11516 Background: Vascular Endothelial Growth Factor (VEGF)-driven angiogenesis is a pivotal factor in creating an immunosuppressive tumor microenvironment. Approximately 80% of Soft Tissue Sarcomas (STS) are characterized by a 'cold' microenvironment, lacking tertiary lymphoid structures (TLS). While the efficacy of VEGF pathway and PD-1/PD-L1 axis blockade has been established in various tumor types, their impact on 'cold' STS remains unexplored. This study aims to evaluate the synergistic effect of anti-angiogenesis and PD-1 blockade in altering the microenvironment of cold STS, potentially enhancing immune response and therapeutic efficacy. Methods: In this phase II, single-arm, open-label, multicentric trial, we explored the efficacy and safety of combining regorafenib (R) and avelumab (A) in advanced TLS-negative STS patients. Patients were administered 160 mg of R daily for 3 weeks in a 4-week cycle, alongside 10 mg/kg of A biweekly. Endpoints included high-throughput analysis of tumor and plasma samples, response rate, progression-free survival (PFS), overall survival (OS), and safety, as per the NCI-CTCAE v5.0 guidelines. Results: From May 2019 to August 2021, 49 TLS-negative STS patients were enrolled, including leiomyosarcoma (45%), synovial sarcoma (18%), and other subtypes. The median age was 57.1 years, with patients having undergone an average of 2 prior treatment lines. High-throughput analysis of sequential plasma samples indicated an upregulation of immune-inducing protein biomarkers such as CXCL10 and soluble CD8 antigen. Multiplex immunofluorescence analysis of sequential tumor samples revealed significant increase in CD8 T cell infiltration on-treatment. The most common severe adverse events were grade 1 or 2 palmar-plantar erythrodysesthesia, fatigue, and diarrhea. The median follow-up was 7.1 months, with 32.6% of patients experiencing tumor shrinkage, and a clinical benefit rate of 48.8%. The 6-month PFS was 22.1%, with a median OS of 15.1 months. Conclusions: The combination of regorafenib and avelumab demonstrates a marked mobilization of antitumor immunity in patients with TLS-negative STS. The observed efficacy appears superior to that of single-agent immune checkpoint inhibition in 'cold' STS, and higher than the 6-month PFS benchmark of 14% set by EORTC. This indicates the potential effectiveness of this treatment combination in managing advanced cold STS, marking a significant stride in precision immunotherapy for this group of tumors. Clinical trial information: NCT03475953 .