Vol. 303, Issue 2, 649-655, November 2002

The Importance of Tumor Glucuronidase in the Activation of Irinotecan in a Mouse Xenograft Model

Helen M. Dodds, Peter J. Tobin, Clinton F. Stewart, Pam Cheshire, Suzan Hanna, Peter Houghton and Laurent P. Rivory

Department of Pharmacology, University of Sydney, New South Wales, Australia (H.M.D., P.J.T., L.P.R.); Medical Oncology, the Sydney Cancer Centre, Camperdown, New South Wales, Australia (L.P.R.); and Departments of Pharmaceutical Sciences (C.F.S., S.H.) and Molecular Pharmacology (P.C., P.H.), St Jude's Children's Research Hospital, Memphis, Tennessee

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 -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 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 of V_max/K_m 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 -glucuronidases may represent an important route of tumor drug activation for CPT-11.