Poster Spotlight 10: Novel Combinations with Endocrine Therapy

M. Bellet¹, P. Tolosa², C. Hernando³, M. Vidal⁴, V. Ortega⁵, M. Tapia⁶, S. González-Santiago⁷, P. Sánchez⁸, Y. Fernández⁹, M. Cruellas¹⁰, M. Alva¹¹, E. Mension¹², M. Espinosa-Bravo¹³, T. Cortadellas¹⁴, S. Aragón¹⁵, M. Gaudio¹⁶, E. Sanfeliu¹⁷, P. Galván¹⁸, R. Olivera¹⁹, G. Villacampa¹⁹, M. Bergamino⁴, N. Santos¹⁹, S. Cano-Crespo¹⁹, X. González-Farré²⁰, A. Prat⁴, J. M. Ferrero-Cafiero¹⁹, A. Hurtado²¹, T. Pascual⁴; ¹Medical Oncology Department, Vall d’Hebron Institute of Oncology (VHIO), Hospital Universitari Vall d’Hebron / Medicine Department, Autonomous University / SOLTI Breast Cancer Research Group, Barcelona, SPAIN, ²Medical Oncology Department, University Hospital 12 de Octubre / Instituto de Investigación Sanitaria Hospital 12 de Octubre (imas12) / SOLTI Breast Cancer Research Group, Madrid, SPAIN, ³Medical Oncology Department, Valencian Institute of Oncology (IVO), IVO Hospital, Valencia, SPAIN, ⁴Medical Oncology Department, Hospital Clinic de Barcelona / Translational Genomics and Targeted Therapies in Solid Tumors, August Pi i Sunyer Biomedical Research Institute (IDIBAPS) / Medicine Department, University of Barcelona / SOLTI Breast Cancer Research Group, Barcelona, SPAIN, ⁵Breast Cancer Unit, Instituto Oncológico Rosell (IOR), Hospital Universitari General de Catalunya, Barcelona, SPAIN, ⁶Medical Oncology Department, Hospital Clínico Universitario de Valencia, Valencia, SPAIN, ⁷Medical Oncology Department, Hospital Universitario San Pedro de Alcántara, Cáceres, SPAIN, ⁸Medical Oncology Department, Hospital Clínico Universitario Virgen de la Arrixaca, Murcia, SPAIN, ⁹Medical Oncology Department, Central University Hospital of Asturias, Asturias, SPAIN, ¹⁰Medical Oncology Department, Hospital Clínico Universitario Lozano Blesa, Zaragoza, SPAIN, ¹¹Medical Oncology Department, University Hospital 12 de Octubre / Instituto de Investigación Sanitaria Hospital 12 de Octubre (imas12), Madrid, SPAIN, ¹²Department of Gynecology, Hospital Clinic de Barcelona / Translational Genomics and Targeted Therapies in Solid Tumors, August Pi i Sunyer Biomedical Research Institute (IDIBAPS), Barcelona, SPAIN, ¹³Breast Surgical Unit, Breast Cancer Center, Hospital Universitari Vall d’Hebron / International Breast Cancer Center (IBCC). Pangaea Oncology. Centro Médico Teknon. QuirónSalud, Barcelona, SPAIN, ¹⁴Breast Cancer Unit, Hospital Universitari Dexeus, Universitat Internacional de Catalunya, Barcelona, SPAIN, ¹⁵Gynecology and Obstetrics Department, University Hospital 12 de Octubre / Instituto de Investigación Sanitaria Hospital 12 de Octubre (imas12), Madrid, SPAIN, ¹⁶Medical Oncology Department, Vall d’Hebron Institute of Oncology (VHIO), Hospital Universitari Vall d’Hebron, Barcelona, SPAIN, ¹⁷Department of Pathology, Hospital Clinic de Barcelona / Medicine Department, University of Barcelona / Translational Genomics and Targeted Therapies in Solid Tumors, August Pi i Sunyer Biomedical Research Institute (IDIBAPS), Barcelona, SPAIN, ¹⁸Medical Oncology Department, Translational Genomics and Targeted Therapies in Solid Tumors, August Pi i Sunyer Biomedical Research Institute (IDIBAPS), Barcelona, SPAIN, ¹⁹Clinical Research, SOLTI Breast Cancer Research Group, Barcelona, SPAIN, ²⁰Breast Cancer Unit, Hospital Universitari General de Catalunya, Universitat Internacional de Catalunya / SOLTI Breast Cancer Research Group, Barcelona, SPAIN, ²¹FICUS Research Group, Cancer Research Center (CIC), University of Salamanca, Salamanca, SPAIN.

Background: Premenopausal women represent nearly 30% of patients (pts) with ER+/HER2- early breast cancer (EBC). Ovarian function suppression (OFS) in combination with tamoxifen or aromatase inhibitors improves disease-free survival but has limitations such as toxicity, suboptimal estrogen suppression, and poor adherence. Elacestrant (ELA) showed superior efficacy vs standard endocrine therapy (ET) in pretreated pts with ER+/HER2- metastatic breast cancer. Mechanistically, ELA acts as a selective ER degrader while exhibiting agonistic activity in non-breast endocrine-responsive tissues like bone, making it particularly suitable for the adjuvant setting. In the SOLTI-ELIPSE trial, 4-week preoperative ELA in ER+/HER2- EBC postmenopausal pts led to a 53% reduction in Ki67 and complete cell cycle arrest (CCCA) rate of 27%. We hypothesize that ELA may serve as an effective ET in the premenopausal EBC setting, with or without OFS. Methods: SOLTI-2104-PremiÈRe (NCT05982093) is a phase 2, non-comparative, open-label, randomized, trial assessing ELA ± triptorelin (T) in premenopausal pts with stage I-IIB ER+/HER2- EBC, age ≥35 years, cT>1 cm, and Ki67 10-35% by local assessment. Pts were stratified by PAM50 subtype (Luminal A vs Non-luminal A) and randomized to ELA 345 mg daily until surgery/biopsy at D28, alone or with T 3.75 mg on days 1 and 29. The primary endpoint was CCCA at D28 (central Ki67 ≤2.7%). Secondary endpoints included CCCA by subtype, changes in central Ki67, changes in gene expression from baseline, and safety. Changes in gene expression were evaluated per arm (paired samples), with P-values corrected using false discovery rate (FDR). Estrogen levels were monitored by LC-MS/MS. Results: From SEP 2023 to MAR 2025, 49 pts were randomly assigned to receive ELA (n=23) and ELA+T (n=26). Baseline characteristics: median age 47; caucasian 92%; cT1-T2 96%, N1 6%; grade 1-2 94%; central Ki67 median 20% in the ELA arm and 16% in ELA+T. PAM50 subtypes were 75.5% Luminal A and 24.5% Luminal B. CCCA was achieved in 28.6% and 26.9% in ELA and ELA+T, arm respectively and in a higher rate in Luminal A tumors. Ki67 was significantly reduced in both arms. Both arms exhibited a shift to a less proliferative phenotype after treatment (Table 1). PAM50 Risk of Recurrence (ROR)-high/medium switched to ROR- low, and PAM50 Luminal B subtype switched to Luminal A/Normal-like. Treatment showed a toxicity profile consistent with previous data. Conclusions: This study provides the first evidence that ELA ± OFS elicits antiproliferative and molecular responses in premenopausal women with ER+/HER2- EBC. Both regimens had comparable and significant reductions in Ki67, CCCA, ROR-P scores, and proliferation gene expression. PremiÈRe trial support the potential role of ELA as a novel ET in this population, potentially sparing the need for OFS.

J. Chien¹, R. M. Mukhtar², C. Yau², A. D. Elias³, A. M. Wallace⁴, N. Chan⁵, C. Omene⁶, N. Chen⁷, J. Tseng⁸, M. S. Trivedi⁹, E. Stringer-Reasor¹⁰, D. Yee¹¹, A. S. Clark¹², A. Thomas¹³, H. Han¹⁴, M. Arora¹⁵, C. Nangia¹⁶, K. Albain¹⁷, C. Falkson¹⁸, C. Isaacs¹⁹, A. Zimmer²⁰, M. Rozenblit²¹, L. van’t Veer²², L. Brown Swigart²², G. Hirst², W. Symmans²³, A. D. Borowsky²⁴, N. Onishi²⁵, N. Hylton²⁵, S. Alkhafaji²², K. V. Giridhar²⁶, C. Vaklavas²⁷, M. Wei²⁷, M. P. Goetz²⁸, O. Olopade²⁹, L. Huppert¹, C. Wu²⁵, L. J. Esserman²; ¹Medicine, University of California San Francisco, San Francisco, CA, ²Surgery, University of California San Francisco, San Francisco, CA, ³Medicine, University of Colorado, Aurora, CO, ⁴Surgery, University of California San Diego, La Jolla, CA, ⁵Medicine, New York University Langone, New York, NY, ⁶Medical Oncology, Rutgers Cancer Institute, New Brunswick, NJ, ⁷Medicine, University of Chicago, Chicago, IL, ⁸Surgery, City of Hope National Medical Center, Duarte, CA, ⁹Hematology/Oncology, Columbia University Irving Medical Center, New York, NY, ¹⁰Hematology/Oncology, UAB O’Neal Comprehensive Cancer Center, Birmingham, AL, ¹¹Medicine and Pharmacology, University of Minnesota, Minneapolis, MN, ¹²Medicine, University of Pennsylvania, Philadelphia, PA, ¹³Medicine, Duke Cancer Center, Durham, NC, ¹⁴Breast Oncology, Moffitt Cancer Center, Tampa, FL, ¹⁵Medicine, University of California Davis Comprehensive Cancer Center, Sacramento, CA, ¹⁶Medical Oncology, HOAG Cancer Center, Irvine, CA, ¹⁷Medicine, Loyola University Chicago Stritch School of Medicine, Maywood, IL, ¹⁸Medicine, University of Rochester Pluta Cancer Center, Rochester, NY, ¹⁹Medicine, Georgetown University Lombardi Cancer Center, Washington, DC, ²⁰Medicine, Oregon Health and Sciences University, Portland, OR, ²¹Medicine, Yale Cancer Center, New Haven, CT, ²²Laboratory Medicine, University of California San Francisco, San Francisco, CA, ²³Pathology, University of Texas MD Anderson Cancer Center, Houston, TX, ²⁴Pathology and Laboratory Medicine, University of California Davis, Sacramento, CA, ²⁵Radiology and Biomedical Imaging, University of California San Francisco, San Francisco, CA, ²⁶Medical Oncology, Mayo Clinic Comprehensive Cancer Center, Rochester, MN, ²⁷Medicine, University of Utah Huntsman Cancer Institute, Salt Lake City, UT, ²⁸Oncology and Pharmacology, Mayo Clinic Comprehensive Cancer Center, Rochester, MN, ²⁹Hematology/Oncology, University of Chicago, Chicago, IL.

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.

E. P. Hamilton¹, M. De Laurentiis², K. Jhaveri³, X. Hu⁴, S. Ladoire⁵, A. Patsouris⁶, C. Zamagni⁷, J. Cui⁸, M. Cazzaniga⁹, T. Cil¹⁰, K. J. Jerzak¹¹, C. Fuentes¹², T. Yoshinami¹³, A. Rodriguez-Lescure¹⁴, O. Valota¹⁵, D. R. Lu¹⁶, M. Martignoni¹⁷, S. Dychter¹⁸, M. Lachowicz¹⁹, X. Zhi²⁰, M. Campone²¹; ¹Breast Cancer Research Program, Sarah Cannon Research Institute, Nashville, TN, ²Department of Breast and Thoracic Oncology, Istituto Nazionale Tumori IRCCS “Fondazione G. Pascale, Napoli, ITALY, ³Medicine Department, Memorial Sloan Kettering Cancer Center, New York, NY, ⁴Shanghai Cancer Center, Fudan University, Shanghai, CHINA, ⁵Oncology Department, Centre Georges Francois Leclerc, Dijon, FRANCE, ⁶Department of Medical Oncology, Institut de Cancérologie de l’Ouest Angers-Saint-Herblain, Angers, FRANCE, ⁷Medical and Surgical Sciences, IRCCS Azienda Ospedaliero Universitaria di Bologna, Bologna, FRANCE, ⁸Department of Oncology, The First Hospital of Jilin University, Changchun, CHINA, ⁹Phase 1 Research Unit, Fondazione IRCCS San Gerardo dei Tintori, Monza, ITALY, ¹⁰Health and Science University, Adana City Hospital, Yüreğir/Adana, TURKEY, ¹¹Odette Cancer Centre, Sunnybrook Health Sciences Centre, University of Toronto, Toronto, ON, CANADA, ¹²Breast Cancer Research Program, Sarah Cannon Research Institute, Buenos Aires, ARGENTINA, ¹³Graduate School of Medicine, Osaka University, Osaka, JAPAN, ¹⁴Medical Oncology Department, Hospital General Universitario de Elche, Alicante, SPAIN, ¹⁵Clinical, Pfizer, S.r.l., Milano, ITALY, ¹⁶Clinical Statistics, Pfizer, Inc., San Diego, CA, ¹⁷Clinical Research, Pfizer, S.r.l., Milano, ITALY, ¹⁸Clinical Research, Pfizer, Inc., San Diego, CA, ¹⁹Clinical Research, Arvinas Operations, Inc., New Haven, CT, ²⁰Biostatistics & Programming, Arvinas Operations, Inc., New Haven, CT, ²¹Medical Oncology Department, Institut deCancérologie de l’Ouest Angers-Nantes, Saint Herblain, FRANCE.

Background: Vepdegestrant is an oral PROTAC ER degrader. The VERITAC-2 study (NCT05654623), the first phase 3 trial of a PROTAC, demonstrated statistically significant and clinically meaningful prolongation of progression-free survival (PFS) with vepdegestrant versus fulvestrant in patients (pts) with ER+/HER2- aBC who were previously treated with endocrine therapy (ET) and a cyclin-dependent kinase 4/6 inhibitor (CDK4/6i) and had ESR1 mutations (ESR1m). Here, we report the results of prespecified subgroup analyses of PFS among pts with ESR1m. Methods: Eligible pts (aged ≥18 years) with ER+/HER2- aBC had received 1 prior line of CDK4/6i + ET, and ≤1 additional line of ET (the most recent line of ET must have been given for ≥6 months before disease progression); pts with prior chemotherapy in the metastatic setting or prior fulvestrant were excluded. Pts were randomized (1:1) to receive vepdegestrant 200 mg orally once daily or fulvestrant 500 mg intramuscularly on days 1 and 15 of cycle 1 and on day 1 of subsequent 28-day cycles; randomization was stratified by ESR1m status and the presence of visceral disease. The primary endpoint, PFS by blinded independent central review (BICR), was assessed across prespecified clinically relevant subgroups without adjustments for multiplicity. Results: In total, 270 pts with ESR1m were randomized to vepdegestrant (n=136) or fulvestrant (n=134). Across groups, the median age was 60.0 years, 21% of pts were premenopausal or perimenopausal, 80% had received prior CDK4/6i therapy for ≥12 months, 45% had liver metastases, and 42% had a PIK3CA, AKT1, or PTEN mutation at baseline. Consistent with the primary analysis, benefit was observed in PFS by BICR with vepdegestrant versus fulvestrant across all prespecified subgroups (Table). Conclusions: Vepdegestrant demonstrated PFS benefit compared with fulvestrant across all prespecified, clinically relevant subgroups of previously treated pts with ESR1m ER+/HER2- aBC. These analyses provide further information in key prognostic patient subgroups that may inform clinical treatment decisions for pts with ESR1m.

NE=not estimable.

A. Brufsky¹, F. Bidard², E. L. Mayer³, Y. Park⁴, W. Janni⁵, C. Ma⁶, M. Cristofanilli⁷, G. Bianchini⁸, H. Iwata⁹, P. A. Fasching¹⁰, Z. Nowecki¹¹, J. Pascual¹², S. Chen¹³, L. Moreau¹⁴, M. Ruiz¹⁵, A. Shai¹⁶, N. Karadurmus¹⁷, K. Jung¹⁸, Y. Kikawa¹⁹, R. Baird²⁰, C. Arizmendi²¹, I. Leddin²², S. McClain²³, C. Huang Bartlett²³, N. Turner²⁴; ¹-, UPMC Magee-Womens Hospital, Pittsburgh, PA, ²-, Institut Curie, Paris, FRANCE, ³-, Dana-Farber Cancer Institute, Boston, MA, ⁴Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, KOREA, REPUBLIC OF, ⁵-, Universitätsklinikum Ulm, Ulm, GERMANY, ⁶-, Washington University School of Medicine, Saint Louis, MO, ⁷-, Weill Cornell Medicine / New York-Presbyterian Hospital, New York, NY, ⁸-, IRCCS Ospedale San Raffaele, Milan, ITALY, ⁹-, Nagoya City University, Nagoya, JAPAN, ¹⁰University Hospital Erlangen, Comprehensive Cancer Center Erlangen-EMN, Erlangen, GERMANY, ¹¹-, Narodowy Instytut Onkologii im. Marii Skłodowskiej-Curie, Warsaw, POLAND, ¹²-, Hospital Universitario Virgen de la Victoria, Málaga, SPAIN, ¹³-, Chang Gung Medical Foundation Linkou Branch, Taoyuan City, TAIWAN, ¹⁴-, Pôle Santé République, Clermont-Ferrand, FRANCE, ¹⁵-, Hospital Universitario Virgen del Rocio, Seville, SPAIN, ¹⁶Department of Oncology, Rambam Health Care Campus, Haifa, ISRAEL, ¹⁷Gülhane Training and Research Hospital, University of Health Sciences, Ankara, TURKEY, ¹⁸Asan Medical Center, University of Ulsan College of Medicine, Seoul, KOREA, REPUBLIC OF, ¹⁹-, Kansai Medical University Hospital, Osaka, JAPAN, ²⁰-, Cancer Research UK Cambridge Centre, Cambridge, UNITED KINGDOM, ²¹Evinova, AstraZeneca, Durham, NC, ²²-, AstraZeneca, Cambridge, UNITED KINGDOM, ²³-, AstraZeneca, Gaithersburg, MD, ²⁴-, Royal Marsden Hospital, London, UNITED KINGDOM.

Background In SERENA-6, switching to CAMI from AI while continuing CDK4/6i at emergence of ESR1m during first-line therapy in pts with HR+/HER2− ABC significantly improved PFS, was well tolerated, and reduced the risk of deterioration in quality of life vs AI + CDK4/6i. Photopsia was the most commonly reported non-hematological adverse event (AE) (20% CAMI + CDK4/6i vs 8% AI + CDK4/6i). Previous data suggest that visual effects with CAMI + CDK4/6i are mild, of short duration and have no/minimal impact on activities of daily living. We report a detailed analysis of pt-reported visual functioning and further details of visual effects in SERENA-6. Methods AE incidence, grade (NCI-CTCAE v5.0), and time to onset were recorded for the visual effects group term (photopsia, vision blurred, visual impairment, diplopia, photophobia, and visual perseveration). Ophthalmologic assessments, including fundoscopy, slit lamp, visual acuity, and optical coherence tomography, were conducted at baseline, as clinically indicated, and at end of treatment. Pt-reported visual functioning (measured by the National Eye Institute 25-Item Visual Function Questionnaire [NEI-25]) was an exploratory endpoint. The NEI-25 was completed at baseline, every 6 months thereafter, and at treatment discontinuation. Data cutoff: Nov 28, 2024. Results Overall, 49 (32%) pts in the CAMI + CDK4/6i arm and 25 (16.1%) pts in the AI + CDK4/6i arm reported a visual effect AE. Among the 49 pts in the CAMI + CDK4/6i arm, visual effects were G1 in 44 (90%) pts, G2 in 4 (8%) pts, and G3 (photopsia) in 1 (2%) pt. Median time to first report of a visual effect AE was 8 days with CAMI + CDK4/6i and 23 days with AI + CDK4/6i. Visual effects did not lead to treatment discontinuation in either arm. Clinically significant ophthalmological findings were rare (<6%), unilateral, non-progressive, and occurred at similar frequencies in both arms. Visual effects did not lead to changes in visual acuity and structural evaluations indicated preserved retinal and optic nerve integrity. Mean scores in NEI-25 items, including ability to drive, perform near/distant activities and role function, at Weeks 26 and 52 were similar to baseline scores and comparable between arms (Table). NEI-25 items not reported here show a similar pattern. Conclusions Pt-reported visual functioning with CAMI + CDK4/6i was similar to the AI + CDK4/6i arm. Visual effects reported with CAMI + CDK4/6i were mostly low grade and occurred early in treatment. Together with the clinical efficacy and well-tolerated safety profile of CAMI + CDK4/6i, these data support this combination as a potential new treatment strategy for pts with HR+/HER2− ABC and emergent ESR1m during first-line AI + CDK4/6i.

S. S. Hammer¹, E. Kirilin², H. de Almeida³, V. Aladinskiy⁴, A. Ustiugova², A. Shneyderman⁵, A. Veviorskiy², M. Korzinkin⁵, S. C. Quay⁶; ¹Research and Development, Atossa Therapeutics Inc., Seattle, WA, ²Insilico Medicine AI Limited, Insilico Medicine AI Limited, Abu Dhabi, UNITED ARAB EMIRATES, ³Insilico Medicine Canada Inc., Insilico Medicine Canada Inc., Montreal, QC, ⁴Insilico Medicine AI Limited, Insilico Medicine AI Limited, Abu Dhabi, QC, ⁵Insilico Medicine Hong Kong Ltd, Insilico Medicine Hong Kong Ltd, Hong Kong, Hong Kong SAR, CHINA, ⁶N/A, Atossa Therapeutics Inc., Seattle, WA.

Background: Mutations in the estrogen receptor gene (ESR1), particularly Y537S and D538G, are key drivers of resistance in estrogen receptor-positive (ER+) metastatic breast cancer (MBC) following aromatase inhibitor therapy. These mutations promote ligand-independent ER activation. In this in silico modeling study, we examined the biophysical behavior and downstream transcriptional impacts of (Z)-endoxifen, an active tamoxifen metabolite, a selective estrogen receptor modulator (SERM), across ESR1 variants to further elucidate its mechanism of action and therapeutic potential. Methods: We conducted full-atom molecular dynamics (MD) and metadynamics simulations of ERα-ligand complexes (wild-type and mutant) to assess conformational dynamics and antagonist state probability. Alchemical Non-Equilibrium Switching (NES) simulations compared binding affinities of (Z)-endoxifen vs. 4-hydroxy-tamoxifen (4-OHT).Luciferase reporter assays and 2D growth assays were performed in 293T and MCF-7 cells expressing ESR1-WT or D538G. Transcriptomic data were analyzed from public datasets, including RNA-seq and microarray profiles from ESR1 mutant tumors (n=56) and drug-treated breast cancer cells (n=27 for (Z)-endoxifen; n=5 for elacestrant). Differential expression and enrichment analyses (gene sets, pathways, TFs) were conducted using the limma and gseapy packages. Results- Biophysical Modeling: MD simulations (4 μs) showed stable (Z)-endoxifen binding across WT and mutant ERα, with no spontaneous receptor reactivation. Alchemical free energy calculations revealed favorable or equivalent binding of (Z)-endoxifen vs. 4-OHT (ΔΔG = 0.09 kcal/mol for WT; 1.30 kcal/mol for D538G). Metadynamic simulations on apo ERα indicated that mutations skewed toward the active state but did not prevent (Z)-endoxifen from stabilizing the antagonist conformation (antagonist state probability: WT 93.5%, Y537S 35.4%, D538G 46.9%). Functional Assays: (Z)-endoxifen potently suppressed ER-Luc activity (>70% inhibition) in 293T cells expressing ESR1-WT and all tested mutants (Y537S, Y537N, D538G, K303R), showing comparable potency across constructs. In MCF-7 cells, (Z)-endoxifen significantly reduced proliferation in both parental and ESR1-D538G lines. D538G expression did not rescue growth; a 60% reduction in 2D proliferation was observed with treatment. Transcriptomic Analysis: Gene expression analyses identified numerous transcripts with reversed regulation between ESR1 mutant tumors and (Z)-endoxifen-treated cells, indicating a functional reversal of mutant-driven expression patterns. Pathways upregulated in ESR1 mutant tumors and downregulated by (Z)-endoxifen included estrogen response, E2F targets, and Myc targets. Conversely, oxidative phosphorylation and p53 signaling were suppressed in ESR1 mutants and upregulated by (Z)-endoxifen (FDR < 0.05).(Z)-endoxifen also reactivated suppressed TF target networks (e.g., AHRR, GLIS2, NFATC4) in mutant tumors. In contrast, elacestrant had minimal transcriptional impact in ESR1 mutant MCF-7 cells, suggesting a narrower effect on disease-relevant pathways. Conclusion: (Z)-Endoxifen demonstrates robust biophysical and functional activity against ESR1 mutations. It stabilizes inactive ERα conformations and reverses mutant-driven transcriptional programs, showing greater transcriptional breadth than next-generation SERDs like elacestrant. These findings support (Z)-endoxifen’s potential as a precision therapy for ESR1-mutant ER+ MBC and justify further clinical investigation in endocrine-resistant settings.

C. Saura¹, G. Curigliano², A. Italiano³, E. Felip⁴, A. Schram⁵, P. Tolosa⁶, A. Schott⁷, B. Pistilli⁸, A. Guerrero Zotano⁹, S. Ehsani¹⁰, K. Wisinski¹¹, R. Nanda¹², J. McGuinness¹³, M. Wei¹⁴, J. Liu¹⁵, V. Debien³, A. Marra², K. Jhaveri⁵, S. Isakoff¹⁶, S. Loi¹⁷, L. Schwartzberg¹⁸, K. Yeung¹⁹, M. George²⁰, E. Hamilton²¹, C. Perez²², C. Ma²³, N. Unni²⁴, J. Rodon²⁵, A. Spira²⁶, E. Puente-Poushnejad²⁷, A. Wagner²⁷, L. Xu²⁷, D. Havkins²⁷, F. Ramirez²⁷, S. Landergan²⁷, G. Tan²⁷, A. Timm²⁷, E. Kwak²⁷, D. Bergstrom²⁷, S. Sammons²⁸, A. Varkaris¹⁶; ¹Medical Oncology, Vall d’Hebron University Hospital. Vall d’Hebron Institute of Oncology (VHIO), Hospitalet De Llobregat, L’, SPAIN, ²Medical Oncology, Istituto Europeo di Oncologia, IRCCS, University of Milano, Milano, ITALY, ³Medical Oncology, Institut Bergonie, Bordeaux, FRANCE, ⁴Medical Oncology, Institut Catalan d’Oncologia Badalona (ICO), Barcelona, SPAIN, ⁵Medical Oncology, Memorial Sloan Kettering Cancer Center, New York, NY, ⁶Medical Oncology, Hospital Universitario 12 de Octubre, Madrid, SPAIN, ⁷Medical Oncology, The University of Michigan, Ann Arbor, MI, ⁸Medical Oncology, Gustave Roussy, Villejuif, FRANCE, ⁹Medical Oncology, Instituto Valenciano de Oncologia, Valencia, SPAIN, ¹⁰Medical Oncology, The University of Arizona Cancer Center, Tucson, AZ, ¹¹Medical Oncology, University of Wisconsin, Madison, Carbone Cancer Center, Madison, WI, ¹²Medical Oncology, The University of Chicago Medicine Comprehensive Cancer Center (UCMC), Chicago, IL, ¹³Medical Oncology, Columbia University Irving Medical Center, New York, NY, ¹⁴Medical Oncology, Huntsman Cancer Institute at the University of Utah, Salt Lake City, UT, ¹⁵Medical Oncology, The Kinghorn Cancer Centre, St. Vincent’s Hospital, and Faculty of Medicine and Health, University of New South Wales, Sydney, AUSTRALIA, ¹⁶Medical Oncology, Massachusetts General Hospital, Boston, MA, ¹⁷Medical Oncology, Peter MacCallum Cancer Centre, Melbourne, AUSTRALIA, ¹⁸Medical Oncology, Renown Health, William N. Pennington Cancer Institute, Reno, NV, ¹⁹Medical Oncology, University of California, San Diego (UCSD) – Medical Center, Moores Cancer Center, San Diego, CA, ²⁰Medical Oncology, Rutgers Cancer Institute of New Jersey, New Brunswick, NJ, ²¹Medical Oncology, Sarah Cannon Research Institute, Nashville, TN, ²²Medical Oncology, Sarah Cannon Research Institute /Florida Cancer Specialists, Orlando, FL, ²³Oncology, Washington University School of Medicine in St. Louis, St. Louis, MO, ²⁴Medical Oncology, University of Texas Southwestern Medical Center – Harold C. Simmons Comprehensive Cancer Center, Dallas, TX, ²⁵Medical Oncology, MD Anderson Cancer Center, Houston, TX, ²⁶Medical Oncology, Virginia Cancer Specialists, Fairfax, VA, ²⁷Clinical, Relay Therapeutics, Cambridge, MA, ²⁸Medical Oncology, Dana-Farber Cancer Institute, Boston, MA.

Background Oncogenic mutations in PIK3CA are present in approximately 40% of HR+/HER2- BC and define a validated target for therapeutic inhibition in this setting. Approved inhibitors yield modest efficacy in combination with endocrine therapy (ET) due to dose-limiting toxicities associated with non-selective PI3K inhibition. RLY-2608 is the first oral, pan-mutant-selective, allosteric α-selective PI3K inhibitor (PI3Kαi) designed to overcome these limitations. The FIH ReDiscover (NCT05216432) study investigated RLY-2608 + fulvestrant (F) in pts with PIK3CA-mutant, HR+/HER2- BC who had been previously treated with CDK4/6i and ET. Here, we report subgroup efficacy analyses according to baseline characteristics, including prior selective estrogen receptor degrader (SERD) treatment and ESR1 mutation status. Methods Previously treated adult pts with evaluable HR+/HER2- advanced BC and PIK3CA mutation per local assessment were eligible. Pts received the RP2D of RLY-2608 (600 mg BID fasted) + standard-dose F. Key objectives were investigator-assessed efficacy per RECIST v1.1 and adverse events (AEs) per CTCAE v5.0. Objective response rate (ORR) is defined as the rate of any confirmed complete or partial response (CR or PR). Progression-free survival (PFS) is defined as the time from date of first dose to the date of progression per RECIST v1.1 or death by any cause in the absence of progression. Tumor response in pts with measurable disease and PFS in pts with evaluable disease, without detectable PTEN/AKT co-alterations, were assessed according to baseline characteristics including prior treatment history (prior SERD vs no prior SERD) and presence of ESR1 mutation. PIK3CA and ESR1 ctDNA were assessed at baseline and at C2D1 of study treatment as a pharmacodynamic marker of biologic activity. Safety was assessed in all pts. Results As of 26MAR25, 64 pts with evaluable disease were treated with the RP2D of RLY-2608 + F. Efficacy was evaluated in the subgroup of 52 pts without detectable PTEN/AKT co-alterations, of whom 42% received ≥2 prior systemic therapies for advanced BC; 52% had received any prior SERD; 31% had detectable ESR1 mutation at baseline. 31/52 pts had measurable disease with 12/31 achieving an objective response (ORR 38.7%, 95% CI 21.8-57.8) and 25/31 (80.6%) experiencing any radiographic tumor reduction. Of 31 pts with measurable disease, 16 received prior SERD with 7/16 achieving a confirmed PR (ORR 43.8%, 95% CI 19.8-70.1). In the 10 pts with measurable disease who had detectable ESR1 mutation at baseline, 6 achieved a confirmed PR (ORR 60.0%, 95% CI 26.2-87.8). Across all 52 pts treated at the RP2D, mPFS was 10.3 mo (95% CI 7.2-18.4). In 27 pts who received prior SERD, the mPFS was 11.0 mo (5.6 NR), and in 16 pts with detectable ESR1 mutation at baseline the mPFS was 8.8 mo (95% CI 3.0-18.4). 97% of pts with paired results experienced decline or clearance of PIK3CA ctDNA across genotypes, and all of those with ESR1 mutation at baseline and paired results experienced ctDNA decline or clearance. Treatment-related AEs (TRAEs) were generally low-grade, manageable, and reversible. Hyperglycemia, when reported, was low grade with most pts not requiring medication management. Severe stomatitis and rash were absent or rare. There were no grade 4/5 TRAEs. Conclusion RLY-2608 + F demonstrates promising efficacy in pts with PIK3CA-mutated HR+/HER2- advanced BC who have progressed on CDK4/6i. These data also highlight the activity of RLY-2608 in pts with prior exposure and resistance to SERD where benefit from fulvestrant is not expected, and support the ongoing pivotal investigation of RLY-2608 + F.

E. Gal-Yam¹, Y. Park², A. Sonnenblick³, S. Breuer⁴, R. Yerushalmi⁵, S. Loi⁶, K. Lee⁷, H. Martin⁸, J. Soong⁹, K. Kallapur¹⁰, R. Schwab¹⁰, C. Cahuzac¹¹, T. Fisher¹², J. Sohn¹³; ¹Breast Cancer Institute, Jusidman Cancer Hospital, Sheba Medical Center, Ramat-Gan, ISRAEL, ²Department of Medicine, Samsung Medical Center, Seoul, KOREA, REPUBLIC OF, ³Breast Cancer Unit, The Oncology Division, Tel Aviv Sourasky Medical Center, Tel Aviv, ISRAEL, ⁴Gyneco-oncology Unit in Young Patients, Helmsley Cancer Center, Shaare Zedek Medical Center, Jerusalem, ISRAEL, ⁵Oncology, Davidoff Center, Rabin Medical Center, affiliated to Tel-Aviv University, Petah Tikva, ISRAEL, ⁶Division of Cancer Research, Peter MacCallum Cancer Centre and The Sir Peter MacCallum Department of Oncology, University of Melbourne, Melbourne, AUSTRALIA, ⁷Center for Clinical Trials, National Cancer Center, Goyang, KOREA, REPUBLIC OF, ⁸Medical Oncology Department, Fiona Stanley Hospital, Perth, AUSTRALIA, ⁹Portfolio Clinical Safety, Product Development Safety, F. Hoffmann-La Roche Ltd, Welwyn Garden City, UNITED KINGDOM, ¹⁰Product Development Oncology, Genentech, Inc., South San Francisco, CA, ¹¹PDD DSS Oncology, F. Hoffmann-La Roche Ltd, Basel, SWITZERLAND, ¹²Clinical Pharmacology, Genentech, Inc., South San Francisco, CA, ¹³Division of Medical Oncology, Yonsei University, Seoul, KOREA, REPUBLIC OF.

BACKGROUND Recommended first-line therapy for patients (pts) with hormone receptor-positive, HER2-negative advanced breast cancer (HR+, HER2- aBC) is endocrine therapy (ET) + a cyclin-dependent kinase 4/6 inhibitor (CDK4/6i). Estrogen receptor-CDK4/6-PI3K pathway crosstalk may promote resistance. PIK3CA mutations are linked to poorer outcomes and are predictive of response to PI3Kis. In INAVO120 (NCT04191499), adding inavolisib (INAVO), a highly potent, oral, selective PI3Kαi, to fulvestrant (FULV) + palbociclib significantly improved progression-free and overall survival vs placebo in pts with PIK3CA-mutated, HR+, HER2-, endocrine-resistant aBC. However, data on the safety and combinability of other CDK4/6is with INAVO + ET are lacking. We present updated interim safety data from the INAVO + abemaciclib (ABEMA) + FULV, INAVO + ribociclib (RIBO) 400 mg + FULV, and INAVO + RIBO 600 mg + FULV / letrozole (LET) arms of the MORPHEUS-pan breast cancer (NCT03424005) umbrella study. METHODS Eligible pts had previously treated PIK3CA-mutated, HR+, HER2- aBC. Pts were randomized to receive INAVO (9 mg orally [PO] daily, days [D]1-28) + ABEMA (150 mg PO twice daily, D1-28) + FULV (500 mg intramuscularly on D1 + 15 of Cycle 1, then D1 of each subsequent cycle); INAVO + RIBO 400 mg (PO daily, D1-21) + FULV; or INAVO + RIBO 600 mg + physician’s choice of FULV or LET (2.5 mg PO daily, D1-28). Data from the INAVO + RIBO 600 mg + FULV / LET arms are combined due to later opening, resulting in smaller sample sizes and shorter follow-up. Adverse events (AEs) were assessed per National Cancer Institute Common Terminology Criteria for AEs v4.0. RESULTS At clinical cutoff (Mar 31, 2025), 7 / 16 pts in the INAVO + ABEMA + FULV arm and 8 / 19 in the INAVO + RIBO 400 mg + FULV arm had discontinued study treatment due to disease progression or symptomatic deterioration (11 / 11 were still on treatment in the INAVO + RIBO 600 mg + FULV / LET arms). Two of the 19 pts in the INAVO + RIBO 400 mg + FULV arm had discontinued the study due to death, assessed as related to progressive disease. Median safety follow-up was 5.7, 4.9, and 2.4 months in the respective arms. Fourteen of 16 pts in the INAVO + ABEMA + FULV arm, 7 / 19 in the INAVO + RIBO 400 mg + FULV arm, and 7 / 11 in the INAVO + RIBO 600 mg + FULV / LET arms had grade 3 AEs. No grade 4 or fatal AEs were reported. Six serious AEs were reported across 2 / 16 pts in the INAVO + ABEMA + FULV arm and 3 / 19 in the INAVO + RIBO 400 mg + FULV arm (none in the RIBO 600 mg arms). Of these, one was assessed as related to INAVO (hyperglycemia). One pt discontinued study treatment due to an AE (grade 2 asthenia in the INAVO + ABEMA + FULV arm; related to INAVO + ABEMA). Dose interruptions of any treatment due to AEs occurred in 13 / 16, 8 / 19, and 5 / 11 pts in the three arms, respectively; dose modifications of any treatment due to AEs, in 13 / 16, 5 / 19, and 6 / 11. Eight of 16 pts in the INAVO + ABEMA + FULV arm had ≥ 1 AE of special interest for INAVO (grade ≥ 3 hyperglycemia in 6 / 16, grade ≥ 2 pneumonitis in 1 / 16, and grade ≥ 3 stomatitis and mucosal inflammation in 1 / 16), as did 3 / 19 in the INAVO + RIBO 400 mg + FULV arm (all grade ≥ 3 hyperglycemia), and 4 / 11 in the INAVO + RIBO 600 mg + FULV / LET arms (all grade ≥ 3 hyperglycemia). CONCLUSIONS AEs observed with INAVO-containing triplet regimens in PIK3CA-mutated, HR+, HER2-, endocrine-resistant aBC were consistent with the known safety profiles of the individual agents. The combinations were tolerable, supporting further investigation. Updated data, including pharmacokinetics and data from additional pts, including those treated with INAVO + ABEMA + LET, will be presented.

F. Ma¹, Q. Zhang², H. Mo¹, J. Cui³, B. Zhao⁴, F. Xu⁵, L. Wang⁶, L. Gan⁷, Y. Liu⁸, F. Luo⁹, Z. Song¹⁰, Y. Lv¹¹, X. Wang¹², T. Sun¹³, X. Ling¹⁴, H. Sun¹⁵, Y. Yin¹⁶, C. Wang¹⁷, Z. Xia¹⁸, H. Su¹⁹, X. Chen²⁰, Y. Li²¹, H. Yang²², Y. Wang²³, Y. Xin²⁴, X. Zhang²⁵, Y. Chen²⁵, X. Luo²⁵, F. Wang²⁵, M. Luo²⁵; ¹Medical Oncology Department, China Cancer Institute and Hospital, Beijing, CHINA, ²Breast Internal Medicine Ward 1, Affiliated Tumor Hospital of Harbin Medical University, Harbin, CHINA, ³Oncology Department, First Hospital of Jilin University, Jilin, CHINA, ⁴Breast Internal Medicine Department, Affiliated Tumor Hospital of Xinjiang Medical University, Xinjiang, CHINA, ⁵Internal Medicine Department, Sun Yat-sen University Cancer Center, Guangzhou, CHINA, ⁶Breast Surgery Department, Xingtai People’s Hospital, Xingtai, CHINA, ⁷Oncology Department, First Affiliated Hospital of Chongqing Medical University, Chongqing, CHINA, ⁸Medical Oncology Department, Xuzhou Central Hospital, Xuzhou, CHINA, ⁹Breast Surgery Department, Shanxi Cancer Hospital, Taiyuan, CHINA, ¹⁰Breast Center, Fourth Hospital of Hebei Medical University, Shijiazhuang, CHINA, ¹¹Medical Oncology Department, General Hospital of Ningxia Medical University, Yinchuan, CHINA, ¹²Medical Oncology Department, Suzhou Municipal Hospital, Anhui Province, Suzhou, CHINA, ¹³Breast Internal Medicine Ward 1, Liaoning Cancer Hospital, Shenyang, CHINA, ¹⁴Medical Oncology Department, First Hospital of Lanzhou University, Lanzhou, CHINA, ¹⁵Oncology Department 1, Jiamusi Tuberculosis Hospital (Jiamusi Tumor Hospital), Jiamusi, CHINA, ¹⁶Oncology Department, Jiangsu Province Hospital, Nanjing, CHINA, ¹⁷Oncology Department, Ganzhou People’s Hospital, Ganzhou, CHINA, ¹⁸Oncology Department, Affiliated Hospital of Jining Medical University, Jining, CHINA, ¹⁹Oncology Department, Second Affiliated Hospital, Air Force Medical University of PLA, Xi’an, CHINA, ²⁰Breast Surgical Oncology Department, Yibin Second People’s Hospital, Yibin, CHINA, ²¹Oncology Department, Guizhou Provincial People’s Hospital, Guiyang, CHINA, ²²Medical Oncology Department, Affiliated Hospital of Hebei University, Baoding, CHINA, ²³Oncology Department, Xiangyang Central Hospital, Xiangyang, CHINA, ²⁴Clinical development, CSPC Zhongqi Pharmaceutical Technology (Shijiazhuang) Co., Ltd, Shijiazhuang, CHINA, ²⁵Clinical development, CSPC Zhongqi Pharmaceutical Technology (Shijiazhuang) Co., Ltd., Shijiazhuang, CHINA.

Background​: There was no standard therapy regimen for HR+/HER2- advanced breast cancer progressing after CDK4/6 inhibitors (CDK4/6i) administration. Sirolimus, also known as rapamycin, is an mTOR specific inhibitor, but its oral bioavailability is low. By binding nanoparticle albumin to sirolimus (nab-Sirolimus) and increasing its water solubility, the bioavailability of sirolimus can be improved. Herein, we reported the efficacy and safety of nab-Sirolimus (HB1901) combined with endocrine therapy (ET) in patients following progression on/after CDK4/6i based regimen. ​​Methods​: This multicenter, open-label, phase 2 trial enrolled patients with pathologically confirmed HR+/HER2- advanced BC who had previously failed aromatase inhibitor (AI)/fulvestrant ± CDK4/6i. Patients failing prior AI ± CDK4/6i received nab-Sirolimus (100 mg/m²) + fulvestrant, while those failing fulvestrant ± CDK4/6i received nab-Sirolimus (100 mg/m²) + AI. Primary endpoint was 6-month progression-free survival (PFS) rate. Secondary endpoints included objective response rates (ORR), disease control rate (DCR), PFS, safety and tolerability. Tumor tissue and/or blood samples from patients were subjected to next-generation sequencing. Results​​: A total of 65 patients were enrolled in this study, with a median age of 54 years (range 31-72). 34 and 31 eligible patients were allocated to receive nab-Sirolimus 100 mg/m² Q2W + fulvestrant or exemestane, respectively. Among them, 64 patients (98.5%) had received ET + CDK4/6i, 51 patients (78.5%) had visceral metastases at baseline, and 33 patients (50.8%) had received at least one type of chemotherapy for advanced/metastatic diseases. At data cut-off (May 31, 2025), with a median follow-up time of 9.1 months, 15 patients remained on study treatment. Among 60 efficacy-evaluable patients, the ORR was 20%, with DCR of 88.3% (12 partial responses [PR] and 41 stable diseases [SD]), median PFS of 5.7 months (95%CI 5.03-9.03), and 6-month PFS rate of 49.5%. In the nab-Sirolimus + fulvestrant cohort (n=31), improved activity was observed: ORR of 29%, DCR of 93.5% (9 PR and 20 SD), median PFS 7.5 months (95%CI 5.13-9.2), and 6-month PFS rate of 58.3%. Twenty-four patients (36.9%) had PIK3CA/AKT1/PTEN alteration. In the nab-Sirolimus + fulvestrant cohort, 11 efficacy-evaluable patients with PIK3CA/AKT1/PTEN alteration showed a median PFS of 9.1 months [95%CI 3.75- not reached] and 6-month PFS rate of 70.1%, with ORR of 27.3% and DCR of 100% (3 PR and 8 SD). Notably, 20 efficacy-evaluable patients with PIK3CA/AKT1/PTEN wild-type showed promising responses (ORR of 30 % and DCR of 90% [6 PR and 12 SD]), with median PFS of 7.4 months [95%CI 4.4 – 9.2] and 6-month PFS rate of 52.6%). The treatment-related adverse events (TRAEs) associated with nab-sirolimus occurred in 98.5% (64/65) of patients, mostly grade 1-2. The most frequently reported TRAEs were hypertriglyceridemia (61.5%), hypercholesterolemia (60%) and hypokalemia (60%). A total of 26 patients (40.0%) had ≥Grade 3 treatment-related adverse events, which had an incidence rate ≥ 5% were hypokalemia (16.9%) and hypertriglyceridemia (10.8%). No drug-related events leading to death occurred. Only 1 patient experienced an event leading to permanent discontinuation. Conclusion:​​ Nab-sirolimus exhibited manageable toxicity and promising antitumor activity, particularly when combined with fulvestrant (achieving an ORR of 29% and a median PFS of 7.5 months). Nab-sirolimus + fulvestrant showed good efficacy in both PIK3CA/AKT1/PTEN altered and wild-type HR+ breast cancer patients. This novel mTOR inhibitor-based regimen may address the unmet need in CDK4/6i-failed HR+/HER2- advanced breast cancer. Clinical trial information: NCT06957379.

G. Sledge¹, H. McArthur², J. Xiu¹, M. J. Oberley³, M. Su⁴, M. Radovich¹, D. Spetzler⁵, V. Kaklamani⁶; ¹Medical Affairs, Caris Life Sciences, Phoenix, AZ, ²Internal Medicine – Hematology/Oncology, UT Southwestern Medical Center, Dallas, TX, ³Clinical Operations, Caris Live Sciences, Phoenix, AZ, ⁴UT Southwestern Medical Center, Caris Life Sciences, Dallas, TX, ⁵Office of the President, Caris Life Sciences, Phoenix, AZ, ⁶Hematology–Medical Oncology, UT Health San Antonio MD Anderson Cancer Center, San Antonio, TX.

Background: ESR1 activating mutations are predominantly located in the ligand-binding domain near helix 12. They drive ligand-independent activation of the estrogen receptor pathway and confer resistance to conventional endocrine therapies. However, tumors often retain sensitivity to selective estrogen receptor degraders (SERDs) like elacestrant. Here, we interrogate a large real-world cohort to explore how specific ESR1 variants and prior endocrine therapy influence elacestrant clinical benefit. Methods: A total of 3,340 breast tumors carrying ESR1 mutations that underwent tumor profiling at Caris Life Sciences (Phoenix, AZ)—including Whole Exome Sequencing and Whole Transcriptome Sequencing—were studied. Real-world clinical data were obtained from insurance claims. Time on treatment (TOT) was defined as the interval from the start to the end of treatment. Cox proportional hazards models were used to calculate hazard ratios (HRs), and log-rank tests were used to determine p-values. Results: In this large real-world database of breast cancer patients, 8.59% (N=3340) of tumors harbored an ESR1 mutation. Mutations most frequently occurred at D538 (G: 1,303; others: 10), Y537 (S: 839; N: 324; C: 192; others: 30), L536 (SNV: 233; others: 8), and E380 (Q/K: 411). While the vast majority of tumors carried a single ESR1 mutation (7.85%), a small fraction harbored more than one ESR1 mutation (0.75%; double mutants: 0.67%; more than double: 0.08%). Among elacestrant-treated patients with a single ESR1 mutation (N=382), the TOT was significantly shorter for patients with a L536 SNV (N = 20; 1.38 months; 95% CI: [0.43-2.17]) compared to patients with other variants: D538G (N = 119; 2.76 months; [1.97-3.29]), Y537 SNV (N = 116; 3.4 months; [2.73-4.11]), and E380 variants (N = 20; 3.45 months; [0.2-5.46]). When comparing L536 with all other variants combined, HR was 2.089 (95% CI: [1.312-3.325], p = 0.001). This coincides with significantly lower expression levels of the ER-responsive gene GREB1 in L536-mutant tumors (7.94 transcripts per million [TPM]) compared to D538 (20.75 TPM), Y537 (38.61 TPM), and E380 (13.65 TPM) tumors (p < 0.001). When investigating tumors carrying double ESR1 mutations, elacestrant TOT was significantly shorter compared to single ESR1 variants (1.94 vs. 2.90 months; HR = 1.716 [1.096-2.686], p = 0.016). While fulvestrant use prior to sequencing was more frequent in patients with multiple ESR1 mutations (29% for single mutants vs. 40% for multiple mutants, p < 0.0001), prior fulvestrant exposure did not impact elacestrant TOT (HR = 1.181 [0.885-1.58], p = 0.259). Conclusions: In a large cohort of patients with HR+HER2- ESR1 mutated breast cancer treated with elacestrant, a relatively common mutation of ESR1 L536 and/or multiple ESR1 mutations conferred significantly less benefit to elacestrant than other variants.