In vitro/in vivo investigations to examine the gender differences in the pharmacokinetics of novel oral Janus kinase (JAK) inhibitor ASP015K and sulfate metabolite M2 in rats

1. Although marked gender differences have been reported for the exposure level of the sulfate metabolite M2 of ASP015K in rats, no such differences have been reported for the unchanged drug. To clarify the cause of these pharmacokinetic gender differences, we investigated the in vitro hepatic sulfation, glucuronidation, and cytochrome P450 (CYP) metabolism of ASP015K in rat liver cytosols or rat liver microsomes. Further, in vivo excretion and metabolic profiles were investigated using rat urine, bile, and feces post-ASP015K administration. 2. In vitro metabolism study using liver cytosols clearly suggested that the gender differences in the M2 exposure were mainly attributed to the female-predominant ASP015K metabolism mediated by sulfotransferase (SULT). Metabolic profiles in urine and bile from male rats suggested that the major elimination pathway of ASP015K is glucuronidation in rats. No remarkable gender differences in the in vitro glucuronidation were observed. 3. The contribution of the sulfation pathway to the clearance of ASP015K was markedly lower than that of the glucuronidation pathway in both male and female rats. These results might explain why gender differences were not marked for ASP015K exposure but were for M2.