Concurrent Flavin-containing Monooxygenase Down-regulation and Cytochrome P-450 Induction by Dietary Indoles in Rat: Implications for Drug-drug Interaction

Our laboratory has previously shown that dietary administration of indole-3-carbinol (I3C) to male Fischer 344 rats has the very unusual property of inducing hepatic levels of a number of cytochrome P450s (CYPs), especially CYP1A1, while markedly inhibiting the levels of flavin-containing monooxygenase (FMO) 1 protein and its catalytic activity. We hypothesized that rats fed I3C or 3,3*-diindolylmethane (DIM), one of its major acid condensation products formed in vivo, should exhibit a marked shift in the metabolic profiles of drugs or xenobiotics that are substrates for both monooxygenase systems. Male rats were fed AIN-76A powdered diets containing 0, 1000, or 2500 ppm I3C or DIM for 4 weeks. Dietary I3C and DIM reduced FMO1 protein levels (8% reduction with I3C and 84% with DIM at 1000 ppm, and 90% reduction with I3C and 97% with DIM at 2500 ppm) in hepatic microsomes. The ratio of FMO (N-oxygenation)to CYP (N-demethylation)-mediated metabolism of N,N-dimethylaniline decreased in liver microsomes from I3Cor DIM-fed rats from near unity to 0.02 at the highest dietary doses. FMO-mediated N-oxygenation (nicotine N-1*-oxide) was decreased, whereas CYP-mediated (nornicotine and nicotine D -iminium ion) metabolism of nicotine was unchanged in liver microsomes from rats fed I3C or DIM. Similarly, the ratio of FMO to CYP metabolites of tamoxifen decreased due to a reduction in N-oxygenation. This study demonstrates alteration of FMOand CYP-mediated drug metabolism in vitro by dietary I3C or DIM and suggests the potential for altered toxicity of tamoxifen and nicotine