Multiplicity of biliary excretion mechanisms for irinotecan, CPT-11, and its metabolites in rats

We have reported previously that a canalicular multispecific organic anion transporter (cMOAT) is responsible for the biliary excretion of carboxylate forms of irinotecan, 7-ethyl-10-[4-(1-piperidino)-1 piperidino] carbonyloxy camptothecin (CPT-11), its active metabolite SN-38, and glucuronide conjugate (SN38-Glu) and the lactone form of SN38-Glu in rats. In this paper, the multiplicity of biliary excretion mechanisms for these four anionic compounds was investigated using isolated liver bile canalicular membrane vesicles (CMVs) obtained from Sprague Dawley rats. For the carboxylate form of CPT-11 and the lactone and carboxylate forms of SN38-Glu, ATP-dependent uptake consisted of both high- and low-affinity components in CMVs. Mutual inhibition studies with S-(2,4-dinitrophenyl)glutathione, a representative substrate for cMOAT, and the uptake study using CMVs from Eisai hyperbilirubinemic rats revealed that cMOAT is responsible for the biliary excretion of the low-affinity component of the carboxylate form of CPT-11 and the high-affinity component of both the lactone and carboxylate forms of SN38-Glu, whereas the high-affinity component for CPT-II and the low-affinity component for SN38-Glu, which are expressed in Eisai hyperbilirubinemic rats, are governed by a transporter different from cMOAT. The carboxylate form of SN-38 was found to be transported by cMOAT alone. We conclude that multiple transporters, including cMOAT, are responsible for the biliary excretion of CPT-II and its metabolites (anionic forms), and the contribution of each transporter differs greatly, depending on the substrates.