CPT-11 and 5Fluorouracil (5FU) combined with leucovorin has become the standard of care for colorectal cancer. PN401 permits treatment with higher than normal doses of 5FU, which could increase its therapeutic potential. It is hypothesized that adding PN401 to the CPT-11, 5FU, leucovorin regimen will reduce toxicity and will allow higher doses of 5FU to be well tolerated and therefore potentially increase effectiveness.