Abstract

The aim of this study was to investigate the functional importance of multidrug resistance-associated protein (Mrp)3/Abcc3 and Mrp4/Abcc4 in the pharmacokinetics of methotrexate. Compared with the corresponding wild-type mice, the plasma concentrations of methotrexate given orally were similar in Abcc4^-/- mice and were significantly lower in Abcc3^-/- mice. Pharmacokinetic parameters related to hepatobiliary transport were determined under steady-state conditions in wild-type and Abcc3^-/- mice that were given a constant intravenous infusion of methotrexate. The biliary clearance, based on the plasma concentration, was 1.6-fold greater in Abcc3^-/- mice than in wild-type mice (23 and 15 ml/min/kg, respectively, P < 0.05). Because the basolateral uptake and canalicular efflux clearances of methotrexate were similar in wild-type and Abcc3^-/- mice, this result suggests that the basolateral efflux clearance of methotrexate is decreased in the liver of Abcc3^-/- mice. Furthermore, a lower fraction of absorption of methotrexate (F_a F_g) was suggested in Abcc3^-/- mice (0.49 and 0.29 in wild-type and Abcc3^-/- mice, respectively). The mucosal-to-serosal transport rate of methotrexate, determined in vitro using everted sacs, was highest in the duodenum and was significantly decreased in Abcc3^-/- mice compared with wild-type mice. This is ascribed to the reduced intrinsic efflux clearance of methotrexate across the serosal membrane (22 and 5.3 μl/min/sac in wild-type and Abcc3^-/- mice, respectively, P < 0.05). These results suggest that Mrp3 mediates basolateral efflux of methotrexate in the liver and duodenum, thereby serving to increase systemic exposure, whereas Mrp4 is likely to play only a limited role in the systemic methotrexate exposure.