Purpose: PARP inhibition (PARPi) has modest clinical activity in recurrent BRCA-mutant (BRCA^MUT) 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 BRCA1/2^MUT HGSOC cells. Tumor growth in vivo was evaluated using a BRCA2^MUT patient-derived xenograft (PDX) model.Results: PARPi monotherapy resulted in a decrease in BRCA^MUT cell survival, colony formation and suppressed but did not eliminate tumor growth at the maximum tolerated dose (MTD) in a BRCA2^MUT 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 with single-agent treatments in BRCA^MUT 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 BRCA2^MUT 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 BRCA^MUT models. Clin Cancer Res; 23(12); 3097-108. ©2016 AACR.

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