Background Acute myeloid leukemia (AML) is a heterogeneous disease characterized by rapid proliferation of leukemic blasts and high rate of acquired resistance to drugs. Differentiating agent ATRA has been established as a backbone of APL treatment, however its activity in other leukemias is limited. Knowing that Cyclin Dependent Kinase 8 (CDK8) can maintain tumor dedifferentiation and embryonic stem cell pluripotency, we investigated whether CDK8 inhibitor SEL120 could effectively target leukemia by induction of lineage commitment. SEL120 is a specific, selective inhibitor of CDK8 and its paralog CDK19. A first-in-human phase Ib clinical trial with SEL120 in AML or HR-MDS patients is currently ongoing. To better understand the mechanism of action of SEL120 in AML, we studied effects of SEL120 on differentiation as one of the important anti-leukemic activities of the compound. Methods Global transcriptional changes were analyzed by RNAseq at different time points to capture early and long-term effects of SEL120. Genome-wide profiling of DNA-binding was performed by CHIPseq. Cell cycle, proliferation and lineage specific markers were studied by flow cytometry. Differentiation potential of AML cells was studied in semi-solid methylcellulose-based media to asses colony formation of SEL120 treated blasts. Results SEL120 treatment leads to decreased CDK8 occupancy and increased RNA Pol II occupancy as well as changes in the peak distribution among promoter and enhancer regions. SEL120 could repress many “stemness” genes and induce the expression of genes involved in lineage commitment, including regulators of erythroid/megakaryocytic fate, such as RGS18, KLF1, FLI1 and GATA1/2. Moreover, for the first time, we showed that prolonged exposure of AML CD34+ cells to SEL120 could lead to colony formation in semi-solid media. Detailed analysis by flow cytometry at early and late time points reflected sequential changes in the expression of lineage-specific surface markers, characterizing differentiation and the presence of cells of myeloid or erythroid/megakaryocytic origin. Conclusion In addition to established role of CDK8 in regulation of tumor suppressor genes we present evidence for an essential role of CDK8 in lineage controlling-functions. Previously we reported specific cytotoxicity of SEL120 on cells positive for stemness markers such as CD34+. We further expanded on these studies, showing profound morphological changes in AML blasts during prolonged exposure to SEL120, correlating with increased expression of myeloid and erythroid/megakaryocytic markers. Based on these findings further studies are warranted to investigate the efficacy of SEL120 in anemia associated with bone marrow failures in AML and MDS. Combination of direct effects of SEL120 on viability of cells, with a strong differentiation potential, represents a promising profile for a drug in successful leukemia treatment. Citation Format: Urszula Pakulska, Elzbieta Adamczyk, Katarzyna Dziedzic, Katarzyna Wiklik, Michal Combik, Michal Mikula, Aniela Golas, Marta Obacz, Magdalena Masiejczyk, Przemyslaw Juszczynski, Magdalena Cybulska, Milena Mazan, Krzysztof Brzozka, Tomasz Rzymski. SEL120, a CDK8/CDK19 inhibitor, possesses strong multilineage differentiation potential in AML [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1018.
Background Acute myeloid leukemia (AML) is characterized by rapid proliferation of myeloid blood cells. Due to its heterogeneity and the high rate of acquired drug resistance, new treatment modalities are needed. SEL120 is a specific type I selective inhibitor of Cyclin-dependent kinase 8 (CDK8) and Cyclin-dependent kinase 19 (CDK19). A first- in- human phase Ib clinical trial with SEL120 in patients with AML or HR-MDS was initiated in June 2019. Preclinical studies demonstrated high efficacy of SEL120 in experimental AML models via mechanisms involving differentiation and induction of apoptosis. Transcriptomic analysis of hematological cell lines demonstrated that SEL120 treatment upregulated expression of an apoptotic activator BIM from BCL-2 family of proteins. Methods Efficacy of the compound alone or in combination was tested in viability assays in a broad panel of cancer cell lines. Activity and mechanism of action of CDK8 inhibitor - SEL120 alone and in combination was investigated by flow cytometry, western blotting, co-immunoprecipitation, differential gene expression and ChIPseq analysis. In vivo efficacy was tested in mice injected with MV4-11 cell line both subcutaneously and intravenously. Results Here we provide a strong rationale for combination of SEL120 and BCL-2-selective inhibitor Venetoclax (ABT-199). We found that SEL120 synergistically induced apoptosis with Venetoclax in AML cells. Combination of both compounds significantly reduced levels of prosurvival protein MCL-1 and induced hallmarks of apoptosis including Caspase-3 activation and PARP cleavage. Previous studies associated Venetoclax resistance with increased sequestration of proapoptotic BIM by high levels of MCL-1. While a SEL120 treatment alone had no effects on MCL-1 levels, combination of both compounds resulted in sensitization of Venetoclax-resistant AML cells. We demonstrated that mechanism of Venetoclax resistance can be abrogated by the cotreatment with SEL120 leading to changes in proportions of BIM and MCL-1 levels. Synergistic interaction between SEL120 and Venetoclax resulted in apoptotic cell death in established cell lines and patient derived AML cells. Finally, using murine models of subcutaneous or disseminated AML, we found complete remissions of AML and associated recovery of normal cells in bone marrow of animals treated with both SEL120 and Venetoclax. Conclusion Taken together, these data provided rationale for a novel clinical strategy that may lead to durable responses in AML patients. Citation Format: Tomasz Rzymski, Marta Obacz, Milena Mazan, Marion Chappelier, Marcus Järås, Michal Mikula, Elżbieta Adamczyk, Katarzyna Wiklik, Michal Combik, Aniela Golas, Magdalena Masiejczyk, Przemyslaw Juszczynski, Jerzy Ostrowski, Krzysztof Brzózka. Synergistic effect of CDK8 and BCL-2 inhibition in AML through regulation of MCL-1 and BIM balance [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6217.
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