LBA1006 Background: The combination of endocrine therapy (ET) with a cyclin-dependent kinase 4/6 inhibitor (CDK4/6i) is the current standard of care (SOC) for 1L treatment of pts with ER+, HER2– LA/mBC. GIRE, a next-generation oral selective full ER antagonist and degrader, has shown superior efficacy vs SOC ET in the adjuvant setting (lidERA BC; Bardia SABCS 2025) and in combination with everolimus vs SOC ET + everolimus in the post-CDK4/6i LA/mBC setting (evERA BC; Mayer ESMO 2025). Primary analysis results of persevERA BC (NCT04546009) are presented. Methods: Pts with de novo mBC or recurrent LA/mBC, measurable disease/evaluable bone disease, and no prior systemic LA/mBC therapy were randomized 1:1 to GIRE (30 mg daily [QD]) + LET placebo (QD) + PALBO (125 mg QD on Days 1–21) or LET (2.5 mg QD) + GIRE placebo (QD) + PALBO on each 28-day cycle (with a luteinizing hormone-releasing hormone agonist in pre-/peri-menopausal women, and men). The primary endpoint was investigator-assessed progression-free survival (INV-PFS). Secondary endpoints included overall survival (OS), objective response rate (ORR), duration of response (DoR), clinical benefit rate (CBR), and safety. Results: 992 pts were randomized (495 to GIRE; 497 to LET). Median age was 63.0 years; 80.1% were post-menopausal; 19.1% had de novo mBC; 69.3% had recurrent disease with treatment-free interval (TFI) >12 months (mo); 60.4% had visceral disease. At data cutoff (01/30/26), median follow-up was 52.2 mo and 623 INV-PFS events were observed. The hazard ratio (HR) for INV-PFS was 0.89 (95% confidence interval [CI] = 0.76, 1.05; p = 0.1553); median INV-PFS was 33.1 mo with GIRE + PALBO vs 28.2 mo with LET + PALBO (Δ 4.9 mo) (Table). The most common grade 3–4 adverse events (AEs) in the GIRE arm were hematologic abnormalities associated with PALBO. ET discontinuations due to AEs were similar (6.5% with GIRE vs 5.5% with LET). Conclusion: 1L GIRE + PALBO resulted in a numerical improvement in INV-PFS vs LET + PALBO in ER+, HER2– LA/mBC, though it did not meet pre-defined statistical significance. The safety profile was tolerable, consistent with individual agents, with no unexpected findings. The ongoing 1L pionERA BC study (NCT06065748) is exploring GIRE vs fulvestrant, in combination with physician’s choice of CDK4/6i in pts who have relapsed on adjuvant ET or with <12 mo TFI. Clinical trial information: NCT04546009 . GIRE + PALBO(n = 495) LET + PALBO(n = 497) Median PFS, mo (95% CI) 33.1 (30.2, 38.3) 28.2 (25.0, 33.1) HR* (95% CI); p-value † 0.89 (0.76, 1.05); 0.1553 Median OS, mo (95% CI) NE (NE) NE (61.3, NE) HR* (95% CI); p-value † 1.03 (0.83, 1.28); 0.777 ORR, % 60.2 58.8 Median DoR, mo (95% CI) 38.5 (30.4, 48.7) 30.4 (25.3, 36.1) HR* (95% CI); p-value † 0.80 (0.63, 1.01); 0.056 CBR, % 82.6 82.1 *Stratified. † Stratified log-rank. NE, not evaluable.

Younger premenopausal women (typically defined as those aged <40 years) diagnosed with oestrogen receptor (ER)-positive early-stage breast cancer have disproportionately poorer outcomes relative to older women, with age-related differences being especially pronounced in this subtype. Emerging evidence suggests that this age-related disparity is underpinned by distinct biological and genomic features — such as enrichment in copy number alterations, homologous recombination deficiency and unique immune microenvironments — that are not fully addressed by current therapeutic strategies. Endocrine therapy remains the cornerstone of treatment for premenopausal women with ER-positive early-stage breast cancer, yet strategies for its use continue to evolve. Clinical studies have highlighted the importance of ovarian function suppression (OFS) in improving the outcomes in patients with high-risk disease, as well as the benefit of adding CDK4/6 inhibitors to standard-of-care (SOC) endocrine therapies and the expanding role of molecular profiling in guiding treatment decisions. In this Review, we describe how treatment paradigms are now challenging the conventional sequencing of chemotherapy and endocrine therapy in younger women. A biology-driven approach — incorporating germline status, gene expression and immune signatures — will better guide therapy in this population and transform clinical decision-making beyond chronological age. We propose that future trials involving women with premenopausal ER-positive disease must prioritize biology over age in defining eligibility, incorporate OFS as a SOC in the control arm and expand biomarker-driven approaches to refine both treatment escalation and de-escalation. A genomically and immunologically informed strategy is essential to improve the outcomes in this under-represented population. Premenopausal women aged <40 years diagnosed with oestrogen receptor-positive early-stage breast cancer have disproportionately poorer outcomes relative to older women. The authors of this Review propose a biology-driven approach to challenge the conventional sequencing of chemotherapy and endocrine therapy in this population.

Tumour-infiltrating lymphocytes (TILs) are prognostic and predictive biomarkers in breast cancer. High pre-treatment TILs are associated with a favourable prognosis and improved response to systemic therapies. However, the role of TILs in mediating response to radiotherapy in breast cancer remains underexplored, with scarce clinical evidence to date. In this Review, we present an overview of current evidence and potential mechanisms, highlight opportunities for integrating TILs into radiation oncology trials and clinical practice, and call for standardised TIL reporting to accelerate biomarker-driven individualisation of radiotherapy in breast cancer.

BACKGROUND:Tumour-infiltrating lymphocytes (TILs) are a robust prognostic marker in patients with triple-negative breast cancer. Artificial intelligence (AI)-derived computational tools assessing TILs could improve efficiency, but require independent validation against clinical outcomes. We aimed to compare the prognostic performance of AI-derived TIL scores with pathologist-scored TILs in a large, prospectively collected dataset pooled from randomised controlled trials. METHODS:CATALINA was an independent, external validation study using prospectively collected long-term clinical outcome data pooled from seven randomised clinical trials conducted at multiple sites. We independently evaluated two previously validated AI pipelines that generate five computationally assessed tumour-infiltrating lymphocyte (cTIL) scores by masked, independent deployment of locked models. cTIL scores were correlated with the mean of the pathologist-scored stromal TILs (sTILs) in 220 digitised haematoxylin and eosin whole slide images in a cohort of patients with early-stage triple-negative or HER-2 positive breast cancer, previously scored by trained pathologists in a TIL-reproducibility study. Prognostic performance was assessed in a separate cohort of patients with early triple-negative breast cancer pooled from seven prospective, randomised adjuvant trials. Multivariable Cox regression models adjusted for clinicopathological factors and study heterogeneity assessed associations of cTIL score and sTIL score with invasive disease-free survival, distant disease-free survival, and overall survival. 5-year discrimination was estimated using time-dependent area under the receiver operating characteristic curve (AUC). FINDINGS:Individual data were collated from 1759 patients, of whom 1356 had complete clinicopathological data, pathologist sTIL scores, and cTIL scores available. Modest correlation (r 0·375-0·473) was observed between cTIL scores and the mean pathologist sTIL score. Both sTIL and cTIL were independently associated with 5-year invasive disease-free survival, distant disease-free survival, and overall survival after adjustment for clinicopathological factors (hazard ratio for invasive disease-free survival was 0·73 [95% CI 0·66-0·82]; q<0·0001, distant disease-free survival was 0·70 [0·61-0·79]; q<0·0001, and overall survival was 0·72 [0·63-0·82]; q<0·0001 for sTIL scores and 0·80 [0·73-0·89]; q<0·0001, 0·77 [0·69-0·86]; q<0·0001, and 0·79 [0·70-0·88]; q=0·0002, respectively, for percentage_lymphocyte scores). In models adjusted for clinicopathological variables and sTIL score, cTIL score did not maintain a statistically significant prognostic association. Both sTIL and cTIL scores improved the 5-year AUC over clinicopathological variables alone, while cTIL score did not significantly further improve AUC when combined with clinicopathological variables and sTIL score. INTERPRETATION:Two cTIL models deployed entirely without retraining or modification provided statistically significant prognostic information and improved risk discrimination compared with clinicopathological variables alone in this large, platform-based, independent validation study. Although cTIL score did not incrementally improve prognostication compared with models combining clinicopathological variables with sTIL score, these findings support the application of cTILs as a reproducible prognostic biomarker, particularly in settings where routine or widespread pathologist assessment is unavailable. FUNDING:Breast Cancer Research Foundation (USA).

Importance:Molecular analyses of biospecimens collected from study participants are essential for identifying biomarkers that can tailor treatments to specific subsets of patients who are most likely to benefit. Sharing of data and biospecimens from clinical trials enables personalized, patient-centric use of cancer therapies and accelerates the development of new treatments. Objective:To describe obstacles to sharing data and biospecimens and to propose strategies to enhance access and collaboration. Evidence Review:This is a Special Communication authored by 53 academic investigators and patient representatives from the breast cancer community with extensive experience in conducting clinical and translational research. The article also evaluates the impact of biomarker research on specifying responsive subpopulations in the 29 registrational clinical trials that have led to approval of a new drug for treatment of breast cancer between 2017 and 2024. Findings:Clinical trial participants are increasingly asked to provide tissue and/or body fluid biospecimens for biomarker research that is typically controlled by the sponsoring pharmaceutical company, but published biomarker studies are rare. Among 29 breast cancer registrational studies reported in the past 8 years, none resulted in biomarker research that restricted a drug's approved indication. Herein, strategies to maximize the value of clinical data and biospecimens contributed by participants are proposed, thereby supporting the shared goals of the pharmaceutical industry and academia to improve patient care. These strategies include (1) establishing coleadership structures involving academia and patients in clinical trial design and conduct, (2) ensuring that informed consent forms state that data and biospecimens will be shared with academia for future research, (3) requiring the sharing of clinical data as a condition for regulatory approval, and (4) enabling access to biospecimens and translational research data for independent studies on biomarkers that may indicate drug efficacy and toxicity. Conclusions and Relevance:Data and biospecimen sharing from registrational trials has been suboptimal. Improving clinical data, biospecimens, and biospecimens' related data sharing requires concrete actions and a multidimensional stakeholder approach to accelerate the impact of clinical cancer research on the quality of patient care.

BACKGROUND:Stromal tumour-infiltrating lymphocytes (sTILs) are prognostic in early-stage HER2-positive breast cancer, but their role in the context of dual HER2 blockade remains undefined. We evaluated manual, digital, and artificial intelligence (AI)-based sTIL quantification, together with AI-derived spatial metrics, for prognostic and treatment-benefit stratification using tumour samples from the phase 3 APHINITY trial. METHODS:In the APHINITY trial, 4805 patients were randomly assigned to receive chemotherapy plus trastuzumab with pertuzumab or chemotherapy plus trastuzumab with placebo. Median follow-up was 74·1 months (IQR 68·3-75·4). We analysed 4262 haematoxylin and eosin-stained images using manual assessment, an automated digital approach, AI-based lymphocyte quantification (AI percentage lymphocytes), and two AI-derived spatial features (AI-TIL and immune hotspot). Interobserver reproducibility was assessed in 262 randomly chosen tumour samples scored independently by five pathologists. Multivariable Cox models were used to assess associations between TIL levels and invasive disease-free survival (primary outcome in APHINITY), distant recurrence-free interval, and overall survival. The heterogeneity of pertuzumab benefit was evaluated using subgroup analyses, subpopulation treatment effect pattern plot analyses, and nested Cox models with treatment-by-biomarker interaction terms. FINDINGS:Manual scoring showed high interobserver reproducibility (intraclass correlation coefficient 0·84 [95% CI 0·79-0·88]). Concordance between manual and automated methods was modest. AI-based scoring (AI percentage lymphocytes) reclassified 120 (11·6%) of 1035 node-positive tumours from immune-low (by manual scoring) to immune-high; this subgroup of patients showed greater separation of 5-year invasive disease-free survival curves between pertuzumab and placebo groups compared with patients whose tumours were concordantly classified as immune-low by both manual and AI-based approaches. Higher levels of TILs were associated with improved invasive disease-free survival for all sTIL measurement approaches and spatial measurements (hazard ratios [HRs] 0·41-0·93). Pertuzumab was associated with improved invasive disease-free survival at higher sTIL levels across all measurement approaches (HRs 0·36-0·48), but was not associated with higher values of spatial measures. The largest 6-year absolute improvements with pertuzumab were observed in patients with node-positive disease whose tumours scored in the highest level of immune infiltration of manual sTIL scoring (≥70·0%; mean absolute improvement 12·1 percentage points [SD 2·8]). In nested prognostic and predictive models, AI-based immune hotspot scores provided the most consistent additional information when combined with any sTIL measurement (all p<0·010). INTERPRETATION:Standardised manual sTIL scoring was reproducible, and digital and AI-based methods showed consistent prognostic stratification and potential for treatment-benefit stratification despite only modest correlation between platforms. AI spatial metrics provided complementary information beyond sTIL density and could support more scalable immune assessment. Future studies are needed to validate these approaches in independent cohorts and to clarify their clinical utility for stratifying contemporary HER2-directed therapies. FUNDING:None.

1013 Background: In the phase 3 ASCENT-04/KEYNOTE-D19 study (NCT05382286) SG + pembro showed significant and clinically meaningful progression-free survival (PFS) improvement vs chemo + pembro in pts with previously untreated PD-L1+ mTNBC. SG + pembro exhibited a manageable safety profile consistent with prior studies for each agent. We present preplanned exploratory efficacy analyses in ASCENT-04 by biomarker subgroups. Methods: 443 pts received (1:1 ratio) SG + pembro in 21-day cycles or chemo (gemcitabine + carboplatin; taxane) + pembro. From centrally tested fresh or archival tumor samples, Trop-2 expression was determined by immunohistochemistry (IHC), tumor BRCA (tBRCA) status by whole exome sequencing, and HER2 expression by in situ hybridization (ISH) + IHC. Pts were grouped by Trop-2 expression quartiles, tBRCA status (wild-type [WT] or mutant [mut] in BRCA1/BRCA2 /both), and HER2 status (IHC 0 vs Low [IHC 1+ or IHC 2+/ISH–]). Biomarker subgroups were analyzed descriptively for association with PFS by blinded independent central review (BICR); other outcomes by biomarker status are forthcoming. Results: Median Trop-2 H-score was 280, ranges: quartile 1 (Q1) 0-224, Q2 225-279, Q3 280-298, Q4 299-300. SG + pembro demonstrated longer PFS by BICR in Q3/Q4 vs Q1/Q2, and benefit was observed for SG + pembro vs chemo + pembro in all quartiles (Table). Hazard ratios (HR; 95% confidence interval [CI]) were: Q1, 0.81 (0.48-1.36); Q2, 0.73 (0.44-1.22); Q3, 0.46 (0.27-0.80); Q4, 0.57 (0.33-0.99). Proportion of pts with tBRCA mutations (~20%) was similar between treatment groups. Longer PFS was observed with SG + pembro with HR (95% CI) of 0.67 (0.49-0.91) in the tBRCA WT and 0.88 (0.45-1.74) in the tBRCA mut subgroups. PFS was longer with SG + pembro vs chemo + pembro in the HER2 IHC 0 and HER2 low subgroups, HR (95% CI) 0.69 (0.46-1.04) and 0.67 (0.48-0.92), respectively. Conclusions: SG + pembro demonstrated longer PFS vs chemo + pembro across all Trop-2, tBRCA, and HER2 subgroups. These analyses strengthen support for the significant, clinically meaningful benefit of SG + pembro as first-line treatment for mTNBC across subgroups. Clinical trial information: NCT05382286 . Efficacy, BICR N Median PFS (95% CI), mo Biomarker Subgroup SG + pembro Chemo + pembro SG + pembro Chemo + pembro HR (95% CI) Trop-2(n = 400) Q1 48 47 9.3(7.4-19.4) 9.0(6.0-10.9) 0.81(0.48-1.36) Q2 50 50 9.6(7.3-16.7) 7.4(6.9-9.7) 0.73(0.44-1.22) Q3 55 50 13.5(9.3-NR) 8.4(5.6-10.8) 0.46(0.27-0.80) Q4 51 49 16.6(8.1-NR) 9.2(5.5-11.3) 0.57(0.33-0.99) tBRCA(n = 332) WT 130 131 9.6(7.6-16.7) 7.4(6.9-9.2) 0.67(0.49-0.91) Mut 39 32 16.6(7.5-NR) 12.9(7.1-NR) 0.88(0.45-1.74) HER2(n = 435) IHC 0 84 82 16.6(9.1-21.2) 9.0(7.2-10.8) 0.69(0.46-1.04) Low 133 136 11.2(9.1-16.6) 7.7(7.0-9.4) 0.67(0.48-0.92)

501 Background: In the NATALEE clinical trial, RIB + NSAI significantly improved invasive disease–free survival (iDFS) vs NSAI alone in patients (pts) with stage II/III HR+/HER2− EBC. We report on the prognostic and predictive value of baseline gene exp in NATALEE. Methods: Pts were randomized 1:1 to RIB + NSAI or NSAI alone. Men and premenopausal women received goserelin. Eligible pts had anatomical stage IIA (if N0 with additional risk factors [G3, or G2 with Ki67 ≥20% or high genomic risk] or N1 [1-3 axillary lymph nodes]), IIB, or III disease per AJCC (8th ed). Gene exp in surgical samples was profiled by NanoString BC360 panel. PAM50-based intrinsic subtype × treatment (tx) interaction was estimated by likelihood ratio test. Prognostic and predictive effects of PAM50 subtypes and genomic risk/proliferation signature scores created from gene exp data were assessed with Cox proportional hazards models. Differences in genomic risk and proliferation scores were analyzed by Wilcoxon test. Associations between gene exp and RIB benefit were estimated with Cox models. Results: In all, 3022 baseline surgical samples were analyzed. iDFS benefit with RIB + NSAI was consistent in the biomarker (hazard ratio [HR], 0.72) and intent-to-treat populations (HR, 0.71). PAM50 subtype distribution was comparable across tx arms and differed between N0 and N1-N3 in pts with luminal A (LumA; 50% vs 68%), luminal B (lumB; 41% vs 26%), and basal-like (BSL; 6.4% vs 2.5%) disease, with similar percentages for HER2-enriched (HER2E; 2.5% vs 3.0%). PAM50 subtypes were strongly prognostic (HRs vs LumA: LumB, 1.39; HER2E, 2.62; BSL, 3.92). RIB had benefit across all PAM50 subtypes (HRs: LumA, 0.77; LumB, 0.71; HER2E, 0.50; BSL, 0.42), with no significant subtype × tx interaction ( P = .34). Higher genomic risk signature or proliferation signature scores showed a trend for increased RIB benefit, again with no significant interaction. Pts with N0 vs N1-N3 had significantly higher genomic risk scores and proliferation scores. Exploratory analysis of the predictive effect of gene exp on RIB benefit identified several genes for which higher (eg, CEACAM6 , NOD2 , and GPX3) or lower exp (eg, GATA3 , SLC39A6 , and MAPT ) was associated with increased RIB benefit. Conclusions: In this analysis of NATALEE, which examined the largest dataset of surgical tumor samples from any adj CDK4/6i trial in HR+/HER2− EBC, RIB had benefit across all PAM50 subtypes, with a trend for increased benefit in pts with higher genomic risk signature or proliferation signature scores. Baseline exp levels of several genes were associated with differential RIB benefit in EBC, showing potential predictive and prognostic value. The findings reinforce the therapeutic benefit of RIB in combination with ET across HR+/HER2− EBC populations. Clinical trial information: NCT03701334 .

LBA1000 Background: In ASCENT-04, first-line (1L) SG + pembro led to a statistically significant and clinically meaningful improvement in PFS vs chemo + pembro (median, 11.2 vs 7.8 mo; hazard ratio [HR], 0.65; 95% CI, 0.51-0.84; P < .001) in pts with previously untreated PD-L1+ mTNBC. Overall survival (OS) data are immature. PFS2 is more strongly associated with OS than PFS and can be used to measure long-term clinical benefit in the absence of mature OS data or when OS may be impacted by crossover. We report PFS2 and subs tx from ASCENT-04. Methods: Pts (N = 443) were randomized 1:1 to SG (10 mg/kg IV, days 1 & 8) + pembro (200 mg, day 1, max 35 cycles) in 21-day cycles or chemo (gemcitabine + carboplatin, paclitaxel, or nab-paclitaxel) + pembro; the primary end point was PFS by blinded independent central review (BICR). Eligible pts in the chemo + pembro group could receive 2L SG provided on study via crossover following BICR-verified progressive disease (PD) or in any subs line commercially; other subs txs per local practice were also permitted. PFS2 was defined as the time from randomization to first documented PD on next-line therapy per investigator assessment or death due to any cause, whichever occurred first. Results: Median follow-up for OS was 14.0 mo; 95 (43%) pts remained on study tx in the SG + pembro group (n = 221) and 52 (23%) in the chemo + pembro group (n = 222). Of the 125 pts in the SG + pembro group who discontinued tx, 69 received any subs tx, the most frequent of which were taxanes (42%), platinum chemo (33%), and capecitabine (33%). Of the 170 pts in the chemo + pembro group who discontinued tx, 119 received any subs tx, the most frequent of which were SG (81%), taxanes (9%), and capecitabine (9%). Median PFS2 and PFS2 rates are in the Table. Median (95% CI) time to first subs tx was 17.3 mo (12.7-not reached [NR]) for SG + pembro and 9.8 mo (8.7-10.9) for chemo + pembro; median (95% CI) time to second subs tx was NR (22.9 mo-NR) and 21.0 mo (16.6-NR). Conclusions: PFS2 was improved in the SG + pembro group vs chemo + pembro group despite crossover tx, with most pts who initiated subs tx in the chemo + pembro group receiving SG. In pts with previously untreated PD-L1+ mTNBC, SG + pembro provided clinically relevant continued benefit beyond first progression, further supporting SG + pembro as a potential new standard of care. Clinical trial information: NCT05382286 . SG + pembro Chemo + pembro Pts with PFS2 events, n/N (%) 55/221 (25) 83/222 (37) Median PFS2 (95% CI), mo NR (NR-NR) 21.0 (16.0-NR) Stratified HR a (95% CI) 0.67 (0.48-0.95) Stratified log-rank nominal P -value a .0224 PFS2 rate (95% CI), % 12 mo 80.0 (73.8-84.9) 75.7 (69.1-81.1) 18 mo 71.9 (64.5-78.0) 53.0 (44.5-60.8) 24 mo 63.7 (51.1-73.9) 45.6 (35.6-55.1) a SG + pembro vs chemo + pembro.

Pre-clinical evidence suggests that stereotactic ablative body radiotherapy (SABR) can enhance systemic anti-tumor responses when combined with immune checkpoint inhibition, although optimal fractionation remains uncertain. Here we conducted a phase II multi-center randomized trial (AZTEC trial; ClinicalTrials.gov identifier NCT03464942) to investigate the efficacy and safety of single-fraction (SF: 20 Gy; n = 27) or multi-fraction (MF: 24 Gy in three fractions; n = 27) SABR followed by anti-PD-L1 atezolizumab in 54 women with advanced triple-negative breast cancer (TNBC) as first or second-line treatment. The primary endpoint was progression free survival (PFS). Secondary endpoints included best overall response rate, clinical benefit rate, overall survival and safety. We observed that both treatment arms had similar efficacy: median PFS was 2.5 months (90% CI: 1.7-4.5) for SF and 3.1 months (90% CI: 1.8-3.9) for MF. The null hypothesis (PFS of 2 months) was not rejected in either arm. Clinical benefit (RECIST complete response/partial response or stable disease for ≥24 weeks) was achieved in 5 (20%) SF and 6 (25%) MF patients, all with oligometastatic disease, independent of PD-L1 expression. Adverse events were similar between arms. Prespecified exploratory analysis of peripheral blood found higher CD8 TPEX cells before and during treatment correlated with improved response duration. Combining SABR with atezolizumab is safe, however further testing is required to determine efficacy for advanced TNBC patients with oligometastatic disease.

1012 Background: T-DXd is approved for adult pts with HER2+ mBC who received a prior anti-HER2–based regimen, or as 1L therapy when given in combination with pertuzumab (P). DESTINY-Breast07 (NCT04538742) is a Phase 1b/2, open-label, platform study exploring the safety, tolerability, and antitumor activity of T-DXd ± other anticancer agents in HER2+ mBC. 1L T-DXd ± P recently showed encouraging clinical activity and safety profiles consistent with previous reports. D, an anti-PD-L1 antibody, has shown efficacy in combination with T-DXd in HER2-low, hormone receptor (HR)–negative mBC. As part of the DESTINY-Breast07 final analysis, here we report the dose-expansion phase for T-DXd + D as a 1L treatment in HER2+ mBC. Methods: Pts had locally assessed HER2+ (IHC 3+ or IHC 2+/ISH+) mBC. A disease-free interval of ≥12 months (mo) from (neo)adjuvant therapy was required; no prior therapy for mBC was allowed. Pts were stratified by HR (positive vs negative), disease (recurrent vs de novo), and PD-L1 status (positive vs negative; positive defined as ≥1% IHC). Pts received T-DXd 5.4 mg/kg IV, in combination with D 1120 mg IV, every 3 weeks. Primary endpoints were safety and tolerability; secondary endpoints included confirmed ORR (cORR), duration of response (DOR) and progression-free survival (PFS) per RECIST 1.1 by investigator, time to progression on subsequent therapy or death (PFS2) by investigator, and overall survival (OS). Results: At data cutoff (DCO) (January 31, 2025), 64 pts were randomized to the T-DXd + D module, and 63 received treatment. Median follow up was 30.1 mo; median total treatment duration was 26.7 mo for T-DXd and 24.6 mo for D. Efficacy results are given in the Table. The most common adverse events (AEs) were nausea (79.4%), vomiting (46.0%), neutropenia (46.0% by grouped term [GT]), anemia (44.4% by GT), and fatigue (38.1%). Grade ≥3 AEs occurred in 58.7% (n=37/63) and serious AEs in 30.2% (n=19/63) of pts. There were two deaths due to AEs (3.2%): one pt with neutropenia and septic shock and one pt with sepsis. Adjudicated drug-related interstitial lung disease (ILD)/pneumonitis events occurred in 11 (17.5%; Grade 1, n=1; Grade 2, n=8; Grade 3, n=2) pts. Additional data by subgroups (stratification factors and biomarkers) will be presented. Conclusions: Encouraging clinical activity was seen for T-DXd + D as a 1L treatment for HER2+ mBC. Safety profiles were consistent with the known profiles for each therapy, with no fatal ILD events. These promising results provide a rationale for further investigation of this treatment combination. Clinical trial information: NCT04538742 . T-DXd + D (n=64) cORR (80% CI), % 82.8 (75.2, 88.8) mDOR* (Q1–Q3), mo 36.1 (23.3, NE) mPFS* (80% CI), mo 37.7 (35.1, NE) PFS rate at 24 mo (80% CI), % 75.5 (67.2, 82.0) mOS (80% CI), mo NE (NE, NE) mPFS2 (80% CI), mo NE (NE, NE) *Most pts were censored at DCO; m, median; NE, not evaluable; Q, quartile.

BACKGROUND:Responses to anti-HER2 therapy can vary based on estrogen receptor expression and HER2 gene amplification. This study assessed the magnitude of benefit by adding pertuzumab to trastuzumab and chemotherapy by estrogen receptor and HER2 levels in the APHINITY trial. METHODS:APHINITY (ClinicalTrials.gov identifier NCT01358877; BIG 4-11) was a randomized, double-blind, phase 3 trial comparing pertuzumab with placebo added to adjuvant trastuzumab and chemotherapy in 4804 patients with HER2-positive early breast cancer. The primary endpoint of this exploratory analysis was invasive disease-free survival (IDFS). Subgroup analyses used Cox models across 4 groups defined by HER2 fluorescence in situ hybridization (FISH) ratio and estrogen receptor status, adjusted for treatment arm, chemotherapy regimen, and a combined variable of nodal status and protocol version. Tumors with a FISH ratio below 2 were excluded, leaving 4782 evaluable cases. The HER2 FISH ratio was classified as low (2 to <5) or high (≥5) and estrogen receptor expression by immunohistochemistry as negative or positive using 1% and 10% cutoffs. IDFS, HER2 FISH ratio, and estrogen receptor expression were also analyzed by intrinsic molecular subtype. RESULTS:All subgroups benefited from pertuzumab, with the largest benefit in HER2 FISH-low/estrogen receptor-positive tumors (hazard ratio = 0.70, 95% CI = 0.51 to 0.95). Other subgroups showed smaller benefits, with HER2 FISH-high/estrogen receptor-negative tumors having the least numerical improvement (hazard ratio = 0.85, 95% CI = 0.59 to 1.25). No statistically significant IDFS differences were observed between HER2-enriched and non-HER2-enriched tumors. CONCLUSIONS:Pertuzumab improved IDFS in all subgroups, with the greatest improvement in HER2 FISH-low/estrogen receptor-positive tumors. These exploratory findings are hypothesis generating and support prospective validation of biomarker-guided strategies. TRIAL REGISTRATION:ClinicalTrials.gov identifier NCT01358877.

PURPOSE:Establish the safety, tolerability, and preliminary activity of trastuzumab deruxtecan (T-DXd) in combination with other anticancer therapies in human epidermal growth factor receptor 2 (HER2)-low metastatic breast cancer (mBC). PATIENTS AND METHODS:DESTINY-Breast08 was a two-part, open-label, multicenter, phase Ib study. Patients with locally confirmed HER2-low mBC received T-DXd plus capecitabine, durvalumab + paclitaxel, capivasertib, anastrozole, or fulvestrant. Eligibility criteria for hormone receptor status varied across modules and between study parts. Primary objectives were safety/tolerability and determining recommended phase II doses (RP2D); secondary endpoints included objective response rate (ORR; per investigator). RESULTS:In the dose-finding phase, 37 patients were assigned to a module. RP2Ds were determined for T-DXd plus capecitabine, capivasertib, anastrozole, or fulvestrant. For strategic reasons, T-DXd + durvalumab + paclitaxel was not pursued beyond the dose-finding phase (n = 3). In the dose-expansion phase, 101 patients were assigned to a module. For T-DXd + capecitabine, grade ≥3 adverse events (AE) occurred in 55% (11/20) of patients, and the ORR was 60%. For T-DXd + capivasertib, grade ≥3 AEs occurred in 67.5% (27/40) of patients, and the ORR was 60%. For T-DXd + anastrozole, grade ≥3 AEs occurred in 47.6% (10/21) of patients, and the ORR was 71.4%. For T-DXd + fulvestrant, grade ≥3 AEs occurred in 55% (11/20) of patients, and the ORR was 40%. Adjudicated drug-related interstitial lung disease/pneumonitis events were reported for T-DXd + capecitabine (3/20; grade 2, n = 2; grade 5, n = 1), T-DXd + capivasertib (8/40; all grade ≤2), and T-DXd + fulvestrant (5/20; all grade 2). CONCLUSIONS:Safety results were generally consistent with known individual profiles for T-DXd and combination drugs. T-DXd plus capecitabine, capivasertib, anastrozole, or fulvestrant demonstrated preliminary clinical activity in patients with HER2-low mBC.

TPS1148 Background: PIK3CA mutations constitutively activate PI3Kα and drive ~40% of HR+/HER2- breast cancer (BC). The PI3K inhibitors alpelisib and inavolisib and the AKT inhibitor capi are approved therapeutic options; however, they are limited by significant toxicity, notably hyperglycemia, rash, and diarrhea, due to non-selective targeting of the pathway. Zovega is a pan-mutant-selective allosteric PI3Kα inhibitor designed to optimize dose intensity and target inhibition with reduced toxicity and improved tolerability. The first-in-human ReDiscover study of zovega demonstrated encouraging antitumor activity with a median progression-free survival (PFS) of 10.3 mo (95% CI: 7.2, 18.4) across a range of PIK3CA genotypes and a favorable safety profile when combined with fulv in patients with PIK3CA -mutated HR+/HER2- advanced BC previously treated with a CDK4/6 inhibitor. Based on these findings, zovega in combination with fulv is being studied in this phase 3 study, ReDiscover-2 (NCT06982521), in PIK3CA -mutated HR+/HER2- advanced BC following recurrence or progression on or after a CDK4/6 inhibitor. Methods: ReDiscover-2 is a global, multicenter, open-label, randomized phase 3 study comparing the efficacy and safety of zovega + fulv to capi + fulv in patients with PIK3CA -mutated HR+/HER2- advanced BC. Approximately 540 patients will be randomized 1:1 to receive zovega (400 mg BID with food) + standard-dose fulv or capi (400 mg BID, 4 days on and 3 days off, with or without food) + fulv. Randomization will be stratified by PIK3CA mutation type, visceral disease, and geographic region. The primary endpoint is PFS assessed by blinded independent central review in patients having tumors with PIK3CA kinase domain mutations and in all patients. Overall survival is a key secondary endpoint within the same populations. Key eligibility criteria: ≥18 years of age with ECOG performance status of 0-1; confirmed diagnosis of HR+/HER2- locally advanced or metastatic BC with radiological or objective evidence of recurrence or progression; presence of one or more oncogenic PIK3CA mutations without evidence of AKT or PTEN alterations; measurable disease per RECIST v1.1 or evaluable bone-only disease; previous treatment for HR+/HER2- advanced BC with at least 1 and no more than 2 lines of endocrine therapy (ET) (prior fulv is allowed) or 1 prior line of CDK4/6 inhibitor therapy; HbA1c <7.0% (<53 mmol/mol) and fasting plasma glucose <140 mg/dL (Type 1 diabetes or Type 2 diabetes requiring antihyperglycemic medication are excluded); no prior PI3K, AKT, or mTOR inhibitors. ReDiscover-2 (NCT06982521) is open for enrollment. For information: clinicaltrials@relaytx.com. Clinical trial information: NCT06982521 .

1064 Background: PIK3CA mutations are a key therapeutic target in ER+ advanced breast cancer (ABC) readily detectable through circulating tumor DNA (ctDNA). The PI3Kα inhibitor alpelisib is approved in ER+ HER2- ABC but it remains unclear when to incorporate it into the current treatment algorithm. To address this, we conducted a phase II study assessing superiority of alpelisib + fulvestrant vs capecitabine in ABC pts with detectable PIK3CA mutant ctDNA following CDK4/6i plus aromatase inhibition (AI). Methods: Eligible pts had ER+ HER2- ABC, prior progression on CDK4/6i+AI and PIK3CA mutations detected via ctDNA ddPCR. Pts were randomized 1:1 to receive alpelisib (300mg daily) + fulvestrant (Arm A), or capecitabine (1000-1250mg/m 2 BD D1-14 q21d) (Arm B); strata were prior chemo for ABC and visceral disease. Primary endpoint was progression free survival (PFS) defined from randomization until progressive disease per RECIST 1.1 or death. Secondary endpoints included objective response rate (ORR), clinical benefit rate (CBR) and adverse events (AE). Imaging was q8 weeks. Exploratory objectives included efficacy according to baseline ctDNA levels. Sample size was reduced from 140 to 66 pts randomized for 53 PFS events to detect median PFS of 9 vs 5 mo (HR=0.56;1-sided α=0.1,80% power). Interim inefficacy analysis (60% events) estimated HR>1.0 and IDMC recommended early termination. Follow-up continued until all pts completed alpelisib. Reported are stratified Cox model HR (2-sided 80% CI) and 2-sided log rank test. Results: From July 2020-Oct 2024, 396 pts were screened, 113 (29%) had PIK3CA mutant ctDNA, 58 were randomized and 55 initiated treatment (Arm A: 29/30; Arm B: 26/28). Median age was 56 (32-82 yrs), 75% had visceral disease; 87% had no prior chemo for ABC. At final analysis, 43 PFS events occurred. Median PFS was 7.4 mo for alpelisib + fulvestrant vs 9.4 mo for capecitabine (HR 1.28, 80% CI: 0.84-1.96, p=0.45). For pts with visceral disease, median PFS was 5.4 mo (80% CI:3.7-5.1) for alpelisib + fulvestrant and 12.1 mo (80% CI:5.4-15) for capecitabine. ORR was 24.1% vs 50.0% and CBR 55.2% vs 61.5% for alpelisib + fulvestrant vs capecitabine. Overall, 79.3% pts on alpelisib + fulvestrant had grade 3/4 AEs with 24% grade 3 hyperglycemia vs 46.2% on capecitabine with 12% grade 3 palmar-plantar erythrodysesthesia. Six of 30 pts discontinued alpelisib due to AEs; 5 of the 6 continued fulvestrant. There was no evidence of association between baseline ctDNA levels (PIK3CA mutant copies/ml; median 90; range 2-13678) and PFS (HR 1.03; 90%CI:0.90-1.19). Conclusions: In our trial of pts with PIK3CA mutant ABC after progression on CDK4/6i plus AI, targeted therapy with alpelisib + fulvestrant was not superior to standard chemotherapy with capecitabine. Compared with the targeted therapy, capecitabine was better tolerated and resulted in longer PFS. Clinical trial information: ACTRN12619001117101.

1079 Background: INAVO (a highly potent and selective PI3Kα inhibitor that also promotes mut p110α degradation) + PALBO + FULV is approved for PIK3CA mut, HR+, HER2–, endocrine-resistant aBC based on statistically significant and clinically meaningful investigator-assessed progression-free survival (PFS) benefit over PBO + PALBO + FULV in INAVO120 (NCT04191499). We report exploratory analyses of treatment (tx) outcomes by lob histology status documented at initial diagnosis. Methods: Baseline characteristics, PFS, overall survival (OS), objective response rate (ORR), and duration of response (DoR) were evaluated for 53 pts with reported lob only and 64 with reported lob only or mixed lob (lob + ≥1 other selected subtype) histology at initial diagnosis. PIK3CA mut distribution, and association of histology and PFS with CDH1 alteration (alt) status, were also assessed. Results: At the clinical data cut-off for the updated PFS and final OS analyses (Nov 15, 2024), 24 and 29 pts with lob only histology, and 29 and 35 pts with mixed lob histology, were randomized to the INAVO and PBO arms, respectively. Baseline characteristics were balanced across tx arms. Median follow-up was 34.2 and 32.3 months (m) in the INAVO and PBO arms, respectively. Pathogenic CDH1 alts were more frequent in the lob only (65.2%) and mixed lob (58.9%) subgroups compared with no lob histology (6.3%). Efficacy by lob status is shown in the Table. PIK3CA mut distribution was similar across histologies. PFS benefit of INAVO over PBO was observed regardless of baseline CDH1 alt status (hazard ratio 0.3 for CDH1 alt and 0.5 for no alt detected). Conclusions: In this INAVO120 exploratory analysis, efficacy was improved with INAVO vs PBO, regardless of lob histology status at initial diagnosis and CDH1 alt status. This further supports the benefit of INAVO + PALBO + FULV in PIK3CA mut, HR+, HER2–, endocrine-resistant aBC. Clinical trial information: NCT04191499 . Lob only: INAVO n = 24 Non-lob: INAVO n = 137 Lob only: PBO n = 29 Non-lob: PBO n = 135 Mixed lob: INAVO n = 29 Non-mixed lob: INAVO n = 132 Mixed lob: PBO n = 35 Non-mixed lob: PBO n = 129 PFS, m (95% CI) 21.7 (9.3–25.8) 17.2 (11.6–24.3) 7.2 (3.7–9.4) 7.4 (5.8–9.7) 21.7 (11.3–27.9) 16.6 (11.2–24.2) 7.2 (3.8–9.4) 7.4 (5.8–9.7) OS, m (95% CI) NR (18.1–NR) 33.0 (27.1–44.8) 24.1 (11.1–NR) 27.0 (22.8–40.7) NR (28.4–NR) 33.0 (27.0–38.0) 24.1 (11.1–NR) 28.0 (22.8–40.7) ORR, n (%; 95% CI) 15 (62.5; 40.6–81.2) 86 (62.8; 54.1–70.9) 6 (20.7; 8.0–39.7) 40 (29.6; 22.1–38.1) 19 (65.5; 45.7–82.1) 82 (62.1; 53.3–70.4) 7 (20.0; 8.4–36.9) 39 (30.2; 22.5–38.9) DoR, m (95% CI) 21.2 (9.3–NR) 18.7 (12.2–28.3) 11.1 (8.5–NR) 10.7 (7.5–20.2) 20.3 (9.6–NR) 18.8 (11.1–28.7) 11.1 (3.1–NR) 11.1 (7.5–20.2) CI, confidence interval; NR, not reached.

554 Background: The 21-gene recurrence score (RS) is a foundational tool for risk-stratifying HR+/HER2- early breast cancer (EBC). However, clinical outcomes vary within RS categories. RlapsRisk BC (RR), an AI pathology-based test, integrates features from H&E-stained whole-slide images with clinical data (age, tumor size, nodal status) and was developed using 7 retrospective cohorts totaling 6,039 patients. We evaluated the clinical validity of RR and its histology-only component (RR-H) beyond RS and standard clinicopathologic factors. Methods: The clinical validity of RR was established through a validation program of 4 cohorts and over 8,521 patients across diverse geographic regions and laboratory settings. This included 3 international cohorts (n=933) and a prospective-retrospective analysis of the TAILORx trial, where RR-H was evaluable in 7,585 (97.5% of analyzable patients). The primary endpoint was distant recurrence-free interval (DRFI). In TAILORX, we assessed the additive value of the RR-H score to a base model (composed of age, tumor size, histological grade and RS) using Cox proportional hazards models and C-index comparison. Results: Across international validation cohorts (median follow-up 7.5 years, 9,8% DRFI events), RR successfully stratified patients (HR=4.91; 95% CI: 3.13-7.71; p < 0.00001), and identified a low-risk population with a 97.5% (95% CI: 95.8%-98.5%) 5-year DRFI rate. In the TAILORx population (n=7,584), the RR-H score was a highly significant independent predictor of DRFI. Incorporating RR-H as a continuous variable to the base model (including RS) significantly increased the C-index from 0.6730 to 0.7007 (p < 0.0001). The estimated HR for a 1-point difference in RR-H was 1.108 (95% CI: 1.078-1.138; p < 0.0001). When analyzed by quartiles, patients in the highest risk group (Q4) exhibited a significantly higher risk of distant recurrence compared to the lowest risk group (Q1) HR= 2.656 (95% CI: 2.003-3.522). Concordance analysis revealed that RR-H is independent of stromal TILs (R 2 =0.01). While RR-H correlated with increasing histological grade, substantial distribution overlap confirms intra-grade prognostic granularity. Furthermore, RR-H distributions were consistent across ILC and non-NLC. Conclusions: RR demonstrated robust and reproducible prognostic value across 8520 patients from diverse populations and settings establishing its clinical validity. Additional findings on TAILORx demonstrate that RR-H provides significant, independent prognostic value that complements existing prognostic tools (including genomic assays and clinicopathological factors), suggesting that integrating AI-pathology can refine precision risk assessment, and thus optimize adjuvant treatment in HR+ HER2- EBC.

Antitumor activity (dose-expansion phase).

BACKGROUND:Dual anti-human epidermal growth factor receptor 2 (HER2) therapy plus chemotherapy followed by maintenance treatment with HER2-targeted and endocrine therapies is standard first-line treatment for hormone-receptor-positive, HER2-positive metastatic breast cancer. On the basis of preclinical and clinical data, the addition of palbociclib (a selective inhibitor of cyclin-dependent kinases 4 and 6) may overcome resistance to both endocrine and HER2-directed therapies. METHODS:In this phase 3, open-label, randomized trial, we enrolled patients with hormone-receptor-positive, HER2-positive metastatic breast cancer who did not have disease progression after four to eight cycles of chemotherapy plus HER2-targeted therapy. Patients were randomly assigned in a 1:1 ratio to receive maintenance HER2-targeted and endocrine therapies with or without palbociclib. The primary end point was investigator-assessed progression-free survival. Secondary end points included the objective response, clinical benefit, safety, and overall survival. RESULTS:A total of 518 patients underwent randomization: 261 were assigned to receive palbociclib and 257 to receive standard therapy. At a median follow-up of 53.5 months, patients in the palbociclib group had significantly longer progression-free survival than those in the standard-therapy group (median duration, 44.3 months vs. 29.1 months; hazard ratio for disease progression or death, 0.75; 95% confidence interval, 0.59 to 0.96; two-sided P = 0.02). Grade 3 and 4 adverse events, predominantly from neutropenia, occurred in 79.7% and 10.0% of the patients, respectively, in the palbociclib group, as compared with 30.6% and 3.6% of the patients, respectively, in the standard-therapy group. CONCLUSIONS:The addition of palbociclib to maintenance anti-HER2 and endocrine therapies led to a significant improvement in progression-free survival over standard therapy, with increased toxic effects, mainly neutropenia. (Funded by Pfizer and others; PATINA ClinicalTrials.gov number, NCT02947685.).