A journey through targeting BRCA2-RAD51 protein-protein interaction to establish synthetic lethality as paradigm for anticancer drug discovery

Nowadays, synthetic lethality (SL) is a known effective paradigm in anticancer therapy.1 However, SL has been mostly limited to the combination of pre-existing mutations with drugs against their synthetic lethal partners.1 In this context, we envisioned SL as paradigm to design synergistic combinations of drugs, targeting pairs of synthetic lethal targets. We selected the synthetic lethal pair PARP/BRCA2, taking advantage of the approved PARPis. We picked BRCA2-RAD51 protein-protein interaction (PPI) as target for our medicinal chemistry campaign, reasoning that its inhibition would mimic BRCAness and trigger SL with PARPi.2 To design small molecule PPI inhibitors, we explored three distinct approaches. First, we used BRC4-RAD51 crystal structure to perform virtual screening against two distinct PPI hot spots, zone I and II. Pyrazoline hit 1 from zone II led to 2 (Fig. 1), which induced SL with PARPi and gave us the first proof-of-concept of a “fully-small-molecule induced SL”.2 In parallel, we used protein-templated dynamic combinatorial chemistry (ptDCC) on RAD51 and identified ten N-acylhydrazones, as micromolar PPI inhibitors. In particular, 3 was found to bind at BRC4 binding site by 19F-NMR (Fig. 1).3 Finally, we used a 19F-NMR fragment screening and identified 4 and subsequently developed the pyrazole pyrimidinone 5, which induced SL in several pancreatic cancer models (Fig. 1).4 Despite issues related to PPI inhibition by small molecules, this work shows the variety of effective approaches that can be used to reach that goal, opening to future development of PPI inhibitors. Moreover, it consolidates SL as paradigm for anticancer drug combinations, promoting future medicinal chemistry efforts against synthetic lethal targets, especially if involved in the DNA damage response.