Oral administration of terbutaline in asthmatic patients

The absorption and excretion profiles of orally administered terbutaline were investigated in 10 asthmatic patients who received a solution containing 5 mg of [3H]terbutaline sulphate. Bronchodilating effect and side-effects were followed. The mean serum concentration curve reached its maximum of 3.0 +/- 0.3 (SEM) ng/mL at 60-90 min. Renal clearance data indicated that terbutaline was excreted only via glomerular filtration. Approximately 30% of the dose was excreted in the urine in 12 h and 40% in 72 h. Three compounds appeared in the urine: unchanged terbutaline, a sulphate conjugate, and a glucuronide, the main metabolite being the sulphate conjugate. The maximum mean increase in volume of air expelled in the first second of forced expiration (FEV1) was 35%, attained 90-120 min after intake of terbutaline. There was a correlation (r = 0.74) between the serum level and the bronchodilator effect. No significant effects on heart rate or blood pressure were found. Tremor, as objectively measured with the aid of an accelerometer, was experienced by the patients when it increased to twice its basal level. The observed tremor did not appear to parallel either the serum level or the bronchodilating effect. A comparative study in 3 of the patients showed that [3H]terbutaline formulated as a tablet had similar bioavailability.