Drug Interaction Study of Tegretol (Carbamazepine) and St. John's Wort in Normal Volunteers

Overview

St. John's Wort is a popular dietary supplement that many patients-including those with epilepsy or seizures-take in addition to their regular medicines to elevate mood or relieve stress. Preliminary research indicates that this supplement can speed the metabolism of the anti-seizure drug Tegretol, causing reduced blood levels of the drug. Patients who take Tegretol to control their seizures may have more frequent seizures if the blood level of the drug drops too low. A recent study shows that this effect is not seen when Tegretol is taken for at least 3 weeks. The present study will examine whether there is a medically important drug interaction between St. John's wort and Tegretol when Tegretol is taken for 1 day. Normal healthy volunteers between 21 and 65 years old who are not taking medicines that can affect the metabolism of drugs in the liver and have not used St. John's wort for at least 30 days may be eligible for this 25-day study. Participants will take a 400-mg dose of Tegretol after fasting overnight. Blood samples will be drawn the next day during a 12-hour clinic stay at the following intervals: just before the Tegretol dose and at 1, 2, 4, 6, 8, 10, 24, 34, 48 and 72 hours after the dose. A catheter will be placed in the vein to prevent the need for multiple needle sticks until after the 10-hour sample. After completing the blood sampling, participants will take 300 mg of St. John's wort 3 times a day with meals for 2 weeks. After 2 weeks, another fasting dose of Tegretol will be given and the 72-hour blood study will be repeated. This study may provide information important for the care of patients with epilepsy who take both Tegretol and St. John's Wort.

Alternative or complementary therapies are widely used by patients with epilepsy. St. John's Wort is one of the most popular herbal dietary supplements with a variety of claims including mood elevation and stabilization, stress relief, anti-viral effects, and enhancement of the immune system. We have previously demonstrated that treatment with St. John's Wort for 14 days does not induce the clearance of carbamazepine under steady state conditions of autoinduction of carbamazepine clearance. One proposed mechanism for this lack of effect is that St. John's Wort is not sufficiently potent to further induce previously induced CYP3A4 enzyme activity. The purpose of this study is to investigate the effect of St. John's Wort on single dose carbamazepine pharmacokinetics under conditions of uninduced CYP3A4. To evaluate this potential drug interaction, as well as the effect of St. John's Wort on epoxide hydrolase activity, eight normal healthy subjects will be enrolled into this pharmacokinetic study. Subjects will take a single 400 mg dose of carbamazepine followed by serial blood sampling over 72 hours for carbamazepine and carbamazepine epoxide plasma concentrations. Subjects will then begin taking a standardized formulation of St. John's Wort three times daily with meals for two weeks. Following 2 weeks of St. John's Wort subjects will then again take a single 400 mg dose of carbamazepine and have serial blood samples collected for carbamazepine and carbamazepine epoxide plasma concentrations. The total carbamazepine and carbamazepine epoxide exposure (AUC), oral clearance (CL/F), and maximal concentrations (Cmax) will be compared between treatment phases to characterize the potential drug interaction.