Analysis of the Disposition of a Novel p38 MAPK Inhibitor, AKP-001, and Its Metabolites in Rats with a Simple Physiologically Based Pharmacokinetic Model

5-[(2-Chloro-6-fluorophenyl)acetylamino]-3-(4-fluorophenyl)-4-(4-pyrimidinyl)isoxazole (AKP-001) is a potent p38 mitogen-activated protein kinase inhibitor that is being developed to specifically target the intestines for the treatment of inflammatory bowel disease. According to the ante-drug concept, AKP-001 was designed to be metabolized to inactive forms via the first-pass metabolism to avoid undesirable systemic exposure. The purpose of this study is to investigate the pharmacokinetic characteristics of AKP-001 and its metabolites (M1 and M2) in rats, utilizing a simple physiologically based pharmacokinetic (PBPK) model. In vitro metabolic activity of AKP-001 in the S9 fraction of rat liver was examined, and plasma concentration-time profiles were developed following intravenous and/or oral administration of AKP-001 and its metabolites. AKP-001 was primarily metabolized to M1; however, M2 was not detected in liver S9 fractions. In accordance with this observation in vitro, M2 was detected in plasma after oral dosing of AKP-001 with a lag time of 1.5 hours, but not after intravenous dosing. To analyze pharmacokinetics in rats in vivo, a simple PBPK model was developed by simultaneous fitting of the plasma concentrations after treatment with AKP-001 and its metabolites. The observed plasma concentration-time profiles of AKP-001 and metabolites were described by the model adequately. Intestinal and systemic exposures of AKP-001 were simulated using the model to assess the relationship between pharmacokinetics and efficacy/safety. Model analysis suggested that oral bioavailability of intestine-targeting ante-drugs should be low to avoid systemic side effects. The pharmacokinetic properties of AKP-001 meet this criterion owing to extensive first-pass metabolism.