Elimination pathways of terbutaline.

The main elimination pathways of tritium-labelled terbutaline have been investigated in the rat, the dog, and man. In the rat, 25% of an intravenous dose is excreted unchanged in the urine. Terbutaline is extensively metabolized to the glucuronic acid conjugate which is eliminated via bile (40% of the dose) and urine (25% of the dose). After oral administration, a high first-pass metabolism (70%) was found. In contrast, in the dog, more than 90% of a parenteral dose is excreted renally, largely as unchanged terbutaline together with a small amount of the sulphate conjugate. Only 1.7% of the dose is excreted via bile. The first-pass metabolism amounted to 13%. The elimination of terbutaline in man exhibits a pattern intermediate between rat and dog. Thus, more than 90% of a parenteral dose is eliminated in the urine, of which about 2/3 is unchanged drug. The main metabolite is the sulphate conjugate which is excreted renally. Less than 1% of the dose is excreted in the bile. A large first-pass metabolism (69%) was confirmed in man.