A randomized controlled clinical study of recombinant high potency antitumor antiviral protein injection (Lefunen) for the treatment of metastatic colorectal cancer that has failed in the second line and beyond. The mechanism of action is different from chemotherapy drugs and other targeted drugs, in that it has the effect of enhancing the immune function of the body while fighting against tumors. Patients aged 18 years or older with a clear pathological diagnosis of metastatic colorectal adenocarcinoma who have received previous chemotherapy with irinotecan and oxaliplatin-based regimens and have failed treatment may be considered for enrollment in this study. The study treatment will continue until the patient develops intolerable toxicity or disease progression, and may be continued after the time point of disease progression is reached if the potential benefit of the study drug is still considered after disease progression. The study drug and related tests and laboratory fees required for the study are provided free of charge.