Cetuximab, Irinotecan, Toripalimab in RAS/BRAF Wild-type Ultraselected Right-sided Colorectal Cancer Study

Inclusion Criteria:

* Histologically confirmed colorectal adenocarcinoma.
* Primary tumor located in the right colon.
* Metastatic disease with at least one measurable lesion according to RECIST v1.1 criteria.
* Histologically tested as RAS/BRAF V600E wild-type and negative ultraselected for mutations including: RAS/BRAF V600E/PIK3CA/PTEN/EGFR (ECD), HER2 and MET amplification, and ALK/RET/NTRK1 gene fusions.
* Patients who have progressed after previous treatments including bevacizumab, irinotecan, oxaliplatin, and 5-fluorouracil, with tumor progression occurring during or within 3 months after irinotecan treatment.
* No prior treatment with anti-EGFR or PD-1 antibodies.
* Normal hematological function (platelets \>90×10\^9/L; white blood cells \>3×10\^9/L; neutrophils \>1.5×10\^9/L).
* Serum bilirubin ≤1.5 times the upper limit of normal (ULN), transaminases ≤5 times ULN.
* No ascites, normal coagulation function, albumin ≥35 g/L.
* Liver function classified as Child-Pugh grade A.
* Serum creatinine less than ULN, or calculated creatinine clearance \>50 ml/min (using the Cockcroft-Gault formula).
* At least one measurable lesion according to RECIST v1.1 criteria.
* ECOG performance status of 0-2.
* Expected survival \>3 months.
* Signed written informed consent.
* Willing and able to undergo follow-up until death or study completion or termination.

Exclusion Criteria:

* Severe arterial thrombosis or ascites.
* Bleeding tendencies or coagulation disorders.
* Hypertensive crisis or hypertensive encephalopathy.
* Severe uncontrolled systemic complications such as infections or diabetes.
* Clinically significant cardiovascular diseases such as cerebrovascular accident (within 6 months prior to enrollment), myocardial infarction (within 6 months prior to enrollment), uncontrolled hypertension despite appropriate medication, unstable angina, congestive heart failure (NYHA grade 2-4), or arrhythmias requiring medication.
* History of or physical examination showing central nervous system diseases (e.g., primary brain tumor, uncontrolled epilepsy, any history of brain metastasis or stroke).
* Other malignancies within the past 5 years (except for basal cell carcinoma of the skin after curative surgery and/or carcinoma in situ of the cervix).
* Use of immunosuppressive drugs within 1 week before treatment, excluding nasal, inhaled, or other topical steroids or physiological doses of systemic steroids (i.e., not exceeding 10 mg/day of prednisone or an equivalent dose of other steroids) or steroids used to prevent contrast agent allergies.
* Steroid-dependent interstitial lung disease.
* Known active autoimmune disease requiring symptomatic treatment or history of such disease within the past 2 years. Patients with vitiligo, psoriasis, alopecia, or -Graves' disease not requiring systemic treatment within the past 2 years, hypothyroidism requiring only thyroid hormone replacement, and type I diabetes requiring only insulin replacement can be enrolled.
* Known history of primary immunodeficiency.
* Known active tuberculosis.
* Known history of allogeneic organ transplantation or allogeneic hematopoietic stem cell transplantation.
* Receipt of any investigational drug treatment within the last 28 days before the study.
* Allergy to any drugs in the study.
* Pregnant or breastfeeding women.
* Women of childbearing potential (within 2 years of last menstruation) or men capable of fathering a child who are not using or refuse to use effective non-hormonal contraceptive methods (e.g., intrauterine device, barrier method combined with spermicide, or sterilization).
* Inability or unwillingness to comply with the study protocol.
* Presence of any other disease, functional impairment caused by metastatic lesions, or suspicious conditions found during a physical examination indicating a contraindication to the study drugs or high risk for treatment-related complications.