Ocular ketone reductase distribution and its role in the metabolism of ocularly applied levobunolol in the pigmented rabbit.

The distribution of ketone reductase activity in the anterior segment tissues of the pigmented rabbit eye and its influence on the ocular metabolism of topically applied levobunolol were studied. A reversed phase high-performance liquid chromatography procedure was used to assay for this drug and its metabolite, dihydrolevobunolol. Ocular ketone reductase activity was 3 to 4 times more dependent on NADPH than on NADH. The rank order of activity was corneal epithelium greater than iris-ciliary body greater than conjunctiva greater than lens. No activity was detected in the tears, corneal stroma, sclera or aqueous humor. Ketone reductase activity was entirely cytosolic. The pigmented rabbit was significantly less active than the albino rabbit in ketone reductase. Its activity in the corneal epithelium, iris-ciliary body and lens was most sensitive to inhibition by quercetin, whereas that in the conjunctiva was most sensitive to metyrapone. The ketone reductase in the corneal epithelium contributed more to the metabolism of topically applied levobunolol than its counterpart in the iris-ciliary body and lens. Moreover, the extent of levobunolol metabolism both during and after corneal penetration was dose-dependent. Overall, these findings indicate that ocularly applied drugs containing the ketone functional group are subject to varying degrees of metabolism by NADPH-dependent ketone reductases in the corneal epithelium, iris-ciliary body and lens.