ENHANCED INHIBITION OF MICROSOMAL CYTOCHROME-P450 3A2 IN RAT-LIVER DURING DILTIAZEM BIOTRANSFORMATION

Pharmacokinetic drug interactions involving the calcium channel blocke r diltiazem (DTZ) have been attributed to inhibition of microsomal cyt ochrome P450 (P450)-mediated drug oxidation. Accumulation of certain D TZ metabolites during dosage with the drug, as well as dose-related di fferences in DTZ pharmacokinetics, suggests that DTZ metabolites may a lso participate in P450 inhibition. The present study evaluated a seri es of putative DTZ metabolites as inhibitors of major constitutive P45 0s in rat liver in vitro, in relation to DTZ biotransformation. The pr incipal finding to emerge was that the N-demethylated metabolite of DT Z was a more potent competitive inhibitor than DTZ of CYP3A2-dependent testosterone 6 beta-hydroxylation. This 2450 appeared to be the prefe rred target for inhibition, because the observed K-i/K-m ratio for inh ibition of CYP3A2-dependent steroid hydroxylation was approximately 4- and 100-fold lower than those for CYP2C11 and CYP2A1-dependent pathwa ys, respectively. It was also established that N-desmethyl-DTZ was a m ajor metabolite formed during microsomal DTZ biotransformation in rat liver in vitro. The other primary metabolites, desacetyl-DTZ and O-des methyl-DTZ, were ineffective inhibitors of any pathways of steroid oxi dation by P450s, but several other potential metabolites, which were n ot detected in microsomal incubations, also inhibited P450 activity. C onsistent with previous reports, there was no evidence of P450 inactiv ation or complexation by DTZ, but the drug and its N-desmethyl metabol ite generated binding inter-actions with ferric P450 in rat hepatic mi crosomes. Considered together, the findings of the present study estab lish that N-desmethyl-DTZ is a preferential inhibitor of CYP3A2 in rat hepatic microsomes, with greater potency than the parent drug. This i s consistent with clinical reports in which this metabolite accumulate s during multiple-dose therapy with DTZ. The competitive nature of the inhibitory interaction suggests that the eventual elimination of N-de smethyl-DTZ should restore normal hepatic oxidation capacity.