Background: Rifampin (INN, rifampicin) is a potent inducer of cytochrome P450 (CYP) enzymes involved in drug metabolism and therefore causes many drug interactions.
Methods: The effects of rifampin on the pharmacokinetics of tamoxifen (study I) and toremifene (study II) were examined in 2 randomized, placebo-controlled crossover studies. Ten (study I) or 9 (study II) healthy male volunteers took either 600 mg rifampin or placebo orally once a day for 5 days. On the sixth day, 80 mg tamoxifen or 120 mg toremifene was administered orally. Blood samples were collected up to 336 hours after drug administration.
Results: Rifampin reduced the area under the plasma concentration-time curve (AUC) of tamoxifen by 86% (P < .001), peak plasma concentration (Cmax) by 55% (P < .001), and elimination half-life (t1/2) by 44% (P < .001). The AUC of toremifene was reduced by 87% (P < .001), Cmax by 55% (P < .001), and t1/2 by 44% (P < .01) with rifampin. During the rifampin phase, the AUC of N-demethyltamoxifen was 38% (P < .001) and the AUC of N-demethyltoremifene was 20% (P < .01) of that during the placebo phase.
Conclusions: Rifampin markedly reduces the plasma concentrations of tamoxifen and toremifene by inducing their CYP3A4-mediated metabolism. Concomitant use of rifampin or other potent inducers of CYP3A4 with tamoxifen and toremifene may reduce the efficacy of these antiestrogens.