REVIEW STUDIES ON SOLVENT DROP SYNTHESIS OF PHARMACEUTICAL COCRYSTAL

Tella A.C1, Ameen O.A2*

1. Department of Chemistry, University of Ilorin, 2. Department of Chemistry, Kwara State Polytechnic Ilorin.

ABSTRACT: An efficient and environmental friendly synthesis of some

pharmaceutical co-crystal has been achieved. The combination of active

pharmaceutical ingredient (api) and co-crystal former was studied and

discovered that this enhance pharmaceutical properties by modification of

chemical stability, moisture uptake, mechanical behaviour, solubility,

dissolution rate and bioavailability.

Keywords: co-crystal, active pharmaceutical ingredient (API), hydrogen bond,

crystalline complex.

Introduction

The design of pharmaceutical crystals that possess different

molecular components is valuable to control pharmaceutical properties of

solids without changing the covalent bonds. These multiple component

crystals, crystalline complexes or co-crystal often rely on hydrogen-

bonded assemblies between neutral molecules of the active

pharmaceutical ingredient (API) and other components.

As a consequence, co-crystals increase the diversity of solid-state

forms of an API, even for non-ionisable API’s and enhance

pharmaceutical properties by modification of chemical stability, moisture

uptake, mechanical behaviour, solubility, dissolution rate and

bioavailability.

Co-crystals can be defined as crystalline complexes of two or more

neutral molecular constituents bond together in the crystal lattice through

non- covalent interaction (primarily hydrogen bond). However

pharmaceutical co-crystal is a single crystalline solid that incorporates

two neutral molecules, one being an active pharmaceutical ingredient

(API) and the other a co-crystal former.

Co-crystal former may be an excipient or another drug.

Pharmaceutical co-crystal technology is used to identify and develop new

proprietary forms of widely prescribed drugs and offer a chance to

increase the number of forms of an API. Scientists showed that modifying

the physical properties of a pharmaceutical compound through

pharmaceutical co-crystal fotmation improved the performance of a drug

know to have poor solubility. Pharmaceutical co-crystallization is a

reliable method to modify physical and technical properties of drugs such

as solubility, dissolution rate, stability hygroscopisity, and

compressibility without alternating their pharmacological behaviour. The

use of co-crystals in drug design and delivery and functional materials

with potential applications as pharmaceutical has recently attracted

considerable interest. Pharmaceutical co-crystal have been described for

many drugs such as acetoaminophen, aspirin, ibuprofen, fluriprofen etc.

ACTIVE PHARMACEUTICAL INGREDIENT CLASSIFICATION

The Classification of Active Pharmaceutical Ingredient (API) solid

form based on structure and composition showed with the schematic

diagram below.

COCRYSTAL VERSUS SOLVATES

The main difference between solvates and co-crystals is the

physical state of the isolated pure components: if one component is a

liquid at room temperature, the crystals are designated as solvates; if both

components are solids at room temperature, the crystals are designated as

co-crystals.

SALT VERSUS CO-CRYSTAL FORMATION

Co-crystal and Salts may sometimes be confused. The

understanding of the fundamental difference between a salt formation and

a co-crystal is very important to both pre-formulation activities and

AMORPHOUS

SOLID CRYSTALLINE SINGLE COMPONENT

POLYMORPHS

MULTIPLE COMPONENT

HYDRATE SALTS SOLVATES COCRYSTAL

chemical/pharmaceutical development aspects. Indeed, salts and co-

crystals can be considered as opposite ends of multi-component

structures. Salt are often chosen instead of the free acid or base as these

can improve crystallinity, solubility and stability of a pharmaceutical

compound. Co-crystals are an alternative to salts when these do not have

the appropriate solid state properties or cannot be formed due to the

absence of ionisable sites in the API.

Salt formation is an acid-base reaction between the API and an

acidic or basic substance. The widespread use of salt formation is

evidence by the large number of marketed crystalline salts of APIs. salt

formation is a three component system having an acid (A), a base (B) and

one or more solvents. A salt is formed by transfer of a proton (H+) from

an acid (A) to base (B)

A-H + B (A-)(B+-H)

Proton transter is thought to mainly depend on the pKa values of

the components. The general rules for the packing of hydrogen bonded

molecules in crystals were developed by etter.

When there is no such transfer and the components are instead

present in the crystal as neutral entities, the product is generally defined

as co-crytal. In other words. A co-crystal is an A-B composite in which

no proton transfer occurred.

The formation of a salt or co-crystal can be predicted from pKa

value of acid (A) and a base (B). salt formation generally requires a

difference of about 2.7 pKa units between the conjugate base and the

conjugate acid (A) i.e [pKa (base) - pKa (acid) ≥ 2.7]. for example,

succinic acid having pKa 4.2 form co-crystal with urea base pKa values

are not sufficiently high to allow proton transfer when co-crystal is

formed. Co-crystal of succinic acid- urea has two hydrogen bonds i.e. the

oxygen atom in urea molecule is bonded to hydrogen atom in succinic

acid molecule while oxygen atom from succinic acid molecule is bonded

to hydrogen atom in urea molecule as show below.

SCREENING OF COCRYSTALS

Co-crystals can be prepared from two molecules of any shape or

size having complementary hydrogen bond functionalities. The ability of

an API to form cocrystal is dependent on a range of variables, including

the types of co-former, the API co-former ratio, the solvent, the

temperature, the pressure, the crystallization technique, etc. experimental

screening for cocrystal former is not trivial. Synthesis/processing of co-

crystals can be accomplished via a number of methods, including slow

solvent evaporation crystallization from solution, solvent-reduced (e.g

slurrying, solvent-drop grinding) and solvent-free (e.g grinding, melt [hot

stage microscopy]), high throughput crystallization and co-sublimation

techniques.

Solution co-crystallization

For solution co-crystallization, the two components must have

similar solubility; otherwise the least soluble component will precipitate

our exclusively. However similar solubility alone will not guarantee

success. It has been suggested that it may be useful to consider

polymorphic compounds, which exist in more than one crystalline form

as co-crystallizing components. If a molecular compound exist in several

polymorphic forms it has demonstrated a structural flexibility and is not

locked into a single type of crystalline lattice or packing mode. Thus, the

chance of bringing such a molecule into a different packing arrangement

in coexistence with another molecule is increased. Clearly polymorphism

alone does not guarantee the functionality of a compound to act as a co-

crystallizing agent, whilst the ability of a molecule to participate in

intermolecular interactions obviously plays a critical role.

Slurry Conversion

Slurry conversion experiments were conducted in different organic

solvents and water. Solvent (100 or 200ml) was added to the co-crystal

(20mg) and the resulting suspension was stirred at room temperature for

some days. After some days, the solvent was decanted and the solid

material was dried under a flow of nitrogen for 5min. the remaining

solids were then characterized using PXRD.

Melt Method

Melts method have also generated an interest in co-crystal

formation. By simply melting two co-crystal formers together and

cooling, a co-crystal may be formed. If a co-crystal is not form from a

melt, a seed from melt may be used in a crystallization solution in order

to form a co-crystal. Another phase change in order to form co-crystal is

that of sublimation. Sublimation may more often than not form hydrate.

Grinding Method (Mechanical Method)

Grinding has attracted interest into the formation of co-crystals.

Both neat and liquid assisted grinding are techniques employed in order

to produce these materials. In neat (dry) grinding, co-crsytal formers are

ground together manually using a mortar and pestle, using a ball mill, or

using a vibratory mill, in liquid-assisted grinding, or kneading, a small or

substoichiometric amount of liquid (solvent) is added to the grinding

mixture. The method was develop in order to increase the rate of co-

crystal formation, but had advantages over neat grinding such as

increased yield, ability to control polymorph production, better product

crystallinity, and applies to a significantly larger scope of co-crystal

formers.

Pharmaceutical co-crystals can also be formed by use of

supercritical fluids. Supercritical fluids act as a new media for the

generation of co-crystals. Supercritical fluid technology offer a new

platform that allows a single-step generation of particles that are difficult

or even impossible to obtain by traditional techniques. The generation of

pure and dried new co-crystals (crystalline molecular complexes

comprising the API and one or more co-formers in the crystal lattice) can

be achieved due to unique properties of supercritical fluids (SCFS) by

using different supercritical fluid properties; supercritical CO2 solvent

power, anti-solvent effect and tits atomization enhancement.

Using intermediate phases to synthesis these solid-state compound

are also employed. Through the use of a hydrate or an amorphous phase

as an intermediate during synthesis is a solid-state route has proven

successful in forming a co-crystal. Also, the used of a metal stable

polymorphic from of one co-crystal former can be employed. In this

method, the metal stable form acts as an unstable intermediate on the

nucleation pathway to a co-crystal. As always, a clear connection

between pair wise components of the co-crystal are needed inn addition

to the thermodynamic requirement in order to form these compounds.

The table below selected examples of pharmaceutical co-crystals and their method of

preparation.

API Co-crystal former Preparation

method

Enhanced

property

Reference

Aspirin 4,4-Dipyridil Slurry conversion Walsh et al 2003

Caffeine Oxalic acid

Glutaric acid

Solven-assisted

grinding

Physical

stability

Trask et al 2005

Carbamazepine Nicotinamide Cooling

crystallization

Physical

stability,

dissolution rate

and oral

bioavailability

Hickey et al 2007

Fluoxetine

hydrochloride

Benzoic acid Solvent evaporation Intrinsic

dissolution rate

Childs et al, 2004

Indomethacin Saccharin Solvent-assisted

grinding or solvent

evaporation

Physical

stability and

dissolution rate

Basavoju et al 2008

CHARACTERIZATION OF CO-CRYSTAL

Characterization of co-crystals involves both structure (infrared

spectroscopy, single crystal x-ray crystallography and powder x-ray

diffraction) and physical properties (e.g melting point apparatus,

differential scanning calorimetry, thermogravimetric analysis).

Single crystal x-ray diffraction is the preferred characterization

technique in determining whether a co-crystalline material has been

generated; how ever, suitable x-ray quality crystals cannot always be

produced. Additionally, even if single crystal can be grown of sufficient

size and quality, the exact location of the hydrogen atom (determination if

proton transfer has occurred from the acid to the base or not) may be

ambiguous. Thus, it is advantageous to utilize a variety of solid-state

spectroscopic techniques (Raman, infrared, and solid-state NMR). When

attempting to characterize potentially new co-crystalline materials.

Infrared spectroscopy can also be a very powerful tool in detecting

co-crystal formation, especially when a carboxylic acid is used as a co-

former.

PROPERTIES OF COCRYSTALS

Co-crystal Structures exhibit long-range order and the Components

interact via non-covalent interactions such as hydrogen bonding, ionic

interactions, vander waals interactions and resulting crystal structures

can generate physical and chemical properties that differ from the

properties of the individual components such properties include melting

point, solubility, chemical stability, and mechanical properties. Some co-

crystals have been observed to exist. As polymorphs, which may display

different physical properties depending on the form of the crystals.

Phase diagram determined from the “contact method” of thermal

microscopy proved valuable in the discovery of new co-crystals. The

construction of these phase diagrams is made possible due to the change

in melting point upon co-crystallization. Two crystalline substances are

deposited on either side of a microscope slide and are sequentially melted

and resolidified. This process creates thin films of each substance with a

contact zone in the middle. A melting point phase diagram may be

constructed by slow heating of the slide under a microscope and

observation of the melting points of the various portions of the slide. For

a simple binary phase diagram, if one eutectic point is observed then the

substances do not form a co-crystal. If two eutectic points are observed,

then the composition between these two points corresponds to the co-

crystal as shown by the graph below

A schematic for the determination of melting point binary phase diagrams from

thermal microscopy.

CASE STUDIES OF SOME PHARMACEUTICAL COCRYSTAL

The Pharmaceutical Co-crystal studied focus on the formation of

pharmaceutical Co-crystal with altered Physical Properties of Clinical

relevance.

Pharmaceutical cocrystal of carbamazepine (tegretol)

Carbamazapine (CBZ) is an important antiepileptic drug that has

been in use for over three decades. Oral administration of (CBZ)

encounters multiple challenges, including low water solubility with high

dosage required for therapeutic effect (i.e > 100mg/day); dissolution,

limited bioavailability and auto induction for metabolism.

The formation of CBZ:Saccharin co-crystal appears to be superior

to existing crystal from of CBZ in the following respect; stability relative

to the anhydrous polymorph of CBZ; favourable dissolution and

suspension stability, favourable oral absorption profile in dogs.

Cocrystal of theophylline:

Theophyline is useful in treatment respiratory disease such as

asthma. From the physiochemical standpoint, theophylline represents

challenge to formulators in that it is known to interconvert between

crystalline anhydrate and monohydrate forms as a function of relative

humidity (RH). The possibility of crystalline hydrate formation

complicates design of a consistent, reproducible for an API in the drug

development process. Reversible hydrate formation is particularly

problematic, as it indicates that neither the anhydrate nor the hydrate is

fully stable across the range of common processing condition,

theophyline is stuctural analogue of caffeine. The cocrystals of the

theophylline were prepared with oxalic acid, malonic acid, maleic acid,

glutaric acid by solvent evaporation technique. The relative humidity

stability comprised of the storage and subsequent PXRD analysis at four

specific RH levels (0%, 43%, 75%, and 98%) across four different time

points ( 1day, 3days, 1 and 7 weeks). Over the course of 7weeks study it

was found that, at 75% RH and below, theophylline anhydrate converted

into theophylline monohydrate. No formation of theophylline hydrate was

found in any case.

The observed RH stability of theophylline co-crystal demonstrates

the physical stabillity improvement, specifically avoidance of hydrate

formation. The co-crystals formed by oxalic acid found to be more stable.

This study demonstrate use of cocrystals in physical property

improvement.

Cocrystals of aceclofenac:

Aceclofenae is an orally effective nonsteoidal antiinflammatory drug of

phenyl acetic acid group, which possesses remarkable antiinflamatory,

analgesic and antipyretic properties. Aceclofenac exhibits slight solubility

in water and a consequence it exhibit low bioavailability after oral

administration. Mutalic prepared cocrystals of aceclofenac by simple

solvent change approach by using chitosan.

Chitosan has been considered to be one of the most promising

biopolymer for drug delivery purpose. Chitosan is a linear hyrophilic

polysaccharide polymer of D-glucosammine. It is non-toxic natural

polcationic polymer that is degraded by the microflora in the colon. It is

abundant in nature and is present in the exoskeleton of crustaceans such

as crabs and shrimps. Chitosan has been demonstrated to a be good

vehicle for enhancing the dissolution properties and bioavailability of a

number of poorly water soluble drugs.

Chitosan was precipitated on acceclofenac crystal using sodium

citrate as a salting out agent. The pure drug and prepared cocrystals with

different concentrations (0.05 to 0.6%) were characterized in terms of

solubility, drug content, particle size, thermal behaviour (differential

scanning calotimetry, DSC), X-ray diffration (X-RD), morphology, in

vivo drug release, stability and phamacokinetic study.

It was observed that particle size of cocrystals was drastically

reduced during the formulation process. The DSC showed a decrease in

melting enthalpy indicating disorder in crystallinity. XRD also reveealed

disorder in crystallinity. The dissolution study showed that marked

increase in dissolution rate in comparison to pure drug. The considerable

dissolution rate of aceclofenac from optimized crystal formulation was

attributed to the welting effect of chitosan, decreased drug crystallinity,

altered morphology and micronization. The optimized crystals showed

excellent stability on storage at accelerated conditions. In vivo study

revealed that the crystals provided a rapid phamacological response in

mice and rat; beside improve in phamacokinetic parameters in rats

Co-crystal of 5-nitrouracil

Co-crystals of 5-nitroracil with solvent molecules, dioxane, pyridine,

DMSO, formamide and ethanol as well as with piperazine, N,N’-

dimethlpiperazine, 3-aminopyridine and diazabicyclooctane obtained by

deliberate inclution,have been examined by X-ray crystallography. The

tape structure found in the parent centric form of nitrouracil is retained

with some modifications in the co-crystals with dioxane, piperazine,

diazabicyclo-octane, N,N’-dimethylpiperazine, pyridine and DMSO, with

the guest molecular tapes exhibit mixed compositions. The observed

bonding patterns have been classified into six schemes, interestingly,

quadruple type of hydrogen bonding patterns are seen in co-crysals

containing 3-aminopyridine or ethanol and water, while a network of

acyclic tetrahedral pentamers of water is found in the cocrystal containing

diazabicyclo-octane and water. This case study reveals that hydrogen

acceptors and donors are necessary to form cocrystal.

Co-crystal of indomethacin

Indomethacin, a non-steroidal antiinflammatory drug (NSAID), is

widely prescribed for patients with athritis. It exist as α, y and amorphous

forms, in the solid state α-form being most stable form at room

temperature. Indomethacin is practically insoluble in water (2.5-4 g/ml; it

belongs to BCS class II) and poses sever chalenges in the formulation

development. Various cocrystal formers, including saccharin, were used

in the screening for indomethacin co-crystals in a series of solvents.

Solution evaporation method was used in the screening phase. DSC,

TGA, IR, Raman and PXRD techniques characterized the potential new

phases. The indomethacin saccharin co-crystal (IND-SAC co-crystal)

structure was determined from single crystal X- ray diffraction data.

Pharmaceutically relevant properties such as the dissolution rate and

dynamic vapour sorption (DVS) of the IND-SAC cocrystal were

evaluated. Solid-state and solvent-drop co-grinding methods were also

applied to indomethacin and saccharin

The IND-SAC co-crystals were obtained from ethl acetate. Physical

characterization showed that the IND.SAC co-crystal is unique vis-^-vis

thermal, spectroscopic and X-ray diffraction properties. The cocrystals

were obtained in a 1:1 ratio with a carboxylic acid and imide dimer

synthons. The dissolution rate of IND-SAC was considerably faster than

that of the stable indomethacin a-form. DVS studies indicated that the

co-crystals gained less than 0.05% in weight at 98% RH. IND-SAC co-

crystals were also obtained by solid state and solvent drop cocrystal

methods. The IND-SAC cocrystal was formed with a unique and

interesting carboxylic acid and amide dimer synthons interconnected by

weak N-H-----O hydrogen bonds. The co-crystals were associated with

significantly faster dissolution rate than indomethacin (a-form) in

phosphate buffer pH 7.4 and were non-hygroscopic.

APPLICATION OF PHARMACEUTICAL CO-CRYSTAL

Compared to other solid-state modification techniques employed by Pharmaceutiical

Industry, Co-crystal Formation appears to be an advantageous alternative for drug

discovery (e.g new molecule synthesis, nutraceutical co-crystals), drug delivery

(solubility, bioavailability) and chiral resolution. Experts are of the opinion that

pharmaceutical intellectual property landscape may benefit through co-crystallization.

LITERATURE REVIEW

According to Lemmere et al (2010), the active pharmaceutical

ingredient 2-chloro-4-nitrobenzoic acid is a potentially novel therapy fro

immunodeficiency diseases as an anti-viral and anti-cancer agent, and

exists as a dimorph in the solid state. Hot stage contacte method was

employed to investigate the potential of preparing a co-crystal with

nicotinamide GRAS compound. The 1:1 co-crystal 1 was made using

liquid-assisted grinding via a carboxylic acid-pyriding hydrogen bond,

while the nic form a centrosymmetric R2(2)(8) dimer to ultimately form a

ribbon architecture compared to other known co-crystals of nic. The

melting point of the cocrystal is higher than the melting of the pure

components, indicating that the pharmaceutical co-crystal is thermally

more stable than the pure pharmaceutical compound. The relative

stability of the interactions in the cocrystal over the pure compounds

supported by molecular modelling calculation.

Also, Andrew et al (2005) studied screening for crystalline salt via

mechanochemistry and found that neat grinding and solvent drop

grinding methods are efffective screening tools for indicating the

potential for crystalline salt formation involving a given acid- base pair

Two structurally similar APIs, the antibacterial drug trimethoprim

(T) and the antimalaria drug pyrimethamine (P) were used with 7

different salt former formate, acetate, malate, fumarate, succinate,

glutarate, salicylate. Results of the mechanochemical salt screening

experiments, as determined by PXRD analysis are summarized in table

below.

The result shows that neat grinding provide 40% overall screening

efficiency, measured as the percentage of experiment that generate

crystalline products distinct from physical mixtures of starting materials.

Counterio

n

Neat grindingb

Solvent-dropgrinding

b

Neat grindingb

Solvent-dropgrindingb

formate + + √ √

acetate + X + ++

maleate X √ X √

fumarate X + X +

succinat

e

X + X √

Glutarate √ √ + +

salicylate x + X +

Upon performing the grinding salt screen with methanol addition, overall

screening efficiency increased from 40% for neat grinding to a

remarkable 100% for solvent-drop grinding.

Likewise, Wenbing et al (2010) examined the high reactivity of

metal-organic frameworks under grinding conditions: parallel with

organic molecular material. These known MOFs were prepared by

grinding 1,4-benzen dicarboxylic acid with zinc oxide (ZnO) or basic

zinc carbonate [ZnCO3]2.[Zn(OH)2]3 in a ball mill in the presence of a

small amount of added liquid (100μL of H2O, MeOH, or DMF) for 20

minutes. The nature of the added liquid determined the product. This

shown according to the diagram below.

The findings improve our insight into the possibilities of grinding-

induced transformations and extend the application of grinding as a

convenient solvent-free or minimal-solvent method.

In accordance with Andrew et al (2004) reported solvent-drop

grinding: green polymorph control of cocrytallization. By grinding with a

minimal addition of solvent of appropriate polarity, control over the

polymorphic outcome of novel cocrystallization involving the model

pharmaceutical compound caffeine maybe achieved. Equimolar quantities

of anhydrous caffeine and glutaric acid were combined in a stainless

grinding jar with two grinding balls and the material was ground together

either with or without the addition of a few drop of solvents. The

resulting material was then characterized by PXRD. When caffeine and

G.A are ground together in the absence of solvent, co-crystal form I

predominantly results. Similarly, the addition of four drops from a pipette

of a non-polar solvent. Such as n-hexane, cyclohexane, or heptane also

produces form I. conversely upon addition of four drops of a more polar

solvent, including chloroform, dichloromethane, acetonitrile, and water,

the grinding experiment result in predominantly form II

CHAPTER FOUR

CONCLUSION

Pharmaceutical co-crystals represent a advantageous class of

crystal form in the context of pharmaceuticals. Co-crystals of drugs and

drug candidates represent a new type of material for pharmaceutical

development. Co-crystals are relatively new to pharmaceutical industry

and pharmaceutical co-crystals have given a new direction to deal with

problems of poorly soluble drugs. Co-crystals have potential to be much

more useful in pharmaceutical products than solvates or hydrates.

The relevance of co-crystals in API formulation includes the ability

of fine-tune physical properties. Characterization of API, identify and

develop new, proprietary forms of prescribed drugs and the opportunity

to generate intellectual property.

Further research is desirable in order to scale up co-crystal systems

and implement manufacturing of final dosage forms on commercial scale.

Screening for solid forms is important to guarantee that the optimum

form is carried forward in development and to minimize the likelihood of

unexpected form conversion. Co-crystals- High throughput gives vital

information on relationship between formation and chemical structure of

the API and co-former. Screening of API’s with library of co-crystal

formaers requires further investigations to include all possible coformers.

Studies regarding polymorphism of co-crystals should be strengthened in

order to accelerate the development of new pharmaceuticals. Additional

developments in screening methodology will further elevate the profile of

co-crystals on the pharmaceutical and intellectual property landscapes.

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