In separate studies, potential pharmacokinetic interactions of topiramate (TPM) with phenytoin (PHT), carbamazepine (CBZ), and valproate (VPA) were evaluated. TPM was added to the baseline antiepileptic drug (AED) at a dosage of up to 800 mg/day, after which the baseline drug was discontinued when possible, Addition of TPM produced no change in plasma levels of CBZ or CBZ epoxide (CBZ-E). Modest increases in PHT plasma levels in six of 12 patients treated with PHT and TPM, and a small mean decrease in VPA levels noted in patients receiving VPA with TPM, were considered unlikely ro require adjustments in the dosage of the concomitant AED when TPM is added or discontinued, When patients were changed from concomitant therapy with PHT or CBZ to TPM monotherapy, TPM clearance was reduced by approximately 50%, suggesting that an adjustment in TPM dose may be required when PHT or CBZ is discontinued from TPM-treated patients. A slight increase in plasma TPM levels during monotherapy compared to concomitant therapy with VPA was considered clinically insignificant and not likely to require TPM dosage adjustment. In another study, oral clearance of digoxin was slightly increased when TPM was added, resulting in a small decrease in peak plasma levels of digoxin. In vitro studies conducted to date on a number of specific cytochrome P450 isoforms show an effect of TPM only on the CYP2C(meph) isoform, The risk for clinically meaningful changes in plasma levels of traditional AEDs when TPM is added to or discontinued from concomitant regimens appears to be minimal, However, adjustments in TPM dosages are likely to be needed when potent enzyme inducers, such as PHT or CBZ, are added or discontinued. TPM has a relatively low propensity for clinically significant drug interactions, and its pharmacokinetic and drug interaction profiles represent a clear advance over those of the traditional AEDs.
