Abstract

Antidepressant drugs that contain alkylaminoalkyl substituents have been associated with serious pharmacokinetic interactions in humans that may be related to the inhibition of cytochrome P450 (P450) enzymes. In this study, the propensity of the tricyclic antidepressant nortriptyline (NOR) to inhibit individual microsomal P450 enzymes in rat liver was investigated to provide a mechanistic explanation for these pharmacokinetic interactions. Enzyme kinetic studies revealed that NOR inhibited steroid 2 alpha-, 6 beta, 7 alpha-, and 16 alpha-hydroxylation in untreated rat liver with Km/Ki ratios of 0.53, 0.59, 0.25, and 0.29, respectively. When the drug was preincubated with microsomes and NADPH before testosterone hydroxylation was conducted, marked increases in the Km/Ki ratios were observed (to 8.8, 3.9, 0.62, and 13, respectively). Thus, enzymic oxidation of NOR enhanced its inhibition capacity against P450 activities. Indeed, the altered Km/Ki ratios indicate 17-, 6.6-, 2.5-, and 47-fold increases in inhibition of the four pathways of testosterone hydroxylation after the biotransformation of NOR to its metabolites. From these experiments it was apparent that testosterone 2 alpha- and 16 alpha-hydroxylations, catalyzed predominantly by P450 2C11, were subject to the most pronounced increase in inhibition. Under these conditions, the apparent content of microsomal P450 was decreased, thus suggesting the formation of a NOR metabolite intermediate (MI) complex with the cytochrome. Further, optical difference spectroscopy of NADPH-supported metabolism of NOR in microsomes and in a reconstituted system incorporating purified P450 2C11 indicated the appearance of an absorbance peak near 454 nm, similar to those produced by triacetyloleandomycin, SKF 525-A, and orphenadrine. Formation of this absorbance peak in microsomes was inhibited by an antibody raised against the male-specific P450 2C11. Because oxidative metabolism of NOR to inhibitory products would not necessarily involve MI complexation, additional experiments were undertaken in which NOR-related free metabolites produced in microsomal incubations were removed on Sep-Pak mini-C18 columns before estimation of testosterone hydroxylation. The principal finding from this experiment was that P450 3A2-dependent steroid 6 beta-hydroxylase activity was inhibited to a much lesser extent after removal of unbound NOR metabolites on Sep-Pak columns (25% inhibition after Sep-Pak extraction, compared with 82% inhibition observed when all NOR metabolites were present during subsequent testosterone hydroxylation); inhibition of P450 2C11-mediated 2 alpha- and 16 alpha-hydroxylation was not noticeably different after Sep-Pak treatment.(ABSTRACT TRUNCATED AT 400 WORDS)