Pemetrexed Plus a Comparator Versus a Combination of 2 Comparators in First-Line Treatment of Colorectal Cancer

In this study, patients will receive either pemetrexed plus irinotecan or 5-fluorouracil (5-FU), leucovorin, and irinotecan. The purposes of this study are to determine: * How pemetrexed plus irinotecan compares with 5-FU, leucovorin, and irinotecan in terms of efficacy. * The safety of pemetrexed plus irinotecan and any side effects that might be associated with it as compared with 5-FU, leucovorin, and irinotecan. * Whether pemetrexed can help patients with colorectal cancer.