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METABOLISM, TRANSPORT, AND PHARMACOGENOMICS

Differential Inhibition of Rat and Human Na⁺-Dependent Taurocholate Cotransporting Polypeptide (NTCP/SLC10A1)by Bosentan: A Mechanism for Species Differences in Hepatotoxicity

School of Pharmacy (E.M.L., K.L.R.B.) and School of Medicine (P.B.W.), University of North Carolina, Chapel Hill, North Carolina; and Division of Clinical Pharmacology, Departments of Medicine and Pharmacology, Vanderbilt University School of Medicine, Nashville, Tennessee (R.B.K.)

Bile acid accumulation in hepatocytes due to inhibition of the canalicular bile salt export pump (BSEP/ABCB11) has been proposed as a mechanism for bosentan-induced hepatotoxicity. The observation that bosentan does not induce hepatotoxicity in rats, although bosentan has been reported to inhibit rat Bsep and cause elevated serum bile acids, challenges this mechanism. The lack of hepatotoxicity could be explained if bosentan inhibited hepatocyte uptake as well as canalicular efflux of bile acids. In the current study, bosentan was found to be a more potent inhibitor of Na⁺-dependent taurocholate uptake in rat (IC₅₀ 5.4 µM) than human (IC₅₀ 30 µM) suspended hepatocytes. In addition, bosentan was a more potent inhibitor of taurocholate uptake by rat Na⁺-dependent taurocholate co-transporting polypeptide (Ntcp/Slc10a1) (IC₅₀ 0.71 µM) than human NTCP (SLC10A1) (IC₅₀ 24 µM) expressed in HEK293 cells. Thus, bosentan is a more potent inhibitor of Ntcp than NTCP, and this should result in less intrahepatocyte accumulation of bile acids in rats during bosentan treatment. To begin characterization of this species difference, two chimeric molecules were generated and expressed in HEK293 cells; NTCP_1–140/Ntcp_141–362 and Ntcp_1–140/NTCP_141–349. The mode of bosentan inhibition was noncompetitive for Ntcp, and competitive for NTCP (K_i 18 µM) and NTCP_1–140/Ntcp_141–362 (K_i 1.7 µM); bosentan affected both the K_m and V_max of Ntcp_1–140/NTCP_141–349 (K_i 7.0 µM). The carboxyl portions of NTCP and Ntcp were found to confer species differences in basal taurocholate transport V_max. In conclusion, differential inhibition of Ntcp and NTCP may represent a novel mechanism for species differences in bosentan-induced hepatotoxicity.

Address correspondence to: Dr. Kim L. R. Brouwer, CB#7360, Kerr Hall, School of Pharmacy, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27599-7360. E-mail: kbrouwer{at}unc.edu