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 have developed a novel RAD51 inhibitor, CYT01B, which we have previously shown to be preferentially active in Activation Induced Cytidine Deaminase (AID) expressing cells, and in other tumor derived cell lines with elevated levels of DNA damage and/or reduced DNA repair capabilities. CYT01B acts by reducing RAD51 focus formation, and depleting the nuclear pool of RAD51. We have observed anti-tumor effects with CYT01B in preclinical models, but have yet to determine if RAD51 could act as a sensitizer to current therapeutics. A main resistance mechanism to PARP inhibitors is the overexpression of RAD51, making it an attractive potential combination for our RAD51 inhibitor. To determine potential drug synergy, a matrix study was performed with CYT01B (concentration range of 20nM to 5µM) and 5 different PARP inhibitors including olaparib (78nM to 5µM), niraparib (78nM to 5µM), veliparib (780nM to 50µM), rucaparib (156nM to 10µM), and talazoparib (39nM to 2.5µM). The combination matrix was tested in cell lines of varying AID expression and PARP inhibitor sensitivity: ARPE19/HPV16 (HPV immortalized epithelial cell line), KYSE-70 (head and neck cancer cell line), Daudi (Burkitt’s Lymphoma cell line), HCC1143 (TNBC), and BT20 (TNBC). We used both the Loewe Additivity model and the Bliss Independence model to determine drug interaction (synergistic, independent, or antagonistic). In general, we observed synergy with all the combinations in the tumor derived cell lines. In the transformed epithelial cell line, synergy was observed only in the olaparib combination. The strength of the synergistic effect differed amongst the PARP inhibitors, with olaparib giving the strongest synergy scores. From this data we hypothesize that the synergistic activity with CYT01B is related to the PARP trapping efficiency of the PARP inhibitors; greater trapping efficiency the greater the synergy. These data suggest that CYT01B may be active as a combinatorial therapy with PARP inhibitors. Overall, we conclude that there is significant potential for combining RAD51 and PARP inhibition in future cancer treatment strategies, and warrants future exploration in vivo.

Citation Format: Tyler Maclay, Melinda Day, Kevin Mills. CYT01B, a novel RAD51 inhibitor, acts synergistically with PARP inhibitors [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 363.