The Role of the Tumor Molecular Profile (CMS), UGT1A1 Genotype and Beta-glucuronidase Activity of the Intestinal Microbiota for Treatment Efficiency, Toxicity, Survival and Quality of Life in Patients With Metastatic or Unresectable Colorectal Cancer During Irinotecan-based Systemic Treatment

Inclusion Criteria: * Adult patient: 18 years of age or older * Patients diagnosed with metastatic or unresectable CRC, who will be treated with irinotecan-based systemic therapy with or without anti-eGFR treatment. * WHO performance status 0-2 * Minimal acceptable safety laboratory values defined as: * ANC of ≥ 1.5 x 109 /L * Platelet count of ≥ 100 x 109 /L * Hepatic function as defined by serum bilirubin ≤ 1.5 x ULN, ALAT and ASAT ≤ 2.5 x ULN; in case of liver metastases ALAT and ASAT ≤ 5 x ULN. * Renal function (eGFR) ≥ 50 ml/min or OR creatinine ≤ 1.5 x ULN * Written informed consent Exclusion Criteria: * Microsatellite instability (MSI) or deficient MMR proteins * Pregnant or nursing * Presence of ileostomy * Asian ethnicity * Other systemic treatment is less than one month before the start of the irinotecan-based treatment * Therapeutic antibiotic use is less than three months before the start of the irinotecan-based treatment * Abdominal radiotherapy is less than two weeks before the start of the irinotecan-based treatment * Physically or mentally incapable or incompetent * More than 25% irinotecan dose reduction at the start of treatment (dose reductions during treatment are allowed), with exception of dose reduction due to UGT1A1 mutation.