How long does the pharmacokinetic interaction between carbamazepine and quetiapine last after carbamazepine withdrawal?

OBJECTIVES

Carbamazepine and quetiapine are drugs that are used as mood stabilizers in the treatment of bipolar disorders. A series of studies has shown that concurrent use of carbamazepine decreases quetiapine serum level due to induction of CYP3A enzymes by carbamazepine.

METHODS

In a 30-year-old bipolar patient with mania treated with quetiapine 1200 mg and carbamazepine 900 mg per day, we measured quetiapine serum level before and after carbamazepine withdrawal.

RESULTS

No serum quetiapine was detected during concurrent use of carbamazepine and was lower than the therapeutic range almost 2 weeks after carbamazepine withdrawal. The patient suffered from sedation when her serum level of quetiapine was 181 ng/ml and because she was quiet we started slowly to decrease to a quetiapine dose of 600 mg. Her serum level (45 ng/ml) was again below therapeutic levels after 3 weeks of carbamazepine withdrawal.

CONCLUSION

We hypothesize that induction of CYP3A lasts even after carbamazepine withdrawal. Our hypothesis was confirmed during the next treatment of mania. The patient had been off carbamazepine for 1 year and her serum level was four times higher (210 ng/ml) on 600 mg of quetiapine than 3 weeks after carbamazepine withdrawal. The influence of carbamazepine on CYP3A enzymes lasted at least 3 weeks after carbamazepine withdrawal which is in accordance with CYP3A de-induction lasting 3 weeks. This could be important information for psychiatrists to know that in some patients it is better to use a minimum washout period of 3 weeks for carbamazepine before new treatment with quetiapine.