Citations

TPS1138 Background: The SV-BR-1-GM breast cancer cell line activates anti-tumor immunity by expressing tumor associated antigens and secreting GM-CSF which enhances dendritic cell activation and promotes adaptive (T-cell mediated) and innate (dendritic and NK cell) immune responses. The cells are also engineered to optimize immune recognition through pt specific HLA antigen matching. SV-BR-1-GM acts through direct antigen presentation and CD4+ T-cell activation and, when combined w/ checkpoint inhibitors (CPIs), has demonstrated clinical benefit in 54 heavily pretreated metastatic breast cancer (MBC) pts. In pts w/ disease progression following CPI therapy, similar or improved progression free survival (PFS) compared to their prior treatment regimen. Disease control following antibody drug conjugates was observed in 40%of pts. Clinical benefit was seen in 5 out of 8 pts w/ untreated intracranial metastases. CD8+ Immuno-PET imaging suggests systemic activation, w/ increased CD8+ tumor infiltrating lymphocytes at both primary and metastatic tumor sites, as well as lymphoid organs. Optimized sequencing of CPI w/ SV-BR-1-GM and its latest phase 3 formulation have shown enhanced clinical outcomes, including improved overall survival (OS) (median 13.4 mos), PFS (3.6 mos), and clinical benefit rate (CBR; 61%).These findings have informed refinements to the ongoing pivotal, registration enabling Phase 3 trial, designed to optimize pt selection and treatment sequencing strategies. Methods: This ongoing multicenter, randomized, open label Phase 3 trial evaluates Bria-IMT + CPI vs. Treatment of Physician’s Choice (TPC) in MBC pts lacking approved curative therapies. Pts are randomized 1:1:1 to Bria-IMT + CPI, TPC, or Bria-IMT monotherapy (discontinued after 150 enrollments to prioritize combination arms). The Bria-IMT regimen consists of: Day -2: Cyclophosphamide 300 mg/m², Day 0: 20M irradiated SV-BR-1-GM cells, Day 2/3: 0.1 mcg pegylated α interferon at each inoculation site. CPI infusion is administered Day -3 to 3. Cycles q3w. TPC regimens follow site specific SOC. Imaging q6w (first 2 cycles), then q8w. Eligibility includes all MBC subtypes, including CNS mets, and permits prior CPI therapy ( >21 days pre-treatment). There will be 100 sites across the U.S., Canada, and ex-North America w/ an enrollment target of 404. The trial is currently active at 59 sites with 217 sub investigators. To date, 67 pts screened: 46 randomized (median age 56 yrs [34–83], median 6 [2–13] prior lines of therapy. The primary endpoint is OS, with an interim analysis at 144 events targeting a hazard ratio of 0.6. Secondary endpoints: PFS, overall response rate, CBR, CNS event free survival, and TWiST. Safety analyses ongoing; pt reported outcomes assess subjective treatment impact. Clinical trial information: NCT03328026 .

1001 Background: Imlunestrant (imlu) is a next-generation, brain-penetrant, oral selective estrogen receptor degrader. The EMBER-3 trial, in patients (pts) with ER+, HER2- ABC who had disease progression on or after aromatase inhibitor-based therapy, showed significant progression free survival (PFS) improvement with imlu vs standard therapy (SOC, fulvestrant or exemestane) in pts with ESR1 mutations ( ESR1 m), and with imlunestrant+abemaciclib (imlu+abema) vs imlu in all pts, regardless of ESR1 m. Exploratory PRO analyses are presented here. Methods: EORTC QLQ-C30 was administered at baseline (BL) and every 8 weeks until treatment discontinuation. Prespecified QLQ-C30 analysis used a longitudinal mixed model for repeated measures to calculate mean change from BL in pts with BL and ≥1 post-BL score. PRO-CTCAE (diarrhea frequency) was administered weekly, reporting 0 (never) to 4 (almost constantly). PRO-CTCAE (injection site reaction [ISR]) was administered to fulvestrant recipients weekly for 2 weeks post-injection, reporting yes/no (pain, swelling, redness). Descriptive analysis was used for PRO-CTCAE. Results: In pts with ESR1 m, imlu monotherapy was associated with many improved or maintained EORTC QLQ-C30 scores, whereas scores with SOC were declined or maintained. Specifically, pts with ESR1 m on imlu had improved global health status (GHS)/quality of life (QOL) and physical function (PF) scores, while scores with SOC declined (mean change differences between treatments: 9.9 [0.1, 19.7] and 6.2 [-0.8, 13.1], respectively). These PRO findings mirror the PFS findings in this group. In the overall population, GHS/QOL scores declined similarly with imlu vs SOC (mean change differences: 0.5 [-4.7, 5.7]), while PF scores were maintained with imlu vs a slight decline with SOC (mean change difference: 2.5 [-1.1, 6.1]). Most fulvestrant recipients (72%) reported ISR at any time while on treatment, with a mean of 31% during the first week of the first 6 cycles. Imlu+abema vs imlu showed broadly similar declines in all pts, with minimal mean change differences in GHS/QOL and PF scores (0.8 [-7.4, 5.9]; -2.2 [-6.6, 2.2], respectively). Pts reported similarly low rates of “frequent” or “almost constant” diarrhea with imlu (3%) and SOC (2%) and higher rates with imlu+abema (22%). Conclusions: PROs from EMBER-3 demonstrated that patients with ESR1 m had better GHS/QOL and PF with imlu vs SOC, mirroring efficacy results. While the frequency of CTCAE defined ISRs was low, the high rate of PRO-CTCAE ISR demonstrates that this clinically relevant adverse event is underappreciated by physicians. Additionally, all pts had generally comparable GHS/QOL and PF with imlu+abema vs imlu. Overall, these results support the efficacy and safety of imlu compared to existing SOC. Clinical trial information: NCT04975308 .

TPS1125 Background: Despite the many available treatments for patients (pts) with HR+, HER2– recurrent/metastatic breast cancer (MBC), optimal treatment after progression on CDK4/6 inhibitors (CDK4/6i) is unclear. One possible CDK4/6i resistance mechanism is increased expression of Aurora kinase A (AURKA), a key mitosis regulator associated with poor prognosis. Further implicated in CDK 4/6i resistance, high c-Myc or RB1 loss of function (LOF) are associated with transcriptional co-regulation or synthetic lethality, respectively, with AURKA. Alisertib is a highly selective, reversible, ATP-competitive, orally administered, small-molecule AURKA inhibitor with antiproliferative activity in HR+ BC-derived cell lines and BC xenograft models. Models with elevated AURKA or c-Myc expression, or RB1 LOF show greater alisertib sensitivity. Alisertib had activity in phase 1 and 2 trials, including objective response rates (ORRs) of 19.6–20% and median progression-free survival (PFS) of 5.4–5.6 months alone or with fulvestrant in pts with HR+/HER2–, endocrine-resistant MBC. The most common treatment-related grade ≥3 adverse events (AEs) were neutropenia, anemia, and leukopenia. Methods: ALISCA-Breast1 (NCT06369285) is a randomized phase 2 study. Primary objective: to determine the optimal alisertib dose administered with endocrine therapy (ET) based on AEs and serious AEs per CTCAE v5.0 and efficacy (ORR, duration of response, disease-control rate, PFS, overall survival). Secondary objectives: to identify biomarkers of efficacy and alisertib pharmacokinetics (PK). Key inclusion criteria: ≥18 years; ECOG performance status 0 or 1; confirmed HR+, HER2–, recurrent/metastatic breast adenocarcinoma not amenable to curative therapy; available tumor tissue for biomarker analyses; progression on or after ≥2 prior ET lines in recurrent/metastatic setting; prior CDK4/6i with ET in recurrent/metastatic setting. Key exclusion criteria: prior chemotherapy in recurrent/metastatic setting; prior AURKA-specific or pan-Aurora-targeted agents; unstable brain metastases. Eligible pts will be randomized 1:1:1 to alisertib 30 mg, 40 mg, or 50 mg orally twice daily on days 1−3, 8–10, and 15–17 every 28 days, plus physician’s choice of anastrozole, letrozole, exemestane, fulvestrant, or tamoxifen not previously used in recurrent/metastatic setting or progressed upon in adjuvant setting; ≤50 pts will be enrolled per arm in the USA and Europe. All pts will undergo sparse PK sampling. Tumor tissue will be centrally assessed for biomarkers, including RB1, MYC, TP53, ESR1, PI3K/AKT pathway, HER2 and AURKA genomic alterations/expression levels. The study will determine the optimal alisertib dose to combine with ET and may identify biomarker(s) defining pts with the greatest benefit from alisertib-based therapy. Clinical trial information: NCT06369285 .

Abstract The Phase 2 IMPRIME 1 study evaluated the combination of the pathogen-associated molecular pattern (PAMP) Imprime with the immune checkpoint inhibitor (ICI) pembrolizumab as second or later line of treatment (2 L+) for patients with metastatic triple-negative breast cancer (mTNBC). Eligible patients with mTNBC received weekly Imprime (4 mg/kg) intravenously in combination with pembrolizumab (200 mg every 3 weeks). Primary endpoints were overall response rate (ORR) and safety. Secondary endpoints included disease control rate (DCR), duration of response (DoR), progression free survival (PFS), and overall survival (OS). Exploratory endpoints included correlations between immune cell activation markers in tumor tissues and blood and response to therapy. Of the 42 evaluable patients, six had a response (one complete, five partial), with an ORR of 14.3% by RECIST v1.1. Median PFS was 2.7 months, median OS was 16.4 months, and DCR was 54.8%, with responders achieving a median DoR of 15.2 months. Therapy was generally well tolerated and resulted in an increase of immune activation markers, with higher levels of activation in peripheral blood associated with response and improved survival. The combination of Imprime and pembrolizumab was safe and demonstrated immune activation in tumor tissues and peripheral blood in patients with TNBC. Improved response rates were observed compared to historical studies of ICI monotherapy in similar patient populations. Study number (ClinicalTrials.gov trial registration): NCT02981303.

Abstract Background: Latin Americans are more likely to be diagnosed with aggressive early-stage breast cancer compared to Non-Hispanic White. Multiple factors may contribute to this, including metabolic factors and ancestry. In a previous study, we identified upregulated immune pathway genes in Luminal B tumors from obese Black patients compared to matched White patients. In this study, we report clinical and transcriptomic profiles of breast tumors from Latin and White patients to enhance understanding of factors contributing to aggressive tumor biology in Latin patients. Methods: We matched 311 Latin and 311 White breast cancer patients enrolled in FLEX by age, T stage, N stage and clinical subtype (ER/PR and HER2 status). FLEX (NCT03053193) is a prospective, observational trial that includes stage I-III breast cancer patients who receive MammaPrint (with or without BluePrint) as standard of care and consent to whole transcriptome and clinical data collection. MammaPrint is a 70-gene risk of distant recurrence signature that classifies patients as Low Risk or High Risk. BluePrint is an 80-gene molecular subtyping signature, categorizes tumors as Luminal-, HER2- or Basal-Type. MammaPrint further groups Luminal into A (Low Risk) and B (High Risk). ImPrint is a 53-gene signature that has been shown to predict the likelihood of achieving pCR with PD1-PDL1 immune checkpoint inhibitors. Statistical analyses on groups were conducted using arsenal R package and p-value < 0.05 was considered significant. Whole transcriptome comparisons were made, using limma R package, between Latin and White patients stratified by BluePrint subtype Luminal and weight categories normal (body-mass-index (BMI) 18.5 to < 25) and obese (BMI ≥30). Significant differentially expressed genes had an adjusted p-value < 0.05. Gene set enrichment analysis (GSEA) was conducted using fgsea R package. Results: Latin patients had significantly higher percentage of type 2 diabetes (23.3% vs 8.5%), BMI obese (49.0% vs 39.4%), BluePrint Basal (14.8% vs 9.3%) and ImPrint Immune Sensitive (12.0% vs 5.1%) compared to matched White patients. When comparing whole transcriptome of tumors from Latin and White patients, only tumors stratified by Luminal B and obesity resulted in differentially expressed genes: Latin patients had higher expression (>2-fold change) of 42 immunoglobulin genes compared to White, with only IGKV6-21 being statistically significant. Additional immune related genes such as IKZF1 and AGER, as well as UTS2 (gene encoding a vasoconstrictor agent and contributor of angiogenesis) were significantly upregulated in Latin patients. Upregulation of immune related pathways such as inflammatory response, interferon alpha/gamma response and downregulation of adipogenesis, oxidative phosphorylation and MYC targets were identified in Latin patients compared to White using GSEA Hallmarks gene sets. Conclusion: There were additional clinical and genomic differences between tumors from Latin and White patients, even when controlling for age, T stage, N stage and clinical subtype. Particularly, there were more type 2 diabetes, BMI obese, BluePrint Basal, and ImPrint Immune Sensitive among Latin patients. In addition, transcriptomic differences between obese Latin and matched White patients were found in Luminal B subgroup that may contribute to the aggressive tumor biology; immunoglobulin genes and immune related pathways were associated with higher expression in Latin patients. This study suggests that biological differences in breast tumors, particularly from obese patients, may result from shared background and reflects the need for inclusion of diverse patient groups in clinical trials. Citation Format: Marcela Mazo-Canola, Virginia Kaklamani, Pooja Advani, Sailaja Kamaraju, Alfredo A. Santillan-Gomez, Robert Maganini, Julie Barone, Sahra Uygun, Lavanya Samraj, William Audeh, Joyce O'Shaughnessy, FLEX Investigators' Group. Racial disparities in breast cancer and effect of obesity: MammaPrint, BluePrint and whole transcriptome analyses of tumors in Latin American patients in FLEX trial [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PS18-04.

Abstract Background: Abemaciclib is approved both for high-risk early breast cancer as well as advanced breast cancer (ABC) in the first- and second-line setting. In MONARCH 2, the addition of abemaciclib to fulvestrant significantly improved both progression-free survival (PFS) and overall survival (OS) in patients with HR+, HER2- ABC with disease progression on prior endocrine therapy (ET). In MONARCH 3, the addition of abemaciclib to a nonsteroidal aromatase inhibitor (NSAI) significantly improved PFS (HR, 0.540; 95% CI, 0.418-0.698; p=0.000002) as initial therapy in HR+, HER2- ABC. At the last interim OS analysis (~252 events, at 5.8 years follow-up [FU]), a numerically favorable median OS difference (12.6 months) was observed (HR, 0.754; 95% CI, 0.584-0.974; p=0.0301, non-significant [NS]). Here, we present the final OS analysis of MONARCH 3 (NCT02246621). Methods: MONARCH 3 is a randomized, double-blind, Phase 3 study of abemaciclib + NSAI (anastrozole or letrozole) vs placebo + NSAI in postmenopausal women with HR+, HER2- ABC without prior systemic therapy in the advanced setting. OS was a gated secondary endpoint and chemotherapy-free survival (CFS) an exploratory endpoint. Final OS analysis was planned after approximately 315 OS events had occurred in the intent-to-treat (ITT) population with a split alpha between the ITT population and the subgroup with visceral disease (sVD) according to a prespecified graphical testing scheme. Kaplan-Meier method and stratified Cox proportional hazards models were used for time-to-event analyses. All reported p-values are two-sided. Results A total of 493 women were randomized 2:1 to receive abemaciclib + NSAI (n=328) or placebo + NSAI (n=165). After a median FU of 8.1 years, 7% of patients were still receiving treatment in the abemaciclib arm vs 3% in the placebo arm. In the ITT population, 314 OS events were observed (198 deaths among 328 patients [60%] in the abemaciclib arm and 116 among 165 [70%] in the placebo arm; HR, 0.804; 95% CI, 0.637-1.015; p=0.0664, NS). Median OS was 66.8 months in the abemaciclib arm and 53.7 months in the placebo arm, a numerical difference of 13.1 months in the ITT population. In the sVD, 178 events were observed (113 deaths among 173 patients [65%] in the abemaciclib arm and 65 among 90 [72%] in the placebo arm; HR, 0.758; 95% CI, 0.558-1.030; p=0.0757, NS). Median OS was 63.7 months in the abemaciclib arm and 48.8 months in the placebo arm, a numerical difference of 14.9 months in the sVD. Consistent OS differences were observed across prespecified subgroups. PFS benefit was sustained (median 29.0 vs 14.8 months; HR, 0.535; 95% CI, 0.429-0.668; nominal p<0.0001) with substantial difference in 6-year PFS rates (23.3% vs 4.3% for abemaciclib vs placebo). CFS was also improved with abemaciclib vs placebo (median 46.7 vs 30.6 months; HR, 0.693; 95% CI, 0.557-0.863; nominal p=0.0010). No new safety signals were observed with longer term use. Conclusion In patients with HR+, HER2- ABC, abemaciclib in combination with a NSAI resulted in numerically longer OS compared to NSAI alone; however, statistical significance was not reached after a median FU of 8.1 years. The clinically meaningful improvement in median OS (>13 months) combined with the sustained significant improvement in median PFS (>14 months) and substantial extension in median CFS (>16 months) continue to support the use of abemaciclib in combination with NSAI as first-line therapy in ABC. Citation Format: Matthew Goetz, Masakazu Toi, Jens Huober, Joo Hyuk Sohn, Olivier Trédan, Inhae Park, Mario Campone, Shin-Cheh Chen, Luis Manual Manso, Shani Paluch-Shimon, Orit C. Freedman, Valerie Andre, Abhijoy Saha, Gertjan van Hal, Ashwin Shahir, Hiroji Iwata, Stephen RD Johnston, Joyce O'Shaughnessy, Xavier Pivot, Sara Tolaney, Sara Hurvitz, Antonio Llombart. MONARCH 3: Final overall survival results of abemaciclib plus a nonsteroidal aromatase inhibitor as first-line therapy for HR+, HER2- advanced breast cancer [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr GS01-12.

Clinical trials frequently include multiple end points that mature at different times. The initial report, typically based on the primary end point, may be published when key planned co-primary or secondary analyses are not yet available. Clinical trial updates provide an opportunity to disseminate additional results from studies, published in JCO or elsewhere, for which the primary end point has already been reported. Two years of adjuvant abemaciclib combined with endocrine therapy (ET) resulted in a significant improvement in invasive disease-free survival (IDFS) and distant relapse-free survival (DRFS) that persisted beyond the 2-year treatment period in patients with hormone receptor–positive, human epidermal growth factor receptor 2–negative, node-positive, high-risk early breast cancer (EBC). Here, we report 5-year efficacy results from a prespecified overall survival (OS) interim analysis. In the intent-to-treat population, with a median follow-up of 54 months, the benefit of abemaciclib was sustained with hazard ratios of 0.680 (95% CI, 0.599 to 0.772) for IDFS and 0.675 (95% CI, 0.588 to 0.774) for DRFS. This persistence of abemaciclib benefit translated to continuous separation of the curves with a deepening in 5-year absolute improvement in IDFS and DRFS rates of 7.6% and 6.7%, respectively, compared with rates of 6% and 5.3% at 4 years and 4.8% and 4.1% at 3 years. With fewer deaths in the abemaciclib plus ET arm compared with the ET-alone arm (208 v 234), statistical significance was not reached for OS. No new safety signals were observed. In conclusion, abemaciclib plus ET continued to reduce the risk of developing invasive and distant disease recurrence beyond the completion of treatment. The increasing absolute improvement at 5 years is consistent with a carryover effect and further supports the use of abemaciclib in patients with high-risk EBC.

Abstract Background: Adjuvant ET has been the standard of care for patients with estrogen receptor-positive (ER+), human epidermal growth factor receptor 2-negative (HER2-) EBC. A significant proportion of patients with increased risk still experience disease relapse despite available ET and more optimum ET is needed to prevent patients developing incurable metastatic cancer. Distant recurrence risk ranges from 20% to 40% after 5 years of adjuvant ET, depending on clinicopathological (clin-path) features at diagnosis. Consequently, there is a need to further optimize adjuvant treatment, particularly in those patients who are at increased risk of recurrence. Imlunestrant is an orally bioavailable selective estrogen receptor degrader (SERD) with pure antagonistic properties and the potential to overcome ET resistance. In early phase trials, imlunestrant monotherapy showed favorable safety with pharmacokinetic (PK) exposures exceeding fulvestrant and preliminary efficacy in ER+, HER2- advanced breast cancer patients (EMBER, Jhaveri 2022) along with robust biological/pharmacodynamic activity and tolerability in EBC (EMBER-2, Neven). Trial Design: EMBER-4 is a randomized, open-label, global phase 3 study comparing imlunestrant versus physicians’ choice of ET, in patients who are at an increased risk of recurrence based on clin-path features and who have received 2 to 5 years of standard adjuvant ET. Approximately 6,000 patients will be randomized 1:1 to receive imlunestrant (400 mg daily) for 5 years or physicians’ choice of adjuvant ET (tamoxifen or an aromatase inhibitor, AI, dosed per label). Study treatment duration is 5 years. Males and pre-/peri-menopausal women will receive concomitant treatment with a GnRH agonist if receiving imlunestrant or an AI. Stratification factors include time from initial adjuvant ET, use of prior adjuvant cyclin dependent kinase 4/6 inhibitors, nodal status, menopausal status, and geographic region. Eligibility criteria: Eligible patients are adult males and females (pre-, peri- or postmenopausal) with ER+, HER2- EBC who have completed definitive locoregional therapy and have received 2 to 5 years of prior adjuvant ET without disease recurrence, but who are at increased risk of recurrence based on clin-path features at diagnosis. Prior (neo) adjuvant chemotherapy and/or targeted therapy with a CDK4/6- or PARP- inhibitor is permitted. Study endpoints: The primary endpoint is invasive disease-free survival (IDFS), excluding second non-breast primary invasive cancers. Key secondary endpoints include distant relapse-free survival, overall survival, IDFS including second non-breast primary invasive cancers, safety, PK and patient reported outcomes. Recruitment for EMBER-4 begins globally in Q4 2022. Citation Format: Komal Jhaveri, Joyce O’Shaughnessy, Fabrice Andre, Matthew P. Goetz, Nadia Harbeck, Miguel Martín, Francois-Clement Bidard, Zachary M. Thomas, Suzanne Young, Roohi Ismail-Khan, Lillian M. Smyth, Michael Gnant. EMBER-4: A phase 3 adjuvant trial of imlunestrant vs standard endocrine therapy (ET) in patients with ER+, HER2- early breast cancer (EBC) with an increased risk of recurrence who have previously received 2 to 5 years of adjuvant ET [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr OT1-01-02.

Abstract Background Adjuvant abemaciclib (a CDK4 and 6 inhibitor) combined with ET resulted in significant and clinically meaningful improvement in invasive disease-free survival (IDFS) and distant relapse-free survival (DRFS) in patients (pts) with HR+, HER2-, node-positive, high risk EBC in the monarchE trial, and is an approved adjuvant therapy for these patients. Here we present efficacy results from a pre-specified overall survival interim analysis (OS IA2) which was planned to occur 2 years (yrs) after the primary outcome analysis. Methods Pts were randomized (1:1) to receive ET for up to 10 yrs +/- abemaciclib for 2 yrs (study treatment period). High-risk EBC was defined as either ≥4 positive axillary lymph nodes (ALN), or 1-3 ALN with either Grade 3 disease and/or tumor ≥5 cm (Cohort 1). While the proliferation biomarker Ki-67 was centrally assessed in all pts with available tissue sample, an additional smaller group of pts with 1-3+ ALN and central Ki-67 ≥20% as the only high-risk feature were included (Cohort 2). The intent-to-treat (ITT) population consisted of both Cohort 1 (5120 pts) and Cohort 2 (517 pts). Hazard ratios (HR) were estimated using Cox proportional hazard model. Results At a median follow-up of 42 months, all pts were off abemaciclib. IDFS and DRFS data illustrate a sustained benefit beyond the treatment period. In the ITT population, the HR for IDFS was 0.664 (95% CI: 0.578, 0.762) and DRFS was 0.659 (95% CI: 0.567, 0.767). At 4 yrs, this reflected an improvement in IDFS rates from 79.4% to 85.8% (absolute difference 6.4%), and in DRFS rates from 82.5% to 88.4% (absolute difference 5.9%). The continued separation of the curves was associated with an increase in absolute benefit in IDFS 4-year rates compared to 2-and 3-year IDFS rates (absolute difference 2.8% and 4.8% respectively). While OS remained immature, there was a lower number of deaths observed in the abemaciclib plus ET arm compared to the ET alone arm (157 [5.6%] vs 173 [6.1%], HR 0.929 [95% CI: 0.748, 1.153], p = 0.503), suggesting that the robust benefit in IDFS and DRFS began to translate into a numerically favorable OS HR. As previously described, within Cohort 1, a Ki-67 index of ≥20% was associated with a worse prognosis, but similar abemaciclib treatment effects were observed regardless of Ki-67 index. No new safety signals were observed. Conclusion The clinically meaningful benefit of adjuvant abemaciclib added to ET in HR+, HER2-, node-positive, high-risk EBC persists beyond completion of abemaciclib therapy, yielding an increase in absolute IDFS and DRFS benefit at 4 yrs. While OS remains immature at this time, the lower number of deaths in the abemaciclib arm compared to the ET arm suggest that a survival signal favoring abemaciclib is emerging. Citation Format: Stephen Johnston, Masakazu Toi, Joyce O’Shaughnessy, Priya Rastogi, Mario Campone, Patrick Neven, Chiun Sheng Huang, Jens Huober, Georgina Garnica Jaliffe, Irfan Cicin, Sara Tolaney, Matthew P. Goetz, Hope Rugo, Elżbieta Senkus, Laura Testa, Lucia Del Mastro, Chikako Shimizu, Ran Wei, Ashwin Shahir, Maria Munoz, Belen San Antonio, Valerie Andre, Nadia Harbeck, Miguel Martín. Abemaciclib plus endocrine therapy for HR+, HER2-, node-positive, high-risk early breast cancer: results from a pre-planned monarchE overall survival interim analysis, including 4-year efficacy outcomes [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr GS1-09.