Metabolism of Terbutaline in Man and Dog

1 The metabolism of terbutaline (1-(3,5-dihydroxyphenyl)-2-tertiarybutylamino-ethanol), a drug which acts selectively on β(2)-adrenoceptors has been studied in man and dog. 2 The sulphate conjugate of terbutaline was the only metabolite identified in plasma or urine from humans or dogs given the drug either orally or intravenously. We have confirmed that terbutaline, a resorcinol derivative, is not a substrate for catechol-0-methyl transferase in man or dog. 3 Metabolism of terbutaline was dependent upon the route of administration in man, but not in dog. In man, oral terbutaline was largely conjugated whereas drug administered intravenously appeared largely unchanged in urine. 4 Conjugation of oral terbutaline, which probably occurred during the `first-pass' through the gut wall or liver, was less than oral isoprenaline. This may explain why the poorly absorbed terbutaline is a more effective oral bronchodilator than isoprenaline, even though the latter drug is completely absorbed following oral dosing.