Abstract P5-03-10: CYC065, a novel CDK2/9 inhibitor: Molecular basis for clinical development in basal-like triple-negative breast cancer
Authors: Blake DG , MacKay C , Frame S , Zheleva D 
Published: May 2016

CYC065 is a novel CDK inhibitor, which inhibits CDK2 and 9 with IC50 values of 5 and 26 nM, respectively. Following completion of IND-enabling studies, CYC065 has been cleared by FDA for first-in-human Phase 1 clinical trials.

Triple-negative breast cancers (TNBC), particularly the basal subtype, often exhibit aggressive characteristics. Despite good initial responses to chemotherapy, patients experience early relapse and diminished 5 year survival. Molecular features of basal-like TNBC include amplification or overexpression of cyclin E and MYC, suggesting potential utility for a CDK2/9 inhibitor such as CYC065. CYC065 is effective in cyclin E-overexpressing tumors, such as uterine serous carcinoma1 and trastuzumab-resistant Her2+ breast cancer2. Moreover, CDK inhibition has also been reported to be synthetic lethal with overexpressed MYC3. This led us to assess the potency and mechanism of action of CYC065 in basal-like TNBC models to evaluate the potential for CYC065 development in this indication.

In vitro cell-based experiments support twice weekly pulse dosing using submicromolar concentrations of CYC065 to achieve maximum impact on cell growth in the majority of breast cancer cell lines tested. Preclinical toxicology data indicate that such levels and durations of exposure are achievable and well tolerated.

As a single agent, CYC065 treatment in breast cancer cells resulted in inhibition of RNA-Pol II phosphorylation, down-regulation of Mcl-1, up-regulation of p53 and rapid induction of apoptosis. The impact of CYC065 on CDK2 targets, cyclin E and MYC was also explored. Interestingly immortalized cell lines obtained from non-malignant tissue displayed similar effects on RNA Pol II, Mcl-1, and p53 but did not undergo apoptosis and consequently exhibited relative resistance to CYC065, indicative of a potential therapeutic window. Cell cycle analysis demonstrated that CYC065 treatment induced an increase in G1 population with no significant induction of cell death in non-malignant derived cell lines, compared to cancer cell lines, in which there was significant induction of cell death.

CDKs have a role in DNA repair which can be exploited to enhance the effectiveness of DNA damaging agents. Seliciclib, an oral, first generation CDK2/9 inhibitor, can be effectively combined with DNA damaging agents, such as the oral nucleoside analogue sapacitabine, or its active metabolite CNDAC. A Phase 1 clinical trial is currently underway to evaluate this combination (NCT00999401). Similarly to seliciclib, we demonstrate that CYC065 is synergistic in combination with CNDAC when given sequentially across multiple breast cancer cell lines. CNDAC-induced double strand breaks persisted for longer when cells were subsequently treated with CYC065, supporting the conclusion that these CDK inhibitors suppress DNA double-strand break repair capacity, which may contribute to the observed synergy.

Taken together the data establish CYC065 as a promising anti-cancer agent in basal-like TNBC, with the potential to be combined effectively in this indication with DNA damaging agents.

1. Cocco, E. et al. AACR, 2015, Abstract 3103.

2. Scaltriti, M. et al. Proc Natl Acad Sci USA. 201, 108, 3761-6.

3. Horiuchi, D. et al. J Exp Med. 2012, 209, 679-96.

Citation Format: Blake DG, MacKay C, Frame S, Zheleva D. CYC065, a novel CDK2/9 inhibitor: Molecular basis for clinical development in basal-like triple-negative breast cancer. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P5-03-10.