This study evaluates the efficacy of two newly developed selective CDK9 inhibitors (CDK9i) across a panel of TNBC cell lines. MDA-MB-453, MFM-223, MDA-MB-468 and MDA-MB-231 TNBC cells were treated with increasing concentrations of two novel and highly selective CDK9 inhibitors and the effect on proliferation, apoptosis and expression of CDK9 targets determined. MDA-MB-453 and -468 cells showed significant growth inhibition with as little as 150 nM of CDK9i, evident 3 days after commencement of treatment. Both MDA-MB-231 and MFM-223 cells were less sensitive to the CDK9 inhibitors, with MDA-231 cells requiring at least 300 nM to suppress growth. MFM-223 cells did not demonstrate any growth inhibition after 7 days of culture with CDK9i concentrations up to 1.2 μM. Protein expression of CDK9 targets, including RNA Polymerase II (RNAPII), phosphorylated-RNAPII, the proto-oncogene C-MYC, and apoptotic marker cleaved caspase-3, were examined by Western blot after optimal CDK9i exposure across each cell line. CDK9i suppressed phosphorylated-RNAPII, but not total RNAPII, indicative of targeted CDK9 inhibition. The master transcription factor C-MYC, which is highly expressed in TNBC, was downregulated, and cleaved-caspase-3 was upregulated with CDK9i treatment. These data demonstrate cell specific efficacy of novel CDK9 inhibitors in cell line models of TNBC via transcriptional suppression of proto-oncogenes and upregulation of apoptotic pathways. Future studies will identify molecular markers of response to CDK9 inhibition and evaluate these novel inhibitors in TNBC patient derived xenografts.
17 Mar 2019 - 20 Mar 2019