Targeting Homologous Recombination in Lymphoid Malignancies: Evaluation of Four Small Molecule Inhibitors of RAD51

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. The homologous recombination (HR) repair pathway is an attractive target. HR deficient cancers are hypersensitive to numerous anticancer drugs, and tumors will often induce expression of HR genes to promote drug resistance. RAD51 is a key component of the HR pathway. RAD51 forms nucleoprotein filaments at sites of DNA damage and replication fork stalls, mediating homologous DNA strand exchange to promote recombinational repair of breaks and damaged replication forks. We utilized four small molecule inhibitors of RAD51-mediated HR for evaluation of RAD51 as a potential therapeutic target. Compounds CYT-0851, CYT-0853, CYT-1027, and CYT-1127 were evaluated for anti-cancer activity in vitro and in vivo. To determine the impact of the small molecules on RAD51 and HR, all four were tested for effects on RAD51 focus formation and sister chromatid exchange (SCE) activity. All the compounds showed a reduction in SCE activity, however only CYT-0851 and CYT-0853 produced a measurable reduction in RAD51 foci. We have previously shown that that RAD51 inhibition leads to accumulation of DNA breaks, and ultimately cell death, in cells expressing the DNA mutator protein Activation Induced Cytidine deaminase (AICDA/AID). Cytotoxicity assays were performed in an AID+ (Daudi, Burkitt's Lymphoma) and AID- (WI-38, fibroblast) cell lines. All four compounds were preferentially active in AID+ cells with little to no cytotoxicity observed in the AID-negative WI-38 cell line. CYT-0853 was the most potent in the Daudi cell line with an EC50 of 8nM. All four compounds were orally bioavailable in all preclinical species tested but showed differences in pharmacokinetics. Preclinical cell line derived xenograft models of AID-high Burkitt's lymphoma (Daudi) and B-cell acute lymphoblastic leukemia (CCRF-SB) were used to determine the in vivo anti-tumor activity of the compounds in lymphoid cancer models. CYT-0851 and CYT-0853 both showed significant anti-tumor activity with tumor growth inhibition of greater than 50% in both models. Further analysis showed drug exposure with CYT-0851 was more consistent in the CDX models than CYT-0853. Overall, these data indicate that RAD51 and HR are attractive therapeutic targets for the treatment of lymphoid malignancies and that CYT-0851 is a viable clinical development candidate.