Blood samples for PK analysis will be collected from patients with colorectal cancer during 1st treatment cycle of irinotecan and 2nd, 3rd infusion. During the 1st treatment cycle, blood will be drawn 0 h (before irinotecan infusion), 0.75 h, 1.5 h and each at time ranges of 2~8 h, 8~16 h, 24~32h and 48~52 hours after the start of irinotecan infusion over 90 min and additional blood will be collected 48~52 hours after the respective 2nd and 3rd infusion.