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Pharmaceuticalapplicationsofcyclodextrins.1.Drugsolubilizationandstabilization.JPharmSci

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REVIEW ARTICLE

Pharmaceutical Applications of Cyclodextrins. 1. Drug Solubilization andStabilization

THORSTEINN LOFTSSON*X AND MARCUS E. BREWSTER†

Received December 29, 1995, from the *Department of Pharmacy, University of Iceland, P.O. Box 7210, IS-127 Reykjavik, Iceland, and†Pharmos Corporation, Two Innovation Drive, Alachua, FL 32615 . Final revised manuscript received March 1, 1996 . Acceptedfor publication March 19, 1996X.

Abstract 0 Cyclodextrins are cyclic oligosaccharides which have recentlybeen recognized as useful pharmaceutical excipients. The molecularstructure of these glucose derivatives, which approximates a truncatedcone or torus, generates a hydrophilic exterior surface and a nonpolarcavity interior. As such, cyclodextrins can interact with appropriately sizedmolecules to result in the formation of inclusion complexes. Thesenoncovalent complexes offer a variety of physicochemical advantagesover the unmanipulated drugs including the possibility for increased watersolubility and solution stability. Further, chemical modification to the parentcyclodextrin can result in an increase in the extent of drug complexationand interaction. In this short review, the effects of substitution on variouscyclodextrin properties and the forces involved in the drug−cyclodextrincomplex formation are discussed. Some general observations are madepredicting drug solubilization by cyclodextrins. In addition, methods whichare useful in the optimization of complexation efficacy are reviewed. Finally,the stabilizing/destabilizing effects of cyclodextrins on chemically labiledrugs are evaluated.

IntroductionAlthough cyclodextrins are frequently regarded as a new

group of pharmaceutical excipients, they have been knownfor over 100 years.1 The foundations of cyclodextrin chemistrywere laid down in the first part of this century2,3 and the firstpatent on cyclodextrins and their complexes was registeredin 1953.4 However, until 1970 only small amounts of cyclo-dextrins could be produced and high production costs pre-vented their widespread usage in pharmaceutical formula-tions. Recent biotechnological advancements have resultedin dramatic improvements in cyclodextrin production, whichhas lowered their production costs. This has led to theavailability of highly purified cyclodextrins and cyclodextrinderivatives which are well suited as pharmaceutical excipi-

ents. These carbohydrates are mainly used to increase theaqueous solubility, stability, and bioavailability of drugs, butthey can also, for example, be used to convert liquid drugsinto microcrystalline powders, prevent drug-drug or drug-additive interactions, reduce gastrointestinal or ocular irrita-tion, and reduce or eliminate unpleasant taste and smell.The following is a short review of the effects of cyclodextrins

on the solubility and stability of drugs in aqueous solutionswith emphasis on the more recent developments. For furtherinformation on cyclodextrins and their physicochemical prop-erties the reader is referred to several excellent books andreviews published in recent years.5-13

Structure and Physicochemical PropertiesCyclodextrins are cyclic (R-1,4)-linked oligosaccharides of

R-D-glucopyranose containing a relatively hydrophobic centralcavity and hydrophilic outer surface. Owing to lack of freerotation about the bonds connecting the glucopyranose units,the cyclodextrins are not perfectly cylindrical molecules butare toroidal or cone shaped. Based on this architecture, theprimary hydroxyl groups are located on the narrow side ofthe torus while the secondary hydroxyl groups are located onthe wider edge (Figure 1). The most common cyclodextrinsare R-cyclodextrin, â-cyclodextrin, and γ-cyclodextrin, whichconsist of six, seven, and eight glucopyranose units, respec-tively. While it is thought that, due to steric factors, cyclo-dextrins having fewer than six glucopyranose units cannotexist, cyclodextrins containing nine, ten, eleven, twelve, andthirteen glucopyranose units, which are designated δ-, ε-, ú-,η-, and θ-cyclodextrin, respectively, have been reported.14,15Of these large-ring cyclodextrins only δ-cyclodextrin has beenwell characterized.16,17 Chemical and physical properties ofthe four most common cyclodextrins are given in Table 1. Themelting points of R-, â-, and γ-cyclodextrin are between 240and 265 °C, consistent with their stable crystal latticestructure.18The parent cyclodextrins, in particular â-cyclodextrin, have

limited aqueous solubility, and their complex formation withX Abstract published in Advance ACS Abstracts, May 1, 1996.

October 1996Volume 85, Number 10

© 1996, American Chemical Society and S0022-3549(95)00534-X CCC: $12.00 Journal of Pharmaceutical Sciences / 1017American Pharmaceutical Association Vol. 85, No. 10, October 1996

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lipophilic drugs, and other compounds with limited aqueoussolubility, frequently gives rise to B-type phase-solubilitydiagrams as defined by Higuchi.19 That is, addition of theseunmodified cyclodextrins to aqueous drug solutions or drugsuspensions often results in precipitation of solid drug-cyclodextrin complexes. The aqueous solubility of the parentcyclodextrins is much lower than that of comparable acyclicsaccharides, and this could partly be due to relatively strongbinding of the cyclodextrin molecules in the crystal state (i.e.,relatively high crystal lattice energy). In addition, â- andδ-cyclodextrin form intramolecular hydrogen bonds betweensecondary OH groups, which detracts from hydrogen bondformation with surrounding water molecules, resulting in lessnegative heats of hydration.5,17 Thus, intramolecular hydro-gen bonding can result in relatively unfavorable enthalpiesof solution and low aqueous solubilities. Substitution of anyof the hydrogen bond forming hydroxyl groups, even byhydrophobic moieties such as methoxy and ethoxy functions,will result in a dramatic increase in water solubility.5 Forexample, the aqueous solubility of â-cyclodextrin is only 1.85%(w/v) at room temperature but increases with increasingdegree of methylation. The highest solubility is obtainedwhen two-thirds of the hydroxyl groups (i.e., 14 of 21) aremethylated, but then falls upon more complete alkylation.That is, the permethylated derivative has a solubility that islower than that of, e.g., heptakis(2,6-O-dimethyl)-â-cyclodex-trin but that is still considerably higher than that of unsub-stituted â-cyclodextrin.7 Other common cyclodextrin deriva-tives are formed by other types of alkylation or hydroxy-alkylation of the hydroxyl groups.5,20 The main reason for thesolubility enhancement in these derivatives is that chemicalmanipulation frequently transforms the crystalline cyclodex-trins into amorphous mixtures of isomeric derivatives. Forexample, (2-hydroxypropyl)-â-cyclodextrin is obtained by treat-ing a base-solubilized solution of â-cyclodextrin with propyleneoxide, resulting in an isomeric system that has an aqueoussolubility well in excess of 60% (w/v).21 The number of isomersgenerated based on random substitution is very large. Sta-tistically, for example, there are about 130 000 possibleheptakis(2-O-(hydroxypropyl))-â-cyclodextrin derivatives, andgiven that introduction of the 2-hydroxypropyl function also

introduced an optically active center, the number of totalisomers, i.e., geometrical and optical, is even much greater.In reality, the chemical alkylation of cyclodextrins is not

totally random, based on relative reactivities of the hydroxyfunctions in the molecule. The secondary OH groups on thecyclodextrin molecule (i.e., OH-2 and OH-3 on the glucopy-ranose units) are somewhat more acidic than the primary OHgroup (i.e., OH-6). Thus, alkylation of OH-6, the leaststerically crowded functionality, is favored in strong basicsolutions while alkylation of OH-2, the most acidic of thehydroxyl groups but also the most hindered, is favored in aweak basic solution.22 Thus, some degree of regioselectivityis possible. Both the molar substitution, i.e., the averagenumber of alkyl or hydroxyalkyl groups that have been reactedwith one glucopyranose unit, and the location of the alkyl orhydroxyalkyl groups on the cyclodextrin molecule will affectthe physicochemical properties of the derivatives includingtheir ability to form drug complexes.23 However, theoreticalstudies have shown that the alkylation and hydroxyalkylationof the cyclodextrins should not introduce significant sterichindrance.24 Some of the commercially available cyclodextrinsare listed in Table 2.

Cyclodextrin Complexes

The central cavity of the cyclodextrin molecule is lined withskeletal carbons and ethereal oxygens of the glucose residues.It is therefore lipophilic. The polarity of the cavity has beenestimated to be similar to that of aqueous ethanolic solution.5It provides a lipophilic microenvironment into which suitablysized drug molecules may enter and be included. No covalentbonds are formed or broken during drug-cyclodextrin complexformation, and in aqueous solutions, the complexes are readilydissociated. Free drug molecules are in equilibrium with themolecules bound within the cyclodextrin cavity. Measure-ments of stability or equilibrium constants (Kc) or the dis-sociation constants (Kd) of the drug-cyclodextrin complexesare important since this is an index of changes in physico-chemical properties of a compound upon inclusion. Mostmethods for determining the K values are based on titratingchanges in the physicochemical properties of the guestmolecule, i.e., the drug molecule, with the cyclodextrin andthen analyzing the concentration dependencies. Additiveproperties that can be titrated in this way to provide informa-tion on the K values include25 aqueous solubility,19,26-28

chemical reactivity,10,29,30 molar absorptivity and other opticalproperties (CD, ORD),31-34 phase solubility measurements,35NMR chemical shifts,23,36 pH-metric methods,37 calorimetrictitration,38 freezing point depression,39 and LC chromato-

a b

Figure 1s(a) The chemical structure and (b) the toroidal shape of the â-cyclodextrin molecule.

Table 1sSome Characteristics of r-, â-, γ-, and δ-Cyclodextrin a

R â γ δ

No. of glucopyranose units 6 7 8 9Molecular weight 972 1135 1297 1459Central cavity diameter (Å) 4.7−5.3 6.0−6.5 7.5−8.3 10.3−11.2Water solubility at 25 °C (g/100 mL) 14.5 1.85 23.2 8.19

a Modified from refs 5 and 17.

1018 / Journal of Pharmaceutical SciencesVol. 85, No. 10, October 1996

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graphic retention times.40 While it is possible to use bothguest or host changes to generate equilibrium constants, guestproperties are usually most easily assessed. Connors hasevaluated the population characteristics of cyclodextrin com-plex stabilities in aqueous solution.41The thermodynamic parameters, i.e., the standard free

energy change (∆G), the standard enthalpy change (∆H), andthe standard entropy change (∆S), can be obtained from thetemperature dependence of the stability constant of thecyclodextrin complex.42 The thermodynamic parameters forseveral series of drugs and other compounds have beendetermined and analyzed.43-45 The thermodynamic param-eters of several other drugs are listed in Table 3. The complexformation is almost always associated with a relatively largenegative ∆H and a ∆S that can be either positive or negative.Also, complex formation is largely independent of the chemicalproperties of the guest (i.e., drug) molecules. The associationof binding constants with substrate polarizability suggeststhat van der Waals forces are important in complex forma-tion.50 Hydrophobic interactions are associated with a slightlypositive ∆H and a large positive ∆S; therefore, classicalhydrophobic interactions are entropy driven, suggesting thatthey are not involved with cyclodextrin complexation since,as indicated, these are enthalpically driven processes. Fur-thermore, for a series of guests there tends to be a linearrelationship between enthalpy and entropy, with increasing

enthalpy related to less negative entropy values.43-45,48 Thiseffect, termed compensation, is often correlated with wateracting as a driving force in complex formation. The maindriving force for complex formation could, therefore, be therelease of enthalpy-rich water from the cyclodextrin cavity.47The water molecules located inside the cavity cannot satisfytheir hydrogen-bonding potentials; therefore, they are ofhigher enthalpy.51 The energy of the system is lowered whenthese enthalpy-rich water molecules are replaced by suitableguest molecules which are less polar than water. Othermechanisms that are thought to be involved with complexformation have been identified in the case of R-cyclodextrin.In this instance, release of ring strain is thought to be involvedwith the driving force for compound-cyclodextrin interaction.Hydrated R-cyclodextrin is associated with an internal hy-drogen bond to an included water molecule which perturbsthe cyclic structure of the macrocycle. Elimination of theincluded water and the associated hydrogen bond is relatedto a significant release of steric strain decreasing the systementhalpy.52 In addition, “nonclassical hydrophobic effects”have been invoked to explain complexation. These nonclas-sical hydrophobic effects are a composite force in which theclassic hydrophobic effects (characterized by large positive ∆S)and van der Waals effects (characterized by negative ∆H andnegative ∆S) are operating in the same system. Usingadamantanecarboxylates as probes, R-, â-, and γ-cyclodextrinswere examined.53 In the case of R-cyclodextrin, experimentaldata indicated small changes in ∆H and ∆S consistent withlittle interaction between the bulky probe and the small cavity.In the case of â-cyclodextrin, a deep and snug-fitting complexwas formed leading to a large negative ∆H and a near zero∆S. Finally, complexation with γ-cyclodextrin demonstratednear zero ∆H values and large positive ∆S values consistentwith a classical hydrophobic interaction. Evidently, the cavitysize of γ-cyclodextrin was too large to provide for a significant

Table 2sSome Currently Available Cyclodextrins Obtained bySubstitution of the OH Groups Located on the Edge of the CyclodextrinRing a

Cyclodextrin Derivatives

R â γ

Alkylated:Methyl Methyl Methyl

EthylButyl Butyl Butyl

Pentyl

Hydroxylalkylated:Hydroxyethyl Hydroxyethyl

2-Hydroxypropyl 2-Hydroxypropyl 2-Hydroxypropyl2-Hydroxybutyl

Esterified:Acetyl Acetyl Acetyl

PropionylButyryl

Succinyl Succinyl SuccinylBenzoylPalmitylToluenesulfonyl

Esterified and Alkylated:Acetyl methylAcetyl butyl

Branched:Glucosyl Glucosyl GlucosylMaltosyl Maltosyl Maltosyl

Ionic:Carboxymethyl ether Carboxymethyl ether Carboxymethyl ether

Carboxymethyl ethylPhosphate ester Phosphate ester Phosphate ester

3-Trimethylammonium-2-hydroxypropyl ether

Sulfobutyl ether

Polymerized:Simple polymers Simple polymers Simple polymersCarboxymethyl Carboxymethyl Carboxymethyl

a Since both the number of substitutes and their location will affect thephysicochemical properties of the cyclodextrin molecules, such as their aqueoussolubility and complexing abilities, each derivative listed should be regarded as agroup of closely related cyclodextrin derivatives.

Table 3sStandard Enthalpy Change ( ∆H) and Standard Entropy Change(∆S) for Several Drug −Cyclodextrin Complexes

Cyclodextrina Drug pH ∆H (kJ/mol) ∆S (J/(mol K)) Ref

HP-R-CD Hydrocortisone −32 −70 49â-CD Phenytoin, un-ionized 7 −38 −67 46

Phenytoin, ionized 7 −21 −21 46â-CD Naproxen −13 18 31â-CD Adenine arabinoside 7 −28 −64 32â-CD Adenosine 7 −21 −53 32â-CD Ibuprofen (pKa 5.2) 2 −29 15 47

4 −32 4 475 −29 3 476 −17 34 47

â-CD Diazepam (pKa 3.3) 2 −0.2 70 473 −3.3 69 474 −17 22 476 −18 19 47

â-CD Hydrochlorothiazide 5 −40 62 47(pKa 8.8 and 10.4) 8 −39 59 47

9 −42 70 47HP-â-CD Acetylsalicylic acid 1 −68 −166 48HP-â-CD Acetazolamide −18 −26 49HP-â-CD 17â-Estradiol −71 −151 49HP-â-CD Hydrocortisone −20 −6 49HP-â-CD Methyl acetylsalicylate 1 −55 −127 48HP-â-CD Methyl salicylate 1 −63 −144 48M/DM-â-CD Acetylsalicylic acid 1 −57 −134 48M/DM-â-CD Methyl acetylsalicylate 1 −20 −28 48HP-γ-CD Acetylsalicylic acid 1 −28 −56 48HP-γ-CD Methyl acetylsalicylate 1 −75 −194 48HP-γ-CD Methyl salicylate 1 −73 −176 48

a HP-R-CD: (2-hydroxypropyl)-R-cyclodextrin . â-CD: â-cyclodextrin. HP-â-CD: (2-hydroxypropyl)-â-cyclodextrin. M/DM-â-CD: mixture of maltosyl- anddimaltosyl-â-cyclodextrin (3:7). HP-γ-CD: (2-hydroxypropyl)-γ-cyclodextrin.

Journal of Pharmaceutical Sciences / 1019Vol. 85, No. 10, October 1996

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contribution by van der Waals-type interactions. Thesevarious explanations show that there is no simple constructto describe the driving force for complexation. Althoughrelease of enthalpy-rich water molecules from the cyclodextrincavity is probably an important driving force for drug-cyclodextrin complex formation, other forces may be impor-tant. These forces include van der Waals interactions,34,54hydrogen bonding,55,56 hydrophobic interactions,34,57 releaseof ring strain in the cyclodextrin molecule,56 and changes insolvent-surface tensions.58

Methods of preparing drug-cyclodextrin complexes havebeen reviewed.25 In the solution phase, the procedure isgenerally as follows: an excess amount of the drug is addedto an aqueous cyclodextrin solution, and the suspension isagitated for up to 1 week at the desired temperature. Thesuspension is then filtered or centrifuged to form a clear drug-cyclodextrin complex solution. For preparation of solid for-mulations of the drug-cyclodextrin complex, the water isremoved from the aqueous drug-cyclodextrin complex solu-tion by evaporation or sublimation. It is sometimes possibleto shorten this process by formation of supersaturated solu-tions through sonication followed by precipitation at thedesired temperature. In some cases, the efficiency of com-plexation is not very high, and therefore, relatively largeamounts of cyclodextrins must be used to complex smallamounts of drug. To add to this difficulty, vehicle additives,osmolality modifiers, and pH adjustments commonly used indrug formulations, such as sodium chloride, buffer salts,surfactants, preservatives, and organic solvents, very oftenreduce the efficiency. For example, in aqueous solutions,ethanol and propylene glycol at low concentrations have beenshown to reduce the cyclodextrin complexation of testosteroneand ibuprofen by acting as competing guest molecules whileat higher concentrations they can reduce complexation througha manipulation of solvent dielectric constant.48,59 Likewise,non-ionic surfactants have been shown to reduce cyclodextrincomplexation of diazepam60 and preservatives to reduce thecyclodextrin complexation of various steroids.61 On the otherhand, additives such as ethanol can promote complex forma-tion in the solid or semisolid state.62 Un-ionized drugs usuallyform a more stable cyclodextrin complex than their ioniccounterparts; thus, the complexation efficiency of basic drugscan be enhanced by addition of ammonia to the aqueouscomplexation media.For example, solubilization of pancratistatin with (hydroxy-

propyl)-cyclodextrins was optimized upon addition of am-monium hydroxide.63 Freeze-drying of the solutions removedammonia, resulting in ammonia-free solid complex prepara-tions which dissolved rapidly to form clear supersaturatedpancratistatin solutions. The resulting solutions were stablefor a few hours, time sufficient for potential use in parenteralpreparations. Finally, enhanced complexation can be obtainedby formation of ternary complexes (or cocomplexes) betweena drug molecule, a cyclodextrin molecule, and a third compo-nent. For instance, addition of a small amount of various

water-soluble polymers to an aqueous complexation medium,followed by heating of the medium in an autoclave, cansignificantly increase the apparent stability constant of thedrug-cyclodextrin complex (Table 4).49,64,65 A somewhatsimilar effect has been obtained through formation of drug-hydroxy acid-cyclodextrin ternary complexes or salts withbasic drugs.66-68

Drug Solubilization

The most common pharmaceutical application of cyclodex-trins is to enhance drug solubility in aqueous solutions. Someof the reports generated on this topic have been reviewed,5-9

and additional data is available from the individual cyclodex-trin manufacturers. The solubilizing effects of various cyclo-dextrins on three different drugs are listed in Table 5.Although prediction of compound solubilization by cyclodex-trins continues to be highly empirical, various historicalobservations permit several general statements. First, thelower the aqueous solubility of the pure drug, the greater therelative solubility enhancement obtained through cyclodextrincomplexation. Drugs that possess aqueous solubility in themicromole/liter range generally demonstrate much greaterenhancement than drugs possessing solubility in the micro-mole/liter range or higher. In Table 5, the enhancement factor,i.e., the solubility in the aqueous cyclodextrin solution dividedby the solubility in pure water, for paclitaxel, for example, ismuch larger than the enhancement factors for hydrocortisoneand pancratistatin. A similar observation was made whenthe solubilizing effect of (2-hydroxypropyl)-â-cyclodextrin on53 different drugs was investigated.9 Second, cyclodextrinderivatives of lower molar substitution are better solubilizersthan the same type of derivatives of higher molar substitution.In Table 5, both randomly methylated â- and γ-cyclodextrinswith molar substitution 0.6 provide for better solubilizationthan the same type of randomly methylated cyclodextrins withmolar substitution 1.8. With the exception of R-cyclodextrin,permethylated derivatives (of â- and γ-cyclodextrin) possessa lower complexing potential (lower Kc value) than the parentcyclodextrins.23 Of the commercially available materials, themethylated cyclodextrins with relatively low molar substitu-tion appear to be the most powerful solubilizers. The chainlength of the alkyl group, on the other hand, appears to be ofless importance.24,70 Third, charged cyclodextrins can bepowerful solubilizers, but their solubilizing effect appears todepend on the relative proximity of the charge to the cyclo-dextrin cavity. The farther away the charge is located, thebetter the complexing abilities. For example, (2-hydroxy-3-(trimethylammonio)propyl)-â- and -γ-cyclodextrin possess ex-cellent solubilizing effects while â-cyclodextrin sulfate has arelatively low complexation potential (Table 5). Sulfobutylether â-cyclodextrin, where the anion has been moved awayfrom the cavity by a butyl ether spacer group, is an excellentsolubilizer.71 (Carboxymethyl)-â-cyclodextrin is another in-teresting anionic cyclodextrin derivative.72 Compared toneutral cyclodextrins, enhanced complexation is frequentlyobserved when the drug and cyclodextrin molecules haveopposite charge but decreased complexation is observed if theycarry same type of charge. For example, (2-hydroxy-3-(trimethylammonio)propyl)-â-cyclodextrin is an excellent solu-bilizer for many acidic drugs capable of forming anions.Another finding is that while many ionizable drugs are able

to form cyclodextrin complexes, the stability constant of thecomplex is much larger for the un-ionized than for the ionizedform. For example, both the un-ionized and the cationic (i.e.,the protonated) form of chlorpromazine give rise to 1:1complexes with â-cyclodextrin but the stability constant forthe un-ionized form is 4 times larger than for the cationic

Table 4sEffect of Poly(vinylpyrrolidone) Concentration on the Value ofthe Apparent Stability Constant (K c) of SomeDrug−(2-Hydroxypropyl)- â-cyclodextrin (1:1) Complexes at RoomTemperature (20 −23 °C)a

Kc (M-1)

PVP (% w/v) Acetazolamide Hydrocortisone 17â-Estradiol

0.00 86.2 1010 529000.10 95.4 1450 588000.25 97.0 c 782000.50 96.2 1190 80400

a From ref 49. b Poly(vinylpyrrolidone). c Not determined.

1020 / Journal of Pharmaceutical SciencesVol. 85, No. 10, October 1996

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form.37 The Kc for the phenytoin-â-cyclodextrin complex isover 3 times larger for the un-ionized form than for the anionicform.46 However, it is frequently possible to enhance cyclo-dextrin solubilization of ionizable drugs by appropriate pHadjustments. Thus, the solubilizing effects of both (2-hydrox-ypropyl)-â-cyclodextrin and dimethyl-â-cyclodextrin on dihy-droergotamine mesylate have been found to increase withdecreasing pH (i.e., formation of the cationic form). Both thesaturation solubility and the slopes of the phase-solubilitydiagrams increase with decreasing pH.73 Similar results havebeen reported for the complexation of phenytoin with â-cy-clodextrin46 and for the complexation of indomethacin,74prazepam, acetazolamide, and sulfamethoxazole75 with (2-hydroxypropyl)-â-cyclodextrin.As mentioned before, it is also possible to enhance com-

plexation and, thus, the solubilizing effect of cyclodextrins byaddition of polymers or hydroxy acids to the cyclodextrinsolutions. It has been shown that polymers, such as water-soluble cellulose derivatives and other rheological agents, canform complexes with cyclodextrins and that such complexespossess physicochemical properties different from those ofindividual cyclodextrin molecules.49,76 In aqueous solutionswater-soluble polymers increase the solubilizing effect ofcyclodextrins on various hydrophobic drugs by increasing theapparent stability constants of the drug-cyclodextrin com-plexes. For example, the solubilizing effect of 10% (w/v) (2-hydroxypropyl)-â-cyclodextrin solution on a series of drugs andother compounds was increased from 12 to 129% when 0.25%

(w/v) poly(vinylpyrrolidone) was added to the aqueous cyclo-dextrin solution.49 Water-soluble polymers are also capableof increasing aqueous solubilities of the parent cyclodextrinswithout decreasing their complexing abilities, thus makingthem more feasible as pharmaceutical excipients. Likewise,addition of hydroxy acids, such as citric, malic, or tartaric acid,can enhance the solubilizing effect of cyclodextrins throughformation of super complexes or salts.67 It is frequentlypossible to obtain even larger solubilization enhancement byapplying several methods simultaneously. For instance,prazepam is a benzodiazepine with a pKa of about 3. (2-Hydroxypropyl)-â-cyclodextrin has a solubilizing effect on boththe un-ionized and the ionized form of the drug, and asexpected, hydroxypropyl methylcellulose has a synergisticeffect on the solubilization. However, the synergistic effectwas more pronounced for the ionized form (Figure 2).75Finally, pharmaceutical formulations should contain as smallan amount of cyclodextrin as possible since excess cyclodextrincan reduce, e.g., drug bioavailability and preservative efficacy.Drug solubility should be determined in the final formulationand under normal production conditions to determine if toomuch, or too little, cyclodextrin is being used.

Effect on Drug StabilityThe effects of cyclodextrins on the chemical stability of

drugs is another useful property of these excipients and hasbeen extensively examined in the literature.10 Cyclodextrin

Table 5sSolubility of Drugs in Different Cyclodextrin Solutions at Room Temperature

Drug Cyclodextrina Concnb (% w/v) Solubility (mM) Enhancementc Factor Ref

Hydrocortisone (MW 362) None 0.993 49Glucosyl-R-CD 10 7.45 7.50 49Maltosyl-R-CD 10 11.3 11.4 49HP-â-CD MS 0.6 10 33.7 33.9 49HE-â-CD 10 48.3 48.6 49RM-â-CD MS 0.6 10 72.2 72.7 27RM-â-CD MS 1.8 10 50.8 51.2 27HTMAP-â-CD MS 0.5 10 30.3 30.1 27CM-â-CD MS 0.6 10 44.6 44.9 27Glucosyl-â-CD 10 46.9 47.2 49Maltosyl-â-CD 10 28.7 28.9 49RM-γ-CD MS 0.6 10 58.8 55.2 27RM-γ-CD MS 1.8 10 38.6 38.9 27

Paclitaxel (Taxol, MW 854)d None 4 × 10-4 69â-CD 1.5 0.005 13 69Dimaltosyl-â-CD 50 0.115 288 69HE-â-CD 50 0.914 2285 69HP-â-CD 50 0.856 2140 69DM-â-CD 50 39.6 99.000 69γ-CD 15 0.020 50 69HP-γ-CD 50 0.080 200 69

Pancratistatin (MW 325) None 0.16 63HTMAP-â-CD MS 1.4 10 0.86 5.4 63S-â-CD Na-salt MS 2.3 10 0.28 1.8 63CM-â-CD Na-salt MS 0.6 10 0.83 5.2 63HP-â-CD MS 0.5 10 1.0 6.3 63Maltosyl-â-CD MS 0.14 10 0.95 5.9 63DM-â-CD MS 2.0 10 1.2 7.5 63HE-â-CD 10 0.83 5.2 63γ-CD 10 0.80 5.0 63HTMAP-γ-CD MS 0.3 10 0.49 3.1 63HP-γ-CD MS 0.7 10 0.83 5.2 63TM-γ-CD MS 3.0 10 0.49 3.1 63

a â-CD: â-cyclodextrin. HP-â-CD: (2-hydroxypropyl)-â-cyclodextrin. HE-â-CD: (hydroxyethyl)-â-cyclodextrin. RM-â-CD: randomly methylated â-cyclodextrin.HTMAP-â-CD: (2-hydroxy-3-(trimethylammonio)propyl)-â-cyclodextrin. CM-â-CD: (carboxymethyl)-â-cyclodextrin. Glucosyl-â-CD: glucosyl-â-cyclodextrin. Maltosyl-â-CD: maltosyl-â-cyclodextrin. DM-â-CD: 2,6-O-dimethyl-â-cyclodextrin. S-â-CD: â-cyclodextrin sulfate. γ-CD: γ-cyclodextrin. RM-γ-CD: randomly methylatedγ-cyclodextrin. HP-γ-CD: (2-hydroxypropyl)-γ-cyclodextrin. HTMAP-γ-CD: (2-hydroxy-3-(trimethylammonio)propyl)-γ-cyclodextrin. TM-γ-CD: trimethyl γ-cyclodextrin.MS: molar substitution (i.e., the average number of OH groups on each glucose repeat unit that have been substituted). Na salt: sodium salt. b Concentration of theaqueous cyclodextrin solution. c The solubility in the aqueous cyclodextrin solution divided by the solubility in water. d pH 7.4

Journal of Pharmaceutical Sciences / 1021Vol. 85, No. 10, October 1996

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interaction with labile compounds can result in severaloutcomes: cyclodextrins can retard degradation, can have noeffect on reactivity, or can accelerate drug degradation. Insome ways, therefore, cyclodextrins can mimic enzymaticcatalysis or inhibition. Similarities include substrate bindingprior to reaction, saturation kinetics, competitive inhibition,and stereospecific interactions. Due to saturation kinetics,the observed first-order rate constants for reaction (kobs)asymptotically approach a maximum (catalysis) or minimum(inhibition) value with increasing cyclodextrin concentration.The concentration dependence of the kobs can be used to deriveboth Kc and kc, the rate constant for the reaction of theincluded compound, by methods analogous to Michaelis-Menten analysis. For the formation of a 1:1 complex, thefollowing equilibrium applies:

where ko represents the observed first-order rate constant forthe degradation of the free drug (D), CD is the cyclodextrin,and D-CD is the drug-cyclodextrin 1:1 complex. In the aboveequilibrium,

where [CD] is the total cyclodextrin concentration in thesolution. The degree of stabilization/destabilization of a drugupon cyclodextrin complexation is dependent on not only therate of drug degradation within the complex (i.e., the valueof kc) but also the fraction of the drug that resides within thecomplex (which again depends on the value of Kc). The

observed first-order rate constant (kobs) is the weight averageof the two rate constants:

where ff is the fraction of free drug in solution, or

A Lineweaver-Burk type of equation25 can be obtained byfurther manipulations of the above equations:

A plot of 1/(ko - kobs) versus 1/[CD] will give rise to a straightline (i.e. if the assumption of a 1:1 complex is correct) with ay-intercept equal to 1/(ko - kc) and a slope equal to 1/Kc(ko -kc), from which the values of kc and Kc can be derived. At lowconcentration most drug-cyclodextrin complexes are of 1:1stoichiometry. Even complexes which are of higher orderstoichiometry at high cyclodextrin and/or drug concentrationform 1:1 complexes at lower concentration. The stoichiometry(i.e., the guest:host molar ratio) will, however, affect thestabilizing/destabilizing effect of the complexation.77-80 Thus,at relatively high concentrations the antiallergic drug, tra-nilast, forms a 2:1 (guest:host) complex with γ-cyclodextrinwhich accelerates the drug degradation (dimerization) byapproximately 5500-fold. With increasing γ-cyclodextrin con-centrations, 1:1 and 1:2 complexes are formed resulting in adecreased rate of dimerization.77 The rate of dimerizationwas, for example, 19 300 times slower within the 1:2 complexthan outside it. Similar observations were made when thedegradation rate of some pilocarpine prodrugs was studiedin aqueous (2-hydroxypropyl)-â-cyclodextrin solutions.80 Asmentioned before, the enthalpy of the system decreases duringcomplex formation, resulting in increased complexation (i.e.,increased Kc value) when the temperature is lowered. Thus,better overall stabilization is frequently obtained at lowtemperatures than at high temperatures.Drug-cyclodextrin complexation can be regarded as mo-

lecular encapsulation, i.e., encapsulation of drug at themolecular level. The cyclodextrin molecule shields, at leastpartly, the drug molecule from attack by various reactivemolecules. That is, the cyclodextrin can insulate a labilecompound from a potentially corrosive environment and, inthis way, reduce or even prevent drug hydrolysis, oxidation,steric rearrangement, racemization, and other forms of isomer-ization, polymerization, and even enzymatic decomposition ofdrugs. For example, the anticancer drug doxorubicin isunstable in aqueous solutions undergoing acid-catalyzedglycosidic bond hydrolysis, A-ring aromatization subsequentto cleavage of the 9-hydroxymethyl ketone function, andphotodecomposition.81-85 Fluorescence, absorbance, circulardichroism, and NMR studies have all indicated complexformation between doxorubicin and both â- and γ-cyclodex-trins.86 The A-ring of the anthraquinonic nucleus of the drugis located inside the cyclodextrin cavity, resulting in asignificantly slower rate of degradation (Table 6). It has beenshown that doxorubicin forms a stronger complex with γ-cy-clodextrin than with â-cyclodextrin86 and that, in general,γ-cyclodextrins are more effective stabilizers of doxorubicinthan, for example, (2-hydroxypropyl)-â-cyclodextrin.89 It ap-pears, however, that complexation of a closely related drug,daunorubicin, with methylated â-cyclodextrin (MâCD) offers

Figure 2sThe effect of ionization and hydroxypropyl methylcellulose (HPMC) onthe (2-hydroxypropyl)-â-cyclodextrin (HPâCD) solubilization of prazepam (pKa 3)in aqueous buffer solutions.

Kc )[D-CD]

[D]([CD] - [D-CD])

or, if [CD] . [D], Kc )[D-CD][D][CD]

d[D]tdt

) -kobs[D]t and kobs ) koff + kc(1 - ff)

ff ) 11 + Kc[CD]

1ko - kobs

) 1Kc(ko - kc)

1[CD]

+ 1ko - kc

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better protection, i.e., a larger stability constant (Kc) and amore favorable ko:kc ratio, than γ-cyclodextrin.Aspirin (acetylsalicylic acid) is a phenolic acetate ester, and

thus, it is unstable in aqueous solutions. In acidic buffersolutions (at pH about 1), the ester is hydrolyzed via an AAC2mechanism whereby it undergoes an acyl-oxy cleavagesubsequent to protonation, attack by water molecules, andformation of an unstable tetrahedral intermediate.90 Un-ionized aspirin forms stable (1:1) inclusion complexes with thevarious â-cyclodextrins. NMR studies have shown that in thecomplex the benzene ring is located well inside the cavity withthe acetyl ester group protruding from cavity. This locationof the acetyl ester does not completely prevent its hydrolysisbut due to steric hindrance, the hydrolysis was determinedto be 4-6 times slower within the complex than outside it(i.e., the ko:kc ratio in Table 7 is between 4 and 6). However,under neutral conditions, where aspirin is in the ionized form,the same cyclodextrins did not affect the observed hydrolyticrate constant. NMR studies indicated that the ionized aspirindoes not form complexes with the â-cyclodextrins tested. Thecyclodextrins did not influence the kinetic behavior (e.g., theorder of reaction) or the degradation mechanism, only the rateof reaction.48

Sulfobutyl ether â-cyclodextrin, which is an anionic â-cy-clodextrin derivative, has been shown to be highly effectivein improving the chemical stability of the antitumor drug O6-benzylguanine. The benzyl moiety of the drug was responsiblefor the cyclodextrin complex formation resulting in an objec-tive increased shelf-life of an aqueous parenteral O6-ben-zylguanine formulation.71 The same cyclodextrin derivativehas been used to increase the shelf-life (and ocular absorption)of pilocarpine in aqueous eye drop solutions.91 The cyclodex-trin stabilization of pilocarpine appeared to be independentof the drug ionization status. Another anionic type cyclodex-trin, i.e., O-(carboxymethyl)-O-ethyl-â-cyclodextrin, has beenused to stabilize prostaglandin E1 in a fatty alcohol propylene

glycol ointment.92 Dihydroergotamine nasal spray has beenused as an acute treatment of migraine. However, dihydro-ergotamine, the free base, has both limited aqueous solubilityand stability. Cyclodextrins, such as (2-hydroxypropyl)-â-cyclodextrin, have been used to solubilize the drug in aqueoussolutions and to stabilize it during autoclaving.73Degradation kinetics in the solid state are, in general, more

complicated and they progress more slowly than in aqueoussolutions. Consequently, there are fewer reports on the effectsof cyclodextrins on the solid-state decomposition of drugs.

Table 6sProposed Structure of the Doxorubicin −γ-Cyclodextrin Complex 86 and Stabilization of Doxorubicin and Related Drugs by CyclodextrinComplexation 87-89

Drug pH Temp ( °C) koa (min-1) Cyclodextrinb kca (min-1) ko/kc Kca (M-1)

Daunorubicin 1.5 50 2.16 × 10-3 M-â-CD 2.64 × 10-4 8.2 19601.5 50 2.00 × 10-3 γ-CD 3.72 × 10-4 5.4 211

Demethoxydaunorubicin 1.5 50 2.16 × 10-3 M-â-CD 5.40 × 10-4 4.0 3690Doxorubicin 1.01 75 0.17 HP-γ-CD 3.02 × 10-2 5.7 69.8

1.84 75 1.86 × 10-2 HP-γ-CD 2.10 × 10-3 8.9 1935.90 75 1.23 × 10-2 HP-γ-CD 4.70 × 10-3 2.6 2437.72 75 5.48 × 10-2 HP-γ-CD 1.03 × 10-2 5.3 1321.5 50 1.71 × 10-3 γ-CD 3.36 × 10-4 5.1 197

a ko represents the observed first-order rate constant for the degradation of the free drug, kc represents the observed first-order rate constant for the degradationof the drug within the complex, and Kc is the observed stability constant for the complex, assuming 1:1 complex formation. b M-â-CD: methylated â-cyclodextrin.γ-CD: γ-cyclodextrin. HP-γ-CD: (2-hydroxypropyl)-γ-cyclodextrin.

Table 7sProposed Structure of the Aspirin −â-Cyclodextrin Complex andStabilization of Aspirin by Cyclodextrin Complexation 48

Drug pH Temp (°C) ko (min-1) Cyclodextrina kc (min-1) ko/kc Kc (M-1)

Aspirin Ca. 1 65 4.76 × 10-3 H-â-CD 1.11 × 10-3 4.3 76.0M/DM-â-CD 8.25 × 10-4 5.8 53.3HP-γ-CD 1.18 × 10-3 4.0 23.2

a HP-â-CD: (2-hydroxypropyl)-â-cyclodextrin. M/DM-â-CD: mixture of maltosyl-and dimaltosyl-â-cyclodextrin (3:7). HP-γ-CD: (2-hydroxypropyl)-γ-cyclodextrin.

Journal of Pharmaceutical Sciences / 1023Vol. 85, No. 10, October 1996

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Solid drug-cyclodextrin complexes are more water-solublethan the pure lipophilic drugs, and thus, moisture-promotedsolid-state decomposition should be accelerated by formationof water-soluble drug-cyclodextrin complexes. This has beenshown to be the case with carmofur, an anticancer agent,where the solid-state rate of degradation was increased uponcomplexation with â-cyclodextrin.93 Under dry conditionscyclodextrin complexation of drugs commonly increases boththeir chemical (e.g., prostaglandin E1

94) and physical stability(e.g., nifedipine95,96).Large drug molecules like peptides and proteins can also

form cyclodextrin complexes, and frequently the complexationresults in both enhanced chemical and physical stability ofthis type of drug.97 Interestingly, the mechanism of stabiliza-tion is qualitatively different than in the case of smallmolecular weight pharmaceuticals. Thus, maximum benefitis usually obtained at low cyclodextrin concentrations, andthe benefits are often only partly concentration dependent.In the case of interleukin-2 (IL-2), for example, (2-hydroxy-propyl)-â-cyclodextrin optimally inhibited aggregation at lev-els of 0.5% in the dosage form.98 The effects of cyclodextrinson improved conformational stability may be related toexcipient interaction with hydrophobic aromatic moieties onthe protein molecules. It is also possible that cyclodextrinscan stabilize proteins similarly fashion to other polyalcoholiccompounds and carbohydrates such as sorbitol.99 On the otherhand, cyclodextrins were more effective than their linearsaccharide analogues in stabilizing human growth hormone.100Other examples of interaction of cyclodextrin with peptidesystems include the decreased proteolytic degradation of basicfibroblast growth factor by pepsin and R-chymotrypsin by theaddition of water-insoluble aluminum salt of â-cyclodextrinsulfate,101 and the inhibition of self-association of insulin inaqueous solutions by maltosyl-â-cyclodextrin complexation.102Thymopentin, which is a small peptide consisting of five aminoacids, has been stabilized in aqueous solutions by complex-ation with (2-hydroxypropyl)-â-cyclodextrin.103Although cyclodextrin complexation of drug molecules usu-

ally results in increased drug stability, there are examples ofaccelerated degradation.10 For example, it has been shownthat the specific-base-catalyzed hydrolysis of the â-lactam ringis facilitated by simultaneous hydrogen bonding between twoadjacent hydroxyl groups on the glucose residue and the amidecarbonyl and â-lactam carbonyl groups of the â-lactamantibiotics.104 The pH-rate profile for the degradation ofcephalothin is characterized by a large pH-independent regionfrom pH about 2-8.105 In this region, the (hydroxypropyl)-cyclodextrins had a significant stabilizing effect, but at pH9.7, where the specific-base catalysis dominates, the samecyclodextrins had a destabilizing effect.106 For aztreonam,specific-base-catalyzed degradation was dominant at pHvalues greater than 6,105 and in this region of the pH-rateprofile, cyclodextrins accelerated the degradation.106 Cyclo-dextrins have also been shown, under some specific conditions,to destabilize other drugs including aspirin,107 the antiallergicdrug tranilast,77 the antiulcer agent 2′-(carboxymethoxy)-4,4′-bis(3-methyl-2-butenyloxy)chalcone,79 and the thromboxanesynthetase inhibitor (E)-4-(1-imidazoylmethyl)cinnamic acid.78Finally, â-cyclodextrin has been shown to catalyze the hydrol-ysis of 2-methoxy-2-phenylacetic acid 4-nitrophenyl ester, butthe (R)-enantiomer was always catalyzed to a greater extentthan the (S)-enantiomer, displaying the possibility of enan-tiomer-selective cyclodextrin stabilization/destabilization.108

Conclusions and Future DirectionsBoth parent and chemically modified cyclodextrins are

rapidly being assimilated into the formulators’ armamen-tarium. In Europe and Japan, oral â-cyclodextrin complexes

of prostaglandins and nonsteroidal antiinflammatory agents(piroxicam) have already been introduced to the market. Inthe United States, a monograph for â-cyclodextrins is availablein the Pharmacopoeia representing the first such citation foran excipient which is not yet available in a marketed (U.S.)product. A monograph for â-cyclodextrin is already in theJapanese Pharmacopoeia, and it will soon appear in theEuropean Pharmacopoeia. It is clear from this and otherperspectives that the introduction of oral â-cyclodextrinformulations is in the offing. Chemically modified cyclodex-trins, especially (2-hydroxypropyl)-â-cyclodextrin, are alsoreceiving attention. A monograph is in preparation for theU.S. Pharmacopoeia, and versions have already appeared insuch compendial sources as the Handbook of PharmaceuticalExcipients.18 (2-Hydroxypropyl)-â-cyclodextrin has been usedas a parenteral (iv) excipient for drugs completing bothpreliminary and advanced human clinical trials, and the firstsuch formulation is expected to garner regulatory approvalwithin the next year or so. Similarly,aqueous eye dropsolutions containing (2-hydroxypropyl)-â-cyclodextrin are cur-rently undergoing clinical testing. Cyclodextrins are, there-fore, proving their usefulness as tools to generate aqueousdrug solutions without the use of organic cosolvents, surfac-tants, or lipids, as formulation adjuncts which increasedissolution rates and oral bioavailability of solid drug com-plexes, and as materials used to generate safe iv dosage formsintended to provide important pharmacokinetic informationor act as potential drug products per se. In addition to therole of the currently applied cyclodextrins, newer derivativesare constantly being developed and reported.

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