Metabolism of a G Protein-Coupled Receptor Modulator, Including Two Major 1,2,4-Oxadiazole Ring-Opened Metabolites and a Rearranged Cysteine-Piperazine Adduct

Metabolites of a G protein-coupled receptor modulator containing 1,2,4-oxadiazole and piperazine substructures were identified in vitro in human, rat, and dog hepatocyte incubates and in vivo in rat plasma, bile, urine, and feces by using C-14-radiolabeled parent compound. Exposure coverage for the major circulating metabolites in humans at steady state and in preclinical species used in drug safety assessments was determined by using pooled plasma samples collected from a human multiple ascending dose study and a 3-month rat toxicokinetic study. Metabolites M1 and M2, which were formed by opening of the 1,2,4-oxadiazole ring, were observed as major metabolites both in vitro and in vivo across species. The carboxylic acid metabolite M2 was presumably formed through reductive N-O bond cleavage of the oxadiazole ring and subsequent hydrolysis. However, the mechanism for the formation of the unusual N-cyanoamide metabolite M1 remains uncertain. Neither M1 nor M2 had any target activity, as did parent drug P. In rat bile, rearranged Cys-piperazine and Gly-Cys-piperazine adducts, involving the formation of a five-membered heteroaromatic imidazole derivative from a six-membered piperazine ring, were observed as minor metabolites. These findings support a previously reported mechanism regarding glutathione detoxification for piperazine bioactivation products.