The metabolic fate of izencitinib, a gut-selective pan-JAK inhibitor, in humans. Identification of unusual fecal metabolites and implications for MIST evaluation

Izencitinib is a novel, orally administered pan-JAK inhibitor designed as a gut-selective therapy that was under development for the treatment of inflammatory bowel disease. The objectives of this study were to define the mass balance, routes and rates of excretion, and metabolic fate of izencitinib after oral administration of [ 14 C]-izencitinib in humans. Six healthy adult male subjects were administered a single 100 mg (~300 μCi) oral dose of [ 14 C]-izencitinib. Fecal excretion was the dominant route of elimination with >90% of the administered dose recovered in the feces. As expected by design, plasma concentrations of total radioactivity and izencitinib were low with the mean terminal half-life of total radioactivity (138 h) exceeding that of izencitinib (32.4 h). Izencitinib represented approximately 17% of the total circulating radioactivity, suggesting the presence of multiple circulating plasma metabolites. However, no individual metabolite exceeded 10% of total drug-related material in plasma. The major metabolites in feces, M18 and M9, were found to have unusual structures that reflected the presence of a nucleophilic carbon center in the naphthyridine ring of izencitinib. Proposed mechanisms for the formation of these metabolites involved oxidation and rearrangement (M18) and a one-carbon addition, potentially occurring through reaction with endogenous formaldehyde. Given the gut-selective properties of izencitinib, it is proposed that these novel fecal metabolites are the most relevant for evaluating the impact of metabolism on the pharmacological and toxicological properties of izencitinib, and that the circulating plasma metabolite profile is of little consequence in the assessment of the safety characteristics of izencitinib metabolites.