Human Breast Cancer Cell Lines Differentially Modulate Signaling from Distant Microenvironments, Which Reflects Their Metastatic Potential.

Metastasis is the stage at which the prognosis substantially decreases for many types of cancer. The ability of tumor cells to metastasize is dependent upon the characteristics of the tumor cells, and the conditioning of distant tissues that support colonization by metastatic cells. In this report, we investigated the systemic alterations in distant tissues caused by multiple human breast cancer cell lines and the impact of these alterations on the tumor cell phenotype. We observed that the niche within the lung, a common metastatic site, was significantly altered by MDA-MB-231, MCF7, and T47 tumors, and that the lung microenvironment stimulated, to differing extents, an epithelial-to-mesenchymal transition (EMT), reducing proliferation, increasing transendothelial migration and senescence, with no significant impact on cell death. We also investigated the ability of an implantable scaffold, which supports the formation of a distant tissue, to serve as a surrogate for the lung to identify systemic alterations. The scaffolds are conditioned by the primary tumor similarly to the lung for each tumor type, evidenced by promoting a pro-EMT profile. Collectively, we demonstrate that metastatic and non-metastatic breast cancers condition distant tissues, with distinct effects on tumor cell responses, and that a surrogate tissue can distinguish the metastatic potential of human breast cancer cell lines in an accessible site that avoids biopsy of a vital organ.

An engineered niche delineates metastatic potential of breast cancer.

Metastatic breast cancer is often not diagnosed until secondary tumors have become macroscopically visible and millions of tumor cells have invaded distant tissues. Yet, metastasis is initiated by a cascade of events leading to formation of the pre-metastatic niche, which can precede tumor formation by a matter of years. We aimed to distinguish the potential for metastatic disease from nonmetastatic disease at early times in triple-negative breast cancer using sister cell lines 4T1 (metastatic), 4T07 (invasive, nonmetastatic), and 67NR (nonmetastatic). We used a porous, polycaprolactone scaffold, that serves as an engineered metastatic niche, to identify metastatic disease through the characteristics of the microenvironment. Analysis of the immune cell composition at the scaffold was able to distinguish noninvasive 67NR tumor-bearing mice from 4T07 and 4T1 tumor-bearing mice but could not delineate metastatic potential between the two invasive cell lines. Gene expression in the scaffolds correlated with the up-regulation of cancer hallmarks (e.g., angiogenesis, hypoxia) in the 4T1 mice relative to 4T07 mice. We developed a 9-gene signature (Dhx9, Dusp12, Fth1, Ifitm1, Ndufs1, Pja2, Slc1a3, Soga1, Spon2) that successfully distinguished 4T1 disease from 67NR or 4T07 disease throughout metastatic progression. Furthermore, this signature proved highly effective at distinguishing diseased lungs in publicly available datasets of mouse models of metastatic breast cancer and in human models of lung cancer. The early and accurate detection of metastatic disease that could lead to early treatment has the potential to improve patient outcomes and quality of life.

A synthetic metastatic niche reveals antitumor neutrophils drive breast cancer metastatic dormancy in the lungs.

Biomaterial scaffolds mimicking the environment in metastatic organs can deconstruct complex signals and facilitate the study of cancer progression and metastasis. Here we report that a subcutaneous scaffold implant in mouse models of metastatic breast cancer in female mice recruits lung-tropic circulating tumor cells yet suppresses their growth through potent in situ antitumor immunity. In contrast, the lung, the endogenous metastatic organ for these models, develops lethal metastases in aggressive breast cancer, with less aggressive tumor models developing dormant lungs suppressing tumor growth. Our study reveals multifaceted roles of neutrophils in regulating metastasis. Breast cancer-educated neutrophils infiltrate the scaffold implants and lungs, secreting the same signal to attract lung-tropic circulating tumor cells. Second, antitumor and pro-tumor neutrophils are selectively recruited to the dormant scaffolds and lungs, respectively, responding to distinct groups of chemoattractants to establish activated or suppressive immune environments that direct different fates of cancer cells.

Prophylactic mastectomy and occult malignancy: Surgical and imaging considerations.

BACKGROUND: Sentinel node biopsy (SLNB) is not routinely recommended for patients undergoing prophylactic mastectomy (PM), yet omission remains a subject of debate among surgeons. A modern patient cohort was examined to determine occult malignancy (OM) incidence within PM specimens to reinforce current recommendations. METHODS: All PM performed over a 5-year period were retrospectively identified, including women with unilateral breast cancer who underwent synchronous or delayed contralateral PM or women with elevated cancer risk who underwent bilateral PM. RESULTS: The study population included 772 patients (598 CPM, 174 BPM) with a total of 39 OM identified: 17 invasive cancers (14 CPM, 3 BPM) and 22 DCIS (19 CPM, 3 BPM). Of the 86 patients for whom SLNB was selectively performed, 1 micrometastasis was identified. In the CPM cohort, risk of OM increased with age, presence of LCIS of either breast, or presence of a non-BRCA high-penetrance gene mutation, while preoperative magnetic resonance imaging was associated with lower likelihood of OM. CONCLUSIONS: Given the low incidence of invasive OM in this updated series, routine SLNB is of low value for patients undergoing PM. For patients with indeterminate radiographic findings, discordant preoperative biopsies, LCIS, or non-BRCA high-penetrance gene mutations, selective SLNB implementation could be considered.

Myeloid cell reprogramming alleviates immunosuppression and promotes clearance of metastatic lesions.

Suppressive myeloid cells, including monocyte and neutrophil populations, play a vital role in the metastatic cascade and can inhibit the anti-tumor function of cytotoxic T-cells. Cargo-free polymeric nanoparticles (NPs) have been shown to modulate innate immune cell responses in multiple pathologies of aberrant inflammation. Here, we test the hypothesis that the intravenous administration of drug-free NPs in the 4T1 murine model of metastatic triple-negative breast cancer can reduce metastatic colonization of the lungs, the primary metastatic site, by targeting the pro-tumor immune cell mediators of metastatic progression. In vivo studies demonstrated that NP administration reprograms the immune milieu of the lungs and reduces pulmonary metastases. Single-cell RNA sequencing of the lungs revealed that intravenous NP administration alters myeloid cell phenotype and function, skewing populations toward inflammatory, anti-tumor phenotypes and away from pro-tumor phenotypes. Monocytes, neutrophils, and dendritic cells in the lungs of NP-treated mice upregulate gene pathways associated with IFN signaling, TNF signaling, and antigen presentation. In a T-cell deficient model, NP administration failed to abrogate pulmonary metastases, implicating the vital role of T-cells in the NP-mediated reduction of metastases. NPs delivered as an adjuvant therapy, following surgical resection of the primary tumor, led to clearance of established pulmonary metastases in all treated mice. Collectively, these results demonstrate that the in vivo administration of cargo-free NPs reprograms myeloid cell responses at the lungs and promotes the clearance of pulmonary metastases in a method of action dependent on functional T-cells.

Single-cell RNA-sequencing identifies anti-cancer immune phenotypes in the early lung metastatic niche during breast cancer.

Microenvironmental changes in the early metastatic niche may be exploited to identify therapeutic targets to inhibit secondary tumor formation and improve disease outcomes. We dissected the developing lung metastatic niche in a model of metastatic, triple-negative breast cancer using single-cell RNA-sequencing. Lungs were extracted from mice at 7-, 14-, or 21 days after tumor inoculation corresponding to the pre-metastatic, micro-metastatic, and metastatic niche, respectively. The progression of the metastatic niche was marked by an increase in neutrophil infiltration (5% of cells at day 0 to 81% of cells at day 21) and signaling pathways corresponding to the hallmarks of cancer. Importantly, the pre-metastatic and early metastatic niche were composed of immune cells with an anti-cancer phenotype not traditionally associated with metastatic disease. As expected, the metastatic niche exhibited pro-cancer phenotypes. The transition from anti-cancer to pro-cancer phenotypes was directly associated with neutrophil and monocyte behaviors at these time points. Predicted metabolic, transcription factor, and receptor-ligand signaling suggested that changes in the neutrophils likely induced the transitions in the other immune cells. Conditioned medium generated by cells extracted from the pre-metastatic niche successfully inhibited tumor cell proliferation and migration in vitro and the in vivo depletion of pre-metastatic neutrophils and monocytes worsened survival outcomes, thus validating the anti-cancer phenotype of the developing niche. Genes associated with the early anti-cancer response could act as biomarkers that could serve as targets for the treatment of early metastatic disease. Such therapies have the potential to revolutionize clinical outcomes in metastatic breast cancer.

Factors influencing scalp cooling discussions and use at a large academic institution: a single-center retrospective review.

PURPOSE: Chemotherapy-induced alopecia (CIA) is a stigmatizing and psychologically devasting side effect of cancer treatment. Scalp cooling therapy (SCT) is the most effective method to reduce CIA, yet it is underutilized. We investigated factors that may impact scalp cooling discussion and use. METHODS: We performed a retrospective review of cancer patients from 2000 to 2019 who had documentation of SCT discussion in the electronic medical record. The University of Michigan Rogel Cancer Center registry was used to identify the total number of cancer patients eligible for SCT during 2015-2019. Chi-square tests were used for outcome and patient characteristic comparisons (p < 0.05). RESULTS: From 2000 to 2019, 194 patients had documentation of SCT discussion. Of those, 72 (43.6%) used SCT, 93 (47.9%) did not use SCT, and the remaining 29 (17.8%) had unknown SCT use. A total of 5615 cancer patients were eligible for SCT from 2015 to 2019. As compared to those who did not have documented SCT discussions, patients who had documentation of SCT discussions in that period (n = 161, 3.0%) were more likely to be female, have breast cancer, be less than 45 years old, and live in a zip code with average income > US $100,000 (all p < 0.0001). Between 2015 and 2019, 57 patients (1.02%) used SCT. On univariate analysis, patient-initiated conversation about SCT (p = 0.01) and age less than 65 (p = 0.03) were significantly associated with decision to use SCT. CONCLUSION: There were distinctions in the types of patients who have documented discussions about SCT. Improving patient knowledge about the availability of SCT and increasing access to this technology for all eligible cancer patients may enable more patients to achieve improved quality of life by reducing or preventing CIA.

Management of early stage HER2 positive breast cancer and increased implementation of axillary imaging to improve identification of nodal metastasis.

BACKGROUND AND OBJECTIVES: Given the significant benefit of targeted therapies for HER2+ breast cancer patients in both the neoadjuvant and adjuvant settings, it is critical to identify all eligible patients for these treatments. We sought to investigate cT1cN0 HER2+ patients to determine the rate of postsurgical nodal positivity, and to identify presurgical factors associated with nodal positivity. We hypothesize there is a subset of underdiagnosed HER2+ patients who would benefit from preoperative axillary imaging and inclusion in neoadjuvant chemotherapy regimens. METHODS: We performed a 10-year retrospective analysis of T1 HER2+ breast cancer patients. Clinicopathologic characteristics were evaluated based on surgical nodal data. RESULTS: We identified 38 patients with cT1cN0 HER2+ cancer. Of this cohort, 24% had positive lymph nodes on final pathology. High tumor grade (p = 0.035) on core needle biopsy and the presence of lymphovascular invasion (p = 0.0036) were associated with an increased likelihood of lymph node positivity. The majority (66%) of lymph node positive patients were clinically T1c. CONCLUSIONS: We identified a 24% nodal positivity rate in clinically node negative T1 HER2+ breast cancer patients. In particular, HER2+ patients with high-grade T1c cancers should undergo preoperative diagnostic axillary imaging to expand potential benefit from targeted therapies.

Identifying Barriers and Facilitators to Scalp Cooling Therapy Through a National Survey of the Awareness, Practice Patterns, and Attitudes of Oncologists.

PURPOSE: Scalp cooling therapy (SCT) is the most effective method to reduce chemotherapy-induced alopecia (CIA), a highly distressing side effect of cancer treatment. Despite data supporting SCT efficacy and safety, SCT use in the United States is not widespread. Oncologists' interactions with scalp cooling were examined to identify facilitators and barriers to SCT implementation. METHODS: A 33-question survey was distributed through the ASCO Research Survey Pool to a nationally representative, random sample of 600 oncology providers. Outcome measures included knowledge of SCT, frequency of initiating conversations about SCT with patients, degree of support, and barriers for SCT. Significance was defined as P < .001. RESULTS: Of 155 (25.8%) responding providers, 62% of providers were in favor of SCT always or most of the time, but only 26% reported initiating discussions about SCT always or most of the time. Providers who treat breast cancer (P ≤ .0001), those who report being very familiar with SCT (P ≤ .0001), those who report having read SCT literature in the past 2 years (P ≤ .0001), and those who work at a facility with machine SCT (P ≤ .0001) were significantly more likely to initiate conversations with patients about SCT. Financial concerns (58%) were the primary reason for not recommending SCT use; efficacy (31%), staff or facility (24%), and safety (15%) concerns were also noted. Although safety concerns have decreased markedly over time, 14% of providers report patients who continue to express these concerns and 17% of providers see safety issues as barriers to supporting SCT. CONCLUSION: Our findings suggest that oncology provider familiarity and experience with SCT lead to increased support for scalp cooling, which may ultimately result in greater availability and utilization of SCT when indicated.

Implications of TGFß Signaling and CDK Inhibition for the Treatment of Breast Cancer.

TGFß signaling enacts tumor-suppressive functions in normal cells through promotion of several cell regulatory actions including cell-cycle control and apoptosis. Canonical TGFß signaling proceeds through phosphorylation of the transcription factor, SMAD3, at the C-terminus of the protein. During oncogenic progression, this tumor suppressant phosphorylation of SMAD3 can be inhibited. Overexpression of cyclins D and E, and subsequent hyperactivation of cyclin-dependent kinases 2/4 (CDKs), are often observed in breast cancer, and have been associated with poor prognosis. The noncanonical phosphorylation of SMAD3 by CDKs 2 and 4 leads to the inhibition of tumor-suppressive function of SMAD3. As a result, CDK overactivation drives oncogenic progression, and can be targeted to improve clinical outcomes. This review focuses on breast cancer, and highlights advances in the understanding of CDK-mediated noncanonical SMAD3 phosphorylation. Specifically, the role of aberrant TGFß signaling in oncogenic progression and treatment response will be examined to illustrate the potential for therapeutic discovery in the context of cyclins/CDKs and SMAD3.

Impact of Breast Cancer Pretreatment Nodal Burden and Disease Subtype on Axillary Surgical Management.

BACKGROUND: The management of clinically node-positive breast cancer after neoadjuvant chemotherapy (NAC) has progressed with the potential to avoid the morbidity of axillary lymph node dissection in patients with complete response to therapy. This study addresses the impact of pretreatment nodal burden and tumor subtype on axillary pathologic complete response (AXpCR) in patients treated with NAC to better inform axillary surgical management. METHODS: A prospective database was reviewed to identify clinically node-positive patients who underwent NAC followed by axillary lymph node dissection. Patients were stratified in accordance with abnormal nodal burden on pretreatment axillary imaging defined as low (1-2 nodes) or high (≥3 nodes), and biologic subtype defined by hormone receptor (HR+, HR-) and HER2 (human epidermal growth factor receptor 2) status. The primary outcome was AXpCR. RESULTS: AXpCR was 43% in the study population. There was no difference in AXpCR between low and high nodal burden groups (44% versus 42%, P = 0.87). Subtype correlated to AXpCR (P < 0.001) with the highest rate (78%) in the HR-/HER2+ group. Overall, HER2+ patients had a significantly higher AXpCR than HER2- subtypes (66% versus 28% P < 0.001). HR and HER2 status were also predictive of AXpCR when comparing patient, tumor, and treatment variables. CONCLUSIONS: Biologic subtype better correlated with rates of AXpCR than nodal burden alone with the highest rates of AXpCR in HER2+ patients. Consideration of tumor biology is more informative than nodal burden when evaluating options for axillary management after NAC.

Predictors and outcomes in breast cancer patients who did or did not pursue fertility preservation.

PURPOSE: Breast cancer is the most common cancer in reproductive age women, and treatment can affect fertility; however, there is often concern regarding the safety of increased estradiol (E2) levels and potential delays in treatment with ovarian stimulation for fertility preservation (FP). The aim of this study was to compare recurrence and survival in breast cancer patients who pursued FP without concurrent letrozole to those who did not (non-FP). METHODS: We reviewed charts of women with breast cancer who contacted the FP patient navigator (PN) at Northwestern University from 01/2005-01/2018. Oncology and fertility outcome data were collected. Data were analyzed by Chi-square test or regression, as appropriate. Kaplan-Meier curves were used to examine breast cancer recurrence and survival. Statistical analyses were performed with SPSS IBM Statistics 26.0 for Windows. RESULTS: 332 patients were included, of which 157 (47.3%) underwent FP. Median days to treatment after consulting the PN was 35 in the FP group and 21 in non-FP (p < 0.05). Cancer recurrence was noted in 7 (4.7%) FP patients and 13 (7.9%) non-FP patients (NS), and mortality in 5 (3.2%) FP patients and 7 (4.2%) non-FP patients (NS). Within the FP group, no significant differences were found in recurrence or mortality based on ER status, age, BMI, peak E2 level or total gonadotropin dose. Likelihood of pursuing FP was primarily a function of age and parity, and was not affected by breast cancer stage. To date, 21 have used cryopreserved specimens, and 13 (62%) had a live birth. CONCLUSIONS: FP is safe and effective in breast cancer patients, regardless of receptor status; E2 elevations and the 2-week delay in treatment start are unlikely to be clinically significant. These findings are unique in that our institution does not use concomitant letrozole during stimulation to minimize E2 elevations in breast cancer patients.

Cargo-free immunomodulatory nanoparticles combined with anti-PD-1 antibody for treating metastatic breast cancer.

The presence of immunosuppressive innate immune cells such as myeloid derived suppressor cells (MDSCs), Ly6C-high monocytes, and tumor-associated macrophages (TAMs) at a tumor can inhibit effector T cell and NK cell function. Immune checkpoint blockade using anti-PD-1 antibody aims to overcome the immune suppressive environment, yet only a fraction of patients responds. Herein, we test the hypothesis that cargo-free PLG nanoparticles administered intravenously can divert circulating immune cells from the tumor microenvironment to enhance the efficacy of anti-PD-1 immunotherapy in the 4T1 mouse model of metastatic triple-negative breast cancer. In vitro studies demonstrate that these nanoparticles decrease the expression of MCP-1 by 5-fold and increase the expression of TNF-α by more than 2-fold upon uptake by innate immune cells. Intravenous administration of particles results in internalization by MDSCs and monocytes, with particles detected in the liver, lung, spleen, and primary tumor. Nanoparticle delivery decreased the abundance of MDSCs in circulation and in the lung, the latter being the primary metastatic site. Combined with anti-PD-1 antibody, nanoparticles significantly slowed tumor growth and resulted in a survival benefit. Gene expression analysis by GSEA indicated inflammatory myeloid cell pathways were downregulated in the lung and upregulated in the spleen and tumor. Upregulation of extrinsic apoptotic pathways was also observed in the primary tumor. Collectively, these results demonstrate that cargo-free PLG nanoparticles can reprogram immune cell responses and alter the tumor microenvironment in vivo to overcome the local immune suppression attributed to myeloid cells and enhance the efficacy of anti-PD-1 therapy.

A View from the past into our collective future: the oncofertility consortium vision statement.

PURPOSE: Today, male and female adult and pediatric cancer patients, individuals transitioning between gender identities, and other individuals facing health extending but fertility limiting treatments can look forward to a fertile future. This is, in part, due to the work of members associated with the Oncofertility Consortium. METHODS: The Oncofertility Consortium is an international, interdisciplinary initiative originally designed to explore the urgent unmet need associated with the reproductive future of cancer survivors. As the strategies for fertility management were invented, developed or applied, the individuals for who the program offered hope, similarly expanded. As a community of practice, Consortium participants share information in an open and rapid manner to addresses the complex health care and quality-of-life issues of cancer, transgender and other patients. To ensure that the organization remains contemporary to the needs of the community, the field designed a fully inclusive mechanism for strategic planning and here present the findings of this process. RESULTS: This interprofessional network of medical specialists, scientists, and scholars in the law, medical ethics, religious studies and other disciplines associated with human interventions, explore the relationships between health, disease, survivorship, treatment, gender and reproductive longevity. CONCLUSION: The goals are to continually integrate the best science in the service of the needs of patients and build a community of care that is ready for the challenges of the field in the future.

Disease-induced immunomodulation at biomaterial scaffolds detects early pancreatic cancer in a spontaneous model.

Pancreatic cancer has the worst prognosis of all cancers due to disease aggressiveness and paucity of early detection platforms. We developed biomaterial scaffolds that recruit metastatic tumor cells and reflect the immune dysregulation of native metastatic sites. While this platform has shown promise in orthotopic breast cancer models, its potential in other models is untested. Herein, we demonstrate that scaffolds recruit disseminated pancreatic cells in the KPCY model of spontaneous pancreatic cancer prior to adenocarcinoma formation (3-fold increase in scaffold YFP + cells). Furthermore, immune cells at the scaffolds differentiate early- and late-stage disease with greater accuracy (0.83) than the natural metastatic site (liver, 0.50). Early disease was identified by an approximately 2-fold increase in monocytes. Late-stage disease was marked by a 1.5-2-fold increase in T cells and natural killer cells. The differential immune response indicated that the scaffolds could distinguish spontaneous pancreatic cancer from spontaneous breast cancer. Collectively, our findings demonstrate the utility of scaffolds to reflect immunomodulation in two spontaneous models of tumorigenesis, and their particular utility for identifying early disease stages in the aggressive KPCY pancreatic cancer model. Such scaffolds may serve as a platform for early detection of pancreatic cancer to improve treatment and prognosis.

Trends in Breast Cancer Treatment De-Implementation in Older Patients with Hormone Receptor-Positive Breast Cancer: A Mixed Methods Study.

INTRODUCTION: Guidelines allow for the omission of sentinel lymph node biopsy (SLNB) and post-lumpectomy radiotherapy in women ≥ 70 years of age with hormone receptor-positive (HR +) breast cancer. Despite this, national data suggest these procedures have not been widely de-implemented. OBJECTIVES: Our objectives were to evaluate trends in SLNB and post-lumpectomy radiotherapy utilization in patients who are eligible for omission, and evaluate patient preferences as a target for de-implementation of low-value care. METHODS: We performed a sequential explanatory mixed-methods study by first analyzing an institutional database of patients ≥ 70 years of age with HR + breast cancer who received surgical treatment from 2014 to 2018. Based on the quantitative data, we conducted semi-structured interviews with women identified as high or low utilizers of breast cancer treatments to elicit patient perspectives on de-implementation. RESULTS: SLNB and post-lumpectomy radiotherapy were performed in 68% and 43% of patients, respectively, who met the criteria for omission. There was a significant decrease in SLNB rates from 2014 to 2018. Forty-nine percent of patients were classified as high utilizers and 26% were classified as low utilizers. Qualitative analysis found that the most important factors influencing decision making regarding SLNB and post-lumpectomy radiotherapy omission for both high and low utilizers were trust in their provider and a desire for peace of mind. CONCLUSIONS: Despite efforts to de-implement low-value care, older women with HR + breast cancer remain at risk of overtreatment. Patient perspectives suggest that multi-level de-implementation strategies will need to target provider practice patterns and patient-provider communication to promote high-quality decision making and reduction in breast cancer overtreatment.

Cyclin E overexpression confers resistance to trastuzumab through noncanonical phosphorylation of SMAD3 in HER2+ breast cancer.

The efficacy of trastuzumab, a treatment for HER2+ breast cancer, can be limited by the development of resistance. Cyclin E (CCNE) overexpression has been implicated in trastuzumab resistance. We sought to uncover a potential mechanism for this trastuzumab resistance and focused on a model of CCNE overexpressing HER2+ breast cancer and noncanonical phosphorylation of the TGF-ß signaling protein, SMAD3. Network analysis of transcriptional activity in a HER2+, CCNE overexpressing, trastuzumab-resistant cell line (BT474R2) identified decreased SMAD3 activity was associated with treatment resistance. Immunoblotting showed SMAD3 expression was significantly downregulated in BT474R2 cells (p < .01), and noncanonical phosphorylation of SMAD3 was increased in these CCNE-overexpressing cells. Also, in response to CDK2 inhibition, expression patterns linked to restored canonical SMAD3 signaling, including decreased cMyc and increased cyclin-dependent inhibitor, p15, were identified. The BT474R2 cell line was modified through overexpression of SMAD3 (BT474R2-SMAD3), a mutant construct resistant to CCNE-mediated noncanonical phosphorylation of SMAD3 (BT474R2-5M), and a control (BT474R2-Blank). In vitro studies examining the response to trastuzumab showed increased sensitivity to treatment for BT474R2-5M cells. These findings were then validated in NSG mice inoculated with BT474R2-5M cells or BT474R2 control cells. After treatment with trastuzumab, the NSG mice inoculated with BT474R2-5M cells developed significantly lower tumor volumes (p < .001), when compared to mice inoculated with BT474R2 cells. Taken together, these results indicate that for patients with HER2+ breast cancer, a mechanism of CCNE-mediated trastuzumab resistance, regulated through noncanonical SMAD3 phosphorylation, could be treated with CDK2 inhibition to help enhance the efficacy of trastuzumab therapy.