Background: ES is a rare, aggressive subtype of STS for which cytotoxic chemotherapy has limited effectiveness. TAZ, an FDA-approved EZH2 inhibitor, has shown single-agent clinical activity and a favorable safety profile in patients with metastatic or locally advanced ES. In preclinical studies, TAZ has shown synergistic antitumor activity with DOX, which is often used as frontline treatment for STS. Here, we present results of the phase 1b study (NCT04204941), designed to assess the recommended phase 3 dose (RP3D), safety, and efficacy of TAZ + DOX in patients with advanced STS. Methods: The open-label, phase 1b portion of this study enrolled adult patients with previously untreated advanced STS. A standard 3 + 3 design was used to assess TAZ 400 mg, 600 mg, and 800 mg orally twice daily in combination with DOX (75 mg/m² intravenously on day 1 of each cycle, for up to 6 cycles) as frontline therapy. Dose-limiting toxicities (DLTs) were predefined in the protocol. The RP3D of TAZ was determined by Scientific Review Committee review of the safety and pharmacokinetic data from the phase 1b trial, with a target DLT rate of < 33%. Results: As of February 1, 2021, 16 patients are enrolled, including 2 with ES; 10 are still receiving TAZ + DOX and 6 have discontinued (5 due to disease progression, 1 due to patient withdrawal). The median age was 49.5 years (range, 29?82) and all had unresectable STS. Median (range) time on treatment was 13 (0.1?51.1+) weeks across all dose levels evaluated. Two DLTs, both of febrile neutropenia, were observed, one in the TAZ 600 mg + DOX cohort (n = 1/6, 17%), and one in the TAZ 800 mg + DOX cohort (n = 1/3, 33%). When used in combination with DOX, the RP3D of TAZ was 800 mg. Grade 3 or 4 treatment-related treatment-emergent adverse events (TR-TEAEs) occurred in 13/16 (81.3%) patients. The most common (≥ 20%) TR-TEAEs were neutropenia (n = 11, 69%), anemia (n = 10, 63%), fatigue (n = 10, 63%), stomatitis (n = 9, 56%), nausea (n = 8, 50%), febrile neutropenia (n = 7, 44%), constipation (n = 6, 38%), vomiting (n = 6, 38%), and decreased appetite (n = 5, 31%). TR-TEAEs were defined as attributable to either study agent. Conclusions: The combination of TAZ + DOX was generally well tolerated in this dose finding study in patients with advanced STS. The RP3D to be tested in the phase 3 randomized, double blind, placebo controlled study is TAZ 800 mg twice daily + DOX. The safety profile of this combination is consistent with the respective safety information for TAZ and for DOX. The TR-TEAEs include known toxicities of DOX or TAZ. Further comparison with DOX + placebo in the phase 3 trial will aid in assessing efficacy and safety of the combination of TAZ + DOX. The global phase 3 confirmatory trial will enroll patients with ES who have unresectable disease and have had no prior systemic therapy. Clinical trial information: NCT04204941