Intramuscular absorption of carbamazepine in rhesus monkeys

The intramuscular absorption characteristics of carbamazepine were investigated in a group of six chair-adapted rhesus monkeys from three parenteral formulations [A:100 mg/ml of carbamazepine in PEG-400; B:50 mg/ml of carbamazepine in PEG-400; and C:50 mg/ml of carbamazepine in a PEG-400-Tween-80 mixture (9:1)]. The absolute bioavailability was determined by administering formulations A or B intravenously. The kinetic profiles obtained after intramuscular administration suggested biphasic absorption in the majority of animals: an initial rapid absorption phase yielding peak concentrations in less than 1 hr followed by a slower phase where absorption was probably rate limiting. The absolute bioavailability was 38% from formulation A, 81% from formulation B and 82% from formulation C. In two of four cases, Tween-80 eliminated the rate-limiting absorption phase. The data suggest that an intramuscular formulation of carbamazepine with acceptable bioavailability may be feasible.