Abstract C14: A novel RAD51 inhibitor, CYT-0851, shows anticancer activity in preclinical models of pancreatic cancer

Genomic instability is recognized as a driver of tumorigenesis and cancer progression. Loss of tumor suppressors or activation of oncogenes can induce DNA damage stress, promoting genomic instability and creating dependencies upon key DNA repair pathways. These dependencies can be targeted therapeutically to induce synthetic lethality. We leverage a “gain-of-function” synthetic lethality approach to selectively target cancers that ectopically express Activation Induced Cytidine Deaminase (AID). AID is a DNA-directed cytidine deaminase normally transiently expressed in maturing B-lymphocytes where it plays a critical role in somatic hypermutation and immunoglobulin class switching. However, AID is dysregulated and inordinately expressed in many cancers, where it acts as a DNA-damaging enzyme mutating widespread locations throughout the genome. This activity leads to the accumulation of point mutations and DNA breaks and promotes DNA replication stress. In the latter context, AID-expressing cells become critically dependent on the homologous recombination factor RAD51 to survive DNA damage-induced replication stress. We have developed a novel small-molecule inhibitor of RAD51, CYT-0851, that is preferentially cytotoxic to AID expressing cells. We have shown in vitro and in vivo that CYT-0851 is efficacious in AID-expressing lymphoma and leukemia mouse models. Genome profiling and gene expression studies have shown many solid tumors, including pancreatic cancer, also aberrantly express AID. To determine whether targeting RAD51 may be an effective approach to the treatment of AID-expressing pancreatic cancer, we evaluated the antitumor activity of CYT-0851 in multiple patient-derived pancreatic cancer xenografts (PDX) models with a range of AID expression levels. All models tested showed tumor growth inhibition (TGI), ranging from 63 to 104% with CYT-0851 treatment. A crossover experiment was performed with one model to determine the activity of CYT-0851 in large, established tumors. We observed tumor regression (137% TGI) in large tumors upon treatment with CYT-0851, resulting in one partial responder and one tumor-free responder. The data presented here demonstrate that CYT-0851 exerts anticancer activity in multiple preclinical models of pancreatic cancer. Considering that recent published studies have shown that these cancers can ectopically overexpress AID, we conclude that CYT-0851 may be an effective treatment for pancreatic cancer. Overall, this provides a rationale for the continued development of CYT-0851 as a novel therapeutic for pancreatic cancer patients. Citation Format: Melinda Day, Jean-Marc Lapierre, Tom O9Shea, Kevin Mills. A novel RAD51 inhibitor, CYT-0851, shows anticancer activity in preclinical models of pancreatic cancer [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Advances in Science and Clinical Care; 2019 Sept 6-9; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2019;79(24 Suppl):Abstract nr C14.