Development of a DNA damage response (DDR) therapeutics platform for oncology.

e15036 Background: Cancer cells respond to increases in DNA damage by deploying their DNA damage response (DDR) pathways. We are building a platform for the discovery and development of target-specific DDR therapeutics, including small molecule inhibitors and targeted protein degradation warheads, founded on fragment- and structure-based drug discovery. Methods: Our DDR platform, which includes hit-to-lead, lead optimization and candidate selection, starts with hit generation from a new technology that uses high-throughput protein X-ray crystallography to directly screen compound libraries. Our hit generation process produces empirical evidence of direct target engagement. The elucidation of high-quality ligand-bound 3D structures reveals the location and pose of the ligand and details of the protein-ligand interactions. Thus we can predict the structure-function consequences of the hit molecule engagement, which sets the stage for rapid assessment of synthetic tractability and intellectual property. After hit identification, we apply a multi-pronged approach in hit-to-lead conversion and lead optimization using iterative biophysical and biochemical assays, coupled with crystallography. We are applying our approach to several new targets in DDR and will present some early progress in this space. Results: Apurinic/apyrimidinic endonuclease 1 (APE1) is the major repair enzyme for abasic sites in DNA and contributes to DNA strand break processing. Many studies have associated increased APE1 levels with enhanced growth, migration, and drug resistance in human tumor cells, and with decreased patient survival. APE1 has been implicated in over 20 human cancers, including glioblastoma, making the protein an attractive target for the development of anticancer therapeutics. Despite intensive effort, there are no clinical endonuclease inhibitors of APE1. We have identified 25 diverse small molecule fragments that bind to APE1 at two distinct sites, including the endonuclease site. Pol eta (or PolH) is a DNA polymerase implicated, among other things, in the development of cisplatin resistance in a subset of ovarian cancers. In our quest to develop PolH inhibitors, we have identified 5 diverse fragments that bind to two distinct sites in the polymerase including a new potential allosteric site. Our results on APE1 and PolH represent the first known cases of crystal structures of small molecules bound to these proteins. Flap endonuclease (FEN1) is implicated in several cancers including for example ER/tamoxifen-resistant breast cancer. We are developing a targeted protein degradation approach using PROTACs (Proteolysis-Targeting Chimeras) toward the development of novel therapeutics against FEN1. Conclusions: Our results will help us develop small molecule inhibitors and targeted protein degradation against DDR targets that may be effective as single therapies or be used to make existing therapies more effective.