HAMNO and CGK733 as BRCAness-driven candidates exhibit synthetic lethality with olaparib in BRCA-proficient ovarian cancer

Olaparib has low efficacy against BRCA-proficient ovarian cancer (OC); improving the efficacy of olaparib is expected to improve the prognosis of OC. In clinical practice, some BRCA-proficient cancers, similar to tumors with BRCA mutations, are sensitive to poly ADP‒ribose polymerase inhibitors (PARPis), a phenomenon known as BRCAness. This study aimed to identify BRCAness-inducing inhibitors that enhance olaparib antitumor efficacy in BRCA-proficient OC using patient-derived organoids (PDOs). We screened 28 inhibitors and evaluated their therapeutic value in combination with olaparib. Ultimately, an RPA70 inhibitor (HAMNO) and an ATR inhibitor (CGK733) significantly increased olaparib toxicity without causing major organ damage in mice. Mechanistically, HAMNO inhibits RPA70 binding to single-strand DNA (ssDNA) and hinders the recruitment of RAD51 at DNA double-strand breaks (DSBs), impairing homologous recombination repair (HRR). CGK733 induced BRCAness by blocking ATR function. Furthermore, the combination therapy with Olaparib was highly effective at inducing G2 phase arrest and inhibiting HRR. In addition, high RPA70 and ATR expression was found to indicate a poor prognosis in OC patients according to data from the Kaplan–Meier plotter database and clinical data from our center. In conclusion, our work provides an innovative approach for improving the therapeutic effect of olaparib on BRCA-proficient OC.