Development of Small Molecules that Specifically Inhibit the D‑loop Activity of RAD51

RAD51 is the central
protein in homologous recombination (HR) DNA
repair and represents a therapeutic target in oncology. Herein we
report a novel class of RAD51 inhibitors that were identified by high
throughput screening. In contrast to many previously reported RAD51
inhibitors, our lead compound 1 is capable of blocking
RAD51-mediated D-loop formation (IC50 21.3 ± 7.8 μM)
at concentrations that do not influence RAD51 binding to ssDNA. In
human cells, 1 inhibits HR (IC50 13.1 ±
1.6 μM) without blocking RAD51’s ability to assemble
into subnuclear foci at sites of DNA damage. We determined that the
active constituent of 1 is actually an oxidized derivative
(termed RI­(dl)-1 or 8) of the original screening
compound. Our SAR campaign also yielded RI­(dl)-2 (hereafter
termed 9h), which effectively blocks RAD51’s D-loop
activity in biochemical systems (IC50 11.1 ± 1.3 μM)
and inhibits HR activity in human cells (IC50 3.0 ±
1.8 μM).