Background: Panitumumab is a monoclonal antibody (mAb) targeting the epidermal growth factor receptor (EGFR) and is a standard therapy in KRAS/NRAS/BRAF WT mCRC. Preclinical data shows that anti-EGFR therapy causes a tumor-specific adaptive immune response and immunogenic apoptosis, with functional adaptive immunity required to mediate efficacy. However, resistance to anti-EGFR antibody therapy inevitably develops and is associated with increased expression of CTLA-4 and PD-L1. We hypothesized that addition of ipilimumab (anti-CTLA-4) and nivolumab (anti-PD-1) to panitumumab will increase response rate in patients with KRAS/NRAS/BRAF WT MSS mCRC. Methods: LCCC1632 was a multicenter, single-arm, Simon’s two stage phase II clinical trial with a pre-specified safety run-in of panitumumab, ipilimumab, and nivolumab in KRAS/NRAS/BRAF WT, MSS mCRC (NCT03442569). Eligible patients must have received 1-2 prior lines of therapy and no prior anti-EGFR or immune checkpoint inhibitor therapy. Subjects received ipilimumab 1 mg/kg IV q6wk, nivolumab 240 mg IV q2wk, and panitumumab 6 mg/kg IV q2wk until progression, toxicity, or patient withdrawal. The primary endpoint was response rate at 12 weeks per RECIST 1.1, and key secondary endpoints included progression-free survival and duration of response. Results: A total of 56 subjects were enrolled 3/2018-6/2020. This included the 6-subject safety run-in, with 0/6 dose-limiting toxicities in first 12 weeks. The first stage of the Simon’s two-stage clinical trial (n=32) had sufficient response rate to merit full enrollment. There were 7 unevaluable subjects for the primary endpoint of 12-week response rate. Among 49 evaluable subjects, 12-week response rate was 35% (95% CI 21-48; n=17 responses). Twenty subjects had at least an unconfirmed response at any time. Median PFS was 5.7 months (95% CI 5.5-7.9). There was one treatment-related grade 5 adverse event of myocarditis. The most common treatment-related grade 3-4 AEs included lipase increased (9%), amylase increased (7%), ALT increased (5%), AST increased (5%), diarrhea (5%), hypophosphatemia (5%), and maculopapular rash (5%). Conclusions: The combination of panitumumab, ipilimumab, and nivolumab demonstrated evidence of activity and met its prespecified primary endpoint of 12-wk response rate criteria to merit further study. The PFS in this single-arm study compares favorably to expected PFS for anti-EGFR monotherapy in RAS wild-type patients, and results suggest activity of immune checkpoint inhibitors combined with anti-EGFR therapy in MSS mCRC. Clinical trial information: NCT03442569