Meeting of Drug Metabolism Group Extrahepatic Metabolism

1. Four non-acidic primary metabolites of N-(5-pyrrolidinopent-3-ynyl)succinimide (BL 14) were identified and quantified using g.l.c. and mass spectrometry. The metabolites are α-hydroxy-N-(5-pyrrolidinopent-3-ynyl)succinimide (A), N-(5-(2-oxopyrrolidino)-pent-3-ynyl)succinimide (B), N-(2-hydroxy-5-pyrrolidinopent-3-ynyl)succinimide (C) and N-(5-pyrrolidinopent-3-ynyl)succinimide N'-oxide (E), the latter analysed after reduction to the parent amine.

2. In rat liver preparations, all metabolites are formed by microsomal, NADPH-dependent enzyme systems, but with different characteristics. The response to inhibitors such as CO and SKF 525A indicates participation of cytochrome P-450 enzymes in the formation of all metabolites. Phenobarbital pretreatment markedly enhances propynylic hydroxylation (C) but has little or no effect on the other metabolic pathways. Succinimide hydroxylation (A) exhibits a pH optimum at 7±0, while the formation of metabolites B and C increases at pH values between 6±4 and 7±7.

3. Kinetic studies on the formation of metabolites A-C revealed differences in the Michaelis constant, while the V_max values were similar. Succinimide hydroxylation (A) is most efficient with a K_m of 3±7 × 10^?5 M, compared with a K_m of 1±7 × 10^?3 M for propynylic hydroxylation (C).

4. The formation of metabolites B and E conforms to the corresponding mechanisms for lactam and N-oxide formation for other xenobiotics. The formation of metabolites A and C represents two extremities, reflected in their different responses to phenobarbital pretreatment, pH changes and in their different K_m values. Although little can be discerned about the mechanisms from the literature, the enzymes cataly sing both reactions appear to be cytochromes.