RATIONALE: Drugs used in chemotherapy, such as irinotecan hydrochloride, fluorouracil, and leucovorin calcium, work in different ways to stop the growth of tumor cells, either by killing the cells or by stopping them from dividing. Zibotentan may be effective in treating metastatic colorectal cancer that has not responded to oxaliplatin. It is not yet known whether combination chemotherapy is more effective when given with or without zibotentan in treating metastatic colorectal cancer.