Abstract 1018: SEL120, a CDK8/CDK19 inhibitor, possesses strong multilineage differentiation potential in AML
Authors: Pakulska Urszula , Adamczyk Elzbieta , Dziedzic Katarzyna , Wiklik Katarzyna , Combik Michal , Mikula Michal , Golas Aniela , Obacz Marta , Masiejczyk Magdalena , Juszczynski Przemyslaw , Cybulska Magdalena , Mazan Milena , Brzozka Krzysztof , Rzymski Tomasz 
Published: July 2021

Abstract
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.