The active ingredient in a drug used alone or with other drugs to treat colon cancer or rectal cancer that has spread to other parts of the body or has come back after treatment with fluorouracil. It is also being studied in the treatment of other types of cancer. Irinotecan blocks certain enzymes needed for cell division and DNA repair, and it may kill cancer cells. It is a type of topoisomerase inhibitor and a type of camptothecin analog.