Phase II Study of Pembrolizumab in Combination with Doxorubicin in Metastatic and Unresectable Soft-Tissue Sarcoma

Purpose: Doxorubicin is standard therapy for advanced soft-tissue sarcoma (STS) with minimal improvement in efficacy and increased toxicity with addition of other cytotoxic agents. Pembrolizumab monotherapy has demonstrated modest activ-ity and tolerability in previous advanced STS studies. This study combined pembrolizumab with doxorubicin to assess safety and efficacy in frontline and relapsed settings of advanced STS. Patients and Methods: This single-center, single-arm, phase II trial enrolled patients with unresectable or metastatic STS with no prior anthracycline therapy. Patients received pembrolizumab 200 mg i.v. and doxorubicin (60 mg/m2 cycle 1 with subsequent escalation to 75 mg/m(2) as tolerated). The primary endpoint was safety. Secondary endpoints included overall survival (OS), objective response rate (ORR), and progression-free survival (PFS) based on RECIST v1.1 guidelines. Results: Thirty patients were enrolled (53.3% female; median age 61.5 years; 87% previously untreated) with 4 (13.3%) patients continuing treatment. The study met its primary safety endpoint by prespecified Bayesian stopping rules. The majority of grade 3+ treatment-emergent adverse events were hematologic (36.7% 3+ neutropenia). ORR was 36.7% [95% confidence interval (CI), 19.9-56.1%], with documented disease control in 80.0% (95% CI, 61.4-92.3%) of patients. Ten (33.3%) patients achieved partial response, 1 (3.3%) patient achieved complete response, and 13 (43.3%) patients had stable disease. Median PFS and OS were 5.7 months (6-month PFS rate: 44%) and 17 months (12-month OS rate: 62%), respectively. Programmed cell death ligand-1 (PD-L1) expression was associated with improved ORR, but not OS or PFS. Conclusions: Combination pembrolizumab and doxorubicin has manageable toxicity and preliminary promising activity in treatment of patients with anthracycline-naive advanced STS.