This is a translational, multicentre and multinational study. The aim of this study is to identify new patient stratification tools in microsatellite stable RAS mutant metastatic Colorectal Cancer

The rational of COLOSSUS is to provide new and more effective stratification tools and therapeutic interventions, specifically tailored to MSS RAS mt mCRC patients

Inclusion criteria

Patient Status

1. Ability to give signed informed consent prior to any study specific procedures and willing and able to comply with the protocol,

2. Age ≥ 18 years,

3. ECOG status of ≤ 2,

4. At least 16 weeks of life expectancy at time of entry into the study.

   Disease-related

5. Histologically confirmed stage IV MSS RAS mt mCRC as per standard diagnostic tests,

6. Unresectable and measurable disease (at least one measurable target lesion) as per clinical and radiologic criteria (RECIST v.1.1),

7. No prior chemotherapy and/or radiation therapy for CRC administered in the metastatic setting,

8. Scheduled to receive or receiving SOC chemotherapy with fluoropyrimidines and oxaliplatin +/- bevacizumab (baseline blood sample should be collected and questionnaires completed during pre-screening and before chemotherapy initiation),

9. Availability of archival formalin-fixed paraffin-embedded (FFPE) tissue from:

   1. Preferred: surgical resection of the primary or metastatic tumour

      or

   2. Biopsy samples (biopsy taken at endoscope, needle core or surgical incision or excision) obtained from the primary or metastatic tumour.

The archival FFPE tissue blocks must be obtained during initial diagnosis (before patient was exposed to any chemotherapy and/or radiation therapy).

Exclusion Criteria

1. Patient with CRC that has received any therapy for CRC before:

   1. surgical resection of tissue that would be utilised in this study

      or

   2. biopsy procedure for tissue that would be utilised in this study,

2. Patient who has received any investigational product within 28 days of the first day of palliative chemotherapy administration,

3. Patient with any significant history of non-compliance to medical treatments or with inability to grant reliable informed consent,

4. Patient with clinical or laboratory contraindication to receive SOC chemotherapy with fluoropyrimidines and oxaliplatin.

All patients with stage IV MSS RAS mutant colorectal cancer