In vitro corneal permeation of unoprostone isopropyl (UI) and its metabolism in the isolated pig eye

The corneal permeability, hydrolysis, and metabolism of unoprostone isopropyl (UI), a docosanoid, were examined in isolated porcine ocular tissues. The apparent permeability coefficient (Papp) of the esterified prodrug and of the acid metabolite were determined in a modified Valia-Chien permeation chamber and quantified by high performance liquid chromatography. Enzymatic hydrolysis and subsequent metabolism were examined in isolated tissue homogenates. The prodrug (ester form) was found to permeate the isolated intact porcine cornea with a Papp of 9.97 x 10(-7) cm/sec. Only the acid metabolite could be detected in the receiver chamber, indicating the requirement of hydrolysis for permeation. The acid metabolite could not permeate the intact cornea but was able to cross an epithelium-denuded cornea with a Papp of 1.22 X 10(-6) cm/sec. Enzymatic hydrolysis of UI was confined to the isolated intact cornea and epithelium, indicating that the esterase activity was localized ill the corneal epithelium. Incubations with different porcine ocular tissues, conjunctiva, iris-ciliary body, trabeculum, as well as aqueous humor, did not reveal other metabolites. These findings demonstrate that the ocular penetration of UI is dependent on its uptake into the epithelium and subsequent hydrolysis prior to its penetration into the anterior chamber, a very common pathway for ophthalmic drugs. In the pig eye, unoprostone does not appear to be further metabolized.