Bcl-xL Inhibition Radiosensitizes PIK3CA/PTEN Wild-type Triple-negative Breast Cancers with Low Mcl-1 Expression

Patients with radioresistant breast cancers, including a large percentage of women with triple-negative breast cancer (TNBC), demonstrate limited response to radiation and increased locoregional recurrence; thus, strategies to increase the efficacy of radiation in TNBC are critically needed. We demonstrate that pan Bcl-2 family inhibition ABT-263, radiation enhancement ratio (rER): 1.52–1.56 or Bcl-xL–specific inhibition (WEHI-539, A-1331852; rER: 1.31–2.00) radiosensitized wild-type PIK3CA/PTEN TNBC (MDA-MB-231, CAL-120) but failed to radiosensitize PIK3CA/PTEN-mutant TNBC (rER: 0.90–1.07; MDA-MB-468, CAL-51, SUM-159). Specific inhibition of Bcl-2 or Mcl-1 did not induce radiosensitization, regardless of PIK3CA/PTEN status (rER: 0.95–1.07). In wild-type PIK3CA/PTEN TNBC, pan Bcl-2 family inhibition or Bcl-xL–specific inhibition with radiation led to increased levels of apoptosis (P 0.001) and an increase in cleaved PARP and cleaved caspase 3. CRISPR-mediated PTEN knockout in wild-type PIK3CA/PTEN MDA-MB-231 and CAL-120 cells induced expression of pAKT/Akt and Mcl-1 and abolished Bcl-xL inhibitor–mediated radiosensitization (rER: 0.94–1.07). Similarly, Mcl-1 overexpression abolished radiosensitization in MDA-MB-231 and CAL-120 cells (rER: 1.02–1.04) but transient MCL1 knockdown in CAL-51 cells promoted Bcl-xL inhibitor–mediated radiosensitization (rER: 2.35 ± 0.05). In vivo, ABT-263 or A-1331852 in combination with radiation decreased tumor growth and increased tumor-tripling time (P 0.0001) in PIK3CA/PTEN wild-type TNBC cell line and patient-derived xenografts. Collectively, this study provides the preclinical rationale for early-phase clinical trials testing the safety, tolerability, and efficacy of Bcl-xL inhibition and radiation in women with wild-type PIK3CA/PTEN wild-type TNBC at high risk for recurrence. Significance: This study proposes a novel strategy for the treatment of radioresistant TNBCs using FDA-approved compounds that target apoptosis to improve local disease control in this patient population.