Vol. 30, Issue 7, 778-787, July 2002

Metabolism of a Thiazole Benzenesulfonamide Derivative, a Potent and Selective Agonist of the Human ₃-Adrenergic Receptor, in Rats: Identification of a Novel Isethionic Acid Conjugate

Wei Tang, Ralph A. Stearns, Randall R. Miller, Jason S. Ngui, Robert J. Mathvink, Ann E. Weber, Gloria Y. Kwei, John R. Strauss, Carol A. Keohane, George A. Doss, Shuet-Hing L. Chiu, and Thomas A. Baillie

Department of Drug Metabolism (W.T., R.A.S., R.R.M., J.S.N., G.Y.K., J.R.S., C.A.K., G.A.D., S-H.L.C., T.A.B.) and Medicinal Chemistry (R.J.M., A.E.W.), Merck Research Laboratories, Rahway, New Jersey.

(R)-N-[4-[2-[[2-Hydroxy-2-(pyridin-3-yl)ethyl]amino]ethyl]phenyl]- 4-[4-(4-trifluoro-methylphenyl)thiazol-2-yl]benzenesulfonamide (1) is a potent and selective agonist of the human ₃-adrenergic receptor. We report herein the data from studies of the metabolism and excretion of 1 in rats. Five metabolites were identified in the bile of male Sprague-Dawley rats administered ³H-labeled 1 by either oral gavage (10 mg/kg) or intravenous injection (3 mg/kg). These included a pyridine N-oxide derivative (M2), a primary amine resulting from N-dealkylation and loss of the pyridinyl-2-hydroxyethyl group (M4), a carboxylic acid derived from N-dealkylation and loss of the pyridyl-2-hydroxyethyl amine (M5), and the corresponding taurine and isethionic acid conjugates (M1 and M3). Metabolites M1 and M3 also were identified in rats treated with M5 and were generated in incubations of M5 with rat liver subcellular fractions in the presence of ATP and coenzyme A with supplementary taurine or isethionic acid. These results suggest that M5 is the precursor of M1 and M3 and that the formation of these conjugated metabolites follows similar mechanisms of amino acid conjugation. On the other hand, M2, M4, and M5 were produced from 1 in an NADPH-dependent manner in incubations with liver microsomes from rats, dogs, monkeys, and humans. In human liver preparations, these routes of biotransformation were shown to be catalyzed by cytochrome P450 3A4. In a bidirectional transport assay, transport of 1 across a monolayer of cells expressing P-glycoprotein (Pgp) was observed to be similar to that of vinblastine, which is an established substrate of the transporter protein. This finding, together with the observation that the parent compound was excreted in the feces of bile duct-cannulated animals following intravenous dosing, suggests that 1 is subject to Pgp-mediated excretion from intestine of rats.