Sellas Life Sciences announces initial data in Phase 2a trial of SLS009

SELLAS Life Sciences Group announced the completion of enrollment as well as initial data from the ongoing Phase 2a trial of SLS009, a highly selective CDK9 inhibitor, in relapsed/refractory acute myeloid leukemia. Thirty heavily pretreated patients were recruited in 5 centers across the US, reflecting the high unmet need of this refractory/relapsed patient population. Except for one, all patients in this Phase 2a trial had unfavorable/poor cytogenetic and/or molecular genetics risk and were treated with continued venetoclax – azacytidine combination therapy after having failed it or similar venetoclax combinations, often more than once. The expected overall survival in those patients is approximately 2.5 months. Key Highlights from the Initial Data: Safety: SLS009 was generally well-tolerated with no safety issues observed across all doses. There were no dose-limiting and no high-grade treatment-related non-hematologic toxicities. Hematologic toxicities profile was not different from that of venetoclax-based regimens alone. 27 patients had at least one efficacy assessment as of May 25, 2024, and were considered evaluable for efficacy. Efficacy: The overall response rate was 29.6% in all evaluable patients, and across all SLS009 doses, with the highest response rate of 50% observed at the dosing regimen of 30 mg BIW. In the first dose level of 45 mg once a week, the median overall survival among evaluable patients followed for survival was 5.4 months, compared to the expected survival of 2.5 months in patients refractory to and relapsed on standard venetoclax in combination with hypomethylating agents. In the second dose level, 60 mg, administered once a week, 3 out of 9 evaluable patients achieved an overall response defined as leukemia-free status that includes complete response, complete response with incomplete hematologic recovery, and morphologic leukemia-free state. Median survival has not been reached. In the third dose level of 30 mg twice a week, 4 out of 8 patients evaluable to date achieved overall response defined as leukemia-free status that includes complete response, complete response with incomplete hematologic recovery, and morphologic leukemia-free state. Median survival has not been reached. Observed efficacy outcomes exceeded the target ORR of 20% and mOS of 3 months. The highest response rates were observed among patients harboring ASXL1 mutations as previously reported. Notably, responses were highly correlated with mutational status, with 6 out of 8 responding patients having myelodysplasia-related somatic mutations and 5 having specifically ASXL1 mutations. A 100% overall response rate was achieved in patients with ASXL1 mutations in the 30 mg BIW cohort to date. Based on the preliminary findings from this Phase 2a trial, the trial has been expanded to include two additional cohorts consisting of dosing at 30 mg BIW. One cohort will enroll AML patients with ASXL1 mutations and the other AML patients with myelodysplasia-related molecular mutations other than ASXL1. Study enrollment continues and additional updates and data are expected in Q3.