Purpose PARP inhibition (PARPi) has modest clinical activity in recurrent BRCA mutant (BRCAMUT) high-grade serous ovarian cancers (HGSOC). We hypothesized that PARPi increases dependence on ATR/CHK1 such that combination PARPi with ATR/CHK1 blockade results in increased cell death and tumor regression. Experimental Design Effects of PARPi (olaparib), CHK1 inhibition (CHK1i;MK8776) or ATR inhibition (ATRi;AZD6738) alone or in combination on survival, colony formation, cell-cycle, genome instability and apoptosis were evaluated in BRCA1/2MUT HGSOC cells. Tumor growth in vivo was evaluated using a BRCA2MUT patient-derived-xenograft (PDX) model. Results PARPi monotherapy resulted in a decrease in BRCAMUT cell survival, colony formation and suppressed but did not eliminate tumor growth at the maximum-tolerated dose in a BRCAMUT PDX. PARPi treatment increased pATR and pCHK1 indicating activation of the ATR-CHK1 fork protection pathway is relied upon for genome stability under PARPi. Indeed, combination of ATRi or CHK1i with PARPi synergistically decreased survival and colony formation compared to single agent treatments in BRCAMUT cells. Notably, PARPi led to G2 phase accumulation, and the addition of ATRi or CHK1i released cells from G2 causing premature mitotic entry with increased chromosomal aberrations and apoptosis. Moreover, the combinations of PARPi with ATRi or CHK1i were synergistic in causing tumor suppression in a BRCAMUT PDX with the PARPi-ATRi combination inducing tumor regression and in most cases, complete remission. Conclusions PARPi causes increased reliance on ATR/CHK1 for genome stability and combination PARPi with ATR/CHK1i is more effective than PARPi alone in reducing tumor burden in BRCAMUT models.

Ovarian cancer (OVCA) inevitably acquires resistance to platinum chemotherapy and PARP inhibitors (PARPi). We show that acquisition of PARPi-resistance is accompanied by increased ATR-CHK1 activity and sensitivity to ATR inhibition (ATRi). However, PARPi-resistant cells are remarkably more sensitive to ATRi when combined with PARPi (PARPi-ATRi). Sensitivity to PARPi-ATRi in diverse PARPi and platinum-resistant models, including BRCA1/2 reversion and CCNE1 -amplified models, correlate with synergistic increases in replication fork stalling, double-strand breaks, and apoptosis. Surprisingly, BRCA reversion mutations and an ability to form RAD51 foci are frequently not observed in models of acquired PARPi-resistance, suggesting the existence of alternative resistance mechanisms. However, regardless of the mechanisms of resistance, complete and durable therapeutic responses to PARPi-ATRi that significantly increase survival are observed in clinically relevant platinum and acquired PARPi-resistant patient-derived xenografts (PDXs) models. These findings indicate that PARPi-ATRi is a highly promising strategy for OVCAs that acquire resistance to PARPi and platinum.

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