In this study the characteristics and alterations of the gut microbiome during chemotherapy for metastasized or irresectable CRC are studied, as well as the relation between the gut microbiome and the effects of chemotherapy.

Although systemic treatment for metastasized or irresectable colorectal cancer (CRC) is becoming increasingly effective, overall survival is still poor and side effects of current treatment modalities are substantial. There is an urgent need for novel therapies, and in addition, better predictive tools are needed to select the right drug to the right patient. New data suggest that modulation of the microbiome of the gut might provide opportunities to increase anti-tumor efficacy of immunotherapy. Whereas the relation between the gut microbiome and immunotherapy is intensively studied, the relation between the gut microbiome and efficacy of conventional chemotherapy is unknown. A better understanding of the composition, function and dynamics of the gut microbiome before and during chemotherapy might help to identify factors that can be influenced during the treatment of patients with metastasized or irresectable CRC.

Therefore, in this study the characteristics and alterations of the gut microbiome during chemotherapy for metastasized or irresectable CRC are studied, as well as the relation between the gut microbiome and the effects of chemotherapy.

Inclusion Criteria:

• Age ≥ 18 years
• Patients with histologically confirmed CRC with an indication for palliative systemic anti-tumor therapy (ANY combination of chemotherapy with/without anti-VEGF of anti-EGFR therapy)
• Measurable disease according to RECIST v1.1.
• Stored pathological specimens available
• Life expectancy ≥ 12 weeks
• Signed Informed Consent Form
• Ability to comply with protocol

Exclusion Criteria:

• Previous (neo)adjuvant chemotherapy < 6 months
• Previous radiotherapy on the small or large intestine < 1month
• Previous surgery of the small or large intestine < 1 month
• Uncontrolled inflammatory bowel disease
• Participation in a study with a potential effect on the gut microbiome

Patients who present in the hospitals of the North Netherlands Colorectal Cancer Network. The participating hospitals together cover 10% of the Dutch population. Given that 13.000 patients yearly are diagnosed with colorectal cancer of which one third will develop metastatic or irresectable disease (www.iknl.nl), in the hospitals participating in this study 400 potential patients will present. Taking into account that about 50% do not fulfil in- and exclusion criteria or will refuse to participate, about 200 patients per year are expected to be included.