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 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 deter...

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Bibliographic Details
Published in:The Annals of pharmacotherapy 2007-02, Vol.41 (2), p.222-228
Main Authors: Trapnell, Carol Braun, Connolly, Margaret, Pentikis, Helen, Forbes, William P, Bettenhausen, Doug K
Format: Article
Language:English
Subjects:
Administration, Oral
Adolescent
Adult
Anti-Bacterial Agents - administration & dosage
Anti-Bacterial Agents - blood
Anti-Bacterial Agents - pharmacology
Biological and medical sciences
Contraceptives, Oral, Combined - blood
Contraceptives, Oral, Combined - pharmacokinetics
Cross-Over Studies
Cytochrome P-450 CYP3A
Cytochrome P-450 Enzyme System - metabolism
Drug Interactions
Ethinyl Estradiol - administration & dosage
Ethinyl Estradiol - blood
Ethinyl Estradiol - pharmacokinetics
Female
Humans
Medical sciences
Middle Aged
Norgestrel - administration & dosage
Norgestrel - analogs & derivatives
Norgestrel - blood
Norgestrel - pharmacokinetics
Pharmacology. Drug treatments
Rifamycins - administration & dosage
Rifamycins - blood
Rifamycins - pharmacology
Rifaximin
Therapeutic Equivalency
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Summary: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.
ISSN:1060-0280
1542-6270
DOI:10.1345/aph.1H395