Targeting fatty acid synthase to overcome PARP inhibitor resistance and to create an artificial synthetic lethality for triple-negative breast cancer.

Despite advances in cancer treatment with targeted therapies and immunotherapies, triple-negative breast cancer (TNBC) has not significantly benefited from these developments. Although poly (ADP-ribose) polymerase (PARP) inhibitors (PARPi) are approved for breast cancer, their clinical use is largely limited to the small subset of HER2-negative patients with germline BRCA1/2 mutations, and resistance is frequently observed. Previously, we demonstrated that proton pump inhibitors (PPIs), including lansoprazole and its metabolite, 5-hydroxy lansoprazole sulfide (5HLS), reduce PARP1 expression by inhibiting fatty acid synthase (FASN), a key enzyme in de-novo lipid synthesis. We also found that PPIs synergize with DNA-damaging agents by regulating PARP1 expression and impairing non-homologous end joining (NHEJ) repair of DNA damage. These findings led to the hypothesis that PPIs synergize with PARPi independently of BRCA mutation, potentially expanding the utility of PARPi to a broader TNBC population. In this study, we show that FASN contributes to PARPi resistance, and that lansoprazole and 5HLS strongly synergize with olaparib and talazoparib in both BRCA1-mutant and wild-type TNBC cells. This synergy occurs through FASN inhibition and subsequent impairment of NHEJ repair of double-strand breaks induced by PARPi trapping. 5HLS also facilitates PARPi-induced PARP1 trapping and inhibits BRCA1 expression by inhibiting FASN, contributing to the synergy with PARPi in both BRCA1 wild-type and mutant TNBC cells. Together, these findings suggest that inhibiting FASN with PPIs creates an artificial synthetic lethality, providing a rationale for combining PPIs with PARPi to expand their utility to TNBC patients without germline BRCA1 mutations and to overcome PARPi resistance.