Abstract PS11-40: A novel and highly selective CDK9 inhibitor (D11) suppresses proliferation of triple negative breast cancer cells

Abstract

Abstract Targeted treatment for triple-negative breast cancer (TNBC) remains an elusive clinical challenge. Cyclin-dependent kinase 9 (CDK9) is a transcriptional regulator shown to promote growth of multiple cancers, including TNBC, and represents a potential new therapeutic target. CDK9 increases RNA Polymerase II (RNAPII) activity, which facilitates sustained expression of normally short-lived oncogenic and anti-apoptotic proteins. However, preclinical evaluation of CDK9 as a therapeutic target has been hampered by the poor selectivity of existing CDK9 inhibitors (CDK9i). Herein, we evaluated the preclinical efficacy of a newly developed, highly selective CDK9i (D-11) across a panel of TNBC cell lines: MDA-MB-453, MDA-MB-468, MDA-MB-231 and MFM-223. Cells were treated with increasing concentrations of D-11 and effects on proliferation, apoptosis, cell cycle and expression of known CDK9 substrates were examined. Treatment with D-11 significantly reduced cell proliferation, induced G2/M cell cycle arrest and increased apoptosis in MDA-MB-453 and MDA-MB-468 cell lines (IC50s of 160 and 258 nM, respectively). Cell proliferation was also inhibited by D-11 in MDA-MB-231 and MFM-223 cells, but with IC50s of 400 and 468 nM, respectively, but without inducing apoptosis. Protein expression of known CDK9 targets, including phosphorylated-RNAPII, the proto-oncogene C-MYC, and the anti-apoptotic marker MCL-1, were examined by Western blot using the optimal D-11 dose determined for each cell line. As expected, D-11 suppressed phosphorylation of RNAPII (Ser2) without affecting total RNAPII expression in all cell lines, indicative of targeted CDK9 inhibition. C-MYC and MCL-1, which highly expressed in the TNBC models, were significantly decreased by treatment with D-11. To determine whether D-11 influenced normal breast epithelial cells, breast tissues were collected from women undergoing reduction mammoplasty and cultured ex vivo as explants treated with or without different doses of D-11 for 48h. A high dose of D-11 (2.7µM) elicited a 20% decrease in the proliferative index (Ki67 positivity) of epithelial cells within normal tissue explants but had no effect on their histology. Collectively, these data demonstrate that D-11 effectively inhibits CDK9 in TNBC cell lines, resulting in growth inhibition with non-toxic effects on histologically normal breast tissues. Citation Format: Ebtihal Mustafa. A novel and highly selective CDK9 inhibitor (D11) suppresses proliferation of triple negative breast cancer cells [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS11-40.