Advanced Drug Delivery Reviews 36 (1999) 3–16

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Mechanisms of drug release from cyclodextrin complexes Valentino J. Stella*, Venkatramana M. Rao, Erika A. Zannou, Vahid Zia Department of Pharmaceutical Chemistry and the Center for Drug Delivery Research, The University of Kansas, Lawrence, KS, USA

Abstract This review addresses the issue of the mechanisms of drug release from cyclodextrin complexes. More specifically, it attempts to answer the question whether drug release from aqueous formulations is slow or incomplete? A critique of the literature, our own work, and various simulations suggests that drug release from cyclodextrin complexes is rapid and quantitative in most cases. In aqueous solution, drug / cyclodextrin complexes are continually forming and dissociating with lifetimes in the range of milliseconds or less. Although the stronger the binding, the slower the relative kinetics of dissociation, the rates are still fast and essentially instantaneous. After parenteral administration, the major driving force for dissociation of weakly to moderately bound drugs appears to be simple dilution. For strongly bound drugs, binding constants of 10 24 M 2 1 or higher, or for those cases where dilution is minimal, contributions from competitive displacement by endogenous materials, drug binding to plasma and tissue components, drug uptake into tissues not available to the complex or the cyclodextrin, rapid elimination of the cyclodextrin and possibly pH and temperature effects, may also be important. After parenteral administration, it does appear that cyclodextrins might cause some alterations in the fraction of free drug eliminated in the urine during that time frame where the cyclodextrin is itself undergoing substantial renal clearance.  1999 Elsevier Science B.V. All rights reserved. Keywords: Complex dissociation; Cyclodextrins; Dilution; Competitive displacement; Pharmacokinetics

Contents 1. Introduction ............................................................................................................................................................................ 2. Kinetics of complex formation and dissociation ......................................................................................................................... 3. Control experiments ................................................................................................................................................................ 3.1. If not solubilized by cyclodextrins then how? ..................................................................................................................... 3.2. Drug pharmacokinetics ..................................................................................................................................................... 4. Simulations: demonstration of the possible role of dilution, displacement and protein binding....................................................... 5. Dilution .................................................................................................................................................................................. 6. Competitive displacement ........................................................................................................................................................ 7. Protein binding ....................................................................................................................................................................... 8. Drug uptake into tissue ............................................................................................................................................................ 9. Cyclodextrin elimination.......................................................................................................................................................... 9.1. Increased renal excretion of lipophilic drugs ...................................................................................................................... 10. Change in ionic state and temperature ..................................................................................................................................... 11. Conclusions .......................................................................................................................................................................... Acknowledgements ...................................................................................................................................................................... References .................................................................................................................................................................................. *Corresponding author. Fax: 11-785-7497393; E-mail: [email protected] 0169-409X / 99 / $ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S0169-409X( 98 )00052-0

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1. Introduction ‘If drugs bind so well to cyclodextrins, how is the drug released in vivo?’ is one of the most frequently asked questions following presentations on cyclodextrins. If a product includes a drug that is substantially complexed with a cyclodextrin, can the release of the drug from the complex itself be incomplete or might the pharmacokinetics, and therefore the pharmacodynamics, of the drug be altered by slow release from the complex? Before attempting to answer this question, it is important to acknowledge that this question raises many others. If the cyclodextrin is being used to solubilize a drug for parenteral administration, then it begs the question, ‘if the drug is insoluble, how will the drug be solubilized if not by a cyclodextrin?’ The solubilization might be by the use of co-solvents, a surfactant, pH-adjustment (only for an ionizable drug), an emulsion or a liposomal formulation etc. The issue then becomes, if the pharmacokinetics of the drug from a cyclodextrin solution is different than that from one of these other formulations, which dosage form is the control? This review will show that drugs are rapidly and quantitatively released from their complexes especially after parenteral administration, and that cyclodextrins do NOT appear to substantially alter the intrinsic pharmacokinetics of drugs. For some examples where this conclusion might be questioned, substantial doubt exists regarding the appropriateness of the control formulations used for comparison. The objective of this review, therefore, is to critique the issue of whether drug release from cyclodextrin complexes of the drug is slow or incomplete. The emphasis will mainly be on drug release from aqueous solutions of the complexes. Drug delivery from solid or other heterogeneous dosage forms is addressed in other chapters of this themed issue. The results from such experiments should provide clearer interpretation of drug release from cyclodextrins. The contributions to drug release from dilution of the complex, competitive displacement of the drug from the complex, drug binding to plasma and tissue components, uptake of the drug by tissues not available to the complex or cyclodextrin, cyclodextrin elimination and possibly pH and temperature adjustments will be the focus of this paper.

2. Kinetics of complex formation and dissociation Complexation of molecules to cyclodextrins occurs, with some exceptions, through a non-covalent interaction between the molecule and the cyclodextrin cavity. This is a dynamic process whereby the guest molecule continuously associates and dissociates from the host cyclodextrin. Although most complexation schemes depict the complex as a single entity, the complex is more realistically composed of a family of species with the depicted complex representing some average. It probably represents the specie with the longest lifetime. Assuming a 1:1 complexation occurring, the association is usually depicted by Scheme 1. Two important parameters can be defined for the inclusion process. First, is the complexation strength or constant (K) defined by Scheme 1 and Eq. (1), where [CyD f ] and [D f ] are the concentrations of free cyclodextrin and free drug molecule, respectively. Second, is the lifetime (t ) of the complex, also defined in Scheme 1 and Eq. (2), measured when the equilibrium is perturbed. The constants k f and k r are the forward and reverse rate constants, respectively, and k obs is the observed rate constant for the reestablishment of the equilibrium after it is perturbed. Note, k f a second order rate constant while k r , is a first order rate constant. If the complex is diluted (see later discussion) such that dissociation of the complex is complete, the half-life will be that associated with k r . K 5 k f /k r 5 [DCyD] /([D f ][CyD f ])

(1)

k obs 5 l /t 5 k f ([CyD f ] 1 [D f ]) 1 k r

(2)

Various methods have been developed in determining the strength and lifetime of a complex. The focus here is on the lifetime of the complex. Temperature-jump technique has been the most common-

Scheme 1. The interaction of drug, D, with a cyclodextrin, CyD, to form an inclusion complex, DCyD, with a binding constant of K, where K5k f /k r .

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ly utilized technique for the determination of kinetic rates [1–9]. It has been shown that the complexation dynamics of Azo derivatives with a-cyclodextrin is fast, but dependent on substrate structure and charge [1,3,4]. Similar results were reported for dyes interacting with b, g, and permethylated cyclodextrins. The lifetime of the complex was in the range of microseconds [2,5–7,9]. Pulse voltammetry [10] and ultrasonic relaxation studies [11–13] have also revealed the association and dissociation rate constants of molecules, to and from cyclodextrins, to be in the range of 10 8 –10 7 M 2 1 s 2 1 (and relatively independent of K), and 10 5 s 2 1 , respectively. Photophysical probes have also been utilized where the dynamic phosphorescence decay was employed to obtain kinetic information on rates of complex formation and dissociation [14–17]. For both strong and weak complexes, the lifetime was found to be in the micro- to millisecond range and dependent on the substrate [14,16]. In general, the formation rate constant, k f , was found to be close to diffusion controlled process and weakly dependent on substrate structure and K, while k r was very structure sensitive and inversely proportional to K [14]. Competitive binding studies altering the equilibrium of cyclodextrin and the guest molecule have also reported similar kinetics [18–20]. Even though most studies utilized unmodified cyclodextrins, cyclodextrins of more pharmaceutical relevance, especially HP–b-CD and (SBE) 7M –bCD, are likely to show similar kinetic behavior. In summary, the association and dissociation of molecules to and from cyclodextrins are dynamic processes. The reactions occur at very rapid rates for both strong and weak complexes with the average lifetime of the molecules in the host cavity being in the milli- to micro-second time range or shorter. With these processes occurring at such rates, the KINETICS of release of a drug molecule from the cyclodextrin cavity should not be a limiting factor.

3. Control experiments

3.1. If not solubilized by cyclodextrins then how? When evaluating in vivo studies which suggest that cyclodextrins either do or do not appear to alter drug pharmacokinetics / pharmacodynamics, the

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question of appropriate controls can and should be raised. Consider the issue of a poorly soluble drug formulated in either a cyclodextrin solution or some other dosage form for parenteral administration. Traditionally, poorly water soluble drugs have been formulated through the use of co-solvents, pH adjustments for weak electrolytes, surfactants, emulsion and liposome formulations, etc. or a combination of these techniques. In all of these strategies the question becomes, ‘is it safe to assume that the dosage form itself, or a component of the formulation may not affect the pharmacokinetics / pharmacodynamics?’ In earlier reviews [21–24], examples were given where, clearly, the alternative to the cyclodextrin formulation proved to be a problem. The best example was the inhibition of carbamazepine metabolism to its epoxide by the co-solvent / surfactant glycofural (PEG–monotetrahydrofurfuryl ether) resulting in altered pharmacokinetics of carbamazepine compared to a HP–b-CD formulation [25]. Frijlink et al. [26] showed that some tissue distribution of flurbiprofen 10 min after a parenteral dose from a HP–b-CD solution was higher than when the drug was administered as a plasma solution and that the difference persisted even after 60 min for brain tissue. The purpose of the Frijlink study was to show that the cyclodextrin could displace both naproxen and flurbiprofen from protein binding sites by competitive binding. There were no differences in the tissue distribution between the cyclodextrin solution and the plasma solution for naproxen. Would the differences for flurbiprofen tissue have been seen if the drug was administered in a purely aqueous vehicle or a co-solvent formulation instead of a plasma solution? Perhaps here, the plasma solution disturbed the initial distribution rather than the cyclodextrin solution. Recently, one of us was asked to explain the difference seen between side effects of a new cyclodextrin formulation of an experimental drug to a previous formulation that utilized a co-solvent combination. Careful analysis and subsequent testing showed that ethanol in the original formulation was masking a reaction from the drug which was unveiled when the cyclodextrin solution was used. The cardiovascular pharmacodynamics of the aesthetic agent propofol was evaluated from a cyclodextrin solution compared to various other formulations including the commercial o / w emulsion [27].

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Enhanced acute cardiovascular effects were seen from the cyclodextrin formulation compared to the emulsion, yet others saw no real differences in central aesthetics effects between similar formulations [28]. The site of injection and the protocol used in the cardiovascular study may have also contributed to the differences or the emulsion may have masked the cardiovascular effects.

3.2. Drug pharmacokinetics Examples of the effects of cyclodextrins on drug pharmacokinetics have been extensively reviewed [21–24]. The reader should consult these reviews for a reasonably updated list of reliable papers that address this issue. At two recent conferences on cyclodextrins, one held in Lawrence, Kansas, USA (‘Pharmaceutical Applications of Cyclodextrins’ June 29–July 2, 1997) and the other in Santiago de Compostela, Spain (‘9th International Symposium on Cyclodextrins’ May 31–June 3, 1998), a number of podium and poster presentations addressed the issue of the possible perturbation of drug pharmacokinetics by cyclodextrins. For example, Piel et al. [29] compared the pharmacokinetics of miconazole in sheep from two parenterally safe cyclodextrins (HP– b-CD and (SBE) 7M –b-CD) to those from a polyoxyl 35 castor oil / lactic acid mixture and concluded that there was no differences between the three dosage forms. The reviews, as well as more recent presentations at major meetings have concluded that cyclodextrins have little or no effect on the intrinsic pharmacokinetics of drugs. In some cases where this conclusion might be challenged, alternative explanations can be found for any differences (see Section 3.1). In Section 9.1 of this paper, a brief discussion of small but significant increases in renal excretion of lipophilic drugs after parenteral administration of drugs in cyclodextrin solutions is discussed. This effect appears to be real and should not be ignored. Section 4, Section 5, Section 6, Section 7, Section 8, Section 9, Section 10 below provide information on the most likely mechanisms that contribute to the quantitative release of drugs from cyclodextrin complexes. However, it does not appear to significantly influence plasma pharmacokinetics.

4. Simulations: demonstration of the possible role of dilution, displacement and protein binding Well designed in vitro and in vivo studies assessing the various contributions to drug release mechanisms from cyclodextrin complexes are not readily available. Simulations allow one to assess these possible contributions by setting up mathematically sound and realistic models based on known facts about the kinetics of association and dissociation, and the strength of drug / cyclodextrin binding. Scheme 2 illustrates three equilibria processes that may be occurring following the administration of a drug / cyclodextrin complex. These equilibria represent the binding of drug (D 1 ) to a cyclodextrin (CyD), binding of drug to a protein (P) and the binding of a competing agent (D 2 ) to the cyclodextrin. K1 , K2 and Kp are the binding constants for 1:1 drug / cyclodextrin complexation, 1:1 competing agent / cyclodextrin complexation and drug / protein binding, respectively. Each of these equilibria can be defined by Eq. (3), Eq. (4), Eq. (5), K1 5 [D 1 CyD] /([D 1,f ][CyD f ]

(3)

K2 5 [D 2 CyD] /([D 2,f ][CyD f ]

(4)

Kp 5 [D 1 P] /([D 1,f ][P])

(5)

where [D 1,f ], [D 2,f ], [P] and [CyD f ] represent con-

Scheme 2. Protein binding and displacement equilibria competing with drug / cyclodextrin inclusion complexation.

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centrations of free components of drug, competing agent, cyclodextrin and protein, respectively, and [D 1 CyD], [D 2 CyD] and [D 1 P] are the bound concentrations of drug to cyclodextrin, competing agent to cyclodextrin and drug to protein, respectively. Eq. (5) assumes a simple equilibrium between drug and protein with a single non-saturable class of binding site. This equation may be further simplified to Eq. (6) by assuming that the total protein concentration is in excess relative to the drug-protein complex. Kapp 5 [D 1 P] / [D 1,f ]

(6)

where Kapp is an apparent association constant. Drug binding to plasma and tissue proteins is a complex phenomenon and generally involves multiple equilibria. A comprehensive model of protein binding involving multiple equilibria was beyond the scope and needs of this paper. Therefore, only this simplified model of protein binding will be considered here. These equations along with the component mass balance equations can be solved to determine the free fraction of drug to cyclodextrin as a function of the extent of dilution, placing realistic values for the binding constants (K1 , K2 , Kapp ), concentrations of drug, competing agent / s, cyclodextrin and protein. It is evident that the free fraction of drug will depend on the concentrations of all species involved, the strengths of binding and extent of dilution. It is impossible to illustrate the effects of the entire range of these parameters on the drug release from drug / cyclodextrin complexes. The objective of this section of the review, therefore, was to present a reasonably realistic picture of the role that all three of these mechanisms might play in the in vivo release of drugs from drug / cyclodextrin complexes. The assumption was made that all of the processes illustrated in Scheme 2 were instantaneous (see Section 3 above). Two hypothetical drugs were chosen, one with a high binding constant (K1 ) of 10 000 M 2 1 and the other with a low binding constant, 610 M 2 1 . The total concentrations of drug and cyclodextrin prior to dilution were fixed at 20 mM and 100 mM, respectively. That is, a hypothetical 100 mM cyclodextrin solution containing 20 mM of drug was administered and the in vivo release of drug from the drug / cyclodextrin complex evaluated. No time dependency was assumed i.e., the figures that follow represent

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instantaneous pictures of the various species immediately after hypothetical drug administration. Uekama et al. [23] and Thompson [24] also presented simulations illustrating the influence of dilution alone on the fraction of drug bound to cyclodextrin, while Stella and Rajewski [21] extended this analysis by including the effects of a competing agent along with dilution effects. However, their examples were limited to cases where both the competing agent and drug were bound to the cyclodextrin with equal strength (K1 5K2 ). Fig. 1 illustrates the effect of dilution on the fraction of drug bound for a weak binding drug (610 M 2 1 ) in the presence of a 0.4 mM competing agent / s with different binding strengths (K2 ). Semi-quantitative amounts of drug were released from the complex by just considering dilution effects alone. That is, in the absence of any competing agent, a 1,000 fold dilution caused the fraction of drug bound to cyclodextrin to decrease from 98% to 5.7%. In the presence of a competing agent with a binding constant of 1,000 M 2 1 and the same 1,000 fold dilution, the fraction of drug bound to cyclodextrin was reduced to 4.2%. Fig. 2 shows the same relationship in the presence of competing agent / s present at 20 mM. For the same competing agent (K2 51,000 M 2 1 ) and a 1,000-fold dilution, the fraction of drug bound to cyclodextrin is reduced to 0.58%.

Fig. 1. The effect of dilution on the fraction of a drug that is bound with a binding constant of 610 M 2 1 , K1 , to a cyclodextrin in the presence of 0.4 mM competing agent / s with variable K2 , values (0–10 000 M 1 ), according to Scheme 2.

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Fig. 2. The effect of dilution on the fraction of a drug that is bound with a binding constant of 610 M 2 1 , K1 , to a cyclodextrin in the presence of 20 mM competing agent / s with variable K2 values (0–10 000 M 21 ), according to Scheme 2.

Fig. 4. The effect of dilution on the fraction of a drug that is bound with a binding constant of 10 000 M 2 1 , K1 , to a cyclodextrin in the presence of 20 mM competing agent / s with variable K2 values (0–10 000 M 2 1 ), according to Scheme 2. Symbols are the same as for Figure 3.

Figs. 3 and 4 show the relationship between the extent of dilution and the fraction complexed for a highly bound drug (K1 510 000 M 2 1 ) in the presence of competing agent / s with variable K2 , values at 0.4 mM and 20 mM, respectively. In the absence of competition, a 1,000 fold dilution caused the fraction of drug bound to cyclodextrin to decrease from 99.9% to 47.5%. However, in the presence of 0.4 mM competing agent with a binding constant (K2 ) of 5,000 M 2 1 and a dilution of 1,000 fold, the fraction bound decreased to 26.0%. In the presence of same competing agent / s at 20 mM, the fraction bound to cyclodextrin was reduced to 1.0%.

Binding of drugs to plasma proteins may be another important mechanism by which drugs may be released from a drug / cyclodextrin complex. Since dilution alone was good enough to account for quantitative release of weakly bound drugs from complexes, only the effect of protein binding for strongly binding drugs (K1 about 10 000 M 2 1 ) was considered for simulation. Fig. 5 shows the relationship between the fraction of drug (K1 510 000 M 2 1 ) bound to a cyclodextrin versus the extent of dilution for drugs with differing protein binding strengths. As before, in the absence of any protein binding, a 1,000 fold dilution caused the fraction of drug bound to

Fig. 3. The effect of dilution on the fraction of a drug that is bound with a binding constant of 10 000 M 2 1 , K1 , to a cyclodextrin in the presence of 0.4 mM competing agent / s with variable K2 , values (0–10 000 M 2 1 ), according to Scheme 2.

Fig. 5. The effect of dilution on the fraction of a drug that is bound with a binding constant of 10 000 M 2 1 , K1 , to a cyclodextrin, CyD, and to plasma proteins to varying extents (Kapp equal to O (h); 1 (앳); 3 (s); 9 (n); 19 (j); and 99 (♦)), according to Scheme 2 and Eq. 5.

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cyclodextrin to decrease to 47.5%. If the same drug has the ability to bind moderately to proteins (Kapp 5 1), a 1,000 fold dilution caused the fraction bound to cyclodextrin to decrease to 31.9%. If the drug has a higher apparent association constant (Kapp 59), a 1,000 fold dilution causes the fraction to be reduced to 8.9%. Kapp values of 1 and 9 may roughly be translated to 50% and 90% protein binding. Even higher protein binding, obviously, would lead to even greater dissociation of the complex. It is evident from Fig. 5 that proteins may effectively compete with cyclodextrins for drug binding and thus facilitate the in vivo release of drugs from drug / cyclodextrin complexes. Fig. 6 illustrates the importance of a number of factors acting in concert. For a drug with strong cyclodextrin binding (K1 510 000 M 2 1 ) which undergoes a 1,000 fold dilution, the fraction of drug bound to cyclodextrin was 47.5%. In the presence of a competing agent / s (0.4 mM, K2 51,000 M 2 1 ) and the same dilution, the fraction is reduced to 16.42%. If the drug is 90% protein bound (Kapp 59), the fraction bound to cyclodextrin reduced from 47.5% to 8.9%. This fraction was further reduced to 2.0% when a 1,000 fold dilution, protein binding (Kapp 59) and competitive displacement (0.4 mM, K2 51,000 M 2 1 ) were considered. In summary, dilution alone may be effective in

Fig. 6. The effect of dilution on the fraction of a drug that is bound with a binding constant of 10 000 M 2 1 , K1 , to a cyclodextrin, CyD; no competing agent / s and protein binding (h); 0.4 mM competing agent (K2 51,000 M 21 ) and no protein binding (앳); no competing agent and protein binding (Kapp 59, s); and 0.4 mM competing agent (K2 51,000 M 2 1 ) and protein binding (Kapp 59, n).

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releasing free drug from weak drug / cyclodextrin complexes. The simulations also suggested that dilution will be a significant contributor even for strongly bound drugs. As the strength of binding between drug and cyclodextrin was increased, mechanisms such as competitive displacement, plasma and tissue protein binding may also play a significant role in the release of drugs from their cyclodextrin complexes. The relative contribution of these mechanisms will however depend on the route of administration (dilution effects), volume of distribution of drug and cyclodextrin (dilution effects), binding strength and concentrations of drug and cyclodextrin (dilution effects), binding constant and concentration of competing agent (competitive displacement) and association constant and protein concentration (protein binding).

5. Dilution What are the realistic dilutions of a formulation following drug administration? After i.v. administration, the minimal volume of distribution for any specie is plasma volume. For a 70 kg subject that would be about 3.5 liters, assuming a plasma volume of about 5% of body weight. Therefore, if a 1 ml drug dosage as a cyclodextrin complex was administered, the dilution factor would be 1:3,500. If the administered volume was 5 ml, the dilution would be 1:700. However, if it is assumed that the volume of distribution of cyclodextrin and the complex is extracellular water, about 30% of body weight, then the dilutions are 1:21 000 or 1:4,200 for 1 and 5 ml administered volumes, respectively. These dilutions are within the range discussed in Section 4 above. The binding of the drug, methylprednisolone, to an SBE–b-CD is about 700 M 2 1 [30]. Following i.v. administration to anaesthetized rats from a 75 mM (SBE) 4M –b-CD solution, a co-solvent mixture consisting of PEG400 / ethanol / water, and isotonic solutions of prodrugs, methylprednisolone phosphate and hemisuccinate, the relative availability of methylprednisolone was 97.164.7%. 100% (control), 59.264.4% and 33.265.3%, respectively [30]. Not only was the release from (SBE) 4M –b-CD quantitative relative to the co-solvent formulation, but also the plasma concentration versus time curves were

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Fig. 7. Plasma concentration (6SE) versus time for methylprednisolone after a 20 mg / kg i.v. dosing of methylprednisolone to rats from a co-solvent mixture (h); 75 mM (SBE) 4M –b-CD (s); and as its 21-phosphate (n) and 21-hemisuccinate prodrugs (j). From Stella et al. [30].

superimposable (Fig. 7), and superior to the two prodrug formulations. Based on the simulations defined earlier, the quantitative and instantaneous release of methylprednisolone from (SBE) 4M –b-CD can be completely accounted for by the probable dilution of the dosage form. Other cases have been discussed in earlier reviews [21,22] and the recent example by Piel et al. for miconazole [29], a more strongly bound drug compared to methylprednisolone [31] supports the probable role of dilution. Dilution is minimal when a drug / cyclodextrin complex is administered ophthalmically. The precorneal fluid volume is about 7 ml, while an eye drop volume is 25–35 ml. Therefore, dilution is likely to account for little, if any, drug dissociation from a cyclodextrin complex. Efficient corneal absorption is further exacerbated by contact time. Excess fluid administered to the precorneal area is rapidly cleared through the nasolacrimal ducts. After oral drug administration, some dilution is likely to occur but again, dilution alone is probably insufficient to account for the relative good absorption of drugs administered as cyclodextrin complexes. Unlike, ophthalmic delivery, residence time in the GI tract is longer, allowing time for other factors to contribute to complex dissociation.

6. Competitive displacement Competitive displacement of drugs from their cyclodextrin complexes probably plays a significant

role in vivo. The simulations described in Section 4 support this contribution. In our laboratories, various in vitro visual observations support this conclusion. For example, some have suggested that the combination of complexation with cyclodextrins and cosolvents or surfactants might be used to help dissolve very insoluble drugs. Our experience has been that most co-solvents, other than methanol (toxic) and ethanol, tend to displace drugs from their complexes leading to decreased rather than increased solubility, with the exceptions of some polymeric solvents. The addition of preservatives such as parabens to parenterals not only leads to decreased antimicrobial activity from the parabens due to complexation, but often to a displacement of the drug from complexes leading to decreased solubility. van Stam et al. [15] showed that alcohols can displace 2-naphthol from b-cyclodextrin complexes. One pharmaceutically relevant example where competitive displacement of a drug from it’s cyclodextrin complex was the work of Tokumura et al. [32,33]. A b-cyclodextrin complex of the poorly water soluble drug, cinnarizine, was more soluble than cinnarizine alone. After oral administration of the complex, improvement in cinnarizine availability were less than expected based on in vitro dissolution experiments. It was suggested that cinnarizine was too strongly bound to the cyclodextrin such that complex dissociation was limiting oral availability. Co-administration of phenylalanine, a displacing agent, improved the availability of cinnarizine from the complex but not from conventional tablets of cinnarizine. When cinnarizine was administered to dogs as HP–b-CD or (SBE) 4M –b-CD aqueous solutions of pH 4.5, or as a (SBE) 4M –b-CD/ cinnarizine complex in capsules, oral availability was essentially complete without the need for a displacing agent [34]. That is, dissociation of cinnarizine from these two cyclodextrins was not limiting.

7. Protein binding The effects of protein binding facilitating drug dissociation from cyclodextrin complexes was easily simulated (Section 4 of this paper). Proof that it actually occurs in vivo is not easily established. Frijlink et al. [26] studied the effects of HP–b-CD

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on the displacement of both naproxen and flurbiprofen from plasma binding sites in vitro. Relatively high concentrations of the cyclodextrin did indeed free drug from protein binding. Thus the assumption can be made that the reverse occurs. After parenteral administration of both naproxen and flurbiprofen to rats as either HP–b-CD solutions or as plasma solutions, tissue distribution was analyzed 10 and 60 min post-dosing. No differences in tissue distribution at both time points were seen for naproxen. Flurbiprofen levels were higher in brain, liver, kidney and spleen from the cyclodextrin solution than from the plasma solution, 10 min post-dosing. The biggest difference was seen in the liver, 7.3960.54 versus 4.9760.67 mg / g, respectively. At 60 min, only the brain showed a significant difference, 0.27560.027 versus 0.21860.043 mg / g, respectively. It would have been instructive to have seen results from a second control experiment where both drugs were administered in purely aqueous solutions. The pH would have to have been raised or another enabling agent may have been necessary to increase the solubility of the molecules. Note, there was actually higher tissue levels from the cyclodextrin solutions, meaning that more drug was free to distribute to the tissues than from the plasma solution suggesting that drug from the cyclodextrin solutions were actually more readily available compared to the plasma solutions.

8. Drug uptake into tissue A potential contributing mechanism for drug release from cyclodextrin is preferential drug uptake by tissues. Scheme 3 illustrates this point. If the drug is lipophilic and has access to tissue not available to the cyclodextrin or the complex, the tissue then acts as a ‘sink’ causing dissociation of the complex based on simple mass action principles. This mechanism may become most relevant for highly bound drugs or when the complex is administered at a site where dilution is minimal, e.g., after ocular, nasal, sublingual, pulmonary, dermal or rectal administration. The literature is rich with examples of cyclodextrin complexes administered at these sites. Some key examples have been described in recent reviews [22,23]. A few examples where drug uptake into the tissue

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Scheme 3. Selective tissue uptake facilitates complex dissociation.

appears to be a relevant release mechanism follow. This mechanism most likely applies only to drugs whose physicochemical properties allow them to rapidly diffuse through biological membranes or for which the membrane possesses a specific transporter / receptor. The complex and the cyclodextrin itself should not be significantly absorbed by these tissues. Some complexes have been shown to be readily absorbed when administered via pulmonary, rectal or dermal (with occlusion) routes [22] probably through paracellular pathways. Also, some cyclodextrin derivatives have absorption enhancing capabilities. This probably results from membrane disruption due primarily to extraction of membrane components [35,36]. The permeation enhancing effects of some cyclodextrins will not be discussed here. Cyclodextrins have been used in ophthalmic delivery of poorly water soluble drugs to increase their solubility and / or stability in the tear fluid and in some cases to decrease irritation [22,39]. Due to solubility increase, complexation also results in a significant increase in drug bioavailability compared to suspensions (common ophthalmic formulations for low water soluble drugs) although this is not always the case [41]. Upon ocular administration, the fate of the complex may be illustrated by Scheme 4. The free, lipophilic drug is transported across the cornea in competition with elimination with the free cyclodextrin and the complex via the nasolacrimal drainage [37,38]. By the laws of mass action, the absorption rate will be dependent on cyclodextrin concentration. Maximal corneal absorption will occur when the concentration of cyclodextrin is minimized to effect formulation, while excess cyclodextrin

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of the cyclodextrin appearing in the urine in the first 1–4 h, post dosing [24,44].

9.1. Increased renal excretion of lipophilic drugs

Scheme 4. Scheme showing the various pathways for corneal drug transport and precorneal loss due to nasolacrimal drainage.

concentrations inhibit absorption [21,37,39,40,42, 43]. Since precomeal fluid is removed rapidly by nasolacrimal duct drainage, significant bioavailability improvement can occur only if rapid complex dissociation followed by fast drug absorption across the cornea occurs. HP–b-CD and (SBE)–b-CD solutions do not alter the cornea permeability and have been recognized as being safe for ophthalmic administration [36,39]. Other drug administration routes where dilution is limited exhibit similar characteristics as the one described for ophthalmic delivery. The only difference being that, for some of the other routes, the residence or exposure time to the tissue is not as limited as for ophthalmic use. In summary, drug tissue uptake may play a major role in complex dissociation when dilution, competitive displacement and protein binding effects may be limited.

9. Cyclodextrin elimination The two unmodified cyclodextrins, a- and bcyclodextrin as well as a number of their alkylated derivatives exhibit significant renal toxicity and are therefore only used nonparenterally [24]. HP–b-CD and (SBE) 7M –b-CD appear to be quite safe and are therefore used both for both parenteral and nonparenteral drug delivery [24,44]. Both HP–b-CD and (SBE) 7M –b-CD are quantitatively cleared unchanged by renal filtration and have volumes of distribution approximately equivalent to extracellular water after parenteral administration [24,44]. Following i.v. administration, these cyclodextrins have halflives of 1–2 h (specie dependent) with the majority

Considering Scheme 1, incomplete dissociation of a drug / cyclodextrin complex could result in increased renal clearance of unchanged drug resulting in a larger than normal fraction of unchanged drug appearing in the urine. However, based on the conclusions in the earlier sections, it is highly likely that most drugs stay minimally associated with the cyclodextrin following parenteral administration. Increased appearance of unchanged drug in the urine, however, may occur via a different mechanism than incomplete dissociation. For example, HP–bCD and (SBE) 7M –b-CD are cleared by filtration as is every other non-plasma protein bound small molecule. Subsequent water reabsorption in the proximal and distal tubules leads to about a hundred fold increase in the concentration of filtered molecules. During this step, lipophilic drugs normally undergo passive reabsorption while polar drug molecules and materials like HP–b-CD and (SBE) 7M –bCD are simply concentrated. The concentration step, as opposed to dilution, encourages complex formation between the renally cleared cyclodextrin and any lipophilic molecule remaining in the kidney tubules. Since the complex is polar, the presence of the cyclodextrins will inhibit passive reabsorption of lipophilic drugs resulting in greater renal clearance of lipophilic molecules. Fig. 8 is a plot of the appearance of the cumulative amount, as a percentage of dose and / or percent absorbed, of unchanged carbamazepine in the urine of dogs against time following either i.v. administration as a HP–b-CD formulation or orally [45]. Clearly, a greater percent of carbamazepine appeared in the urine from the i.v. formulation, even correcting for the incomplete availability from the oral dosage form. Note, the increased appearance of carbamazepine occurred in the first two hours when the cyclodextrin itself was probably undergoing renal elimination. After two hours, the urinary excretion rate appeared to be independent of the dosage form. As a percent of the total dose administered, the renal route still accounts for only a small fraction of the

V. J. Stella et al. / Advanced Drug Delivery Reviews 36 (1999) 3 – 16

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most of the increased renal excretion occurred in the first few hours and that only a small percentage of the total dose appeared in the urine from either dosage form. HP–b-CD did not alter the plasma pharmacokinetics of dexamethasone [46].

10. Change in ionic state and temperature

Fig. 8. Cumulative urinary excretion of carbamazepine after i.v. administration of 20 mg / kg as a 10 mg / ml solution in 22.5% mM HP–b-CD (h), or orally as a suspension (n, and s) to five dogs. From the work of Brewster et al. [45].

total elimination, ,5%, with the majority of the drug eliminated via metabolism. A similar observation to that for carbamazepine was seen by Dietzel et al. [46] following the i.v. administration of dexamethasone in either a HP–bCD formulation or as its phosphate prodrug, see Fig. 9. Because the plasma AUC values for dexamethasone from the two delivery forms were not significantly different, it was safe to conclude that the increased appearance of unchanged dexamethasone in urine was due to the cyclodextrin. Again, note that

For weak electrolytes, the strength of binding to a cyclodextrin is dependent on the charged state of the drug, which is dependent on dissociation constant / s of the drug and the pH of the environment. For most molecules, the ionized or charged form of the molecule has poorer binding to cyclodextrins compared to the non-ionized or neutral form of the drug especially when bound to a neutral cyclodextrin [31]. Weak acids and bases can form complexes with various cyclodextrins as shown in Schemes 5 and 6, where Ka and Ka9 are the ionization constants for the free and bound form, and K 19 and K 29 are the complexation constants for the unionized and ionized

Scheme 5. Scheme showing the various equilibria involved in the binding of an acidic drug, AH, to a cyclodextrin, CyD, and the effect of drug ionization.

Fig. 9. Cumulative urinary excretion of dexamethasone after i.v. administration of 5 mg / kg as a 25 mg / ml solution in 40% mM HP–b-CD (h), or as its phosphate ester prodrug (s) to six dogs. From the work of Dietzel et al. [46].

Scheme 6. Scheme showing the various equilibria involved in the binding a basic drug, B, to a cyclodextrin, CyD, and the effect of drug protonation.

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form of either a weak acid (AH) or base (B), respectively. For most weak electrolytes forming complexes with neutral cyclodextrins, the ionized or charged form of a molecule shows a poorer binding compared to the non-ionized or neutral form [31,47– 51]. For example, both Okimoto et al. [31], and Krishnamoorthy and Mitra [50], demonstrated decreased complexation of molecules with HP–b-CD as the substrate becomes ionized. This decrease in stability was attributed to the overall increased hydrophilicity of the substrate upon ionization which reduces the substrates interaction with the more hydrophobic cavity of the cyclodextrin. Therefore, conditions favoring the ionization of drug substances will increase the fraction of non-complexed molecules in solution. If the cyclodextrin is itself charged, this effect may be even more apparent when the charge of the drug and cyclodextrin are similar, resulting in coulombic repulsion. If the charge on the drug and the cyclodextrin are opposite, a coulombic attraction component to the binding may exist such that the binding may not change dramatically as the charge on the drug molecule changes with pH [31,53]. For example, the complexation of negatively charged molecules to cyclodextrins was improved by creating a cyclodextrin torus which was positively charged [52,53]. The increased stability was attributed to a coulombic attraction of the opposite charges. The effect of substrate charge on complexation has also been reported for a negatively charged cyclodextrin [31]. Release of drugs from cyclodextrins which are themselves weak electrolytes such as O-carboxymethyl-O-ethyl b-cyclodextrin is also pH dependent [54,55]. The combination of drug ionization and the use of cyclodextrins to improve solubility and stability of molecules has been studied [56,57]. This technique requires a balance between the ionic state of the molecule and the ability of the molecule to complex with a cyclodextrin, since upon ionization of a drug, the aqueous solubility improves, however the ability to form complexes with cyclodextrins deteriorates. In summary, it is possible that a situation may be envisioned whereby a drug, formulated as a cyclodextrin complex, may dissociate due to a decrease in K as the complex is exposed to a pH which changes either the charge status of the drug or the cyclodextrin.

Additionally, binding of substrates to cyclodextrins has been shown to be an exothermic process (DH is negative) [58,59]. Hence, any increase in temperature results in weakening of the complex, and thus increasing the free fraction of substrate. Most drug / cyclodextrin complexes are prepared and stored at or below room temperature. Since, normal body tissue temperatures can be as high as 378C, another contributing factor to drug dissociation, in vivo, may be this temperature difference. However, the extent of this effect will be dependent on the DH value for the specific complex.

11. Conclusions Upon administration, drugs appear to be rapidly and quantitatively released from cyclodextrin inclusion complexes, especially following oral and parenteral administration. Dissociation due to dilution appears to be the major release mechanism although other factors such as competitive displacement of the drug from the complex, drug binding to plasma and tissue components, uptake of the drug by tissues not available to the complex or cyclodextrin, and cyclodextrin elimination may also contribute for more strongly bound drugs. Some possible perturbations in early time point pharmacokinetics after i.v. dosing have been seen for highly bound drugs, but in some of these studies, questions exist about the appropriateness of the control formulations used for comparisons. Local drug administration and release from matrices can be affected by the presence of cyclodextrins. Because cyclodextrins themselves are rapidly renally cleared, some increase in the renal clearance of lipophilic drugs may be seen.

Acknowledgements We would like to thank Drs. Diane Thompson and Roger Rajewski for their contributions to the sulfoalkylether cyclodextrin development. We would also like to acknowledge the Kansas Technology Enterprise Corporation, Centers of Excellence program, the National Cancer Institute and The University of Kansas for their financial support of our cyclodextrin research.

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