Targeting the ATR/CHK1 Axis with PARP Inhibition Results in Tumor Regression in <i>BRCA</i> -Mutant Ovarian Cancer Models

Abstract

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