Background: Regorafenib (R) has been shown to modulate anti-tumor immunity by different mechanisms including reduction of tumor-associated macrophages (TAMs). Synergy between R and anti–PD-1/PD-L1 antibodies has been shown in pre-clinical models. Methods: This is a single-arm open-label multicentric phase II trial assessing the efficacy and safety of R (160 mg QD 3weeks/4) + Avelumab (A) (10 mg/kg every 2 weeks) combination in non MSI-H mCRC patients (pts). The primary endpoint was the confirmed objective response rate, based on central review according to RECIST 1.1. Secondary endpoints included: 1-year progression free survival (PFS), 1-year overall survival (OS), and Safety using NCI-CTCAE v5.0. Correlative studies were planned from pts tumor samples obtained at baseline and C2D1. Results: Between Nov. 2018 and Oct. 2019, 48 pts were enrolled in 4 centers. Median age was 61.8 (range: 26.3-78.7). Median follow-up was: 7.2 months. Median number of previous treatment lines was: 3 (range: 1-7). 41 (87.2%) pts experienced at least 1 dose modification or treatment interruption. The most common grade 3/4 adverse events were palmar-plantar erythro-dysesthesia syndrome (29.8%), hypertension (23.4%) and diarrhea (12.8%). No death was related to the treatment. Among 40 pts who had at least one imaging tumor assessment, 12 (30%) had reduction in tumor burden. Best response was stable disease for 23 pts (57.5%) and progressive disease for 17 pts (42.5%). The median PFS and OS were 3.6 months (CI_95%: [1.8 – 5.4]) and 10.8 months (CI_95%: [5.9 – NA]) respectively. Baseline tumor samples and paired biopsies were available for 24 and 15 pts respectively. High infiltration by TAMs at baseline was significantly associated with adverse outcome (PFS: 1.9 vs 3.7 months, p=0.045; OS: 4.8 months vs NR, p=0.027). Increased tumor infiltration by CD8+ at C2D1 compared to baseline was significantly associated with better PFS (p=0.011). Combining low TAMs infiltration and low tumor cells to CD8+ T cells distance enabled the identification of a subgroup of pts (n= 6/24, 25%) more likely to benefit from R+A combination: median PFS: 5.3 vs 1.9 months (p=0.037); median OS: NR vs 5.3 months (p=0.02). Conclusions: The R+A combination achieved PFS and OS that compared favourably with historical data of R alone in this clinical setting. High-resolution analysis of tumor samples identified a composite score based on TAMs infiltration and tumor cell to CD8+ T cells distance which could be used as a biomarker in further studies investigating this approach in mCRC pts. Clinical trial information: NCT03475953.