Beta-cyclodextrins enhance bioavailability of pilocarpine

Antinociceptive Effect of a p-Cymene/β-Cyclodextrin Inclusion Complex in a Murine Cancer Pain Model: Characterization Aided through a Docking Study

Pain is one of the most prevalent and difficult to manage symptoms in cancer patients, and conventional drugs present a range of adverse reactions. The development of β-cyclodextrins (β-CD) complexes has been used to avoid physicochemical and pharmacological limitations due to the lipophilicity of compounds such as p-Cymene (PC), a monoterpene with antinociceptive effects. Our aim was to obtain, characterize, and measure the effect of the complex of p-cymene and β-cyclodextrin (PC/β-CD) in a cancer pain model. Initially, molecular docking was performed to predict the viability of complex formation. Afterward, PC/β-CD was obtained by slurry complexation, characterized by HPLC and NMR. Finally, PC/β-CD was tested in a Sarcoma 180 (S180)-induced pain model. Molecular docking indicated that the occurrence of interaction between PC and β-CD is favorable. PC/β-CD showed complexation efficiency of 82.61%, and NMR demonstrated PC complexation in the β-CD cavity. In the S180 cancer pain model, PC/β-CD significantly reduced the mechanical hyperalgesia, spontaneous nociception, and nociception induced by non-noxious palpation at the doses tested (p < 0.05) when compared to vehicle differently from free PC (p > 0.05). Therefore, the complexation of PC in β-CD was shown to improve the pharmacological effect of the drug as well as reducing the required dose.

Cyclodextrins: structure, physicochemical properties and pharmaceutical applications

Since their discovery over 100 years ago cyclodextrins (CDs) have been the subject of numerous scientific publications. In 2016 alone CDs were the subject of over 2200 research articles published in peer-reviewed journals and mentioned in over 2300 patents and patent applications, many of which were on pharmaceutical applications. Natural CDs and their derivatives are used as enabling pharmaceutical excipients that enhance aqueous solubility of poorly soluble drugs, increase drug permeability through biological membranes and improve drug bioavailability. Unlike conventional penetration enhancers, their hydrophilic structure and high molecular weight prevents them from penetrate into lipophilic membranes leaving biological membranes intact. The natural CDs and some of their derivatives have monographs in pharmacopeias and are also commonly used as food additives and in toiletry products. CDs form inclusion complexes with lipophilic moieties of hydrophobic drugs. Furthermore, CDs are able to form non-inclusion complexes and self-assembled aggregates; small and large complex aggregates with micellar-like structures that can enhance drug solubility. Excipients commonly used in pharmaceutical formulations may have additive or inhibiting effect on the CD solubilization. Here various methods used to investigate CD aggregate formation are reviewed as well as techniques that are used to increase the solubilizing effects of CDs; methods that enhance the apparent intrinsic solubility of drugs and/or the complexation efficacy and decrease the amount of CD needed to develop CD-containing pharmaceutical formulations. It will be explained how too much or too little CD can hamper drug bioavailability, and the role of CDs in solid dosage forms and parenteral formulations, and examples given on how CDs can enhance drug delivery after ocular, nasal and pulmonary administration.

Cyclodextrins and topical drug delivery to the anterior and posterior segments of the eye

It is generally believed that it is virtually impossible to obtain therapeutic drug concentrations in the posterior segment of the eye after topical application of aqueous, low viscosity eye drops. Thus, intravitreal drug injections and drug implants are currently used to treat diseases in the posterior segment such as macular edema. Here it is described how, through proper analysis of the drug permeation barriers and application of well-known pharmaceutical excipients, aqueous eye drops are designed that can deliver lipophilic drugs to the posterior segment as well as how such eye drops can maintain high drug concentrations in the anterior segment. Through stepwise optimization, eye drops containing solid drug/cyclodextrin complex microparticles with a mean diameter of 2-4μm, dissolved drug/cyclodextrin complex nanoparticles and dissolved drug molecules in an aqueous eye drop media of low viscosity were designed. After administration of the eye drops the microparticles slowly dissolved and maintained close to saturated drug concentrations in the aqueous tear fluid for several hours. Studies in rabbits and clinical evaluations in humans, using dorzolamide and dexamethasone as sample drugs, show that the eye drops deliver significant amounts of drugs to both the posterior segment and anterior segment of the eye. Clinical studies indicate that the eye drops can replace intravitreal injections and implants that are currently used to treat ophthalmic diseases and decrease frequency of drug administration, both of which can improve patient compliance.

Pharmaceutical applications of cyclodextrins: effects on drug permeation through biological membranes

Objectives

Cyclodextrins are useful solubilizing excipients that have gained currency in the formulator's armamentarium based on their ability to temporarily camouflage undesirable physicochemical properties. In this context cyclodextrins can increase oral bioavailability, stabilize compounds to chemical and enzymatic degradation and can affect permeability through biological membranes under certain circumstances. This latter property is examined herein as a function of the published literature as well as work completed in our laboratories.

Key findings

Cyclodextrins can increase the uptake of drugs through biological barriers if the limiting barrier component is the unstirred water layer (UWL) that exists between the membrane and bulk water. This means that cyclodextrins are most useful when they interact with lipophiles in systems where such an UWL is present and contributes significantly to the barrier properties of the membrane. Furthermore, these principles are used to direct the optimal formulation of drugs in cyclodextrins. A second related critical success factor in the formulation of cyclodextrin-based drug product is an understanding of the kinetics and thermodynamics of complexation and the need to optimize the cyclodextrin amount and drug-to-cyclodextrin ratios. Drug formulations, especially those targeting compartments associated with limited dissolution (i.e. the eye, subcutaneous space, etc.), should be carefully designed such that the thermodynamic activity of the drug in the formulation is optimal meaning that there is sufficient cyclodextrin to solubilize the drug but not more than that. Increasing the cyclodextrin concentration decreases the formulation ‘push’ and may reduce the bioavailability of the system.

Conclusions

A mechanism-based understanding of cyclodextrin complexation is essential for the appropriate formulation of contemporary drug candidates.

Recent perspectives in ocular drug delivery

Anatomy and physiology of the eye makes it a highly protected organ. Designing an effective therapy for ocular diseases, especially for the posterior segment, has been considered as a formidable task. Limitations of topical and intravitreal route of administration have challenged scientists to find alternative mode of administration like periocular routes. Transporter targeted drug delivery has generated a great deal of interest in the field because of its potential to overcome many barriers associated with current therapy. Application of nanotechnology has been very promising in the treatment of a gamut of diseases. In this review, we have briefly discussed several ocular drug delivery systems such as microemulsions, nanosuspensions, nanoparticles, liposomes, niosomes, dendrimers, implants, and hydrogels. Potential for ocular gene therapy has also been described in this article. In near future, a great deal of attention will be paid to develop non-invasive sustained drug release for both anterior and posterior segment eye disorders. A better understanding of nature of ocular diseases, barriers and factors affecting in vivo performance, would greatly drive the development of new delivery systems. Current momentum in the invention of new drug delivery systems hold a promise towards much improved therapies for the treatment of vision threatening disorders.

Intraocular availability of topically applied mycophenolate mofetil in rabbits

The efficacy of mycophenolate mofetil (MMF), a prodrug of mycophenolic acid (MPA), in high risk keratoplasties and ocular immune-mediated diseases has been shown in recent years. As peroral administration of MMF can cause various side effects, topical application should be considered. This study investigates the intraocular availability of MMF and MPA in the rabbit eye. An eye drop solution (MMF-CD; 1% MMF/10% hydroxypropyl-beta-cyclodextrin (HP-beta-CD)) or a 1% aqueous suspension (MMF-SP) was instilled into the lower cul-de-sac of the right eye of each rabbit. Rabbits (each group: n = 4) were put down after 30, 60 and 240 min. Aqueous humor, vitreous, cornea, sclera, conjunctiva, iris-ciliary-body, and plasma were isolated. Several extraction procedures were performed in order to quantify the drug by HPLC. The aqueous humor concentration of the active metabolite MPA was 24 microg/mL after 30 min for both preparations. The ratio of the MPA concentrations after 30, 60, and 240 min was 1 : 2 : 0.07 for MMF-CD and 1 : 0.6 : 0.04 for MMF-SP, respectively. MPA levels in the cornea were 90.78 / 56.90 / 4.08 x 10(-6) microg/microg for MMF-CD, whereas MMF-SP resulted in MPA levels of 102.65 / 31.18 / 2.59 x 10(-6) microg/microg at the three time points. As a high concentration of the active drug MPA in cornea and aqueous humor is desired, e.g. following corneal transplantation, the MMF/HP-beta-CD formulation could be an useful topical treatment. Furthermore, the present study shows that MMF-CD is superior to MMF-SP by increasing intraocular availability.

Influence of hydroxypropyl beta-cyclodextrin on the corneal permeation of pilocarpine

The influence of hydroxypropyl beta-cyclodextrin (HPbetaCD) on the corneal permeation of pilocarpine nitrate was investigated by an in vitro permeability study using isolated rabbit cornea. Pupillary-response pattern to pilocarpine nitrate with and without HPbetaCD was examined in rabbit eye. Corneal permeation of pilocarpine nitrate was found to be four times higher after adding HPbetaCD into the formulation. The reduction of pupil diameter (miosis) by pilocarpine nitrate was significantly increased as a result of HPbetaCD addition into the simple aqueous solution of the active substance. The highest miotic response was obtained with the formulation prepared in a vehicle of Carbopol 940. It is suggested that ocular bioavailability of pilocarpine nitrate could be improved by the addition of HPbetaCD.

Cannabinoids in the treatment of glaucoma

The leading cause of irreversible blindness is glaucoma, a disease normally characterized by the development of ocular hypertension and consequent damage to the optic nerve at its point of retinal attachment. This results in a narrowing of the visual field, and eventually results in blindness. A number of drugs are available to lower intraocular pressure (IOP), but, occasionally, they are ineffective or have intolerable side-effects for some patients and can lose efficacy with chronic administration. The smoking of marijuana has decreased IOP in glaucoma patients. Cannabinoid drugs, therefore, are thought to have significant potential for pharmaceutical development. However, as the mechanism surrounding their effect on IOP initially was thought to involve the CNS, issues of psychoactivity hindered progress. The discovery of ocular cannabinoid receptors implied an explanation for the induction of hypotension by topical cannabinoid applications, and has stimulated a new phase of ophthalmic cannabinoid research. Featured within these investigations is the possibility that at least some cannabinoids may ameliorate optic neuronal damage through suppression of N-methyl-D-aspartate receptor hyperexcitability, stimulation of neural microcirculation, and the suppression of both apoptosis and damaging free radical reactions, among other mechanisms. Separation of therapeutic actions from side-effects now seems possible through a diverse array of novel chemical, pharmacological, and formulation strategies.

Ocular preparations: the formulation approach

The main aim of pharmacotherapeutics is the attainment of an effective drug concentration at the intended site of action for a sufficient period of time to elicit the response. A major problem being faced in ocular therapeutics is the attainment of an optimal concentration at the site of action. Poor bioavailability of drugs from ocular dosage forms is mainly due to the tear production, non-productive absorption, transient residence time, and impermeability of corneal epithelium. This article reviews: (1) the barriers that decrease the bioavailability of an ophthalmic drug; (2) the objectives to be considered in producing optimal formulations; and (3) the approaches being used to improve the corneal penetration of a drug molecule and delay its elimination from the eye. The focus of this review is on the recent developments in topical ocular drug delivery systems, the rationale for their use, their drug release mechanism, and the characteristic advantages and limitations of each system. In addition, the review attempts to give various analytical procedures including the animal models and other models required for bioavailability and pharmacokinetic studies. The latter can aid in the design and predictive evaluation of newer delivery systems. The dosage forms are divided into the ones which affect the precorneal parameters, and those that provide a controlled and continuous delivery to the pre- and intraocular tissues. The systems discussed include: (a) the commonly used dosage forms such as gels, viscosity imparting agents, ointments, and aqueous suspensions; (b) the newer concept of penetration enhancers, phase transition systems, use of cyclodextrins to increase solubility of various drugs, vesicular systems, and chemical delivery systems such as the prodrugs; (c) the developed and under-development controlled/continuous drug delivery systems including ocular inserts, collagen shields, ocular films, disposable contact lenses, and other new ophthalmic drug delivery systems; and (d) the newer trends directed towards a combination of drug delivery technologies for improving the therapeutic response of a non-efficacious drug. The fruitful resolution of the above-mentioned technological suggestions can result in a superior dosage form for both topical and intraocular ophthalmic application.

Cyclodextrins in eye drop formulations: enhanced topical delivery of corticosteroids to the eye

Cyclodextrins are cylindrical oligosaccharides with a lipophilic central cavity and hydrophilic outer surface. They can form water-soluble complexes with lipophilic drugs, which 'hide' in the cavity. Cyclodextrins can be used to form aqueous eye drop solutions with lipophilic drugs, such as steroids and some carbonic anhydrase inhibitors. The cyclodextrins increase the water solubility of the drug, enhance drug absorption into the eye, improve aqueous stability and reduce local irritation. Cyclodextrins are useful excipients in eye drop formulations of various drugs, including steroids of any kind, carbonic anhydrase inhibitors, pilocarpine, cyclosporins, etc. Their use in ophthalmology has already begun and is likely to expand the selection of drugs available as eye drops. In this paper we review the properties of cyclodextrins and their application in eye drop formulations, of which their use in the formulation of dexamethasone eye drops is an example. Cyclodextrins have been used to formulate eye drops containing corticosteroids, such as dexamethasone, with levels of concentration and ocular absorption which, according to human and animal studies, are many times those seen with presently available formulations. Cyclodextrin-based dexamethasone eye drops are well tolerated in the eye and seem to provide a higher degree of bioavailability and clinical efficiency than the steroid eye drop formulations presently available. Such formulations offer the possibility of once per day application of corticosteroid eye drops after eye surgery, and more intensive topical steroid treatment in severe inflammation. While cyclodextrins have been known for more than a century, their use in ophthalmology is just starting. Cyclodextrins are useful excipients in eye drop formulations for a variety of lipophilic drugs. They will facilitate eye drop formulations for drugs that otherwise might not be available for topical use, while improving absorption and stability and decreasing local irritation.

Cyclodextrins in topical drug formulations: theory and practice

Cyclodextrins are cyclic oligosaccharides with a hydrophilic outer surface and a somewhat lipophilic central cavity. Cyclodextrins are able to form water-soluble inclusion complexes with many lipophilic water-insoluble drugs. In aqueous solutions drug molecules located in the central cavity are in a dynamic equilibrium with free drug molecules. Furthermore, lipophilic molecules in the aqueous complexation media will compete with each other for a space in the cavity. Due to their size and hydrophilicity only insignificant amounts of cyclodextrins and drug/cyclodextrin complexes are able to penetrate into lipophilic biological barriers, such as intact skin. In general, cyclodextrins enhance topical drug delivery by increasing the drug availability at the barrier surface. At the surface the drug molecules partition from the cyclodextrin cavity into the lipophilic barrier. Thus, drug delivery from aqueous cyclodextrin solutions is both diffusion controlled and membrane controlled. It appears that cyclodextrins can only enhance topical drug delivery in the presence of water.

Influence of cyclodextrins on the in vitro corneal permeability and in vivo ocular distribution of thalidomide

The aim of the present study has been to develop aqueous Thalidomide (THA) eye drops in order to minimize systemic side effects and to improve bioavailability following topical application. Cyclodextrins (CDs), suitable vehicles to improve aqueous solubility of THA, were evaluated with regard to their ability to influence in vitro corneal permeability of THA. Additionally, rabbit eyes received either THA-suspension (0.04%) (THA-SP) or THA (0.04%)/hydroxypropyl-beta-cyclodextrin (HP-beta-CD) (12.5%) solution (THA-CD). In vitro corneal permeation studies demonstrated that the absolute amount of THA permeated could not be increased by means of CDs. The percentile release of THA was extensively decreased using saturated THA/CD solutions. Following loading doses of either THA-CD or THA-SP onto the rabbit eye, significantly increased aqueous humor levels were obtained for THA-CD 30 min (THA-CD:THA-SP=4.6:1) and 60 min (THA-CD:THA-SP=3.1:1) post instillation (p<0.05). In the iris-ciliary body, significantly increased THA levels were obtained using THA suspension (THA-CD(60 min):THA-SP(60 min)=1:6.1) (p<0.05). In the cornea, conjunctiva, vitreous and sclera, differences between the THA tissue levels were not statistically significant. Cyclodextrins might be a useful tool to formulate aqueous THA eye drop solutions and modify intraocular drug bioavailability.

Ophthalmic drug delivery systems--recent advances

Eye-drops are the conventional dosage forms that account for 90% of currently accessible ophthalmic formulations. Despite the excellent acceptance by patients, one of the major problems encountered is rapid precorneal drug loss. To improve ocular drug bioavailability, there is a significant effort directed towards new drug delivery systems for ophthalmic administration. This chapter will focus on three representative areas of ophthalmic drug delivery systems: polymeric gels, colloidal systems, cyclodextrins and collagen shields. Hydrogels generally offer a moderate improvement of ocular drug bioavailability with the disadvantage of blurring of vision. In situ activated gel-forming systems are preferred as they can be delivered in drop form with sustained release properties. Colloidal systems including liposomes and nanoparticles have the convenience of a drop, which is able to maintain drug activity at its site of action and is suitable for poorly water-soluble drugs. Among the new therapeutic approaches in ophthalmology, cyclodextrins represent an alternative approach to increase the solubility of the drug in solution and to increase corneal permeability. Finally, collagen shields have been developed as a new continuous-delivery system for drugs that provide high and sustained levels of drugs to the cornea, despite a problem of tolerance. It seems that new tendency of research in ophthalmic drug delivery systems is directed towards a combination of several drug delivery technologies. There is a tendency to develop systems which not only prolong the contact time of the vehicle at the ocular surface, but which at the same time slow down the elimination of the drug. Combination of drug delivery systems could open a new directive for improving results and the therapeutic response of non-efficacious systems.

Influence of alpha-cyclodextrin and hydroxyalkylated beta-cyclodextrin derivatives on the in vitro corneal uptake and permeation of aqueous pilocarpine-HCl solutions

Interactions in aqueous solution between pilocarpine hydrochloride (P-HCl), a rather hydrophilic drug with good water solubility, and various cyclodextrins (CDs) were described recently. To assess the influence of CDs on the diffusion behavior of pilocarpine, in vitro studies were performed using porcine or bovine corneas as diffusion barriers. The affinity of P-HCl for porcine cornea in the presence of alpha-cyclodextrin (alpha-CD) and (hydroxyethyl)-beta-cyclodextrin (HE-beta-CD) was determined by drug uptake experiments. Additionally, in vitro permeation experiments through bovine corneas were conducted with a modified diffusion device optimized for corneal perfusion studies. The results obtained from the corneal uptake studies indicate that the addition of alpha-CD led to increased tissue drug levels. The increase in permeability of pilocarpine in the presence of alpha-CD was approximately 10-fold (log Papp = -4.87 +/- 0.03) in comparison with plain P-HCl solution (log Papp = -5.89 +/- 0.06). Permeation studies with corneas pretreated with alpha-CD solution revealed enhanced corneal permeability of pilocarpine due to alpha-CD induced membrane effects. The hydroxyalkylated beta-CD derivatives HE-beta-CD (log Papp = -6.27 +/- 0.09) and (hydroxypropyl)-beta-cyclodextrin (HP-beta-CD; log Papp = -6.40 +/- 0.03), however, seemed to cause slightly decreased permeation rates, supporting the concept of an interaction between pilocarpine and the hydroxyalkylated-beta-CD derivatives. Considering physiological compatibility, the addition of CDs seems to be an effective tool to modify and optimize the ocular availability of pilocarpine.

Cyclodextrins: their future in drug formulation and delivery

Since their discovery, cyclodextrins and their ability to form inclusion complexes have fascinated chemists, formulators and recently, entrepreneurs. This mini-review has as its objective, a critical assessment of the current status of cyclodextrins in the formulation and delivery of pharmaceuticals and commentary on their potential future uses. The emphasis will be on answers to common questions often asked of pharmaceutical scientists working in this area. Why use cyclodextrins for drug solubilization and stabilization when alternative techniques are available? Why the greater interest in modified cyclodextrins and not the parent cyclodextrins? If a drug forms a strong cyclodextrin inclusion complex, how is the drug released in vivo? Dose the injection of a cyclodextrin/drug complex alter the pharmacokinetics of the drug? Are there drug products on the market which contain cyclodextrins? What is the regulatory status of cyclodextrins? Although definitive answers to all these questions are not possible at this time, many of these questions are answerable, and educated and informed responses are possible for the rest.

Pharmaceutical applications of cyclodextrins. III. Toxicological issues and safety evaluation

The objective of this review is to summarize recent findings on the safety profiles of three natural cyclodextrins (alpha-, beta- and gamma-CDs) and several chemically modified CDs. To demonstrate the potential of CDs in pharmaceutical formulations, their stability against non-enzymatic and enzymatic degradations in various body fluids and tissue homogenates and their pharmacokinetics via parenteral, oral, transmucosal, and dermal routes of administration are outlined. Furthermore, the bioadaptabilities of CDs, including in vitro cellular interactions and in vivo safety profiles, via a variety of administration routes are addressed. Finally, the therapeutic potentials of CDs are discussed on the basis of their ability to interact with various endogenous and exogenous lipophiles or, especially for sulfated CDs, their effects on cellular processes mediated by heparin binding growth factors.

Pharmaceutical applications of cyclodextrins. 2. In vivo drug delivery

The objective of this Review is to summarize and critique recent findings and applications of both unmodified and modified cyclodextrins for in vivo drug delivery. This review focuses on the use of cyclodextrins for parenteral, oral, ophthalmic, and nasal drug delivery. Other routes including dermal, rectal, and pulmonary delivery are also briefly addressed. This Review primarily focuses on newer findings concerning cyclodextrin derivatives which are likely to receive regulatory acceptance due to improved aqueous solubility and safety profiles as compared to the unmodified cyclodextrins. Many of the applications reviewed involve the use of hydroxypropyl-beta-cyclodextrins (HP-beta-CDs) and sulfobutylether-beta-cyclodextrins (SBE-beta-CDs) which show promise of greater safety while maintaining the ability to form inclusion complexes. The advantages and limitations of HP-beta-CD, SBE-beta-CD, and other cyclodextrins are addressed.

Ophthalmic arachidonylethanolamide decreases intraocular pressure in normotensive rabbits

Arachidonylethanolamide (AEA) was the first anandamide to be identified as an endogenous ligand for the cannabinoid receptor of porcine brain. Since cannabinoids have shown some value in the reduction of ocular hypertension, the title compound was evaluated in normotensive rabbits as a possible topically applied agent for reducing intraocular pressure. AEA was dissolved in an aqueous solution of 2-hydroxy-propyl-beta-cyclodextrin. Single eyedrops (25 microliters) containing 3.13, 6.25, 31.25, 62.5 or 125.0 micrograms of AEA were instilled unilaterally into eyes of normotensive albino and pigmented rabbits. The intraocular pressures (IOPs) of these rabbits were then measured at fixed time intervals. The effect of AEA on IOP in treated and untreated (contralateral) eyes was similar in both types of rabbits. Administration of 31.25 micrograms of AEA caused an immediate IOP reduction in the treated eyes. AEA doses of 62.5 micrograms caused an initial increase and subsequent decrease of IOP in the treated eyes. In the untreated eyes, a marginal ocular hypotensive response of limited duration occurred immediately after administration of AEA at doses 31.25 or 62.5 micrograms. A significant increase (without subsequent decrease below baseline) in IOP occurred in treated eyes after a dose of 125.0 micrograms. The lowest dose (3.13 micrograms) did not have an effect on IOP. This study constitutes the first published demonstration that topical, unilateral administration of AEA significantly decreases IOP in normotensive albino and pigmented rabbits. Although the mechanism of action by which this compound produces its hypotensive effect in the eye is not known, the results suggest that AEA may prove useful in the investigation of glaucoma therapy.

Hydroxypropyl-beta-cyclodextrin increases the aqueous solubility and stability of pilocarpine prodrugs

PURPOSE

The effects of 2-hydroxypropyl-beta-cyclodextrin (HP-beta-CD) on the aqueous solubility and stability of two lipophilic bispilocarpine prodrugs were investigated at pH 7.4.

METHODS

The solubility of prodrugs was studied by phase-solubility method (0-72.5 mM HP-beta-CD). The stability of one of the prodrugs was investigated as a function of temperature (40 degrees C-70 degrees C) and HP-beta-CD concentration (0-72.5 mM). The apparent rate constants (k1, k2) for degradation of prodrug in 1:1 and 1:2 inclusion complexes and apparent stability constants (K1:1, K1:2) were calculated by the curve-fitting method.

RESULTS

The phase-solubility diagrams were classified as Ap-type and the apparent stability constants (K1:1, K1:2) for 1:1- and 1:2-inclusion complexes were calculated to be 143-815 M-1 and 29-825 M-1, respectively. The stability of prodrug increased as a function of HP-beta-CD concentration over the studied temperature range. The shelf-life (t90%, calculated by the Arrhenius equation) of the prodrug in 72.5 mM HP-beta-CD solution increased 5.1-fold and 6.1-fold at 25 degrees C and 4 degrees C, respectively.

CONCLUSIONS

The solubility of the prodrugs was shown to increase markedly in phase-solubility studies. The degradation rate of prodrug in stability studies was shown to be slower in the 1:2-complex than in the 1:1-complex and the relative amounts of complex species were found to be dependent on CD concentration.

The effect of a modified beta-cyclodextrin, SBE4-beta-CD, on the aqueous stability and ocular absorption of pilocarpine

In the present study, the effects of a novel, modified beta-cyclodextrin derivative (SBE4-beta-CD; a variably substituted sulfobutyl ether of beta-cyclodextrin with an average degree of substitution of four) on the aqueous stability of pilocarpine and on its ocular absorption in albino rabbits were studied. For stability reasons, commercial pilocarpine eyedrops are formulated at pH 4-5, a pH range where pilocarpine (pKa approximately 7) is almost completely ionized. As shown in the present and past studies, increasing the pH of the pilocarpine solution from 4.5 to 7.0 increases the ocular absorption of pilocarpine. SBE4-beta-CD increased the aqueous stability of pilocarpine (0.36 mM) at pH 7.0 (4 degrees C, projected values from Arrhenius data at 25 degrees C, 37 degrees C and 50 degrees C); in the absence of SBE4-beta CD, t90% was 236 days. In the presence of 1 mM and 25 mM of SBE4-beta-CD, t90% was 382 days and 2054 days, respectively suggesting that indeed, pilocarpine does interact with SBE4-beta-CD. SBE4-beta-CD did not damage the corneal epithelium in vitro and was well-tolerated by the rabbit eye in vivo. Coadministered SBE4-beta-CD did not significantly affect the miotic response of pilocarpine solutions at pH values of 4.5 or 7.0 when the molar ratio of SBE4-beta-CD to pilocarpine was between 0.2:1-7:1. The effect of the coadministered SBE4-beta-CD on the miotic response of pilocarpine solutions was also compared to that of 2-hydroxypropyl-beta-cyclodextrin (HP-beta-CD) which has recently been suggested to increase ocular bioavailability of pilocarpine in rabbits.(ABSTRACT TRUNCATED AT 250 WORDS)