PENETRATION OF BETA-BLOCKERS THROUGH OCULAR MEMBRANES IN ALBINO RABBITS

The purpose of this study was to investigate the barrier properties of ocular membranes for controlling the extent and pathway of ocular absorption of instilled beta-blockers. The penetration of beta-blockers was measured across the isolated corneal, conjunctival and scleral membranes of the albino rabbit using a two-chamber glass diffusion cell. beta-Blockers tested were atenolol, carteolol, tilisolol, timolol and befunolol. Corneal penetration of befunolol was much higher than that of atenolol. Scraping the epithelium increased corneal penetration of beta-blockers. Conjunctival membranes showed higher permeability than corneal and scleral membranes. The penetration parameters were estimated according to Fick's equation. The corneal permeability coefficient showed an apparent linear relationship with penetrant lipophilicity. The lipophilic character of the corneal barrier was determined by the partition coefficient of drug to corneal surface, not by the diffusion coefficient. Conjunctival and scleral permeability coefficients were not determined by the lipophilicity of beta-blockers. These results indicate that the conjunctiva, sclera and cornea of the rabbit eye are sufficiently different in permeation character to control the extent and pathway for ocular absorption.