RAD51 Inhibitors
RAD51 is a central protein in the homologous recombination (HR) pathway, which is responsible for the high-fidelity repair of DNA double-strand breaks (DSBs). During HR, RAD51 is recruited to single-stranded DNA (ssDNA) tails generated at the site of the break. It forms a nucleoprotein filament that searches for a homologous DNA sequence on a sister chromatid and catalyzes strand invasion, forming a D-loop structure. This process is tightly regulated by several mediator proteins, most notably BRCA2, which facilitates the loading of RAD51 onto ssDNA. Because HR is essential for maintaining genomic stability, its dysregulation is a hallmark of many cancers, and conversely, its inhibition can be a powerful therapeutic strategy. RAD51 inhibitors are primarily being developed as anticancer agents.
They are particularly attractive for use in "synthetic lethality" strategies. For instance, tumors that are already deficient in other DNA repair pathways, such as those with BRCA1/2 mutations, may be hypersensitive to RAD51 inhibition. Furthermore, RAD51 overexpression is often associated with resistance to radiotherapy and conventional chemotherapies like cisplatin or topoisomerase inhibitors, which induce DSBs. By blocking RAD51, researchers hope to sensitize these resistant tumors to standard treatments. The landscape of RAD51 inhibitors includes several small molecules that have shown efficacy in preclinical studies. B02 and RI-1 are two of the most well-characterized tool compounds.
B02 inhibits the DNA-binding activity of RAD51 and its ability to catalyze strand exchange, while RI-1 covalently binds to a specific cysteine residue on the surface of RAD51, preventing its multimerization into a functional filament. More recently, inhibitors that disrupt the interaction between RAD51 and BRCA2 have been developed. While no RAD51 inhibitor has yet reached the market, several candidates are moving through early-phase clinical trials or advanced preclinical development. Inhibitors can be classified into direct RAD51 binders, which interfere with its ATPase activity or DNA binding, and indirect inhibitors, which target the regulatory proteins or upstream signaling pathways that control RAD51 expression and localization. Another classification is based on the mode of binding, such as reversible versus irreversible (covalent) inhibitors. One of the primary challenges in developing RAD51 inhibitors is the high degree of structural conservation in the RecA-like fold across different species and related proteins, which can lead to off-target effects.
Additionally, since HR is also used by normal dividing cells, achieving a therapeutic window is crucial. Current research is focusing on identifying biomarkers to select patients who would most benefit from RAD51 inhibition and exploring combinations with PARP inhibitors to overcome or prevent resistance in HR-proficient and HR-deficient cancers.
