Cyteir Therapeutics (United States)

Abstract 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 O'Shea, 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.

Abstract AID, a DNA-directed cytidine deaminase, plays a critical role in somatic hypermutation and immunoglobulin class switching in maturing B-lymphocytes. Unlike site specific recombinases such as RAG1/2, AID is a promiscuous DNA damaging enzyme that deaminates cytidines at sites throughout the genome. AID expressing cells become critically dependent on the homologous recombination factor RAD51 to repair DNA double strand breaks that result from these off target deamination events. We have developed a novel small molecule, CYT01B, which inhibits RAD51 response to DSBs and is potent to a much greater extent in AID expressing cells. Previously, we have shown this molecule to have activity in mouse lymphoma xenograft models that constitutively express AID. Here we present new preclinical characterization data of CYT01B in solid cancer models. We first analyzed the frequency and levels of AID overexpression in solid tumor data in The Cancer Genome Atlas. Multiple solid tumor types displayed ectopic AID expression, including breast cancer, sarcoma, melanoma, pancreatic cancer, lung cancer, and head and neck cancers with about 30% of the patient samples 4-fold above the baseline expression level. We then tested CYT01B in several solid tumor derived human cell lines. We observed a correlation between AID expression and activity. Cell lines with low ectopic expression of AID had EC50 values in the low micromolar range (~2µM), while those without AID expression gave EC50 values of about 5µM and higher. Next, we examined the activity of our small molecule in 14 different PDX models with various levels of AID expression. These models included renal, head and neck, lung, pancreatic, ovarian, colorectal, and breast cancer samples. We observed a wide range of tumor growth inhibition across the different models (0% to 60%+), which tended to correspond with AID expression; the higher the AID expression the greater the observed tumor growth inhibition. Taken together, these data demonstrate that CYT01B shows anti-cancer activity in a range of solid tumor models. Overall, this provides the basis for continued development of CYT01B as an AID/RAD51 synthetic lethal therapeutic for solid cancers. Citation Format: Melinda Day, Tyler Maclay, Kevin Mills. A novel RAD51 inhibitor, CYT01B, shows anti-cancer activity in preclinical models of AID expressing solid tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4730.

Abstract A key characteristic of many cancers is genomic instability, and is often associated with poor prognosis. While genomic instability can promote tumorigenesis, it also provides a therapeutic opportunity for synthetic lethality. The induction of DNA damage creates stress that necessitates highly active DNA repair as a critical survival system for many transformed cells. Recently, therapeutics that target PARP1 have been approved by the FDA as synthetic lethal therapeutics for cancers with deficiencies in BRCA1/2 and associated pathways. Here we present the early development of a new synthetic lethal therapy that leverages gain-of-function abnormalities to selectively target cancer cells. Activation Induced Cytidine Deaminase (AICDA or AID) is a DNA-directed cytidine deaminase that is normally expressed exclusively and transiently in activated B-lymphocytes, where it plays critical roles in somatic hypermutation and immunoglobulin class switching. AICDA is a DNA damaging enzyme, producing DNA base pair mismatches which can subsequently be converted into mutations, DNA single strand breaks (SSB), or DNA double strand breaks (DSB). Numerous cancers show constitutive overexpression of AICDA, leading to hypermutation, genomic instability, and tumor evolution. We have previously demonstrated that cells expressing AICDA are critically dependent upon the DNA repair factor RAD51. Here we present data illustrating the effectiveness of a novel small molecule, CYT02A, in targeting the AICDA/RAD51 axis. Cell culture assays showed CYT02A to be a potent DNA repair inhibitor that targets RAD51 subcellular localization and filament formation. Although CYT02A was stable when incubated with liver microsomes. Although CYT-02A showed limited oral bioavailability, it was stable in liver microsome assyas assays and the pharmacokinetic profile following intravenous injection revealed dose proportionality and acceptable in vivo exposures. CYT02A was efficacious in a human-to-mouse xenograft model of chronic lymphocytic leukemia (CLL). CYT02A was administered intravenously at a concentration of 50 mg/kg to AID+ CLL xenograft mice daily for up to 9 days. Treated mice showed a significant reduction in CLL burden in the bone marrow compared to vehicle control animals. CYT02A was well tolerated by the animals, with no observable change in behavior or complete blood counts (CBC). Taken together, these data validate a novel gain of function synthetic lethal approach targeting the RAD51/AICDA axis. Preclinical efficacy data support this strategy for the treatment of AICDA-expressing cancers. We continue to build upon this foundation to produce a new therapeutic paradigm that may be effective in a wide range of both hematologic malignancies and solid tumors. Citation Format: Muneer Hasham, Kin-hoe Chow, Tyler Maclay, Amber Cyr, Darryl Patrick, Melinda Day, Kevin D. Mills. Targeting the AICDA/RAD51 axis: A novel gain-of-function synthetic lethal therapy for the treatment of AICDA-expressing cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 347.