ABSTRACT
Esters of ethacrynic acid and partial structural analogs were synthesized and evaluated for topical antiglaucoma activity in rabbits. Maximum activity was shown by analogs 2 and 6 (34% and 30% reduction in intraocular pressure recovery rate, respectively). Among the esters, only the ethyl ester (2) was found to be active; the methyl and n-propyl esters (1 and 3) were inactive. Analogs 1-3 were subjected to an estimation of physicochemical properties and chemical stability. However, no correlation was found to exist between the biological activity/inactivity and the physicochemical properties of the analogs.
The analogs were evaluated for ex vivo hydrolysis using rabbit aqueous humor (AH), corneal (C) homogenate and iris-ciliary body (ICB) homogenate. For all tissues, the rate of enzymatic hydrolysis increased significantly with an increasing ester chain length. The ICB-mediated hydrolysis was the fastest among the three tissues for all of the analogs. The relationship between the rate constants for the tissue-mediated hydrolyses were: analog 1, ICB>C>AH; analog 2, ICB>C=AH and analog 3, ICB>AH>C. Apparent Michaelis-Menten kinetic parameters were determined for the three analogs using corneal homogenate. Analog 2 showed the highest v0 for all substrate concentrations studied. The conventional Michaelis-Menten equation did not fit the data as well as a sigmoidal model. Both fits of the data showed the fastest enzyme-mediated hydrolysis for analog 2. The parameters of the sigmoidal fit of the data correlated with the activity/inactivity of the analogs. The data indicate that the major factors responsible for the observed activity/inactivity are the differences in the corneal enzymatic hydrolysis of the esters in conjunction with the rapid dynamics of ocular prodrug absorption.
