Bioavailability of theophylline following a rectally administered concentrated aminophylline solution.

A two-way complete crossover bioavailability study of solutions of theophylline given orally or rectally in 16 healthy subjects was conducted. Blood samples were obtained prior to dosing and at 0.25, 0.5, 0.75, 1, 1.5, 2, 2,5, 3, 3,5, 4, 4.5, 5, 6, 7, 8, 10, 12, and 24 hr. Plasma theophylline was determined by high-pressure liquid chromatography. In the fasting subjects solutions given orally were absorbed faster (0.95 versus 1.95 hr to peak), reaching a higher peak concentration (7.26 versus 4.87 microgram/ml) than the solutions given rectally. Depending on the method of computation, solutions given rectally were estimated to be 86% to 87% as biovailable as the solutions given orally, with 75% of the subjects having 80% or greater relative bioavailability.