The transformation of irinotecan (CPT-11) to its active metabolite SN-38 by human liver microsomes Differential hydrolysis for the lactone and carboxylate forms

Abstract

Irinotecan (CPT-11) is a new camptothecine derivative presently in development for the treatment of several advanced malignancies. It is converted in vivo to a highly potent metabolite, SN-38, by carboxylesterases. All camptothecine derivatives undergo lactonolysis in a pH-dependent reversible manner, generating inactive carboxylate forms. We have investigated in vitro the kinetics of transformation of CPT-11 to SN-38 by human liver microsomes originating from several donors. Microsomes from seven livers were studied individually or as a pooled preparation. CPT-11, either in its lactone or its carboxylate form, was added at a range of concentrations. The SN-38 formed was measured by HPLC with fluorometric detection. In the deacylation-limited carboxylesterase reaction, the linear steady-state kinetics between 10 and 60 min were determined. At all concentrations of CPT-11, the steady-state velocity of SN-38 formation as well as the intercept concentrations of SN-38 were about 2-fold higher when the substrate was under the lactone form than under the carboxylate form. We estimated the values (+/-SD) of K'm and Vmax to be 23.3 +/- 5.3 microM and 1.43 +/- 0.15 pmol/min/mg for the lactone and 48.9 +/- 5.5 microM and 1.09 +/- 0.06 pmol/min/mg for the carboxylate form of CPT-11, respectively. We conclude that the greater rate of conversion of CPT-11 lactone may contribute to the plasma predominance of SN-38 lactone observed in vivo. The inter-individual variation of SN-38 formation was relatively high (ratio of 4 between extreme values) but no large age- or gender-related differences were seen. The effect of twelve drugs of different therapeutic classes (antibiotics, antiemetics, antineoplastics, antidiarrhoeics, analgesics), which could be administered in association with irinotecan in the clinical setting, was evaluated in this system (drug concentration: 100 microM; CPT-11 lactone concentration: 10 microM). Loperamide and ciprofloxacine where the only drugs exerting a weak inhibition of CPT-11 conversion to SN-38.

In vitro conversion of irinotecan to SN‐38 in human plasma

“…Although our hypothesis was not supported by this study, other mechanisms such as hepatobiliary secretion, which has already been shown to have inter-individual variability, 11,12) might have interfered in some way. Although conversion from irinotecan to SN-38 is evident in liver, 1,13,14,17,18) small intestine, 13) colon, 14) plasma 15) and tumor tissue, 14,16) their mutual correlations have not been reported. A convenient assay system for CCE activity would facilitate such research.…”

“…Actually, this issue is complicated by the fact that ex vivo studies have shown that irinotecan is also converted to SN-38 in the small intestine, 13) colon, 14) plasma 15) and tumor tissue, 14,16) and not only in the liver. 1,13,14,17,18) The mechanisms involved, including inter-individual variability of conversion efficiency, however, have not yet been clarified in detail.We investigated the inter-individual variability of the conversion efficiency from irinotecan to SN-38 by incubating irinotecan in vitro with human plasma samples and determining the time-concentration curves of SN-38. The pharmacodynamics and pharmacokinetics of the agent were also investigated in relation to the in vitro conversion efficiency.…”

“…Actually, this issue is complicated by the fact that ex vivo studies have shown that irinotecan is also converted to SN-38 in the small intestine, 13) colon, 14) plasma 15) and tumor tissue, 14,16) and not only in the liver. 1,13,14,17,18) The mechanisms involved, including inter-individual variability of conversion efficiency, however, have not yet been clarified in detail.…”

A randomised phase II multicentre trial of irinotecan (CPT-11) using four different schedules in patients with metastatic colorectal cancer

“…We also found a significant difference between the metabolic ratio of SN-38 between arms A and B, and B and C. A low, but very significant inverse correlation between dose and metabolic ratio of SN-38 was observed for arms A, B and C. A possible explanation for this change in metabolic ratio is saturation of the carboxylesterase reaction. The carboxylesterase reaction converting irinotecan into SN-38 in humans is very inefficient and deacylation dependent (Rivory et al, 1996;Haaz et al, 1997;Takimoto et al, 2000). The saturability of this enzymatic reaction may be of relevance for prolonged infusion schedules of irinotecan.…”

Determination of Drug Interactions Occurring with the Metabolic Pathways of Irinotecan: Figure 1

“…Irinotecan is a prodrug that needs to be converted to SN-38 1 (7-ethyl-10-hydroxycamptothecine; Kawato et al, 1991) through the action of carboxylesterases (Rivory et al, 1996a;Haaz et al, 1997b). SN-38 is a very potent inhibitor of topoisomerase I able to stabilize the cleavable complexes DNAtopoisomerase I. Irinotecan and SN-38 are detoxified through two major metabolic pathways.…”