Metabolism of methoxyphenamine in extensive and poor metabolizers of debrisoquin

Abstract

Urine and plasma concentrations of methoxyphenamine (MP) and three of its metabolites were determined after a single oral 60.3 mg dose of MP hydrochloride to healthy subjects of known debrisoquin (D) phenotype. Urine was collected from five extensive (EM) and five poor (PM) metabolizers of D for 12 hours and analyzed after treatment with beta-glucuronidase/sulfatase. There were marked interphenotype differences in the total urinary excretion of O-demethylmethoxyphenamine (ODMP) and 5-hydroxymethoxyphenamine (5HMP), as well as in MP/ODMP and MP/5HMP ratios. In contrast, the urinary output of N-demethylmethoxyphenamine (NDMP) or MP/NDMP ratios showed no interphenotype differences. Plasma data from two EMs and two PMs showed that the mean values for maximum concentration t1/2, and total AUC for MP were two-, three-, and sixfold greater, respectively, in PMs than in EMs. The plasma levels of ODMP and 5HMP were higher in EMs than in PMs, whereas the converse was true for NDMP. Thus, O-demethylation and aromatic 5-hydroxylation of MP are defective in PMs of D, resulting in increased MP and NDMP plasma levels. The form of cytochrome P-450 involved in the N-demethylation of MP is different from that responsible for O-demethylation and aromatic 5-hydroxylation.