The purpose of this study is to determine the treatment effect of QL1203 in combination with mFOLFOX6 compared to Placebo in combination with mFOLFOX6 as first line therapy for metastatic colorectal cancer.

The study is a randomized, double-blind, Placebo-controlled, multi-center Phase III study. It is planned to enroll 590 patients with previously untreated wild-type RAS metastatic colorectal cancer. Subjects are randomized into the QL1203 combined with Oxaliplatin/5-fluorouracil/ Leucovorin or Placebo combined with Oxaliplatin/5-fluorouracil/ Leucovorin treatment group by a ratio of 2:1.

Inclusion Criteria:

• Diagnosed as metastatic colorectal adenocarcinoma and is not suitable for local treatment such as radical resection, radiotherapy, radiofrequency and so on.
• Man or woman at least 18 years old.
• At least one measurable lesion can be evaluated according to Response Evaluation Criteria In Solid Tumors 1.1（RECIST1.1） criteria.
• Eastern Cooperative Oncology Group (ECOG) performance status should be 0-1 before randomization.
• The level of organ function must meet the requirements before randomization.
• Prior to randomization, the damage caused by other treatments had recovered to < grade 2 (CTCAE version 4.03).

Exclusion Criteria:

• Prior systemic or local chemotherapy for colorectal cancer，except in the following cases: the interval between the last dose of neoadjuvant or adjuvant therapy and recurrence> 6 months。
• Prior epithelial growth factor receptor(EGFR)-targeted drugs for colorectal cancer.
• Presence of central nervous system (CNS) metastases before the informed consent was signed, except for those who had stabilized CNS metastases for more than 4 weeks and had no symptoms after treatment.
• History of malignancies other than colorectal cancer within 5 years prior to randomization, excluding cutaneous basal cell carcinoma, cervical carcinoma in situ, and thyroid papillary adenocarcinoma of non-melanoma after radical treatment.
• History of interstitial lung disease.
• Existing intestinal obstruction before randomization, active inflammatory bowel disease.
• Patients with non-healing abdominal fistula and gastrointestinal perforation before randomization.
• There were severe active infections or uncontrollable infections that required systemic treatment and could not be enrolled at the decision of the investigator within 14 days before randomization.