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 (RT) and increased locoregional recurrence; thus, strategies to increase the efficacy of RT in TNBC are critically needed. We demonstrate that pan Bcl-2 family inhibition (ABT-263, 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 mutant PIK3CA/PTEN 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 RT 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 RT 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 RT in women with wild-type PIK3CA/PTEN wild-type TNBC at high risk for recurrence.

RB expression confers sensitivity to CDK4/6 inhibitor-mediated radiosensitization across breast cancer subtypes

Standard radiation therapy (RT) does not reliably provide locoregional control for women with multinode-positive breast cancer and triple-negative breast cancer (TNBC). We hypothesized that CDK4/6 inhibition (CDK4/6i) would increase the radiosensitivity not only of estrogen receptor–positive (ER⁺) cells, but also of TNBC that expresses retinoblastoma (RB) protein. We found that CDK4/6i radiosensitized RB WT TNBC (n = 4, radiation enhancement ratio [rER]: 1.49–2.22) but failed to radiosensitize RB-null TNBC (n = 3, rER: 0.84–1.00). RB expression predicted response to CDK4/6i + RT (R² = 0.84), and radiosensitization was lost in ER⁺/TNBC cells (rER: 0.88–1.13) after RB1 knockdown in isogenic and nonisogenic models. CDK4/6i suppressed homologous recombination (HR) in RB WT cells but not in RB-null cells or isogenic models of RB1 loss; HR competency was rescued with RB reexpression. Radiosensitization was independent of nonhomologous end joining and the known effects of CDK4/6i on cell cycle arrest. Mechanistically, RB and RAD51 interact in vitro to promote HR repair. CDK4/6i produced RB-dependent radiosensitization in TNBC xenografts but not in isogenic RB1-null xenografts. Our data provide the preclinical rationale for a clinical trial expanding the use of CDK4/6i + RT to difficult-to-control RB-intact breast cancers (including TNBC) and nominate RB status as a predictive biomarker of therapeutic efficacy.

Short-term CDK4/6 Inhibition Radiosensitizes Estrogen Receptor-Positive Breast Cancers

PURPOSE

Cyclin-dependent kinase 4/6 (CDK4/6) inhibitors have improved progression-free survival for metastatic, estrogen receptor-positive (ER⁺) breast cancers, but their role in the nonmetastatic setting remains unclear. We sought to understand the effects of CDK4/6 inhibition (CDK4/6i) and radiotherapy in multiple preclinical breast cancer models.

EXPERIMENTAL DESIGN

Transcriptomic and proteomic analyses were used to identify significantly altered pathways after CDK4/6i. Clonogenic assays were used to quantify the radiotherapy enhancement ratio (rER). DNA damage was quantified using γH2AX staining and the neutral comet assay. DNA repair was assessed using RAD51 foci formation and nonhomologous end joining (NHEJ) reporter assays. Orthotopic xenografts were used to assess the efficacy of combination therapy.

RESULTS

Palbociclib significantly radiosensitized multiple ER⁺ cell lines at low nanomolar, sub IC₅₀ concentrations (rER: 1.21-1.52) and led to a decrease in the surviving fraction of cells at 2 Gy (P < 0.001). Similar results were observed in ribociclib-treated (rER: 1.08-1.68) and abemaciclib-treated (rER: 1.19-2.05) cells. Combination treatment decreased RAD51 foci formation (P < 0.001), leading to a suppression of homologous recombination activity, but did not affect NHEJ efficiency (P > 0.05). Immortalized breast epithelial cells and cells with acquired resistance to CDK4/6i did not demonstrate radiosensitization (rER: 0.94-1.11) or changes in RAD51 foci. In xenograft models, concurrent palbociclib and radiotherapy led to a significant decrease in tumor growth.

CONCLUSIONS

These studies provide preclinical rationale to test CDK4/6i and radiotherapy in women with locally advanced ER⁺ breast cancer at high risk for locoregional recurrence.