Corneal esterases are utilized in the activation of topically applied ester prodrugs. Esterases may also be involved in the metabolism of drugs in posterior eye tissues, but their physiological activity is unknown. Furthermore, extrapolation of the esterase activity from protein level to the tissues is missing. The aims of the current study were to determine esterase activities in porcine and albino rabbit ocular tissues, calculate the activities for whole tissues and compare esterase activity between the species. We conducted a hydrolysis study with ocular tissue homogenates using an esterase probe substrate 4-nitrophenyl acetate. The hydrolysis rates were first normalized to protein content and then scaled to whole tissues. The hydrolytic rate normalized to protein content was high in the cornea and iris-ciliary body and low in the lens and aqueous humor, and in general, the rabbit tissues had higher hydrolytic rates than the porcine ones. Esterase activity scaled to whole tissue was high in cornea and iris-ciliary body and low in aqueous humor and retinal pigment epithelium in both species. The current study revealed differences in esterase activities among the ocular tissues and the species. This basic knowledge on ocular esterases provides background information particularly for posterior segment drug development.
Keywords: Esterases; Ocular ADME; Ocular drug bioavailability; Ocular metabolism; Ocular prodrugs.
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