Regiochemistry and substrate stereoselectivity of O-demethylation of verapamil in the presence of the microsomal fraction from rat and human liver.

The oxidative O-demethylation of verapamil (1), a calcium channel antagonist, in the presence of rat and human liver microsomes was examined. By using GC/MS methodology and synthesized regioisomeric standards, we showed that three of the four possible monophenolic metabolites, alpha-[3-([2-(3,4-dimethoxyphenyl)ethyl]methyl-amino) propyl]-3-methoxy-4-hydroxy-alpha-(1-methylethyl)phenyl-acetonitrile (2), alpha-[3-([2-(3,4-methoxyphenyl)ethyl]methyl-amino) propyl]-3-hydroxy-4-methoxy-alpha-(1-methylethyl)phenylaceto nitrile (3), and alpha-[3-([2-(3-methoxy-4-hydroxyphenyl)ethyl]methylamino) propyl]-3,4-dimethoxy-alpha-(1-methylethyl)phenylacetonitrile (4) were formed. The other possible regioisomeric monophenolic metabolite 5 was not observed. Substrate stereoselectivity for the O-demethylation process was determined when pseudoracemic verapamil [equimolar (S)-(-)-verapamil-d6 and (R)-(+)-verapamil-d0] was used as substrate. In the presence of rat liver microsomes, significant substrate stereoselectivity was observed for formation of 4 (S/R ratio 2.28), whereas marginal substrate stereoselectivity was observed in the formation of both 3 and 2 (S/R ratio approximately 0.8). Substrate stereoselectivity for the O-demethylation process in the presence of human liver microsomes was slight and variable (six samples). Quantitatively, the ratio of O-demethylation products obtained (4:2:3) was similar in the presence of rat and human liver microsomes. In both systems, more than one-half of the total O-demethylation occurred in the aromatic ring of the phenethylamine moiety, and of the total O-demethylation process, more para- than meta-O-demethylation was observed. The similarity of regioselectivity for O-demethylation in the presence of rat and human liver microsomes suggests a similar cytochrome P-450 isozyme or set of isozymes may be responsible for the O-demethylation process.