A Study of Tazemetostat in Adult Participants With Soft Tissue Sarcoma

This is a Phase II, multicenter, open-label, single arm, 2-stage study of tazemetostat 800 mg BID (twice daily) and 1600 mg QD (once daily). Subjects will be screened for eligibility within 21 days of the planned date of the first dose of tazemetostat and enrolled into one of 8 cohorts: Cohort using tazemetostat 800 mg BID * Cohort 1 (Closed for enrollment): malignant rhabdoid tumor (MRT), rhabdoid tumor of the kidney (RTK), atypical teratoid rhabdoid tumor (ATRT), and selected tumors with rhabdoid features, including small cell carcinoma of the ovary hypercalcemic type (SCCOHT), also known as malignant rhaboid tumor of the ovary (MRTO) * Cohort 2 (Closed for enrollment): Relapsed or refractory synovial sarcoma with SS18-SSX rearrangement * Cohort 3 (Closed for enrollment): Other integrase interactor 1 (INI1) negative tumors or any solid tumor with an enhancer of zeste homologue-2 (EZH2) gain of function (GOF) mutation, including: epithelioid malignant peripheral nerve sheath tumor (EMPNST), extraskeletal myxoid chondrosarcoma (EMC), myoepithelial carcinoma, other INI1-negative malignant tumors with Sponsor approval (e.g., dedifferentiated chordoma) any solid tumor with an EZH2 GOF mutation including but not limited to Ewing's sarcoma and melanoma * Cohort 4 (Closed for enrollment): Renal medullary carcinoma (RMC) * Cohort 5 (Closed for enrollment): Epithelioid sarcoma (ES) * Cohort 6 (Closed for enrollment): Epithelioid sarcoma (ES) undergoing mandatory tumor biopsy * Cohort 7 (Closed for enrollment): Poorly differentiated chordoma (or other chordoma with Sponsor approval) Cohort using tazemetostat 1600 mg QD • Cohort 8 (Closed for enrollment): Epitheliod sarcoma Participants will be dosed in continuous 28-day cycles. (Note: if treatment with study drug is discontinued prior to completing 2 years, subjects will be followed for a maximum duration of 2 years from start of study drug dosing.) Response assessment will be performed every 8 weeks while on study. Treatment with tazemetostat will continue until disease progression, unacceptable toxicity or withdrawal of consent, or termination of the study.