The Metabolism and Disposition of Brepocitinib in Humans and Characterization of the Formation Mechanism of an Aminopyridine Metabolite

<s><s></s></s> Brepocitinib is an oral once-daily Janus kinase 1 and Tyrosine kinase 2 selective inhibitor currently in development for the treatment of several autoimmune disorders. Mass balance and metabolic profiles were determined using accelerator mass spectrometry in six healthy male participants following a single oral 60 mg dose of ¹⁴C-brepocitinib (~300 nCi). The average mass balance recovery was 96.7% {plus minus} 6.3% with the majority of dose (88.0% {plus minus} 8.0%) recovered in urine and 8.7% {plus minus} 2.1% of the dose in recovered in feces. Absorption of brepocitinib was rapid, with maximal plasma concentrations of total radioactivity and brepocitinib achieved within 0.5 hours after dosing. Circulating radioactivity consisted primarily of brepocitinib (47.8%) and metabolite M1 (37.1%) derived from hydroxylation at the C59 position of the pyrazole ring. Fractional contributions to metabolism via cytochrome P450 (CYP) enzymes were determined to be 0.77 for CYP3A4/5 and 0.14 for CYP1A2 based on phenotyping studies in HLM. However, additional clinical studies are required to understand the potential contribution of CYP1A1. Approximately 83% of the dose was eliminated as N-methylpyrazolyl oxidative metabolites, with 52.1% of the dose excreted as M1 alone. Notably, M1 was not observed as a circulating metabolite in earlier metabolic profiling of human plasma from a multiple ascending study with unlabeled brepocitinib. Mechanistic studies revealed M1 was highly unstable in human plasma and phosphate buffer, undergoing chemical oxidation leading to loss of the 5-hydroxy-1-methylpyrazole moiety and formation of aminopyrimidine cleavage product M2. Time dependent inhibition and trapping studies with M1 yielded insights into the mechanism of this unusual and unexpected instability. Significance Statement This work describes the mass balance and metabolic profile of brepocitinib in human, a JAK1/TYK2 inhibitor being developed for treatment of autoimmune diseases.