Presence of a diffusional barrier on metabolite kinetics: enalaprilat as a generated versus preformed metabolite

Studies in the once-through perfused rat liver with the simultaneous delivery of 14 C-enalapril and its polar diacid metabolite, 3H-enalaprilat, revealed different extents of elimination (exclusively by biliary excretion) for the generated (14C-enalaprilat) and preformed (3H-enalaprilat) metabolite (18 and 5% dose) [Pang, Cherry, Terrell, and Ulm: Drug Metab. Dispos. 12, 309-313 (1984)]. The present re-examination of data provided an explanation for these discrepant observations: enalaprilat, being a polar dicarboxylic acid, encounters more of a diffusional barrier than its precursor, enalapril, an ethyl ester of enalaprilat. Programs written in Fortran 77 on mass balance relationships were employed to simulate data upon varying the diffusional clearances for drug (CLd) and metabolite [CLd(mi)] from 1 to 5000 ml/min. The metabolic and biliary intrinsic clearances for drug and metabolite were found by trial and error such that the combinations of all clearance parameters yielded data similar to enalaprilat, and 3H-enalaprilat. Our finding indicated that the diffusional clearance for enalaprilat was low (2 ml/min) compared to that of enalapril (75 ml/min). The presence of a diffusional barrier for enalaprilat retards entry of the preformed metabolite into hepatocytes but prevents efflux of the intracellularly formed generated metabolite into sinusoidal blood, thereby enhancing generated metabolite elimination.