Background: Refractory RAS mutated MSS mCRC represents a critical unmet need with minimal response to immune checkpoint blockade (ICB). Preclinical CRC models reveal that KRAS mutations activate the CXCR2 axis promoting an immunosuppressive tumor microenvironment (TME). This occurs via KRAS repression of interferon regulatory factor 2 (IRF2) resulting in upregulation of CXCL3 chemokines, which bind CXCR2 and recruit myeloid-derived suppressor cells (MDSC; Liao et al, Cancer Cell 2019; Grover et al, Cancer Discovery, 2022). SX-682 is a novel oral small-molecule inhibitor of the CXCR1/2 chemokine receptors involved in MDSC-recruitment to the TME. This proof-of-concept study investigates whether de novo immunotherapy resistance in MSS RAS mutated mCRC can be overcome by treatment with a small molecule CXCR1/2 antagonist in combination with anti-PD-1 therapy. Methods: STOPTRAFFIC-1 (NCT04599140, enrolling) is a phase I/II, open-label, dose-escalation/dose expansion study of SX-682 in combination with nivolumab to evaluate safety and clinical activity for patients (pts) with refractory RAS (KRAS and NRAS) mutated MSS mCRC. Key eligibility criteria: patients (pts) with MSS mCRC with measurable disease, progression or intolerance to at least 2 prior lines of standard therapy, and ECOG 0 or 1. Pts will receive SX-682 (5 dose levels: 25 mg, 50 mg, 100mg, 200mg, or 400 mg by mouth twice daily) administered in an 8-week cycle with intravenous nivolumab (480 mg) on days 1 and 29 of each cycle. Adverse events are assessed according to CTCAE v5.0. Response assessments (per RECIST) occur every 8 weeks. In dose escalation, pts enter a 3-week monotherapy safety run-in of SX-682 followed by 3-week combination with nivolumab for a six-week dose-limiting toxicity (DLT) period. Cohorts 1-4 have been completed without DLT. Enrollment to cohort 5 began in January 2022. Following determination of maximum tolerated dose (MTD), dose expansion design is a Simon's optimal two-stage design. Efficacy will be assessed in the first 15 pts with a requirement of at least 2 responses in order to enroll 14 additional pts (N=29) in the second stage. Pre- and on-treatment tissue biopsies will be collected in the expansion phase. The primary objectives are to determine the safety profile of SX-682 alone and in combination with nivolumab, including the MTD, recommended phase 2 dose, and the DLT. The secondary objectives include overall response rate, progression-free survival, overall survival, and pharmacokinetic profiles of SX-682. Translational analyses include correlations of clinical outcomes with genomic and immune biomarkers from paired tissue and plasma samples. Clinical trial information: NCT04599140.