Abstract 2226: Combined inhibition of MEK and HDACs improves the cytotoxicity of CD4 and CD8 T cells in NRAS;ASXL1-driven acute myeloid leukemia mice

Abstract Acute myeloid leukemia (AML) is a heterogeneous malignant blood cancer that occurs mainly in the elderly. Its treatment outcome is greatly influenced by leukemia-driving mutations. Oncogenic NRAS mutations are associated with both AML progression and multi-drug resistance and treatment failure in AML. We have previously developed a NRAS; ASXL1-driven secondary AML (NA-AML) mouse model, which is characterized by a suppressive immune microenvironment resulting in reduced leukemia killing activities of T cells. Targeting hyperactive RAS/MEK signaling via trametinib (T, a MEK inhibitor) attenuates the exhaustion of CD8 T cells and prolongs the survival of NA-AML mice. To further boost the efficacy of T, we performed a re-purpose screen of ∼2, 500 drugs, either approved by FDA for treating various human diseases or currently under clinical evaluation, in the absence or presence of T. We identified that quisinostat (Q), a 2nd generation of HDAC inhibitor, potently inhibited the growth of both mouse and human NA leukemia cells and significantly synergized with T in vitro. In addition, combined T and Q (TQ) downregulated AP-1 transcription factors and expression of immune checkpoint ligands (PD-L1/L2, CD80, and CD86) while upregulated expression of MHC I/II in mouse NA-AML cells. We further validated our results in NA-AML mice. Vehicle (Veh) -treated mice rapidly succumbed to AML and died within 4-5 weeks post-treatment. T alone and Q alone slowed down AML progression and prolonged the survival of leukemia mice, while all TQ-treated mice remained alive ∼22 weeks post-treatment. Surprisingly, TQ only provided moderate survival benefits in immunodeficient NSG mice engrafted with NA-AML cells, suggesting an important role of cytotoxic T cells in TQ-mediated AML alleviation. We then analyzed CD4 and CD8 T cells from moribund vehicle-treated mice and age-matched drug-treated mice. TQ decreased the exhaustion of CD4 and CD8 T cells and expanded the activated effector/memory T (Tem) cells. Single-cell RNA-Seq analysis showed increased expression of gene signatures associated with cytotoxicity of Tem cells, including CXCR3 and CXCR5. Consistent with these findings, leukemia: T cell co-culture experiments demonstrated significantly enhanced cytotoxicity of TQ-treated CD4 and CD8 T cells, which appeared to be mediated by increased expression of MHC I and II in AML cells. We are currently pursuing the underlying molecular mechanisms. Our data provide a strong rationale to develop immunotherapies via epigenetic modulation of both leukemia and leukemia-associated T cells. Citation Format: Meher Gayatri Bolisetti, Jing Li, Bei Jia, Esra’a Keewan, Erik A. Ranheim, Kalyan V.G Nadiminti, Hong Zheng, Xiaona You, Jing Zhang. Combined inhibition of MEK and HDACs improves the cytotoxicity of CD4 and CD8 T cells in NRAS;ASXL1-driven acute myeloid leukemia mice [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 2226.