Increased theophylline clearance in asthmatic patients due to terbutaline

Summary The pharmacokinetic mechanism of the theophylline-terbutaline interaction has been studied. Sustained release theophylline 200–400 mg b.d. was given with placebo or terbutaline 2.5 mg t.d.s. to six adult asthmatic patients.Terbutaline decreased the serum trough theophylline levels from 8.1 to 7.3 µg/ml, improved daily the clinical score from 1.51 to 1.26 and increased the peak expiratory flow rate from 316 to 370 l/min. In a single dose study following the chronic therapy, it was shown that there was no change in the peak theophylline concentration or in the timing of the peak, but the t_1/2 was reduced from 9.0 to 7.5 h, and the systemic clearance was increased from 20.2 to 24.8 ml·h^–1·kg^–1.Thus, terbutaline reduced the serum theophylline concentration by increasing its systemic clearance.