Mechanisms of Intrinsic Radioresistance in Breast Cancer Identify Potential Therapeutic Vulnerabilities

PURPOSE/OBJECTIVE(S)

Clinical management of breast cancer (BC) includes radiation therapy (RT) for most women, though the molecular mechanisms that underly RT response and intrinsic radioresistance are poorly understood. Both in vitro and in vivo models aid in our understanding of radiobiology, and we hypothesized that transcriptional changes caused by radiation in vitro in BC cell lines would be recapitulated in an in vivo mouse xenograft model and uncover targetable mechanisms of radioresistance in BC.

MATERIALS/METHODS

Radiosensitivity was measured with clonogenic survival assays in 16 cell lines. RNA-seq experiments in vitro and in vivo were performed in an RT resistant (SUM-159) and RT sensitive (ZR-75) cell line 24 hrs after 4 Gy or after 2 Gy x 6 fractions, respectively. Differentially expressed genes (DEGs) were identified from RNA-seq data with DeSeq2 followed by pathway analysis with iPathwayGuide.

RESULTS

RT sensitivity was subtype independent in 16 BC cell lines, with SUM-159 radioresistant (SF 0.88) and ZR-75-1 radiosensitive (SF 0.29). There were 75 unique pathways that were significantly altered after RT in SUM-159 cells (53 pathways in vivo only, 36 pathways in vitro only, 14 both conditions; adjusted p-value < 0.05) and 85 unique pathways that were significantly altered after RT in ZR-75-1 cells (16 pathways in vivo only, 72 in vivo only, 3 both conditions; adjusted p-value < 0.05). Pathways that were significantly affected in both cell lines exclusively in the in vitro condition include canonical RT response pathways such as cell cycle, cellular senescence, and DNA replication, though the direction of DEGs were opposite in the two cell lines for each of these pathways. The IL-17 signaling pathway was significantly altered for both cell lines in vivo. Of the pathways that were significantly altered in both conditions for SUM-159 cells, inflammation, including chemokine signaling pathway and cytokine-cytokine receptor interaction, were among the most significant. Significantly more cytokines were upregulated following RT in vivo than in vitro. Cytokines were not upregulated in ZR-75-1 cells in vitro or in vivo.

CONCLUSION

Taken together, the significant changes in the IL-17 pathway and the upregulation of cytokines only in vivo indicate a potential of the tumor microenvironment in the in vivo condition that the in vitro condition lacks. Increased heterogeneity in vivo relative to in vitro may also explain the absence of several canonical RT response pathways in the in vivo conditions for each cell line. Notably, the opposite direction of DEG changes in the canonical RT response pathways between the 2 cell lines with disparate radiosensitivity levels may point to important biologic vulnerabilities that may be targeted in the resistant SUM-159 cells. Future studies are underway using additional BC cell lines and single-cell analysis to better understand RT response heterogeneity.

Transcriptomic Analysis to Uncover the Mechanism of Radiosensitization of AR-Positive Triple Negative Breast Cancers with AR Inhibition

PURPOSE/OBJECTIVE(S)

The androgen receptor (AR) has been shown to drive tumor growth in triple negative breast cancers (TNBC), and previous work demonstrated AR inhibition as a strategy for radiosensitization in AR-positive (AR+) TNBC. Despite its role in radioresistance, the mechanistic role of AR in response to radiation therapy (RT) remains unknown, as does the benefit of 2nd generation anti-androgens in this context. We hypothesized that all 2nd generation anti-AR therapy would radiosensitize similarly and that canonical AR transcriptional function was responsible for radioresistance in these models.

MATERIALS/METHODS

Radiosensitization was assessed using 2nd generation AR antagonists (apalutamide, enzalutamide, and darolutamide) using clonogenic survival assays in MDA-MB-453, SUM185, MFM-223, and MDA-MB-231 cells at 2-6Gy. Cellular fractionation experiments were performed and quantitated to determine the location of the AR protein in cells treated with AR agonists +/- RT. RNA Seq was performed and transcriptomic approaches were used (Advaita iPathway analysis) to investigate AR-mediated effects in response to RT.

RESULTS

Inhibition with the 2nd generation anti-androgens enzalutamide and apalutamide is sufficient to radiosensitize AR+ TNBC models (rER: 1.34-1.41); while darolutamide had no effect on radiosensitivity (rER: 0.96-1.11). Additionally, TNBC cells with low AR expression were not radiosensitized by AR inhibition with any drug (rER: 0.96-1.03). While stimulation with the synthetic androgen methyltrienolone R1881 is sufficient to induce nuclear translocation of AR in AR+ TNBC cells, AR inhibition with enzalutamide, apalutamide, or darolutamide blocked AR nuclear translocation under growth conditions with charcoal stripped serum or fetal bovine serum. When cells are treated with R1881+RT, nuclear translocation of AR was induced at similar or greater levels compared to R1881 alone in AR+ TNBC cells. Combination treatment of RT with enzalutamide in the presence of hormones reduced nuclear localization of AR (32-39% reduction) compared to RT alone. RNA-sequencing after RT identified transcriptional changes potentially regulated by AR+RT, including changes in the NHEJ pathway genes. Additionally, pathway analyses in these models demonstrated changes in the MAPK/ERK signaling pathway, among others, that may regulate RT resistance in AR+ TNBC models.

CONCLUSION

Most 2nd generation anti-androgens confer radiosensitization in AR+ TNBC models with cellular localization changes of AR noted after RT. The known structural differences amongst 2nd generation anti-androgens may account for differences in radiosensitization noted. Furthermore, AR-mediated radioresistance may be due, at least in part, to downstream MAPK/ERK signaling. This work builds on the mechanistic understanding of AR-mediated radioresistance in AR+ TNBC and may expose vulnerabilities to overcome resistance to combination treatment with AR inhibition and RT.

Abstract 2401: Molecular mechanisms of intrinsic radioresistance in breast cancer

Background: Clinical management of BC includes radiation therapy (RT), with most women receiving RT as part of their treatment. Although effective, many women develop locoregional recurrence, including a disproportionate number of women with triple-negative or inflammatory BC. Unfortunately, the molecular mechanisms that underly RT response and intrinsic radioresistance are poorly understood. We hypothesized that transcriptomic and proteomic changes that occur after ionizing radiation in intrinsically radiosensitive and resistant BC models would offer mechanistic insight into mediators of this differential response.

Methods: Intrinsic radiosensitivity across all 10 cell lines was measured with clonogenic survival assays as the surviving fraction (SF) after 2 Gy RT. Gene expression changes were assessed by RNA-Seq 24 hours after 4 Gy RT. For long-course RT, cell lines were treated with fractionated RT (2 Gy x 5 fractions). For in vivo mouse xenograft experiments mice received fractionated RT (2 Gy x 6 fractions). Differential gene expression analysis with DeSeq2 was performed on all samples, followed by pathway analysis with Advaita Bioinformatics’ iPathwayGuide. Protein was collected 1, 12, and 24 hours after RT for RPPA analysis evaluating expression changes in 100 proteins and phospho-proteins with SuperCurve.

Results: Clonogenic survival identified a wide range of radiation sensitivity in human BC cell lines (SF 83% - 19%) with no significant correlation (r %lt 0.3) to intrinsic BC subtype. The most highly affected pathways in both resistant and sensitive cell lines 24 hours after RT include cell cycle, cellular senescence, and estrogen signaling pathways. For the long-course RT samples, several pathways were significantly altered in fractionated samples only, including MAPK and Hippo signaling and EGFR tyrosine kinase inhibitor resistance. From the in vivo experiments, pathways uniquely affected in the in vivo samples include IL-17 signaling and transcriptional misregulation in cancer. From the proteomic data, we found that proteins including p53, Bcl-2 family proteins, and cell cycle proteins exhibit expression changes after 1 hour. A significant number of pathways (N=69, p %lt 0.01, FDR 0.05) were affected in radioresistant BC models compared to radiosensitive cell lines and these pathways may underlie intrinsic radioresistance.

Conclusions: Ionizing radiation induces transcriptomic and proteomic expression changes that differ between intrinsically sensitive and resistant BC models in both single fraction and fractionated studies. Pathways identified in these analyses offer potential insight into the mechanisms underlying intrinsic radioresistance and suggest biologic vulnerabilities that may be targeted to more effectively treat women at a high risk of local BC recurrence. Genome wide CRIPSR-Cas9 screens are currently underway in these breast cancer models to confirm these vulnerability targets.

Citation Format: Breanna N. McBean, Anna R. Michmerhuizen, Kari Wilder-Romans, Benjamin C. Chandler, Lynn M. Lerner, Connor Ward, Meilan Liu, Alan P. Boyle, Corey W. Speers. Molecular mechanisms of intrinsic radioresistance in breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2401.

Androgen and oestrogen receptor co-expression determines the efficacy of hormone receptor-mediated radiosensitisation in breast cancer

PURPOSE

Radiation therapy (RT) and hormone receptor (HR) inhibition are used for the treatment of HR-positive breast cancers; however, little is known about the interaction of the androgen receptor (AR) and estrogen receptor (ER) in response to RT in AR-positive, ER-positive (AR+/ER+) breast cancers. Here we assessed radiosensitisation of AR+/ER+ cell lines using pharmacologic or genetic inhibition/degradation of AR and/or ER.

METHODS

Radiosensitisation was assessed with AR antagonists (enzalutamide, apalutamide, darolutamide, seviteronel, ARD-61), ER antagonists (tamoxifen, fulvestrant) or using knockout of AR.

RESULTS

Treatment with AR antagonists or ER antagonists in combination with RT did not result in radiosensitisation changes (radiation enhancement ratios [rER]: 0.76-1.21). Fulvestrant treatment provided significant radiosensitisation of CAMA-1 and BT-474 cells (rER: 1.06-2.0) but not ZR-75-1 cells (rER: 0.9-1.11). Combining tamoxifen with enzalutamide did not alter radiosensitivity using a 1 h or 1-week pretreatment (rER: 0.95-1.14). Radiosensitivity was unchanged in AR knockout compared to Cas9 cells (rER: 1.07 ± 0.11), and no additional radiosensitisation was achieved with tamoxifen or fulvestrant compared to Cas9 cells (rER: 0.84-1.19).

CONCLUSION

While radiosensitising in AR + TNBC, AR inhibition does not modulate radiation sensitivity in AR+/ER+ breast cancer. The efficacy of ER antagonists in combination with RT may also be dependent on AR expression.

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.

Abstract 2697: Inhibition of estrogen receptor signaling as a strategy for radiosensitization of ER+ breast cancers

Purpose: The estrogen receptor (ER) is expressed in over 80% of breast tumors and has been shown to be a significant driver of breast cancer (BC) pathogenesis and therefore a target of effective first-line therapies. While both ionizing radiation (RT) and endocrine therapies (ET) are used for the treatment of ER+ BC, the effect of ET on tumor radiosensitization remains unclear, with concerns it may be radioprotective based on G1 cell arrest with ET treatment. Here we assessed the efficacy and mechanism of ER-mediated radiosensitization using various pharmacologic approaches in ER+ BC.

Methods: Radiosensitization with ER inhibitors (tamoxifen [TAM], fulvestrant [FULV], AZD9496) was assessed using clonogenic survival assays. DNA damage was assessed by the neutral comet assay. Efficiency of homologous recombination (HR) or non-homologous end joining (NHEJ) as well as changes in cell cycle, apoptosis, and senescence were assessed. The efficacy of TAM with RT in vivo was assessed with an MCF-7 xenograft model.

Results: The selective estrogen receptor modulator TAM radiosensitized ER+ MCF-7 (enhancement ratio [enhR]: 1.14-1.50) and T47D (enhR: 1.33-1.60) cells but not ER-negative SUM-159 cells (enhR: 0.99-1.02). The selective estrogen receptor degrader (SERD) FULV had similar radiosensitization effects in MCF-7 (enhR: 1.33-1.76) and T47D cells (enhR: 0.97-2.81) with no radiosensitization observed in SUM-159 cells (enhR: 1.01-1.03). The novel oral SERD AZD9496 radiosensitized MCF-7 cells (enhR: 1.36-1.56). MCF-7 cells treated with TAM and RT had an increase in dsDNA breaks compared to RT alone as measured by the comet assay (p&lt;0.05) and a decrease in NHEJ-mediated repair with TAM (p&lt;0.05). No changes were observed in HR-mediated repair by Rad51 foci or a reporter (p=NS). RT alone and in combination with TAM or FULV induced similar levels of cell cycle arrest, suggesting that radiosensitization with the combination therapy is cell-cycle independent. There were no significant changes in apoptosis with TAM, FULV, RT, or the combination (p=NS). Although TAM or FULV did induce senescence, ET with RT increased senescence induction (p&lt;0.05). In vivo, combination RT and TAM led to a significant delay in days to tumor doubling (control: 17, TAM: 40, RT: 32, TAM+RT: undefined; p&lt;0.0001), and a significant difference in tumor growth between mice treated with TAM or RT alone compared combination treatment, with no increased toxicities or skin lesions from the combination treatment. Conclusion: Our data suggest that TAM, FULV, or AZD9496 can radiosensitize ER+ breast tumors, and these agents with RT may be more effective for radiosensitization. This work also supports further clinical investigation of the timing of RT for patients receiving ET, including using ET during RT, especially as initiating ET prior to RT has been increasingly utilized as a bridging therapy followed by concurrent ET+RT during the COVID-19 pandemic.

Citation Format: Anna R. Michmerhuizen, Lynn M. Lerner, Andrea M. Pesch, Connor Ward, Rachel Schwartz, Kari Wilder-Romans, Meilan Liu, Charles Nino, Kassidy Jungles, Ruth Azaria, Alexa Jelley, Nicole Zambrana Garcia, Alexis Harold, Amanda Zhang, Bryan Wharram, Daniel F. Hayes, James M. Rae, Lori J. Pierce, Corey W. Speers. Inhibition of estrogen receptor signaling as a strategy for radiosensitization of ER+ breast cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2697.

Abstract 3307: Multiomics analysis to uncover the mechanism of radiosensitization of AR-positive triple negative breast cancers with AR inhibition

Purpose: Expression of the androgen receptor (AR) has been identified as a driver of tumor growth in triple negative breast cancers (TNBC), and previous work has nominated AR inhibition as a strategy for radiosensitization in AR+ TNBC. Despite its role in radioresistance in AR+ TNBC, the mechanistic role of AR and specifically its role in mediating DNA damage repair in response to radiation therapy (RT) remains unknown.

Methods: Nuclear fractionation experiments were performed to assess cellular localization of AR protein in AR+ TNBC cell lines (ACC-422, MDA-MB-453). Cells were cultured in media containing hormones (FBS) with treatment of enzalutamide (ENZA), apalutamide (APA), or darolutamide (DARO). Cells were alternatively cultured in media containing charcoal stripped serum (CSS) without hormones with R1881 stimulation. RNA-sequencing was performed to compare AR+ TNBC cells treated with CSS or R1881 stimulation alone or in combination with ionizing radiation. Reverse phase protein arrays were performed in cells treated with ENZA, RT, or combination treatment.

Results: While stimulation with R1881 was sufficient to induce nuclear translocation of AR in MDA-MB-453 cells, AR inhibition with ENZA, APA, or DARO blocked AR nuclear translocation under CSS or FBS growth conditions. When cells were treated with R1881+RT, AR nuclear translocation was induced at similar or greater levels compared to R1881 alone in MDA-MB-453 and ACC-422 cells. Combination treatment of RT with ENZA in the presence of hormones reduced AR nuclear localization (39% reduction in MDA-MB-453 cells and 32% reduction in ACC-422 cells) compared to RT alone. These results suggest that decreased promoter region binding, and gene expression upregulation may be a mechanism of radiosensitization with AR inhibition. In addition, transcriptomic analyses demonstrated at least 979 genes differentially expressed in multiple models. Pathway analyses in these models showed common affected pathways included ECM-receptor interaction, PPAR-gamma activation, PI3K-Akt signaling pathway, and the MAPK/ERK signaling pathway. Proteomic analysis in the same cell lines identified apoptosis, DNA damage, and cell cycle pathway changes after RT when AR-signaling was blocked. Common affected pathways in combined analyses identified PI3K-Akt and MAPK/ERK signaling pathway changes that may be responsible for this radiosensitizing phenotype.

Conclusions: Our data suggest that AR inhibition in AR+ TNBC is sufficient to inhibit AR nuclear translocation suggesting that AR may play a nuclear role in response to RT to promote DNA repair and radioresistance. We identify potential pathways, including ECM-receptor interaction, PI3K-Akt signaling pathway, and the MAPK/ERK signaling pathway that may be regulated by AR in response to RT and therefore may be responsible for radioresistance.

Citation Format: Anna R. Michmerhuizen, Andrea M. Pesch, Benjamin C. Chandler, Lynn M. Lerner, Connor Ward, Leah Moubadder, Stephanie The, Breanna McBean, Caleb Cheng, Lori J. Pierce, Corey W. Speers. Multiomics analysis to uncover the mechanism of radiosensitization of AR-positive triple negative breast cancers with AR inhibition [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3307.

Abstract 216: Expression of DNA damage response proteins modifies the efficacy of CDK4/6 inhibitor-mediated radiosensitization in breast cancer models

Purpose: CDK4/6 inhibitors (CDK4/6i) are standard of care for the treatment of locally advanced and metastatic estrogen receptor-positive (ER+), HER2-negative metastatic breast cancer (BC). CDK4/6 inhibition + radiation therapy (RT) is synergistic in both ER+ and triple negative breast cancers (TNBC), but the underlying mechanism is not entirely understood. In this study, we evaluated how pre-existing or genetically engineered deficits in DNA damage response genes (BRCA1/2, RAD51, RB1, XRCC6, TP53) influence radiosensitization. We hypothesized that inhibition of homologous recombination (HR) would prevent CDK4/6i-mediated radiosensitization and blocking non-homologous end joining (NHEJ) would be synergistic.

Methods: Cellular proliferation assays determined the half-maximal inhibitory concentrations (IC50) of the 3 approved CDK4/6i palbociclib, ribociclib, and abemaciclib. Clonogenic survival assays determined the radiation enhancement ratios (rERs) and evaluated the efficacy of CDK4/6i + RT. Immunofluorescence assays measured RAD51 foci formation and quantified micronuclei formation following RT and/or CDK4/6 inhibition. Immunoprecipitation with myc-RAD51 and GFP-RB assessed potential protein-protein interactions.

Results: While ER+ and TNBC cell lines with wild type BRCA1 expression are radiosensitized by CDK4/6i, BRCA1-deficient SUM-149 cells are not radiosensitized by CDK4/6i at concentrations up to 1µM (rER: 0.92-1.01). In an MCF-7 isogenic model of BRCA2 knockout, CDK4/6i-mediated radiosensitization was abolished compared to Cas9 control or parental cell lines. In ER+ BC cell lines (MCF-7-p53 wt, T47D-p53 mutant), transient or genetic knockdown of RAD51 prevented CDK4/6i-induced radiosensitization. The total quantity of RT-induced RAD51 foci increased in vitro following overexpression of RB-a tumor suppressor and downstream target of CDK4/6. RB overexpression also rescued CDK4/6i-mediated radiosensitization in RB-deficient cell lines through changes in HR efficiency but not via NHEJ or altered micronuclei formation. Moreover, immunoprecipitation of RAD51 in ER+ (MCF-7) and TNBC (MDA-MB-231) cells exhibited an interaction with RB. Conversely, loss of the NHEJ-associated protein Ku70 (XRCC6) was synergistic with palbociclib + RT in MCF7 (rER: 1.76-2.44) and T47D (rER: 1.61-3.88) cells. Finally, CRISPR Cas9-mediated loss of the tumor suppressor p53 (TP53) did not affect radiosensitization induced by CDK4/6i in isogenic p53 wt ER+ (MCF-7, rER: 1.19-1.33) and p53 wt TNBC (CAL-51, rER: 1.23-1.52) cell lines with TP53 loss.

Conclusions: Taken together, our results in multiple non-overlapping isogenic models of ER+ and TNBC suggest that CDK4/6i-mediated radiosensitization of BC cell lines occurs through impaired HR activity and RB signaling, and not through the actions of p53 or NHEJ-mediated DNA repair.

Citation Format: Kassidy M. Jungles, Andrea M. Pesch, Nicole Hirsh, Anna R. Michmerhuizen, Kari Wilder-Romans, Benjamin C. Chandler, Meilan Liu, Lynn Lerner, Lori J. Pierce, James M. Rae, Corey W. Speers. Expression of DNA damage response proteins modifies the efficacy of CDK4/6 inhibitor-mediated radiosensitization in breast cancer models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 216.

Abstract P4-02-04: Endocrine therapy treatment radiosensitizes estrogen receptor-positive breast cancers

Purpose: Estrogen receptor (ER) expression is present in over 80% of breast tumors and has been shown to be a significant driver of breast cancer (BC) pathogenesis and therefore a target of first-line therapies for ER-positive (ER+) BC patients. While both ionizing radiation (RT) and endocrine therapies (ET) are used for the treatment of ER+ BC, the sequencing of therapy and the effect of ET on tumor radiosensitization remain unclear. Recently, this question has become much more clinically relevant when many physicians started offering ET as a bridging strategy to surgery and RT during the COVID-19 pandemic. Here we assessed the efficacy and mechanism of ER inhibition in ER+ BC in combination with RT in preclinical models. Methods: Clonogenic survival assays were used to assess radiosensitization. Inhibition of ER signaling was accomplished by treating ER+ MCF-7 and T47D cells with the selective ER modulator (SERM), tamoxifen, or the selective ER degrader (SERD), fulvestrant. The ER-negative SUM-159 cells were used as a negative control. DNA damage was assessed by the neutral comet assay. Efficiency of homologous recombination (HR) was measured by Rad51 foci or a GFP reporter system. Non-homologous end joining (NHEJ) efficiency was assessed with a pEYFP reporter. Cell cycle effects were measured using flow cytometry with propidium iodide (PI) staining. Apoptosis was assessed by annexin V/PI via flow cytometry. Senescence was measured using β-galactosidase staining. Western blotting was used to quantify expression of proteins and phospho-proteins involved in cell cycle and apoptosis. An MCF-7 xenograft model was used to assess the efficacy of tamoxifen with RT in vivo. Synergy was determined using the fractional tumor volume (FTV) method. Results: ER inhibition with tamoxifen radiosensitized ER+ MCF-7 (10-250 nM, enhR: 1.14-1.50) and T47D (500 nM-2.0 µM, enhR: 1.33-1.60) cells but not ER-negative SUM-159 cells (500 nM-2.0 µM, enhR: 0.99-1.02). ER degradation with fulvestrant had similar radiosensitization effects in MCF-7 (1-25 nM, enhR: 1.33-1.76) and T47D cells (0.5-5 nM, enhR: 0.97-2.81) with no radiosensitization observed in SUM-159 cells (1-25 nM, enhR: 1.01-1.03). MCF-7 cells treated with 500 nM tamoxifen and 4 Gy RT had an increase in dsDNA breaks compared to RT alone as measured by the comet assay (p&lt;0.05), and there was a decrease in NHEJ-mediated repair with tamoxifen treatment (p&lt;0.05). No changes were observed in HR-mediated repair by Rad51 foci or an HR reporter (p=NS). RT alone and in combination with tamoxifen and fulvestrant induced similar levels of cell cycle arrest, suggesting that radiosensitization with the combination therapy is a cell-cycle independent effect. In addition, there were no significant changes in apoptosis in MCF-7 or T47D cells with endocrine therapy, RT, or the combination (p=NS). Although treatment with ET did induce senescence in ER+ MCF-7 and T47D cells, the combination treatment of ET with RT induced senescence to a much greater level suggesting this mechanism may contribute to radiosensitization (p&lt;0.05). In vivo, combination RT and tamoxifen led to a significant delay in time to tumor doubling (17 days in control, 40 days with tamoxifen alone, 32 days with RT alone, and undefined with combination; p&lt;0.0001) and a significant difference in tumor growth between mice treated with tamoxifen or RT alone compared to mice treated with tamoxifen and RT with synergy noted with combination treatment (FTV 1.297). Conclusion: Our data suggest that ET can radiosensitize ER+ breast tumors, and ET with RT may be more effective for radiosensitization. Ongoing studies will address concurrent versus sequential ET with RT. This work also supports further clinical investigation of the timing of RT for patients receiving ET, especially as ET prior to RT is increasingly used as a bridging therapy during the COVID-19 pandemic.

Citation Format: Anna R Michmerhuizen, Lynn Lerner, Andrea M Pesch, Connor Ward, Rachel Schwartz, Kari Wilder-Romans, Meilan Liu, Bryan Wharram, Alexis Harold, Ruth Azaria, Nicole Zambrana Garcia, Daniel F Hayes, James M Rae, Lori J Pierce, Corey W Speers. Endocrine therapy treatment radiosensitizes estrogen receptor-positive breast cancers [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P4-02-04.

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.

Abstract 1386: The role of MDM2 inhibition in the radiosensitization of ER+ breast cancers

Background​: Radiation therapy (RT) is standard in the treatment of many women with breast cancer (BC). Despite this, women with estrogen receptor positive (ER+) BC respond heterogeneously to RT. Radiosensitization methods for aggressive ER+ disease are needed. We performed a radiosensitizer screen paired with transcriptomic and proteomic data from ER+ models treated +/-RT to identify potential mediators of RT resistance.

Methods​: Clonogenic survival assays were used to determine RT sensitivity of 21 BCC lines as well as radiosensitization with drug treatment. IC50 values were determined for 130 clinical compounds and correlation coefficients were calculated using IC50 values and SF-2Gy. Microarray and RPPA data was used for differential gene/protein expression and pathway analysis. AlamarBlue was used to determine IC-50 values of the MDM2 inhibitor AMG-232. Western blot analysis of Cleaved PARP and Annexin V staining for FLOW was used to measure apoptosis and Cyclins A, E, B and p-Histone H3 and flow cytometry to measure cell cycle progression. yH2AX immunofluorescence was used to measure dsDNA breaks.

Results​: A MDM2 inhibitor (JNJ-26854165) was nominated as an effective drug in treatment for RT-resistant BC cell lines (R2​ = 0.43, p-value &lt;0.01) in our novel radiosensitizer screen. Differential gene expression and pathway analysis in multiple non-overlapping ER+ BC cell lines treated +/-RT identified apoptosis, cell cycle, and p53 signaling as the top pathways induced in ER+ cell lines by RT. Within these MDM2 was significantly overexpressed after RT+ compared to RT- in ER+ p53 wild-type (WT) cells. In p53 mutant (MT) cell lines, however, MDM2 was not differentially expressed. This suggests MDM2 may mediate radioresistance in a p53 dependent manner. Cell growth in the p53​ ​WT cell lines MCF-7 and ZR-75-1 was inhibited by AMG-232, an MDM2 inhibitor (IC-50 values of 554nM and 264nM). p53 MT ER+ cell lines were not sensitive to MDM2 inhibition with this drug (IC-50&gt; 10uM). Clonogenic survival assays demonstrated that at sub-IC50 doses MDM2 inhibition leads to radiosensitization in p53 WT ER+ cell lines (MCF-7 rER: 1.17-2.13; ZR751 rER: 1.30-1.65), however, p53 MT ER+ cells were not radiosensitized (T47D rER: 0.94-1.11; CAMA-1 rER: 0.88-0.95). AMG-232 and RT combined led to an increase in apoptosis compared to RT alone in ER+ p53 WT cells but not p53 MT cells. Combination treatment led to differential cyclin and p-Histone H3 expression in p53 WT cells but not p53 MT cells. ​G1 cell cycle arrest was a secondary effect of MDM2 inhibition and radiation. Experiments investigating the role of dsDNA breaks in radiosensitization are ongoing.

Conclusions​: Our novel radiosensitizer screen identifies MDM2 as a potential mediator of radioresistance in ER+ BC in a p53-dependent manner and suggests that MDM2 targeting concurrent with RT may represent a tractable clinical strategy in women with locally advanced ER+, p53 WT BC.

Citation Format: Cassandra Lynne Ritter, Benjamin C. Chandler, Andrea M. Pesch, Anna R. Michmerhuizen, Nicole Hirsh, Tanner Ward, Amanda Zhang, Mattia Cremona, Lori J. Pierce, Bryan Hennessy, Corey W. Speers. The role of MDM2 inhibition in the radiosensitization of ER+ breast cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1386.

Abstract 737: Estrogen receptor inhibition with tamoxifen mediates radiosensitization of ER+ breast cancer models

Purpose: Estrogen receptor (ER) expression is present in over 80% of breast tumors and has been shown to be a significant driver of tumor initiation and progression. Therefore, patients with ER-positive (ER+) breast cancers are given first-line therapies which target the ER and downstream ER signaling. Ionizing radiation (RT) has been shown to significantly improve locoregional control and increase overall survival in patients with ER+ breast cancer. Similarly, endocrine therapy (ET) has also been shown to improve metastasis-free and overall survival in women with ER+ breast cancer. While both radiation and ET are used in women with ER+ breast cancer, the effect of endocrine therapies on tumor radiosensitization remains unclear. Here we assessed the efficacy and mechanism of ER inhibition in ER+ breast cancers in combination with radiation therapy.

Methods: Clonogenic survival assays were performed to assess radiosensitization and calculate radiation enhancement ratios (enhR) with the selective estrogen receptor modulator (SERM), tamoxifen, in ER+ MCF-7 and T47D cells or ER-negative (ER-) SUM-159 cells. DNA damage was assessed by yH2AX foci. Efficiency of homologous recombination (HR) or non-homologous end joining (NHEJ) was measured by RAD51 foci or using a pYFP reporter, respectively. Cell cycle effects were measured using flow cytometry with propidium iodide (PI) staining. Apoptosis was assessed by annexin V/PI via flow cytometry. Western blotting was used to quantify expression of proteins and phospho-proteins involved in DNA repair, cell cycle, and apoptosis. An MCF-7 xenograft model was used to assess the efficacy of tamoxifen with RT in vivo.

Results: ER inhibition with tamoxifen radiosensitized ER+ MCF-7 (enhR: 1.14-1.50) and T47D (enhR: 1.33-1.60) cells but not ER- SUM-159 cells (enhR: 0.99-1.02). MCF-7 and T47D cells treated with tamoxifen did not have changes in the kinetics of dsDNA break repair as measured by yH2AX foci (p&gt;0.05) but demonstrated a decrease in NHEJ-mediated repair (p&lt;0.05). No changes were observed in HR-mediated repair by Rad51 foci (p&gt;0.05). While cell cycle arrest was induced at 24 hours after RT, no changes were observed with tamoxifen treatment in combination with RT. In addition, there were no significant changes in apoptosis in MCF-7 or T47D cells with treatment of tamoxifen, radiation, or the combination (p&gt;0.05). In vivo xenograft studies demonstrate a significant delay in time to tumor doubling and a significant difference in tumor growth between mice treated with tamoxifen or RT alone compared to mice treated with tamoxifen and RT.

Conclusion: Our data suggest that tamoxifen may be effectively used to radiosensitize ER+ breast tumors. This work also supports further clinical investigation of the timing of radiation for patients receiving endocrine therapy as concurrent use may be more effective than sequential.

Citation Format: Anna R. Michmerhuizen, Andrea M. Pesch, Rachel Schwartz, Kari Wilder-Romans, Meilan Liu, Ruth Azaria, Alexa Jelley, Lori J. Pierce, Corey W. Speers. Estrogen receptor inhibition with tamoxifen mediates radiosensitization of ER+ breast cancer models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 737.

Abstract 1943: Radiosensitization of PIK3CA wild type triple negative breast cancers with Bcl-family inhibition

Purpose: Compared to other breast cancer subtypes, triple negative breast cancers (TNBC) derive the least benefit from adjuvant radiation (RT) which contributes to higher rates of locoregional recurrence. Thus, there is a critical need to identify clinical strategies to increase the effectiveness of RT therapy in TNBC.

Methods: Alamar blue proliferation assays were used to calculate half maximal inhibitory concentration (IC50) values for each Bcl-2 family inhibitor 72 hours after drug treatment. Clonogenic survival assays were used to evaluate radiosensitivity and to calculate the radiation enhancement ratio (rER) after combination treatment. Apoptosis was assessed through formation of cleaved PARP and annexin V/PI-based flow cytometry. Xenograft models with MDA-MB-231 cells and TNBC patient-derived xenografts (PDX4664) were used to assess radiosensitization in vivo.

Results: A novel radiosensitizer screen identified Bcl-2 family inhibition as a potentially effective treatment strategy in radioresistant breast cancer models. Single-agent response to pan Bcl-2 family inhibition (ABT-263) or Bcl-xL inhibition (WEHI-539, A-1331852) was more effective in PIK3CA wild type (wt) TNBC (IC50 &lt; 1µM) compared to PIK3CA mutant TNBC. Inhibition of apoptosis with ABT-263 led to radiosensitization of PIK3CA/PTEN wild-type TNBC cell lines (rER: 1.09-1.74), but had no effect on PIK3CA/PTEN mutant TNBC (rER: 0.87-1.18). Radiosensitization was observed to be Bcl-xL-dependent, with Bcl-xL inhibitor-specific radiosensitization (rER: 1.12-2.38) but a lack of Bcl-2 inhibitor (ABT-199, rER: 0.94 - 1.21) or MCL-1 inhibitor-mediated radiosensitization (S63845, rER: 0.91 - 1.06). In PIK3CA wt TNBC, combination treatment of Bcl-2 family inhibition and RT significantly increased the percent of apoptotic cells (p &lt; 0.001) and led to increased formation of cleaved PARP 48 hours after RT. Sensitivity to RT was dependent on expression of MCL-1, an anti-apoptotic protein that is overexpressed in PIK3CA/PTEN mutant TNBC. Overexpression of MCL-1 in PIK3CA/PTEN wild type TNBC rescued radioresistance (rER: 0.99-1.09), whereas co-inhibition of MCL-1 and Bcl-xL in PIK3CA/PTEN mutant TNBC was sufficient to overcome radioresistance (rER: 2.32 - 2.35). In vivo, nonspecific Bcl-2 family inhibition or specific Bcl-xL inhibition in combination with RT decreased tumor growth and increased time to tumor tripling (p &lt; 0.0001) in PIK3CA wt models of TNBC.

Conclusions: In this study, we demonstrated that inhibition of Bcl-2 family proteins in combination with RT led to increased levels of apoptosis and cell death in PIK3CA/PTEN wt - but not PIK3CA/PTEN mutant - TNBC and we identified MCL-1 as a critical mediator of this radiosensitIvity. Together, these results indicate that Bcl-xL inhibition may be a feasible clinical strategy for the radiosensitization of PIK3CA/PTEN wild-type TNBC.

Citation Format: Andrea M. Pesch, Benjamin C. Chandler, Anna R. Michmerhuizen, Nicole Hirsh, Kari Wilder-Romans, Meilan Liu, Tanner Ward, Dana Messinger, Charles Nino, Cassandra Ritter, James M. Rae, Corey W. Speers. Radiosensitization of PIK3CA wild type triple negative breast cancers with Bcl-family inhibition [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1943.

Abstract 1952: CDK4/6 inhibition radiosensitizes RB1 wild type triple negative breast cancers through impaired homologous recombination

Purpose: Cyclin-dependent kinase 4&6 (CDK4/6) inhibitors have been approved for the treatment of metastatic, estrogen receptor positive (ER+) breast cancers, but there is growing interesting in CDK4/6 inhibition as a therapeutic strategy in other breast cancer subtypes, including triple negative breast cancer (TNBC). Previous studies have shown that CDK4/6 inhibition radiosensitizes ER+ breast cancers, but the interaction between CDK4/6 inhibition and radiation (RT) in TNBC is incompletely understood.

Methods: Cellular viability was quantified 72 hours after drug treatment (in the absence of RT) to calculate a half maximal inhibitory concentration (IC50) value of proliferation. Radiation enhancement ratios (rER) and surviving fractions of cells after RT were calculated using clonogenic survival assays in RB1 wild type and mutant TNBC cell lines. Homologous recombination (HR) was assessed using RAD51 foci formation and a stable HR reporter system. G1 cell cycle arrest was quantified using propidium iodide-based flow cytometry. CRISPR-induced knockout of RB1 and transient siRNA-mediated knockdown of RB1 in TNBC cell lines was used in both clonogenic survival assays and immunofluorescence experiments. In vivo efficacy of CDK4/6 inhibition + RT was assessed using TNBC patient-derived xenograft models (PDX4664).

Results: Although most TNBC cell lines are resistant to CDK4/6 inhibitor monotherapy (IC50 &gt; 250nM) compared to ER+ cells, treatment with 250nM-1μM palbociclib radiosensitized RB1 wild type TNBC (MDA-MB-231, CAL-51, SUM-159, CAL-120; rER 1.08 – 2.22) but failed to radiosensitize RB1 mutant TNBC (CAL-851, MDA-MB-468; rER: 0.84 – 1.00). Radiosensitization of TNBC cell lines also occurred with short term ribociclib or abemaciclib pretreatment. At 6 and 16 hours following RT, significant suppression of RT-induced homologous recombination (HR) activity (RAD51 foci) was observed in RB1 wild type (p &lt; 0.001) but not RB1 mutant (p &gt; 0.05) TNBC cell lines. Cell cycle arrest after short term CDK4/6 inhibition was dependent on the presence of RB1. In addition, genetic knockdown of RB1 in RB1 wild type TNBC lead to a loss of CDK4/6 inhibitor-mediated HR suppression (p &gt; 0.05) and diminished radiosensitization.

Conclusions: In TNBC, CDK4/6 inhibition and RT leads to suppression of HR activity in an RB1-dependent manner. While ongoing studies seek to elucidate the role of RB1 in HR suppression in the context of CDK4/6 inhibitor-mediated radiosensitization of TNBC, our data suggests that CDK4/6 inhibition + RT could be a valuable clinical strategy to radiosensitize a wide range of breast cancer subtypes, including RB1 wild type TNBC.

Citation Format: Andrea M. Pesch, Nicole Hirsh, Anna R. Michmerhuizen, Benjamin C. Chandler, Kari Wilder-Romans, Meilan Liu, Lori J. Pierce, James M. Rae, Corey W. Speers. CDK4/6 inhibition radiosensitizes RB1 wild type triple negative breast cancers through impaired homologous recombination [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1952.

Abstract PO-028: RB loss mitigates CDK4/6 inhibitor-mediated radiosensitization of estrogen receptor positive (ER+) breast cancers

Purpose: Fractionated radiation (RT) is used in the adjuvant setting for locoregional control and to prevent the development of metastatic lesions in estrogen receptor positive (ER+) breast cancers. We previously demonstrated that inhibition of cyclin-dependent kinases 4 and 6 (CDK4/6) along with RT leads to the radiosensitization of ER+ breast cancers, but the exact mechanism by which this occurs is unknown. We hypothesized that the presence of RB is necessary for effective double strand repair of radiation-induced DNA damage mediated through homologous recombination (HR), and that this is prevented using CDK4/6 inhibition. Methods: Pharmacological CDK4/6 inhibition was achieved using three FDA approved CKD4/6 inhibitors: palbociclib, ribociclib, and abemaciclib. Genetic knockdown of RB1 was performed using siRNA and knockout was achieved using a CRISPR-Cas9 system. Parental and RB1 knockdown/CRISPR MCF-7 and T47D cells were treated for 72 hours to assess drug sensitivity. Cells were pretreated with a CDK4/6 inhibitor one hour prior to RT and colony formation was quantified to assess changes in radiosensitivity. MCF-7 cells expressing an HR-specific GFP reporter were used to assess HR competency. RB, yH2AX, and RAD51 cellular localization following RT and CDK4/6 inhibition was assessed using immunofluorescence assays. Flow cytometry with propidium iodide staining was used to assess cell cycle distribution. Protein expression was assessed by immunoblotting. Results: CDK4/6 inhibition with palbociclib, ribociclib, and abemaciclib + RT radiosensitizes ER+ breast cancer cells at sub-IC50 concentrations in vitro (rER: 1.21 – 2.05) through impaired HR, which we confirmed using MCF-7 cells that express a stable HR-GFP reporter system (p &lt; 0.01). RB1 knockdown decreased single-agent efficacy of CDK4/6 inhibition on the proliferation of ER+ breast cancer cell lines, leading to an increase in the IC50 for each CDK4/6 inhibitor. Palbociclib, ribociclib, and abemaciclib failed to induce G1 cell cycle accumulation after RB1 knockdown (p &gt; 0.05). Genetic knockdown of RB1 led to a decrease in the ability of breast cancer cells to perform HR-directed DNA repair (p &lt; 0.01) independent of drug treatment, and further suppression of HR with CDK4/6 inhibition was lost in MCF-7 and T47D cells lacking RB expression (p &gt; 0.05). Furthermore, RB protein is necessary for CDK4/6i mediated radiosensitization as evidenced by the abrogation of radiosensitization in RB null isogenic models of ER+ breast cancer (MCF-7 rER: 0.97 ± 0.13). Conclusions: Our data suggests that CDK4/6 inhibitor-mediated radiosensitization and HR suppression is dependent on RB expression. Thus, RB might serve as an effective biomarker for patient selection in future clinical trials that seek to combine CDK4/6 inhibition + RT.

Citation Format: Andrea M. Pesch, Nicole Hirsh, Anna R. Michmerhuizen, Benjamin C. Chandler, Kari Wilder-Romans, Meilan Liu, Erin Cobain, Lori J. Pierce, James M. Rae, Corey Speers. RB loss mitigates CDK4/6 inhibitor-mediated radiosensitization of estrogen receptor positive (ER+) breast cancers [abstract]. In: Proceedings of the AACR Virtual Special Conference on Radiation Science and Medicine; 2021 Mar 2-3. Philadelphia (PA): AACR; Clin Cancer Res 2021;27(8_Suppl):Abstract nr PO-028.

ARe we there yet? Understanding androgen receptor signaling in breast cancer

The role of androgen receptor (AR) activation and expression is well understood in prostate cancer. In breast cancer, expression and activation of AR is increasingly recognized for its role in cancer development and its importance in promoting cell growth in the presence or absence of estrogen. As both prostate and breast cancers often share a reliance on nuclear hormone signaling, there is increasing appreciation of the overlap between activated cellular pathways in these cancers in response to androgen signaling. Targeting of the androgen receptor as a monotherapy or in combination with other conventional therapies has proven to be an effective clinical strategy for the treatment of patients with prostate cancer, and these therapeutic strategies are increasingly being investigated in breast cancer. This overlap suggests that targeting androgens and AR signaling in other cancer types may also be effective. This manuscript will review the role of AR in various cellular processes that promote tumorigenesis and metastasis, first in prostate cancer and then in breast cancer, as well as discuss ongoing efforts to target AR for the more effective treatment and prevention of cancer, especially breast cancer.

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.

Abstract 6280: CDK4/6 inhibitor-mediatated radiosensitization of estrogen receptor positive breast cancer

Purpose: Although cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors like palbociclib, ribociclib, and abemaciclib have improved progression free survival in patients with metastatic, estrogen receptor positive (ER+) breast cancer, acquired resistance to these drugs limits their efficacy. Despite promising new studies defining the utility of CDK4/6 inhibitors in the upfront, non-metastatic setting, there is limited data available on the effects of concurrent CDK4/6 inhibition and radiation (RT).

Methods: Transcriptomic and proteomic expression data was used to quantify changes in RNA and protein expression in ER+ breast cancer cell lines (MCF-7, T47D) after short term (16 hour) CDK4/6 inhibition. Proliferation assays were used to determine the half-maximal inhibitory concentration (IC50) of palbociclib, ribociclib, and abemaciclib. Clonogenic survival assays were performed to calculate the radiation enhancement ratio (rER) and the surviving fraction at 2 Gy for each treatment. Homologous recombination (HR) proficiency was assessed using RAD51 and γH2AX foci formation and a pYFP reporter was used to assess non-homologous end joining (NHEJ) efficiency. Western blots were used to quantify protein expression. MCF-7 xenografts were used to study the efficacy of combination (palbociclib + RT) therapy in vivo. MCF-7 and T47D cell lines with acquired resistance to CDK4/6 inhibition (IC50 &gt;1uM) were used for comparison in all assays.

Results: Transcriptomic and proteomic analyses identified changes in expression of DNA damage response mediators and cell cycle machinery with short term CDK4/6 inhibition. Palbociclib significantly radiosensitized ER+ cell lines at concentrations at or below the IC50 value in clonogenic survival assays (MCF-7 rER: 1.22-1.52, T47D rER: 1.23-1.50) and led to a decrease in the surviving fraction of cells at 2 Gy (p &lt; 0.001). Similar results were observed in ribociclib- (rER: 1.08 - 1.68) and abemaciclib-treated (rER: 1.19 - 2.05) cells. MCF-7 and T47D cells treated with CDK4/6 inhibition and RT showed a decrease in RAD51 foci formation, suggesting a decrease in HR efficiency (p &lt; 0.001). However, CDK4/6 inhibition did not affect NHEJ efficiency (p &gt; 0.05). CDK4/6 inhibition + RT led to a decrease in expression of protein expression of HR meditators like p-CHK1 but did not affect phosphorylation of NHEJ proteins like pKu80/pKu70. Cells with acquired resistance to CDK4/6 inhibition did not demonstrate radiosensitization (MCF-7 rER: 0.93 - 1.03, T47D rER: 0.96 - 1.11) or changes in RAD51 foci formation with combination treatment.

Conclusions: Our data suggests that CDK4/6 inhibitor-mediated radiosensitization may be effective in ER+ breast cancers prior to the development of CDK4/6 inhibitor resistance. These studies provide preclinical rationale to test CDK4/6 inhibition + RT in women with locally-advanced ER+ breast cancer at high risk for locoregional recurrence.

Citation Format: Andrea M. Pesch, Nicole Hirsh, Benjamin C. Chandler, Anna R. Michmerhuizen, Cassandra L. Ritter, Marlie Androsiglio, Kari Wilder-Romans, Meilan Liu, Christina L. Gersch, Jose M. Larios, James M. Rae, Corey W. Speers. CDK4/6 inhibitor-mediatated radiosensitization of estrogen receptor positive breast cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6280.

Abstract 6271: Hormone receptor inhibition as a strategy for radiosensitization of breast cancer

Purpose: Expression of the androgen receptor (AR) has been identified as a driver of tumor growth in triple negative breast cancers (TNBC), and previous work has nominated AR as a target for radiosensitization. In addition, 70-95% of all estrogen receptor (ER) positive (ER+) breast cancers also have coexpression of AR, suggesting extended utility of AR inhibition in the radiosensitization of these AR+, ER+ tumors. Here we assessed the efficacy of AR inhibition in ER+, AR+ breast cancers to better understand the role of AR signaling across breast cancer models. Further, we also investigated the effect of ER inhibition on radiosensitization of ER+ breast cancer models.

Methods: IC50 values were determined for MDV3100 (enzalutamide), ARN-509 (apalutamide), and ODM-201 (darolutamide) in TNBC cell lines (AR+ TNBC: MDA-MB-453, ACC-422, and SUM-185PE, and AR- TNBC: MDA-MB-231) and ER+ breast cancer cell lines (AR+, ER+: ZR-75-1, BT-474, CAMA-1, and AR-, ER+: MCF-7). IC50 values for tamoxifen were determined for ER+ breast cancer cell lines (MCF-7, T47D, ZR-75-1), and ER- (SUM-159) cells. Clonogenic survival assays were performed to assess radiosensitization with ER or AR inhibition with tamoxifen or second generation anti-androgens, respectively, in TNBC and ER+ breast cancer models.

Results: AR inhibition with enzalutamide, apalutamide, and darolutamide showed limited single agent growth inhibition efficacy in AR+ TNBC and AR+, ER+ breast cancer cell lines (IC50 &gt; 10 μM). AR inhibition with enzalutamide did not induce radiosensitivity in vitro. In AR+, ER+ CAMA-1 cells, AR blockade with enzalutamide had a radioprotective effect with enhancement ratios (enhR) of 0.76-0.83. No radiosensitization was observed in BT-474 (enhR: 0.92-1.01) or ZR-75-1 cells (enhR: 0.94-1.00). Radiosensitization was also assessed with anti-androgens apalutamide and darolutamide in AR+ breast cancer models. Inhibition of ER with tamoxifen, however, induced radiosensitization in MCF-7 (enhR: 1.14-1.50) and T47D (enhR: 1.33-1.60) cells. No radiosensitization was observed with tamoxifen in ER- SUM-159 cells.

Conclusion: Although AR is a mediator of radioresistance in AR+ TNBC, AR inhibition does not provide comparable radiosensitization in AR+, ER+ models and may actually confer a radioprotective effect. In contrast, our results demonstrate ER inhibition is an effective radiosensitizing strategy in ER+ breast cancers, independent of AR status. This work highlights the complexities of androgen and estrogen receptor signaling in AR+, ER+ breast tumors and underscores the necessity for understanding context dependent effects when translating into patients with AR+ breast cancer.

Citation Format: Anna R. Michmerhuizen, Amanda Zhang, Rachel Schwartz, Andrea M. Pesch, Benjamin C. Chandler, Cassandra L. Ritter, Meilan Liu, Kari Wilder-Romans, Daniel E. Spratt, Daniel R. Wahl, Shyam Nyati, Lori J. Pierce, Corey Speers. Hormone receptor inhibition as a strategy for radiosensitization of breast cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6271.

Abstract 6270: A radiosensitizer screen identifies a novel role for MDM2 inhibition in the radiosensitization of ER+ breast cancers in a p53 dependent manner

Background: Radiation therapy (RT) is a mainstay of treatment for most women with breast cancer (BC). Despite this treatment, response remains heterogenous for women with estrogen receptor positive (ER+) BC. Thus, approaches that result in radiosensitization of aggressive ER+ disease are critically needed. We performed a radiosensitizer screen paired with transcriptomic and proteomic data from ER+ models treated +/-RT to identify potential mediators of RT resistance.

Methods: Clonogenic survival assays were used to determine RT sensitivity of 21 BCC lines as well as radiosensitization with drug treatment. IC50 values were determined for 130 clinical compounds and correlation coefficients were calculated using IC50 values and SF-2Gy. Microarray and RPPA data was used for differential gene/protein expression and pathway analysis. AlamarBlue was used to determine IC50 values of the MDM2 inhibitor AMG-232. Western blot analysis of Cleaved PARP was used to measure apoptosis and Cyclins A and E to measure cell cycle progression.

Results: Our radiosensitizer screen nominated the MDM2 inhibitor (JNJ-26854165) as one of the most effective drugs in treating RT-resistant BC cell lines (R2= 0.43, p-value &lt;0.01). In addition, differential gene expression and pathway analysis in multiple non-overlapping ER+ BC cell lines treated +/-RT identified apoptosis, cell cycle, and p53 signaling as the top pathways induced by RT in ER+ cell lines. Within these pathways MDM2 was significantly overexpressed after RT compared to RT- in ER+ p53 wild-type (WT) cells. However, in p53 mutant (MT) cell lines MDM2 was not differentially expressed suggesting MDM2 may mediate radioresistance in a p53 dependent manner. The MDM2 inhibitor AMG-232 inhibited cell growth in the p53 WT cell lines MCF-7 and ZR-75-1 (IC-50 values of 554nM and 264nM, respectively). In contrast, p53 MT ER+ cell lines were not sensitive to MDM2 inhibition (IC-50&gt; 10uM). Clonogenic survival assays demonstrated that MDM2 inhibition at sub-IC50 doses leads to radiosensitization in p53 WT ER+ cell lines (MCF-7 rER: 1.37-1.66;ZR751 rER: 1.30-1.65). In contrast, p53 MT ER+ cells did not demonstrate significant radiosensitization (T47D rER: 0.94-1.11). Combination of AMG-232 and RT led to an increase in apoptosis compared to RT alone in ER+ p53 WT cells but not p53 MT cells. Additionally, combination treatment led to differential cylin expression in p53 WT cells but not p53 MT cells. In vivo studies testing MDM2 inhibition with RT in p53 WT and MT orthotopic and PDX models are ongoing.

Conclusions: Our novel radiosensitizer screen identifies MDM2 as a potential mediator of radioresistance in ER+ BC. Additionally, MDM2 inhibition confers radiosensitization in a p53 dependent manner in ER+ BC and may represent a tractable clinical strategy in women with p53 WT BC.

Citation Format: Cassandra L. Ritter, Benjamin C. Chandler, Andrea M. Pesch, Anna R. Michmerhuizen, Nicole Hirsh, Amanda Zhang, Tanner Ward, Mattia Cremona, Bryan Hennessy, Lori J. Pierce, Corey W. Speers. A radiosensitizer screen identifies a novel role for MDM2 inhibition in the radiosensitization of ER+ breast cancers in a p53 dependent manner [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6270.

TTK inhibition radiosensitizes basal-like breast cancer through impaired homologous recombination

Increased rates of locoregional recurrence are observed in patients with basal-like breast cancer (BC) despite the use of radiation therapy (RT); therefore, approaches that result in radiosensitization of basal-like BC are critically needed. Using patients' tumor gene expression data from 4 independent data sets, we correlated gene expression with recurrence to find genes significantly correlated with early recurrence after RT. The highest-ranked gene, TTK, was most highly expressed in basal-like BC across multiple data sets. Inhibition of TTK by both genetic and pharmacologic methods enhanced radiosensitivity in multiple basal-like cell lines. Radiosensitivity was mediated, at least in part, through persistent DNA damage after treatment with TTK inhibition and RT. Inhibition of TTK impaired homologous recombination (HR) and repair efficiency, but not nonhomologous end-joining, and decreased the formation of Rad51 foci. Reintroduction of wild-type TTK rescued both radioresistance and HR repair efficiency after TTK knockdown; however, reintroduction of kinase-dead TTK did not. In vivo, TTK inhibition combined with RT led to a significant decrease in tumor growth in both heterotopic and orthotopic, including patient-derived xenograft, BC models. These data support the rationale for clinical development of TTK inhibition as a radiosensitizing strategy for patients with basal-like BC, and efforts toward this end are currently underway.

Seviteronel, a Novel CYP17 Lyase Inhibitor and Androgen Receptor Antagonist, Radiosensitizes AR-Positive Triple Negative Breast Cancer Cells

Increased rates of locoregional recurrence (LR) have been observed in triple negative breast cancer (TNBC) despite multimodality therapy, including radiation (RT). Recent data suggest inhibiting the androgen receptor (AR) may be an effective radiosensitizing strategy, and AR is expressed in 15-35% of TNBC tumors. The aim of this study was to determine whether seviteronel (INO-464), a novel CYP17 lyase inhibitor and AR antagonist, is able to radiosensitize AR-positive (AR+) TNBC models. In cell viability assays, seviteronel and enzalutamide exhibited limited effect as a single agent (IC50 > 10 μM). Using clonogenic survival assays, however, AR knockdown and AR inhibition with seviteronel were effective at radiosensitizing cells with radiation enhancement ratios of 1.20-1.89 in models of TNBC with high AR expression. AR-negative (AR-) models, regardless of their estrogen receptor expression, were not radiosensitized with seviteronel treatment at concentrations up to 5 μM. Radiosensitization of AR+ TNBC models was at least partially dependent on impaired dsDNA break repair with significant delays in repair at 6, 16, and 24 h as measured by immunofluorescent staining of γH2AX foci. Similar effects were observed in an in vivo AR+ TNBC xenograft model where there was a significant reduction in tumor volume and a delay to tumor doubling and tripling times in mice treated with seviteronel and radiation. Following combination treatment with seviteronel and radiation, increased binding of AR occurred at DNA damage response genes, including genes involved both in homologous recombination and non-homologous end joining. This trend was not observed with combination treatment of enzalutamide and RT, suggesting that seviteronel may have a different mechanism of radiosensitization compared to other AR inhibitors. Enzalutamide and seviteronel treatment also had different effects on AR and AR target genes as measured by immunoblot and qPCR. These results implicate AR as a mediator of radioresistance in AR+ TNBC models and support the use of seviteronel as a radiosensitizing agent in AR+ TNBC.

PARP1 Inhibition Radiosensitizes Models of Inflammatory Breast Cancer to Ionizing Radiation

Sustained locoregional control of disease is a significant issue in patients with inflammatory breast cancer (IBC), with local control rates of 80% or less at 5 years. Given the unsatisfactory outcomes for these patients, there is a clear need for intensification of local therapy, including radiation. Inhibition of the DNA repair protein PARP1 has had little efficacy as a single agent in breast cancer outside of studies restricted to patients with BRCA mutations; however, PARP1 inhibition (PARPi) may lead to the radiosensitization of aggressive tumor types. Thus, this study investigates inhibition of PARP1 as a novel and promising radiosensitization strategy in IBC. In multiple existing IBC models (SUM-149, SUM-190, MDA-IBC-3), PARPi (AZD2281-olaparib and ABT-888-veliparib) had limited single-agent efficacy (IC₅₀ > 10 μmol/L) in proliferation assays. Despite limited single-agent efficacy, submicromolar concentrations of AZD2281 in combination with RT led to significant radiosensitization (rER 1.12-1.76). This effect was partially dependent on BRCA1 mutational status. Radiosensitization was due, at least in part, to delayed resolution of double strand DNA breaks as measured by multiple assays. Using a SUM-190 xenograft model in vivo, the combination of PARPi and RT significantly delays tumor doubling and tripling times compared with PARPi or RT alone with limited toxicity. This study demonstrates that PARPi improves the effectiveness of radiotherapy in IBC models and provides the preclinical rationale for the opening phase II randomized trial of RT ± PARPi in women with IBC (SWOG 1706, NCT03598257).