Relative bioavailability of topiramate administered rectally

Objective: To determine the relative bioavailability and tolerability of a topiramate (TPM) suspension after rectal administration. Design/method: Seven healthy men and five healthy non-pregnant women were enrolled. A 100 or 200mg tablet of TPM was given orally and a 200mg dose was given rectally in a randomized, open-label, crossover study with at least a 2-week washout period between doses. Plasma samples were collected prior to dosing and the following times after each dose: 0.5, 1, 1.5, 2, 3, 4, 6, 8, 12, 16, 24, 48, 72, and 96h. Relative bioavailability was determined by calculating the ratio of the dose-normalized area under the curve (AUC/D) for the rectal and oral doses. Results: Ten subjects completed the study. Two of the first seven subjects who received a 200mg initial oral dose, withdrew because of side effects. The remaining subjects received a 100mg oral dose. Three subjects received a 200mg dose orally and rectally, and seven subjects received 100mg orally and 200mg rectally. The average AUC/D was 0.72±0.18h/l for the rectal dose and 0.76±0.20h/l for the oral dose. The relative bioavailability (n=10) for TPM administered rectally was 0.95±0.17 with a range of 0.68-1.2. There were no statistically significant differences between the oral or rectal pharmacokinetic parameters. Conclusions: In healthy adults, rectally administered TPM is absorbed to a similar extent as the oral dosage form. Rectal administration is an acceptable route of administration for TPM, when the oral route is temporarily unavailable.