Cyclodextrin formulation of dorzolamide and its distribution in the eye after topical administration
Introduction
Dorzolamide is a carbonic anhydrase inhibitor (CAI) used in the treatment of glaucoma. Carbonic anhydrase (CA) is responsible for generation of bicarbonate anions secreted by the ciliary process into the posterior chamber of the eye. Inhibition of CA results reduction in intraocular pressure (IOP) [1], [2]. Orally administered CAIs, such as acetazolamide, are very effective ocular hypotensive agents but their oral administration also results in myriad of systemic side effects including general malaise, depression, loss of appetite, fatigue, weight loss, gastrointestinal disturbances, paresthesias and renal calculi [3]. Studies in the 1960s showed that, when applied topically, acetazolamide did not have any IOP lowering effect and therefore topical administration of CAIs was considered impossible [4]. There are at least seven different isoenzymes of CA and two of them, CA-I and CA-II, are relevant to the human eye. Isoenzyme II is thought to play a major role in aqueous humor secretion [5]. However, this enzyme has to be almost 100% inhibited to obtain IOP lowering and topically applied acetazolamide, or other CAIs synthesized before 1980, were not topically active due to limited bioavailability. Dorzolamide hydrochloride ((4-S-trans)-4-ethylamino-5,6-dihydro-6-methyl-4H-thieno-[2,3-b]thiopyran-2-sulfonamide-7,7-dioxide monohydrochloride) (Fig. 1) was synthesized in the 1980s [6]. Dorzolamide was shown to be an about 20 times more potent CAI, with regard to isoenzyme II, than acetazolamide, and topically active [7]. Topically effective aqueous dorzolamide eye drop solution (Trusopt®) became available in 1995. The concentration of dorzolamide HCl in Trusopt is 2.2% (w/v), corresponding to 2.0% of the free base, at pH 5.65. Hydroxyethyl cellulose is used to increase the viscosity of the eye drops. Increased viscosity leads to increased corneal contact time and, consequently, to increased bioavailability. However, the relatively low pH and high viscosity have been shown to generate local irritation after topical administration of the eye drops [8]. The eye presents unique challenges when it comes to delivery of drug molecules. In general, less than 5% of an applied dose is absorbed into the eye and more typically, less than 1% is absorbed [9], [10]. The cornea consists of five layers, i.e., the epithelium, Bowman's membrane, stroma, Descement's membrane and the endothelium. Studies have shown that the outermost layer, the epithelium, is generally the rate-limiting barrier to transcorneal drug transport and that drug molecules must possess sufficient lipophilicity to be able to penetrate this barrier [11]. The task of formulating hydrophilic dorzolamide hydrochloride as a lipophilic base, at pH around 7.4, is therefore especially interesting. The corneal contact time of eye drops can be increased by increasing the viscosity of the aqueous eye drop vehicle in the lower viscosity region (5–25 cps) [12], [13].
The study of dorzolamide and its effect on ocular blood flow and oxygenation of the retina has gained much attention in recent years. Dorzolamide has been shown to raise the oxygen tension in optic nerve in pigs when administered intravenously [14]. However, the large amounts of data yielded about dorzolamide and its effect on ocular blood flow has not been consistent [15]. Some reports have indicated that dorzolamide eye drops used for treatment of glaucoma have direct pharmacological effect on the blood flow of the retina and optic nerve [14], [16], [17]. Other reports indicate that dorzolamide has no measurable vascular effect in both glaucoma patients and healthy individuals when given topically [18], [19]. Data from experimental studies have not been in agreement either. Some studies have shown that dorzolamide has effect on retinal arteries [20] but other studies do not show this effect [21]. Measuring the effect of dorzolamide on the blood flow in retina and optic nerve is rather difficult and therefore it is easy to miss this effect. One aspect of this study is to show that it is possible that dorzolamide has a direct effect on the human CA isoenzyme II in the back of the eye.
Cyclodextrins are cyclic oligosaccharides with a hydrophilic outer surface and a lipophilic cavity in the center. They are able form water-soluble drug/cyclodextrin inclusion complexes of lipophilic water-insoluble drugs. No covalent bounds are formed or broken during the complex formation, and in aqueous solution the complexes are readily dissociated upon dilution. In general, cyclodextrin molecules do not penetrate biological membranes but act as penetration enhancers by assuring high concentration of dissolved drug at the membrane surface. Cyclodextrins increase the aqueous solubility of lipophilic water-insoluble drugs without decreasing the intrinsic ability of the lipophilic drug molecules to penetrate lipophilic biological membranes. Cyclodextrin can act as a drug carrier that delivers the drug molecule through the aqueous exterior of the membrane, i.e., the mucin layer, and releases it to the relatively lipophilic biological membrane such as the cornea [22]. Cyclodextrins do not disrupt the ophthalmic barrier like conventional penetration enhancers do (like, for example, benzalkonium chloride) [23]. Care must be taken to use the right amount of cyclodextrin since too much cyclodextrin can decrease topical bioavailability of drugs.
The purpose of this study was to formulate low viscosity aqueous dorzolamide eye drop solution containing the unionized drug at pH 7.45 using randomly methylated β-cyclodextrin (RMβCD) as a solubilizer, and to evaluate the formulations in rabbits. In a previous study we have used RMβCD to solubilize dexamethasone in aqueous eye drop solutions and shown that eye drops containing RMβCD are well tolerated in humans [24].
Section snippets
Materials
Dorzolamide HCl, internal standard (L-662614-002J005) and Trusopt® eye drops were obtained from Merck (USA). Randomly methylated β-cyclodextrin with some degree of substitution 1.8 (RMβCD) was purchased from Wacker-Chemie GmbH (Germany). Analytical grades of disodium phosphate dihydrate, monosodium phosphate monohydrate and disodium edetate dihydrate (EDTA) were purchased from Merck (Germany). Hydroxypropyl methylcellulose (HPMC) was obtained from Mecobenzon (Denmark). Benzalkonium chloride was
Results
Dorzolamide has two pKa values of 6.35 (pKa1) and 8.5 (pKa2) corresponding to the protonized secondary amino group and the sulfonamide group, respectively (Fig. 1). It is mainly in its hydrophilic cationic form at pH below 6.4 and mainly in its hydrophilic anionic form at pH above 8.5. The largest fraction of the lipophilic unionized form exists at pH right between the two pKa values or at pH 7.45. Dorzolamide exists in two polymorphic forms: form I which is the more thermodynamically stable
Discussion
Our results are in agreement with the results of Sugrue [5], [25] who reported similar dorzolamide concentrations in the cornea, aqueous humor and iris-ciliary body after topical administration of the drug. However, Sugrue did not measure dorzolamide concentration in the vitreous gel or optic nerve. Our results show that the drug levels in the vitreous gel were always lower than those in retina and optic nerve. Possible explanation could be the binding of the drug to carbonic anhydrase and
Conclusion
Aqueous dorzolamide/cyclodextrin eye drop solutions, with pH 7.4 and viscosity of 3 to 5 cps, were successfully formulated and compared to Trusopt®, with pH 5.65 and viscosity of 100 cps. No irritation or other side effects could be observed after topical administration of the cyclodextrin eye drop solutions to rabbits. The topical availability of dorzolamide from the cyclodextrin-containing eye drops appeared to be good and the drug reached retina and optic nerve to give measurable
Acknowledgments
This study was supported in part by a research grant from the Icelandic Research Council.
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