Biliary Excretion Mechanism of CPT-11 and Its Metabolites in Humans: Involvement of Primary Active Transporters

Abstract

After administration of CPT-11, a camptothecin derivative exhibiting a wide spectrum of antitumor activity, dose-limiting gastrointestinal toxicity with great interpatient variability is observed. Because the biliary excretion is a major elimination pathway for CPT-11 and its metabolites [an active metabolite, 7-ethyl-10-hydroxy-camptothecin (SN-38), and its glucuronide, SN38-Glu], several hypotheses for the toxicity involve biliary excretion. Here, we investigated whether primary active transport is involved in the biliary excretion of anionic forms of CPT-11 and its metabolites in humans using bile canalicular membrane vesicles (cMVs). Uptake of the carboxylate form of CPT-11 and the carboxylate and lactone forms of SN38-Glu by cMVs prepared from five human liver samples was ATP dependent. The concentration dependence of the ATP-dependent uptake of the carboxylate form of CPT-11 and SN38-Glu suggests the involvement of at least two saturable transport components, both with lower affinity and higher capacity than in rats. The ATP-dependent uptake of the carboxylate form of SN-38 showed a single saturable component but was detectable only in one human cMV sample. Both carboxylate and lactone forms of SN38-Glu uptake also showed a large intersample variability, although the variability was less than that observed for the carboxylate form of SN-38. On the other hand, the carboxylate form of CPT-11 exhibited much less variability. The carboxylate forms of SN38-Glu and SN-38 almost completely inhibited the ATP-dependent uptake of leukotriene C₄, a well-known substrate of canalicular multispecific organic anion transporter, whereas the inhibition by the carboxylate form of CPT-11 was not as marked. Thus, multiple primary active transport systems are responsible for the biliary excretion of CPT-11 and its metabolites, and the major transport system for CPT-11 differs from that for the other two compounds. A greater degree of inter-cMV variability in the uptake of SN-38 and SN38-Glu may imply that interindividual variability in biliary excretion of these metabolites might contribute to interpatient variability in the toxicity caused by CPT-11.