Citations

Abstract Background: Triple-negative breast cancer (TNBC) is an aggressive yet immunogenic subtype of breast cancer (BC) and the only one with FDA-approved immune checkpoint inhibitor (ICI) - pembrolizumab. Currently, PD-L1 (22C3) is the companion biomarker for predicting pembrolizumab benefit, but its utility is limited by spatial/temporal heterogeneity, assay variability, and modest correlation with treatment response. Chemokines such as CXCL9, CXCL10, and CCL5 recruit CD8+ T cells into the tumor microenvironment and have been associated with improved response to ICI, but their predictive value alongside PD-L1 remains unclear. Here, we examined the relationship between these chemokines, PD-L1, and pembrolizumab efficacy in TNBC. Methods: We analyzed 3662 TNBC tumors profiled by NGS (592, NextSeq; WES/WTS, NovaSeq; Caris Life Sciences). Immune cells were estimated using WTS deconvolution (Quantiseq). PD-L1 positivity was defined as >10% CPS (PharmDx 22C3). CXCL9/CXCL10/CCL5-high (H) and -low (L) TNBC were classified by RNA expression above or below 50th percentile. Real-world median overall survival (mOS) was derived from insurance claims and calculated from start of pembrolizumab to last contact using Kaplan-Meier. Statistical significance was assessed by chi-square and Mann-Whitney U with multiple comparison adjustments (q<0.05). Results: TNBC expressed higher levels of CXCL9, CXCL10, and CCL5 (N = 3662, median TPM: 5.3, 14.8, and 15.1) compared to HER2+ (N = 1344; 4.8, 10.1, 10.6) and HR+HER2- BC (N = 6097; 2.7, 6.6, 9.4), all q<0.05. Moderate correlations were observed between PD-L1 CPS and each chemokine (r = 0.41, p<.01). PD-L1-positive tumors had better mOS (24.3 months (m) vs 18.6 m, HR 0.75, 95% CI 0.6-0.92, p=0.007) compared to PD-L1-negative TNBC post-pembrolizumab. Among PD-L1-negative tumors, those with CXCL9-H had better mOS (26.9 m vs 15.8 m for CXCL9-L, HR 0.61, 95% CI 0.45-0.82, p=0.001) post-pembrolizumab. Among PD-L1-positive tumors, CXCL10-H and CCL5-H was associated with improved mOS (CXCL10-H: 25.7 m vs. 20.7 m for CXC10-L, p = 0.03; CCL5-H: 26.7 vs. 18.5 m for CCL5-L, p = 0.03) post-pembrolizumab. PD-L1 negative CXCL9-H, PD-L1 positive CXCL10-H and PD-L1 positive CCL5-H groups showed higher infiltration of CD8+ T cell, B cells, M1/M2 macrophages, Tregs. They also exhibited higher T cell inflamed score, IFNy score and MAPK activation score vs. CXCL9-L, CXCL10-L and CCL5-L groups (Table). Conclusions: In TNBC, a chemokine signature comprising CXCL9, CXCL10 and CCL5 refines PD-L1 based prediction of pembrolizumab benefit. Specifically, high CXCL10 or CCL5 expression in PD-L1-positive tumors, and high CXCL9 expression in PD-L1-negative tumors, were associated with improved survival post pembrolizumab and immune-rich microenvironment. These findings support the use of chemokine profiling to complement PD-L1 and enhance patient selection for ICI therapy in TNBC. Citation Format: S. Gandhi, S. Deshmukh, S. Wu, J. Xiu, G. Sledge Jr., D. Trapani, J. Leone, S. Chumsri, M. Lustberg, C. Hong, S. Yao, K. Takabe, M. Ernstoff, M. Opyrchal, P. Chalasani, C. Paulos, G. Lesinski, A. Madabhushi, S. Badve, K. Kalinsky, P. Kalinski. Chemokine signatures improve PD-L1-based prediction of pembrolizumab response in triple-negative 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 PS2-08-14.

BACKGROUND We constructed multi-trait polygenic risk scores (PRSs) predicting chronic obstructive pulmonary disease (COPD) and exacerbations, validated their performance in diverse cohorts, and identified PRS-related proteins for potential therapeutic targeting. METHODS PRSmix + , a multi-trait PRS framework, is used to train a composite PRS (PRS multi ) in COPDGene non-Hispanic White participants ( n = 6,647). Associations of PRS multi with COPD status (GOLD 2–4 vs. GOLD 0 or ICD) and exacerbation frequency were tested in COPDGene African American ( n = 2,466), ECLIPSE ( n = 1,858), Mass General Brigham Biobank ( n = 15,152), and All of Us ( n = 118,566). Protein prediction models were applied to GWAS summary statistics from traits contributing to PRS multi and were validated with proteomic data in COPDGene ( n = 5,173) and UK Biobank ( n = 5,012). RESULTS PRSmix + selected 7 traits for PRS multi . In multivariable models, PRS multi was associated with COPD status (meta-analysis random effects [RE] OR 1.58 [95% CI: 1.28–1.94]) and exacerbation frequency (meta-analysis RE β 0.21 [95% CI: 0.11–0.31]), with higher effect sizes observed in smoking-enriched cohorts. PRS multi outperformed traditional single-trait PRS in all tested cohorts. Using protein prediction models, we identified 73 proteins associated with the PRSs that were also validated with measured protein levels in COPDGene and UK Biobank. Of these proteins, 25 were linked to approved or investigational drugs. Notable targets include RAGE/sRAGE, IL1RL1, and SCARF2, all implicated in COPD pathogenesis and exacerbations. CONCLUSIONS Multi-trait PRS improves prediction of COPD and exacerbation risk. Integration with proteomic data identifies druggable protein targets, offering a promising avenue for precision medicine in COPD management. TRIAL REGISTRATION COPDGene: ClinicalTrials.gov NCT00608764; ECLIPSE: ClinicalTrials.gov NCT00292552.

Abstract Background While immune checkpoint inhibitors (ICIs) provide durable clinical benefit in a subset of patients, current biomarkers to predict response lack accuracy. Ex vivo cytokine profiling has shown promise for improved accuracy but has been mostly limited to ICIs targeting PD-1 [Voabil et al, 2021]. This poses the question of whether ICIs beyond αPD-1 – such as CTLA4 blockade, which has been reported to act within the draining lymph node – would be amenable to similar approaches. The Elephas platform characterizes response to ICIs in live tumor fragments (LTFs) from core needle biopsies (CNBs) and resections using longitudinal cytokine profiling. Using a panel of 7 clinically relevant ICI regimens, we demonstrate cytokine responses across diverse tumor types, characterize similarities and differences in cytokine response profiles across different ICIs, and explore specimen variability in responses across ICI treatments. Methods CNBs were collected through 3 observational clinical trials (NCT05478538, NCT05520099, NCT0634962) and a biobank collection study, while resections were procured from NCI CHTN. LTFs were generated using an automated cutting device and encapsulated in hydrogel. LTFs were treated with mono (αPD-1, αPD-L1, αCTLA4, αLAG-3) or combination therapy (αPD-1/αCTLA4, αPD-1/αLAG-3, αPD-L1/αCTLA4) using a sequential treatment strategy in which isotype control followed by ICI were added to the same tissue in a single well. This approach enabled the comparison of control and treatment conditions while mitigating the effects of tumor heterogeneity. Production rates for 46 cytokines were measured using a multiplex assay. Remaining T-cell reactivity was also evaluated at the end of the assay to better understand the presence of functional T cells in LTFs. Results Unsupervised hierarchical clustering of cytokine profiling data from CNB LTFs across 129 patients identified defined responder and non-responder clusters. Within the response cluster, 4 subclusters with distinct immune phenotypes were identified. While tight clustering of samples treated with the same ICI was not observed, some enrichment of samples treated with combination ICIs was observed within individual subclusters. Further, cytokine responses to αCTLA4 monotherapy were observed, suggesting that CTLA4 blockade may act beyond the draining lymph node. LTFs from resections enabled the characterization of cytokine response profiles across treatment conditions within individual specimens, including the magnitude of cytokine response and variability in cytokine response profiles. Evaluation of T-cell reactivity provided further insight into the presence of functional T-cells in LTFs and the extent of T-cell activation achieved by ICI treatment. Conclusions This study demonstrates the platform’s ability to recapitulate diverse mechanisms of immune activation ex vivo, enabling the characterization of cytokine responses to clinically relevant ICI mono or combination therapy in live tumor tissue and supporting future efforts to improve ICI response prediction beyond αPD-1. Citation Format: Pichet Adstamongkonkul, Pierre Cunin, Santanu Bhattacharya, Tanmay Kulkarni, Amreen Nasreen, Nathan Marhefke, Nina Chen, T S. Ramasubramanian, Laura Hrycyniak, Julie Zweng, Erika von Euw, Christina Vivelo, Sean Caenepeel, Debabrata Mukhopadhyay, Hinco Gierman, Hilario Ramos. Characterization of αPD1, αPD-L1, αCTLA4, and αLAG3 as mono and combination therapy in live tumor fragments using an ex vivo cytokine profiling platform [abstract]. In: Proceedings of the AACR Immuno-Oncology Conference (AACR IO): Discovery and Innovation in Cancer Immunology: Revolutionizing Treatment through Immunotherapy; 2026 Feb 18-21; Los Angeles, CA. Philadelphia (PA): AACR; Cancer Immunol Res 2026;14(2 Suppl):Abstract nr B032.

Abstract Few prospective benchmark studies exist to characterize the evolving contemporary real-world practice for peripheral T-cell lymphoma (PTCL). We report the patterns of first-line care and outcomes for 720 patients with systemic PTCL enrolled in 2 related prospective cohort studies, Lymphoma Epidemiology of Outcomes (LEO) from 2015 to 2020 and Molecular Epidemiology Resource (MER) from 2002 to 2015, both followed to 2024. The primary end points were event-free survival (EFS) and overall survival (OS) using Kaplan-Meier estimator and Cox regression model. Secondary end points included correlations of clinical and treatment factors with survival. The most common induction regimens were CHOP (cyclophosphamide, doxorubicin, vincristine, prednisone) based (70%), given as CHOP (36%), CHOP plus etoposide (23%), or CHOP-like plus novel agents (11.5%, including 5% BV-CHP [brentuximab plus cyclophosphamide, doxorubicin, and prednisone]). Consolidative autologous stem cell transplant was performed in 102 patients (14%). Within nodal PTCL, EFS and OS were adversely associated with International Prognostic Index 2 to 5, prognostic index for T-cell lymphoma score 1 to 4, and non–anaplastic large-cell lymphoma (ALCL) subtypes. Within LEO, which captured increasing first-line etoposide and brentuximab vedotin, adding etoposide to CHOP was associated with better OS in anaplastic lymphoma kinase-negative ALCL. BV-CHP showed a trend toward OS improvement in ALCL. Patients failing EFS6 and EFS24 had 5-year subsequent OS of 12% and 17%, respectively. The inferior outcomes in non-ALCL subtypes and patients failing EFS6 and EFS24 highlight unmet needs with CHOP-based induction, where clinical trials with targeted therapy should be prioritized. This trial was registered at www.clinicaltrials.gov as NCT02736357.

Abstract Background: Pathogenic variants (PVs) in PALB2 are known to confer high-to-moderate risk for breast cancer. However, breast cancer risk estimates for PALB2 PVs for different populations are not well defined and associations with other cancers are still unclear. Using data from a large clinical genetic testing cohort we have established cancer-specific risks associated with PALB2 PVs. Methods: We analyzed the associations of PALB2 PVs with breast, ovarian, prostate, and pancreatic cancers in individuals tested at Ambry Genetics. Cancer-specific odds ratios (ORs) and 95% confidence intervals (CI) were estimated by comparing PV carrier frequencies in cancer cases to population controls from AllofUs matched by sex and adjusting for age and genetic ancestry and gnomAD v4.1 matched by sex and adjusting for ancestry. Two-sided p-values are reported. Results: Of 1,119,667 individuals tested, 357,109 had breast cancer, 37,468 ovarian cancer, 22,112 pancreatic cancer and 32,738 prostate cancer. PALB2 PVs were observed in 0.79% of breast cancer cases, 0.30% of ovarian cancer cases, 0.78% of pancreatic cancer cases, and 0.47% of prostate cancer cases, with mean age at diagnosis of 54.2 years for breast, 60.1 years for ovarian, 62.8 years for pancreatic and 66.7 years for prostate. Whereas only 2.4% of breast cancer cases with PVs had a personal history of ovarian or pancreatic, 13.4% of ovarian cases and 33.3% of pancreatic cases had a history of breast cancer. Analysis of PV types found that frameshift PVs accounted for 50% of PVs, nonsense for 30%, canonical splice sites for 5% and copy number variants for 8-9% of cases in all cancer types. Among 10,077 individuals with multiple primary cancers, PALB2 PVs were detected in 0.65% of cases, with the highest frequencies observed in combinations of breast and pancreatic cancer (1.87%) and pancreatic and prostate cancer (0.79%). To refine estimates of risks for breast cancer, association studies were performed separately with GnomAD v4.1 and AllofUs controls. PALB2 PVs were consistently associated with high risks of breast cancer with lower 95%CI above 4 (GnomADv.4.1 OR=6.85, 95%CI=5.94-7.96, p=3.05x10-145; AllofUs OR=5.65, 95%CI=4.83-6.65, p=2.54x10-155). This was significantly different and increased over breast cancer risk estimates from the CARRIERS and BRIDGES population-based studies (OR=4.67, 95%CI=3.73-5.85, p=3.95x10-27) and the UK Biobank population-based study (OR=3.87, 95%CI=3.09-4.83, p=4.62x10-27). Similarly, high risks of pancreatic cancer were confirmed (GnomADv.4.1 OR=5.14, 95%CI=4.15-6.32, p=3.38x10-52; AllofUs OR=5.59, 95%CI=3.92-7.85, p=2.78x10-18). Importantly, associations between PALB2 PVs and moderate risks of both ovarian cancer (GnomADv.4.1 OR=2.68, 95%CI=2.11-3.40, p=5.94x10-16; AllofUs OR=2.15, 95%CI=1.62-2.85, p=2.97x10-7) and prostate cancer (enriched for aggressive disease) (GnomADv.4.1 OR=2.45, 95%CI=2.01-2.97, p=1.52x10-19) were observed. Conclusions: This large-scale study provides robust estimates of PALB2 PV prevalence and associated cancer risks in a U.S. clinical genetic testing population. The findings provide refined risk estimates for breast and pancreatic cancers but also establish that PALB2 PVs confer increased risk for ovarian and prostate cancers. These results help clarify ongoing uncertainty, particularly for ovarian and prostate cancer, and highlight the importance of PALB2 in broader hereditary cancer risk assessment and multigene panel testing. Citation Format: Y. Y. Tan, C. Hu, H. Huang, N. J. Boddicker, W. Chen, M. Kaul, T. J. Rao, A. N. Monteiro, T. Pesaran, R. Karam, S. M. Domchek, S. Yadav, M. E. Richardson, F. J. Couch. Cancer-specific risks associated with germline PALB2 pathogenic variants from a large clinical genetic testing cohort [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 RF5-01.

Abstract Background: Approximately 50% of newly diagnosed invasive breast cancers are stage 1, with the majority being ER/PR-positive, HER2-negative. Genomic assays such as the Oncotype DX® have identified patients (pts) with reduced risk of distant metastasis and without benefit from chemotherapy added to endocrine therapy (ET), freeing them from excess toxicity. Genomic assays are also recognized as prognostic for in-breast recurrence (IBR) after breast-conserving surgery (BCS) and could similarly allow de-escalation of adjuvant radiotherapy (RT). Reducing overtreatment is of interest to pts, providers, and payers. Methods: We hypothesize that BCS alone is non-inferior to BCS plus RT for IBR and breast preservation in women intending ET for stage 1 invasive breast cancer (ER and/or PR-positive, HER2-negative with an Oncotype DX Recurrence Score [RS] of ≤18). Stratification is by age (<60; ≥60), tumor size (≤1 cm; >1-2cm), and RS (≤11, >11-18/MammaPrint Low). Pts are randomized post-BCS to Arm 1 with breast RT using standard methods (moderate or ultra hypo- or conventional-fractionated whole breast RT with/without boost, or APBI) with ≥5 yrs of ET (tamoxifen or AI) or Arm 2 with ≥5 yrs of ET (tamoxifen or AI) alone. The specific regimen of ET in both arms is at the treating physician’s discretion. Eligible pts are stage 1: pT1 (≤2 cm), pN0, age ≥50 to <70 yrs, s/p BCS with negative margins (no ink on tumor), s/p axillary nodal staging (SNB or ALND), ER and/or PR-positive (ASCO/CAP), HER2-negative (ASCO/CAP), and Oncotype DX RS ≤18 (diagnostic core biopsy or resected specimen). A “low risk” MammaPrint is permissible if completed as part of usual care prior to screening. Primary endpoint is IBR (invasive breast cancer or DCIS). Secondary endpoints are breast conservation rate, invasive in-breast recurrence, relapse-free interval, distant disease-free survival, overall survival, patient-reported breast pain, patient-reported worry about recurrence, and adherence to ET. We assume a clinically acceptable difference in IBR of 4% at 10 yrs to judge omission of RT as non-inferior (10-yr event-free survival for RT group is 95.6% v 91.6% for the omission-of-RT group). BR007 is powered to detect non-inferiority with 80% power and a one-sided α=0.025, assuming that there would be a ramp-up in accrual in the first two years (leveling off in Yrs 3-5); 1,670 pts (835 per arm) are required for randomization. Conservative loss to follow-up is 1%/yr. Some T1a pts screened may have Oncotype DX scores >18, making them ineligible for the study. In the accrual process, 1,714 pts will be required to register to ensure that our final randomized cohort is 1,670 pts. As of July 9, 2025, 1,490 pts have been screened and 1,349 randomly assigned. NCT #: NCT04852887 Support: U10 CA180868, -180822, U24 CA196067, UG1 CA189867 Citation Format: J. R. White, R. S. Cecchini, E. E. Harris, E. P. Mamounas, J. G. Bazan, D. G. Stover, P. A. Ganz, R. Jagsi, S. J. Anderson, C. Bergom, V. Théberge, M. B. El-Tamer, R. C. Zellars, D. A. Shumway, G. Chen, T. B. Julian, N. Wolmark. A phase III trial evaluating De-escalation of Breast Radiation (DEBRA) following breast-conserving surgery of stage 1, HR+, HER2-, RS ≤18 breast cancer: NRG-BR007 [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 PS5-08-20.

Abstract Background PALLAS trial compared palbociclib plus endocrine therapy (ET) vs. ET alone in stage II-III HR+/HER2- breast cancer. SETER/PR index quantifies transcriptional activity related to estrogen and progesterone receptors and serves as a measure of endocrine sensitivity. In CALGB 9741, low SETER/PR (< 0.75) - reflecting endocrine resistance - predicted survival benefit from adjuvant anthracycline/taxane-based treatment. In PALLAS low SETER/PR was associated with worse prognosis and early recurrence. Early recurrences driven by endocrine-resistant tumors may obscure the benefit of palbociclib by disproportionately contributing events from a more resistant population. We hypothesized that palbociclib efficacy might vary by endocrine sensitivity and prior chemotherapy regimen, with greatest benefit in patients with intermediate SETER/PR index. Methods Invasive disease-free survival (iDFS) was summarized with Kaplan-Meier plots and the treatment arms were compared with a log-rank test. Cox models estimated the hazard ratio (HR) with 95% confidence interval (95%CI) (significance level used was a two-sided 0.05). SETER/PR index was successfully obtained for 3,090 PALLAS patients (TransPALLAS ITT cohort, Metzger et al. ASCO 2024). SETER/PR index cut points (0.75 and 1.50) were proposed from previous studies comparing different adjuvant chemotherapy regimens. Results Table 1 summarizes the efficacy of adjuvant palbociclib by prior (neo)adjuvant chemotherapy regimen. Palbociclib was associated with longer iDFS in the group who received prior anthracycline-paclitaxel chemotherapy (AC-P), though the chemo regimen x palbociclib interaction term was not significant (Table 1). In the 1,132 patients from the TransPALLAS cohort who received AC-P chemotherapy, the 518 (46%) patients with intermediate SETER/PR (0.75≤ SETER/PR <1.50) had longer iDFS with palbociclib (HR 0.58; 95% CI: 0.40-0.84; p=0.004; 7-yr iDFS 80.2% vs. 68.4%), but this subset did not predict palbociclib benefit relative to all others, i.e., 206 (18%) patients with low SETER/PR (<0.75) and 408 (36%) high SETER/PR (≥1.50); P interaction=0.11. Exploratory analysis of SETER/PR index as a continuous variable in patients who received prior AC-P chemotherapy revealed a non-linear relationship with the hazard ratio favoring palbociclib observed in the subset with SETER/PR values between 1.1 and 1.8, compared to all others (P interaction=0.01). Conclusions In PALLAS, we observed a numerical benefit favoring palbociclib among pts treated with adjuvant AC-P; in this subset a moderate level of endocrine transcriptional activity (SETER/PR index between 1.1 and 1.8) predicted palbociclib benefit. These results are hypothesis-generating. Citation Format: O. Metzger, P. O'Brien, K. Ballman, M. Gnant, M. Watson, E. Chen, K. Tran, D. Hlauschek, M. Martin, J. Balko, Z. Nowecki, O. Hahn, C. Denkert, C. Curtis, F. Liu, A. Dueck, C. Fesl, E. Mayer, A. De Michele, W. Symmans. Exploratory analysis of palbociclib benefit in the PALLAS trial by SETERPR index and prior chemotherapy regimens (ABCSG-42/AFT-05/BIG-14-13) [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 PS3-08-13.

Abstract Background: BRCA2 variants with partially aberrant RNA splicing have been associated with a relatively lower breast cancer risk (reduced penetrance) relative to canonical pathogenic variants. However, the existence of partial loss of function (hypomorphic) missense variants, conferring reduced penetrance, is less certain. Identification and assessment of the clinical relevance of such reduced penetrance pathogenic variants (RPPVs) is needed to provide accurate risk estimates. Methods: Comprehensive functional characterization of hypomorphic missense variants (n=70) in the BRCA2 DNA-binding domain, initially identified through homology-directed DNA repair (HDR) assays, was conducted through multiple independent functional assays, including HDR, CRISPR/Cas9-based endogenous targeting, RAD51 foci formation assay, and Poly(ADP-ribose) polymerase (PARP) inhibitor response assay. Risk estimates were obtained using two large datasets from patients referred for hereditary cancer testing. A ClinGen/ACMG/AMP model was used for clinical classification. Results: The variants consistently demonstrated hypomorphic HDR activity and endogenous survival in HAP1 targeted cells, decreased RAD51 foci formation, and increased sensitivity to PARP inhibitors. These variants were associated with a moderate increase in breast cancer risk, as shown in two independent studies: one using multivariable logistic analysis adjusted for age, ancestry and family history (OR=2.41, 95%CI=1.51-3.85) and another comparing frequencies of variants in breast cancer cases to public reference controls adjusted for ancestry (OR=2.09, 95% CI=1.16-4.08). The mean age of breast cancer diagnosis for carriers of hypomorphic variants was 57 years, which was significantly older than the mean age of diagnosis of 51 years for carriers of protein truncating variants. Functional data were combined with other evidence sources in a ClinGen/ACMG/AMP model to classify 14 variants as “likely pathogenic”. Conclusions: We identified partially functional, hypomorphic missense variants associated with reduced penetrance for breast cancer. Cancer risk management guidelines for this new category of variants should be revisited given that the associated breast cancer risks warrant high risk screening but do not meet the threshold at which risk reducing mastectomy is offered. This study supports a shift in paradigm to considering variant-based risks on a continuum, with implications for risk-appropriate cancer risk management. Citation Format: H. Huang, C. Hu, J. Na, M. Kucera, T. Simmons, E. Mundt, C. C. Young, Z. Heidari, P. C. Lyra, Y. Y. Tan, K. L. Nathanson, T. Pal, R. Karam, T. Pesaran, S. Yadav, S. M. Domchek, A. N. Monteiro, E. Hughes, N. Boddicker, W. Chen, M. E. Richardson, F. J. Couch. Partially functional (hypomorphic) missense variants in BRCA2 are reduced penetrance pathogenic variants [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 PD4-05.

Abstract BACKGROUND: EOP is an I-SPY2 sub-study designed to test novel endocrine-based strategies in patients (pts) predicted to have modest benefit from chemotherapy. Vepdegestrant is a selective oral PROTAC estrogen receptor (ER) degrader that has demonstrated efficacy in mutant ER+ HER2-negative MBC. METHODS: EOP eligibility included pts with Stage 2/3 HR+ HER2-negative, MammaPrint (MP) Low Risk BC. Patients with MP High1 signatures were eligible if clinically node-negative or SET 2,3 High. Pts were randomized to one of 3 oral treatment (tx) arms: vepdegestrant 200 mg/d (V), vepdegestrant 200 mg/d +letrozole (VL), vepdegestrant 200 mg/d + abemaciclib 150 mg bid (VA). Pts received tx for six 28-day cycles followed by surgery. Premenopausal (PreMN) pts received ovarian function suppression (OFS) starting C1D1 in the VL arm, and C2D1 in the V and VA arms. The primary endpoint was feasibility, defined as >75% of pts completing >75% of study therapy. Breast MRI functional tumor volume (FTV) and ctDNA (tumor-informed assay) were assessed at baseline (T0), 3 weeks (T1), 12 weeks (T2), and 6 months (T3). Ki67 and ER/PR by IHC were centrally assessed from tumor biopsies at T0, T1, and the surgical specimen. Results: Between 4/2023 and 1/2025, 121 pts were enrolled: 40 in V, 41 in VL, and 40 in VA. Median age was 54 years, 50% preMN. 60% of pts had ductal, 35% lobular, 5% mixed histology. 55% were cN+, and 59% were not breast conservation surgery (BCS) candidates at baseline. Mean baseline Ki67 was 12.5%, 85.1% had MP Low Risk signature, 79.3% were SET 2,3 High. All 3 tx arms met the primary endpoint with 88%, 88%, and 83% completing > 75% study therapy in V, VL, and VA (thus far), respectively. Most common all grade (AG) TRAEs in V and VL arms include fatigue (53% V, 71% VL), hot flashes (48% V, 71% VL), arthralgias (33% V, 49% VL), all G1 or G2 except for 1 G3 fatigue in VL arm. G2 and G3 neutropenia was seen in 1 pt each in V and VL arms. The most common AG TRAEs in VA include diarrhea (80%), fatigue (65%), nausea (58%), hot flashes (50%), neutropenia (43%). High grade TRAEs in VA include neutropenia (23% G3, 5% G4), ovarian cyst requiring surgery (5% G3), diarrhea (15% G3), nausea (3% G3), transaminitis (3% G3), thromboembolic event (8% G3), acute kidney injury (3% G3). Table 1 summarizes key response endpoints by tx arm. CONCLUSION: Six months of neoadjuvant vepdegestrant, alone or in combination with letrozole or abemaciclib, is feasible and demonstrates robust ER/PR degradation. Encouraging anti-tumor activity through reduction of Ki67 and MRI FTV, ctDNA clearance, and achievement of BCS was seen in pre- and postMN women. The neoadjuvant setting provides a rich platform to identify the dynamic range of response and non-response using a multi-modal approach and can be leveraged to inform future trial design and adjuvant therapy. Citation Format: J. Chien, R. M. Mukhtar, C. Yau, A. D. Elias, A. M. Wallace, N. Chan, C. Omene, N. Chen, J. Tseng, M. S. Trivedi, E. Stringer-Reasor, D. Yee, A. S. Clark, A. Thomas, H. Han, M. Arora, C. Nangia, K. Albain, C. Falkson, C. Isaacs, A. Zimmer, M. Rozenblit, L. van't Veer, L. Brown Swigart, G. Hirst, W. Symmans, A. D. Borowsky, N. Onishi, N. Hylton, S. Alkhafaji, K. V. Giridhar, C. Vaklavas, M. Wei, M. P. Goetz, O. Olopade, L. Huppert, C. Wu, L. J. Esserman. I- SPY2 Endocrine Optimization Pilot (EOP): Neoadjuvant vepdegestrant monotherapy or in combination with letrozole or abemaciclib in molecularly selected patients with stage 2/3 HR+ HER2-negative breast cancer (BC) [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 PD10-02.