Purpose: Because enzyme-inducing antiepileptic drugs (AEDs) can affect pharmacokinetics of oral contraceptives and thereby cause contraceptive failure, the potential effect of topiramate, a new AED, on the pharmacokinetics of the combination oral contraceptive norethindrone/ethinyl estradiol was evaluated.
Methods: Twelve women receiving stable valproic acid (VPA) monotherapy for epilepsy received a combination norethindrone 1.0 mg/ethinyl estradiol 35-microg tablet daily for 21 days followed by seven daily doses of inert tablets for four 28-day cycles. After a baseline cycle (cycle 1), topiramate 100, 200, and 400 mg every 12 h was administered in cycles 2 through 4, respectively. Serial blood samples were obtained on day 20 of each cycle and were analyzed for norethindrone, ethinyl estradiol, and progesterone by using validated radioimmunoassay methods.
Results: Compared with cycle 1, none of the norethindrone pharmacokinetic parameters changed significantly in the presence of topiramate, 100-400 mg every 12 h. Individual patient serum progesterone concentrations measured during each cycle were at or close to the limit of quantification with no apparent differences among cycles. However, mean area under the concentration-versus-time curve over the 24-h period (AUC(0-24)) values for ethinyl estradiol were 18-30% lower in cycles 2 through 4 compared with cycle 1 (p < or = 0.05 for all pairs), whereas mean oral serum clearance (CL/F) values were 14.7-33.0% higher (p < or = 0.05 for cycles 2 and 4 vs. cycle 1). Mean time of peak concentration (T(max)) values determined during topiramate therapy were not significantly different from those at baseline.
Conclusions: When prescribing an oral contraceptive for patients receiving topiramate, clinicians should consider initial therapy with an agent containing > or = 35 microg of ethinyl estradiol.
PIP: The efficacy of combined oral contraceptives (OCs) is diminished in women taking enzyme-inducing anti-epileptic drugs such as phenytoin, phenobarbital, and carbamazepine. In preliminary in vitro studies, a new anti-epileptic drug derived from D-fructose, topiramate, produced no clinically relevant inhibitory effects on the metabolism of such drugs as barbiturates, classic neuroleptics, and tricyclic antidepressants. To assess this new drug, 12 women with documented histories of epilepsy took an OC containing 1 mg norethindrone and 35 mcg ethinyl estradiol as well as topiramate (100-400 mg every 12 hours) for 4 menstrual cycles. Serial blood samples were obtained on day 20 of the 4 cycles. None of the norethindrone pharmacokinetic parameters changed significantly in the presence of topiramate. Ethinyl estradiol serum levels were reduced by an average of 30% from baseline. The mean area under the concentration-versus-time curve over the 24-hour period values for ethinyl estradiol were 18-30% lower in cycles 2-4 than the baseline cycle and mean oral serum clearance values were 14.7-33.0% higher. This compares favorably with the 40-72% reductions in progestin and estrogen levels recorded in women taking a levonorgestrel-containing OC and enzyme-inducing anti-epileptics. Although topiramate's modest interaction with OCs is not likely to interfere with contraceptive efficacy, the reduction in serum estrogen concentrations has the potential to increase the incidence of breakthrough bleeding, indicating the OC should contain at least 35 mcg of estrogen.