Tissue Distribution and Characterization of Drug-Related Material in Rats and Dogs after Repeated Oral Administration of Casopitant

Casopitant [1-piperidinecarboxamide,4-(4-acetyl-1-piperazinyl)-N-((1R)-1-(3,5-bis(trifluoromethyl)phenyl)-ethyl)-2-(4-fluoro-2-methylphenyl)-N-methyl-(2R,4S)] has been shown to be a potent and selective antagonist of the human neurokinin 1 receptor, the primary receptor for substance P. During long-term toxicity studies conducted in rat and dog, evidence of cardiomyopathy and increased cardiac weight were observed. The distribution and metabolism of casopitant were studied in both species evaluating the accumulation of drug-related material (DRM) after repeat dosing and its potential relationship with pathological findings observed in myocardium. After repeat oral administration of [(14)C] casopitant to rats (20 days) and dogs (14 days), DRM was quantifiable in all of the tissues examined with lung and liver containing the highest level of radioactivity. The concentration of radioactivity was significantly higher in tissues than in plasma, declining slowly and still quantifiable after a recovery period of 20 days. The principal circulating components identified in both species were casopitant, M12 (oxidized deacetylated), M13 (hydroxylated piperazine), and M31 and M134 (two N-dealkylated piperazines). In tissues, a similar metabolic pattern was observed, in which casopitant, M31, M134, M76 (N-deacetylated), and M200 (N-deacetylated N,N-deethylated) were the major components quantified. After a 26-week repeat dose study in dog, casopitant and M13 were the major circulating components, whereas in myocardium, M200 and M134 were the major ones and their levels increased over time, reaching considerable concentrations (millimolar magnitude). After a washout period, all circulating derivatives decreased to undetectable levels, whereas M200 was still the major component in myocardium. Overall DRM in plasma did not correlate with the respective concentrations in tissues.