CYT01B, a Novel RAD51 Inhibitor, Act Synergistically with Both Targeted and Chemotherapeutic Anti-Cancer Agents
Authors: Maclay Tyler , Vacca Joseph , McComas Casey , Castro Alfredo , Day Melinda , Mills Kevin 
Published: October 2019

Abstract
We have developed CYT01B, a novel RAD51 inhibitor, that sensitizes cells to Activation Induced Cytidine Deaminase (AID) activity. In cancer cells, AID causes significant genotoxic stress through DNA replication fork damage, creating a dependency upon the homologous recombination repair factor, RAD51, for survival. CYT01B acts by destabilizing RAD51 focus formation, leading to its premature nuclear export and degradation. We have shown CYT01B to be effective in AID expressing cells, however, we had yet to address if inhibition of RAD51 could act as a sensitizer to current therapeutics. To determine potential drug combinations, a matrix study was performed with CYT01B (concentration range of 20nM to 5μM) and six different targeted agents or chemotherapeutics in three different cell lines: ARPE19/HPV16 (HPV immortalized normal epithelial cell line), KYSE-70 (head and neck cancer cell line) and Daudi (Burkitt's Lymphoma cell line). We then used the Bliss Independence model to determine drug interaction (synergistic, independent, or antagonistic). The compounds tested were the ATR inhibitor VE-822 (concentration range of 39nM to 2.5μM), the RPA inhibitor TDRL-505 (concentration range of 39nM to 5μM), the proteasome inhibitor Bortezomib (concentration range of 39nM to 2.5μM), Carboplatin (concentration range of 156nM to 10μM), and the PARP inhibitors Olaparib and Niraparib (concentration range of 78nM to 5μM). With VE-822 we observed synergy in the KYSE-70 cell line with ambiguous results in Daudi and ARPE19/HPV16. In ARPE19/HPV16 cell line, synergy was observed with CYT01B at 39nM with all concentrations of VE-822, but antagonistic activity was seen at the high and low concentrations. In Daudi, antagonism was observed at the highest concentrations of VE-822, but an additive effect was noted at the lower concentrations of VE-822. Antagonism was observed at all concentrations of CYT01B with TDRL-505 in both Daudi and ARPE19/HPV16. Weak synergy was observed in KYSE70 cells at 156 and 312nM CYT01B. CYT01B was synergistic with Bortezomib in ARPE19/HPV16 at all concentrations but was consistently antagonistic in KYSE-70 and Daudi. We observed synergy with carboplatin in all cell lines, with the effect consistent across the full concentration range in the cancer cell lines. Synergy was also observed consistently across the full concentration range in all three cell lines with both PARP inhibitors. However, Olaparib showed a stronger synergistic effect than Niraparib. These data suggest that CYT01B may be effective as a combinatorial therapy with platinum based chemotherapeutics and with PARP and ATR inhibitors. Overall, we conclude that there is significant potential for RAD51 inhibition to be used in future combination cancer treatment strategies and warrants further exploration in vivo.

Disclosures
Maclay: Cyteir Therapeutics: Employment. Vacca:Cyteir Therapeutics: Consultancy. McComas:Cyteir Therapeutics: Consultancy. Castro:Cyteir Therapeutics: Consultancy. Day:Cyteir Therapeutics: Employment. Mills:Cyteir Therapeutics: Employment, Equity Ownership, Membership on an entity's Board of Directors or advisory committees.