Increase in aqueous solubility, stability and in vitro corneal permeability of anandamide by hydroxypropyl-β-cyclodextrin

Improved Topical Ophthalmic Natamycin Suspension for the Treatment of Fungal Keratitis

Purpose: Natamycin (NT) is used as a first-line antifungal prescription in the treatment of fungal keratitis (FK) and is commercially available as a 5% w/v ophthalmic suspension. NT shows poor water solubility and light sensitivity. Thus, the present investigation is aimed to enhance the fraction of NT in solution in the commercial formulation by adding cyclodextrins (CDs), thereby improving the delivery of the drug into deeper ocular tissues. Methods: The solubility of NT in different CDs, the impact of ultraviolet (UV) light exposure, stability at 4°C and 25°C, in vitro release, and ex vivo transcorneal permeation studies were performed. Results: NT exhibited the highest solubility (66-fold) in randomly methylated-β-cyclodextrin (RM-βCD) with hydroxypropyl-βCD (HP-βCD) showing the next highest solubility (54-fold) increase in comparison to market formulation Natacyn® as control. The stability of NT-CD solutions was monitored for 2 months (last-time point) at both storage conditions. The degradation profile of NT in NT-RM-βCD and NT-HP-βCD solutions under UV-light exposure followed first-order kinetics exhibiting half-lives of 1.2 h and 1.4 h, respectively, an almost 3-fold increase over the control solutions. In vitro release/diffusion studies revealed that suspensions containing RM-βCD and HP-βCD increased transmembrane flux significantly (3.1-fold) compared to the control group. The transcorneal permeability of NT from NT-RM-βCD suspension exhibited an 8.5-fold (P < 0.05) improvement compared to Natacyn eyedrops. Furthermore, the addition of RM-βCD to NT suspension increases the solubilized fraction of NT and enhances transcorneal permeability. Conclusion: Therefore, NT-RM-βCD formulations could potentially lead to a decreased frequency of administration and significantly improved therapeutic outcomes in FK treatment.

A Composite System Based upon Hydroxypropyl Cyclodextrins and Soft Hydrogel Contact Lenses for the Delivery of Therapeutic Doses of Econazole to the Cornea, In Vitro

Fungal keratitis, a disease in which the cornea becomes inflamed due to an invasive fungal infection, remains difficult to treat due in part to limited choices of available treatments. Topical eye drops are first-line treatment, but can be ineffective as low levels of drug reach the target site due to precorneal losses and the impenetrability of the cornea. The aim of this study was to determine the corneal delivery of econazole using a novel topical enhancement approach using a composite delivery system based upon cyclodextrins and soft hydrogel contact lenses. Excess econazole nitrate was added to hydroxypropyl-α-cyclodextrin (HP-α-CD) and hydroxypropyl-β-cyclodextrin (HP-β-CD) solutions, and the solubility determined using HPLC. Proprietary soft hydrogel contact lenses were then impregnated with saturated solutions and applied to freshly enucleated porcine eyeballs. Econazole nitrate ‘eye drops’ at the same concentrations served as the control. After 6 h, the corneas were excised and drug-extracted, prior to quantification using HPLC. Molecular dynamic simulations were performed to examine econazole−HP-β-CD inclusion complexation and dissociation. The minimum inhibitory concentration (MIC) of econazole was determined against four fungal species associated with keratitis, and these data were then related to the amount of drug delivered to the cornea, using an average corneal volume of 0.19 mL. The solubility of econazole increased greatly in the presence of HP-β-CD and more so with HP-α-CD (p < 0.001), with ratios >> 2. Hydrogel contact lenses delivered ×2.8 more drug across the corneas in comparison to eye drops alone, and ×5 more drug delivered to the cornea when cyclodextrin was present. Molecular graphics demonstrated dynamic econazole release, which would create transient enhanced drug concentration at the cornea surface. The solution-only drops achieved the least satisfactory result, producing sub-MIC levels with factors of ×0.81 for both Fusarium semitectum and Fusarium solani and ×0.40 for both Scolecobasidium tshawytschae and Bipolaris hawaiiensis. All other treatments delivered econazole at > MIC for all four fungal species. The efficacies of the delivery platforms evaluated were ranked: HP-α-CD contact lens > HP-β-CD contact lens > contact lens = HP-α-CD drops > HP-β-CD drops > solution-only drops. In summary, the results in this study have demonstrated that a composite drug delivery system based upon econazole−HP-β-CD inclusion complexes loaded into contact lenses can achieve significantly greater corneal drug delivery with the potential for improved clinical responses.

Progress in drug formulation design and delivery of medicinal substances used in ophthalmology

Due to the very low bioavailability of drugs administered to the surface of the eyeball, issues related to the formulation of an ophthalmic drug pose a technological challenge. The essence of an ophthalmic drug is the selection of an appropriate active substance (API), but also auxiliary substances that determine the desired drug quality and API availability. The ophthalmic drug is not only classic eye drops. Therefore, on the basis of the literature data, the properties and application of auxiliary substances increasing the pharmaceutical availability of API, improving the penetration of API into the eye structures and modifying the viscosity of eye drops were characterized. The possibility of chemical modification of API and the use of prodrugs in ophthalmic drug forms was also noted. Taking into account the progress in the field of ophthalmic drug formulation, the use of multi-compartment systems (lipid particles, nanoparticles, microparticles, liposomes, niosomes, dendrimers) and modern ophthalmic drug delivery systems (inserts, implants, microneedles, contact lenses, ionophoretic systems) have been indicated. Examples of solutions already used by manufacturers, as well as those in the phase of laboratory or clinical trials, were indicated.

Cyclodextrin-based formulation of carbonic anhydrase inhibitors for ocular delivery - A review

Carbonic anhydrase inhibitors (CAIs) are used as systemic and topical agents for lowering intraocular pressure (IOP) in patients with glaucoma. Owing to the wide distribution of CAs and their physiological functions in various tissues, systemic administration of CAIs may lead to unwanted side effects. Thus, exploration of drugs targeting the specific CA isoenzyme in ocular tissues and application of the same as topical eye drops would be desirable. However, the anatomical and physiological barriers of the eyes can limit drug availability at the site. The very low aqueous solubility of CAI agents can further hamper drug bioavailability, consequently resulting in insufficient therapeutic efficacy. Solubilization of drugs using cyclodextrin (CD) complexes can enhance both solubility and permeability of the drugs. The use of CD for such purposes and development and testing of topical CAI eye drops containing CD have been discussed in detail. Further, pharmaceutical nanotechnology platforms were discussed in terms of investigation of their IOP-lowering efficacies. Future prospects in drug discovery and the use of CD nanoparticles and CD-based nanocarriers to develop potential topical CAI formulations have also been described here.

Polysaccharides in Ocular Drug Delivery

Polysaccharides, such as cellulose, hyaluronic acid, alginic acid, and chitosan, as well as polysaccharide derivatives, have been successfully used to augment drug delivery in the treatment of ocular pathologies. The properties of polysaccharides can be extensively modified to optimize ocular drug formulations and to obtain biocompatible and biodegradable drugs with improved bioavailability and tailored pharmacological effects. This review discusses the available polysaccharide choices for overcoming the difficulties associated with ocular drug delivery, and it explores the reasons for the dependence between the physicochemical properties of polysaccharide-based drug carriers and their efficiency in different formulations and applications. Polysaccharides will continue to be of great interest to researchers endeavoring to develop ophthalmic drugs with improved effectiveness and safety.

Encapsulating curcumin in ethylene diamine-β-cyclodextrin nanoparticle improves topical cornea delivery

Curcumin is a powerful scavenger of reactive oxygen species and could prevent the corneal cells from oxidative damage. However, the clinical efficacy of curcumin is limited by its low aqueous solubility and stability, leading to poor bioavailability. β-cyclodextrin, with a hydrophilic surface and a hydrophobic cavity and self-assembling properties, can form inclusion complexes with lipophilic drugs such as curcumin for ocular delivery. We synthesized ethylene diamine (EDA)-modified β-cyclodextrin and prepared the curcumin complexation using the solvent evaporation method. The EDA-β-cyclodextrin provided a better thermodynamic stability and higher complex yield for curcumin complexes, compared to β-cyclodextrin, which were demonstrated on the analysis of their van't Hoff plots and phase solubility diagrams. We characterized EDA-β-cyclodextrin curcumin nanoparticles and determined that the EDA modified β-cyclodextrin is a more suitable carrier than parental β-cyclodextrin, using FT-IR, XRD, TEM, and analyses of solubility and storage stability. In addition, the curcumin-EDA-β-cyclodextrin nanoparticles had better in vitro corneal penetration and 3 -h cumulative flux in a porcine cornea experiment, and displayed an improved biocompatibility, confirmed by the histological examination of porcine corneas and cell viability of bovine corneal epithelial cells. These results together revealed a role of EDA modification in the β-cyclodextrin carrier, including the improvement of curcumin complex formation, thermodynamic properties, cytotoxicity, and the in vitro corneal penetration. The EDA-β-cyclodextrin inclusion can provide curcumin a higher degree of aqueous solubility and corneal permeability.

Penetration Enhancers in Ocular Drug Delivery

There are more than 100 recognized disorders of the eye. This makes the development of advanced ocular formulations an important topic in pharmaceutical science. One of the ways to improve drug delivery to the eye is the use of penetration enhancers. These are defined as compounds capable of enhancing drug permeability across ocular membranes. This review paper provides an overview of anatomical and physiological features of the eye and discusses some common ophthalmological conditions and permeability of ocular membranes. The review also presents the analysis of literature on the use of penetration-enhancing compounds (cyclodextrins, chelating agents, crown ethers, bile acids and bile salts, cell-penetrating peptides, and other amphiphilic compounds) in ocular drug delivery, describing their properties and modes of action.

Crown Ethers: Novel Permeability Enhancers for Ocular Drug Delivery?

Crown ethers are cyclic molecules consisting of a ring containing several ether groups. The most common and important members of this series are 12-crown-4 (12C4), 15-crown-5 (15C5), and 18-crown-6 (18C6). These container molecules have the ability to sequester metal ions, and their complexes with drugs are able to traverse cell membranes. This study investigated 12C4, 15C5, and 18C6 for their ability to increase solubility of ocular drugs and enhance their penetration into the cornea. Phase solubility analysis determined crown ethers' ability to enhance the solubility of riboflavin, a drug used for the therapy of keratoconus, and these solutions were investigated for ocular drug permeation enhancing properties. Atomic absorption spectroscopy demonstrated crown ether solutions' ability to sequester Ca²⁺ from corneal epithelia, and crown ether mediated adsorption of riboflavin into the stroma was investigated. Induced corneal opacity studies assessed potential toxicity of crown ethers. Crown ethers enhanced riboflavin's aqueous solubility and its penetration into in vitro bovine corneas; the smaller sized crown ethers gave greatest enhancement. They were shown to sequester Ca²⁺ ions from corneal epithelia; doing so loosens cellular membrane tight junctions thus enhancing riboflavin penetration. Induced corneal opacity was similar to that afforded by benzalkonium chloride and less than is produced using polyaminocarboxylic acids. However, in vivo experiments performed in rats with 12C4 did not show any statistically significant permeability enhancement compared to enhancer-free formulation.

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.

Enhancement in corneal permeability of riboflavin using calcium sequestering compounds

Ethylenediaminetetraacetic acid, ethylenediamine-N,N'-disuccinic acid and ethylene glycol-bis(2-aminoethylether)-N,N,N',N'-tetraacetic acid are polyaminocarboxylic acids that are able to sequester metal ions. Calcium is implicated in maintenance of intercellular matrix, zonula occludens (tight junctions) and zonula adherens of epithelium and endothelium cells. Corneal epithelium is impervious to many aqueous formulations due to it being lipophilic, whereby transcellular drug transit is resisted, whilst tight junctions restrict access via the paracellular route. Research has shown that integrity of tight junctions breaks down through loss of Ca(2+) for endothelial and epithelial cells. This study investigates different Ca(2+) sequestering compounds and their effect on corneal permeability of riboflavin at physiological pH. Riboflavin is a topically administered ocular drug applied during UV-induced corneal cross-linking for the treatment of keratoconus.

Endocannabinoid-Goα signalling inhibits axon regeneration in Caenorhabditis elegans by antagonizing Gqα-PKC-JNK signalling

The ability of neurons to regenerate their axons after injury is determined by a balance between cellular pathways that promote and those that inhibit regeneration. In Caenorhabditis elegans, axon regeneration is positively regulated by the c-Jun N-terminal kinase mitogen activated protein kinase pathway, which is activated by growth factor-receptor tyrosine kinase signalling. Here we show that fatty acid amide hydrolase-1, an enzyme involved in the degradation of the endocannabinoid anandamide (arachidonoyl ethanolamide), regulates the axon regeneration response of γ-aminobutyric acid neurons after laser axotomy. Exogenous arachidonoyl ethanolamide inhibits axon regeneration via the Goα subunit GOA-1, which antagonizes the Gqα subunit EGL-30. We further demonstrate that protein kinase C functions downstream of Gqα and activates the MLK-1-MEK-1-KGB-1 c-Jun N-terminal kinase pathway by phosphorylating MLK-1. Our results show that arachidonoyl ethanolamide induction of a G protein signal transduction pathway has a role in the inhibition of post-development axon regeneration.

Cyclodextrin-mediated enhancement of riboflavin solubility and corneal permeability

Cyclodextrins are water-soluble cyclic oligosaccharides consisting of six, seven, and eight α-(1,4)-linked glucopyranose subunits. This study reports the use of different cyclodextrins in eye drop formulations to improve the aqueous solubility and corneal permeability of riboflavin. Riboflavin is a poorly soluble drug with a solubility up to 0.08 mg mL(-1) in deionized water. It is used as a drug topically administered to the eye to mediate UV-induced corneal cross-linking in the treatment of keratoconus. Aqueous solutions of β-cyclodextrin (10-30 mg mL(-1)) can enhance the solubility of riboflavin up to 0.12-0.19 mg mL(-1), whereas the higher concentration of α-cyclodextrin (100 mg mL(-1)) achieved a lower level of enhancement of 0.11 mg mL(-1). The other oligosaccharides were found to be inefficient for this purpose. In vitro diffusion experiments performed with fresh and cryopreserved bovine cornea have demonstrated that β-cyclodextrin enhances riboflavin permeability. The mechanism of this enhancement was examined through microscopic histological analysis of the cornea and is discussed in this paper.

Anandamide-loaded nanoparticles: preparation and characterization

OBJECTIVE

Preparation and characterization of anandamide (N-arachidonoyl-ethanolamine, AEA) loaded polycaprolactone nanoparticles (PCL NP) as a research tool to clarify the presence of an AEA transporter in cell membranes and to avoid AEA plastic adsorption and instability.

MATERIALS AND METHODS

High performance liquid chromatography and light scattering were used to determine encapsulation efficiency, particle size, drug release, permeability and stability.

RESULTS

A high encapsulation efficiency 96.05 ± 1.77% and a particle size of 83.52 ± 21.38 nm were obtained. Nearly 40% of AEA remained in the NP after a 99.9% dilution and only 50% was released after 24 h at 37 °C with a 99% dilution. PCL NP prevented the adsorption of the drug to polypropylene or polystyrene, but not to acrylic multiwell plates. Drug permeability through artificial membranes was low (10⁻⁷ to 10⁻⁸ cm/s) and was affected by the presence of NP. NP increased AEA stability in suspension (drug half-life 431 h vs. 12 h) and freeze-dried with 5% sucrose.

CONCLUSION

This article presents the first study where stable AEA-loaded NP with high encapsulation efficiencies have been obtained.

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.

Effects of Cyclodextrins on Drug Delivery Through Biological Membranes

Cyclodextrins have proven themselves to be useful functional excipients. Cyclodextrin derivatives can be hydrophilic or relatively lipophilic based on their substitution and these properties can give insight into their ability to act as permeability enhancers. Lipophilic cyclodextrins such as the methylated derivatives are thought to increase drug flux by altering barrier properties of the membrane through component extraction or fluidization. The hydrophilic cyclodextrin family also modulate drug flux through membranes but via different mechanisms. The current effort seeks to provide various explanations for these observations based on interactions of hydrophilic cyclodextrins with the unstirred water layer that separates the bulk media from biological membranes such as the gastric mucosa, cornea and reproductive tract. Theories on the serial nature of resistances to drug flux are used to explain why hydrophilic cyclodextrins can enhance drug uptake in some situation (i.e., for lipophilic material) but not in others. In addition, the nature of secondary equilibria and competition between cyclodextrins and rheologically important biopolymers such as mucin are assessed to give a complete picture of the effect of these starch derivatives. This information can be useful not only in understanding the actions of cyclodextrin but also in expanding their application and uses.

Preparation of hydrophobic drugs cyclodextrin complex by lyophilization monophase solution

A novel method was evaluated for preparation of hydrophobic drugs cyclodextrin (CD) complex in this study. To obtain sterilized drug-CD complex lyophilized powder for injection or other purpose, the CD solution in water and the hydrophobic drug in tertiary butyl alcohol (TBA) were mixed in a suitable volume ratio, filtered through 0.22 microm millpores, and subsequently freeze-dried. A high drug concentration was obtained in the co-solvent due to the good solvency of TBA, which is miscible with water in any proportion, for hydrophobic drugs. Moreover, TBA could be removed rapidly and completely by freeze-drying because of its high vapor pressure and high melting point. The chemical stability of some labile active compounds was also improved in TBA-water co-solvent. Based on the data from differential scanning calormetry (DSC) and X-ray diffractometry (XRD), drug was amorphous in freeze-dried complex. The fourier transform infrared spectra indicated drug-CD interaction was present in drug-CD complex. An enhanced dissolution rate was also obtained in drug-CD complex. These results proved drug-CD complex had been formed after this technique. Thus, this report provided a simple, efficient, and economic technique for preparation of hydrophobic drugs CD complex, which may be useful practically in modifying hydrophobic drugs physicochemical properties and improving their absorption and pharmacodynamics.

In-vitro corneal permeation of cannabinoids and their water-soluble phosphate ester prodrugs

Topically administered cannabinoids have been shown to reduce intraocular pressure by interacting with the ocular cannabinoid receptor. Most cannabinoids have very poor aqueous solubility, which limits their pharmaceutical development and usefulness. In this study, permeation of three cannabinoids (arachidonylethanolamide, R-methanandamide and noladin ether) and their water-soluble phosphate ester prodrugs across isolated rabbit cornea was investigated in vitro. Hydroxypropyl-beta-cyclodextrin (HP-beta-CD) was used to solubilize the parent cannabinoids in permeation studies to achieve the required concentration in donor and receiving cells. Highest fluxes were obtained with lipophilic parent compounds administered with HP-beta-CD, and the fluxes of phosphate esters were 45-70% that of their corresponding parent compounds. Phosphate esters hydrolysed on the surface of the cornea or during the permeation to release the lipophilic parent compound, which further permeated the cornea. No phosphate esters were detected on the endothelial side of the cornea. Although the phosphate esters had lower fluxes than their corresponding parent compounds in these HP-beta-CD formulations, the results are promising and the fluxes of phosphate esters are significantly higher than the fluxes of parent compounds administered as a suspension (due to their low aqueous solubility) without HP-beta-CD.

Cyclodextrins in drug delivery

Cyclodextrins are a family of cyclic oligosaccharides with a hydrophilic outer surface and a lipophilic central cavity. Cyclodextrin molecules are relatively large with a number of hydrogen donors and acceptors and, thus, in general they do not permeate lipophilic membranes. In the pharmaceutical industry cyclodextrins have mainly been used as complexing agents to increase aqueous solubility of poorly soluble drugs, and to increase their bioavailability and stability. Studies in both humans and animals have shown that cyclodextrins can be used to improve drug delivery from almost any type of drug formulation. However, the addition of cyclodextrins to existing formulations without further optimisation will seldom result in acceptable outcome. Currently there are approximately 30 different pharmaceutical products worldwide containing drug/cyclodextrin complexes on the market.

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.

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 and drug permeability through semi-permeable cellophane membranes

Determinations of drug fluxes through semi-permeable cellophane membranes are used to evaluate cyclodextrin complexes and cyclodextrin containing drug formulations. In the present study we investigated how the cyclodextrin concentration, the membrane thickness and the molecular weight cut off (MWCO) of the membrane influence drug fluxes. The cyclodextrin used was 2-hydroxypropyl-beta-cyclodextrin (HPbetaCD) and the sample drug was hydrocortisone. The MWCO of the membranes ranged from 500 to 14,000 and the HPbetaCD concentration ranged from 0 to 25% (w/v). The hydrocortisone flux from saturated solutions through the MWCO 500 membrane was unaffected by the cyclodextrin concentration. When MWCO of the membrane was greater than the molecular weight of the complex the flux from solutions saturated with hydrocortisone increased with increasing HPbetaCD concentration. This increase showed negative deviation from linearity. When the flux was corrected for the viscosity increase with increasing HPbetaCD concentration then the flux pattern could be described on the basis of Fick's first law and Stokes-Einstein equation. However, the flux did not correlate with the viscosity when it was increased by adding polymer to the saturated drug solutions. It was shown that the observed flux pattern was consistent with self-association of cyclodextrin complexes in the aqueous donor phase.

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.

Cyclodextrins as permeation enhancers: some theoretical evaluations and in vitro testing

It is well known that cyclodextrins can enhance the permeation of poorly soluble drugs through biological membranes. However, the permeability will decrease if cyclodextrin is added in excess of the concentration needed to solvate the drug. The mechanism of cyclodextrin effect on drug permeability has not been fully explained. The effect of cyclodextrins can not be explained as solely due to increased solubility of the drug in the aqueous donor phase nor can it be explained by assuming that cyclodextrins act as classical permeation enhancers, i.e. by decreasing the barrier function of the lipophilic membrane. In the present work we have modeled the effect of cyclodextrins in terms of mixed barrier consisting of both diffusion and membrane controlled diffusion, where the diffusion of the drug in the aqueous diffusion layer is significantly slower than in the bulk of the donor. This diffusion model is described by simple mathematical equation where the properties of the system are expressed in terms of two constants P(M)/Kd and M1/2. Data for the permeation of hydrocortisone through hairless mouse skin in the presence of various cyclodextrins, and cyclodextrin polymer mixtures, were fitted to obtain values for these two constants. The rise in flux with increased cyclodextrin complex concentration and fall with excess cyclodextrin was accurately predicted. Data for the permeation of drugs through semi-permeable cellophane membrane could also be fitted to the equation. It was concluded that cyclodextrins act as permeation enhancers carrying the drug through the aqueous barrier, from the bulk solution towards the lipophilic surface of biological membranes, where the drug molecules partition from the complex into the lipophilic membrane.

Hydroxypropyl-beta-cyclodextrin and its combination with hydroxypropyl-methylcellulose increases aqueous solubility of delta9-tetrahydrocannabinol

Delta9-tetrahydrocannabinol (THC) is the main psychoactive constituent of Cannabis sativa L. and its therapeutic effects are currently under intensive study. However, THC has a very low aqueous solubility (1-2 microg/mL), which restricts its use as a pharmaceutical. The present study demonstrates that THC forms a drug-cyclodextrin complex in an aqueous solution of hydroxypropyl-beta-cyclodextrin (HP-beta-CD), resulting in a thousand-fold increase in THC solubility. This improvement in solubility can be further increased by adding 0.1% hydroxypropylmethylcellulose to the HP-beta-CD solution. The present results suggest that the use of cyclodextrins might be a simple and useful method to overcome the poor water solubility of THC.