The theophylline-enoxacin interaction: I. Effect of enoxacin dose size on theophylline disposition.

Theophylline interacts pharmacokinetically with a variety of other drugs. Recently enoxacin was found to change theophylline's disposition. In a four-subject, four-way crossover study enoxacin was administered every 12 hours at four levels (0, 25, 100, and 400 mg) for 14 doses. With the ninth dose of enoxacin, 200 mg theophylline was coadministered. Blood and urine samples were assayed by sensitive and specific assays for the parent drugs and their metabolites. Significant reduction in the formation of theophylline's three major metabolites occurred on coadministration of enoxacin. At the 400 mg dose level, enoxacin caused a threefold decrease in theophylline's plasma clearance, a fourfold decrease in the urinary recovery of 3-methylxanthine and 1,3-dimethylurate, and a threefold decrease in the recovery of 1-methylurate.