Abstract 1309: CYC065, a novel CDK2/9 inhibitor, is an effective inducer of cell death and synergizes with BCL2 and BET inhibitors in B-cell lymphoma, including double-hit lymphomas
Authors: Frame Sheelagh M. , Pohler Elizabeth , MacKay Craig , Zheleva Daniella , Blake David 
Published: July 2016

Abstract
Introduction: Double-hit lymphomas (DHL), defined by concurrent MYC and BCL2 rearrangements, have poor prognosis compared to standard-risk diffuse large B cell lymphomas (DLBCL). Current treatment regimens involve multiple chemotherapies, but do not specifically exploit these molecular features of the disease.
Several studies have established that: (i) DLBCL show frequent overexpression of Mcl-1, an anti-apoptotic member of the Bcl-2 family (30% and 50% in the ABC and GCB subtypes, respectively), (ii) MYC-driven lymphomas are highly sensitive to depletion of Mcl-1, (iii) MYC overexpression and inhibition of CDK activity are synthetically lethal and (iv) resistance to BH3 mimetics targeting Bcl-2 can be conferred by upregulation of Mcl-1.
CYC065 is a specific and potent CDK2/9 inhibitor, currently in a Phase 1 trial in patients with advanced cancer (NCT02552953). The mechanism of action of CYC065 involves rapid reduction of the levels of both Mcl-1 and MYC, suggesting a therapeutic rationale for investigating this agent in DLBCL. This preclinical study explored the single agent activity of CYC065 in B cell lymphoma, and its potential to combine with other molecularly targeted agents of interest, including venetoclax (ABT-199, a Bcl-2 inhibitor), and BET inhibitors.
Methods: Single agent activity of CYC065 was explored using short pulse treatments (6-8 h) in B cell lymphoma cell lines. Viability and total cell number were assessed 24, 48 or 72 h following treatment. Levels of MYC, Bcl-2 and Mcl-1 were determined by Western blotting at baseline and following treatment with CYC065. Cell fate was examined by flow cytometry. CYC065 was combined with venetoclax or BET inhibitors ((+)-JQ-1, GSK525762 and OTX-015), which were administered concomitantly for a 6 h pulse or up to 72 h. Combination data were analyzed by the Chou & Talalay method.
Results: The median IC50 for CYC065 in 13 B cell lymphoma cell lines was 0.43 μM. No obvious difference was observed between ABC and GCB subtypes of DLBCL, and DHL DLBCL lines had similar IC50 values to non-DHL DLBCL lines (median 0.47 μM vs 0.29 μM; p = 0.95). As expected from the target inhibitory profile, CYC065 caused a rapid decrease in the phosphorylation of S2 of the CTD of RNA polymerase II followed by downregulation of Mcl-1 and MYC, and rapid induction of apoptosis in sensitive cell lines. CYC065 had no impact on Bcl-2 levels. Combining CYC065 with venetoclax was highly synergistic in DHL lines (median CI values range from 0.1-0.8), and resulted in &gt;90% cell death. The combination of CYC065 and BET inhibitors was also highly synergistic.
Conclusions: CYC065 targets key oncogenic and survival pathways in DLBCL. CYC065 is a potent and effective inducer of cell death and combines synergistically with Bcl-2 or BET inhibitors in B cell lymphoma cell lines, including DHLs, which represent an unmet clinical need.
Citation Format: Sheelagh M. Frame, Elizabeth Pohler, Craig MacKay, Daniella Zheleva, David Blake. CYC065, a novel CDK2/9 inhibitor, is an effective inducer of cell death and synergizes with BCL2 and BET inhibitors in B-cell lymphoma, including double-hit lymphomas. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1309.