CYC065 is a novel CDK2/9 inhibitor. Following completion of IND-enabling studies CYC065 has been cleared by the FDA for first-in-human clinical trials. This study aims to establish the mechanistic rationale and dosing schedule for targeting tumors that are either dependent on sustained expression of certain CDK9 transcriptional targets including Mcl-1 and MYC, or on activation of CDK2, e.g. by overexpression of cyclin E.

CYC065 is efficacious in ALL and AML preclinical models, including MLL rearranged (MLLr) leukemia1. Mcl-1 is critical for survival of AML2, and MLLr AML are dependent on CDK9-driven expression of MLL target genes3. CYC065 is effective in cyclin E-overexpressing tumor models, such as uterine serous carcinoma4 and trastuzumab-resistant Her2+ breast cancer5. Pharmacologic CDK inhibition or CDK knockdown is synthetically lethal with overexpressed MYC in triple negative breast cancer (TNBC)6. A common feature of the basal-like subtype of TNBC is amplification or overexpression of cyclin E and MYC, suggesting potential utility for a CDK2/9 inhibitor. The efficacy and mechanism of action of CYC065 was analyzed in AML and basal-like TNBC cell panels by measuring the effects on cell proliferation, levels of key CDK9-dependent proteins, cell cycle distribution and apoptosis induction.

We show that submicromolar CYC065 inhibits CDK9-dependent RNA Polymerase II phosphorylation, resulting in downregulation of target proteins and rapid induction of apoptosis. Sensitive tumor subtypes undergo rapid apoptosis after short pulse treatments at doses and durations predicted to be clinically achievable and well tolerated. However, significant apoptosis induction was not observed in non-malignant cells despite showing similar upstream effects. The data predicts a therapeutic window between cancer and normal proliferating cells and indicates the potential for stratification markers to enrich for sensitive patients.

Combinability of anti-cancer agents is an important consideration for clinical development. In preclinical models Cyclacel's first generation CDK inhibitor seliciclib combines effectively with the novel nucleoside analogue sapacitabine (or its active metabolite CNDAC). This combination is currently being evaluated in a phase 1 clinical trial (NCT00999401)7. We show that CYC065 combines effectively with CNDAC in a TNBC panel. Increased expression and activity of other anti-apoptotic Bcl-2-family members can counteract the pro-apoptotic effects of Mcl-1 downregulation by CYC065. This can be overcome by combined treatment with CYC065 and Bcl-2 inhibitors such as ABT-199, resulting in a highly synergistic combination.

CYC065 has the potential to be efficacious in cancers that require sustained expression of CDK9-dependent transcripts or activation of CDK2. These include Mcl-1-dependent and MLLr leukemia, MYC-driven lymphoma, and MYC- or cyclin E-dependent breast or gynecological cancers. CYC065 can act synergistically with selected DNA damaging or targeted agents. Together this data suggests that CYC065 may be a promising novel anti-cancer agent.

1) Saladino et al. AACR 2015, Abs 1650

2) Glaser et al. Genes Dev. 2012, 26, 120-5

3) Dou & Hess. Int J Haematol. 2008, 87, 10-8

4) Cocco et al. AACR, 2015, Abs 3103

5) Scaltriti et al. Proc Natl Acad Sci USA. 201, 108, 3761-6

6) Horiuchi et al. J Exp Med. 2012, 209, 679-96

7) Shapiro et al. AACR, 2013, Abs LB-202

Citation Format: Craig MacKay, Sheelagh Frame, Chiara Saladino, Elizabeth Pohler, Daniella Zheleva, David G. Blake. Molecular basis for clinical development of the novel CDK2/9 inhibitor CYC065 in oncology. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr B182.