TY - JOUR T1 - The Importance of Tumor Glucuronidase in the Activation of Irinotecan in a Mouse Xenograft Model JF - Journal of Pharmacology and Experimental Therapeutics JO - J Pharmacol Exp Ther SP - 649 LP - 655 DO - 10.1124/jpet.102.039040 VL - 303 IS - 2 AU - Helen M. Dodds AU - Peter J. Tobin AU - Clinton F. Stewart AU - Pam Cheshire AU - Suzan Hanna AU - Peter Houghton AU - Laurent P. Rivory Y1 - 2002/11/01 UR - http://jpet.aspetjournals.org/content/303/2/649.abstract N2 - 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 β-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, Km andVmax values of 1.6 μM and 4.4 pmol/min/mg of protein, respectively, were observed. By comparison, these parameters were estimated to be 6.9 μM and 9.4 pmol/min/mg for mouse liver and 2.1 μM 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 μM) and time-independent (0–60 min). The derived values ofVmax/Km were 0.65 μl/min/mg for the tumor and 2.12 μl/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 β-glucuronidases may represent an important route of tumor drug activation for CPT-11. The American Society for Pharmacology and Experimental Therapeutics ER -