Targeting de novo pyrimidine synthesis confers vulnerability to copper-mediated ATR inactivation in PARP inhibitor-resistant ovarian cancer.

Although poly(ADP-ribose) polymerase (PARP) inhibitors (PARPi) as monotherapy or in combination with other DNA-damaging agents exhibit promising clinical efficacy, the therapeutic responses are usually transient, with subsequent development of acquired resistance posing a significant challenge. Here, through a small-molecule compound screening, we identify elesclomol, a potent copper ionophore, which sensitizes BRCA-proficient ovarian cancer cells to PARPi by inhibiting activation of the ATR-CHK1 pathway. Mechanistically, we demonstrate that copper directly binds to ATRIP, a critical cofactor of ATR activation, disrupting the ATR-ATRIP interaction, further impairing ATR-mediated DNA damage repair signaling and potentiating PARPi sensitivity. Importantly, we reveal a secondary metabolic vulnerability in PARPi-resistant ovarian cancer associated with de novo pyrimidine synthesis, suggesting that targeting this pathway as an effective strategy to eradicate drug-adaptive residual tumors and resistant patient-derived xenograft models following ATR and PARP co-inhibition. These findings propose de novo pyrimidine synthesis as an adaptive metabolic vulnerability that can be therapeutically targeted to overcome PARPi resistance in BRCA-proficient ovarian cancer.