Abstract 694: Development of selective and potent CDK7 inhibitors for breast cancer therapy.

Cyclin dependent kinases (CDK) function as either regulators of cell cycle progression or transcription. CDK7 is unique from other CDKs because it has a dual role in the regulation of cell cycle and transcription making it a desirable target for drug development. It forms a complex with Cyclin H and MAT1 to form the CDK-activating kinase (CAK) that regulates CDK1, CDK2, CDK4 and CDK6, which are important in cell cycle progression. The CAK complex can also act as a transcriptional regulator as part of the TFIIH transcription factor by phosphorylating the COOH-terminal domain of RNA polymerase II (Pol II CTD). In addition, CDK7 as part of the CAK complex can inhibit the phosphorylation of serine 118 on the estrogen receptor (ER), a key player in breast cancer pathogenesis. Our target validation studies using siRNA have shown that CDK7 reduces growth of the human colorectal cancer cell line, HCT116, induces apoptosis and reduces phosphorylation of key biomarkers (RNA Poll II, CDK1 and CDK2). Using computer modelling of the CDK7 structure, we have designed CDK7 inhibitors, which were selected from a library of pyrazolo [1,5-a]pyrimidine-derived compounds. BS-181, is our first CDK7 inhibitor which inhibits CAK activity with an IC50 of 21 nmol/L. BS-181 has poor pharmacokinetic properties and is not orally bioavailable therefore after extensive studies and chemical modifications we have developed a more potent orally bioavailable CDK7 selective inhibitor, BS-181-L. Extensive in vitro ADME studies have highlighted that BS-181-L has good aqueous solubility, plasma proteins binding and no hERG liability. BS-181-L inhibits in vivo tumour cell growth in two cancer cell line models, MCF7 and HCT116 cells. In these studies BS-181-L shows no organ and haematological toxicity. We show BS-181-L down regulates key biomarkers in both the peripheral blood mononuclear cells (PBMCs) by flow cytometry and tumour tissue by immunohistochemistry (IHC) and western blotting. In addition, we show that BS-181-L reduces ER activity in reporter assays and ER (serine 118) phosphorylation. Furthermore, combination studies of BS-181-L and anti-estrogens show additive growth inhibitory effects on breast cancer cells. In summary, here we present data for a potent CDK7 inhibitor, BS-181-L, which has improved CDK7 selectivity, potency and bioavailability from our previously identified CDK7 inhibitor, BS-181. Furthermore, dual inhibition of CDK7 and ER has an additive effect in breast cancer cell growth. Taken together, this data highlights a potential new therapeutic approach for breast cancer. Citation Format: Hetal Patel, Silvia Ottaviani, Manikandan Periyasamy, Alexander Bondke, Brian Slafer, Richard Starkey, Sebastian H.B Kroll, Sean Delaney, Zahida Zahoor, Matthew J. Fuchter, Anthony G.M Barrett, R. Charles Coombes, Simak Ali. Development of selective and potent CDK7 inhibitors for breast cancer therapy. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 694. doi:10.1158/1538-7445.AM2013-694