A Novel RAD51-BRCA2 Complex Stabilizing Drug: A Medical Countermeasure for Mitigating Accidental Radiation Exposure - Abstract Ionizing radiation (IR) exposure from accidents or warfare can cause severe damage to radiosensitive tissues, such as bone marrow, intestines, and blood vessels, leading to acute radiation syndrome (ARS) and delayed effects of acute radiation exposure (DEARE) with high mortality and morbidity. These effects are mainly due to DNA double-strand breaks (DSBs) induced by IR, which require efficient repair to maintain genomic integrity and cell survival. We have developed small-molecule protein ligand interface stabilizers (SPLINTS), referred to as MH analogs, that target the binding pocket of the BRCA2-RAD51 complex, a key component of homologous recombination (HR), the most accurate and efficient pathway for DSB repair in S/G2 phases of the cell cycle. Our preliminary results show that MH01, the most promising SPLINTS, can be administered hours after IR exposure and mitigates cytotoxicity in a RAD51- and BRCA2-dependent manner, and enhances the repair kinetics of radiation-induced DSBs in cell culture models. However, the precise mechanism by which MH01 stabilizes the BRCA2-RAD51 complex and enhances its functions during DSB repair and break-induced replication is still elusive. Remarkably, we found that MH01 treatment 24 hours after lethal whole-body irradiation (WBI) rescues 80% of mice from IR-induced death and reverses intestinal toxicity. However, the optimal dose, pharmacokinetics, biodistribution, pharmacodynamics, and mechanism of action of MH01 are still unknown. We hypothesize that MH01 is a potent and safe SPLINTS that mitigates tissue damage caused by IR exposure by enhancing HR-mediated DSB repair in the S/G2 phases of the cell cycle through BRCA2-RAD51 complex stabilization. Our specific aims are: 1. Optimize the MH01 regimen for mitigating radiation toxicity by integrating pharmacokinetics, pharmacodynamics, and biophysical assays; 2. Elucidate the mechanism underlying MH01- mediated enhancement of the BRCA2-RAD51 complex's functioning during DSB repair following IR; and 3. Evaluate MH01 efficacy in mitigating radiation gastrointestinal tract and cardiac toxicities. Our approach on strengthening the stability of the BRCA2-RAD51 complex using SPLINTS at the peak of BRCA2-RAD51 activity during radiation-induced DSB repair, signifies a paradigm shift in the modulation of DNA DSB repair. This novel strategy presents a therapeutic avenue for mitigating radiation injuries in normal tissues with cells capable of undergoing division. Our overarching objective is to establish SPLINTS as a new and potentially significant inclusion in the current array of medical countermeasures designed to address accidental radiation exposure.
