Pentobarbital absorption from capsules and suppositories in humans

Serum pentobarbital levels following administration of the sodium salt as a 100-mg capsule orally and as two 120-mg suppository formulations (A and B) rectally were measured. From these data and previously determined kinetic constants after intravenous administration, the absorption rates and bioavailability of pentobarbital from each dosage form were determined. All three dosage forms were 100% absorbed. Peak serum pentobarbital levels occurred at 1, 4, and 10 hr for the capsule, Suppository A, and Suppository B, respectively. In vitro studies agreed with the serum data in that Suppository A released drug in an in vitro aqueous pH 1.4 system at a much greater rate that Suppository B. The capsule and Suppository A both appeared to be absorbed by simple first-order processes; however, Suppository B had a complex absorption pattern, which was modeled using sequential zero-order and first-order absorption.