Molecular mechanisms restoring olaparib efficacy through ATR/CHK1 pathway inhibition in olaparib-resistant BRCA1/2MUT ovarian cancer models

Resistance to olaparib inevitably emerges in ovarian cancer (OC) patients, underscoring the urgent need for strategies to enhance its therapeutic efficacy. In this study, we developed a novel olaparib-resistant patient-derived xenograft (PDX) model of high-grade serous OC harboring BRCA1/2 mutations to investigate the molecular mechanisms underlying acquired resistance and resensitization to olaparib in treatment-naïve tumors in vivo. Resistant xenografts were treated with olaparib, an ATR inhibitor (ATRi, ceralasertib), a CHK1 inhibitor (CHK1i, MK-8776), or combinations thereof. Tumor proliferation, apoptosis, ATR/CHK1 activity, PARP signaling, DNA damage response (DDR), epithelial-to-mesenchymal transition (EMT), and MDR1 expression were assessed using RT-qPCR, western blot, and immunohistochemistry.

Resistant tumors demonstrated defects in PARP and ATR/CHK1 signaling, along with altered expression of proteins involved in DDR and EMT. Notably, rechallenging resistant tumors with olaparib in combination with ATR or CHK1 inhibitors produced synergistic tumor growth inhibition. Mechanistic analysis revealed that combined treatments suppressed tumor proliferation without significantly increasing apoptosis or necrosis but induced tumor cell vacuolization, suggestive of cell death. Co-treatment with ATRi and olaparib led to the downregulation of ATR, CHK1, PARP1, PARG, BRCA1, γH2AX, and PARylated proteins, while reversing olaparib-induced upregulation of vimentin, BRCA2, and 53BP1.

These findings suggest that ATR/CHK1 pathway inhibition restores olaparib sensitivity in BRCA1/2-mutant high-grade serous OC, providing a promising strategy for overcoming resistance in non-responsive patients. The mechanisms uncovered in this study deepen our understanding of olaparib resistance and resensitization, offering potential avenues for improved therapeutic approaches in ovarian cancer.