The Food and Drug Administration approved a new drug for people whose colorectal cancer has returned despite all available treatments. Irinotecan, to be sold by Pharmacia & Upjohn Inc. under the brand name Camptosar, becomes the first alternative for these patients in 40 years. About 134,500 Americans are found to have colon or rectal cancer each year. Ninety percent of patients whose tumors are caught early survive. When colorectal cancer recurs or spreads to other parts of the body, the standard therapy is 5-fluorouracil.