Toxicity/Benefit Ratio Optimization of Chemotherapy in Colorectal Cancer (CRC) Patients by Determination of Individual Genotypic Determinants

Inclusion Criteria: * Has provided written informed consent prior to study-specific screening procedures, with the understanding that the patient has the right to withdraw from the study at any time without prejudice * Ages between 18 and 85 years * Histologically confirmed colorectal cancer * No treatment for metastatic disease * No irinotecan previously administered * World Health Organization (WHO) performance status \< 3 * Laboratory values : * neutrophils \> 1.5 x 10\^9/L; * platelet count \> 100 x 10\^9/L; * serum creatinine \< 130µmol/L; * serum bilirubin \< 2 x upper limit of normal (ULN); * ASAT and ALAT \< 2.5 x ULN; * alkaline phosphatase \< 5 x ULN. * At least one measurable lesion by Response Evaluation Criteria in Solid Tumors (RECIST) criteria Exclusion Criteria: * History of another malignancy except cured basal cell carcinoma of the skin or carcinoma in situ of the uterine cervix, breast or bladder. * Other concomitant anticancer therapy. * Pregnant or lactating women. * Women of childbearing potential unless using a reliable and appropriate contraceptive method. * Symptomatic cerebral or leptospiral metastasis. * Intestinal obstruction. * Uncontrolled seizures (diabetes, severe infection). * Clinically significant cardiac disease. * Central nervous system disorders or severe psychiatric disability. * Participation in any investigational study within 4 weeks.