In ophthalmic pharmacotherapy, the cornea is considered to be a major pathway for ocular permeation of topically applied drugs.1,2) The cornea, however, is impermeable to hydrophilic compounds because of a poor paracellular route due to the epithelial tight junctions. 3,4) Passive diffusion across the lipid cell membrane primarily contributes to the penetration of most ophthalmic drugs through the intact cornea. Many drugs had an almost parabolic relationship between the corneal permeability and the drug lipophilicity. 5,6) On the other hand, several transporters are expressed in ocular tissue. Jain-Vakkalagadda et al. identified an active transport system for L-arginine on rabbit corneal epithelium and human cornea and showed that amino acid ester prodrugs of acyclovir appeared to be substrates for this transporter.7) Sodium dependent nucleoside transporter was functionally expressed on the rabbit cornea although acyclovir and idoxuridine were not substrates for this transporter. 8)We previously demonstrated an expression of P-glycoprotein (P-gp) on cultured normal rabbit corneal epithelial cells (RCEC). 9) Dey et al. reported that P-gp restricts topical erythromycin absorption across the cornea, which can be inhibited by known P-gp inhibitors. 10)There was a relationship between the permeability of beta-blockers, which are often used in ophthalmic therapy, through an excised rabbit cornea and their lipophilicities. 6,11) Among them, however, permeability of acebutolol was much lower than that expected by its lipophilicity. This results indicated the possibility that acebutolol was secreted by specific transporter in corneal epithelium.Therefore, in the present study, the permeation mechanism of acebutolol in the corneal epithelium is investigated by RCEC system that we previously establised. 12) MATERIALS AND METHODS Materials and AnimalsAcebutolol, FITC-dextran (FD-4, molecular weight 4400), 6-carboxyfluorescein (6-CF) were purchased from Sigma Chemicals (St. Louis, MO, U.S.A.). Cyclosporin A (CyA) was kindly supplied from Sandoz Pharmaceuticals (Basel, Switzerland). Sodium azide (NaN 3 ) and verapamil hydrochloride (VER) were obtained from Nacalai Tesque Inc. (Kyoto, Japan). All other chemicals were commercial products of reagent grade.Primary cultured cells were obtained from Kurabo Industries Ltd. (Osaka, Japan). Transwell-COL ® cell culture chambers (pore size 0.4 mm, diameter 12 mm, surface area 1 cm 2 ) were purchased from Costar (Bedford, MA, U.S.A.). Dulbecco's modified Eagle's medium/nutrient mixture F-12 (DMEM/F-12), fetal bovine serum (FBS) and another culture reagents were purchased from GIBCO (Grand Island, NY, U.S.A.). Epidermal growth factor (EGF), choleratoxin (CTX), hydrocortisone (HCS), and insulin-transferrin sodium selenite media supplement (ISL) were purchased from Sigma Chemicals. Penicillin G and streptomycin were obtained from Wako Pure Chemical Industries Ltd. (Osaka, Japan). Human fibronectin was purchased from Boehringer Mannheim GmbH (Mannheim, Germany). 14 C-Mannitol (specific act...

Beta-blockers decrease aqueous humor formation in the ciliary processes after instillation and are very often indispensable in the treatment of glaucoma.1) However, most of the instilled amount is rapidly eliminated from the precorneal area and easily absorbed into the systemic circulation.2) Betablockers in the precorneal area should also penetrate the tight barrier of the corneal epithelium into the eye.3) Such behavior can result in poor bioavailability in the anterior segment and increase the severity of systemic adverse effects.