Metabolism and Ocular Tissue Distribution of an Antiglaucoma Prostanoid, Tafluprost, After Ocular Instillation to Monkeys

Abstract

To investigate the metabolism of a new antiglaucoma difluoroprostaglandin, tafluprost, in ocular tissues and evaluate the distribution of the parent drug and its metabolites in ocular and systemic tissues after a single ocular administration to cynomolgus monkeys (Macaca fascicularis). A single dose of an ophthalmic solution containing 0.0005%, 0.005%, or 0.05% [(3)H]tafluprost was topically instilled (20 μL/eye) to male and/or female cynomolgus monkeys to study tissue distribution and metabolism. Blood, ocular/systemic tissues, or excreta were collected until 24 h after dosing. The radioactivity of each sample was measured by liquid scintillation counting, and metabolites were characterized by liquid chromatography-mass spectrometry. The major metabolites found in ocular tissues were intracameraly administered to monkeys to confirm their effect on intraocular pressure (IOP). Soon after dosing, high concentrations of drug-related radioactivity were observed in the cornea and bulbar/palpebral conjunctiva, followed by the iris, sclera, choroid with retinal pigmented epithelium, and aqueous humor. The highest concentration of radioactivity concentrations occurred in the anterior and posterior ocular tissues within 2 h after dosing. The radioactivity measured in the plasma and ocular tissues was proportional to the dose administered. The major metabolites of tafluprost identified in the ocular tissues were tafluprost acid and 1,2-dinor- and 1,2,3,4-tetaranor-tafluprost acid. The estimated concentration of tafluprost acid in the aqueous humor and ciliary body was enough to stimulate prostanoid FP-receptors. After hydrolysis to the acid form, the primary metabolic pathway of tafluprost was via β-oxidation and, subsequently, oxidation. No metabolic reactions to the 15-carbon position were observed. Tafluprost acid was shown to significantly lower the IOP, whereas 1,2-dinor- and 1,2,3,4-tetaranor-tafluprost acid did not. Topically administered [(3)H]tafluprost was well absorbed into the ocular and systemic tissues of the primary nonclinical species, monkey. The amount of the pharmacologically active form, that is, tafluprost acid, was high enough to occupy the target FP receptors at the site of action. The pharmacokinetic and metabolic properties of this difluorinated prostaglandin in primates are believed to result in clinical benefits of a long-term IOP-lowering effect.