Ocular metabolism of levobunolol.

Dihydrobunolol is an ocular metabolite equipotent to levobunolol. In order to understand the formation and distribution of dihydrobunolol after an ophthalmic dose of levobunolol, studies in vitro and in vivo were initiated. The metabolism of levobunolol to dihydrobunolol was investigated using an organ-culture technique. The corneal formation of dihydrobunolol was pH-dependent and increased as the pH of the incubation fluid increased from 5.3 to 8.3. Its formation from levobunolol was saturable with Vmax and Km values (pH 7.4) of 13.2 nmol/min/gm of cornea and 1.48 mM, respectively. After a topical dose of 0.5% levobunolol hydrochloride to rabbit eyes, rapid absorption of levobunolol and facila formation of dihydrobunolol were noted. The drug concentration in the eye drop (approximately 17 mM) was much higher than Km and would saturate the epithelial reductase system in the cornea during drug absorption. The total concentrations of levobunolol and dihydrobunolol in ocular tissues were in the micromolar range throughout the experimental period. Dihydrobunolol, after distribution equilibrium, was the major drug-derived species in the cornea, aqueous humor, and iris-ciliary body. The study results indicated pH-dependent and capacity-limited formation of dihydrobunolol in the cornea. Buffering capacity and the drug concentration in the ophthalmic dose are important formulation strategies because they may affect the rate and the extent of dihydrobunolol formation in the epithelial cell layers of the cornea.