Background: The scavenger receptor CLEVER-1 mediates the clearance of “unwanted” self-components and is highly expressed on tumor associated macrophages (TAMs). CLEVER-1 expression is associated with immunotherapy resistance and poor survival in several cancers. Pre-clinical studies demonstrate that CLEVER-1 inhibition increases TAM pro-inflammatory cytokine secretion and antigen presentation reactivating CD8 T cell responses with robust anti-tumor activity. Targeting CLEVER-1 could therefore overcome the immunosuppressive tumor microenvironment and has led to the development of FP-1305, a humanized anti-CLEVER-1 IgG4-antibody. Methods: The MATINS (Macrophage Antibody To INhibit immune Suppression) trial is a multicenter first-in-human phase I/II study (NCT03733990) to assess the tolerability, safety and preliminary efficacy of FP-1305 in patients (pts) with advanced cancers including immunotherapy-refractory melanoma, cholangiocarcinoma, hepatocellular carcinoma, ovarian cancer, colorectal (CRC), and pancreatic ductal adenocarcinoma. Part 1 consisted of a dose escalation phase; 30 pts (median age 65, range 30-81) were enrolled to examine 5 dose levels (0.1, 0.3, 1.0, 3.0, 10 mg/kg), to determine the optimal dose of FP-1305 for Parts 2 and 3. Two-stage time-to-event continual reassessment method (TITE-CRM) was utilized for the dose escalation in Part 1. Pts received 1-8 cycles (median 3) of FP-1305 Q3w. FP-1305 was well tolerated without dose-limiting toxicities. A consistent increase in blood NK cells, CD8/CD4 T cell ratio, B cells and a decrease in regulatory T cells was demonstrated. FP-1305 dosing led to the activation (CD25⁺) and Th1 skewing (CXCR3⁺) of T cell populations including increase in effector CD8 T-cells with downregulation of several inhibitory immune checkpoint molecules (PD-1, PD-L1, CTLA-4, and LAG3). Increased circulating IFNɣ levels were detected, with the highest levels in a heavily pretreated metastatic, microsatellite stable (MSS) colorectal cancer patient leading to a partial tumor response (-52%). FP-1305 is the first macrophage checkpoint inhibitor candidate promoting immune switch with promising tolerability and clinical anti-tumor activity. FP-1305 represents a novel treatment option to provoke immune response especially in non-inflamed tumors. Full safety, pharmacokinetic and efficacy results of MATINS trial (Part 1) will be presented for the first time in a final late breaking abstract. Clinical trial information: 2018-002732-24.