Preoperative immunotherapy with atezolizumab and tiragolumab in patients with colorectal liver metastases—the PURPLE trial

PURPLE is an international, open-label, multicenter, randomized phase II 'window of opportunity' trial. Patients with resectable CRLM are randomized 2 : 1 to receive two cycles of atezolizumab (840 mg) plus tiragolumab (420 mg) before surgery (experimental arm) or immediate surgery (control arm). The primary endpoint is the percentage of patients with complete or major pathological regression of the resected metastases (tumor regression grade 1/2, Rubbia-Brandt criteria). The statistical design follows Simon's two-stage approach with interim and final analyses comparing pathological response rates using descriptive and exploratory methods. Secondary endpoints include feasibility, safety, post-operative complications, and metabolic response by positron emission tomography. Exploratory studies characterize immune cell infiltration, tumor mutational burden, and circulating tumor DNA dynamics.