Received for publication December 28, 2005.
Revised May 6, 2006.
Accepted for publication May 9, 2006.

Species Differences in Metabolism and Pharmacokinetics of a Sphingosine-1-Phosphate Receptor Agonist in Rats and Dogs: Formation of a Unique Glutathione Adduct in the Rat

Abstract

The pharmacokinetics and metabolism of MRL-A, a selective agonist for the S1P₁ receptor, were investigated in rats and dogs. In both species, more than 50% of the dose was excreted in bile. Specific to the rat, and observed in bile, were a taurine conjugate of MRL-A and a glucuronide conjugate of an azetidine lactam metabolite. In dog, a smaller portion of the dose (54% of administered dose) was excreted intact in bile and the major metabolites detected were an azetidine N-oxide of MRL-A and an acylglucuronide of an N-dealkylation product. This latter metabolite was also observed in rat bile. Stereoselective formation of the N-oxide isomer was observed in dogs while the rat produced comparable amounts of both isomers. The formation of a unique glutathione adduct was observed in rat bile, which was proposed to occur via N-dealkylation, followed by reduction of the putative aldehyde product to form the alcohol, and dehydration of the alcohol to generate a reactive quinone methide intermediate. Incubation of a synthetic standard of this alcohol in rat microsomes fortified with reduced glutathione or rat hepatocytes resulted in formation of this unique glutathione adduct.

Key words: biliary excretion, bioactivation, cytochrome P450 catalyzed oxidations, glutathione conjugates, metabolite identification, pharmacokinetics, reactive intermediate, UDP glucuronyltransferases