Background: Soft tissue sarcomas (STS) are rare tumors and exhibit substantial histological diversity. Efficacious targeted 1^st line treatment for advanced or metastatic STS is not available, and the standard therapy has been anthracycline-based for decades. This strategy shows poor efficacy, as demonstrated by a median Overall Survival (OS) of 12-20 months. Several STS subtypes, however, have been shown to express PD-L1, and immune checkpoint inhibitors (ICI) have demonstrated principle anti-tumor activity in pretreated STS. Our ongoing clinical trial tests the activity and safety of ICI combination therapy in the first-line setting. We hypothesize that the dual checkpoint blockade with Durvalumab (PD-L1) and Tremelimumab (CTLA-4) improves overall survival in STS when compared to the standard of care doxorubicin. Methods: MEDISARC is a multi-center phase II trial that is enrolling adult treatment-naïve patients with histologically confirmed STS of intermediate or high grade (FNCLCC grade 2 or 3) not amenable to surgery with curative intent and ECOG performance status 0-2. Chemosensitive histologic STS are eligible. 100 patients will be randomized 1:1, stratified by ECOG status (0 vs. 1/2). Patients in the experimental arm are treated with fixed doses of durvalumab (1.5 g Q4W) and tremelimumab (75 mg Q4W for 3 cycles, then Q12W) until Progressive Disease (PD) or for a maximum of 12 months. Doxorubicin treatment in the standard arm is at 75 mg/m² Q3W and limited to 6 cycles. OS is the primary endpoint. Secondary endpoints include 2-year OS rate, PFS, ORR according to conventional and modified RECIST 1.1, safety and tolerability and health-related quality of life (EORTC QLQ-C30). OS analysis may be performed when the required number of events (E=70) has been observed. All randomized and treated subjects will be included in the efficacy and safety analysis. The accompanying translational research aims to identify prognostic and predictive biomarkers in blood and tumor tissue. Enrollment of patients started in April 2018 and is ongoing. As of February 2019, 32 patients have been enrolled. The study is sponsored by AIO-Studien-gGmbH, Berlin, Germany. Clinical trial information: 2016-004750-15.