INHIBITION OF MICROSOMAL CYTOCHROMES P450 IN RAT-LIVER BY THE TRICYCLEC ANTIDEPRESSANT DRUG DESIPRAMINE AND ITS PRIMARY OXIDIZED METABOLITES

N-Monoalkyl substituted tricyclic antidepressants like desipramine (DE S) undergo cytochrome P450 (P450)-mediated biotransformation in liver to produce inhibitory metabolite-intermediate (MI) complexes with the enzyme. However, additional oxidation pathways that generate isolable metabolites have also been identified, so that the relationship betwee n MI complexation and total oxidative metabolism is unclear. The prese nt study investigated the capacity of DES and three putative metabolit es (2-hydroxy- and 10-hydroxy-DES and N,N-didesmethylimipramine; DIDES ) to elicit MI complexation and inhibit P450-dependent activities in r at liver. MI complexation of P450 was produced by DES, but not with th e three metabolites, in NADPH-supplemented microsomes. Consistent with this finding, inhibition of testosterone hydroxylation pathways was e nhanced markedly by prior incubation of DES with NADPH and microsomes. Direct addition of DIDES to incubations resulted in significant inhib ition of P450 activities (IC(50)s of 35 and 29 mu M against estradiol 6 beta- and 16 alpha-hydroxylation mediated by P450s 3A2 and 2C11, res pectively). Neither 2-hydroxy- nor 10-hydroxy-DES directly inhibited t estosterone hydroxylation (IC(50)s > 100 mu M). However, after a prein cubation step between these metabolites and NADPH-fortified microsomes , enhanced inhibition of reactions mediated by P450 3A2 and P450 2C11/ 2A1 was produced by 2-hydroxy-DES and 10-hydroxy-DES, respectively. Me tabolism of DES to DIDES and 2-hydroxy-DES was estimated as 7.77 +/- 0 .48 nmol/mg protein/hr (10-hydroxy-DES was not detected). It is likely that secondary oxidized metabolites derived from 2-hydroxy-DES, as we ll as the primary metabolite DIDES, may contribute to the inhibition o f P450 activity during DES biotransformation. These results indicate t hat the 2-hydroxy-, 10-hydroxy-, and N-desmethyl-metabolites of DES ar e not involved in MI complexation, but complexation is not the sole me chanism by which DES inhibits microsomal drug oxidation that may lead to pharmacokinetic drug interactions.