Solubilization and dissolution of tamoxifen-hydroxybutenyl cyclodextrin complexes

Solubilization and Dissolution of Tamoxifen-HydroxybutenylCyclodextrin Complexes

CHARLES M. BUCHANAN, NORMA L. BUCHANAN, KEVIN J. EDGAR, JUANELLE L. LAMBERT,JESSICA D. POSEY-DOWTY, MICHAEL G. RAMSEY, MICHAEL F. WEMPE

Research Laboratories, Eastman Chemical Company, P.O. Box 1972, Kingsport, Tennessee 37662

Received 6 February 2006; revised 5 May 2006; accepted 6 May 2006

Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/jps.20710

ABSTRACT: The solubility and dissolution of tamoxifen base and tamoxifen citratewith and without hydroxybutenyl-b-cyclodextrin (HBenBCD) in aqueous and organicmediawere examined. The solubility of tamoxifenwas greatly enhanced by complexationwith HBenBCD; pH of the medium, and choice of buffer significantly impacted theamount of drug that could be solubilized. Different tamoxifen:HBenBCD formulationswere prepared, including liquid fill capsule formulations, and their dissolution profileswere obtained. These dissolution studies demonstrated that enhanced solubilization oftamoxifen with HBenBCDwas effective across a wide variety of formulation options. Bycomplexation of tamoxifen base with HBenBCD, it was possible to obtain solubility anddissolution profiles for tamoxifen base thatwere essentially identical to that of tamoxifencitrate. � 2006Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 95:2246–

2255, 2006

Keywords: hydroxybutenyl-b-cyclodextrin; HBenBCD; tamoxifen; complex; liquidfill; solubility; dissolution

INTRODUCTION

In designing drug delivery systems for poorlywater-soluble drugs, the relationship betweendrug solubility in a physiological environment,drug absorption across biological barriers, andbioavailability or efficacy is of paramount impor-tance. There are many aspects to this problem;crystallinity, intrinsic aqueous solubility, drugstructure, and charge state of the drug moleculeare important issues. The ability of a deliveryvehicle to solubilize a drug, drug loading, andinteraction of the delivery vehicle with membranetransporters (e.g., P-glycoprotein) are also impor-tant issues. The medium in which the drugdelivery system is contained is a very significantissue.

Tamoxifen (Fig. 1) is a member of a class ofcompounds known as selective estrogen receptormodulators which have the capability of acting asestrogen receptor agonists in some tissues and asantagonists in other tissues.1,2 Tamoxifen is anestrogen receptor agonist in bone, the cardiovas-cular system, and the endometrium, but acts as anantagonist in breast tissue. Tamoxifen is usedclinically as a nonsteroidal antiestrogen for first-line endocrine treatment as well as adjuvanttherapy in early and metastatic breast cancers inpostmenopausal women. Tamoxifen is alsoapproved as a prophylactic agent for women athigh risk of developing breast cancer.

Tamoxifen is a weak base (pKa� 8.8) with verylow aqueous solubility and a melting point of1008C.3 Commercially, tamoxifen base is con-verted to the citrate salt as a means of increasingthe aqueous solubility and efficacy of tamoxifen.4

However, the increase in tamoxifen solubility viasalt formation is limited due to the higher meltingpoint of tamoxifen citrate (1468C) which inhibits

2246 JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 95, NO. 10, OCTOBER 2006

Correspondence to: Charles M. Buchanan (Telephone:423.229.8562; Fax: 423.229.4558; E-mail: [email protected])

Journal of Pharmaceutical Sciences, Vol. 95, 2246–2255 (2006)� 2006 Wiley-Liss, Inc. and the American Pharmacists Association

dissolution.5 The Z-isomer of tamoxifen is biologi-cally active but the E-isomer has limited biologicalactivity.6,7 Tamoxifen is a low-dose therapeuticagent typically given to patients for long periods oftime.

Cyclodextrins (CD) are cyclic oligomers ofglucose, which typically contain 6, 7, or 8 glucosemonomers joined by a-1,4 linkages; these oligo-mers are commonly called a-CD, b-CD, and g-CD,respectively. Topologically, cyclodextrins form atorus that has a hydrophobic interior and ahydrophilic exterior which allows the CD to bedissolved in water where it acts as a host moleculeand forms inclusion complexes with hydrophobicguest molecules. This feature has led to the use ofCD in pharmaceutical formulations.8,9

Unfortunately, unmodified cyclodextrins, par-ticularly b-CD, are relatively crystalline and havelimited aqueous solubility. In parenteral formula-tions this limited solubility is a very serious issueas renal concentration of the unmodified CD canlead to crystallization of the CD and necroticdamage.10 Fortunately, the solubility of unmodi-fied cyclodextrins in water can be significantlyincreased by the addition of a small number ofsubstituents to the hydroxyl groups of the anhy-droglucose monomers.11 It is important to notethat cyclodextrin derivatives that are similar instructure can provide significantly different solu-bilization, dissolution rates, and stabilization ofdrugs under otherwise similar conditions.

With regard to complexation of tamoxifen withcyclodextrins, Loftsson et al.,12 have described thesolubilization and stabilization of drugs, includingtamoxifen, using hydroxypropyl-b-CD (HPBCD)and other cyclodextrin derivatives. Loftsson foundthat the stabilizing effect in aqueous solutions washighly dependent upon the structure of thecyclodextrin derivative. Szejtli has described theuse of drug/cyclodextrin/organic acid multicompo-nent systems in pharmaceutical formulationsinvolving tamoxifen.13,14 Szejtli found that incor-

poration of hydroxy acids, for example, tartaricacid, in formulations in a specific molar ratiobased on cyclodextrin and drug, increased thesolubilization of tamoxifen base. Similarly, Lofts-son described the increase in complexation effi-ciency of a basic drug (tamoxifen) by addition ofvolatile acid (e.g., acetic acid) to anaqueousmediumof basic drug and cyclodextrin. The volatile acidwasthen removed resulting in formation of solidcyclodextrin complexes of the unionized drug.15

Fischer et al.16 investigated the utility of HPBCDand sulfobutyl-i-cyclodextrin in the solubilizationand stabilization of Z-4-hydroxytamoxifen, a meta-bolite of tamoxifen (Fig. 1). Fischer found that astable complex of 4-hydroxytamoxifenand cyclodex-trin could be obtained in pH 7 saline solution.

Recently we have described the preparation andcharacterization of highly water-soluble (>50%w/v) hydroxybutenyl-b-cyclodextrin (HBenBCD).17

In this initial account, we reported that HBenBCDwas very effective in solubilizing a broad spectrumof drugs including ibuprofen and glibenclamide.In this report, we describe our investigationsinto the ability ofHBenBCD to solubilize tamoxifenbase and tamoxifen citrate, the preparation andisolation of solid tamoxifen:HBenBCD complexes,novel formulations of tamoxifen and HBenBCD inorganic (PEG 400) liquid formulations, and thedissolution of drugs from different tamoxifen:HBenBCD formulations. Our goals were to demon-strate increased solubility and enhanced dissolu-tion profiles of tamoxifen due to complexationwith HBenBCD and the development of suitableformulations for use in preclinical studies.18

EXPERIMENTAL

General Methods

Hydroxybutenyl-b-cyclodextrin (HBenBCD, molarsubstitution¼ 4.7) was prepared according to

Figure 1. Structure of tamoxifen and the two major metabolites of tamoxifen,4-hydroxytamoxifen and desmethyltamoxifen.

TAMOXIFEN-HYDROXYBUTENYL CYCLODEXTRIN COMPLEXES 2247

DOI 10.1002/jps JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 95, NO. 10, OCTOBER 2006

previously described methods.17 The HBenBCDwas dried at ca. 5 mmHg at ambient temperaturefor ca. 14 h prior to use. The water content (KarlFisher analysis) after drying was typically 2.2%.Tamoxifen base and tamoxifen citrate wereobtained from Apin Chemicals and, with oneexception, used without further purification orprocessing. As noted below, we did attempt toobtain amorphous tamoxifen by freeze drying asolution of tamoxifen base dissolved in EtOH. Wewere unsuccessful as the freeze-dried tamoxifenhad virtually the same Tm and DH as the nativetamoxifen base. Although no attempt was made tocharacterize differences in morphology betweenthe two physical forms, we did note thatfreeze dried tamoxifen powder had a much lowerbulk density. Water was prefiltered through aMilli-QTM Water System (Millipore Corporation)and had very low total organic and pyrogencontent and low-ionic strength. Buffers wereprepared using Millipore water.

Modulated differential scanning calorimetry(DSC) curves were obtained using a Q1000 TAspectrometer. First scan DSC heating curves wereobtained by heating from 0 to 1608C at 58/minwithamodulation of 28 every 60 s. Prior to collecting thespectra, the instrument was calibrated versusindium-tin-water. Tamoxifen:HBenBCD com-plexes and physical mixtures were dried at 258Cat ca. 5 mm Hg for 5 days then taken fromthe vacuum oven and immediately sealed in ahermetic pan.

Solubility Studies

Solubility measurements were made using amodified version of the traditional shaking flaskmethod:19–21 To each well of a 2 mL 96-wellpolypropylene mixing plate was added excessdrug (ca. 5–10 mg). To each well of the preloaded96-well mixing plate was added 300–500 mL ofwater or buffer (blanks with no CD) or theappropriate CD solution. Each determinationwas made in triplicate. The blanks were used todetermine the intrinsic solubility (So) of drug inthat solution, and the wells containing the CDsolutions were used to determine the solubility ofdrug due to CD (St). After addition of the stocksolutions, the plate was sealed using aluminumfoil with a nonvolatile adhesive on one surface.The plate was placed on a rotary shaking plate(Heidolph Titramax 1000) and the plate wasshaken at 800–1200 rpm at 23� 28C for 48–72 h. Preliminary experiments showed that less

that 24 h was required to reach equilibrium.Longer mixing times were used both for conve-nience and to insure that we were well above thetime required to reach equilibrium. During themixing period, the plate was inspected to ensurethat each well contained undissolved, excessdrug. Additional drug was added if necessary.

Following the mixing period, the solutions ineach well were transferred to the correspondingwells of a 96-well 2 mL multiscreen filter plateusing a multichannel pipette. The bottom of eachwell was a hydrophilic membrane. The filter platewas placed on top of a vacuum manifold and thesolutions were filtered at ca. 20 mm Hg intothe corresponding wells of a 2 mL storage plate.The duration of the filtration period was typicallyno more than 60 s. The storage plate was thensealed with a silicon mat and samples wereremoved for analysis as appropriate.

The drug content in each well was determinedby UV spectroscopy using a SpectraMax Plus 384Molecular Devices multiwell plate reader. Typi-cally, 10–20 mL of drug solutionwas transferred tothe corresponding well of a 96-well measurementplate (UV-STAR plates from Greiner with aspectral range of 190–400 nm) and diluted with1:1 water:ethanol so that the absorbance was inthe linear response range. Absorbance was con-verted to drug concentrationusing the appropriateabsorptivity for the drug. The drug concentrationswere then exported to the appropriate softwarepackage for final analysis. Additionally, the finalpH of each well was measured to insure that thepH had not drifted significantly due to lack ofbuffering capacity.

The method described for solubility analysiswas developed as a high-throughput method foranalyzing many samples on a daily basis.Although HPLC can be used to determine drugconcentration, we have found that UV spectro-scopy is a much faster and cheaper method ofanalysis and can be used with >90% of the drugswehave examined.Typically,HPLC is onlyneededwhen the drug has both very poor water solubilityand low-molar absorptivity or when there arequestions about drug stability under the testconditions. In this study, there was no evidence(UV, NMR, LC/MS/MS) of tamoxifen degradationduring complex preparation, dissolution studies,or in animal studies.18 The maximum absorptionfor HBenBCD is less than 200 nm while themaximum absorption or assay wavelength formany drugs are typically 260–290 nm. In thepresent case, tamoxifen absorptivity at 280 nm is

2248 BUCHANAN ET AL.

JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 95, NO. 10, OCTOBER 2006 DOI 10.1002/jps

32.5 abs/cm/(g/L) versus 0.003 abs/cm/(g/L) forHBenBCD. Hence, at the normal drug concentra-tions we obtain in these experiments and whenthe drug has a reasonable molar absorptivity,absorbance contributions by HBenBCD can beneglected. When absorbance contributions byHBenBCD become significant, a correction canbe applied by collecting a background spectrumof HBenBCD (no drug) in the measurementsolvent.19–21

Typical Methods for the Preparationof Tamoxifen:HBenBCD ComplexesOr Physical Mixtures

The following are example procedures for prepar-ing complexes or mixtures of tamoxifen withHBenBCD. These procedures have not beenoptimized.

Physical Solid Mixtures

Appropriate amounts of tamoxifen base or citrateand HBenBCD were loaded into the holder of afreezer mill (Spex Industries, Inc., Metuchen, NJ).After cooling to liquid nitrogen temperature, thesamples were milled for 12 min at maximum millspeed. The samples were then transferred to glassjars and dried at 27 mm Hg for 1 h prior to use.

Solid Tamoxifen Base:HBenBCD Complex

A 20 wt% HBenBCD solution was prepared bydissolving 6 g of HBenBCD in phosphate buffer(0.05 M, pH 3). Tamoxifen base (901 mg) wasadded to this solution. The mixture was brieflyvortexed and placed in a preheated (508C) ultra-sonic bath for 30 min. The sample was then placedon a rotary shaker and the temperature of thesample was maintained at 308C. After ca. 66 h,solids were still present. The sample was removedfrom the shaker and the pH of the solution wasadjusted from 6.5 to 3.1 before returning thesample to the shaker. After an additional 24 h,solids were still evident and the solution pH was5.6. The sample was filtered through a 0.45 mmfilter and the clear solution was freeze driedwhich provided 6.49 g of a white powder (94%yield). Using UV spectroscopy, the sample wasdetermined to contain 7.2 wt% of tamoxifen base.

Solid Tamoxifen Citrate:HBenBCD Complex

A 20 wt% HBenBCD solution was prepared bydissolving 6 g of HBenBCD in phosphate buffer

(0.05 M, pH 3). To this solution was added 900 mgof tamoxifen citrate. The mixture was brieflyvortexed and placed in a preheated (508C) ultra-sonic bath for 30 min. The sample was then placedon a rotary shaker and the temperature of thesample was maintained at 308C. After ca. 66 h,the solution pHwas 3.7 and no solids were evidentby the naked eye (use of a laser pointer revealedthe presence of small particles in the solution).The sample was filtered through a 0.45 mm filterand the clear solution was freeze dried to give6.8 g of a white powder (>98% yield). Using UVspectroscopy, the sample was determined tocontain 11.6 wt% of tamoxifen citrate.

Tamoxifen Base:HBenBCD PEG 400/PG Solution

To a 14 g solution of PEG 400 (39.1 wt%),propylene glycol (56.5 wt%), and water (4.4 wt%)was added 6 g of HBenBCD. The sample wasvortexed and then mixed until a clear solutionwas obtained. Tamoxifen base (450mg) was addedand the sample was placed on a rotary shaker forca. 96 h. Excess drug was removed by filtration at608C through a 45 mm filter. The sample wasdetermined to contain 0.62 wt% of tamoxifen baseby UV spectroscopy.

Tamoxifen Citrate:HBenBCD PEG 400/PG Solution

Tamoxifen citrate (751 mg) was suspended in 14 gof a 42.9 wt% solution of HBenBCD in PEG 400(39.1 wt%), propylene glycol (56.5 wt%), andwater (4.4 wt%). The sample was placed on arotary shaker for ca. 96 h. Excess drug wasremoved by filtration at 608C through a 45 mmfilter. The sample was determined to contain2.46 wt% of tamoxifen citrate by UV spectroscopy.

Tamoxifen Base:HBenBCD Preparedby Comixing of Solutions

Thus, 6 g of HBenBCD (6 g) was dissolved in 9 mLof water. In a separate vessel, 422mg of tamoxifenbase (7 wt%) was dissolved in 5 mL of absoluteEtOH. The solution of tamoxifen base in EtOHwas added slowly to the HBenBCD aqueoussolution while stirring. The solution remainedclear and nothing was observed to crystallize orprecipitate when the solution was stored atambient temperature overnight. The water/EtOHwas removed in vacuo providing 6.5 g of a whitesolid. Thermal analysis of this material by DSCrevealed the complete absence of a meltingtransition for tamoxifen in the first heating scan.



Dissolution Studies

Dissolution studies were conducted at 378Caccording to USP 28-NF 23 711 (United StatesPharmacopeia, 2004). For these studies, the solidor PEG 400 liquid fill formulations were filled intohard shell Torpac lock ring gelatin capsules (Size#1 or #0). The apparatus used for the testing was aVarian VK 7000 dissolution tester. The pH valuesused for the experiments were selected on thebasis of the pH normally found in the stomach orupper and lower GI tracts of humans.

RESULTS AND DISCUSSION

Solubility Studies

The first step in our investigation into the abilityof HBenBCD to solubilize tamoxifen was tomeasure the solubility of tamoxifen base andtamoxifen citrate in water and a series of relevantbuffers; one set of these media did not containHBenBCD and a second set contained HBenBCD(10 wt%). We typically conduct this type of initialscreening using 5–10 wt% HBenBCD as we havefound that increases in drug solubility are usuallylinear in this range while at higher concentra-tions, departure from linearity is often observed(vide infra).22 Furthermore, we routinely deter-mine the appropriate buffer strength to be used inthis type of screening.

Use of buffers in drug formulations is often abalance between having sufficient buffering capa-city and salting out effects due to the ionic strengthof the media. In this context, we examined thesolubility of tamoxifen base in the presence of5 wt% HBenBCD in pH 3 phosphate buffers. In0.025 and 0.05 M phosphate buffer, the amount oftamoxifen base solubilized by HBenBCD wasessentially identical (134 mg/g HBenBCD) but in0.10 M phosphate buffer, the amount of tamoxifenbase solubilized was less (117 mg/g HBenBCD).For this reason, 0.05 M buffer was used in allsubsequent studies in the report.

Figure 2 shows the results of this investigationfor tamoxifen base. The intrinsic solubility (So) oftamoxifen base in water was quite low (24 mg/mL)and the increase in solubility of tamoxifenbasedueto complexation with HBenBCD (St) was onlymarginal (St/So¼ 5). At pH 1.2, HBenBCD sig-nificantly increased tamoxifen base solubility(So¼ 37 mg/mL,St/So¼ 241). At pH3, the solubilityof tamoxifen was not only dependent upon the pHof the medium but, was also apparently upon the

type of buffer. At pH 3, the lowest solubility wasobserved with the phosphate buffer (St/So¼ 49)while the highest solubility was observed with thetartrate buffer (St/So¼ 175). Aswould be expected,as the pH of the medium (4.9 and 7.4) appro-ached the pKa of tamoxifen the solubility oftamoxifen base in the presence of HBenBCDwas greatly diminished.5 Figure 3 shows theequivalent solubility data for tamoxifen citrate.The So (So¼ 329 mg/mL) of the citrate salt oftamoxifen in water was much higher than that of

Figure 2. Tamoxifen base solubility in water andbuffers without cyclodextrin and with HBenBCD(10 wt%).

Figure 3. Tamoxifen citrate solubility in water andbuffers without cyclodextrin and with HBenBCD(10 wt%).



the free base but was somewhat diminished in thebuffer solutions due to increased ionic strength ofthe media.5 In the presence of HBenBCD, thesolubility of tamoxifen citrate was significantlyincreased in allmedia;St/So ranged from54 to 293.

We next examined the solubility of tamoxifenbase and tamoxifen citrate in different buffersystems at different concentrations of HBenBCD.Figure 4 shows the results obtained with tamox-ifen base:HBenBCD in 0.05 M citrate and phos-phate buffers. In the case of the citrate buffer,tamoxifen base solubility increases linearly withHBenBCD concentration until the HBenBCDconcentration reaches ca. 25 wt% at which pointthe apparent drug solubility diminishes slightly.The equilibrium binding constant for the linearpart of the curve was 681/M.23,24 In the case of thepH 3 phosphate buffer, tamoxifen base concentra-tion begins to flatten out much earlier at ca. 5 wt%HBenBCD. This behavior is indicative of forma-tion of tamoxifen base:HBenBCD inclusion com-plexes with finite solubility (Bs solubility) whichmay inpart rationalize thedifferences between thepH 3 buffer systems noted in Figure 2.22 Incontrast, the equilibrium solubility curves fortamoxifen citrate (Fig. 5) in water and 0.05 Mcitrate and phosphate buffers were essentiallyidentical andwere indicative ofAn phase solubilitybehavior. In pH 3 phosphate buffer, the equili-brium binding constant for tamoxifen citrate:H-BenBCD in the linear part of the curve was 305/M.

Dissolution Studies

Figure 6 shows the dissolution of tamoxifen baseand tamoxifen citrate (no HBenBCD) at pH 1.2

and 4.5. The initial dissolution of tamoxifen baseat pH 1.2 was relatively slow (16% at 15 min) anda maximum dissolution of ca. 45% was reached inca. 3 h. At pH 4.5, the dissolution of tamoxifenbase was minimal with only 5 wt% being dissolvedafter 6 h. The dissolution profiles for tamoxifencitrate were relatively insensitive to pH. At pH 1.2and 4.5, the dissolution profiles overlapped andthe maximum dissolution reached was ca. 5%.

We also studied the dissolution of dry milledphysical mixtures of tamoxifen base andHBenBCD in which the molar ratios of tamoxifenbase and HBenBCD were varied (Fig. 7). In allcases, the initial dissolution of tamoxifen base wasrapid, reaching a plateau after ca. 20–50 min. Ascan be seen, the total amount of tamoxifen basedissolved increased as the molar HBenBCD:ta-moxifen base ratio increased. These observations

Figure 4. Tamoxifen base solubility in HBenBCDbuffer solutions with varying HBenBCD concentration.

Figure 5. Tamoxifen citrate solubility in HBenBCDbuffer solutions with varying HBenBCD concentration.

Figure 6. Dissolution of tamoxifen base (TB) andtamoxifen citrate (TC) at pH 1.2 and 4.5 (no HBenBCD)at 378C from gelatin capsules.



indicate that complex formation can be rapid andthat it is not necessary to preform a complexwith HBenBCD to achieve increased tamoxifendissolution.

In order to compare the dissolution of tamoxifenbase:HBenBCD and tamoxifen citrate:HBenBCDcomplexes, these complexes were prepared eachcontaining equivalent amounts of tamoxifen onthe basis of tamoxifen base (80 mg/mL, 1:3 molarratio of tamoxifen:HBenBCD). The dissolution ofthe complexes was then evaluated at pH 1.2, 6.0,and 7.5 (Fig. 8). At pH 1.2, 100% dissolution oftamoxifen base was achieved in ca. 30 min versus85% for tamoxifen citrate. At pH 6.0, the dissolu-tion profiles of tamoxifen base and citrate wereequivalent reaching ca. 92% dissolution within

30 min. At pH 1.2 and 6.0, precipitation oftamoxifen was not observed during the time scaleof the experiment. At pH 7.5, the dissolutionprofiles for tamoxifen base and citrate weresimilar. The dissolution rate of tamoxifen citratewas only slightly faster than that of tamoxifenbase; the total amount of tamoxifen dissolvedwas essentially equivalent. In both cases, thepercentage of drug dissolved reached a maximumof 26% then decreased as the drugs precipitated.Precipitation of a basic drug can often be expectedas the pH of themedium approached its pKa.Whatis noteworthy is that, in the presence ofHBenBCD,the dissolution profile of tamoxifen base wasequivalent to the salt.

As noted above, acceptable dissolution of tamox-ifen can be achieved simply by preparing physicalmixtures. However, if one compares the resultspresented inFigures 7and8, it canbe seen that thepreformed tamoxifen:HBenBCD complexes pro-vided more rapid dissolution rates with increaseddissolution of tamoxifen. In the case of the 1:3.5tamoxifen base:HBenBCD physical mixture, thedissolution of tamoxifen reached a plateau atca. 52% after 50 min. In the case of the 1:3tamoxifen base:HBenBCD complex, the dissolu-tion of tamoxifen leveled out at ca. 92% after30 min. This difference in dissolution profilesmay in part be due to the difficulty in achievinga completely amorphous physical mixture oftamoxifen-HBenBCD by milling. Analysis bymodulated DSC of the physical mixtures revealedthe presence of a melting endotherm correspond-ing to that of tamoxifen; DH for the meltingendotherm increased as the amount of HBenBCDdecreased. In contrast, DSC analysis of thepreformed complex revealed the complete absenceof a melting endotherm (Fig. 9).

In this context, we prepared tamoxifen base:-HBenBCD mixtures by first comixing solutions ofHBenBCD dissolved in H2O and tamoxifen basedissolved in EtOH followed by concentrationin vacuo. As a control, part of the tamoxifenbase:HBenBCD solid from comixing of the solventmixtures was dissolved in water then isolated byfreeze drying. The dissolution profiles for thesetwo samples are shown in Figure 10. The initialdissolution of both samples was quite rapidwith ca. 80% of the samples being dissolved after20 min. The complex prepared by comixingreached a plateau at ca. 85% dissolution whilethe control sample leveled out at ca. 90%. Collec-tively, the results presented in Figures 7–10indicate that tamoxifen:HBenBCD complexes

Figure 7. Dissolution of tamoxifen at 378C fromgelatin capsules filled with tamoxifen base (TB) andtamoxifen base:HBenBCD physical mixtures (pH 6.0).

Figure 8. Dissolution of tamoxifen at 378C fromgelatin capsules filled with solid tamoxifen base:HBenBCD and tamoxifen citrate:HBenBCD complexesat pH 1.2, 6.0, and 7.45.



with favorable dissolution profiles can be preparedby different techniques provided that the drug inthe final mixture is amorphous.

The observations with comixing of solutionsencouraged us to explore the feasibility of prepar-ing drug:HBenBCD mixtures in water-solubleorganic solvents that may be suitable for liquid fillcapsule formulations. With HBenBCD, thisseemed plausible as we had found that HBenBCDwas soluble (ca. 50 wt%) in solvents commonlyused in liquid fill formulations. We had noexpectation that the drug and HBenBCD wouldform complexes in these solvents but we antici-pated that during dissolution ormixing in aqueous

media, complexation could be faster than precipi-tation of drug.25 The dissolution profiles obtainedfor capsules filled with solid tamoxifen:HBenBCDcomplex and capsules filled with PEG 400 solu-tions of tamoxifen-HBenBCD are compared inFigure 11. For comparison purposes, we have alsoincluded dissolution profiles for capsules filledwith tamoxifen dissolved in PEG400 and capsulesfilled with tamoxifen powder obtained from freezedrying a solution of tamoxifen dissolved in EtOH.As can be seen, the dissolution profiles from thetamoxifen-HBenBCD solid filled capsules and theliquid fill capsules were essentially identical. Inboth cases, dissolution was rapid and no precipita-tion of tamoxifen was observed over the course ofthe experiment. In the case of the tamoxifensolution (no HBenBCD), upon rupture of thecapsule 30%–40% of the drug remained in solu-tion; the remaining drug was observed to pre-cipitate virtually immediately. In the case ofcapsules filled with freeze-dried tamoxifen (noHBenBCD), the tamoxifen slowly dissolved overtime. The rate and extent of tamoxifen dissolutionwasmuch less than that observedwhenHBenBCDwas present. However, comparison of the dissolu-tion profile of the freeze-dried tamoxifen to thatof the native tamoxifen (Fig. 6) shows thatfreeze drying of the tamoxifen improved the rateand extent of dissolution relative to the native

Figure 9. DSC spectra for a preformed tamoxifenbase:HBenBCD complex and for physical mixtures oftamoxifen base and HBenBCD. The melting point ofnative tamoxifen base is 1008C.

Figure 10. Dissolution of tamoxifen at 378C, pH 6from gelatin capsules filled with solid tamoxifen base:-HBenBCD complexes prepared by (a) comixing oftamoxifen base/ethanol solution with HBenBCD/watersolution and (b) prepared by mixing in water.

Figure 11. Dissolution of tamoxifen at 378C fromgelatin capsules filled with solid tamoxifen:HBenBCDcomplex, tamoxifen-HBenBCD dissolved in PEG400,tamoxifen dissolved in PEG400 (no HBenBCD), ortamoxifen (no HBenBCD) obtained by freeze drying. Allexperiments were conducted in pH 6 buffer excepttamoxifen (noHBenBCD)whichwas conductedatpH4.5.



tamoxifen. These findings are significant in thatliquid fill formulations could provide additionalformulation flexibility for drug:HBenBCD com-plexes and in some cases will offer a means ofhigher drug loading relative to solid oral formula-tions.

CONCLUSIONS

In this account, we have shown that complexationof tamoxifen base or citrate with HBenBCD was avery effective means for increasing the solubilityof tamoxifen in aqueous media. The pH of themedium and the choice of buffer significantlyimpacted the amount of drug that could besolubilized. Solid and liquid formulations oftamoxifen:HBenBCD inclusion complexes or phy-sical mixtures were prepared by a number ofdifferent methods. Dissolution studies using thesecomplexes or physical mixtures revealed that thedissolution profile of tamoxifen could be signifi-cantly improved relative to the parent drug.

Collectively, these results indicate that byformulating tamoxifen base with HBenBCD, itis possible to obtain solubility and dissolutionprofiles essentially identical to that of tamoxifencitrate. These formulations potentially could elim-inate the need to convert this basic drug to asalt in order obtain improved solubility andefficacy. The ability to formulate tamoxifen basewith HBenBCD in liquid fill capsule formula-tions offers additional flexibility in oral drugformulations.

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17. Buchanan CM, Alderson SR, Cleven CD, DixonDW, Ivanyi R, Lambert JL, LowmanDW, OffermanRJ, Szejtli J, Szente L. 2002. Synthesis andcharacterization of water-soluble hydroxybutenylcyclodextrins. Carbohydr Res 327:493–507.

18. Buchanan CM, Buchanan NL, Edgar KJ, Little JL,Malcolm MO, Ruble KM, Wacher VJ, Wempe MF.Preclinical studies involving intravenous and oraladministration of these complexes to Sprague–Dawley rats have shown that these complexesprovided a 10–14� increase in the oral bioavail-ability of tamoxifen. J Pharma Sci (in press).

19. Pan L, Ho Q, Tsutsui K, Takahashi L. 2001.Comparison of chromatographic and spectroscopicmethods used to rank compounds for aqueoussolubility. J Pharma Sci 90:521–529.

20. Chen TM, Shen H, Zhu C. 2002. Evaluationof a method for high throughput solubility



determination using a multi-wavelength UV platereader. Comb Chem High Throughput Screen5:575–581.

21. Buchanan CM, Buchanan NL. Unpublishedresults.

22. Higuchi T, Conners KA. 1965. Phase-solubilitytechniques. Adv Anal Chem Instr 4:117–212.

23. Thompson DO. 1997. Cyclodextrins-enabling exci-pients: Their present and future use in pharma-ceuticals. Crit Rev Ther Drug Carrier Syst 14:1–104.

24. The apparent binding constant is obtained byplotting the molar concentration of drug versus

molar concentration of complexing agent. A fit tothe initial linear portion of the curve allows one tocalculate the apparent binding constant; K¼ slope/So(1� slope). Although this equation is commonlyused, one should note that the intrinsic solubilities(So) for poorly water-soluble drugs are typicallyvery small and difficult to measure accurately.Small variations in So will significantly impact thereported binding constant.

25. The details of cyclodextrin liquid fill formulationsexceed the scope of this paper and will be presentedelsewhere. Buchanan CM, Buchanan NL, LambertJL, Ramsey MG. Unpublished results.



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Solubilization and dissolution of tamoxifen-hydroxybutenyl cyclodextrin complexes

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