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PHARMACOKINETICS

Absence of Effect of Oral Rifaximin on the Pharmacokinetics of Ethinyl Estradiol/Norgestimate in Healthy Females

at time of study, Vice President, Clinical Research and Development, ICON Development Solutions, Ellicott City, MD; now, Head, Clinical Research and Development, AkaRx, Inc., Columbia, MD

Margaret Connolly, PhD

Director, Biostatistics, ICON Development Solutions

Helen Pentikis, PhD

at time of study, Global Vice President, Pharmacokinetics and Pharmacodynamics, ICON Development Solutions; now, Head, Clinical Pharmacology, AkaRx, Inc.

William P Forbes, PharmD

Vice President, Research & Development and Chief Development Officer, Salix Pharmaceuticals, Morrisville, NC

Doug K Bettenhausen, PharmD

Executive Director, Medical Affairs, Salix Pharmaceuticals

Reprints: Dr. Bettenhausen, Salix Pharmaceuticals, 1700 Perimeter Park Dr., Morrisville, NC 27560, fax 919/228-4215, Doug. Bettenhausen{at}salix.com

BACKGROUND: Rifaximin is an oral rifampin analog, and its activity is targeted within the gastrointestinal tract. Some analogs induce the cytochrome P450 family of oxidative enzymes. Ethinyl estradiol (EE), commonly found in oral contraceptives (OCs), is a known CYP3A4 substrate.

OBJECTIVE: To determine the potential effect of rifaximin on EE and norgestimate pharmacokinetics.

METHODS: In an open-label, crossover study, healthy females received a single dose of OC (EE 0.07 mg/norgestimate 0.50 mg). Following a 1 week washout period, individuals received rifaximin 200 mg every 8 hours for 3 days, with a single dose of OC administered with the ninth rifaximin dose. During both treatment periods, blood samples were collected periodically for up to 96 hours after each OC dose. Plasma concentration-time profiles and pharmacokinetic parameters were characterized for EE and 2 major metabolites of norgestimate, norgestrel (NG) and 17-deacetyl norgestimate (17-DNGM). A drug-drug interaction was confirmed if the 90% CI for the 2 treatment period comparison was outside the 80-125% limit.

RESULTS: Twenty-six of 28 women completed the study. No differences in pharmacokinetic parameters were observed for EE, NG, or 17-DNGM when a single dose of the OC was administered alone or with rifaximin. In addition, the 90% CI for the bioavailability contrasts (OC alone vs OC with rifaximin) for the maximum plasma concentration, area under the plasma concentration-time curve from zero to the last measurable plasma concentration or to infinity for EE, NG, and 17-DNGM all ranged from 86-118%. These intervals were within the predefined range for equivalence; therefore, no interaction was observed between OC 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.

Key Words: norgestrel, pharmacokinetics, rifaximin

Published Online, February 6, 2007. www.theannals.com, DOI 10.1345/aph.1H395

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