Irinotecan Hydrochloride (CPT-11 hydrochloride) is the hydrochloride salt of a semisynthetic derivative of camptothecin. Irinotecan, a prodrug, is converted to a biologically active metabolite 7-ethyl-10-hydroxy-camptothecin (SN-38) by a carboxylesterase-converting enzyme. One thousand-fold more potent than its parent compound irinotecan, SN-38 inhibits topoisomerase I activity by stabilizing the cleavable complex between topoisomerase I and DNA, resulting in DNA breaks that inhibit DNA replication and trigger apoptotic cell death. Because ongoing DNA synthesis is necessary for irinotecan to exert its cytotoxic effects, it is classified as an S-phase-specific agent.

Increasing concentrations of Irinotecan (0.1, 1, 10, 100, and 1000 μg/mL) inhibits the growth of all cell lines in a dose-dependent manner. COLO-357 cells are most sensitive and HT29 most resistant to Irinotecan in the MTT assay on incubation for 90 min. The IC50 concentrations are 100, 50, 5.4, 23 and 46 μg/mL Irinotecan for HT29, NMG 64/84, COLO-357, MIA PaCa-2 and PANC-1, respectively[1].

The time course of body weight change after Irinotecan (100 mg/kg i.p.) injection shows a significant dosing time-dependent difference (P<0.01). Mean maximum body weight loss is observed between days 3 and 4 after Irinotecan (CPT-11) injection. The minimum mean body weight loss is observed after Irinotecan (CPT-11) injection at 1700 hr. Moreover, the maximum mean body weight loss is observed after Irinotecan injection at 0500 or 0900 hr[2].

Irinotecan hydrochloride is dissolved in DMSO and stored, and then diluted with appropriate medium before use[1]. To determine the effects of Irinotecan in combination with 5-FU, the MTT assay is used. Depending on the cell lines, 10,000 to 20,000 cells per well are seeded in 96-well plates and incubated for 24 h in complete medium. On day 2, cells are incubated in the absence or presence of Irinotecan for 30 min followed by 5-FU for 24 h. After another 24 h in complete medium without any additives, MTT reagent is added on day 4 to initiate the assay and the cells are incubated for an additional 4 h at 37°C. After removal of the medium and dissolving the crystals with acidified isopropanol, the samples are analyzed using an ELISA plate reader at 570 nm. The value at 650 nm is subtracted as background[1].