Background: Patients with advanced soft tissue sarcomas (STS) treated with single agent Dox have a median progression-free survival (PFS) of 4.6 months (mo) and response rate (RR) of 14%. Dox sensitizes tumors to Pem through calreticulin release and killing of immunosuppressive cells. Thus we hypothesize that combining Dox + Pem will improve patient outcomes. Methods: A phase I/II trial (NCT02888665) evaluating Dox+Pem was designed for Dox naïve STS and select bone sarcomas with a 1° endpoints of safety (CTCAE v4.03) and response rate (RR) by RECIST 1.1. Patients received one “priming” dose of Pem (200mg IV) prior to starting Dox+Pem Q3wks. Dox+Pem was continued for up to 6 cycles, followed by Pem monotherapy for up to 2 years or progression. The phase I portion used a 3+3 design with 2 Dox doses (45 & 75 mg/m²), followed by a Simon 2-stage expansion. A retrospective study of patients treated at our center on non-ifosfamide containing Dox trials (DoxT) was performed in order to compare our observed PFS with a comparable historic population. Results: Treatment was well tolerated; detailed safety data will be presented. No additional cardiac risk was observed. No DLTs were observed during phase I and 75mg/m² was selected as the phase 2 Dox dose. The study met criteria for expansion to the 2nd stage. Though the planned enrollment was 41, the study closed after 37 as it was clear that the RR (22% , including phase I patients) would not meet the phase 2 RR target of 29%. However, 59% of patients had stable disease (disease control rate = 81%) with tumor regression in a majority of patients. The median PFS on Dox + Pem was 8.1 mo (95% CI: 6.3, 10.8). Patients treated with Dox + Pem had a significantly longer median PFS compared to the DoxT cohort (4.1 mo, 95%CI 3.0 – 6.6, p < 0.001). Conclusions: Dox+Pem is well-tolerated. While this study failed to meet its 1° RR endpoint, a highly significant improvement in PFS was observed compared with historical controls. This is consistent with findings in other cancers, such as head & neck, where improved clinical outcomes were observed without significant increase in RR by RECIST. A randomized trial of Dox +/- Pem should be carefully considered in light of recent negative trials in STS. Clinical trial information: NCT02888665