Goetz, Matthew P.

Abstract Background: Endocrine therapies remain central to estrogen receptor alpha-positive (ERα+) breast cancer treatment, yet their predominantly cytostatic effects highlight the need for combinatorial approaches that promote tumor cell death. We therefore tested BH3 mimetics as single agents and in combination with (Z)-endoxifen (ENDX) across endocrine sensitive and endocrine resistant ERα+ breast cancer models. Methods: Six BH3 mimetics were tested in MCF7 and T47D cells and matched ENDX-resistant and fulvestrant-resistant derivatives. ENDX combinations were assessed using 2D proliferation and synergy assays, 3D RASTRUM® bioprinted cultures, migration assays, and in vivo MCF7 xenograft studies. Results: BH3 mimetics showed broad anti-proliferative activity as single agents in endocrine sensitive and resistant models. ENDX combinations were predominantly additive in 2D assays, with limited synergy confined to specific dose windows. In 3D culture, selected combinations of an MCL1 inhibitor (MCL1i) or navitoclax with ENDX produced greater suppression than single agents and, in several conditions, reduced growth below baseline, consistent with loss of viability. Combination treatment also enhanced suppression of migration under select concentrations. In MCF7 xenograft studies, ENDX robustly suppressed tumor growth; however, adding MCL1i or navitoclax did not provide additional antitumor benefit at the doses, schedules, and timeframes tested. Conclusion: BH3 mimetics show activity in ERα+ breast cancer models in vitro , including endocrine resistant models. Although ENDX combination benefit did not translate in vivo under the conditions tested, these data support further study of optimized BH3 mimetic combinations, including evaluation of dose, ratio, schedule, tumor target engagement, and other clinically advanced agents.

The NCCN Clinical Practice Guidelines for Breast Cancer include recommendations for clinical management of patients with carcinoma in situ, invasive breast cancer, Paget’s disease, Phyllodes tumor, inflammatory breast cancer, and management of breast cancer during pregnancy. The panel convenes annually to update recommendations based on a review of recently published clinical trials. This selection from the NCCN Guidelines for Breast Cancer focuses on the recommendations for the management of recurrent or stage IV (M1) metastatic breast cancer. It summarizes the treatment strategies stratified by tumor biology, including hormone receptor status and HER2 expression, as well as clinical factors such as prior therapy and disease-free interval. Throughout all treatment lines, supportive care, quality of life preservation, toxicity minimization, and clinical trial participation are emphasized as integral components of patient care.

Background Tumor-infiltrating lymphocytes (TILs) are established determinants of clinical outcomes in breast cancer. 1–6 While stromal TILs (sTILs) are commonly evaluated due to easier scoring and higher interobserver reproducibility, intraepithelial TILs may provide additional insights into anti-tumor immunity. 7 We aimed to comprehensively characterize immunotype-associated cellular landscapes in triple-negative breast cancer (TNBC) and identify features associated with clinical outcomes. Methods 212 TNBC tumors were classified into three immunotypes based on immune infiltrate location and density: immune desert (ID), immune excluded (IE), and immune inflamed (IN). 8 Single-cell spatial transcriptomics was performed on 75 tumors to define the cellular composition and functional states of the tumor microenvironment across immunotypes. Findings were validated and integrated with bulk RNA sequencing data and clinical outcome datasets from FinXX, CALGB-40603, I-SPY 2, and Real-World Clinico-Genomic Data (RWCGD) cohorts. Results Patients with IN tumors had significantly improved outcomes compared with ID tumors. Despite high sTILs, IE tumors showed poor outcomes similar to ID tumors. Single-cell spatial analysis revealed that ID and IE tumors exhibited reduced major histocompatibility complex I/II expression and fewer tumor-resident plasmacytoid dendritic cells (pDCs) compared with IN tumors, which were enriched for interferon-alpha (IFNα) and interferon-gamma (IFNγ) responses. High IFN response scores were associated with favorable outcomes across multiple therapy types in independent datasets. Deconvolution of RWCGD bulk RNA-seq data confirmed that pDC abundance correlated with improved outcomes specifically in hormone receptor–negative subtypes. Conclusions Our study highlights pDCs and IFN signaling as hallmarks of effective anti-tumor immunity in TNBC. Immunotype-based profiling underscores the limited prognostic value of sTILs alone in immune-excluded tumors and supports pDCs and IFN pathways as potential biomarkers for prognosis and therapeutic development in TNBC.

TPS655 Background: Early (14-28 day) on-treatment suppression of tumor proliferation during neoadjuvant endocrine therapy (NET) is strongly associated with favorable long-term outcomes in estrogen receptor–positive (ER+) HER2– breast cancer. In premenopausal women, tamoxifen with or without ovarian function suppression (OFS) results in suboptimal Ki-67 suppression compared with aromatase inhibitor + OFS. (Z)-endoxifen (ENDX), the active metabolite of tamoxifen, dually inhibits ER signaling and PKCβ1-mediated AKT activation, supporting its evaluation as an alternative NET strategy in this population. Methods: EVANGELINE (NCT05607004) is an ongoing, multicenter, open-label Phase 2 study evaluating daily 40 mg ENDX plus goserelin administered every 28 days as neoadjuvant therapy in premenopausal women with ER+/HER2–, cT2–3, cN0–1 breast cancer. The primary objective is to determine the proportion of patients with baseline Ki-67 >10% who achieve Ki-67 ≤10% after 4 weeks of therapy. A Simon two-stage design is used to test whether the Week 4 Ki-67 ≤10% rate is at least 65%, with 20 patients enrolled into the first stage and, if promising, another 25 patients enrolled into the second stage (cohort A). A parallel cohort of 20 patients with baseline Ki-67 ≤10% (cohort B) is enrolled to assess objective response rate (ORR) at 24 weeks per RECIST v1.1. Secondary objectives include examining safety and tolerability, residual cancer burden, and PEPI score. Correlative analyses include examining effect of treatment on select tumor and plasma biomarkers. Clinical trial information: NCT05607004 .

560 Background: Patients (pts) with early stage hormone receptor-positive, human epidermal growth factor receptor 2-negative (HR+/HER2-) breast cancer (BC) remain at both short and long term risk of recurrence. While clinico-pathological and gene-based biomarkers provide insight into early and late recurrence risk, additional prognostic tools are needed. We previously developed (Saha et al. Clin Cancer Res , 2025) a new breast MRI image-based artificial intelligence prognostic tool (TumorSight Risk) composed of both radiographic and clinical information. Here we sought to validate this tool in an independent cohort. Methods: In this multi-center, blinded, prospective/retrospective validation study, we sought to validate the 5-year recurrence-free survival (RFS) according to pre-defined risk scores (0-100) and categories (“high” or “low”) via pre-specified cutoff (≥47) for pts with HR+/HER2- BC. Kaplan-Meier and Cox regression analyses were then used to assess the unadjusted and adjusted association between risk category/score and the 5-year RFS. Results: 2,129 pts with HR+/HER2- BC diagnosed between January 1, 2010, and December 31, 2024, who underwent breast MRI prior to surgery (no neoadjuvant therapy) were included from four institutions. Median age was 57 years, 16% were Black, 15% had grade 3 disease, 93% were clinical T1 or T2, and 85%/15% were clinical N0/N1. At a median follow up of 6.24 years, the unadjusted hazard ratio (HR) for RFS comparing low (n=1808) and high risk (n=321) pts was 2.71 (95% CI = 1.95 – 3.76, p = 3x10 -9 ). The estimated 5 year RFS rate according to risk score (low and high) was 95.5% (95% CI: 94.63 – 96.3%) and 89.6% (95% CI = 86.4 – 92.6%) while the 10 year RFS estimates were 88.9% (95% CI = 86.5 - 90.9%) and 69.6% (95% CI = 59.1 – 77.9%). Univariate subgroup analysis demonstrated that the risk score was prognostic by age (< 50 vs. ≥50 years), pathologic nodal status, grade, and surgery type, regardless of chemo-, endocrine, or radiation therapy administration. The observed continuous risk score was strongly associated with risk of recurrence, with a 22% increase in relative risk per 10-unit change in risk score (0-100), translating to a 2.4% (95% CI = 1.2 – 4.5%) and 25.5% (95% CI = 15.4 – 40.4%) risk of recurrence at 5-years for pts at the lowest (0-9) versus highest (90-100) risk score. The prognostic association between risk score and RFS remained after adjusting for pathologic T- and N-stage, grade, age, race, and treatment (p = 0.002). Conclusions: TumorSight Risk is a new image-based biomarker for HR+/HER2- BC that provides prognostic information independent of standard clinico-pathological parameters. Given the ready availability of breast MRI in most pts with BC, TumorSight Risk may be a convenient tool for estimating risk for recurrence.

545 Background: ILC is characterized by loss of E-cadherin expression, with distinct biological and clinical features, and is often perceived to be less sensitive to chemotherapy compared with IDC. Oncologists and pts may face a treatment dilemma regarding the benefits of adjuvant chemotherapy in pts with lower biologic risk but higher anatomic risk (N2/N3), stage III ILC. In this study, we sought to compare the benefit of adjuvant chemotherapy in pN2/N3 ILC compared with IDC. Methods: We used the National Cancer Database (NCDB) to identify adults with primary, non-metastatic, ER-positive/HER2-negative invasive breast cancer diagnosed between 2010 and 2022 who underwent upfront surgery. Patients receiving neoadjuvant systemic therapy were excluded. Analyses were limited to pathologic stage I–III disease with known nodal involvement (N2–N3) and tumor category (T1–4). Propensity score matching (1:3, caliper 0.2; using R 4.5.1) was performed to balance age, sex, race, insurance status, grade, pathologic stage, nodal category, tumor category, Charlson-Deyo comorbidity score, and year of diagnosis. Overall survival (OS) was assessed using Kaplan–Meier methods and Cox proportional hazards models used to estimate hazard ratios (SPSS v28). Results: After propensity matching, we analyzed 28,115 pts with pN2/N3 disease (ILC 9,603 and IDC 18,512). Upstaging from cN0 to pN2/3 was higher for ILC than IDC (20.3% vs 13.3%, p<0.001). ILC pts were more likely to undergo mastectomy compared to IDC (80.2% vs 71.2%, p<0.001). Receipt of adjuvant chemotherapy for pN2/N3 disease was similar between IDC and ILC (78.0% vs 78.4%, p=0.37), as was use of adjuvant radiation and endocrine therapy. Among patients receiving chemotherapy, treatment was associated with a 65% reduction in death in ILC (HR 0.35, 95% CI 0.32–0.38, p<0.001) and a 73% reduction in IDC (HR 0.27, 95% CI 0.26–0.29, p<0.001), compared with no chemotherapy. 5-year OS among chemotherapy-treated patients was lower for ILC compared with IDC (HR 1.4, 95% CI 1.33-1.49; p<0.001, Table 1). In contrast, no OS difference was observed between ILC and IDC among patients who did not receive chemotherapy (HR 1.03, 95% CI 0.96-1.11; p=0.38). Chemotherapy was associated with higher 5-year OS in both IDC (87.7% vs 58.9%) and ILC (84.3% vs 59.2%). Conclusions: Adjuvant chemotherapy was associated with a substantial improvement in 5-year OS among patients with pN2/N3 disease in both IDC and ILC. Pts with ILC and extensive nodal involvement benefit from adjuvant chemotherapy. 5-year survival of IDC and ILC. IDC (18,512) ILC (9,603) No Chemotherapy (4,078) Adjuvant Chemotherapy (14,434) No Chemotherapy (2,071) Adjuvant Chemotherapy (7,532) pN2 (18,669) 62.8% 90.1% 64.4% 87.6% pN3 (9,446) 48.3% 82.2% 51.4% 79.4% pN2/pN3 Combined (28,115) 58.9% 87.7% 59.2% 84.3% P <0.001, log rank.

525 Background: Adjuvant endocrine therapy (ET) decreases breast cancer (BC) recurrence and improves overall survival (OS) for patients (pts) with estrogen receptor positive (ER+) BC. Omission of adjuvant ET and nonadherence are major issues contributing to worse clinical outcomes. Recent data suggest that the number of pts who omit adjuvant ET may be increasing in Europe [1]. These data have yet to be evaluated in the U.S. We previously reported data from the National Cancer Database (NCDB) that ET omission was more likely in pts with ER+/HER2+ disease who achieved a pathologic complete response (pCR) after neoadjuvant chemotherapy (NAC) [2]. Herein we report an updated analysis with a larger cohort designed to examine trends in ET omission over time and the impact on OS in a group of pts at high risk of recurrence. Methods: The NCDB was queried from 2010-2022 for pts with stage I-III ER+BC treated with NAC and surgery. pCR was defined as ypT0, ypN0. Use of adjuvant ET and the impact of adjuvant ET omission on OS in pts with and without pCR were assessed separately based on HER2 status. Adjuvant ET initiation by 1 year was summarized using the cumulative incidence estimate to account for the competing risk of death. OS was analyzed with ET as a time-dependent covariate using multivariable Cox proportional hazards regression. Results: We identified 130,438 pts with ER+BC (78,778 ER+/HER2-, 51,660 ER+/HER2+) who received NAC. Median follow-up was 5 years. Overall, pCR was achieved in 10.0% of ER+/HER2- and 35.9% of ER+/HER2+. On multivariable analysis, ET omission was significantly associated with worse OS in pts with residual disease (RD) [ER+/HER2- (HR 1.80 p<0.001) ER+/HER2+ (HR 1.66 p<0.001] and in those with a pCR [ER+/HER2- (HR 1.38 p=0.002) ER+/HER2+ (HR 1.42 p<0.001)]. Between 2010-2017, adjuvant ET omission was stable, with approximately 10.0% of pts not having initiated ET by 1 year after surgery. The proportion of pts omitting ET overall increased thereafter to 12.9% in 2018, rising to 24.4% in 2022. A sharp increase in ET omission was observed 2021-2022 in ER+/HER2- BC who achieved pCR, with 42.4% omitting ET compared to 25.1% for 2018-2020 and 18.8% for 2010-2017 (p<0.001). Adjuvant ET omission showed more modest but significant increases for other subgroups (p<0.001): ER+/HER2- with RD (8.5% 2010-2017, 12.3% 2018-2020, 18.7% 2021-2022), ER+/HER2+ with pCR (15.1% 2010-2017, 14.0% 2018-2020, 24.4% 2021-2022), and ER+/HER2+ with RD (9.5% 2010-2017, 12.5% 2018-2020, 22.2% 2021-2022). Conclusions: In ER+BC treated with NAC, omission of ET is increasing over time and associated with a higher risk of death, regardless of HER2, RD, or pCR status. These findings have substantial public health implications as they may result in a reversal of the 4-decade improvements in breast cancer mortality. Additional intervention is critical to improve adherence to guidelines. [1] Verreck et al. Breast 2025 [2] Choong et al. SABCS 2022.

10513 Background: Prior reports of high frequency of BRCA1 and BRCA2 ( BRCA1/2 ) PVs among South Asian women from Bangladesh, India, Nepal, and Pakistan with breast or ovarian cancer require validation in large prospective unselected cohorts. Methods: Individual-level anonymized patient data from two prospective cohorts were pooled to evaluate the prevalence of germline PVs among South Asian women: the South Asian Regional Alliance for Hereditary Cancers (SARAH Consortium) in Bangladesh, Nepal, and Pakistan, and the previously reported Nurse-led Genetic Counseling and Awareness (NuGenA) expansion cohort in India. Both studies enrolled consecutive cases of histologically confirmed newly diagnosed breast or epithelial ovarian cancer irrespective of age at diagnosis or family history. In both studies, participants completed questionnaires on personal and family history (first-, second- or third-degree) and provided blood for germline genetic testing after written consent. PVs in cancer predisposition genes including ATM, BARD1, BRCA1, BRCA2, BRIP1, CHEK2, MLH1, MSH2, MSH6, PALB2, RAD51C, RAD51D, and TP53 were identified as part of the studies utilizing a multigene panel and the results were returned to ordering clinicians. The data cutoff date for this analysis was November 30, 2025. Results: The analysis included 1,711 South Asian women with breast (n=1,210) or ovarian (n=501) cancer. A family history of breast or ovarian cancer was noted in 177 (10.3%). Among breast cancers, 94% were invasive, 84% were ductal, 35% were triple-negative and over two-thirds presented with nodal involvement. Germline PVs across 13 predisposition genes were observed in 17.7% of breast and 35.9% of ovarian cancers. Notably, BRCA1/2 PVs accounted for most of these PV (13.6% of breast and 28.3% of ovarian cancer cases) with PVs in all genes except BRCA1/2 under 1%. The frequency of BRCA1/2 PVs was 24.4% in triple-negative breast cancer and 8.0% in ER+/HER2-. BRCA1/2 PV carriers were diagnosed with breast cancer at a younger age than non-carriers (Median age: 42 vs 46 years, P<0.001), with no age difference observed in ovarian cancer (50 vs. 51 years, P=0.79). Notably, among the 306 BRCA1/2 PV carriers identified, 229 (74.2%) did not report a family history of breast or ovarian cancer. Conclusions: We confirm a high prevalence of BRCA1/2 PVs among unselected South Asian women with either breast or ovarian cancer. The observations of younger age at cancer diagnosis, and low rates of family history support the need for population-specific germline genetic testing and risk management strategies in South Asia.

Abstract Introduction: Epoxy fatty acid (EpFA) mechanisms of hormone therapy resistance (HTR) in ER+HER2- breast cancer (BC) are poorly understood. EETs are EpFAs that contribute to BC progression and HTR by promoting oxidative phosphorylation (OXPHOS). However, it remains unclear how EETs contribute to HTR. In this study, two fulvestrant resistant (FR) subclones of the letrozole resistant (LR) MCF-7AC1 line were isolated (termed LR/FR) and were incidentally resistant to the CDKi palbociclib (PR) (termed MCF-7AC1 LR/FR/PR or multiply resistant - MCF-7AC1 MR). These HTR cell lines exhibited CYP3A4 upregulation (2.3- to 4.3-fold) and increased cellular (±)14,15-EET levels, which were suppressed by biguanide treatment (P&lt;0.05). Structure activity studies led to the design of hexyl cuban-1-yl biguanide (HCB) and fluorinated HCB derivatives C5F2-HCB and C6F3-HCB, which inhibited CYP3A4-mediated EET biosynthesis and growth of the HTR lines. HCB fluorination improved pharmacokinetics (PK) and toxicity. Methods: HCB and fluorinated HCBs were tested for impact on EET biosynthesis, OXPHOS, PK, and growth inhibition and reversal of hypoxia in the MCF-7AC1 MR tumor model. Results: Fulvestrant selection for MCF-7AC1 MR cell lines resulted in undetectable cyclin D1 and estrogen receptor (ERα) expression, while cyclin E1, CDK4, CDK6, CYP3A4, and c-MYC were upregulated (P&lt;0.001). MCF-7 MR cells demonstrated 1.8, 1.4, and 1.2- fold higher levels of (±)8,9, (±)11,12, and (±)14,15-EET compared to parental MCF-7 cells (P&lt;0.05). The most potent fluorinated HCB, C6F3-HCB, suppressed cellular levels of (±)8,9-EET, (±)11,12-EET, and (±)14,15-EET by 53-60% (P&lt;0.029). The MCF-7AC1 MR cells were more potently inhibited by HCBs and fluorinated derivatives compared to the MCF-7 cell line. Comparing an MCF-7 MR line vs. MCF-7, HCB exhibited IC50 values of 4.1 vs. 7.0 uM, C5F2-HCB 10 vs. 25 uM, and C6F3-HCB 5.4 vs. 12 uM (P values all &lt;0.05). C5F2-HCB exhibited the best PK and treatment of the MCF-7AC1 MR xenograft model showed reversal of hypoxia (P&lt;0.05) and transient inhibition by C5F2-HCB (P=0.025 at 20 days). P/F also transiently inhibited this model (P=0.006 at 20 days). C5F2-HCB and P/F showed synergy in vitro (CI=0.46) leading to testing of C5F2-HCB/P/F in vivo enabled by acceptable toxicity of the triplet in mice. This combination showed synergy in vivo vs. standard of care palbociclib/fulvestrant by nonparametric statistical tests (Mann Whitney; P=0.0389; Kolmogorov-Smirnov; P=0.0473), providing a pathway for possible clinical development of the combination. Conclusions: MR BC cell lines exhibit upregulation of CYP3A4 and biosynthesis of EETs, indicating a role for EETs in HTR. The C5F2-HCB/P/F exhibits activity against the highly resistant MCF-7 MR xenograft with acceptable toxicity indicating an avenue for clinical development. Citation Format: Zhijun Guo, Jianxun Lei, Christian T. Wells, Allison M. Makovec, Andrew C. Yates, Swaathi Jayaraman, John R. Hawse, Joshua A. McCarra, Qing Cao, Michael J. Pryzbilla, Brenda L. Koniar, Beverly J. Norris, Craig M. Florey, Robert J. Schumacher, Michael A. Farrar, Kaylee L. Schwertfeger, Elizabeth A. Ambrose, Henry Wong, Gunda I. Georg, Antonino D'Assoro, Matthew P. Goetz, David A. Potter. Fluorinated cuban-1-yl biguanides overcome hormone therapy resistance to standard of care fulvestrant, and palbociclib [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(8_Suppl):Abstract nr LB196.

Abstract Breast cancer accounts for nearly a quarter of all cancers in women with an estimated 2.3 million new diagnoses and over 670,000 deaths annually. Triple-negative breast cancer (TNBC) is an aggressive subtype characterized by lack of estrogen receptor alpha (ERα) and progesterone receptor (PR) expression in the absence of HER2 amplification. TNBC accounts for ∼15% of all incident breast cancers and is associated with poor patient outcomes. Problematically, TN tumors exhibit vast inter-patient molecular heterogeneity making the identification of novel therapeutic vulnerabilities difficult and the development of alternative treatment strategies problematic. We and others have demonstrated that estrogen receptor beta (ERβ) is expressed in about 20% of TN tumors and potently suppresses proliferation, invasion, and migration in TNBC cells in vitro and in vivo. We thus hypothesized that endocrine therapies that selectively activate ERβ would be an effective treatment approach for this subset of patients. We also anticipated that some tumors would display de novo or acquired resistance to ERβ targeted therapies despite ERβ protein expression. We therefore sought to define mechanisms of resistance to ERβ targeted therapy and to identify synthetic lethal vulnerabilities. We generated the first models of ERβ resistant TNBC through chronic exposure to estradiol or an ERβ specific agonist (LY500307) and through a CRISPR genome wide knockout screen. We identified the LncRNA XIST as one of the most upregulated transcripts in multiple models of ERβ resistant disease, and knockdown of XIST expression was shown to resensitize resistant cell to estradiol and LY500307 treatment. XIST was not found to be directly regulated by ERβ in TNBC further suggesting that its overexpression is a bona fide mechanism of resistance to ERβ mediated suppression of proliferation. The phenomena of chronic ligand exposure leading to activation of XIST expression in ERβ+ cells is not shared with ERα suggesting that the mechanism by which XIST expression is elevated during the development of resistance is distinct between the two sister receptors. We further characterized XIST expression in patient samples of TNBC and found that XIST expression varies, but largely exhibits a bimodal distribution with most tumors displaying either low or high transcript levels. In conclusion, we have identified the X-inactivating transcript as a driver of resistance to ERβ targeted therapies in TNBC. Future studies aimed at uncovering the consequences of XIST over-expression on global gene expression, functionality of ERβ, and aggressiveness of TNBC are ongoing. Citation Format: Michael Emch, Kirsten Aspros, Matthew P. Goetz, John Hawse. The LncRNA XIST mediates resistance to estrogen receptor beta targeted therapies in triple negative breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 2293.