Molecular characterization of inhibition of the DNA repair protein RAD51 by the small molecule JKYN-1

The ability of cancer cells to form large tumors and rapidly adapt to changing environmental conditions results from inherent genetic instability that also confers upon them sensitivity to cytotoxic agents. Cancer cells rely heavily on the action of the protein RAD51, overexpressed in many tumors, to survive DNA damage and replication stress (RS) (Cells 12: 1169-1189, 2023. https://doi.org/10.3390/cells12081169). The IBR-family RAD51 inhibitors not only inhibit tumor cell proliferation as single agents at low micromolar concentrations, but also work synergistically with a variety of established anticancer agents to stop growth and induce cell death (J Pharmacol Expt Ther, 364: 46-54, 2018. doi.org/10.1124/jpet.117.241661). To better understand the synergistic interaction and help improve selectivity against cancer cells, and thus potential for clinical use, we have characterized the activity of IBR2, IBR120, and JKYN-1 using a variety of methods. JKYN-1, the structure of which is a modification of IBR120, is 5-fold stronger as a single-agent inhibitor of proliferation. As demonstrated previously for IBR2 and IBR120, JKYN-1 synergistically inhibited proliferation of cell lines representative of numerous tumor sources in combination with established anticancer drugs. In hydroxyurea-treated human breast carcinoma MCF-7 cells, B02, an established RAD51 inhibitor (Mol Cancer Ther 20:1257-1269, 2021. doi: 10.1158/1535-7163.MCT-20-0252), and JKYN-1 markedly suppressed formation of RAD51 foci while increasing pRPA foci formation, a marker of RS. In human osteosarcoma U2OS cells exposed to ionizing radiation, JKYN-1 alone reduced RAD51 foci formation. Mechanistically, JKYN-1 impaired RAD51-dependent D-loop formation. JKYN-1 inhibited multimerization of RAD51 as well as its ATPase activity, responsible for normal dissociation of RAD51 from DNA following completion of repair. Therefore, JKYN-1 acts as expected in inhibiting RAD51 function. How this contributes to the synergy observed with anticancer agents is being investigated. Derivatives of JKYN-1 are currently being studied for improvements to pharmacokinetic properties. Funded by Sarissa, Inc., Breast Cancer Canada, and London Health Sciences Foundation.