Irinotecan is an antineoplastic enzyme inhibitor primarily used in the treatment of colorectal cancer. It is a derivative of camptothecin that inhibits the action of topoisomerase I. Irinotecan prevents religation of the DNA strand by binding to topoisomerase I-DNA complex, and causes double-strand DNA breakage and cell death.
Irinotecan is dissolved in DMSO and stored, and then diluted with appropriate medium before use. 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.