Absence of effect of oral rifaximin on the pharmacokinetics of ethinyl estradiol/norgestimate in healthy females

BACKGROUND: Rifaximin is an oral rifampin analog, and its activity is targeted astrointestinal tract. Some analogs induce the cytochrome P450 idative enzymes. Ethinyl estradiol (EE), commonly found in oral es (OCs), is a known CYP3A4 substrate. OBJECTIVE: To determine the potential effect of rifaximin on EE and norgestimate pharmokinetics. METHODS: In an open-label, crossover study, healthy females received a single (EE 0.07 mg/norgestimate 0.50 mg). Following a 1 week washout iduals received rifaximin 200 mg every 8 hours for 3 days, with a of OC administered with the ninth rifaximin dose. During both eriods, blood samples were collected periodically for up to 96 hours C dose. Plasma concentration-time profiles and pharmacokinetic were characterized for EE and 2 major metabolites of norgestimate, (NG) and 17-deacetyl norgestimate (17-DNGM). A drug-drug interaction was confirmed if the 90% CI for the 2 treatment period comparison the 80-125% limit. RESULTS: Twenty-six of 28 women completed the study. No differences in netic parameters were observed for EE, NG, or 17-DNGM when a of the OC was administered alone or with rifaximin. In addition, the he bioavailability contrasts (OC alone vs OC with rifaximin) for the lasma concentration, area under the plasma concentration-time ero to the last measurable plasma concentration or to infinity for EE, -DNGM all ranged from 86-118%. These intervals were within the range for equivalence; therefore, no interaction was observed and rifaximin. Rifaximin was well tolerated. CONCLUSIONS: Administration of a 3 day dosing regimen of oral rifaximin was well tolerated and did not alter the pharmacokinetics of a commonly used combination OC containing EE and norgestimate.