Advani, P. P.

Abstract Background: Male breast cancer (MaBC) is a rare malignancy, accounting for approximately 1% of all breast cancer cases. Our previous studies suggest that MaBC is biologically distinct from female breast cancer (FeBC), exhibiting a unique mutational profile and immune-genomic microenvironment. While the impact of these molecular alterations on site-specific metastases has been explored in FeBC, it remains poorly understood in MaBC. This gap limits accurate risk stratification and prediction of disease trajectory in male patients. In this study, we investigated site-specific genomic and immune alterations across metastatic sites in MaBC. Methods: 238 MaBC samples were tested by NGS (592, NextSeq; WES, NovaSeq) and WTS (NovaSeq; Caris Life Sciences, Phoenix, AZ). We performed analysis in hormone receptor-positive (HR+) and HER2-negative (HER2-) subtypes which are most common in male. HR+HER2- was defined as estrogen receptor (ER) or progesterone receptor (PR) positive and HER2 IHC 1+ or 2+ with negative chromogenic in situ hybridization (CISH) assay. Immune cells were estimated using WTS deconvolution (Quantiseq). Gene expression profiles were analyzed for T-cell inflamed score, interferon-gamma (IFN-gamma) score and MAPK Activation Score. We classified patients by site of biopsy/specimen site. Statistical significance was assessed with chi-square and Mann-Whitney U tests, p<0.05 is considered statistically significant. Results: Our cohort included 86 HR+HER2- MaBC samples biopsied from primary breast cancer (pBC) (n=39), bone (n=15), liver (n=8), lung (n=8), lymph nodes (n=7), and skin (n=9) mets. We report here only the data on statistically significant differences, but the overall landscape has been depicted across all defined-sites investigated. pBC showed a significantly lower prevalence of PIK3CA mutations (23.6% vs. 53.3%, p<0.05) compared to bone mets. ESR1 mutations were absent in pBC but present in 25% of liver and 25% of lung mets (all comparisons p<0.05). Immune profiling revealed reduced infiltration of M2 macrophages (4.3% vs. 6%, p<0.05) and neutrophil (2.2% vs. 6.9%, p<0.05) in pBC relative to bone Mets. Further, CD4+ T cells were lower in pBC compared to lymph nodes (0% vs. 2.4%, p<0.05). pBC had lower expression of immune checkpoint genes (PDCD1LG2, HAVCR2 and CD47) compared to other mets. pBC exhibited lower MAPK pathway activation score (-1.6 vs. 0.17, p=0.03) and a trend toward lower T cell-inflamed gene expression scores (-65 vs. 8, p=0.06) compared to bone mets. Bone mets had higher infiltration of M2 macrophage compared to lung (6% vs. 4.4%, p<0.05) and lymph node (6% vs. 4.5%, p<0.05). Neutrophil infiltration was elevated in bone versus liver (6.9% vs. 2%, p<0.05) but T regulatory cells were lower in bone compared to lymph nodes (1% vs. 2.4%, p<0.05). Bone showed reduced expression of CD47 (median TPM 50.6 vs. 109.9, p<0.05) compared to skin. Further bone had lower IFN-gamma scores (-0.41 vs. -0.17, p<0.05) compared to lymph node. Data on prior number and types of cancer treatment to give a context to the findings will be presented at the meeting. Conclusions: Our study highlights that each MaBC metastatic site harbors a unique genomic and immune landscape that can influence tumor behavior and potentially impact patient prognosis. Better understanding of metastatic site-specific molecular features in larger cohorts can help in the future research efforts and guiding treatment decisions. Citation Format: D. Trapani, S. Deshmukh, S. Wu, J. Xiu, N. Lin, G. Curigliano, P. Spanheimer, P. Advani, P. Barreto Coelho, M. Lustberg, G. Sledge Jr, S. Tolaney, J. Leone. Site specific Genomic and Immune Landscapes Underpinning in metastatic Male Breast Cancer [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS2-08-04.

Abstract Background: DNA methyltransferase (DNMT) isoforms 1, 3A and 3B are enzymes that regulate DNA methylation, controlling gene expression, chromatin stability and genetic imprinting. We have previously shown that DNMT3A protein expression in early triple negative breast cancer (TNBC) is associated with more aggressive clinicopathologic features (higher tumor grade, tumor proliferation and frequency of nodal metastases) and worse clinical outcomes in the absence of chemotherapy (Leon-Ferre et al, SABCS 2024). Pre-clinically, we have shown that DNMT3A protein expression predicts sensitivity to DNMT inhibitors (Yu J et al., J Clin Invest 2018), and that DNMT inhibitor treatment leads to downregulation of DNMT3A, enhanced expression of immune-related pathways, and halted tumor growth, particularly in combination with immunotherapy and paclitaxel. Methods: This is a phase 1, open-label, multicenter study evaluating the safety and recommended phase 2 dose (RP2D) of the oral DNMT inhibitor ASTX727 + paclitaxel + pembrolizumab in metastatic TNBC. ASTX727 is an oral fixed dose combination (FDC) tablet of 35 mg decitabine (active agent) and 100 mg cedazuridine (a cytidine deaminase inhibitor that prevents decitabine degradation in the bowel and liver, allowing oral delivery). The study consists of two parts: Part 1 is a dose-finding cohort, using a 3+3 design, with 5 dose levels (Table), and its primary objective is to determine the RP2D. Part 2 is an expansion cohort (n=6), with the primary objective to confirm tolerability of the regimen at the RP2D. Secondary objectives are to record the antitumor activity of the regimen, describe the AE profile, explore the association of baseline gene expression profiles with clinical benefit, and to evaluate impact of treatment on tumor and ctDNA methylation. Exploratory objectives will evaluate the association of baseline DNMT3A protein expression and antitumor activity, and the impact of treatment on the tumor and peripheral blood immune phenotype. Eligible patients are adults 18 years or older with metastatic TNBC (ER/PR ≤10%, and HER2-negative per ASCO/CAP guidelines), independent of PD-L1 expression. Patients are allowed to have received 0-3 prior lines of systemic therapy in the metastatic setting, including prior immune checkpoint inhibitors in the early and/or metastatic settings. Patients must be eligible for taxanes and have adequate organ and bone marrow function. Patients with treated brain metastases are eligible if previously treated without evidence of progression after 4 weeks. Leptomeningeal carcinomatosis is not allowed. Patients must have an ECOG performance status of ≤2 and have no gastrointestinal disorders impacting absorption of oral medications. As of June 20, 2025, the study is enrolling patients, with 10 patients dosed in the dose-finding phase. Citation Format: R. Leon-Ferre, D. M. Zahrieh, P. P. Advani, D. Quiroga, K. V. Giridhar, P. J. Dizona, D. Stover, X. Wang, A. J. Tevaarwerk, G. M. Choong, S. Yasir, K. R. Kalari, J. Foldi, R. Parajuli, M. H. Hackney, H. K. Chew, R. Wajeeha, H. Bear, E. Sharon, B. Ko, G. I. Shapiro, S. Gore, L. Pelosof, L. Wang, M. P. Goetz. Trial in progress: Phase 1 Study Targeting DNA Methyltransferases in Metastatic Triple-Negative Breast Cancer (NCI Protocol #10546) [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS5-07-20.

Abstract Introduction: Metaplastic breast cancer (MpBC) is a rare and histologically diverse subtype of breast cancer (BC) that is frequently triple negative (TN) and exhibits poor responsiveness to chemotherapy (CT). Our group previously reported a low pathologic complete response (pCR) rate among patients (pts) with MpBC treated with neoadjuvant CT in the pre-immunotherapy (IO) era. The standard neoadjuvant approach for pts with stage 2-3 TNBC has evolved with the results of the KEYNOTE-522 (KN522) trial, which demonstrated improved pCR rates, event-free survival (EFS), and overall survival (OS) with the addition of pembrolizumab to CT. However, pts with MpBC were not well-represented in KN522, and their response to IO remains poorly characterized. In this study, we examine the pCR rate with the addition of IO to neoadjuvant CT in pts with TN MpBC. Methods: Pts diagnosed with MpBC and treated at Miami Cancer Institute between 2017-2024 were retrospectively identified by COTA real-world Analytics® platform following IRB approval. Eligible pts had stage 2-3 ER-negative (defined as ER < 10%), PR-negative, HER2-negative MpBC, received neoadjuvant systemic therapy, and subsequently underwent definitive surgery. Demographic, clinical, treatment and outcome data were extracted from the electronic health record. pCR was defined as the absence of residual invasive cancer in the breast and axillary lymph nodes at the time of surgery. Kaplan-Meier method was used to estimate median disease-free survival (mDFS) and median OS (mOS). Comparison between treatment groups for pCR were performed using Fisher’s exact test. No multivariable or survival modeling was performed due to limited statistical power in this small cohort. All p values were two-sided. Results: A total of 27 pts were included in the analysis; 16 pts received CT (in the pre-IO era) with 19.2 months (m) median follow up and 11 pts received CT + IO with 15.5m median follow up. Spindle cell histology was reported in 18.5% of tumors, non-spindle cell in 59.3% and 22.2% were not further specified. Most patients (81.5%) had clinical stage 2 disease, 18.5% had stage 3; median tumor size was 3.2 cm. Anthracycline-based CT was administered to 81.5% of pts, and the remainder received non-anthracycline regimens. pCR was achieved in none of the 16 pts (0.0%) who received CT alone and in 3 of 11 pts (27.3%) who received CT + IO (p=0.046). Among the 3 pts who achieved pCR, 2 had non-spindle cell histology and 1 unspecified. All 3 pCR pts had stage 2 disease, received IO and anthracycline, and experienced immune-related adverse events. mDFS was 23.9m in the CT group and 14.6 m in the CT + IO group (p = 0.641), though follow-up duration differed between groups. Among all pts, mDFS was 15.3 m in those who did not achieve pCR and was not reached in those that did (p = 0.226). mOS was not reached in the CT group and was 20.7m in the CT + IO group (p = 0.192). Conclusion: In this small, retrospective real-world analysis of pts with early-stage TN MpBC, the addition of IO to neoadjuvant CT was associated with a higher pCR rate compared to CT alone. Although the observed pCR rate of 27.3% with CT+IO is lower than that reported in KN522 for unselected TNBC, this likely reflects the small sample size and the inherently CT-resistant biology of MpBC. None of the pts in the CT alone group achieved pCR, suggesting that IO may help to overcome CT resistance in MpBC. Differences in mDFS and mOS were not statistically significant, possibly due to limited follow up and sample size. Despite these limitations, our findings highlight the importance of incorporating IO into treatment for MpBC. Future multicenter analyses are ongoing to validate these results in a larger, more diverse population and to further explore the role of histologic subtypes and immune biomarkers in predicting response. Citation Format: N. Dempsey, L. Hodgson, A. Sandoval-Leon, L. Carcas, M. Ahluwalia, C. Wang, H. Kim, P. Advani, R. Mahtani. Pathologic complete response rates in early-stage triple negative metaplastic breast cancer in the neoadjuvant immunotherapy era: a retrospective review [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS4-10-07.

Abstract Background: Triple-negative breast cancer (TNBC) is highly aggressive subtype with poor prognosis and there is an urgent need to develop novel therapies. Deregulated cellular energetics is a known hallmark of cancer. Stearoyl-CoA desaturase-1 (SCD1), is a key enzyme in fatty acid (FA) synthesis, converting saturated to monounsaturated FA. It is upregulated in TNBC, and its expression correlates with poor survival. Hence, targeting SCD1 is a novel strategy for cancer therapy. We have developed an SCD1 inhibitor, MTI-301 and shown anti-tumor activity in murine TNBC cell lines with MTI-301 alone and in combination with paclitaxel. Here, we study the efficacy of MTI-301 in TNBC mouse model and, molecular and immunological association of SCD1 in TNBC. Methods: MTI-301 activity (10 mg/kg oral gavage daily) was evaluated in E0771 immunocompetent TNBC mouse models alone and with immune checkpoint inhibitors (ICI) and paclitaxel (anti-PD-L1 Ab 200ug/mouse; IP and 25 mg/kg IP both given 2/week). Immune markers such as CD4+ T cell and CD8+ T cell in mice tumor tissue were assessed by immunohistochemistry (IHC). Tumor volume was measured by calipers 2/week and compared between groups by ANOVA test. Molecular and immune characterization was performed in 1,581 invasive ductal (ID) TNBC patient (pt) samples tested by NGS (592-gene panel, NextSeq; WES/WTS, NovaSeq; Caris Life Sciences, Phoenix, AZ). ID-TNBC SCD1-high(H) and SCD1-low(L) RNA expression were classified as above or below the 50 percentiles, respectively. Immune cell fractions were calculated by deconvolution of WTS: Quantiseq. Pathway enrichment was determined by GSEA (Broad Inst). Statistical significance was determined using chi-square and Mann-Whitney U test with p-values adjusted for multiple comparisons (q<0.05). Results: Compared to vehicle control, MTI-301 showed significant reduction in mean tumor volume and was associated with macrophage phagocytosis and intratumor infiltration of CD4 and CD8 T cells. We have also showed that MTI-301 + anti-PD1 Ab + paclitaxel has significant antitumor activity compared to either agent alone. IHC analysis is ongoing. In real-word pt data, SCD1-H TNBC (N=791) had higher immune cell infiltration of M2 MΦ (2.95% vs. 2.58%) and neutrophils (4.5% vs. 4.1%) compared to SCD1-L (N=790), all q<0.05. SCD1-H had decreased IFNγ score (-0.29 vs -0.24), but increased MAPK activation score (-0.57 vs -1.77), all q<0.05. SCD1-H had higher levels of metabolic regulator genes (IL10, IL10RA, SOCS3, HIF1A, DUSP6, FC: 1.4-1.8), immune checkpoint genes (PDCD1, CTLA4, LAG3, TIGIT, VISTA, CD244, CD160, BTLA, HAVCR2, FC: 1.3-1.6), stem cell related genes (CD44, ALDH1A1, ALDH1A2, ALDH1A3, KLF4, NANOG, FC: 1.4-1.6) and markers of exhausted T cells (CXCL13, CD38, CD101, ENTPD1, CD69, CD103, KLRG1, PRF1, GZMB, FC: 1.2-1.7), all q<0.05. SCD1-H had enrichment of pathways related to oxidative phosphorylation, FA metabolism, PI3K/AKT/MTOR, EMT, DNA repair, TGF beta, P53 and inflammatory response (NES: 1.7-3.8, all FDR<0.25). SCD1-H had higher frequency of PIK3CA (18.1% vs 8.8%), PTEN (10.7% vs 5.2%), PIK3R1 (6.4% vs 2.3%), AKT1 (3.3% vs 1.3%) and NF1 (8.3% vs 2.7%) mutation compared to SCD1-L, all q<0.05. Conclusion: SCD1 high TNBC tumors are associated with immune suppressive TME, FA metabolism, and dysregulated PI3K pathway, positioning SCD1 as a metabolic driver of therapy resistance and immune evasion and support its targeting to enhance chemo-ICI efficacy. SCD1 inhibitor, MTI-301 significantly reduced tumor growth and enhanced effector immune cell infiltration in TNBC murine models with combinatorial activity with chemo-ICI, indicating that pharmacologic modulation of SCD1 is a promising strategy for cancer therapy. We will be conducting Phase I clinical trial with MTI-301 in pts with advanced tumors including TNBC (NCT NCT06911008). Citation Format: P. P. Advani, S. K. Deshmukh, R. Whetten, M. Redig, M. Pawlush, S. Wu, J. Xiu, J. P. Leone, S. Gandhi, M. Lustberg, G. Sledge Jr, J. Copland. Metabolic and immune reprogramming via SCD1 inhibition enhances chemo-immunotherapy response in TNBC [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS2-12-07.

Abstract Background: Breast cancer remains the leading cause of cancer-related mortality in women globally, with metastatic disease accounting for >90% of deaths. Current staging and histopathological features inadequately predict individual recurrence risk. Approximately 20-30% of early-stage patients develop distant metastases despite optimal therapy, while others are overtreated. Circulating microRNAs (MIRNAs) represent a paradigm shift toward liquid biopsy, offering real-time tumor assessment through minimally invasive blood sampling. These stable molecules reflect dynamic tumor behavior and metastatic potential, making them ideal for personalized surveillance strategies. Methods: This prospective proof-of-concept study included 46 breast cancer patients from Sri Shankara Cancer Hospital, India: 37 with metastatic disease (plasma collected at progression, 2024) and 9 disease-free controls (≥3-year follow-up). MIRNA isolation used MIRVANA method with samples processed within 2 hours, stored at -80°C. Expression levels of MIR-21, MIR-30d, and MIR-425 were quantified by TaqMan qRT-PCR, normalized to MIR-16 control. Logistic regression analysis of the transcript levels yielded a probability of predictive accuracy ranging from 0-1. Cut-off of high expression was chosen based on the best sensitivity and specificity by ROC curve analysis. Clinicopathological parameters (CPP) were correlated with MIR expression(Mann-Whitney U test). A combined model of MIR and CPP was built using logistic regression and ROC curve analysis. Results: Patient demographics(in Table) Difference in median age, subtypes and nodal status are interesting to note. MIR-21 showed elevated expression in metastatic cases, particularly hormone receptor-positive tumors. MIR-30d demonstrated significant correlation with grade 3 and node-positive status (p=0.038 for N0 vs N1, p=0.006 for N0 vs N3), with high expression associated with poor progression-free survival (p=0.0057, HR=3.24). MIR-425 exhibited significant differential expression between metastatic and non-metastatic groups with strong correlation to advanced nodal stages (p=0.0018). Univariate analysis with all clinicopathological parametres with DFS revealed LN status as significant predictor (AUC=0.68). Similarly combined MIR score of MIR-425 and MIR-30d achieved 72% predictive accuracy of DFS (AUC=0.716). Interestingly, multivariate analysis of MIR score and LN with DFS, enhanced the AUC to 0.81. Conclusions: This proof-of-concept study suggests circulating MIR-30d and MIR-425 may serve as potential biomarkers for breast cancer recurrence risk stratification. The integrated signature shows promise for enhancing surveillance strategies. Assay also standardized methods for using plasma to measure circulating MIRNA levels. Validation in larger multicenter cohorts is underway for clinical assay development. Citation Format: A. Korlimarla, V. Sasimouli, L. Krishna, S. Shree, H. P.S, B. Sivaraman, S. Appachu, S. Srihari, G. Guhan, P. Advani, S. B.S. Circulating MIRNA Signature Predicts Breast Cancer Metastasis: A Pilot Study for Non-Invasive Risk Stratification [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS4-01-04.

Abstract Background: Immune checkpoint inhibition (ICI) in combination with chemotherapy is associated with improved outcomes in a subset of patients with triple negative breast cancer (TNBC). However, biomarkers for predicting patient response, such as PD-L1 expression, have limited predictive efficiency. Voabil et al. has shown that ex vivo cytokine profiling of live tumor tissue resections treated with αPD-1 can predict patient response to ICI (Voabil et al., Nat Med. 2021). However, this method requires large amounts of tissue to address intra-specimen tumor heterogeneity, which is not feasible with standard of care (SOC) diagnostic core needle biopsies (CNBs) for breast cancer, where tissue is scarce. We show for the first time that ex vivo cytokine profiling is possible in live tumor fragments (LTFs) generated from CNBs in breast tissue. Methods: We present a cytokine profiling platform to assess response to ICI treatment in LTFs generated from CNBs. CNBs were collected from patients who were identified as potential candidates for SOC ICI (by treating physician) and enrolled in one of two prospective observational clinical trials (NCT05520099 and NCT06349642). CNBs were cut by an automated, proprietary cutting instrument to generate LTFs ofapproximately 300 µm in thickness. To address tissue heterogeneity and mitigate tissue scarcity in CNBs, we used a sequential treatment strategy in which control and ICI treatment were applied to LTFs in the same well, in a phased approach. This strategy provides a solution for the variability observed in cross-well comparisons. LTFs were encapsulated in hydrogel and treated with IgG control followed by αPD-1 (BioXCell). Cytokine production rates were measured longitudinally using multiplex assays assessing>45 cytokines, including 15 cytokines assayed by Voabil et al., and the fold changes of cytokine production rates in response to ICI over IgG control were calculated. Results: Using non-breast tumor tissue, we validated that our platform can measure changes in cytokine production in LTFs following sequential treatment, and that these measurements are comparable to those observed in a cross-well setting. In a small cohort of TNBC specimens, we show changes in cytokine production rates in response to ex vivo sequential control and ICI treatment. Ten cytokines, including IFN-γ and CXCL10, shown to be predictive of patient response to ICI in Voabil et al. were found upregulated in response to αPD-1 in TNBC specimens. Future determination of correlation with clinical response data (pathological complete response for early stage and RECIST 1.1 for metastatic patients) gathered from NCT05520099 and NCT06349642 will be used to validate platform responders and non-responders. To date, 11 patients with confirmed TNBC diagnoses have enrolled in these clinical studies. Conclusion: We report, for the first time to our knowledge, the ability to detect immune checkpoint inhibitor responses in live TNBC tissue obtained via CNBs. This proof-of-concept lays the groundwork for integrating functional tissue responses with clinical correlation data, with important implications for guiding current ICI therapies and advancing this platform for use in next-generation immuno-oncology treatments for TNBC. Citation Format: P. P. Advani, J. Harvey, J. Polley, M. Weidner, A. Arnold, D. Haley, R. Bollam, R. Rao, S. Chumsri, K. Hugghins, M. Acampora, C. Vivelo, H. Hernan, L. Hrycyniak, J. Zweng, H. Gierman, S. Bhattacharya, D. Mukhopadhyay. Ex vivo cytokine profiling of live triple negative breast cancer patient specimens from core needle biopsies illustrates proof of concept to assess tumor response to immune checkpoint inhibition [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS4-01-08.

Abstract Background: 18F-Fluoroestradiol (FES) positron emission tomography (PET) is approved for non-invasive detection of estrogen receptor (ER)-positive breast cancer (BC) and recent updates to the National Comprehensive Cancer Center Network clinical practice guidelines include FES-PET for staging recurrent/metastatic ILC. Given the wide range of FES-PET indications, we retrospectively reviewed the management of ER+ BC patients after FES-PET at Mayo Clinic. Methods: We identified ER+ BC patients who underwent FES-PET from April 2021 to June 2025 across the Mayo Clinic. BC context at the time of FES-PET requisition was categorized as early (EBC), locally recurrent (LRBC), or metastatic (MBC). Management changes were classified as either affecting BC or non-BC care (not mutually exclusive). For pts who underwent 18F-flurodeoxyglucose (FDG) PET-CT and had a discrepant result with FES-PET, we assessed the clinical impact of these discrepancies. Finally, we examined the change in clinical management when FES-PET was ordered to investigate rising tumor markers CA 15-3 or CA 27.29. Results: 166 pts underwent ≥1 FES-PET scan. At diagnosis, 57% had invasive lobular carcinoma (ILC), 33% invasive ductal carcinoma (IDC), and 5.4% mixed histology. ER expression ≥ 50% was observed in 81.4% and 1.2% were HER2+. Overall, FES-PET led to a change in management in 54.5% (n=90) of pts with 43.0% (n=71) experiencing a change in BC care and 16.4% (n=27) in non-BC care. Changes in BC care occurred in 36% of EBC, 68% of LRBC, and 43.7% of MBC. The most common BC management changes after FES-PET were change in treatment strategy in 23.6% (n=39), additional BC-related imaging including further FES-PET scans in 23.0% (n=38), upstaging in 10.3% (n=17), change in surgical planning in 10.3% (n=17), local therapy in 4.2% (n=7), and biopsy avoidance in 3% (n=5). Surgical planning changes were mostly deciding to proceed with curative intent surgery (n=11), though some surgeries were aborted upon FES-PET upstaging (n=6). Non-BC management changes included additional imaging to evaluate incidental findings in 11.5% (n=19) and procedures in 6.7% (n=11), including 5 biopsies, 2 bronchoscopies, 2 endoscopies, 1 hysterectomy, and 1 cholecystectomy. FDG/FES discordancy was observed in 22.3% of the overall population (n=37). The most common pattern was FDG-/FES+ (n=19), leading to upstaging of BC in 8 and the sole use of FES-PET for future restaging in 13 BC patients. The second most common pattern was FDG+/FES- (n=18), attributed to non-BC related FDG uptake in 11 (false-positive FDG) or post-treatment ER loss in 7. Three FDG+/FES- patients were found to have additional cancers: two non-small cell lung cancers and one lymphoma. Non-malignant causes of false positive FDG were inflammation (n=3), silicone implants (n=1), thymic hyperplasia (n=1), and unknown (n=5). Six patients initially upstaged with FDG-PET imaging proceeded to curative-intent surgery after negative FES-PET confirmatory scans (FDG+/FES-). When FES-PET was ordered to investigate rising CA 15-3 or CA 27.29 tumor markers, this led to upstaging/progression in 3 of 21 cases (1 EBC, 2 MBC). The mean tumor marker increase was 57.0% in patients with upstaging/progression versus 19.0% in those without (p=0.02, two-tailed Student’s t-test). Conclusions: FES-PET altered clinical management in approximately half of BC cases in this study. Changes in BC-specific management were most frequent in LRBC, followed by MBC and EBC. One in six patients had a non-BC clinical management change after FES-PET. FDG/FES discordance provided value in upstaging FDG non-avid cancers and downstaging FDG-avid lesions unrelated to BC, influencing curative treatment decisions. FES-PET ordered for rising tumor markers rarely changed management, though larger tumor marker increases were associated with upstaging or progression. Citation Format: N. Wiest, K. Bullock, T. Pai, M. Kornas, B. McKinley, E. Parent, P. Advani. FES-PET imaging in estrogen receptor-positive breast cancer: implications for clinical management [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS1-06-16.

Abstract BACKGROUND: Anthracycline chemotherapy-related cardiomyopathy (CCM) is a serious adverse event that can occur several years after completion of breast cancer (BC) therapy. Demographic and clinical risk factors have failed to predict which patients will experience CCM. Genetic variants can account for a significant proportion of inter-individual variation. Knowledge of genetic risk variants prior to chemotherapy will be informative for risk stratification and early intervention. The goal of our study is to identify genetic variants that predispose patients to CCM. Methods: We developed a cardiotoxicity registry at Mayo Clinic, Florida that enrolls patients who are to undergo standard of care chemotherapy in the early BC setting. Patients were (and continue to be) enrolled and consented for chart review and DNA sequencing. We sequenced whole exomes of the first 136 patients (anthracycline N=55, anti-HER2 (no anthracycline) N=71, other chemotherapy, N=10), primarily focusing on Titin (TTN) truncating variants, known to be present in ∼25% of patients with primary dilated cardiomyopathy, and previously reported in 7.5% of patients with CCM, followed by exploration of increased burden of rare non-synonymous variants in 60 established genes for primary cardiomyopathy. Filtering of rare non-synonymous variants included minor allele frequency <0.5% (in GnomAD database European, African and Asian ancestries) and CADDv1.7 PHRED score ≥20. Results: 18/55 (33%) patients treated with anthracycline experienced CCM. TTN truncating variants in were identified in 2/18 (11%) CCM patients and absent in 37 patients who did not experience CCM. Both patients with TTN variants had severe HF and required cardiac transplant. We identified a pathogenic variant in LMNA (p.Arg190Gln) in 1/18 (5.5%) patients and the same rare (p.Glu1127Gly) variant in RYR2 occurred in 2/18 (11%) of patients (Table 1). We noted enrichment of rare missense variants in anthracycline CCM patients compared to HER2 treated without anthracycline, CCM, p=0.0001. Conclusions: Pathogenic variants in TTN, LMNA, are enriched in BC patients (17%) who develop anthracycline-related cardiomyopathy. Early genetic screening may improve risk stratification and guide personalized cardiac monitoring to prevent CCM. Further research is needed to clarify the role of additional rare variants like RYR2 and to develop a CCM risk score based in idiopathic CM genes. Incorporating genetic risk assessments into treatment planning has the potential to enhance oncologic outcomes as well as long-term survivorship by preserving cardiac health. Citation Format: P. P. Advani, A. McPherson, J. Reddy, J. Schneider, S. Baheti, T. Nguyen, J. C. Ray, N. Norton. Genomic Insights into Anthracycline Cardiotoxicity: Enhancing Survivorship Care through Early Risk Identification [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS1-01-22.

Abstract Background: Assessing invasive lobular carcinoma (ILC) with standard-of-care (SOC) imaging, such as 18F-fluorodeoxyglucose (FDG) PET-computed tomography (CT), is challenging. In May 2025, National Comprehensive Cancer Center Network guidelines added FES PET for recurrent/metastatic ILC staging. However, perceptions of the value added by FES PET to SOC imaging may vary, as only small ILC cohorts have been published. We evaluated FES PET’s clinical utility in the largest-studied cohort, to our knowledge, of ILC pts who have undergone FES PET. Methods: We identified all ILC pts who had an FES PET [PET-magnetic resonance (MR) or PET-CT)] between April 1, 2021, and May 1, 2025, at Mayo Clinic. Clinical and radiological data, including indications and management (mgmt) changes related to pts’ first FES PET, were obtained from clinical records. FES and SOC FDG PET were compared. Data were summarized with descriptive statistics. Results: 102 female pts were included: 88% White and 4.9% Black. Median age at initial breast cancer (BC) diagnosis was 57 (range, 36-79). At initial ILC diagnosis, most tumors were strongly estrogen receptor (ER) positive. Highest tumor grade was 1 in 25% of pts and 2 in 57%, with grade not documented in 19%; none had documented grade 3 ILC. Initial clinical prognostic stage was as follows: 26% stage 1, 21% stage 2, 10% stage 3, and 16% stage 4. At the time of FES PET, 37% had biopsy-confirmed metastatic ILC at the time of FES PET. The most common indications for FES PET were (re)staging ILC (49%), detecting suspected metastatic/recurrent BC (30%), and evaluating rising tumor markers like CA 15-3 or CA 27.29 (15%). 16% of pts were upstaged due to the FES PET result: 14 pts to stage 4 and 2 pts to stage 3. FES PET led to a mgmt change in 38 pts (37%), for whom the most common FES PET indications were (re)staging ILC (21 pts), detecting suspected recurrent/metastatic BC (11 pts), and detecting ER expression when biopsy was not feasible/nondiagnostic (7 pts). 19 scans (50%) led to a systemic therapy (tx) decision, 10 (26%) to a biopsy, 4 (11%) to additional imaging like MRI, 3 (8%) to locoregional tx, and 2 (5%) to a non-oncologic specialty referral to evaluate a suspected new metastasis. 11 pts had stage 4 BC pre-FES PET; their FES PET was ordered primarily for restaging (5 pts) or detecting ER expression (5 pts). Notably, of the 19 pts with a systemic tx-related change, 4 began a triple-negative BC regimen due to low FES uptake on the scan. Of the 15 pts who had FES PET to evaluate a rising tumor marker, a direct change in systemic tx occurred in 2 pts. 63/102 pts (62%) had FDG PET-CT prior to FES PET. Of these, 57% had discordant FDG and FES PET results. 39% had a higher number of avid metastatic lesions on FES PET as compared to FDG PET, primarily in the bone (8 pts) and lymph nodes (4 pts). 25% had bone, nodal, or soft tissue metastatic lesions that were FDG but not FES avid. 13 pts had FES PET-MRI; 9 of these had a prior FDG PET-CT. 2 PET-MRI pts had FES-avid metastatic lesions in unusual sites (brain, peritoneum) that were not FDG avid. Among the 38 pts with a mgmt change due to their FES PET result, 63% had prior FDG PET; 32% of the 38 pts had FES-avid metastatic lesions that were not seen on FDG PET, and 8% had FDG-avid but not FES-avid lesions. Conclusions: FES PET was useful for evaluating not only stage 4, but also early-stage ILC, with 1/3 of pts with an FES PET-driven mgmt change having early-stage BC. FES PET was most helpful for ILC (re)staging, detection of suspected recurrent/metastatic BC, and detection of ER expression when biopsy was not feasible/nondiagnostic. FES PET added clinically meaningful information beyond SOC imaging, with detection of FES-avid but not FDG-avid metastatic lesions in 1/3 of pts with an FES PET-driven mgmt change. Notably, FES PET-MRI demonstrated potential superiority over FDG PET in identifying atypical ILC metastases. Citation Format: T. Pai, C. Bullock, N. Wiest, E. Parent, P. Advani. Clinical utility of 18F-fluoroestradiol positron emission tomography (FES PET) in patients (pts) with lobular breast cancer [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PD9-09.