Increased theophylline clearance in asthmatic patients due to terbutaline

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.