Tazemetostat, an EZH2 inhibitor, in solid tumors harboring SWI/SNF alterations: a phase II basket study.

Advanced tumors harboring SWI/SNF mutations are rare and aggressive. Often presenting in young patients, treatment options are limited. This phase II basket study (NCT02601950) assessed oral tazemetostat, an EZH2 inhibitor, at 800 mg twice daily (N = 129). Cohorts included malignant rhabdoid tumors (n = 32, Cohort 1), synovial sarcoma (n = 33, Cohort 2), INI1-negative tumors (n = 32, Cohort 3), renal medullary carcinoma (n = 14, Cohort 4), and poorly differentiated chordoma (n = 18, Cohort 7). Efficacy assessments used a 2-stage Green-Dahlberg design. Stage 1 futility was predefined as objective responses (complete/partial [CR/PR]) <5% (Cohorts 1, 3, 4, 7) or 16-week progression-free survival (PFS) <15% (Cohort 2). Sufficient clinical activity was declared in Stage 2 if ≥5 patients had a CR/PR (Cohorts 1,3,4,7) or ≥9 patients had a CR/PR, or stable disease (Cohort 2; primary endpoints). Responses were observed in malignant rhabdoid tumors (9%; n = 2 PR, n = 1 CR), INI1-negative tumors (9%; n = 3 PR, n = 0 CR), and poorly differentiated chordoma (6%; n = 1 PR, n = 0 CR) but not the remaining 2 cohorts (both 0%). 16-week PFS for synovial sarcoma was 15.2% (n = 5). Grade ≥3 treatment-related adverse events occurred in 10% of patients. Tazemetostat demonstrated preliminary antitumor activity in a subset of poor-prognosis cancers, underscoring the need for combination strategies when EZH2 signaling is not the sole disease driver.