Vol. 25, Issue 12, 1424-1428, 1997

In Vitro and In Vivo Effects of the Arylamine Human Immunodeficiency Virus Protease Inhibitor 4R-(4,5,6,7)-1-[(3-(1-Imidazoylcarbamoyl)phenyl)methyl]-3-[(3-aminophenyl)methyl]hexahydro-5,6-dihydroxy-4,7-bis(phenylmethyl)-2H-1,3-diazepin-2-one (SD894) on Rat Hepatic Cytochrome P450 2B and 3A

Mary F. Grubb, Sharon Diamond, and David D. Christ

Drug Metabolism and Pharmacokinetics Section, The DuPont Merck Pharmaceutical Company

The human immunodeficiency virus-1 protease inhibitor SD894 was evaluated as an inhibitor and inducer of cytochromes P450 (CYPs) in rats. After addition of 10 µM SD894 and 2 mM NADPH to liver microsomes from dexamethasone-treated rats, a type II spectrum appeared. Within 2 min, it was replaced by a type III spectrum, with absorbance maxima at 426 and 456 nm, similar to those observed with alkylamines (SKF-525A) and arylamines (p-chloroaniline). Preincubation of microsomes from dexamethasone-treated rats with SD894 and NADPH resulted in a time-dependent inhibition of testosterone 6-hydroxylation (CYP 3A1/2 activity), which was decreased to 25% of controls after 30 min. Testosterone 16-hydroxylation (CYP 2B1/2 activity) was unaffected under these conditions. Testosterone 6-hydroxylation rates in liver microsomes from pregnenolone 16-carbonitrile-treated rats incubated with 10 µM SD894 and NADPH, washed, and reisolated by ultracentrifugation were reduced by 71%, whereas 16-hydroxylation was unaffected by SD894. Immunoblots of liver microsomes from rats dosed iv with SD894 or ip with TAO displayed increased CYP 2B1 and CYP 3A1 levels, respectively. Testosterone 6-hydroxylase activity in microsomes from TAO-treated rats was greater than controls. Preincubation of these microsomes with potassium ferricyanide produced an additional 50% increase, consistent with disruption of a metabolite-CYP complex. Microsomes from SD894-treated rats displayed a 3-fold increase in testosterone 16-hydroxylation. Potassium ferricyanide preincubation did not increase activity. Thus, although SD894 appears to inhibit CYP in vitro in a manner typical of other amine-containing, mechanism-based inhibitors, in vivo induction by 10 mg/kg daily doses of SD894 affects a different isozyme than does inhibition. The mechanism of induction is unknown.