Oxazepam esters. 1. Correlation between hydrolysis rates and brain appearance of oxazepam

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Abstract
Esters of the central depressant oxazepam were studied to find quantitative correlations between the pharmacokinetics of the parent drug and in vitro biotransformation rates and physicochemical properties of its prodrugs. The 14C-labeled aliphatic and .omega.-phenyl-substituted esters were administered i.v. to mice. Brain levels of the esters and oxazepam were determined and the latter was fitted to a simplified exponential equation. In vitro hydrolysis rate of the esters catalyzed by the hepatic microsomal fraction was measured with a pH stat. Pharmacokinetic constants characterizing the rising part of oxazepam brain levels correlate with the chromatographic RM values and with in vitro maximal hydrolysis rates of the esters. The hydrolysis is capacity-limited in the liver. In a closely related set of aliphatic esters, oxazepam brain penetration correlates with the steric constant (ES) of its esters.