Inhibition of CDK9 enhances AML cell death induced by combined venetoclax and azacitidine

Relapsed/refractory (R/R) disease is a major hurdle to long-term survival of acute myeloid leukemia (AML) patients treated with intensive cytarabine (AraC)-based chemotherapy. R/R AML salvage treatment with venetoclax (VEN) + azacitidine (AZA) results in overall response rates between 20% and 60%, and responses are not durable, highlighting the need for new therapies. Here, we report elevated mTORC1 signaling in AraC-resistant AML cell lines, primary AML patient samples, and patient-derived xenograft (PDX) AML cells derived from patients at relapse postchemotherapy. The CDK9 inhibitor AZD4573 suppresses mTORC1 signaling and downregulates c-MYC and MCL-1, inducing AraC-resistant AML cell death. AZD4573 in combination with VEN + AZA significantly increases AML cell death compared to any of the two-drug combinations and suppresses AML progenitor cells but spares normal hematopoietic progenitor cells. The efficacy of this triple combination remains even with a 10-fold reduction of VEN concentration. The roles of MCL-1 and c-MYC in the three-drug combination were confirmed by knockdown. This study demonstrates that AZD4573 enhances the activity of VEN + AZA against AraC-resistant AML by downregulating c-MYC and MCL-1 and to a lesser extent cellular respiration.