Glucuronidation of SN-38, the active metabolite of irinotecan, by human hepatic microsomes.
We have investigated the glucuronidation in vitro of SN-38, the active metabolite of irinotecan, a semi-synthetic anticancer drug derived from 20(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 MgCl2, in the presence of UDP-glucuronic 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 beta-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 microM 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 microM; SN-38 concentration: 5 microM). These produced little if any interference with SN-38 glucuronidation. Therefore, major interferences of this transformation by comedications are unlikely to occur in vivo.
Haaz
M C
MC
Bergonié Institut, Bordeaux, France.
Rivory
L
L
Jantet
S
S
Ratanasavanh
D
D
Robert
J
J
D000972:Antineoplastic Agents, Phytogenic / Q000378:metabolism / Q000493:pharmacokinetics*; D001711:Biotransformation; D002166:Camptothecin / Q000031:analogs & derivatives* / Q000378:metabolism / Q000493:pharmacokinetics; D003902:Detergents / Q000494:pharmacology; D005965:Glucuronates / Q000378:metabolism; D006801:Humans; D007206:Individuality; D000077146:Irinotecan; D008099:Liver / Q000378:metabolism; D008862:Microsomes, Liver / Q000378:metabolism*
D000972:Antineoplastic Agents, Phytogenic; D003902:Detergents; D005965:Glucuronates; D000077146:Irinotecan; D002166:Camptothecin