The study will combine a new investigational drug with standard anti-cancer drugs for the treatment of advanced colorectal cancer. The standard and approved treatment for colorectal cancer is to undergo chemotherapy with a combination of irinotecan (also known as CPT-11, Camptosar), 5-fluorouracil (also known as 5-FU), and leucovorin (also known as LV). This is known as the triple therapy. One of the major side effects of CPT-11/5-FU/LV chemotherapy treatment is diarrhea. The purpose of this research study is to see whether adding this investigational drug to the standard treatment for advanced colorectal cancer can reduce the amount of diarrhea a patient experiences. This study will also determine if adding the investigational drug to triple therapy has a positive effect on tumors.