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.