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School of Pharmacy, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina (X.T., M.J.Z.-G., J.L., A.S.B., K.N., K.L.R.B.); and Eli Lilly and Company, Drug Disposition, Indianapolis, Indiana (N.J.P., T.J.R.)
Previous studies implicated P-glycoprotein (P-gp) as the major transport protein responsible for the biliary excretion of fexofenadine (FEX). However, FEX biliary excretion was not impaired in P-gp- or breast cancer resistance protein (Bcrp)-knockout mice or multidrug resistance-associated protein 2 (Mrp2)-deficient rats. The present study tested the hypothesis that species differences exist in the transport protein primarily responsible for FEX biliary excretion between mice and rats. Livers from Mrp2-knockout (Mrp2KO) mice and Mrp2-deficient (TR-) rats were perfused in a single-pass manner with 0.5 µM FEX. N-(4-[2-(1,2,3,4-Tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl]-phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide (GF120918) (10 µM) was employed to inhibit P-gp and Bcrp. The biliary excretion rate of FEX was decreased 85% in Mrp2KO relative to wild-type mice (18.4 ± 2.2 versus 122 ± 34 pmol/min/g liver). In mice, more than 50% of FEX unbound intrinsic biliary clearance (
= 3.0 ml/h/g liver) could be attributed to Mrp2 (Mrp2-dependent 
1.7 ml/h/g liver), with P-gp and Bcrp playing a minor role (P-gp- and Bcrp-dependent 0.3 ml/h/g liver). Approximately one third of FEX was attributed to unidentified mechanisms in mice. In contrast to mice, FEX biliary excretion rate (245 ± 38 and 250 ± 25 pmol/min/g liver) and (9.72 ± 2.47 and 6.49 ± 0.68 ml/h/g liver) were comparable between TR- and control Wistar rats, respectively, suggesting that unidentified transport mechanism(s) can completely compensate for the loss of Mrp2 function in rats. Mrp2 clearly plays a major role in FEX biliary excretion in mice. In conclusion, remarkable species differences exist in FEX hepatobiliary transport mechanisms.
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