Biliary excretion mechanism of CPT-11 and its metabolites in humans: involvement of primary active transporters.
After administration of CTP-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.
Chu
X Y
XY
Graduate School of Pharmaceutical Sciences, University of Tokyo, Bunkyo-ku, Japan.
Kato
Y
Y
Ueda
K
K
Suzuki
H
H
Niinuma
K
K
Tyson
C A
CA
Weizer
V
V
Dabbs
J E
JE
Froehlich
R
R
Green
C E
CE
Sugiyama
Y
Y
D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D000972:Antineoplastic Agents, Phytogenic / Q000378:metabolism* / Q000494:pharmacology; D001646:Bile / Q000378:metabolism*; D001648:Bile Canaliculi / Q000378:metabolism; D002166:Camptothecin / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D006801:Humans; D000077146:Irinotecan; D017997:Leukotriene C4 / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley
D000972:Antineoplastic Agents, Phytogenic; C106003:SN38-Glu; D017997:Leukotriene C4; D000077146:Irinotecan; D000255:Adenosine Triphosphate; D002166:Camptothecin