Abstract:We have investigated the glucuronidation in vitro of SN-38, the active metabolite of irinotecan, a semi-synthetic anticancer drug derived from ZO(S)camptothecin. Preparations of human hepatic microsomes (final concentration : 1 mg prot./ml), were incubated for 1 hr in 0.1 M Tris buffer, pH 7.4, containing 10 mM MgC12, in the presence of UDPglucuronic acid (4 mM), saccharolactone (4 mM), and a detergent. Microsomes from five livers were studied individually or as a pooled preparation. SN-38, either in its lactone or its carboxylate form, was added at a range of concentrations. The SN-38 P-glucuronide formed was measured by HPLC with fluorometric detection. The glucuronidation reaction appeared linear over 1 hr in these conditions and Brij 35 at 0.5 mg/mg prot. was the best activator. The apparent parameters of the reaction were independent of the molecular form of the substrate. The half-saturation constant was 17-20 pM and Vmax was 60-75 pmol/min./mg prot. The interindividual variation of SN-38 glucuronidation was relatively low (ratio of 1.8 between extreme values). In addition, the effect of twelve drugs currently associated with irinotecan in clinics was evaluated in this system (drug concentration: 100 pM; SN-38 concentration: 5 pM). These produced little if any interference with SN-38 glucuronidation. Therefore, major interferences of this transformation by comedications are unlikely to occur in viva Irinotecan (7-ethyl-l0-[4-( 1-piperidin0)-1-piperidinolcarbonyloxycamptothecin), also known as CPT-11, is a semisynthetic derivative of 20(S)camptothecin with promising activity in colorectal cancer (Rougier & Bugat 1996). Camptothecins stabilize transient "cleavable" complexes formed between DNA and the nuclear enzyme topoisomerase I, which is required for DNA processing leading to DNA damage. This action appears to be more important than inhibition of the catalytic activity of topoisomerase I per se, and camptothecins are, therefore, considered to be "poisons" of this enzyme (Pommier 1996). Irinotecan is thought to exert its anticancer activity following transformation in vivo to a highly potent metabolite, SN-38 (7-ethyl-10-hydroxycampthothecin), and can be assumed to be essentially a prodrug of SN-38 (Kawato et al. 1991).During the study of plasma pharmacokinetics of irinotecan in 19 patients treated with this drug during phase 1/11 studies, we recently identified two major circulating metabolites and identified one of these as SN-38 p-glucuronide (Rivory & Robert 1995). The total plasma concentrations of this metabolite were found to be systematically higher than those of SN-38 in all patients studied . The elimination of SN-38 p-glucuronide from plasma closely paralleled that of SN-38 itself, suggesting that the transformation of SN-38 to the glucuronide was the rate-limiting step in the elimination Author for correspondence: Jacques Robert, Institut Bergonie, 180, rue Saint-Genes, F-33076 Bordeaux Cedex, France (fax +33 556 333389).of SN-38 and could play a key role in its pharmacokinetics (Rivo...