Abstract 518: The CDK2/9 inhibitor fadraciclib is active in Richter transformation and lymphoma cell lines by targeting both cell survival and proliferation

Abstract Richter transformation (RT), characterized by the transformation of chronic lymphocytic leukemia (CLL) to an aggressive large-cell lymphoma, remains a challenge for the therapy. Genetic aberrations, such as Myc activation, TP53 abnormality, NOTCH1 gene mutation or loss of CDKN2A are associated with uncontrolled RT cell proliferation and escaping of apoptosis. Among those changes, gain of Myc activity, found in around 70% of RT, plays a nexus role in the pathogenesis of RT. As a transcription factor, Myc activation has a profound effect on cell survival, proliferation, adhesion and metabolism. Because of its essential role in lymphoma transformation and aggressiveness, Myc is a vital target for therapy toward RT. Fadraciclib (CYC065) is a second-generation CDK inhibitor with selective potency toward CDK2 and CDK9. We have shown previously that by inhibiting CDK9-mediated transcription, fadraciclib reduced expression of the short-lived anti-apoptotic protein Mcl-1 to initiate apoptosis in primary CLL cells. Like Mcl-1, Myc is a quintessential example of an oncogene with rapid turnover in both its mRNA and protein, therefore a promising target of CDK9 inhibition. Thus, we proposed that fadraciclib would target both the survival and proliferation pathways of RT pathogenesis through collaborative inhibition of Mcl-1, Myc and CDK2. First, our data showed that fadraciclib effectively induced apoptosis in the RT cell line HPRT1, as well as the lymphoma cell lines with Myc amplification, including Raji, Ramos and SU-DHL-2. The IC50 for apoptosis was 0. 3 µM at 24h in the HPRT1 cells, which is less than the average IC50 for the primary CLL cells (0. 8 µM). The IC50 for the lymphoma lines ranged from 0. 8 µM to 1. 3 µM. Immunoblots showed reduction of RNA Pol2 phosphorylation by fadraciclib, consistent with the inhibition of CDK9. As a result of loss of RNA Pol2 activity and pausing of transcription, there was a clear reduction of both Mcl-1 and Myc mRNA, measured by real-time RT-PCR, that was associated with the reduction of their protein levels. CDK2 was also inhibited by fadraciclib, shown by the reduction of the phosphorylation of both RB and NPM. Second, Fadraciclib inhibited cell proliferation of the HPRT1 cells at an IC50 of 0. 25 µM measured by the CellTiter-Glo assay. The IC50 for growth inhibition in the lymphoma lines ranged from 0. 5 to 0. 7 µM. CFSE tracing demonstrated halting of cell division upon fadraciclib incubation. Cell cycle analysis by Click-iT EdU showed reduction of S-phase population and arresting of cells in both G1 and G2. We are currently working on dissecting the contributions of inhibiting Myc, Mcl-1 and CDK2 individually and together, in apoptosis induction and proliferation blockade. Taken together, these data suggest that fadraciclib is active in RT cells and Myc-dependent lymphoma cell lines and support expansion of its clinical testing in patients with RT. Citation Format: Rong Chen, Yuling Chen, Ping Xiong, William Plunkett. The CDK2/9 inhibitor fadraciclib is active in Richter transformation and lymphoma cell lines by targeting both cell survival and proliferation abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 518.