title: The importance of tumor glucuronidase in the activation of irinotecan in a mouse xenograft model.
abstract: The anticancer drug irinotecan (CPT-11) is activated to the potent topoisomerase I inhibitor, SN-38 (7-ethyl-10-hydroxycamptothecin), by esterases. SN-38 is in turn conjugated to the inactive SN-38 glucuronide (SN-38G). The reverse reaction is mediated by beta-glucuronidases. Hence, production of SN-38 may occur through either pathway. In this study we conducted in vitro studies to examine these two reactions in neuroblastoma xenograft tumors (NB1691) and compared the rates of SN-38 production with those observed in the liver and plasma of the host SCID (severe-combined immunodeficient) mice. The rate of formation of SN-38 from CPT-11 by esterases slowed considerably during a 60-min incubation, consistent with the known deacylation-limited nature of this reaction. For xenograft tumor tissue, K(m) and V(max) values of 1.6 microM and 4.4 pmol/min/mg of protein, respectively, were observed. By comparison, these parameters were estimated to be 6.9 microM and 9.4 pmol/min/mg for mouse liver and 2.1 microM and 40.0 pmol/min/mg for mouse plasma, respectively. The formation of SN-38 from SN-38G was very pronounced in both liver and xenograft tumor tissue, in which it was nonsaturable (0.125-50 microM) and time-independent (0-60 min). The derived values of V(max)/K(m) were 0.65 microl/min/mg for the tumor and 2.12 microl/min/mg for the liver preparations. Microdialysate experiments revealed the concentrations of SN-38G and CPT-11 in tumor to be comparable. At equal substrate concentrations, production of SN-38 from SN-38G in tumor extracts was comparable with that from CPT-11. Therefore, reactivation of SN-38 in the tumor by beta-glucuronidases may represent an important route of tumor drug activation for CPT-11.
authors.affiliation: Department of Pharmacology, University of Sydney, New South Wales, Australia.
authors.forename: Helen M
authors.initials: HM
authors.lastname: Dodds
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Tobin
authors.forename: Clinton F
authors.initials: CF
authors.lastname: Stewart
authors.forename: Pam
authors.initials: P
authors.lastname: Cheshire
authors.forename: Suzan
authors.initials: S
authors.lastname: Hanna
authors.forename: Peter
authors.initials: P
authors.lastname: Houghton
authors.forename: Laurent P
authors.initials: LP
authors.lastname: Rivory
mesh_terms: D000465:Algorithms; D000818:Animals; D000972:Antineoplastic Agents, Phytogenic / Q000008:administration & dosage / Q000378:metabolism*; D001711:Biotransformation / Q000187:drug effects; D001932:Brain Neoplasms / Q000378:metabolism; D002166:Camptothecin / Q000008:administration & dosage / Q000031:analogs & derivatives* / Q000378:metabolism*; D002265:Carboxylic Ester Hydrolases / Q000378:metabolism; D002851:Chromatography, High Pressure Liquid; D005260:Female; D005966:Glucuronidase / Q000378:metabolism*; D006801:Humans; D000077146:Irinotecan; D007700:Kinetics; D051379:Mice; D016513:Mice, SCID; D017551:Microdialysis; D009368:Neoplasm Transplantation; D009447:Neuroblastoma / Q000378:metabolism; D014183:Transplantation, Heterologous
chemical_list: D000972:Antineoplastic Agents, Phytogenic; D000077146:Irinotecan; D002265:Carboxylic Ester Hydrolases; D005966:Glucuronidase; D002166:Camptothecin