Abstract 4426: Selective inhibition of CDK9 induces apoptosis of TNBC cells independent of Rb status

The family of cyclin-dependent kinases (CDKs) are serine/threonine protein kinases important for cell cycle control and/or regulation of transcription. CDK9/CyclinT1 is a key regulator of transcription in eukaryotic cells and has been shown to be dysregulated at the level of protein and kinase activity in both hematologic and solid tumors. CDK9/CyclinT1 forms the active P-TEFb complex and phosphorylates Ser2 residues in the carboxy-terminal domain of RNA polymerase II (RNApol II) to initiate elongation of mRNA transcripts. CDK9 activity regulates transcription of a variety of short-lived transcripts that promote survival and directly suppress apoptosis in cancer cells, including MYC,XRN2, MCL-1,and XIAP. MYC-driven tumor types with Rb-loss or high expression levels of cyclin E, such as triple negative breast cancer (TNBC), are difficult to treat and are resistant to existing CDK4/6 inhibitors. Since CDK9 is upstream of these oncogenic drivers, inhibition of CDK9 could potentially bypass innate resistance mechanisms and induce cell death in TNBC through blocking expression of MYC and MCL-1, for example. To target CDK9 in TNBC, we used structure-based drug design and developed a library of novel, potent and selective CDK9 inhibitors (CDK9i). The lead CDK9i exhibit single digit nanomolar potency against CDK9/CyclinT1 complexes and good selectivity against other CDK family members in biochemical assays. In addition, they also display high selectivity over 502 other kinases. Lead CDK9 inhibitors were found to potently inhibit phosphorylation of RNApol II Ser2 in a time and dose dependent manner in TNBC cells and decrease MYC, MCL-1 and XIAP at both mRNA and protein levels. Additionally, treatment of TNBC cells led to a potent G2/M cell cycle arrest, inhibition of cell proliferation and induction of apoptosis within 24 hours, regardless of Rb status of the tumor cells. Primary human bone marrow and normal human fibroblast cells, however, did not undergo apoptosis within 72 hours of treatment with CDK9i, suggesting a potential therapeutic window for CDK9i treatment. Further studies are ongoing to assess the effect of dose and scheduling on tumor efficacy in mouse TNBC tumor models. Selective inhibition of CDK9 presents a novel treatment strategy for difficult-to-treat tumor types, including those resistant to existing targeted therapeutics. Citation Format: Hailey E. Brighton, Claire R. Hall, Kerry A. Dillon, John E. Bisi, Jay C. Strum. Selective inhibition of CD