes that could be administered orally.
1.1.9.4. Storage temperature, light, and packing materials:
During storage, the stability of SLNs might be affected by strong light, high temperature,
and packing material, all of which will decrease particles' zeta potential and induce particle
aggregation (Freitas and Miiller, 1998). For instance, it was found that by storing SLNs in
siliconized vials at 8 °C in the dark, a significant stability over 3 years for Compritol® SLNs
dispersions was achieved (Freitas and Miiller, 1998).
1.1.10. Possible administration routes of SLNs and their in vivo fate:
1.1.10. 1. Oral administration
SLNs can be formulated as tablets, pellets or capsules for oral drug delivery. Some times,
the acidity and high ionic strength of the microclimate of the stomach favors particle
aggregation. However, oral administration of SLNs has the advantages of reproducible
bioavailability (less variability in drug plasma levels) and prolonged drug plasma levels.
Bioavailability enhancement and reproducibility after oral administration could be demonstrated
by Cyclosporine A, which was formulated into SLNs for oral administration (Miiller et al., 2006;
Penkler et al., 2003).
1.1.10. 1.1. Mechanism of oral absorption:
Mechanism of oral absorption enhancement and reproducible bioavailability, the
increased absorption and reproducible bioavailability, observed in certain drugs formulated in
SLNs after oral administration, might be attributed to adhesiveness of nanoparticles to the GIT
membrane (Muchow et al., 2008). Adhesiveness due to the small particle size and large surface
area of SLNs leads to fast and specific drug release at the site of absorption (Liversidge and
Cundy, 1995). Furthermore, lipid nanoparticles are ultrafme dispersions, and thus they have the
Dept. of Pharmaceutical Technology CPS, JNTUH
26
26
potential to enhance oral bioavailability. For instance, after the digestion of lipids in the gut, the
formation of surface active monoand diglycerides may produce micelles that can entrap drugs
within their cores leading to drugs solubilization (Figure 4) (Muller and Keck, 2004). The
solubilized drug molecules may interact with the biological bile salts to produce mixed micelles
with a consequent increase in drug absorption (Muller and Keck, 2004). It was also reported that
in SLNs the fatty acid chain length may affect the site of absorption (Porter and Charman,
2001a). For instance, fatty acids with C-14 to C-18 chains promote lymphatic drug absorption
(Porter and Charman, 2001a). By this specific drug delivery to the lymphatics, it is possible to
avoid first pass metabolism, and thereby enhancing the oral bioavailability of drugs (Porter and
Charman, 200la). Schematic representation of the mechanisms of drug absorption from SLNs
and the promoting effect of lipids (Miiller and Keck, 2004) Camptothecin was prepared in SLNs
using stearic acid and a blend of lecithin and poloxamer 188 as stabilizers (Yang et al., 1999). In
addition to protecting the drug against hydrolysis, offered by SLNs, the plasma profile of SLNs
exhibited a first burst drug release attributed to the free drug with a subsequent controlled drug
release that was attributed to prolonged gut uptake of SLNs (Yang et al., 1999).
Fig: 4. Drug solubilizatin in the GIT
1.1.10. 1.2. Potential effect of lipids and lipidic excipients on drug absorption :
There are three primary mechanisms by which lipids and lipophilic excipients affect drug
absorption, bioavailability and disposition after oral administration. Lipids can affect drug
absorption (Porter et al., 2007):
Dept. of Pharmaceutical Technology CPS, JNTUH
27
By enhancing drug (D) solubilization in the intestinal milieu through alterations to the
composition and character of the colloidal environment — for example, vesicles, mixed
micelles and micelles,
By interacting with enterocyte-based transport and metabolic processes, thereby
potentially changing drug uptake, efflux, disposition and the formation of metabolites
(M) within the enterocyte,
By altering the pathway (portal vein versus intestinal lymphatic system) of drug transport
to the systemic circulation — which in turn can reduce first-pass drug metabolism as
intestinal lymph travels directly to the systemic circulation without first passing through
the liver. Cellular junctions are represented by green ovals, and a representative transport
protein is depicted by a blue oval.
Fig: 5 Effect of lipids on drug absorption
1.1.10. 2. Parenteral administration:
SLNs could be injected either intravenously, intramuscularly, or subcutaneously. When
injected intravenously, SLNs could be used to target drugs to specific tissue or organ. However,
the particle size must be below 5 (am to avoid embolism. The hydrophobic surfaces of SLNs
cause rapid clearance from the circulation by the RES and uptake by the liver, spleen and other
parts of the RES. In order to produce particles with longer circulation times, SLNs should be 100
nm or less in diameter with a hydrophilic surface in order to reduce clearance by macrophages
(Storm et al., 1995). Thus, to facilitate drug targeting to tumor tissue, "stealth SLNs" could be
Dept. of Pharmaceutical Technology CPS, JNTUH
28
28
prepared by pegylation, or by incorporating polyethylene glycol, to form a hydrophilic sheath
around SLNs. Such modification creates chains at the particle surface, which will repel plasma
proteins and thereby evading the RES and increasing tumor accumulation (Chen et al., 2001).
SLNs may also act as a sustained or depot release after subcutaneous administration in which
case drug release rate is controlled by the nature of the lipid, surfactant, particle size, and inner
structure of SLNs.
1.1.10. 3. Topical administration:
SLN’s posses a number of advantages for the topical route of administration due to small
particle size.SLN’s ensure close contact to stratum corneum and there by increases penetration of
encapsulated drug in to the viable skin.Sustained release of the drug from SLN’s supplies the
drug to the skin over a prolongrd period and there by reduces systemic absorption.SLN’s showed
occlusive properties as a result of film formation on the skin, which reduces transdermal water
loss. Increase of water content in the skin reduces the symptoms of atopic eczema and also
improves the appearance of healthy human skin .Occlusion also favours the drug penetration in
the skin (K.Manjunath, J.Suresh Reddy and V.Venkateswarlu).
1.1.10. 4. Rectal administration:
It is the preffered route of administration in pediatric patients due to ease of application.
Advantage of submicron emulsions and SLN’s over conventional rectal solution is that organic
solvents present in commercial preparations can be avoided. But lower relative bioavailability
was observed for SLN’s compared to solution;the reason repoeted for this is lack of efficient
diffusion through lipid matrix. Therefore the lipid matrix solid at room temperature is not an
advantageous system for rectal delivery of drugs, even if delivered as submicron dispersions.
Thus low melting point lipids were to be selected for formulations of rectal delivery drugs.to
achieve prolonged release of drug as well as higher absorption and bioavailabilites
(K.Manjunath, J.Suresh Reddy and V.Venkateswarlu).
1.1.10. 5. Ocular administration
Dept. of Pharmaceutical Technology CPS, JNTUH
29
SLN’s appear as a promising delivery system for occlular administration of pilocarpine
and tobramycin. It was observed that when tobramycin loaded SLN’s were administered
topically there is significantly higher bioavailability in the aqueous humor when compared with
the standard commercial eye drops. The increased tobramycin availability in aqueous humour
might be due to entrapment and prolonged retention of SLN’s in the mucin layer covering the
corneal epithelium and /or enhancement of corneal penetration of drug (K.Manjunath, J.Suresh
Reddy and V.Venkateswarlu).
1.1.11. Concluding remarks about SLNs
SLNs represent an innovative and alternative approach for the administration of
challenging drug molecules by overcoming the solubility, permeability, physical stability, and
toxicity problems associated with these drugs. In contrast to polymeric nanoparticles, SLNs
ingredients are particularly safe and free from cytotoxicity problems. Furthermore, large-scale
production of SLNs is feasible by high-pressure homogenization. SLN-based systems are not
only limited to lipophilic compounds, rather, hydrophilic and charged agents can be efficiently
encapsulated. Therefore, SLNs were shown to be an effective delivery of a vast variety of drug
molecules including analgesics, anticancer, antianxiety, antibiotics, and antiviral agents.
Table 2: List of some molecules incorporated in to SLN’s
Molecule Therapeutic useDoxorubicin Various cancers
PrednisoloneInflammation and arthritis
Tetracaine Ophthalmic treatment
Etomidate Anesthetic
Dept. of Pharmaceutical Technology CPS, JNTUH
30
2. LITERATURE REVIEW
2.1. Review Articles,
Wolfgang Mehnert, Karsten Mader presented overview about the selection of the ingredients
used in formulation, different ways of SLN’s production and applications. Aspects of SLN’s
stability , sterilization by lyophilization and spray drying of SLN’s are discussed.Drug
incorporation complexity of SLN’s dispersion,physical state of lipid,analytical methods for
characterization and stability of SLN’s.Administration of SLN’s by various routes , invivo fate
of SLN’s are presented in this article.
K.Manjunath, J.Suresh Reddy and V.Venkateswarlu discussed various lipid
matrices,surfactants,and other excipients used in formulation along with preparation methods,
sterilization and lyophilization of SLN’s ,entrapment efficiency of drug carrier and its effect on
physical parameters ,drug release,and release mechanisms of various compositions,
characterization and stability of SLN’s.Various invivo studies carried out by different research
groups ,administration of SLN’s by various routes ,passive and active targeting using SLN’s are
also presented in this article.
S.A.Wissing, O.Kayser, R.H.Muller describes the use of nanoparticles based on solid lipids for
parenteral application of drugs and structural differences among different types of nanoparticles
such as solid lipid nanoparticles (SLN,nanostructured lipid carriers (NLC) and lipid drug
conjugate (LDC) along with that they described different production methods including
suitability for large scale production,stability issues,drug incorporation mechanisms in to tha
particles,biological activity of parenterally applied SLN and biopharmaceutical aspects as well as
toxicity aspects of SLN.
Anusha Rupenagunta, I.Somasundaram, V.Ravichandiran, J.Kausalya, B.Senthilnathan
reviewed the recent advances ,various method of preparation,methods of evaluation,various
routes of administration,stability and pharmaceutical applications of SLN were discussed and
solid lipid nanoparticles (SLN’S) have been proposed as suitable colloidal carriers for delivery of
drugs with limited solubility.
Dept. of Pharmaceutical Technology CPS, JNTUH
LITERATURE REVIEW REVIEW
32
Katja Jores, Wolfgang Mehnert, Markus Drechsler, HeikeBunjes, Christoph
Johann,Karsten Mader investigated the structures of SLN and NLC based on glyceryl behenate
and medium chain triglycerides were characterized by photon correlation spectroscopy (PCS)
and laser diffraction (LD), field flow fractionation (FFF) with multi-angle light scattering
detection (MALS),and cryo transmission electron microscopy (cryo TEM).
2.2. Research Articles
Kopparam Manjunath & Vobalaboina Venkateswarlu incorporated Nitrendipine in to SLN
prepared by hot homogenization followed by ultra sonication method using various triglycerides
(trimyristin,tripalmitin and tristearin),soy phosphatidylcholine 95%,polaxamer 188 and charge
modifiers (dicetyl phosphate,DCP and stearyl amine,SA) . Dispersion was investigated for
particle size and charge, they studied pharmacokinetics after intravenous (i.v.) and intraduodenal
(i.d.) administration to conscious male wistor rats and tissue distribution studies were carried out
in Swiss albino mice after intravenous (i.v.) administration and compared to Nitrendipine
suspension.
Roberta Cavalli, Otto Caputo, Maria Rosa Gasco describes the development of stealth and
non-stealth solid lipid nanospheres (SLN’s) as colloidal carriers for paclitaxel by using lipid
materials such as tripamitin and phosphatidyl choline. These particulates were evaluated for
particle size, drug release and finally sterilized and freeze-dried.They conducted thermal analysis
(DSC), stability studies.
YiFan Luo, DaWei Chen, LiXiang Ren, XiuLi Zhao, Jing Qin prepared the vinpocetine
loaded Glyceryl monostearate nanoparticles with narrow size distribution by ultra-Solvent
emulsification .To increase the lipid load this process was conducted at 50oC, and these
nanoparticles were evaluated for particle size and size distribution, drug loading capacity, drug
entrapment efficiency (EE%), zeta potential and drug release by using a dialysis bag method.
Michele Trotta, Francesca Debernardi, Otto Caputo prepared the Glyceryl monostearate
nanoparticles with narrow size distribution by Solvent emulsification-diffusion technique using
butyl lactate or benzyl alcohol solvents and lecithin, taurodeoxycholic acid sodium salt, as
surfactants. To increase the lipid load this process was conducted at 47+2oC.
Dept. of Pharmaceutical Technology CPS, JNTUH
LITERATURE REVIEW
33
Kopparam Manjunath & Vobalaboina Venkateswarlu incorporated Clozapine in to SLN
prepared by hot homogenization followed by ultra sonication method using various triglycerides
(trimyristin,tripalmitin and tristearin),soy phosphatidylcholine 95%,polaxamer 188 and charge
modifiers (dicetyl phosphate,DCP and stearyl amine,SA) . Dispersion was investigated for
particle size and charge, they studied pharmacokinetics after intravenous (i.v.) and intraduodenal
(i.d.) administration to conscious male wistor rats and tissue distribution studies were carried out
in Swiss albino mice after intravenous (i.v.) administration.
Zaida Uraban-Morlan, Adriana Ganem-Rondero,Luz Maria Melgoza-Contreras,Jose Juan
Escobar-Chavez,Maria Guadalupe Nava-Arzaluz,David Quintanar-Guerrero prepared
cyclosporine loaded solid lipid nanoparticles by the emulsification- diffusion method using
Glyceryl behenate (Compritol ATO 888) and lauroyl macrogol glycerides (Gelucire 44/14) as
matrix materials and evaluated for particle size, zeta potential ,XRD,SEM analysis.
Severine Jaspart, Pascal Bertholet, Geraldine Piel, Jean-Michel Dogne, Luc Delattre,
Brigitte Evrard studied sustained release profile of salbutamol acetomide (SA) loaded soli lipid
microparticles, which are produced by hot emulsion technique followed by high shear
homogenization and investigated for particle size, XRD,SEM analysis.
Annete zur Muhlen, Cora Schwarz, Wolfgang Mehnert incorporated the model drugs
tetracaine, etomidate and prednisolone in to solid lipid nanoparticles by high pressure
homozgenization for parenteral drug administration using Compritol 888 ATO and Dynasan 112
as matrix material and these particulate systems are investigated for particle size, drug load
capacity, effect of drug incorporation on the structure of the lipid matrix and the release profiles
and mechanism.
Chrysantha Freitas, Rainer H.Muller assess the destabilizing factors by using a poloxamer
188 stabilized Compritol SLN formulation and stability was investigated as function of storage
temperature,light exposure and packing material (untreated and siliconized vials of glass quality
I)
Dept. of Pharmaceutical Technology CPS, JNTUH
LITERATURE REVIEW
34
3. OBJECTIVE OF THE STUDY
The objective of the present study is to develop Solid lipid nanopaticles (SLN’s) for
Valsartan to increase its saturation solubility in low pH conditions and dissolution velocity for
enhancing bioavailability while reducing variability in systemic exposure.
Valsartan is poorly soluble and the aqueous solubility, solubility in low pH is reported to
be less than 1mg/ml and is having pH dependent solubility.The drug is rapidly absorbed
following oral administration, with a bioavailability of about 23%,peak plasma concentrations of
valsartan occur 2 to 4 h after an oral dose.Therefore,it is necessary to enhance the aqueous
solubility,solubility in low pH and dissolution rate of valsartan to obtain faster onset of
action.The approach used in this study is to formulate the poorly soluble model drug, Valsartan
as drug loaded solid lipid nano dispersion which is converted into a solid form by vaccum drying
.The solid state characteristics of the dried product shall be investigated with DSC studies. The
dissolution studies of the solid dosage form of valsartan SLN’s will be carried out in
discriminating dissolution conditions and shall be compared with the commercially available
Valsartan capsule dosage form (DIOVAN).
Dept. of Pharmaceutical Technology CPS, JNTUH
OBJECTIVE
36
4.1. API Information
Valsartan (Merck Index, rxlist.com, drugs.com, medline plus)
Valsartan (marketed as Diovan® by Novartis Company) is a nonpeptide, orally active
and specific angiotensin II antagonist acting on the AT1 receptor subtype.
Category: Angiotensin receptor blocker.
Structure:
Fig: 6 Structure of Valsartan
Chemical name: N-(1-oxopentyl)-N-[[2´-(1H-tetrazol-5-yl) [1, 1´-biphenyl]-4-yl] methyl]-L-
valine.
Empirical formula: C24H29N5O3
Molecular weight: 435.5
Dept. of Pharmaceutical Technology CPS, JNTUH
DRUG PROFILE
37
Aqueous solubility: 2.34e-02 g/l
LogP: 5.8
Melting point: 116-117oC
State: Amorphous Solid
Description:
Valsartan is a white to practically white fine powder. It is soluble in ethanol and methanol
and slightly soluble in water.
.Mechanism of Action:
Angiotensin II is formed from angiotensin I in a reaction catalyzed by angiotensin-
converting enzyme (ACE, kininase II). Angiotensin II is the principal pressor agent of the renin-
angiotensin system, with effects that include vasoconstriction, stimulation of synthesis and
release of aldosterone, cardiac stimulation, and renal reabsorption of sodium. Valsartan blocks
the vasoconstrictor and aldosteronesecreting effects of angiotensin II by selectively blocking the
binding of angiotensin II to the AT1 receptor in many tissues, such as vascular smooth muscle
and the adrenal gland. Its action is therefore independent of the pathways for angiotensin II
synthesis. Table No.3: Characteristics of Valsartan
Systematic (IUPAC) name(2S)-3-methyl-2-[N-({4-[2-(2H-1,2,3,4-tetrazol-5-yl)phenyl]phenyl}methyl)pentanamido]butanoic acid
CAS number 137862-53-4Formula C24H29N5 O3
Mol. mass 435.5188 g/molBioavailability 23% with high variabilityProtein binding 94 - 97% bound to serum proteinsVolume of distribution (Vd) 17 L (low tissue distribution)Metabolism Hepatic 4-hydroxylation
Half-lifeThe initial phase t1/2 α is < 1 hour while the terminal phase t1/2 β is 5-9 hours
Excretion feces Legal status ℞ Prescription only
Dept. of Pharmaceutical Technology CPS, JNTUH
DRUG PROFILE
38
Routes Oral
Pharmacodynamics:
Valsartan is a specific and selective type-1 angiotensin II receptor (AT1) antagonist
which blocks the blood pressure increasing effects angiotensin II via the renin-angiotensin-
aldosterone system (RAAS). During sympathetic stimulation or when renal blood pressure or
blood flow is reduced; renin is released from granular cells of the juxtaglomerular apparatus in
the kidneys. Renin cleaves circulating angiotensinogen to angiotensin I, which is cleaved by
angiotensin converting enzyme (ACE) to angiotensin II. Angiotensin II increases blood pressure
by increasing total peripheral resistance, increasing sodium and water reabsorption in the kidneys
via aldosterone secretion, and altering cardiovascular structure. Angiotensin II binds to two
receptors: AT1 and type-2 angiotensin II receptor (AT2). AT1 mediates the vasoconstrictive and
aldosterone-secreting effects of angiotensin II. Angiotensin receptor blockers (ARBs) are non-
peptide competitive inhibitors of AT1. ARBs block the ability of angiotensin II to stimulate
pressor and cell proliferative effects. The overall effect of ARBs is a decrease in blood pressure.
Pharmacokinetics:
Absorption — Valsartan peak plasma concentration is reached 2 to 4 hours after dosing.
Valsartan shows bi-exponential decay kinetics following intravenous administration, with an
average elimination half-life of about 6 hours. Absolute bioavailability for Diovan is about 25%
(range 10%-35%). The bioavailability of the suspension is 1.6 times greater than with the tablet.
With the tablet, food decreases the exposure (as measured by AUC) to valsartan by about 40%
and peak plasma concentration (Cmax) by about 50%. AUC and Cmax values of valsartan
increase approximately linearly with increasing dose over the clinical dosing range. Valsartan
does not accumulate appreciably in plasma following repeated administration.
Distribution — the steady state volume of distribution of valsartan after intravenous
administration is small (17 L), indicating that valsartan does not distribute into tissues
extensively. Valsartan is highly bound to serum proteins (95%), mainly serum albumin.
Dept. of Pharmaceutical Technology CPS, JNTUH
DRUG PROFILE
39
Metabolism — Valsartan is excreted largely as unchanged drug (80%) and is minimally
metabolized in humans. The primary circulating metabolite, 4-OH-valsartan, is
pharmacologically inactive and produced CYP2C9. 4-OH-valsartan accounts for approximately
9% of the circulating dose of valsartan. Although valsartan is metabolized by CYP2C9, CYP-
mediated drug-drug interactions between valsartan and other drugs are unlikely.
Excretion — 83% of absorbed valsartan is excreted in feces and 13% is excreted in urine,
primarily as unchanged drug
Indications:
Treatment of hypertension
Treatment of heart failure (NYHA class II-IV);
Reduction of cardiovascular mortality in clinically stable patients with left ventricular
failure or left ventricular dysfunction following myocardial infarction
Table No.4: Recommended dose of Valsartan
Recommended Dosing:
Indication Starting Dose Dose Range Target MaintenanceDose*
Adult Hypertension 80 or 160 mg once daily 80-320 mg once daily ---
Pediatric Hypertension
1.3 mg/kg once daily (up to 40 mg total)
1.3-2.7 mg/kg once daily (up to 40-160 mg total)
---
Heart Failure 40 mg twice daily 40-160 mg twice daily 160 mg twice daily Post-Myocardial Infarction 20 mg twice daily 20-160 mg twice daily 160 mg twice daily
* As tolerated by patient No initial dosage adjustment is required for elderly patients, for patients
with mild or moderate renal impairment, or for patients with mild or moderate liver
insufficiency. Care should be exercised with dosing of Diovan in patients with hepatic or severe
renal impairment. Diovan may be administered with or without food. In heart failure patients,
consideration should be given to reducing the dose of concomitant diuretics. Following
myocardial infarction, consideration should be given to a dosage reduction if symptomatic
hypotension or renal dysfunction occurs.
Dept. of Pharmaceutical Technology CPS, JNTUH
DRUG PROFILE
40
Use in specific populations:
Nursing Mothers: Nursing or drug should be discontinued.
Pediatrics : Efficacy and safety data support use in 6-16 year old patients.
Geriatrics : overall difference in efficacy or safety vs. younger patients is not found,
but greater sensitivity of some older individuals cannot be ruled out.
Contraindications:
None Warnings and precautions:
Avoid fetal or neonatal exposure
Observe for signs and symptoms of hypotension
Use with caution in patients with impaired hepatic or renal function
Adverse reactions:
Hypertension: Most common adverse reactions are headache, dizziness, viral infection, fatigue
and abdominal pain.
Heart Failure: Most common adverse reactions are dizziness, hypotension, diarrhea, arthralgia,
back pain, fatigue and hyperkalemia.
Post-Myocardial Infarction: Most common adverse reactions which caused patients to
discontinue therapy are hypotension, cough and increased blood creatinine.
Drug Interactions:
Potassium sparing diuretics, potassium supplements or salt substitutes may lead to
increases in serum potassium, and in heart failure patients increases in serum creatinine.
NSAID use may lead to increased risk of renal impairment and loss of antihypertensive
effect.
Dept. of Pharmaceutical Technology CPS, JNTUH
DRUG PROFILE
41
Marketed formulation:
Diovan (Novartis Company) (FDA)
Diovan is available as capsules for oral administration, containing either 80 mg or 160 mg of
valsartan. The inactive ingredients of the capsules are cellulose compounds, crospovidone,
gelatin, iron oxides, magnesium stearate, povidone, sodium lauryl sulfate, and titanium dioxide.
The present drug was approved by the FDA (NDA) on December 23, 1996, as an
antihypertensive drug. The brand name of this drug is DIOVAN manufactured by NOVARTIS
Company. It is new molecular entity (NME) standard review drug with capsule dosage forms of
80mg, 160mg strengths (discontinued). Tablet dosage form of this drug was approved (NDA) on
July 18, 2001 as chemical type new formulation under review classification standard review drug
with 40mg,80mg,160mg,320mg. (ANDA)Generic drugs: marketing status none (tentative
approval) valsartan tablet oral 320mg by IVAX PHARMS approved on June 10, 2008, tablets
multiple strengths by RANBAXY approved on October 25, 2007.
Dept. of Pharmaceutical Technology CPS, JNTUH
DRUG PROFILE
42
43
4.2. Marketed product specifications
DiovanFig: 7 Diovan (160 mg capsule)
Generic name Valsartan capsules
Brand name DIOVAN
Market India
Manufactured and Marketed by Novartis pharmaceuticals corp.
Strength 160 mg
Dosage form Immediate release tablet
Colour Gray / Pink
Shape Capsule shaped
Imprint CG GOG
Weight 265 mg (avg of 10 caps)
Storage conditionsBelow 25º C, protected from moisture and
humidity.
Patents United States,Canada
Table No.5: Specifications of marketed product (DIOVAN)
Dept. of Pharmaceutical Technology CPS, JNTUH
DRUG PROFILE
44
5. PLAN OF WORK
5.1. Solubility studies
5.2. Development of analytical method by UV/Visible spectrophotometer/HPLC
5.3. Formulation Development
Selection of Excipients
a. Screening of Lipids
b. Screening of Solvents
c. Screening of Surfactants
Evaluation of SLN’s
Determination of particle size and zeta potential.
Determination of drug entrapment and drug loading.
In-Vitro drug release study
Optimization of formula
Conversion of lipid dispersion (liquid) into a solid form.
Solid state charecreterization of optimized by DSC analysis.
Stability analysis.
5.4. Development of analytical methods
Dissolution methhod
Method development for particle size analysis and zeta potential by laser diffraction
technique (Malvern zeta sizer)
Method development for differential scanning calorimetry(DSC) analysis
Dept. of Pharmaceutical Technology CPS, JNTUH
PLAN OF WORK
46
6.1. List of materials
Table No.6: List of materials and their suppliersS.No. Name of the material Category Supplier
1 ValsartanSelective Estrogen Receptor Modulator (SERM)
Dr.Reddy’s Laboratories Ltd.
2 Compritol Phospholipid Avanti polar lipids, USA
3 Glyceryl monostearate Phospholipid Avanti polar lipids, USA
4 Tristearate Phospholipid Avanti polar lipids, USA
9 Tween-80 Surfactant Merck, Mumbai
10 Polaxamer 188 AdsorbentEvonik Degussa GmBH, Germany
11 Cremophore EL Adsorbent SD Fine chem, Mumbai
12 Dichloro methane Solvent Fischer scientifics, USA
13 Ethanol Solvent Standard Reagents
14 Potassium Chloride Analytical reagent Rankem, New Delhi
15 Hydrochloric acid Analytical reagent SD Fine Chem, Mumbai
16 Potassium hydrogen Pthalate Analytical reagent Merck, Mumbai
17Monobasic Potassium phosphate
Analytical reagent Fischer scientifics, USA
18 Sodium Hydroxide Analytical reagent Merck, Mumbai
19 Triethyl amine Analytical reagent Merck, Mumbai
20 Methanol Analytical reagent Rankem, New Delhi
21 Ortho phosphoric acid Analytical reagent Rankem, New Delhi
Dept. of Pharmaceutical Technology CPS, JNTUH
EXPERIMENTAL STUDIES
22 Capsules Hard Gelatin capsules Associated capsules Ltd
6.1.1. Classification of materials
Fig: 8 Classificationsof materials
Drug:
Valsartan is used as drug, which is Angiotensin II blocker.
Lipids:
Lipids such as Compritol 888, Glyceryl monostearate, Glyceryl tristearate, Dynasan 118 are used to entrap the drug (Lipid matrix)
Surfactants
1. Surfactants in SLN’s are used to disperse the molten lipid in to aqueous phase and then to stabilisze the lipid/aqueous interface by covering nanoparticles surfaces after cooling.
2. Tween 80 and Polaxamer 188 are used as surfactants.
Solvents:
Dept. of Pharmaceutical Technology CPS, JNTUH
Materials
Drug SurfactantsLipids Solvents
EXPERIMENTAL STUDIES
Reagents
48
1. DCM, Ethanol, Acetone, Chloroform are used to dissolve the drug and lipid for the formation of emulsion in Solvent emulsification-Diffusion technique.
2. Methanol is used to prepare the HPLC mobile phase
Reagents:
1. Potassium Chloride,Hydrochloric acid,Potassium hydrogen Pthalate,Monobasic Potassium Phosphate,Sodium Hydroxide are used to prepare 1.2pH,3 pH,4.5 pH,6.8 pH,7.4 pH, 8 pHbuffers.
2. Triethyl amine and Ortho phosphoric acid are used to prepare triethylamine buffer (Triethylamine: Methanol =45:55), which is used as HPLC mobile phase.
Dept. of Pharmaceutical Technology CPS, JNTUH
EXPERIMENTAL STUDIES
49
6.2. List of instruments
Table No.7: List of Instruments and Apparatus along with their ManufacturersS.No. Name of the Equipment Model Manufacturer
1 Semi micro analytical balance LE 225D Sartorius, India
..2 Top loading balance CP 622 Sartorius, India
3 Rota shaker SW 23 Julabo, North America
4 Heating mantle -Shital scientific industries, Mumbai
5 pH meter Orion 420 A+ Thermo orion,
6 Overhead Stirrer RZR 2051control Heidolph, Germany
7 Magnetic stirrer MR-3001,HeiTec Heidolph, Germany
8 Sonicator - Bandelin sonorex
12 Dissolution apparatus Disso 2000 Labindia
13Alliance HPLC, dual Y absorbance detector
2695 Waters, USA
14 HPLC Column4.6 ×150mm, RP18, 5µm
Xterra
15 Nano-ZS (Zeta sizer) ZEN3600 Malvern
16Differential Scanning Calorimeter
DSC Q-1000 V9.8AA 124
TA instruments, USA
17 X-Ray Diffractometer XPert Pro AA 198 PANalytical BV, Netherland
Dept. of Pharmaceutical Technology CPS, JNTUH
EXPERIMENTAL STUDIES
50
18Scanning Electron Microscope
S 3000 N Hitachi
19 Vacuum Evaporator HUSGB Hitachi
20 Ultra centrifuge - Heraeus
21 Hot air oven - Cintex
22 Milli Q Water purifier - Millipore (India) Pvt Ltd
23 Storage bottles - Schott Duran, North America
24 Glass ware - Merck, Rankem, Borosil
25 Pipettes - Vensil
26Syringe filters (0.45 µ- 47,25,13 mm)
NX047100, ZWGSFN 13045
Pall Life sciences,India
Zodiac Life sciences, India
27 Syringes - Dispo van
Dept. of Pharmaceutical Technology CPS, JNTUH
EXPERIMENTAL STUDIES
51
6.3. Methods
6.3.1. Solubility studies
The solubility studies for the drug were carried out using the Rotary shaking method.
These studies included the rotary shaking at 200 rpm and addition of excess drug till saturation
was observed for 48 Hrs. Then the samples were filtered and required dilutions were made to the
sample and was analyzed using HPLC.
6.3.1.1. Solubility in purified water:
Solubility of the drug in purified water was investigated using Rotary Shaking method at
two temperatures i.e. Room temperature (25º C). Excess drug was added to 20 ml of water in
stoppered conical flasks and were agitated continuously in a Rota shaker for 48 Hrs at 200 rpm
and respective temperatures, till saturation was observed. Then, the samples were filtered using
0.45 µ Nylon (47 mm) syringe filters. From this, samples equivalent to standard concentrations
were prepared by diluting with mobile phase and were analyzed using HPLC at 225 nm.
6.3.1.2. Solubility across pH:
Solubility of the drug across different buffers was studied. The pH ranged from 1.2 to 8.0
(1.2, 3.0, 4.5, 6.8, 7.4 and8.0). All the buffers were prepared according to USP NF. Excess drug
was added to 20 ml of water in stoppered conical flasks and were agitated continuously in a Rota
shaker for 48 Hrs at 200 rpm and Room temperature (25° C), till saturation was observed. Then,
Dept. of Pharmaceutical Technology CPS, JNTUH
EXPERIMENTAL
52
the samples were filtered using 0.45 µ Nylon (47 mm) syringe filters. From this, samples
equivalent to standard concentrations were prepared by diluting with mobile phase and were
analyzed using HPLC at 225 nm.
6.3.2. Analytical Method
6.3.2.1. Determination of max
Stock solution: Accurately weighed 100 mg of API was dissolved in little amount of methanol
and then volume was made up to 100 ml using 0.1N HCl buffer (1mg/ml)
Scanning : From the stock solution, 15-25 µg /ml solutions were prepared by pipetting 5ml to a
series of 200 ml volumetric flasks and the volume was made up to 200 ml with phosphate buffer
pH 6.8. These solutions were scanned in UV range between 200-400 nm.
6.3.2.2. HPLC Method:
The analytical methodology for estimation of drug content in the solubility samples was
adapted from in-house analytical method for estimation of valsartan. The details of the method
were given below.
Preparation of mobile phase: The mobile phase consisted of 55% Methanol and 45% of
triethylamine buffer with pH adjusted to 3.0 with o-phosphoric acid. The mobile phase was
prepared daily and degassed by sonication under reduced pressure and filtered before use.
Diluent: Methanol
Preparation of standard solution: 50 mg of pure Valsartan was added to 100 ml volumetric
flask and add about 70 ml of diluent and sonicate for 5 minutes or completely dissolved and
Dept. of Pharmaceutical Technology CPS, JNTUH
EXPERIMENTAL STUDIES
53
dilute to volume using methanol and mix well. Then 5 ml of the above solution is in to a 50 ml
volumetric flask and dilute to volume with diluents and mix well. After filtration with 0.45 µ
PVDF (25 mm) syringe filter.
Preparation of test solution: Transfer an accurately weighed portion of sample equivalent to
about 100 mg of valsartan in to 200 ml volumetric flask. Add about 100 ml of diluent and
mechanically shake for 30 minutes. Further add about 50 ml of diluents and sonicate for 30
minutes or completely dissolved and dilute to volume using methanol and mix well. After
filtration with 0.45 µ PVDF (25 mm) syringe filter.
The solubility of the drug was calculated using the formulas:
Where,
Q = Percent of drug dissolved (% w/v)
A =Standard Area
B = Test Area
C = Standard concentration (µg/mL)
Wt. = Total weight of drug added
V1 = volume of test sample taken for dilution
V2 = diluted volume of test sample
Dept. of Pharmaceutical Technology CPS, JNTUH
EXPERIMENTAL STUDIES
54
V3 = volume of diluted sample (V2) taken for dilution
V4 = diluted volume of sample (V3)
m = Amount/quantity (mg) of drug dissolved
S = Solubility (mg/mL)
6.3.3. Method development for particle size analysis by laser diffraction technique:
Laser diffraction is now one of the widely used techniques for particle size analysis. it
offers flexibility,wide dynamic range of speed of operation and yield significant advantages
compared to other methods of particle size analysis such as sieving,microscopy and
electrozone Sensing (coulter counter) Particle size analysis using wet dispersion is widely used
for obtaining Reproducible results using laser diffraction .wet analysis provides a method of
dispersion for sample across a wide particle Size range from submicron pigments to sand and
sediments. Malvern Mastersizer is used to measure the particle size distribution of drug
nanosuspension, is based on the laser diffraction Technique.
6.3.3.1. Principle and working of malvern mastersizer
The interaction of a particle and light incident upon it gives rise to four different but
inherently related scattering phenomena, namely, diffraction, refraction, reflection and
absorption of the incident beam. The magnitude of each phenomenon will vary depending upon
the nature and size of the particle and the beam. Size analysis by interpretation of the scattered
light Patterns formed due to diffraction of the incident light is of primary interest .Diffraction of
light occurs at the surface of the particle and can be thought of as the bending of light waves by
the surface of The particle .diffraction arises due to slight differences in the path length of the
light waves created upon interaction with the particle surface. These differences in the path
Dept. of Pharmaceutical Technology CPS, JNTUH
EXPERIMENTAL
55
length cause constructive and destructive interference between the sinusoidal Light waves
leading to characteristic diffraction patterns. The diffracted waves are scattered in different
diections. The Direction of scatter depends on the size and shape of the particle. Large, spherical
particles scatter mostly in the forward Direction. As the particle size gets smaller, the scattering
occurs over a broader range of angles.In practice; scattering is significantly more complex and is
influenced by the nature of polarization of the incident light, Optical properties of the particle
and surface roughness of the particle.
6.3.3.2. Light diffraction instruments are based on three basic assumptions;
a) The particles scattering the light are spherical in nature,
b) There is little to no interaction between the light scattered from different particles ( i.e.,
no multiple scattering phenomena),and
c) The scattering pattern at the detectors is the sum of the individual scattering patterns
generated by each particle interacting with the incident beam in the sample volume.
Diviations from these assumptions will introduce some degree of error due to the
inability of the mathematical algorithms for the deconvolution and inversion procedures to
account for the deviations. The assumpation of spherical particle shape is particularly
important as most algorithms in commercial instruments use the mathematical solution for
Mie, Fraunhofer and Rayleigh scattering from spherical particles.
6.3.3.2.1. Fraunhofer Approximation
The Fraunhofer approximation (also referred to as the Fraunhofer theory )is applicable
when the diameter of the particle scattering the incident ligh Is larger than the wave length of the
incident light [ d >e.. ].Particles showing Fraunhofer scattering have a very strong forward
scattering. Intensity of Scattered light is very high. By its very nature, this model does not need
Dept. of Pharmaceutical Technology CPS, JNTUH
EXPERIMENTAL STUDIES
56
any information about the refractive index of the particle and so is extremely useful for analysis
of powders coarser than about 1 im to 2 im.
6.3.3.2.2. Mie Scattering
The Mie theory is applicable when the particle size is equal to, or smaller than, the wave
length of the incident light [d e..]. Smaller particles with Mie scattering Show decreased forward
scattering and intensity. The intensity of the light scattered decreases linearly with decrease in
particle size.
6.3.3.2.3. Rayleigh Scattering
When diameter of the particle is very small compared to the wave length of the incident
light [d<< e..], the solutions for scattering are best represented by Rayleigh scattering models.
Particles showing Rayleigh scattering have a very symmetric scattering pattern, when forward
and backward scattered light intensity is compared.these patterns have no angular information
any more; therefore particles with Rayleigh characteristics cannot be analyzed laser
Diffractometry.
6.3.3.3. Instrumentation
Light diffraction instruments comprise of a light source, tyoically a low power
(approximately 10 mw Helium – Neon, in the region of 632 nm wavelength) Laser source
optical elements to process the incident beam, a sample cell within which the sample is
introduced .sample cells have built-in ultrasonicators or agitators to keep the specimen
sample dispersed and to prevent agglomeration.sample cells also possess pumping systems to
keep the specimen circulating. Light diffraction instruments lack the ability to distinguish
between well-dispersed powders and agglomerates and thus prevention of agglomeration is a
key factor in ensuring reliability and reproducibility. Light scattered from the sample is then
focused on to a detection system, that can be a multi-element array or numerous detectors
placed at discrete locations .The detectors convert the scattered light intensity incident upon
Dept. of Pharmaceutical Technology CPS, JNTUH
EXPERIMENTAL STUDIES
57
them into electrical signals that are then processed to obtain information about the particle
size and size distribution.
Fig: 9 schematic of components in a typical laser diffraction instrument.
A typical light pattern is shown below (Fig.10).Each bar in the histogram represents the light
scattering from a particular detector.
Fig: 10 Histogram showing typical light scattering pattern
6.3.3.4. Method development for particle size determination
Dept. of Pharmaceutical Technology CPS, JNTUH
EXPERIMENTAL STUDIES
58
The wet method development steps include;
1. Representative sampling
2. Dispersant selection
3. Measurement settings
4. Pump and stirrer setting
5. Sample concentration
6. Sonication Energy
7. Solid particle Measurement
The sampling is the most important aspect of particle size analysis. Laser diffraction
is a volume –based measurement Technique and is therefore sensitive to small changes in
the amount of large particles in the sample. If sampling is controlled it should be easy to
obtain a measurement – to – measurement reproducibility within the limits defined in
ISO13320-1[2], the ISO Standard for laser diffraction measurements ( within 3% at the
d( v,0.5) and within 5% at the d( v,0.1) and d(v,0.9)). If sampling is not controlled then
measurement to measurement variations of up to 20% can be observed. The process of
wet method development for measurement of particle size distribution is as follows;
1. The particle size is measured following intitial wetting of the particles.
2. The particle size is measured during the application of ultrasound for different time
periods and at different stirrer speeds.
3. The ultrasound probe should be switched off and the particle size has to be monitored
to ensure that the dispersion is stable.
Following initial wetting of the sample in the dispersion unit the particle the particle size
may slowly decrease – this is due to the dispersion of loosely bound agglomerates under the
action of the pump and stirrer. If the obscuration reduces rapidly at this point it may suggest that
the material under test is soluble in the chosen dispersant. If this is the case, the obscuration drop
will often be associated with increase in the measured particle size, as the fines present in the
Dept. of Pharmaceutical Technology CPS, JNTUH
EXPERIMENTAL STUDIES
59
60
sample will be dissolved most rapidly. If this is observed a different dispersant should be used.
Following the initial stage of dispersion, ultrasound shoul be applied and the particle size
measurement followed in real – time. Rapid dispersion is normally observed at this stage as
strongly bound agglomerates are dispersed. As the time of sonication is increased the particle
size should reach a plateau where the particle size becomes stable – this represents the fully
dispersed state. If the particle size continues to reduce over time this may suggest that particle
breakup occurs during sonication. Sonication may also cause agglomeration to occur – this
would suggest that the dispersion is unstable, requiring that analyst to adjust the dispersion
conditions. Finally, the particle size should be monitored with the ultra probe switched off, once
full dispersion is achieved. If the particle size remains stable then it indicates that the dispersion
conditions are optimized. The particle size distribution of nanosuspension is represented as 10%,
50%, 90% [d (10), d (50) and d (90)]. Diameter 10%means, 10% of the partcles were below the
indicated size, diameter 50% means, 50%of the particle were below the indicated size and
diameter 90% means that 90% of the particles are below the indicated size. Water was selected
as dispersant as drug is in soluble. Different concentration of the drug (5% w/w and 10%w/w) in
polymer-stabilizer dispersions were prepared for the method development.
6.3.4. Formulation Development
When dealing with a BCS Class II or IV compound, the main formulation objective is to
increase the apparent water solubility of the API in gastro-intestinal fluid (GIF) and to maintain
it in a solubilized state until it reaches the site of absorption, which consequently means that the
API will be fully solubilized in the final dosage form.
6.3.4.1. Screening of excipients
The development of solid lipid nanoparticles begins with screening of excipients in order
to identify those that provide the best solubilization and chemical compatibility with Valsartan.
There are several types of excipients routinely used in screening. These can be classified by their
functional role in a formulation system: Water dispersible surfactant, lipid carrier and a solvent.
Dept. of Pharmaceutical Technology CPS, JNTUH
EXPERIMENTAL STUDIES
Fig: 11 Schematic representation of formulation development
6.3.4.1.1. Screening of lipids
Choice of the lipid cores for SLN’s preparation is dependent on many factors like their
degree of crystallinity, fatty acid chain length, and drug loading capacity in the lipids. The
loading capacity depends upon solubility of drug in lipid melt.
In the present SLN’s development, API solubility screening of lipids was carried out by visual
examination as follows
1. Lipid is allowed to melt by slowly increasing the temperature.The amount of lipid can
be used in the ratio of 1: 5 (minimum) - 1:10 (maximum) (Annette zur Muhlen,Cora
Schwarz,Wolfgang).
2. Add API to the above lipid melt which is maintained at its melting temperature. The
concentration of API used in the initial binary mixture should correspond to the target
dose
3. The evaluation of API solubility is performed by visual observation. When the active
drug is partially solubilized, particles will be visible, when the API is completely
solubilized particles are no longer observed the composition remains visually clear.
4. The method is repeated to determine the maximum/minimum concentration of API that
is completely solubilized in the lipid excipient.
Dept. of Pharmaceutical Technology CPS, JNTUH
Screening of excipients
Screening of lipidsScreening of solvents Selection of surfactants
EXPERIMENTAL STUDIES
61
5. Depending on visual appearance, drug solubility in respective lipid is evaluated. If the
composition is clear in appearance then it is considered as ‘soluble’ and if the
composition is hazy in appearance then it is inferred as insoluble.
6.3.4.1.2. Screening of solvents:
In the preparation of SLN’s the selection of solvent is of paramount important. The
solvent used for the preparation should be partially miscible and solubilize both drug and lipid.
The miscibility of solvent is performed by visual examination of water – solvent mixture.
Add the specified quantity of solvent selected as per IIG limits in to 100 mL of purified water.
Depending on the visual examination, solvents were graded as “miscible” where the solvent
forms clear solution, “partially miscible” where the solvent is dispersed as stable fine globules
and “immiscible” where phase separation of solvent occurs.
The quantity of partially miscible solvent is optimized by visual observation of different
percentages of (3%v/v, 5%v/v, 7.5%v/v) solvent-water mixtures as described above. Finally the
selected solvent was analyzed for its ability to solubilize both drug targeted dose and minimum
amount of lipid.
6.3.1.3. Screening of Surfactants:
The selection of water miscible surfactants was done as follows
1. Add the specified amount of surfactant selected as per IIG limits in to 100% of purified
water.
2. The water miscibility of surfactant is performed by visual observation. Depending on the
visual appearance, surfactants were graded as “miscible” when the surfactant solution
appears visually clear and “water immiscible” when the surfactant solution appears
hazy.
The concentrations of surfactant and lipid were selected based on the resultant particle
size and drug entrapment efficiency when dispersed in water. Different formulations were
Dept. of Pharmaceutical Technology CPS, JNTUH
EXPERIMENTAL STUDIES
62
developed with different concentrations of surfactants. Finally formulations with optimal
surfactant concentration were studied further by increasing the concentration of lipid (1:10).
6.3.4.2. Preparation of SLN’S:
DCM and water were mutually saturated at 40+2oC for 10min in order to ensure initial
thermodynamic equlibrium of both liquids. After the equilibrium was reached add GMS to 3ml
of above water saturated solvent and stirr the mixture for 10 min until it forms clear solution.
Then add specified amount of drug to the above mixture slowly by continuous stirring. Finally
this organic solution was emulsified at 40+2oC with 50ml mixture of aqueous solution containing
different surfactants, using magnetic stirrer at 900rpm for 10 min. The SLN’s were precipitated
by adding cold water (50ml) maintained at 0 oC and stirred continuously until all the organic
solvent (DCM) was evaporated.
Figure: 12 Schematic Procedure for the preparation of solid lipid nanoparticles.
s
Dept. of Pharmaceutical Technology CPS, JNTUH
DCM and water are saturated for 10min
Add GMS and stirr for 10 min
Above organic solution is emulsified with 50ml mixture of aqueous surfactant solution, stir at 900rpm for 10 min
Add drug and stirr for 5 min
Heated at 40+ 2oC
EXPERIMENTAL STUDIES
63
6.3.4.3. Evaluation of SLN’s:
Particle size analysis and Z-potential analysis:
Particle size and zeta potential of SLNs in the dispersion was determined by photon
correlation spectroscopy (PCS) using Malvern zeta sizer at a fixed angle of 90° at 25 °C using
water as dispersant. Before measurement, SLN dispersions were diluted 50- fold with the
original dispersion preparation medium for size determination and zeta potential measurement.
All the measurements were performed in triplicate.
Determination of drug load and entrapment efficiency:
Ten milliliter SLN dispersion was centrifuged for 15 min at 8000 rpm−1. After centrifugation
supernant is separated.The drug content in the supernatant was measured by HPLC. The HPLC
Dept. of Pharmaceutical Technology CPS, JNTUH
Add cold water (50ml) at 0 oC to the initial
emulsion and stir continuously until organic
solvent is evaporated
after continual stirring for 60min.
emulsio
The dispersion is centrifuged and the sediment was vaccum dried
EXPERIMENTAL STUDIES
64
system (WATERS, USA), with a UV–VIS detector and Xterra column (4.6 ×150mm, RP18,
5µm) were utilized for drug separation, using methanol: Triethyl amine buffer (55:45) as mobile
phase. The flow rate and UV wavelength were 1.0 mL min−1 and 225 nm, respectively. The
equations for the drug content and loading efficiency are as follows
Entrapment efficiency (%) = WS/Wtotal ×100%
Load content (%) = WS/Wlipid ×100%
Ws = amount of VAL in the SLNs;
Wtotal =amount of VAL used informulation:
Wlipid =weight of the vehicle.
Solid state characterization:
These studies are important in estimating the crystallinity, entrapment nature, surface
morphology, particle size etc. of the developed formulation. Following studies were carried out
to characterize the SLNs:
Differential Scanning Calorimetry (DSC)
Preparation of samples:
API: The pure API was taken alone.
Placebo: The placebo was prepared with the excipients, similar to that of the formulation
excluding the pure drug.
Formulation: The formulation containing Valsartan SLNs.
Differential scanning calorimetry (DSC):
Dept. of Pharmaceutical Technology CPS, JNTUH
EXPERIMENTAL STUDIES
65
This gives us the information about the thermal properties of the sample like crystalline
or amorphous nature, entrapment of drug with lipids and transition temperature of lipids. The
thermal properties were investigated using DSC Q – 1000, Differential scanning calorimeter with
a refrigerated cooling system.
Process:
Weighed samples of 3 – 5 mg were taken in an open aluminum DSC pans and then sealed
and crimped. These samples were scanned at a ramp of 10º C / min over a range of 10 – 300 º C.
The samples included API, Placebo and SLN Formulation.
In Vitro drug release:
Selection of Discriminating Dissolution medium:
The selection of a discriminating medium was done based on the solubility of the drug in
purified water and across pH buffers. Considering the drug’s solubility, the medium with lowest
solubility of drug was selected for dissolution testing. Based on the drug’s solubility in the
discriminating medium, the conditions viz., rpm, temperature, volume of medium, sink / non-
sink conditions, time points for sample collection, etc were selected.
Dissolution:
The dissolution was carried out in discriminating conditions as mentioned above. In this the
capsule formulation representing unit dose (40 mg) of the drug was taken and dropped in the
dissolution bowl, which contained 900 mL of 1.2pH buffer and maintained at 37 ± 0.5º C. At pre
determined time intervals of 15, 30, 45 min and 60 minutes. Aliquots of sample (10 mL) were
withdrawn and were filtered through 0.45µ PVDF (13 mm) syringe filter. Then 1 mL of this
filtered sample was diluted to 10 mL using diluent (1.2pH buffer) and were analyzed using
Dept. of Pharmaceutical Technology CPS, JNTUH
EXPERIMENTAL STUDIES
66
HPLC. The cumulative percent drug dissolved at various intervals was calculated and plotted
against time.
Preparation of standard solution: 50 mg of pure Valsartan API was added to 1000 ml volumetric
flask and add 900 ml of methanol and final volume was made with water. Now it is kept in an
ultrasonic bath for 10 min and left sealed to stand overnight. After filtration with 0.45 µ PVDF
(25 mm) syringe filter.
6.3.4.4. Stability analysis: (Chrysantha Freitas, Rainer H.Muller)
Solid lipid nanoparticles are basically stable for up to 3 years; however some systems
show particle growth & drug leakage (reference). The GMS formulation stabilized with Tween-
80 was investigated for its stability as a function of storage temperature, light exposure. In
general introduction of energy to the system (temperature, light) led to particle growth and drug
leakage. This process was accompanied by a decrease in zeta potential from approximately -
25mv to -15mv.the SLN was filled in to glass vials and stored at varying conditions. Storage was
performed at different temperatures (8 ºC, 25 ºC and 50 ºC) and light exposures (dark, artificial
illumination). For each storage condition, the SLN formulation was placed in 5ml white and
brown glass vials (glass quality I). After 2 weeks cumulative % drug release of the formulation
sample was established.
Dept. of Pharmaceutical Technology CPS, JNTUH
67
7. RESULTS
AND DISCUSSIONS
7.1. Solubility studies
7.1.1. Solubility in purified water:
Dept. of Pharmaceutical Technology CPS, JNTUH
RESULTS AND DISCUSSIONS
68
The solubility of Valsartan was investigated in purified water at 25º C using the Rotary
shaking method and the results were summarized in Table No.8.
Table No.8: Solubility studies of Valsartan in water
From the above data it was stated that Valsartan is a poorly water soluble drug.
7.1.1. 2.Solubility across pH:
The solubility of Valsartan was also investigated in various buffers differing in their pH
using the Rotary shaking method at 25º C. The solubility results of drug in various buffers were
summarized in Table No.9.
Table No.9: Solubility studies of Valsartan across pH
MediumImmediate Saturation
Solubility at 25°C (mg/mL)
Saturation
Solubility(mg/ml)(48hrs)
pH-1.2 (0.1 N HCl) 0.014 0.01823
pH-3.0 (0.001 N HCl) 0.324 0.4283
pH-4.5 (Acetate buffer) 0.7234 0.7854
pH-6.8 (Phosphate buffer) 7.5 8.5
pH-7.4 (Phosphate buffer) 8.3 9.48
pH-8.0 (Phosphate buffer) 12.5 13.68
Dept. of Pharmaceutical Technology CPS, JNTUH
MediumImmediate Saturation
Solubility at 25°C (mg/mL)
Saturation
Solubility(mg/ml) (48hrs)
Water 0.013 0.01793
RESULTS AND DISCUSSIONS
69
The solubility results indicate that the solubility of Valsartan is pH dependent. At low pH, the
solubility was found less and on increasing pH the solubility was significantly increased. The pH
solubility data clearly shows that drug is highly soluble in pH-8.0 buffer and less soluble in 0.1 N
HCl (pH-1.2).
Solubility enhancement is the single most important driving force in prototype
formulation selection and optimization. As a general rule, if solubility is < 1% (w/v) or 10 mg/ml
in the aqueous or buffers, it is necessary to further evaluate solubility in pharmaceutical
excipients and vehicles.
7.2. Analytical method
7.2.1. Determination of max of API in 0.1N HCl:
The spectrum obtained in scanning of the drug was as shown in Figure 13.
Figure 13: UV spectra of API
From the above spectrum (Figure 12), the absorption maximum (max) of API was
found to be 248.0 nm and this wavelength was used for HPLC method.
Dept. of Pharmaceutical Technology CPS, JNTUH
70
7.2.2. HPLC Method:
The HPLC chromatographic conditions used for anlaysing the samples are summarized in Table
No.10
Two washing solvents were prepared using purified water and Acetonitrile (90:10 and
10:90 respectively).
7.3. Method for particle size analysis by laser diffraction technique:
The specification used for particle size analysis of the drug nanodispersion was
summarized in Table 11. Water was selected as dispersant since the drug is insoluble in aqueous
media.
Dept. of Pharmaceutical Technology CPS, JNTUH
Table No.10: Chromatographic conditions for anlalytical method by HPLC
HPLC SYSTEM WATERS 2695 Separations module
DETECTOR WATERS 2996 Photo Diode Array Detector
COLUMN 4.6 ×150mm, Xterra RP18, 5µm or equivalent.
COLUMN TEMPERATURE Ambient
FLOW RATE 1.0 ml/min
RUN TIME 15 min
LOAD VOLUME 10 µL
UV WAVELENGTH 225 nm
TYPE OF FLOW Isocratic
RESULTS AND DISCUSSIONS
Table: 11. Parameters used for particles size analysis using Malvern Mastersizer
7.4. Dissolution method:
The conditions for discriminating dissolution were summarized in Table No.12.
The formulation samples were evaluated in the discriminating dissolution media using
the following parameters
Dept. of Pharmaceutical Technology CPS, JNTUH
Parameter Value
Refractive Index of water 1.33
Refractive Index of drug 1.50
Obscuration range 5-15
Pump speed 2000 rpm
Sonication Not applicable
Table No.12: Dissolution conditions for the Discriminating medium
Medium Purified (De mineralized) Water
Volume 900 mL
Replacement Volume 10 mL
Temperature 37º ±0.5ºC
Method USP I (basket)
Rpm 100
Time Points 10, 30, 45min, 60 minutes
RESULTS AND DISCUSSIONS
71
7.5. Formulation Development
7.5.1. Screeninig of Excipients:
Screeninig of Lipids:
The solubility of drug in various lipids is summerized in Table 13. The solubility was
evaluated based on the Visual clarity of the solution after addition of drug to the lipid at its
melting temperature.
Table No.13: Solubility of drug in various Lipids
S. NoAmount of
Drug Lipids (Qty) Meltingpoint Visual Observation1 40 mg Compritol 888 (400 mg) 59.3-70.5°C Hazy
240 mg Glyceryl Monostearate
(400 mg) 68°C Clear3 40 mg Dynasan 118 (400 mg) 71-73 ºC Hazy4 40 mg Tristearin (400 mg) 68°C Hazy
The results described in Table 13, indicates that Compritol 888, Dynasan 118 and tristearin were
‘hazy’ in appearance when the drug was added to the lipids individually and Glyceryl
monostearate was ‘clear’ in appearance indicating the drug solubility in GMS. Based on these
observations Glyceryl monostearate was selected as suitable lipid carrier for developing SLN
formulation.
The concentration of Glyceryl monostearate required for formulation development was
selected by studying the solubility of targeted dose (40 mg) of drug in various amounts of lipid
and the results were summarized in Table 14.
Dept. of Pharmaceutical Technology CPS, JNTUH
RESULTS AND DISCUSSIONS
72
Table No.14: Solubility of drug targeted dose in Glyceryl monostearate
Amount of lipid (mg)
SNO Lipids Meltingpoint
Visual obseravation
40 80 160 200
1 Glyceryl Monostearate 68°C Hazy Hazy
Slightly Hazy Clear
The results from Table 14 indicate that 200 mg of GMS is required for solubilizing the drug
therefore this amount was choosen for formulation development.
Screeninig of Solvents:
The results of water miscibility of various solvents were summarized in the following Table
No.15
Table No.15: Water miscibility of various solvents
SNO Solvents Qty Miscibility with 100 mL
water1 Methanol Miscible 2 Alcohol Miscible 3 Acetone Miscible 4 DCM Partially miscible5 Chloroform Immiscible
From the Table No.15 it was clear that DCM is the partially water miscible solvent and
therefore it was selected as solvent for the formulation development.
Dept. of Pharmaceutical Technology CPS, JNTUH
RESULTS AND DISCUSSIONS
73
After the selection of suitable solvent, the solubility of targeted dose (40 mg) of drug in
presence of lipid (200 mg) by appling gentle heat (40° C) was studied by visual clarity and the
results were summarized in the following Table 16.
Table No.16: Solubility of drug in the drug and lipid
SNO Ingredients Solvent Visual Observation
1 Drug (40 mg) + GMS (200 mg) DCM (3 mL) Clear
From the solubility data given in the Table No.16 it was clear that dichloromethane
(DCM) can solubilise both drug and the lipid.
Screeninig of surfactants:
In solvent emulsification-diffusion technique preparation of aqueous surfactant solution is the
secondary step for preparing the SLNs. For this purpose Tween-80 and Poloxamer 188 were
selected for prototype formulation development. Various concentrations of Tween 80 and
Polaxamer 188 were evaluated by characterizing SLN’s Particle size analysis and drug
entrapment. The composition evaluated for prototype formulation development are summarized
in Table 17 Table 17 selection of surfactants with optimal concentrations
Weights (mg)SNO Ingredients VSX 1 VSX 2 VSX 3 VSX 4 VSX 5 VSX 6 VSX 7
1 Drug 40 40 40 40 40 40 40
2Glyceryl monostearate 200 200 200 200 200 200 200
Dept. of Pharmaceutical Technology CPS, JNTUH
RESULTS AND DISCUSSIONS
74
Surfactants
3Polaxamer-188 30 50 70 - - - -
4 Tween-80® - - - 100 200 300 400 Solvents(ml)5 DCM 3 3 3 3 3 3 36 Purified water 50 50 50 50 50 50 507 Total weight 270 290 307 340 440 540 640
Formula explanation (in brief):
VSX-1: In this formulation 0.75% of Polaxamer 188 was used as surfactant
VSX-2: In this formulation 1.25% of Polaxamer 188 was used as surfactant
VSX-3: In this formulation 1.675% mg of Polaxamer 188 was used as surfactant
VSX-4: In this formulation 2.5% mg of Tween 80 was used as surfactant
VSX-5: In this formulation 5% mg of Tween 80 was used as surfactant.
VSX-6: In this formulation 7.5% of Tween 80 was used as surfactant
VSX-7: In this formulation 10% of Tween 80 was used as surfactant
In the above formulations drug: lipid ratio is 1:5
7.5.2. Evaluation of SLNs
The results of partcle size and zeta potential of various formulations (Table 20) were summarized
in the Table 18.
Particle size analysis and measurement of potential
Table: 18 Z-Average size (d.nm), zeta potential and PDI of different SLN formulations (0 day)
Particle size & Z-Potential analysis
Formulae Z-Average size (d.nm) PDI Z-Potential (mv)
Dept. of Pharmaceutical Technology CPS, JNTUH
RESULTS AND DISCUSSIONS
75
VSX 1 8492 + 30 1 -18.9 + 2
VSX 2 6738 + 25 1 -0.95 + 1
VSX 3 3522 + 10 0.824 -0.68 + 1
VSX 4 6300 + 20 0.923 -17.8 + 0.23
VSX 5 2939 + 5 0.7 -15.6 + 3
VSX 6 772 + 20 0.432 -14.8 + 3
VSX 7 723 + 5 0.841 -11.9 + 5
Fig: 14 Graph representation of Z- average size (d.nm) of different SLN formulations
The mean particle size of VAL-SLN prepared with different formulations, ranged from
723 + 5to 8492 + 30 nm (Table 21). Higher surfactant concentrations reduce the lipid/water
interfacial tension, resulting in a decrease in particle size subsequent increase in surface area.
The mean diameters, PDI of VSX 3 and VSX 6 were in the range of approximately 3522 + 10
nm, 0.8-0.9 and 772 + 20 nm, 0.4-0.5 respectively (Table 21.). The VSX 3 and VSX 6 SLNs had
a zeta potential 0.68 + 1,-14.8 + 3 mV respectively (Table 21). As concentration of Polaxamer
188 and Tween 80 increased, the zeta potential decreased significantly.
Fig: 15 Peak showing Z-average size (d.nm) of formulae VSX 6 in zeta sizer (0 day)
Dept. of Pharmaceutical Technology CPS, JNTUH
RESULTS AND DISCUSSIONS
76
Determination of drug load and entrapment efficiency:
The results of entrapment efficiency of various formulations were summarized in the Table
19.
Table: 19 Entrapment efficiency of different SLN formulations
Determination of drug entrapmentFormulae Entrapment efficiency (%)
VSX 1 28.17 + 5VSX 2 25.78 + 4VSX 3 23.44 + 3VSX 4 28.34 + 5VSX 5 32.78 + 7VSX 6 78 + 4VSX 7 62.5 + 2
Fig: 16 Graph representation of Entrapment efficiency of different SLN formulations
Dept. of Pharmaceutical Technology CPS, JNTUH
RESULTS AND DISCUSSIONS
77
Fig: 17 Chromatogram representing standard preparation in determination of entrapment efficiency
Fig: 18 Chromatogram representing VLX 6 (blank) in determination of entrapment efficiency
Dept. of Pharmaceutical Technology CPS, JNTUH
RESULTS AND DISCUSSIONS
78
Fig 19 Chromatogram representing VLX 6 in determination of entrapment efficiency
From Table 22 it was clear that VAL has high entrapment efficiency in formula VSX 6
(78 + 4 %) SLN’s, and the entrapment efficiency increases as the amounts of the surfactants
were increased. The entrapment efficiency of formulations containing Polaxamer 188 was lower
than that of formulations containing Tween 80 and (Table 22). These results may have
contributed to the lower stabilization of polaxamer 188 compared with Tween 80.
From the above discussions of Table 21 and Table 22, it was clear that VSX 6
formulation was selected as best formulation with optimal particle size (772 + 20 nm) and drug
entrapment (78 + 4 %) among all other Tween 80 and polaxamer 188 formulations, so VSX 6
was studied against increase in lipid concentration to 400 mg (1:10) as shown in the Table 20.
Table No.20: Optimization of the lipid concentration.
Dept. of Pharmaceutical Technology CPS, JNTUH
RESULTS AND DISCUSSIONS
79
VSX-8: In this formulation drug: lipid is 1:10 and 7.5% of Tween 80 was used.
The results of partcle size and zeta potential analysis of VSX 8 were summarized in the
Table 21
Table: 21 Effect of lipid concentration on the Partcle size (0 day)
Particle size & Z-Potential analysis
Formulae Z-Average size (d.nm) PDI Z-Potential (mv)
VSX 6 772 + 20 0.432 -14.8 + 3
Dept. of Pharmaceutical Technology CPS, JNTUH
Weights (mg)SNO Ingredients VSX 6 VSX 8
1 Drug 40 40
2 Glyceryl monostearate 200 400 surfactants
3 Polaxamer-188 - 4 Tween-80® 300 300 Solvents(ml)5 DCM 3 36 Purified water 50 507 Total weight 540 740
RESULTS AND DISCUSSIONS
80
VSX 8 3832 +10 1 -18.5 + 2
Fig: 20 Graph representation of the effect of lipid concentration on the Partcle size
The results of drug entrapment and load content of formulation VSX 8 were summarized in the
following Table 22.
Table: 22 Effect of lipid concentration on the drug entrapment (0 day)
Determination of drug entrapment
Formulae Entrapment efficiency (%)
VSX 6 78 + 4
VSX 8 80 + 5
Dept. of Pharmaceutical Technology CPS, JNTUH
RESULTS AND DISCUSSIONS
81
Fig: 21 Graph representation of the effect of lipid concentration on the drug entrapment (0 day)
Table: 22 reports that as the concentration of lipid increases particle Z-Average size, PDI
and zeta potential also increases. Formula VSX 6 where drug: lipid is 1:5 was showing Z-
Average, Z-Potential and PDI in the range of 772 + 20 nm,-14.8 + 3 mv, and 0.432 to 0.5
respexctively. Formula VSX 8 where drug: lipid is 1:10 was showing Z-Average, Z-Potential
and PDI in the range of 3832 +10nm, -18.5 + 2 mv, and 0.932 to 1. From Table 25, even though
VSX 8 has having high drug entrapment (80 + 5 %) than VSX 6 (78 + 45%) , it has shown larger
particle size and PDI when compared to formulation VSX 6. Therefore VSX 6 was selected as
the final formulation for in-vitro characterization.
Drug release of VAL from SLNs
Drug release from VSX 6 was observed in pH 1.2 buffer by comparing with API
Solution and marketed formulation (DIOVAN) at time intervals of 10, 20,30,45,60
min.dissolution was carried out using Purified (De mineralized) Water 800 mL,0 mL (Non-sink
conditions),USP I (basket) and at temperature37º ±0.5ºC.
Dept. of Pharmaceutical Technology CPS, JNTUH
RESULTS AND DISCUSSIONS
82
Table: 23 Cumulative % release of VAL from VSX 6, API and Marketed formulation (DIOVAN) in
dissolution media: 0.1 M HCl (0 day)
Fig: 22: Chromatogram corresponding to the standard preparation
Fig: 23 Chromatogram corresponding to the VSX 6 Placebo
Dept. of Pharmaceutical Technology CPS, JNTUH
Time(min) VSX 6(%) API (%)Marketed formulation
(%)
0 0 0 0
10 53.57 0.3 5
20 64.43 0.9 10
30 75.8 1.9 13
45 76.13 4.3 17
60 80.16 6.8 20
RESULTS AND DISCUSSIONS
83
Fig: 24 Chromatogram corresponding to the VSX 6 formulation
From the above chromatograms, it was clear that, there was no interference of the blank
and placebo in the analysis of formulation. The peak represented by the test preparation’s
chromatogram completely elucidated the pure drug’s peak. Also the Retention time of the peak
and standard were almost equivalent along with a good shape and peak purity.
Fig: 25 Comparative Cumulative % release profiles of VSX 6 formulations API and Marketed formulation
(DIOVAN) in dissolution media: 0.1 M HC (0 day)
Dept. of Pharmaceutical Technology CPS, JNTUH
RESULTS AND DISCUSSIONS
84
Table: 26 Reports that the %Cumulative drug release of VSX 6 was higher than API
solution and marketed formulation (DIOVAN) in pH 1.2 media.
7.5.3. Optimization of formula
From the above results, we concluded that the surfactants (polaxamer 188 and Tween 80)
made an important contribution to the differences between the release from the two SLN
formulations, diffusion from API and marketed formulation (DIOVAN). Surfactants altered the
barrier properties of the aqueous boundary layer around drug particle, resulting in a high release
velocity of VAL in SLN dispersion. The API would not have this effect. In addition, the
concentration of VAL in SLN dispersion was close to saturation (maximal thermodynamic
activity), while in API, although the overall concentration of VAL was identical with that in SLN
dispersion, with the appearance of microcrystals the real concentration of drug dissolved in
solution would be greatly lowered, since thermodynamic activity is the driving force for
transport, so the diffusion of VAL was slow compared with that in the SLN dispersion. The
above dissolution results indicated convincing for the formulation VSX 6 on comparison with
API and DIOVAN. Hence, it was concluded that, the solid lipid nanoparticles (VSX 6) showed
greater dissolution profile, when compared to that of, API and DIOVAN and better particle
Dept. of Pharmaceutical Technology CPS, JNTUH
85
size,PDI,Zetapotential,drug Entrapment when compared to that of formulations (VSX 3,VSX 8).
Hence, VSX 6 was concluded as the optimized formula for Valsartan SLN’s and further solid
state characterization studies were coducted on the formula VSX 6.The optimized concentration
of ingredients for the lipid based formulation is mentioned below in Table No.24.
Table No: 24 Optimized Formula for Valsartan SLN’s Formulation
Weights (mg)SNO Ingredients VSX 6
1 Drug 40
2 Glyceryl monostearate 200 surfactants
3 Polaxamer-188 -4 Tween-80® 300 Solvents(ml)5 DCM 36 Purified water 507 Total weight 540
7.5.4. Solid state characterization of optimized formula:
The solid state characterization studies were carried out for API, Placebo Lipid
formulation and Granulated Lipid formulation.
Table No: 25 Sample compositions for Solid State Characterization
S.No. Sample Drug (mg) Excipients (mg)
1 API 40 0
2 Placebo 0 500
3 Formulation 40 500
Dept. of Pharmaceutical Technology CPS, JNTUH
RESULTS AND DISCUSSIONS
RESULTS AND DISCUSSIONS
86
Differential Scanning Calorimetry (DSC)
Figure 37 represents the thermograms of pure API, Placebo and Lipid Formulation.
The DSC curve of pure Valsatan exhibited a single endothermic peak at 98.1º C corresponding to
the melting of the drug and the sharp peak indicated its crystallinity.The DSC curve of Placebo
lipid formulation and VSX 6 formulation exhibits broad endotherms at 75.3º C, 86.68 º C
respectively, corresponding to Glyceryl monostearate and Tween 80 respectively, but the drug’s
peak was no longer observed. It could be attributed to complete entrapment of the drug in the
lipids. Solid state studies did not indicate chemical decomposition of the components (drug and
excipients), showing compatibility and formation of homogenous systems.
Fig: 26 Thermograms of API, Placebo and VSX 6 Formulation
Dept. of Pharmaceutical Technology CPS, JNTUH
RESULTS AND DISCUSSIONS
87
7.5.5. Stability Studies:
A 2 weeks storage stability study was conducted on the lipid based formulation.
Dissolution runs were conducted on the stressed samples at 2 weeks to assess any changes in
release behavior of Valsartan. Accordingly, the formulations was placed in 5ml white and brown
glass vials (glass quality I) and charged for stability studies at at different temperatures (8 ºC,25
ºC and 50 ºC) and light exposures (dark, artificial illumination). At pre-determined time of 2
weeks the samples were analyzed for the percent drug content and the drug dissolution rates. The
dissolution studies were carried out in the developed 100 % release medium. Table No.26
corresponds to the dissolution conditions and results carried out for stability studies.
Table: 26 Stability results for optimized formula VSX 6
VSX 6 LIGHT DARK
Parameter Initial 2 weeks 2 weeks
Temperature 8oC 25oC 50oC 8oC 25oC 50oC Dissoultion Profile
10 min 53.57 53.53 50.6 51.8 53.53 43.45 40.3215 min 64.43 59.1 50.12 54.23 60.12 55.35 50.2430 min 75.8 60.8 55.29 57.15 67.25 56.2 51.345 min 76.13 53.13 58.25 56.19 68.23 54.1 52.660 min 80.16 65.25 60.23 57.25 70.8 58.2 51.8
The above results indicated the intermediate stability of the formulation. As the storage temperature increased, there was a slight decrease in the release of Valsartan from the formulation. A decrease in the dissolution rate of the drug in light exposure conditions when compared to dark conditions was observed in the formulation
The decrease in stability as a result of decreasing dissolution rates in the formulation indicating the effect of light and temperature on formulation.
Dept. of Pharmaceutical Technology CPS, JNTUH
RESULTS AND DISCUSSIONS
88
8. SUMMARY AND CONCLUSION
The goal of any drug delivery system is to provide a therapeutic amount of drug to the proper
site in the body and also to achieve and maintain the desired plasma concentration of the drug for
a particular period of time. However, poor water solubility, incomplete release of the drug,
shorter residence times of dosage forms in the upper GIT leads to lower oral bioavailability.
Such limitations of the conventional dosage forms have paved way to an era of novel drug
delivery systems.
Valsartan is a poorly water soluble drug having an oral bioavailability of 23 to 25%.
Valsartan belongs to the category of Angitotensin II blocker, used in the treatment of
Hypertension. Thus in order to overcome these drawbacks, solid lipid nanoparticles based
formulation approach was selected.
The solubility studies across pH and water indicated the poor solubility of Valsartan in
water.
Biologically compatible Lipid excipients, stabilizers were screened in order to reduce the
particle size of drug to enhance the solubility of Valsartan in water. Based on solubility
results appropriate excipients were selected for formulation development.
Prototype formlations were prepared using the Solvent emulsification-diffusion
technique.
The prepared prototype formulations were characterized for Particle size, zeta potential,
drug entrapment and in-vitro relase characteristics.
The final optimized formulation was characterized for its physical properties Viz.,
particle size, zeta potential, DSC and drug release rate. The final SLN formulation has
shown significant increase in drug relase when comared to API and marketed product
(DIOVAN) in physiologically relevant media.
The optimized lipid based formulations was charged on stability studies for a period of
two weeks and the results indicated that temperature and light have significant effect on
the formulation stability. Therefore storing the formuation in light resistant containers
and at low temperatures and also use of stabilizers in the formulation can provide a stable
drug product.
Dept. of Pharmaceutical Technology CPS, JNTUH
Summary AND conclusion
90
To sum up a poorly aqueous-soluble drug VAL was successfully incorporated into SLNs by a
Solvent emulsification-Diffusion technique. The physicochemical characterization and short-
term physical stability were investigated. The in-vitro release tests showed that the release rate of
the SLNs is significantly higher compared with the API and marketed product. Thus the prepared
solid lipid nanoparticles were proved to be a potential technology for enhancing the
bioavailability.
Dept. of Pharmaceutical Technology CPS, JNTUH 91
Summary AND conclusion
9. BIBLIOGRAPHY
1. Abdelbary, G.; Fahmy, R. H. Diazepam-loaded solid lipid nanoparticles: design and
characterization. AAPS PharmSciTech 2009, 10, (1), 211-219.
2. Ahlin, P.; Kristl, J.; Smid-Kobar, J. Optimization of procedure parameters and physical
stability of solid lipid nanoparticles in dispersions. Acta Pharm. 1998, 48, 257-267.
3. Al-Khouri-Fallouh, N.; Roblot-Treupel, L.; Fessi, H.; Devissaguet, J. P.; Puisieux, F.
development of a new process for the manufacture of polyisobutylcyanoacrylate
nanocapsules. Int. J. Pharm. 1986, 28,125-132.
4. Ali, H.; Nazzal, M.; Zaghloul, A. A.; Nazzal, S. Comparison between lipolysis and
compendial dissolution as alternative techniques for the in vitro characterization of alpha
-tocopherol self emulsified drug delivery systems (SEDDS). Int. J. Pharm. 2008, 352, (1-
. 2), 104-114
5. Ali, H.; Shirode, A.; Sylvester, P.; Nazzal, S. Preparation and antiproliferative effect of
Tocotrienol loaded lipid nanoparticles. Colloids Surf. A 2010, 353, (1), 43-51.
6. Allemann, E.; Gurny, R.; Doelker, E. Drug loaded nanoparticles - preparation methods
and drug targeting issues Eur. J. Pharm. Biopharm. 1993, 39, (5), 173-191.
7. Almeida, A. J.; Runge, S.; Muller, R. H. Peptide-loaded solid lipid nanoparticles (SLN):
Influence of production parameters. Int. J. Pharm. 1997, 149, (2), 155-165.
8. Aungst, B. J. Novel formulation strategies for improving oral bioavailability of drugs
with poor membrane permeation or presystemic metabolism. J. Pharm. Sci. 1993, 82,
(10), 979-987.
9. Bioistelle, R., Fundamentals of nucleation and crystal growth. In Crystallization and
Polymorphism of Fats and Fatty Acids, Garti, N.; Sato, K., Eds. Marcel Dekker Inc.: NY,
Basel, 1988; pp 189-226.
10. Brockerhoff, H.; Jensen, R. G., Pharmacokinetics of lipolysis. In Lipolytic Enzymes,
Academic Press: New York 1974; pp 10-24.
11. Bunjes, H.; Westesen, K.; Koch, M. H. J. Crystalization tendency and polymorphic
transitions in triglyceride nanoparticle. Int. J. Pharm. 1996, 129, 159-173.
12. Bunjes, H.; Drechsler, M.; Koch, M. H.; Westesen, K. Incorporation of the model drug
ubidecarenone into solid lipid nanoparticles. Pharm. Res. 2001, 18, (3), 287-293.
13. Bunjes, H.; Koch, M. H. Saturated phospholipids promote crystallization but slow down
Dept. of Pharmaceutical Technology CPS, JNTUH
BIBLIOGRAPHY
93
Polymorphic transitions in triglyceride nanoparticles. J. Control. Release 2005, 107, (2),
229- 243.
14. Carey, M. C.; Small, D. M.; Bliss, C. M. Lipid digestion and absorption. Annu. Rev.
Physiol. 1983, 45,651-677.
15. Carriere, F.; Barrowman, J. A.; Verger, R.; Laugier, R. Secretion and contribution to
lipolysis of gastric and pancreatic lipases during a test meal in humans. Gastroenterology
1993, 105, (3), 876-888.
16. Carrigan, P. J.; Bates, T. R. Biopharmaceutics of drugs administered in lipid-containing
dosage forms: GI absorption of griseofulvin from oil in water emulsion in the rat. J. Pharm.
Sci. 1973, 62, 1476-1479.
17. Castelli, F.; Puglia, C.; Sarpietro, M. G.; Rizza, L.; Bonina, F. Characterization of
indomethacin-loaded lipid nanoparticles by differential scanning calorimetry. Int. J. Pharm.
2005, 304, (1-2), 231-238.
18. Cavalli, R.; Caputo, O.; Carlotti, M. E.; Trotta, M.; Scarnecchia, C.; Gasco, M. R.
Sterilization and freeze-drying of drug-free and drug-loaded solid lipid nanoparticles Int. J.
Pharm. 1997, 148, 47-54.
19. Cavalli, R.; Caputo, O.; Marengo, E.; Pattarino, F.; Gasco, M. R. The effect of components
of microemulsions on both size and crystalline structure of solid lipid nanoparticles (SLN)
containing a series of model molecules. Pharmazie 1998, 53, 392-396.
20. Charcosset, C.; El-Harati, A.; Fessi, H. Preparation of solid lipid nanoparticles using a
membrane contactor. J. Control. Release 2005, 108, (1), 112-120.
21. Charman, W. N. Lipids, lipophilic drugs, and oral drug delivery-some emerging concepts.
J. Pharm. Sci. 2000, 89, (8), 967-978.
22. Chen, D. B.; Yang, T. Z.; Lu, W. L.; Zhang, Q. In vitro and in vivo study of two types of
long-circulating solid lipid nanoparticles containing paclitaxel. Chem. Pharm. Bull. (Tokyo)
2001, 49, (11), 1444-1447.
23. Chen, Y.; Dalwadi, G.; Benson, H. A. Drug delivery across the blood-brain barrier. Curr.
Drug Deliv. 2004, 1, (4), 361-376.
24. Couvreur, P.; Dubernet, C.; Puisieux, F. Controlled drug delivery with nanoparticles:
current possibilities and future trends. Eur. J. Pharm. Biopharm. 1995, 41, 2-13.
Dept. of Pharmaceutical Technology CPS, JNTUH
BIBLIOGRAPHY
94
25. Dahan, A.; Hoffman, A. Enhanced gastrointestinal absorption of lipophilic drugs, in: E.
Touitou, B.W. Barry (Eds.), Enhancement in drug delivery, CRC press,. 2006a, pp. 111-
127.
26. Dahan, A.; Hoffman, A. Use of a dynamic in vitro lipolysis model to rationalize oral
formulation development for poor water soluble drugs: correlation with in vivo data and the
relationship to intra-enterocyte processes in rats. Pharm. Res. 2006b, 23, (9), 2165-2174.
27. Dong, X.; Mattingly, C. A.; Tseng, M.; Cho, M.; Adams, V. R.; Mumper, R. J.
Development of new lipid-based paclitaxel nanoparticles using sequential simplex
optimization. Eur. J. Pharm. Biopharm. 2009, 72, (1), 9-17.
28. Esposito, E.; Fantin, M.; Marti, M.; Drechsler, M.; Paccamiccio, L.; Mariani, P.; Sivieri, E.;
Lain, F.; Menegatti, E.; Morari, M.; Cortesi, R. Solid lipid nanoparticles as delivery
systems for bromocriptine. Pharm. Res. 2008, 25, (7), 1521-1530.
29. Ford, J. L.; Timmins, P., Pharmaceutical Thermal Analysis. Ellis Horwood: Chichester,
1989.
30. Freitas, C.; Miiller, R. H. Effect of light and temperature on zeta potential and physical
stability in solid lipid nanoparticles (SLN) dispersions. Int. J. Pharm. 1998, 168, 221-229.
31. Freitas, C.; Miiller, R. H. Stability determination of solid lipid nanoparticles (SLN) in
aqueous dispersion after addition of electrolyte. J. Microencapsul. 1999a, 16, (1), 59-71.
32. Freitas, C.; Miiller, R. H. Correlation between long-term stability of solid lipid
nanoparticles (SLN) and crystallinity of the lipid phase. Eur. J. Pharm. Biopharm. 1999b,
47, (2), 125-132.
33. Fundaro, A.; Cavalli, R.; Bargoni, A.; Vighetto, D.; Zara, G. P.; Gasco, M. R. Non-stealth
and stealth solid lipid nanoparticles (SLN) carrying doxorubicin: pharmacokinetics and
tissue distribution after i.v. administration to rats. Pharmacol. Res. 2000, 42, (4), 337-343.
34. Gasco, M. R. Method for producing solid lipid microspheres having a narrow size
distribution. US Patent No. 52502361993.
35. Goppert, T. M.; Muller, R. H. Protein adsorption patterns on poloxamer- and poloxamine-
stabilized solid lipid nanoparticles (SLN). Eur. J. Pharm. Biopharm. 2005, 60, (3), 361-
372.
36. Heurtault, B.; Saulnier, P.; Pech, B.; Proust, J. E.; Benoit, J. P. Physico-chemical stability
of colloidal lipid particles. Biomaterials 2003, 24, (23), 4283-4300.
Dept. of Pharmaceutical Technology CPS, JNTUH
BIBLIOGRAPHY
95
37. Holm, R.; Porter, C. J.; Mullertz, A.; Kristensen, H. G.; Charman, W. N. Structured
triglyceride vehicles for oral delivery of halofantrine: examination of intestinal lymphatic
transport and bioavailability in conscious rats. Pharm. Res. 2002, 19, (9), 1354-1361.
38. Huang, G.; Zhang, N.; Bi, X.; Dou, M. Solid lipid nanoparticles of temozolomide: potential reduction of cardial and nephric toxicity. Int. J. Pharm. 2008,355, (1-2), 314-320.
39. Jannin, V., Musakhanian, J., Marchaud D., 2008. Appraches for the development of solid
and semi-solid lipid-based formulations. Adv. Drug Deliv. Rev. 60, 734-746.
40. Jenning, V.; Gohla, S. Comparison of wax and glyceride solid lipid nanoparticles (SLN).
Int. J. Pharm. 2000, 196, (2), 219-222.
41. Jenning, V.; Gysler, A.; Schafer-Korting, M.; Gohla, S. H. Vitamin A loaded solid lipid
nanoparticles for topical use: occlusive properties and drug targeting to the upper skin. Eur.
J. Pharm. Biopharm. 2000a, 49, (3), 211-218.
42. Jenning, V.; Mader, K.; Gohla, S. H. Solid lipid nanoparticles (SLN) based on binary
mixtures of liquid and solid lipids: a H-NMR study. Int. J. Pharm. 2000b, 205, (1-2), 15-
21.
43. Jenning, V.; Thunemann, A. F.; Gohla, S. H. Characterisation of a novel solid lipid
nanoparticle carrier system based on binary mixtures of liquid and solid lipids. Int. J.
Pharm. 2000c, 199, (2), 167-177.
44. Jenning, V.; Gohla, S. H. Encapsulation of retinoids in solid lipid nanoparticles (SLN). J.
Microencapsul. 2001, 18, (2), 149-158.
45. Jores, K. Characterization of solid lipid nanoparticles (SLN™)-how to optimize the
quantity of surfactants. Proc. Intern. Symp. Control. Rel. Bioact. Mater. 2000; pp 1092-
1093.
46. Jores, K.; Mehnert, W.; Mader, K. Physicochemical investigations on solid lipid
nanoparticles and on oil-loaded solid lipid nanoparticles: a nuclear magnetic resonance and
electron spin resonance study. Pharm. Res. 2003, 20, (8), 1274- 1283.
47. Jores, K.; Mehnert, W.; Drechsler, M.; Bunjes, H.; Johann, C.; Mader, K. Investigations on
the structure of solid lipid nanoparticles (SLN) and oil-loaded solid lipid nanoparticles by
photon correlation spectroscopy, field-flow fractionation and transmission electron
microscopy. J. Control. Release 2004, 95, (2), 217-227.
Dept. of Pharmaceutical Technology CPS, JNTUH
BIBLIOGRAPHY
96
48. Kaukonen, A. M.; Boyd, B. J.; Charman, W. N.; Porter, C. J. Drug solubilization behavior
during in vitro digestion of suspension formulations of poorly watersoluble drugs in
triglyceride lipids. Pharm. Res. 2004b, 21, (2), 254-260.
49. Kaukonen, A. M.; Boyd, B. J.; Porter, C. J.; Charman, W. N. Drug solubilization behavior
during in vitro digestion of simple triglyceride lipid solution formulations. Pharm. Res.
2004a, 21, (2), 245-253.
50. Kaur, I. P.; Bhandari, R.; Bhandari, S.; Kakkar, V. Potential of solid lipid nanoparticles in
brain targeting. J. Control. Release 2008a, 127, (2), 97-109.
51. Kuo, Y. C.; Chen, H. H. Entrapment and release of saquinavir using novel cationic solid
lipid nanoparticles. Int. J. Pharm. 2009, 365, (1-2), 206-213.
52. Ladbrooke, B. D.; Williams, R. M.; Chapman, D. Studies on lecithin-cholesterol-water
nteractions by differential scanning calorimetry and X-ray diffraction. Biochim Biophys.
Acta 1968, 150, 333-340.
53. Lai, F.; Wissing, S. A.; Muller, R. H.; Fadda, A. M. Artemisia arborescens L essential oil-
loaded solid lipid nanoparticles for potential agricultural application: preparation and
characterization. AAPS PharmSciTech 2006, 7, (1), E10-E18.
54. Lander, R.; Manger, W.; Scouloudis, M.; Ku, A.; Davis, C.; Lee, A. Gaulin
homogenization: a mechanistic study. Biotechnol. Prog. 2000, 16, (1), 80-85.
55. Lee, M. K.; Lim, S. J.; Kim, C. K. Preparation, characterization and in vitro cytotoxicity of
paclitaxel-loaded sterically stabilized solid lipid nanoparticles. Biomaterials 2007, 28, (12),
2137-2146.
56. Lim, S. J.; Lee, M. K.; Kim, C. K. Altered chemical and biological activities of all-trans
retinoic acid incorporated in solid lipid nanoparticle powders. J. Control. Release 2004,
100,(1), 53-61.
57. Ma, P.; Dong, X.; Swadley, C. L.; Gupte, A.; Leggas, M.; Ledebur, H. C.; Mumper, R. J.
Development of Idarubicin and Doxorubicin Solid Lipid Nanoparticles to Overcome Pgp-
Mediated Multiple DrugResistance in Leukemia. J. biomed. nanotechnol. 2009,5,151-161
58. Manjunath, K.; Reddy, J. S.; Venkateswarlu, V. Solid lipid nanoparticles as drug delivery
systems. Methods Find. Exp. Clin. Pharmacol. 2005, 27, (2), 127-144.
Dept. of Pharmaceutical Technology CPS, JNTUH
BIBLIOGRAPHY
97
59. Mehnert, W.; Mader, K. Solid lipid nanoparticles: production, characterization and
applications. Adv. DrugDeliv. Rev. 2001, 47, (2-3), 165-196.
60. Muchow, M.; Maincent, P.; Muller, R. H. Lipid nanoparticles with a solid matrix (SLN,
NLC, LDC) for oral drug delivery. Drug Dev. Ind. Pharm. 2008, 34, (12), 1394- 1405.
61. Muller, B. G.; Leuenberger, H.; Kissel, T. Albumin nanospheres as carriers for passive
drug targeting: an optimized manufacturing technique. Pharm. Res. 1996a, 13, (1), 32-37.
62. Müller, R. H., Mäder, K., and Gohla, S. (2000) Solid lipid nanoparticles (SLN) for controlled drug delivery – a review of the state of the art. European Journal of Pharmaceutics and Biopharmaceutics 50, 161-17
63. Muller, R. H.; Ruhl, D.; Runge, S. A. Biodegradation of solid lipid nanoparticles as a function of lipase incubation time. Int. J. Pharm. 1996b, 144, 115-121.
64. Muller, R. H.; Mehnert, W.; Lucks, J. S.; Schwarz, C.; Zur Muhlen, A.; Weyhers, H.;
Freitas, C.; Ruhl, D. Solid lipid nanoparticles (SLN) - An alternative colloidal carrier
system for controlled drug delivery. Eur. J. Pharm. Biopharm. 1995,41, 62-69.
65. Muller, R. H.; Mader, K.; Gohla, S. Solid lipid nanoparticles (SLN) for controlled drug
delivery - a review of the state of the art. Eur. J. Pharm. Biopharm. 2000, 50, (1), 161-177.
66. Miiller, R. H.; Radtke, M.; Wissing, S. A. Solid lipid nanoparticles (SLN) and
nanostructured lipid carriers (NLC) in cosmetic and dermatological preparations. Adv.
Drug Deliv. Rev. 2002a, 54 Suppl 1, S131-155.
67. Miiller, R. H.; Keck, C. M. Challenges and solutions for the delivery of biotech drugs—a
review of drug nanocrystal technology and lipid nanoparticles. J. Biotechnol. 2004, 113,(1-
3), 151-170.
68. Miiller, R. H.; Runge, S.; Ravelli, V.; Mehnert, W.; Thunemann, A. F.; Souto, E. B. Oral
bioavailability of cyclosporine: solid lipid nanoparticles (SLN) versus drug nanocrystals.
Int. J. Pharm. 2006, 317, (1), 82-89.
69. Mumper, R. J.; Cui, Z.; Oyewumi, M. O. Nanotemplate engineering of cell specific
nanoparticles. J. Disp. Sci. technol. 2003, 24, (3&4), 569-588.
70. O'Driscoll, C. M. Lipid-based formulations for intestinal lymphatic delivery. Eur. J.
Pharm. Sci. 2002, 15, (5), 405-415.
71. Olbrich, C.; Miiller, R. H. Enzymatic degradation of SLN-effect of surfactant and
surfactant mixtures. Int. J. Pharm. 1999, 180, (1), 31-39.
Dept. of Pharmaceutical Technology CPS, JNTUH
BIBLIOGRAPHY
98
72. Paliwal, R.; Rai, S.; Vaidya, B.; Khatri, K.; Goyal, A. K.; Mishra, N.; Mehta, A.; Vyas, S.
P. Effect of lipid core material on characteristics of solid lipid nanoparticles designed for
oral lymphatic delivery. Nanomedicine 2009, 5, (2), 184-191.
73. Pardeike, J.; Hommoss, A.; Muller, R. H. Lipid nanoparticles (SLN, NLC) in cosmetic and
pharmaceutical dermal products. Int. J. Pharm. 2009, 366, (1-2), 170-184.
74. Patton, J. S.; Carey, M. C. Watching fat digestion. Science 1979, 204, (4389), 145-148.
75. Porter, C. J.; Charman, W. N. Intestinal lymphatic drug transport: an update. Adv. Drug
Deliv. Rev. 2001a, 50, (1-2), 61-80.
76. Porter, C. J.; Trevaskis, N. L.; Charman, W. N. Lipids and lipid-based formulations:
optimizing the oral delivery of lipophilic drugs. Nat. Rev. Drug Discov. 2007, 6, (3), 231-
248.
77. Porter, C. J. H.; Charman, W. N. Uptake of drugs into the intestinal lymphatics after oral
administration Adv. Drug Deliv. Rev. 1997, 25, 71-89.
78. Porter, C. J. H.; Pouton, C. W.; Cuine, J. F.; Charman, W. N. Enhancing intestinal drug
solubilisation using lipid-based delivery systems. Adv. Drug Deliv. Rev. 2008, 60, (6), 673-
691.
79. Pouton, C. W. Formulation of poorly water-soluble drugs for oral administration:
physicochemical and physiological issues and the lipid formulation classification system.
Eur. J. Pharm. Sci 2006,29, (3-4), 278-287.
80. Sato, K., Crystallization of fats and fatty acids. In Crystallization and Polymorphism of
Fats and Fatty Acids, Garti, N.; Sato, K., Eds. Marcel Dekker Inc.: NY, Basel, 1988; pp
227-266.
81. Saupe, A.; Gordon, K. C.; Rades, T. Structural investigations on nanoemulsions, solid lipid
nanoparticles and nanostructured lipid carriers by cryo-field emission scanning electron
microscopy and Raman spectroscopy. Int. J. Pharm. 2006, 314, (1), 56-62.
82. Scholer, N.; Olbrich, C.; Tabatt, K.; Muller, R. H.; Hahn, H.; Liesenfeld, O. Surfactant, but
not the size of solid lipid nanoparticles (SLN) influences viability and cytokine production
of macrophages. Int. J. Pharm. 2001,221, (1-2), 57-67.
83. Scholer, N.; Hahn, H.; Muller, R. H.; Liesenfeld, O. Effect of lipid matrix and size of solid
lipid nanoparticles (SLN) on the viability and cytokine production of macrophages. Int. J.
Pharm. 2002,231, (2), 167-176.
Dept. of Pharmaceutical Technology CPS, JNTUH
BIBLIOGRAPHY
99
84. Schubert, M. A.; Muller-Goymann, C. C. Solvent injection as a new approach for
manufacturing lipid nanoparticles—evaluation of the method and process parameters. Eur.
J. Pharm. Biopharm. 2003, 55, (1), 125-131.
85. Schubert, M. A.; Harms, M.; Muller-Goymann, C. C. Structural investigations on lipid
nanoparticles containing high amounts of lecithin. Eur. J. Pharm. Sci. 2006, 27, (2-3), 226-
236.
86. Schwarz, C.; Mehnert, W.; Lucks, J. S.; Muller, R. H. Solid lipid nanoparticles (SLN) for
controlled drug delivery. I. Production, characterization and sterilization. J. Control.
Release 1994, 30, 83-96.
87. Schwarz, C. Feste Lipidnanopartikel: Herstellung, Charakterisierung,
Arzneistoffinkorporation und -freisetzung, Sterilisation und Lyophilisation. Ph.D. Thesis,
Free University of Berlin, Berlin, 1995.
88. Schwarz, C.; Freitas, C.; Mehnert, W.; Muller, R. H. Sterilization and physical stability of
drug-free and etomidate-loaded solid lipid nanoparticles. Proc. Intern. Symp. Control. Rel.
Bioct. Mater. 1995; pp 766-767.
89. Schwarz, C.; Mehnert, W. Sterilization of drug-free and tetracaine-loaded solid lipid
nanoparticles (SLN). Proc 1st World Meeting APGI/APV, Budapest 1995; pp 485- 486.
90. Schwarz, C.; Mehnert, W. Freeze-drying of drug-free and drug-loaded solid lipid
nanoparticles (SLN). Int. J. Pharm. 1997, 157, (2), 171-179.
91. Scow, R. O.; Desnuelle, P.; Verger, R. Lipolysis and lipid movement in a membrane
model. Action of lipoprotein lipase. J. Biol. Chem. 1979,254, (14), 6456-6463.
92. Shahgaldian, P.; Gualbert, J.; Aissa, K.; Coleman, A. W. A study of the freeze-drying
conditions of calixarene based solid lipid nanoparticles. Eur. J. Pharm. Biopharm. 2003,
55,(2), 181-184.
93. Siekmann, B.; Westesen, K. Submicron-sized parenteral carrier systems based on solid
lipids, pharm. pharmacol. Lett. 1992, 1, 123-126.
94. Siekmann, B.; Westesen, K. Melt-homogenized solid lipid nanoparticles stabilized by the
nonionic surfactant tyloxapol. I. Preparation and particle size determination. Pharm.
Pharmacol. Lett. 1994a, 3, 194-197.
Dept. of Pharmaceutical Technology CPS, JNTUH
BIBLIOGRAPHY
100
95. Siekmann, B.; Westesen, K. Thermoanalysis of the recrystallization process of
melthomogenmized glyceride nanoparticles. Colloids Surf. B Biointerfaces 1994b, 3, 159-
175.
96. Siekmann, B.; Westesen, K. Investigation on solid lipid nanoparticles prepared by
precipitation in o/w emuslion. Eur. J. Pharm. Biopharm. 1996,43,104-109.
97. Siekmann, B.; Westesen, K., Submicron lipid suspensions (solid lipid nanoparticles) versus
lipid nanoemulsions: similarities and differences. In Submicron Emulsions in Drug
Targeting and Delivery, Benita, S., Ed. Harwood Academic Publishers: Amsterdam, 1998;
pp 205-218.
98. Souto, E. B.; Mehnert, W.; Muller, R. H. Polymorphic behaviour of Compritol888 ATO as
bulk lipid and as SLN and NLC. J. Microencapsul. 2006,23, (4), 417-433.
99. Storm, G.; Belliot, S. O.; Daemen, T.; Lasic, D. Surface modification of nanoparticles to
oppose uptake by the mononuclear phagocyte system. Adv. Drug Deliv. Rev. 1995, 17,31-
48.
100. Tian, J.; Pang, X.; Yu, K.; Liu, L.; Zhou, J. Preparation, characterization and in vivo
distribution of solid lipid nanoparticles loaded with cisplatin. Pharmazie 2008, 63, (8), 593-
597.
101. Tiyaboonchai, W.; Tungpradit, W.; Plianbangchang, P. Formulation and characterization
of curcuminoids loaded solid lipid nanoparticles. Int. J. Pharm. 2007, 337, (1-2), 299-306.
102. Trotta, M.; Debernardi, F.; Caputo, O. Preparation of solid lipid nanoparticles by a
solvent emulsification-diffusion technique. Int. J. Pharm. 2003, 257, (1-2), 153- 160.
103. Tso, P. Gastrointestinal digestion and absorption of lipid. Adv. Lipid Res. 1985, 21, 143-
186.
104. Venkateswarlu, V.; Manjunath, K. Preparation, characterization and in vitro release
kinetics of clozapine solid lipid nanoparticles. J. Control. Release 2004, 95, (3), 627-638.
105. Westesen, K.; Siekmann, B.; Koch, M. H. J. Investigations on the physical state of lipid
nanoparticles by synchrotron X-ray diffraction. Int. J. Pharm. 1993, 93, 189-199.
106. Westesen, K.; Bunjes, H. Do nanoparticels prepared from lipids solid at room
temperature always possess a solid matrix? Int. J. Pharm. 1995, 115,129-131.
Dept. of Pharmaceutical Technology CPS, JNTUH
BIBLIOGRAPHY
101
107. Westesen, K.; Bunjes, H.; Koch, M. H. J. Physicochemical characterization of lipid
nanoparticles and evaluation of their drug loading capacity and sustained release potential.
J. Control. Release 1997,48, 223-236.
108. Westesen, K.; Siekmann, B. Investigation of the gel formation of phospholipid-stabilized
solid lipid nanoparticles. Int. J. Pharm. 1997, 151, 35-45.
109. Westesen, K.; Drechsler, M.; Bunjes, H., Colloidal dispersions based on solid lipids. In
Food Colloids: Fundamentals of Formulation, Dickinson, E.; Miller, R., Eds.
110. Royal Society of Chemistry: Cambridge, 2001; pp 103-115. Wong, H. L.; Li, Y.;
Bendayan, R.; Rauth, M. A.; Wu, X. Y., Solid lipid nanoparticles for anti-tumor drug
delivery. In Nanotechnology for Cancer Therapy, Amiji, M. M., Ed. CRC Press: Boca
Raton, 2007a; pp 741 -776.
111. Wong, H. L.; Bendayan, R.; Rauth, A. M.; Li, Y.; Wu, X. Y. Chemotherapy with
anticancer drugs encapsulated in solid lipid nanoparticles. Adv. Drug Deliv. Rev. 2007b,
59, (6), 491-504.
112. Wong, H. L.; Bendayan, R.; Rauth, A. M.; Wu, X. Y. Development of solid lipid
nanoparticles containing ionically complexed chemotherapeutic drugs and
chemosensitizers. J. Pharm. Sci. 2004, 93, (8), 1993-2008.
113. Xu, Z.; Chen, L.; GU, W.; GAO, Y.; Lin, L.; Zhang, Z.; Xi, Y.; Li, Y. The performance
of docetaxel-loaded solid lipid nanoparticles targeted to hepatocellular carcinoma.
Biomaterials 2009, 30, (2), 226-232.
114. Yang, S.; Zhu, J.; Lu, Y.; Liang, B.; Yang, C. Body distribution of camptothecin solid
lipid nanoparticles after oral administration. Pharm. Res. 1999, 16, (5), 751-757.
115. Ying, X. Y.; Du, Y. Z.; Chen, W. W.; Yuan, H.; Hu, F. Q. Preparation and
characterization of modified lipid nanoparticles for doxorubicin controlled release.
Pharmazie 2008, 63, (12), 878-882.
116. Yuan, H.; Miao, J.; Du, Y. Z.; You, J.; Hu, F. Q.; Zeng, S. Cellular uptake of solid lipid
nanoparticles and cytotoxicity of encapsulated paclitaxel in A549 cancer cells. Int. J.
Pharm. 2008, 348, (1-2), 137-145.
117. Yuan, H.; Jiang, S. P.; Du, Y. Z.; Miao, J.; Zhang, X. G.; Hu, F. Q. Strategic approaches
for improving entrapment of hydrophilic peptide drugs by lipid nanoparticles. Colloids
Surf. B Biointerfaces 2009, 70, (2), 248-253.
Dept. of Pharmaceutical Technology CPS, JNTUH
BIBLIOGRAPHY
102
118. Zara, G. P.; Cavalli, R.; Fundaro, A.; Bargoni, A.; Caputo, O.; Gasco, M. R.
Pharmacokinetics of doxorubicin incorporated in solid lipid nanospheres (SLN).
Pharmacol. Res. 1999, 40, (3), 281-286.
119. Zara, G. P.; Cavalli, R.; Bargoni, A.; Fundaro, A.; Vighetto, D.; Gasco, M. R.
Intravenous administration to rabbits of non-stealth and stealth doxorubicinloaded solid
lipid nanoparticles at increasing concentrations of stealth agent: pharmacokinetics and
distribution of doxorubicin in brain and other tissues. J. Drug Target. 2002, 10, (4), 327-
335.
120. Zimmermann, E.; Liedtke, S.; Muller, R. H.; Mader, K. 'H-NMR as a method to
characterize colloidal carrier systems. Proc. Intern. Symp. Control. Rel. Bioact. Mater.
1999,26.
121. Zimmermann, E.; Muller, R. H.; Mader, K. Influence of different parameters on
reconstitution of lyophilized SLN. Int. J. Pharm. 2000, 196, (2), 211-213.
122. Zur Miihlen, A. Feste Lipid-Nanopartikel mit prolongierter Wirkstoffliberation:
Herstellung, Langzeitstabilitat, Charakterisierung, Freisetzungsverhalten und
mechanismen. Ph.D. Thesis, Free University of Berlin, 1996.
123. Zur Miihlen, A.; Zur Miihlen, E.; Niehus, H.; Mehnert, W. Atomic force microscopy
studies of solid lipid nanoparticles. Pharm. Res. 1996, 13, (9), 1411-1416.
124. Zur Miihlen, A.; Mehnert, W. Drug release and release mechanism of prednisolone
loaded solid lipid nanoparticles. pharmazie 1998, 53, 552-555.
125. Zur Miihlen, A.; Schwarz, C.; Mehnert, W. Solid lipid nanoparticles (SLN) for controlled
drug delivery—drug release and release mechanism. Eur. J. Pharm. Biopharm. 1998, 45,
(2), 149-155.
Dept. of Pharmaceutical Technology CPS, JNTUH
BIBLIOGRAPHY
103