No effect of phenobarbital pretreatment of rats on methotrexate pharmacokinetics in the isolated liver perfused in a single-pass way

Pretreatment of the rat with phenobarbital (PB) is known to increase gene e xpression of the canalicular multispecific organic anion transporter (cMOAT ) and hepatobiliary transport of its substrates (glutathione, sulfobromopht halein), To determine the effect of PB on the hepatobiliary transport of me thotrexate (MTX, another substrate of cMOAT) and its metabolism to 7-hydrox ymethotrexate (7-OHMTX) in the rat, we compared the steady-state pharmacoki netics of MTX in the isolated liver of either PB-pretreated (80 mg/day/kg b w for 4 days, i.p.) or nonpretreated rats. The livers were perfused in a si ngle-pass way at a flow rate of 15 ml/min using a perfusate which consisted of Krebs-Henseleit buffer containing glucose, taurocholate, bovine albumin and erythrocytes. During the perfusion with 50 mu mol/l MTX, the steady-st ate biliary clearance (1.26 +/- 0.24 ml/min) in 7 nonpretreated rats accoun ted for a major proportion of the hepatic clearance (1.30 +/- 0.33 ml/min), metabolism of MTX to 7-OHMTX was minor (partial metabolic clearance = 0.04 1 +/- 0.023 ml/min). MTX concentrations in bile surpassed those in the inpu t perfusate by approximately 100-fold, Pretreatment of rats (n = 7) with PB did not change significantly the steady-state hepatic, biliary and partial metabolic clearances of 50 mu mol/l MTX. An interesting result is a 38% in crease in the hepatic vascular resistance of non-pretreated livers caused b y MTX. The results suggest that in rats, pretreatment with PB has no effect on the hepatobiliary transport and hydroxylation of MTX, Copyright (C) 200 1 S. Karger AG, Basel.