Study of Petosemtamab Plus Chemotherapy Versus Cetuximab Plus Chemotherapy in RAS and BRAF Wild-type, Unresectable or Metastatic, Left-sided Colorectal Cancer (LiGeR-CRC1)

Key Inclusion Criteria: * Histologically or cytologically confirmed left-sided colorectal adenocarcinoma that is unresectable or metastatic. * Must have documented KRAS and NRAS wild type (wt) colorectal cancer (CRC), as determined by medical record of results from local testing or as assessed by central testing. Local testing must have been conducted in accordance with local guidelines using an Food and Drug Administration (FDA)-approved test or a laboratory-developed test that is validated in a Clinical Laboratory Improvement Amendments (CLIA)-certified laboratory (sites in the United States) or an accredited local laboratory (sites outside of the United States). Next-generation sequencing (NGS)-based test results from tumor tissue are required for determining eligibility. Polymerase chain reaction (PCR)-based tests, sanger sequencing, or pyrosequencing test results are not allowed. * Has not received any prior systemic therapy for unresectable or metastatic CRC. * Must be eligible for treatment with mFOLFOX6 (if assigned to receive mFOLFOX6) or FOLFIRI (if assigned to receive FOLFIRI) according to local regulatory approvals and SOC guidelines. Key Exclusion Criteria: * BRAF mutation, and/or microsatellite instability-high (MSI-H)/deficient mismatch repair (dMMR) and/or protocol specified tumor status as documented by local test results in the medical record or from central testing or known documented activating HRAS mutation identified prior to enrollment from local testing results in the medical record, if available. * Prior exposure to any agents that target epidermal growth factor receptor (EGFR) (including but not limited to protein products, monoclonal antibodies, tyrosine kinase inhibitors, or antisense oligonucleotide therapy). * Known complete dihydropyrimidine dehydrogenase (DPD) deficiency or known homozygous/compound heterozygous dihydropyrimidine dehydrogenase gene (DPYD) variants associated with complete loss of DPD activity. Testing for DPD deficiency should be performed per local guidelines. * For a participant who is to receive FOLFIRI: known to be homozygous for the UGT1A1\*28 or \*6 alleles or compound or double heterozygous for the UGT1A1\*28 and \*6 alleles. Testing for UGT1A1 should be done in accordance with local guidelines. * Participants with non-colorectal adenocarcinomatous disease. Note: Other protocol-defined Inclusion and Exclusion criteria may apply.