In vitro activity: Irinotecan is activated to SN-38 by carboxylesterases to become able to interact with its target, topoisomerase I. Irinotecan induces similar amounts of cleavable complexes at its IC50 in LoVo cells and HT-29 cell lines. SN-38 induces a concentration-dependent formation of cleavable complexes, which is not significantly different in LoVo cells and HT-29 cell lines. Cell accumulation of Irinotecan is markedly different, reaching consistently higher levels in HT-29 cells than in LoVo cells. The lactone E-ring of Irinotecan and SN-38 hydrolyses reversibly in aqueous solutions, and the interconversion between the lactone and carboxylate forms is dependent on pH and temperature. Liver is primarily responsible for the activation of Irinotecan to SN-38. At equal concentrations of Irinotecan and SN-38 glucuronide, the rate of beta-glucuronidase-mediated SN-38 production is higher than that formed from Irinotecan in both tumour and normal tissue.

Cell Assay: Exponentially growing cells (LoVo and HT-29 cells) are seeded in 20 cm2 Petri dishes with an optimal cell number for each cell line (2 × 104 for LoVo cells, 105 for HT-29 cells). They are treated 2 days later with increasing concentrations of Irinotecan or SN-38 for one cell doubling time (24 hours for LoVo cells, 40 hours for HT-29 cells). After washing with 0.15 M NaCl, the cells are further grown for two doubling times in normal medium, detached from the support with trypsin-EDTA and counted in a hemocytometer. The IC50 values are then estimated as the Irinotecan or SN-38 concentrations responsible for 50% growth inhibition as compared with cells incubated without Irinotecan or SN-38.

Cancer Chemother Pharmacol. 2002 Apr;49(4):329-35; Int J Cancer. 1997 Jan 27;70(3):335-40.