Physiological disposition and metabolism of (3H)bitolterol in man and dog

The metabolism and disposition of bitolterol, the di-p-toluate ester of N-t-butylarterenol (tBA) was studied in man after a single oral dose and in dog after intraduodenal, iv, or oral administration. The mean (+/- SE) peak plasma radioactivity in man (dose, 70 mug/kg) was 180 +/- 18 ng equivalents of [3H]bitolterol per ml or approximately 11% of the dose, whereas peak plasma radioactivity in dog (dose, 200 mug/kg) was 144 +/- 23 ng equivalents per ml or approximately 4% of the dose. For both man and dog, the time for maximum plasma level of radioactivity varied from 0.5 to 2 hr. In man, only 1% of the plasma radioactivity represented intact [3H]bitolterol 1.0 hr after medication. In the dog, radioactivity was concentrated in lung tissue after iv administration of [3H]bitolterol. Recovery of intact [3H]bitolterol in lung at 4.5 hr ranged from 26 to 46% of total tissue radioactivity after iv dosage and from 4 to 14% total tissue radioactivity after intraduodenal administration. Radioactivity recovered in human urine and feces (0-72 hr) accounted for 86 and 8.1% of the dose, respectively. Recovery of radioactivity in dog urine and feces accounted for 58 and 23% of the dose, respectively, in the same time period. Radiochromatograms of urine samples from man and dog revealed similar patterns of metabolites including free and conjugated forms of both tBA and the 3-O-methyl metabolite, N-t-butylmetarterenol. The major radioactive components of the feces were bitolterol and tBA. The results indicate that bitolterol is absorbed orally and retained as the intact ester in lung. The prolonged bronchodilator activity of bitolterol is due to the slow release of the ester from lung and hydrolysis to tBA, an active beta2-adrenoceptor agonist. Pharmacological activity is terminated by metabolism of tBA via conjugation or 3-O-methylation.