Constellation Pharmaceuticals, Inc. summarized in a poster presentation preliminary data from the ongoing Phase 1/2 trial of CPI-0209, a novel, second-generation, small molecule inhibitor of Enhancer of Zeste Homolog 2 (EZH2). These data were published online in conjunction with the American Society of Clinical Oncology (ASCO) Annual Meeting, and show comprehensive target engagement and durable exposure of CPI-0209. Anti-tumor activity of CPI-0209 in pre-clinical models was demonstrated across several advanced hematologic and solid tumor types. A total of 40 patients were treated across 14 tumor types. A 350mg oral, once-daily dose of CPI-0209 has been selected for evaluation in Phase 2. As of the data cut of March 9, 2021, of the 4 BAP1 loss mesothelioma patients, 1 patient had a durable partial response (PR) after four cycles of treatment and 2 had stable disease (SD). Subsequently, to the data cut, the fourth patient also had SD. High levels of target engagement observed preclinically are now corroborated clinically. A total of 40 patients were evaluated for safety. CPI-0209 was generally well tolerated, with a manageable adverse event profile. Across all dose cohorts, 43% of patients had at least one Grade 3 or greater treatment emergent adverse event (TEAE), 28% of patients had at least one serious adverse event (SAE). The most common TEAEs (= 15%) included thrombocytopenia (reversible and dose dependent), diarrhea, asthenic conditions, nausea, anemia, dysgeusia, abdominal pain and alopecia. 23% of patients reported a TEAE that led to dose reduction or interruption. Four patients discontinued treatment because of TEAEs. One patient in the highest dose cohort (375mg) experienced Grade 4 thrombocytopenia, and one patient experienced a Grade 5 adverse event due to progressive disease.