Inhibition of Cyclin Dependent Kinase 8(CDK8): A Novel Approach to Target the Leukemia Initiating Cells (LICs) in T-Cell Acute Lymphoblastic Leukaemia (T-ALL)

Abstract Background: T cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematopoietic malignancy characterized by an aberrant proliferation of immature thymocytes affecting preferentially children and adolescents. Approximately 70% of T-ALL cases have activating mutations of the NOTCH1 followed by oncogenic pathways PI3K/Akt, the anti-apoptotic BCL-2 family, and CDKN2A/2B cell cycle regulators 1, 2. Unlike other leukemias such as chronic myeloid leukemia (CML) and Philadelphia-positive ALL, which are kinase-driven malignancies 3, 4, the initiating events in T-ALL cause the ectopic expression of transcription factors that drive leukemogenesis, hence targeted therapies needed to focused on this field. Cyclin Dependent Kinase 8 (CDK8) and its paralog CDK19, restrain activation of super-enhancer-associated tumor suppressors and lineage commitment genes in leukemia. Moreover CDK8 phosphorylates intracellular domain (ICD) and regulates activation and turnover of NOTCH1 5, 6, indicating that targeting CDK8 with specific inhibitors may be considered as a novel therapeutic strategy for T-ALL. RVU120 is a first-in-class, specific and selective inhibitor of CDK8/CDK19, currently in phase Ib dose-escalation trial in patients with relapsed or refractory (R/R) AML and HR-MDS (NCT04021368). Objectives: The goals of these studies were to investigated the effectiveness of RVU120 and other non-related CDK8/19 inhibitors against T-ALL cell lines/PDXs, including GSI-resistant lines. Comprehensive analysis of target inhibition and apoptotic signatures were evaluated. The clinical relevance of RVU120 in LICs was confirmed in Patient derived xenografts (PDXs). Results: T-ALL cell lines and PDXs were treated with RVU120 and other chemically non-related CDK8/19 inhibitors in different concentration for 12-18 days, and the changes in cell number and apoptosis using flow cytometry analysis were determined. High differential efficacy in double-digit nM range has been observed for NOTCH1 PEST domain mutant cell lines including GSI-sensitive HPB-ALL cells and GSI-resistant cells PF-382, KOPT-K1, and MOLT4, where both inhibition of cell proliferation and induction of apoptosis were observed. GI-resistant cell lines SUP-T1, SUP-T11, MOLT16 and JURKAT were considered as moderately sensitive and LOUCY (early T-cell phenotype ALL), and CCRF-CEM devoid those phenotype, were considered as resistant to RVU120. Results form etsblished cell lines were further corroborated in patient-derived xenograft cells (PDX) treated with RVU120. In T-ALL PDX 6522288 (Notch1 HD /PEST mut, TET2 mut, U2AF1 mut, WT1 mut and CUL76- CDKN2A/B mut, PTEN-/-) treatment with RVU120 was followed by a reduction in cell number and induction of apoptosis in phenotypically defined leukemia initiating cells (LICs) CD34+CD7+CD19-, with calculated IC50s value for bulk and LICs as 92.8 nM and 141.8 nM, respectively. The clinical relevance of these findings and impact on LICs are under the evaluation in PDX mice model. Mechanistically, treatment with RVU120 reduced the phosphorylation of STAT5 and STAT1 as well as the level of MYC and anti-apoptotic proteins MCL1, but induced expression of BAX in sensitive cells. Hence, inhibition of CDK8 is a novel approach to target the LICs in T-ALL. Conclusion: Inhibition of CDK8 by RVU120 triggers inhibition of cell proliferation and apoptosis in cell lines and LICs in T-ALL. Further clinical study is warranted to study the efficacy of RVU120 and CDK8 inhibition in T-ALL. Keywords: CDK8, RVU120, apoptosis, T-ALL, LICs in PDX model Reference 1 Girardi, T., Vicente, C., Cools, J. & De Keersmaecker, K. Blood129, 1113-1123, doi:10.1182/blood-2016-10-706465 (2017). 2 Sanchez-Martin, M. & Ferrando, A. Blood 129, 1124-1133, doi:10.1182/blood-2016-09-692582 (2017). 3 Braun, T. P., Eide, C. A. & Druker, B. J. Cancer cell 37, 530-542, doi:10.1016/j.ccell.2020.03.006 (2020). 4 Gazeau, N. et al. Leukemia 34, 2230-2233, doi:10.1038/s41375-020-0715-2 (2020). 5 Pelish, H. E. et al. Nature526, 273-276, doi:10.1038/nature14904 (2015). 6 Fryer, C. J., White, J. B. & Jones, K. A. Molecular cell 16, 509-520, doi:10.1016/j.molcel.2004.10.014 (2004). Disclosures Golas: Ryvu Therapeutics: Current Employment. Windak: Ryvu Therapeutics: Current Employment. Rzymski: Ryvu Therapeutics: Current Employment, Current equity holder in publicly-traded company. Andreeff: ONO Pharmaceuticals: Research Funding; AstraZeneca: Research Funding; Glycomimetics: Consultancy; Daiichi-Sankyo: Consultancy, Research Funding; Breast Cancer Research Foundation: Research Funding; Karyopharm: Research Funding; Reata, Aptose, Eutropics, SentiBio; Chimerix, Oncolyze: Current holder of individual stocks in a privately-held company; Amgen: Research Funding; Aptose: Consultancy; Medicxi: Consultancy; Oxford Biomedica UK: Research Funding; Novartis, Cancer UK; Leukemia & Lymphoma Society (LLS), German Research Council; NCI-RDCRN (Rare Disease Clin Network), CLL Foundation; Novartis: Membership on an entity's Board of Directors or advisory committees; Syndax: Consultancy; Senti-Bio: Consultancy. Borthakur: GSK: Consultancy; Astex: Research Funding; Takeda: Membership on an entity's Board of Directors or advisory committees; Ryvu: Research Funding; University of Texas MD Anderson Cancer Center: Current Employment; Protagonist: Consultancy; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; ArgenX: Membership on an entity's Board of Directors or advisory committees.