Endocrine therapy (ET) resistance in estrogen receptor positive (ER+) advanced breast cancer is often linked to ESR1 mutations, yet responses to oral selective ER degraders vary within mutant subgroups. Through a biomarker analysis of acelERA Breast Cancer (NCT04576455), we show that tumor ER transcriptional activity as well as circulating tumor DNA (ctDNA) genomics and dynamics effectively stratify response to ET, including giredestrant. We find that following first-line therapy, the ctDNA genomic landscape is diverse and influenced by CDK4/6 inhibitor exposure. Despite this complexity, ER activity in ESR1-mutant tumors remains comparable to early breast cancer but is reduced in most non-mutant cases. This maintained ER activity is associated with giredestrant benefit. Furthermore, early ctDNA clearance identifies responding patients, and the combination of low ER activity and high ctDNA burden predicts rapid clinical progression. These findings provide a framework for personalizing future breast cancer therapies by integrating liquid biopsies with tissue-based signatures.

Abstract Background: Endocrine resistance limits benefit durability in ER+/HER2− advanced breast cancer (aBC), particularly after CDK4/6 inhibition. Prior studies have shown that tumor RNA-based estrogen receptor (ER) activity predicts response to the oral SERD giredestrant and serves as a pharmacodynamic (PD) biomarker of ER pathway suppression. However, tumor RNA profiling requires serial biopsies which are often challenging in aBC. The Precede ER Dependence Index (PERDI) quantifies ER-driven cis-regulatory enhancer activity from circulating chromatin, enabling noninvasive monitoring of ER pathway dependence and treatment response. Methods: 87 patients (pts) with ER+/HER2− aBC received second or third line giredestrant, alone or in combination. One milliliter of plasma from 87 pre-treatment (tx) and 9 on-tx samples was profiled using Precede’s comprehensive epigenomic liquid biopsy assay. PERDI scores were evaluable for 80 pre-tx and 8 on-tx samples. Baseline ESR1 mutation (m) status was determined using the FoundationOne®Liquid CDx assay. PERDI concordance was assessed against tissue ER activity from matched RNA-seq. Plasma-based gene expression was inferred using Precede’s algorithms that integrate enhancer, promoter and DNA methylation features. Results: Plasma PERDI strongly correlated with tumor tissue ER activity in pts without detectable ESR1m and effectively discriminated ESR1m status. Plasma-derived predictions of ER signaling-related genes were concordant with tumor RNA-seq results, with identified genes offering biological insights into clinical response. At baseline, PERDI distinguished RECIST 1.1-evaluated responders well compared to tumor ER activity or ctDNA level. Similar to tumor ER activity, a median split of PERDI demonstrated strong predictive power (HR=0.43; CI: 0.20-0.93), with median PFS of 11.4 months in the PERDI-high group and 2.0 months in the PERDI-low group. The effect size was further enhanced when comparing the top versus bottom PERDI tertiles (HR=0.19; CI: 0.07-0.52). PERDI complemented ESR1m status and ctDNA levels in predicting response, identifying rapid progressors with ESR1m as well as durable responders despite high ctDNA or absent ESR1m. PERDI declined on-tx in all partial responders (3/3), suggesting pathway-specific modulation beyond ctDNA effects. Conclusions: PERDI quantifies plasma-based ER activity and predicts giredestrant benefit, particularly when integrated with ESR1 genotype or ctDNA tumor fraction. PERDI also showed early evidence of PD change directly from plasma, mitigating the need for serial tumor biopsies. cfDNA-based gene expression modeling enables noninvasive assessment of ER dependence and response-associated transcriptional programs, offering a scalable framework for monitoring endocrine response and adaptation in ER+/HER2− aBC. Citation Format: Ann E. Collier, Jon Beagan, Travis Clark, Kristian Cibulskis, Corrie Painter, Mary McGillicuddy, Aparna Gorthi, Khoi Nguyen, James Sullivan, Richard Schwab, Jing Zhu, Pablo Perez-Moreno, Tharu M. Fernando, Matthew Eaton, Carl J. Barrett. A liquid biopsy assay of estrogen receptor activity predicts response to giredestrant in ER+/HER2− advanced breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 2606.

TPS8651 Background: The KRAS G12C mutation, present in ~12% of NSCLC patients, drives oncogenic signaling and cancer formation and is associated with poor prognosis. The current first-line treatment for advanced KRAS G12C+ NSCLC is checkpoint inhibitor (CPI) ± chemotherapy (CT). Novel combinations using a more targeted, biomarker-directed approach are supported by pre-clinical evidence and may further improve outcomes. Divarasib is an oral KRAS G12C inhibitor with potent pre-clinical and clinical anti-tumor activity. We hypothesize that divarasib + CPI ± CT may improve outcomes for patients with KRAS G12C+ NSCLC. Methods: Krascendo-170 Lung (NCT05789082) is a phase Ib/II, open-label study evaluating the safety and activity of divarasib + pembrolizumab in patients with PD-L1 tumor cell expression ≥1% (Cohort A) and of divarasib + pembrolizumab with platinum-based CT and pemetrexed in patients with any PD-L1 tumor cell expression level (Cohort B). Patients must be ≥18 years old with untreated unresectable/metastatic non-squamous NSCLC (measurable per RECIST v1.1), a confirmed KRASG12C mutation, and an Eastern Cooperative Oncology Group performance status 0/1. Each cohort will have two stages: divarasib combination dose finding and dose expansion, with two planned dose levels of divarasib (Table). Tumor assessments will be performed at baseline and every 6 weeks for 48 weeks, then every 9 weeks thereafter. Plasma samples will be taken at various timepoints before and after divarasib and pembrolizumab dosing to characterize pharmacokinetics. Patients will be treated until disease progression per RECIST v1.1 or unacceptable toxicity. The co-primary endpoints are adverse events and change from baseline in targeted safety parameters. Key secondary endpoints include objective response rate, progression-free survival and duration of response (all investigator assessed per RECIST v1.1). Enrollment into the combination dose finding stage of Cohort A has been completed without dose-limiting toxicities and enrollment into the dose expansion stage is continuing. Clinical trial information: NCT05789082 . [Table: see text]

Abstract Background: Racial and ethnic disparities are highly prevalent in cancer care and impact treatment outcomes, with an underrepresentation of minority populations in clinical studies. The impact of genetic ancestry on the genomic landscape of tumors remains understudied. To address this, we sought to comprehensively characterize the ancestry-based genomic co-alteration landscape and immunotherapy-related biomarkers in KRAS and EGFR-altered tumors, two major driver populations in non-squamous non-small cell lung cancer (non-Sq NSCLC). Methods: Our study consisted of 68,197 adult patients with non-Sq NSCLC who underwent comprehensive genomic profiling using FoundationOne® or FoundationOne®CDx during routine clinical care in the United States. Genetic ancestry was inferred using a SNP-based approach. Tumor mutational burden (TMB) was calculated across 0.8-1.2 megabases. PD-L1 expression was determined by immunohistochemistry using the Dako 22C3 PD-L1 antibody. Results: Overall, 81% of the patients were of European (EUR; n= 55,430), 10% of African (AFR; n=7,062), 5% of East Asian (EAS; n=3,297), 3% of Admixed American (AMR; n=2,011) and less than 1% of South Asian ancestry (SAS; n=497). KRAS was the most frequently altered oncogene in EUR (39%) and AFR (33%), whereas EGFR was most frequently altered in EAS (53%), SAS (36%), and AMR (30%) ancestry groups. While STK11 and KEAP1 alterations co-occurred with KRAS across all ancestry groups, they were significantly (FDR p <= 0.05) less frequent in EAS (16%) and AMR (15%) compared to EUR (28%). Additional ancestry-specific co-alteration patterns in KRAS-altered tumors included co-occurrence with GNAS alterations in AMR (Odds ratio, OR: 3.5, p < 10-5), co-occurrence with ARID1A alterations in SAS (OR: 4.7, p = 0.02), and mutual exclusivity with NF1 alterations in EUR (OR: 0.4, p < 10-5) and AFR (OR: 0.4, p < 10-5). In contrast, EGFR-altered tumors had a more conserved co-alteration landscape across all ancestry groups. Despite the known mutual exclusivity of KRAS and EGFR alterations, 13% of patients of EAS ancestry (compared to 2-4% of other ancestry groups) with KRAS alterations had co-occurring EGFR alterations; notably, a majority of these patients had an amplification in KRAS and/or EGFR, potentially representing treatment resistance mechanisms. Among immunotherapy-associated biomarkers, PD-L1 expression was similar across ancestries. Of note, patients of AFR ancestry with KRAS or EGFR alterations had higher TMB and the SAS ancestry group had the lowest TMB. Conclusion: Our study provides a comprehensive landscape of ancestry-specific patterns in KRAS and EGFR-altered non-Sq NSCLC. These findings can help better understand cancer disparities, aid in the development of new therapeutic strategies and inform more inclusive clinical trials and treatment decisions. Citation Format: Saumya D. Sisoudiya, Armande A. Houle, Tharu M. Fernando, Timothy R. Wilson, Jennifer L. Schutzman, Jessica K. Lee, Alexa B. Schrock, Ethan S. Sokol, Smruthy Sivakumar, Zhen Shi, Gaurav Pathria. Genomic profiling of KRAS and EGFR-altered non-squamous non-small cell lung cancer reveal ancestry-specific co-alterations with therapeutic implications [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 6120.

Racial/ethnic disparities mar NSCLC care and treatment outcomes. While socioeconomic factors and access to healthcare are important drivers of NSCLC disparities, a deeper understanding of genetic ancestry-associated genomic landscapes can better inform the biology and the treatment actionability for these tumors. We present a comprehensive ancestry-based prevalence and co-alteration landscape of genomic alterations and immunotherapy-associated biomarkers in patients with KRAS and EGFR-altered non-squamous (non-Sq) NSCLC. KRAS was the most frequently altered oncogene in European (EUR) and African (AFR), while EGFR alterations predominated in East Asian (EAS), South Asian (SAS), and Admixed American (AMR) groups, consistent with prior studies. As expected, STK11 and KEAP1 alterations co-occurred with KRAS alterations while showing mutual exclusivity with EGFR alterations. EAS and AMR KRAS-altered non-Sq NSCLC showed lower rates of co-occurring STK11 and KEAP1 alterations relative to other ancestry groups. Ancestry-specific co-alterations included the co-occurrence of KRAS and GNAS alterations in AMR, KRAS, and ARID1A alterations in SAS, and the mutual exclusivity of KRAS and NF1 alterations in the EUR and AFR ancestries. Contrastingly, EGFR-altered tumors exhibited a more conserved co-alteration landscape across ancestries. AFR exhibited the highest tumor mutational burden, with potential therapeutic implications for these tumors.

Estrogen receptor (ER) α is expressed in the vast majority of breast cancers and is one of the most successfully prosecuted drug targets in oncology, with multiple classes of endocrine therapies approved for the treatment of ER+ breast cancer. These existing agents are highly active, both as single agents and as combination partners for other targeted therapies, and have significantly benefited patients. However, each of these standard-of-care (SOC) therapies has liabilities that allow for the reengagement of ER signaling as a mechanism of resistance. Data supporting the continued dependence of tumors on ER signaling following exposure to SOC agents have underpinned an extraordinary reenergizing of academic, biotechnology, and pharmaceutical groups pursuing next-generation ER-targeted therapies. The hypothesis that there remains an opportunity to bring further meaningful benefit to patients through fully optimized ER-targeted therapies is currently being investigated in the clinic.

Supplementary Figures 1-8 Supplementary Tables 1-2 Supplementary Materials and Methods

Background The development of more potent selective oestrogen receptor antagonists and degraders (SERDs) that can be orally administered could help to address the limitations of current endocrine therapies. We report the primary and final analyses of the coopERA Breast Cancer study, designed to test whether giredestrant, a highly potent, non-steroidal, oral SERD, would show a stronger anti-proliferative effect than anastrozole after 2 weeks for oestrogen receptor-positive, HER2-negative, untreated early breast cancer.Methods In this open-label, randomised, controlled, phase 2 study, postmenopausal women were eligible if they were aged 18 years or older; had clinical T stage (cT)1c to cT4a-c (& GE;1 & BULL;5 cm within cT1c) oestrogen receptor-positive, HER2negative, untreated early breast cancer; an Eastern Cooperative Oncology Group performance status of 0-1; and baseline Ki67 score of at least 5%. The study was conducted at 59 hospital or clinic sites in 11 countries globally. Participants were randomly assigned (1:1) to giredestrant 30 mg oral daily or anastrozole 1 mg oral daily on days 1-14 (window-of-opportunity phase) via an interactive web-based system with permuted-block randomisation with block size of four. Randomisation was stratified by cT stage, baseline Ki67 score, and progesterone receptor status. A 16-week neoadjuvant phase comprised the same regimen plus palbociclib 125 mg oral daily on days 1-21 of a 28-day cycle, for four cycles. The primary endpoint was geometric mean relative Ki67 score change from baseline to week 2 in patients with complete central Ki67 scores at baseline and week 2 (window-of-opportunity phase). Safety was assessed in all patients who received at least one dose of study drug. The study is registered with ClinicalTrials.gov (NCT04436744) and is complete.Findings Between Sept 4, 2020, and June 22, 2021, 221 patients were enrolled and randomly assigned to the giredestrant plus palbociclib group (n=112; median age 62 & BULL;0 years [IQR 57 & BULL;0-68 & BULL;5]) or anastrozole plus palbociclib group (n=109; median age 62 & BULL;0 [57 & BULL;0-67 & BULL;0] years). 15 (7%) of 221 patients were Asian, three (1%) were Black or African American, 194 (88%) were White, and nine (4%) were unknown races. At data cutoff for the primary analysis (July 19, 2021), the geometric mean relative reduction of Ki67 from baseline to week 2 was -75% (95% CI -80 to -70) with giredestrant and -67% (-73 to -59) with anastrozole (p=0 & BULL;043), meeting the primary endpoint. At the final analysis (data cutoff Nov 24, 2021), the most common grade 3-4 adverse events were neutropenia (29 [26%] of 112 in the giredestrant plus palbociclib group vs 29 [27%] of 109 in the anastrozole plus palbociclib group) and decreased neutrophil count (17 [15%] vs 16 [15%]). Serious adverse events occurred in five (4%) patients in the giredestrant plus palbociclib group and in two (2%) patients in the anastrozole plus palbociclib group. There were no treatment-related deaths. One patient died due to an adverse event in the giredestrant plus palbociclib group (myocardial infarction).Interpretation Giredestrant offers encouraging anti-proliferative and anti-tumour activity and was well tolerated, both as a single agent and in combination with palbociclib. Results justify further investigation in ongoing trials.Funding F Hoffmann-La Roche.Copyright & COPY; 2023 The Author(s). Published by Elsevier Ltd. This is an Open Access article under the CC BY 4.0 license.

Supplementary Data from VERONICA: Randomized Phase II Study of Fulvestrant and Venetoclax in ER-Positive Metastatic Breast Cancer Post-CDK4/6 Inhibitors – Efficacy, Safety, and Biomarker Results

BACKGROUND The cell proliferation biomarker Ki67 is expressed during every phase of the cell cycle and has long been used as a diagnostic tool for cancer prognosis, especially for hormone receptor positive breast cancer (HR+ BC). More recently, Ki67 has emerged as a companion diagnostic to select patients for the medicines targeting high risk HR+ BC. We report survey results of local Ki67 immunohistochemistry (IHC) testing practices across 99 pathology labs in 11 countries supporting clinical trial sites in 2020-2021 for the coopERA Breast Cancer clinical study in neoadjuvant HR+ BC (NCT04436744). METHODS The survey was disseminated to pathology labs across five continents to assess local Ki67 IHC staining, analysis, and scoring methodologies. Metrics included pre-analytical considerations (e.g. sample type and requirements) and analytical considerations (e.g. test validation status, antibody, scoring methods, reporting). RESULTS All pathology labs reported requiring formalin-fixed, paraffin-embedded (FFPE) tissue for local Ki67 testing with 89% using sections with thickness of 2-5 microns. For the Ki67 test, the majority (65%) reported using an in vitro diagnostic assay, 23% used a validated test, and 8% utilized a research-use-only assay. Ki67 antibody selection varied among the labs with 46% using the MIB-1 mouse monoclonal (Dako Agilent), followed by 33% using the 30-9 rabbit monoclonal (Ventana) and 12% reporting the SP6 rabbit monoclonal (Thermo Fisher). A majority (65%) reported using single pathologist visual assessment for scoring, and 17% reported using two or more pathologists. Use of automated digital image analysis (ADIA) was reported by 18% of labs, either alone or in combination with pathologist visual assessment. A significant portion (75%) reported using the International Ki67 in Breast Cancer Working Group (IKWG) recommendations, whereas 7% reported using only digital image analysis (e.g. Ventana Virtuoso). A minority (7%) indicated neither and instead described variations of “eyeball” or “hot spot” visual estimates. Most labs (65%) reported counting at least 500 cells with 15% of these counting more than 1000 cells. Remaining labs (30%) counted less than 500 or no cells. Predominantly, 85% reported counting cells in at least 3 or more high power fields. Most labs (96%) report Ki67 scores as a percentage of positive nuclei and the remaining minority reported using other methods (e.g. ranges [< 10%, 10-20%, etc.] or H-score [0-300]). CONCLUSIONS The survey results suggest high global variability of local Ki67 testing practices with the highest variability observed in the test validation status, Ki67 clone, and scoring methods. Despite efforts by the IKWG to harmonize and increase the clinical validity of Ki67 as a biomarker, many labs indicating IKWG compliance had survey answers that were discordant with the specific guidelines set forth by the working group. Taking into account the totality of all answers provided by each respondent, only 51% of the surveyed labs fully conformed to the IKWG recommendations. Moreover, a small fraction conducts global estimations without specific cell counting or use “hot spot” scoring methods, despite the high variability and low reproducibility of these scores both intra- and inter-lab. This study demonstrates the benefits of using a central assay in clinical studies to reduce the variability of local Ki67 results in identifying high risk HR+ BC patients and suggests more work is needed to streamline the analytical practices of local Ki67 methodologies, which may directly impact clinical decisions such as the use of neoadjuvant therapies in HR+ BC. Citation Format: Heather M. Moore, Wendy W. Lin, Tharu M. Fernando, Celine Lopez, Emma Kent, Karine Ellouk, Jennifer M. Giltnane. Variations in diagnostic practice of Ki67 scoring suggest standard guidelines needed for pathological assessment: survey results of global Ki67 testing methods [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 P6-04-09.

Background: Endocrine therapy remains the mainstay treatment for ER+ BC. CDK4/6 inhibitors induce cell cycle arrest and decrease tumor cell proliferation, as measured by the biomarker Ki67, when used in combination with aromatase inhibitors (AI) such as anastrozole (A). Giredestrant is an oral, well-tolerated, and highly potent selective ER degrader (SERD) that achieves robust ER suppression and has demonstrated antitumor activity in the metastatic setting either as monotherapy or in combination with the CDK4/6 inhibitor palbociclib. The randomized, phase 2 coopERA Breast Cancer study (NCT04436744) evaluated giredestrant in postmenopausal women with untreated ER+/HER2- early BC and met its primary endpoint, demonstrating superior Ki67 suppression with giredestrant vs A after two weeks of single agent treatment. This suppression was maintained at surgery where giredestrant vs A was evaluated in combination with palbociclib. Here, we present gene expression analysis and associations with Ki67 response. Methods: 221 eligible patients with measurable ER+/HER2– untreated early BC and baseline Ki67 ≥ 5% were randomized 1:1 to receive 30 mg oral daily (PO QD) giredestrant or 1 mg PO QD A on Days 1–14 of a neoadjuvant window-of-opportunity phase, followed by four 28-day cycles of PO QD giredestrant or A with 125 mg PO QD palbociclib on Days 1–21 before surgery. FFPE specimens were collected at baseline, week 2 and surgery; and RNA-sequencing (seq) was performed. Gene expression analysis included ER pathway activity, PAM50 intrinsic subtypes, and other pathway analyses, assessed by Ki67 response. Results: 112 and 92 patients had paired tumor samples at baseline/week 2 and baseline/surgery, respectively, that were evaluable for RNA-seq and Ki67. The trend for greater Ki67 protein suppression by giredestrant vs A from baseline to week 2 was maintained in the RNA-seq evaluable subset. Interestingly, the same subset revealed similar suppression of both proliferation gene signatures and ER pathway activity between A and giredestrant. PAM50 subtyping showed that 69% of tumors were luminal (Lum) A and 29% were LumB at baseline. Less than 1% were classified as basal or HER2. Interestingly, giredestrant (G) showed greater suppression of both Ki67 and ER pathway activity vs A in LumB tumors (Ki67: -82% [G] vs -62% [A]; ER activity: -0.83 [G] vs -0.66 [A]) compared to LumA at week 2 (Ki67: -74% [G] vs -71% [A]; ER activity: -0.60 [G] vs -0.70 [A]). Moreover, at week 2, 83% (13/18) of LumB tumors at baseline transitioned into a LumA subtype after giredestrant treatment compared to 46% (5/11) of A-treated tumors. Giredestrant-treated tumors also achieved lower mean ER pathway activity compared to those treated with A at surgery (p=0.023). Gene set enrichment analysis showed downregulation of cell-cycle and ER-related pathways at week 2 and surgery in both treatment arms. A subset of cytokine signaling and immune response pathways were increased at week 2 compared to baseline after treatment with A but not with giredestrant. These pathways were also associated with Ki67 resistance (Ki67 ≥ 7.4%) in A-treated tumors. This was consistent with differential expression analysis of samples collected at week 2, in which cytokine signaling pathways were enriched in A compared to giredestrant. Notably, IL12 signaling was enriched in tumors resistant to A but not giredestrant. Conclusions: Giredestrant has a greater effect on Ki67 protein suppression in ER+/HER2- early BC compared to A, which is more pronounced in LumB tumors. This benefit may involve differential regulation of cytokine and immune responses. These exploratory findings reveal novel mechanisms that may differentiate the activity of SERDs vs AIs, which warrant further validation. Citation Format: Alejandro M. Chibly, Tharu M. Fernando, Ciara Metcalfe, Marc Hafner, Gilbert Owusu-Manu, Sara Hurvitz, Aditya Bardia, Peter A. Fasching, Yeon H. Park, Vanesa Quiroga, Jutta Steinseifer, Pablo Perez-Moreno, Heather M. Moore. PD13-02 Exploratory gene expression analysis of coopERA Breast Cancer (BC): a study evaluating neoadjuvant giredestrant versus anastrozole alone and in combination with palbociclib in ER-positive, HER2-negative untreated early BC [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 PD13-02.

Background Endocrine therapy, the therapeutic mainstay for estrogen receptor-positive breast cancer, targets estrogen receptor activity and/or estrogen synthesis. CDK4/6 inhibitors cause cell cycle arrest and significantly decrease expression of the proliferation biomarker Ki67 when used in conjunction with aromatase inhibitors such as anastrozole. Giredestrant, a highly potent, nonsteroidal, oral, selective estrogen receptor antagonist and degrader, achieves robust estrogen receptor occupancy, is well tolerated, and has encouraging antitumor activity as a monotherapy and in combination with the CDK4/6 inhibitor palbociclib in metastatic breast cancer. coopERA Breast Cancer (NCT04436744) is a phase II study investigating 2 weeks of giredestrant versus anastrozole in a window-of-opportunity phase, followed by 4 months of giredestrant plus palbociclib versus anastrozole plus palbociclib in a neoadjuvant phase in postmenopausal women with estrogen receptor-positive, HER2-negative, untreated early breast cancer. We will report the results of the primary analysis. Methods Eligible patients who had measurable cT1c (≥1.5 cm)-cT4a-c estrogen receptor-positive, HER2-negative, untreated early breast cancer and baseline Ki67 score ≥5% were randomized 1:1 to 1 mg oral, daily anastrozole or 30 mg oral, daily giredestrant on Days 1-14 (window-of-opportunity phase lasting 14 days) followed by daily dosing for four 28-day cycles in combination with 125 mg oral palbociclib on Days 1-21 (neoadjuvant phase lasting 16 weeks) before surgery. Patients were stratified according to T status, Ki67 score, and progesterone receptor status. The primary efficacy endpoint was centrally assessed geometric mean relative Ki67 score change from baseline to Week 2 during the window-of-opportunity phase, which is reflective of the ability of endocrine therapies to suppress tumor-cell proliferation, and is a surrogate marker for clinical outcomes. The secondary efficacy endpoint is complete cell cycle arrest rate (CCCA), defined as Ki67 score ≤2.7%, at Week 2. Safety was also assessed. Results Results of a previous interim analysis (including 83 of the planned 202 patients) demonstrated a greater relative reduction of Ki67 at 2 weeks with giredestrant (reduction from baseline to Week 2 geometric mean = 80%; 95% CI = -85%, -72%) compared with anastrozole (reduction from baseline to Week 2 geometric mean = 67%; 95% CI = -75%, -56%; P = 0.0222). Similarly, consistent Ki67 suppression was observed in patients with baseline Ki67 ≥20% (83% reduction with giredestrant versus 71% reduction with anastrozole) or <20% (65% versus 24% reductions). At Week 2, 25% of tumors exhibited CCCA with giredestrant versus 5.1% with anastrozole (a 20% difference; 95% CI = -37%, -3%). Safety results were consistent with the known safety profile for giredestrant. Fewer patients experienced adverse events (AEs) related to giredestrant (28%) than to anastrozole (38%), and no grade ≥3 AEs or serious adverse events were assessed as related to giredestrant. We will present the results of the primary analysis, which will include data from all enrolled patients, and will report the primary and secondary efficacy endpoints (including patients with Ki67 >20%), and updated safety. Conclusions The study will proceed to the primary analysis. We expect to see encouraging results based on the favorable interim analysis data that demonstrated the superior activity of giredestrant, an oral selective estrogen receptor antagonist and degrader, compared with anastrozole. Citation Format: Sara A Hurvitz, Vanesa Quiroga, Yeon Hee Park, Aditya Bardia, Vanesa López-Valverde, Jutta Steinseifer, Tharu M Fernando, Gonzalo Spera, Cloris Xue, Peter A Fasching. Neoadjuvant giredestrant (GDC-9545) + palbociclib versus anastrozole + palbociclib in postmenopausal women with estrogen receptor-positive, HER2-negative, untreated early breast cancer: Primary analysis of the randomized, open-label, phase II coopERA breast cancer study [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr PD13-06.

Abstract Purpose: Despite promising activity in hematopoietic malignancies, efficacy of the B-cell lymphoma 2 (BCL2) inhibitor venetoclax in solid tumors is unknown. We report the prespecified VERONICA primary results, a randomized phase II clinical trial evaluating venetoclax and fulvestrant in estrogen receptor (ER)-positive, HER2-negative metastatic breast cancer, post–cyclin-dependent kinase (CDK) 4/6 inhibitor progression. Patients and Methods: Pre-/postmenopausal females ≥18 years were randomized 1:1 to venetoclax (800 mg orally daily) plus fulvestrant (500 mg intramuscular; cycle 1: days 1 and 15; subsequent 28-day cycles: day 1) or fulvestrant alone. The primary endpoint was clinical benefit rate (CBR); secondary endpoints were progression-free survival (PFS), overall survival, and safety. Exploratory biomarker analyses included BCL2 and BCL extra-large (BCLXL) tumor expression, and PIK3CA circulating tumor DNA mutational status. Results: At primary analysis (cutoff: August 5, 2020; n = 103), venetoclax did not significantly improve CBR [venetoclax plus fulvestrant: 11.8% (n = 6/51; 95% confidence interval (CI), 4.44–23.87); fulvestrant: 13.7% (7/51; 5.70–26.26); risk difference –1.96% (95% CI, –16.86 to 12.94)]. Median PFS was 2.69 months (95% CI, 1.94–3.71) with venetoclax plus fulvestrant versus 1.94 months (1.84–3.55) with fulvestrant (stratified HR, 0.94; 95% CI, 0.61–1.45; P = 0.7853). Overall survival data were not mature. A nonsignificant improvement of CBR and PFS was observed in patients whose tumors had strong BCL2 expression (IHC 3+), a BCL2/BCLXL Histoscore ratio ≥1, or PIK3CA-wild-type status. Conclusions: Our findings do not indicate clinical utility for venetoclax plus fulvestrant in endocrine therapy–resistant, CDK4/6 inhibitor–refractory metastatic breast tumors, but suggest possible increased dependence on BCLXL in this setting.

BACKGROUND: Venetoclax (VEN) is a potent and selective inhibitor of the anti-apoptotic protein, BCL2. Preclinical studies have implicated the BCL2 family members, BCLXL and MCL1, in VEN resistance, and clinical studies in hematological malignancies have demonstrated subgroups with high ratios of BCL2/BCLXL and BCL2/MCL1 have the greatest VEN antitumor activity. The randomized phase 2 VERONICA study (NCT03584009) evaluated VEN in combination with fulvestrant (F) vs F alone in ER-positive, HER2-negative MBC pts who experienced disease recurrence/progression during or after a CDK4/6 inhibitor. Previously reported results from VERONICA (Lindeman et al. ASCO 2021) did not show an improved clinical benefit rate or progression-free survival (PFS) with VEN+F vs F alone. Here we present exploratory biomarker analyses of the expression of BCL2 family members and genomic alterations in circulating tumor DNA (ctDNA) and association with clinical outcomes from VEN+F vs F. METHODS: Tumor specimens were obtained during screening from 103 patients enrolled in the study, and expression levels of BCL2, BCLXL and MCL1 were analyzed by IHC. Baseline plasma-derived ctDNA was evaluated using the FoundationOne® Liquid assay. Expression of BCL2, BCLXL, MCL1 and mutations in ctDNA were correlated with PFS from VEN+F vs F based on the primary analysis (cutoff: Aug 5, 2020). RESULTS: In the overall population, protein levels of BCL2, BCLXL and MCL1 were similar between the VEN+F vs F arms. Patients whose tumors were BCL2 3+ trended towards having the greatest difference in median (m) PFS (3.9 months [mo] in VEN+F vs 1.7 mo in F; hazard ratio [HR] 0.38 [95% CI 0.09, 1.62]) albeit in a small sample size (n=13). Similarly, subgroup analysis suggested a trend for increasing mPFS and improved HR in VEN+F vs F alone in patients with the lowest BCLXL expression. mPFS in patients with a BCL2/BCLXL ratio ≥1 was 3.7 mo for VEN+F vs 1.8 mo for F (HR 0.67 [95% CI 0.3-1.49]) whereas patients with a BCL2/BCLXL ratio <1 had no difference in mPFS between the arms (2.0 mo in both arms, HR 1.21 [95% CI 0.7-2.1]). In the ctDNA-evaluable population, ESR1 (42.6%), TP53 (41.5%) and PIK3CA (35%) were the most prevalent genomic alterations observed and well-controlled between arms. PFS was similar between the ESR1 wildtype (wt) and mutant (mut) subgroups. TP53 mut status was a poor prognostic factor in both treatment arms. The PIK3CA wt subgroup had increased mPFS with VEN+F vs F alone (HR 0.66 [95% 0.38-1.17]) compared to PIK3CA mut (HR 1.59 [95% 0.74-3.34]). Patients with PIK3CA wt and BCL2 high tumors had the largest difference in mPFS between VEN+F (3.7 mo) vs F alone (1.9 mo) (HR 0.58 [95%CI 0.28-1.19]), compared to PIK3CA wt-BCL2 low tumors (2.4 vs 1.9 mo; HR 0.67 [95%CI 0.26-1.72]). CONCLUSION: Our data suggest that a high ratio of BCL2 to BCLXL conferred a trend towards a greater benefit to VEN+F compared to F alone, consistent with other clinical studies evaluating VEN. These analyses highlight the need to profile BCL2 and its family members to identify the VEN-sensitive subgroups, especially in indications where high expression of BCLXL or MCL1 may be observed. The ctDNA profile of VERONICA patients indicate a heavily pretreated patient population. The benefit observed in PIK3CA wt patients from VEN+F suggest increased dependence on BCL2 in this subgroup, while PIK3CA mut tumors likely rely on PI3K/AKT/mTOR survival programs or other BCL2 family members to evade apoptosis. Exploratory biomarker analyses are ongoing to further understand the VERONICA patient population. Citation Format: Geoffrey J. Lindeman, Tharu M. Fernando, Rebecca Bowen, Ching-Wei Chang, Rupal Desai, Kushagra Gupta, Aulde Fléchais, Timothy R. Wilson, Aditya Bardia. Exploratory biomarker analysis in VERONICA, a phase 2 study of venetoclax + fulvestrant versus fulvestrant in patients with estrogen receptor (ER)-positive HER2-negative metastatic breast cancer (mBC) [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P5-13-06.

589 Background: Endocrine therapy (ET) is the therapeutic mainstay for ER+ BC. Giredestrant is a highly potent, nonsteroidal, oral, selective ER antagonist and degrader (SERD) which has demonstrated robust ER occupancy, is well tolerated, and has previously shown encouraging antitumor activity as monotherapy and in combination with P in metastatic BC. coopERA BC (NCT04436744) evaluated giredestrant in eBC and met its primary endpoint, highlighting superior Ki67 suppression with single-agent giredestrant vs A at Week 2. Giredestrant was well tolerated. Here, we report the final analysis. Methods: Eligible patients (pts) with measurable ER+/HER2– untreated eBC and baseline Ki67 score ≥5% (202 planned) were randomized 1:1 to receive, on Days 1–14 of a neoadjuvant window-of-opportunity phase, 30 mg oral daily (PO QD) giredestrant or 1 mg PO QD A followed by a 16-week neoadjuvant phase of QD giredestrant or A for four 28-day cycles with 125 mg PO P on Days 1–21. Randomization was stratified by tumor size, baseline Ki67 score, and progesterone receptor status. Endpoints assessed here included Ki67 suppression from baseline to surgery, complete cell cycle arrest (CCCA; Ki67 ≤2.7%) at surgery, objective response rate (ORR), and safety. Results: At final analysis (cutoff: Nov 24, 2021), 112 and 109 pts were randomized to the giredestrant and A arms, respectively (median age: 62 years each; stage I/IIa disease: 60% vs 54%). Consistent with the primary analysis, greater suppression of Ki67 was observed at surgery with giredestrant + P (–81% [95% confidence interval (CI): –86%, –75%]) vs A + P (–74% [95% CI: –80%, –67%]). Similarly, greater CCCA was achieved at surgery with giredestrant + P (20%) vs A + P (14%). ORR was similar between the two arms (giredestrant + P: 50% [95% CI: 40%, 60%]; A + P: 49% [95% CI: 39%, 59%]). ET-related adverse events (AEs) were non-serious and occurred at similar rates between the two arms. Related Grade ≥3 AE rates were also similar at 6% each. Interruption/withdrawal of ET due to AEs was low and similar for both arms. Conclusions: In this final analysis of coopERA BC, the greater suppression of Ki67 with giredestrant vs A observed at Week 2 in the primary analysis was maintained at surgery, and safety data remained consistent with the known safety profile of giredestrant. coopERA BC is the first randomized study to show superior antiproliferative activity of an oral SERD (giredestrant) over an aromatase inhibitor (A) in ER+/HER2– eBC; studies are ongoing to further assess giredestrant’s clinical benefit. Clinical trial information: NCT04436744.

Abstract Background ~15-20% of primary, invasive breast cancer (BC) overexpresses HER2 and, despite survival improvements, there remains an unmet need for further progress. The antibody-drug conjugate T-DM1 is approved for HER2-positive LA and/or MBC that has previously been treated with trastuzumab and a taxane (separately or in combination), and as adjuvant therapy for HER2-positive early BC, where there is residual invasive disease after neoadjuvant taxane and trastuzumab- or HER2-based treatment. Venetoclax (GDC-0199/ABT-199), an oral, selective small-molecule inhibitor of the antiapoptotic protein BCL-2, is approved for treatment of chronic lymphocytic leukemia, small lymphocytic lymphoma, and acute myeloid leukemia. BCL-2 may play a key role in HER2-positive BC, and venetoclax has shown promising activity in estrogen receptor-positive, BCL-2-positive MBC. We describe VICKI (Venetoclax in Combination with Kadcyla), a Phase Ib/II, randomized, double-blind, placebo-controlled, study of venetoclax plus T-DM1 in previously treated HER2-positive LA/MBC (NCT04298918). Trial design The study comprises a Phase Ib stage (dose escalation and expansion cohorts) and a randomized Phase II stage. Phase II will be initiated following identification of the recommended Phase II dose of venetoclax in Phase Ib (400 mg or 800 mg). Pts will be randomized 1:1 to T-DM1 (intravenous 3.6 mg/kg q3w) plus venetoclax or placebo. Randomization will be stratified per BCL-2 status (BCL-2 high vs. low), visceral disease (Yes vs. No), and HER2 immunohistochemistry (IHC) 3+ status (Yes vs. No). Eligibility Adult pts with HER2-positive (IHC 3+ or IHC 2+/in situ hybridization-positive), previously treated, unresectable, histologically or cytologically confirmed invasive LA/MBC are eligible. Pts will have measurable disease per RECIST v1.1 and an Eastern Cooperative Oncology Group performance status of 0 or 1. Pts in Phase II will have BCL-2 expression status by IHC (≥50% of pts BCL-2 high) and will not have received prior treatment with T-DM1, venetoclax, or anti-HER2 drug conjugates. Aims The Phase II co-primary efficacy endpoints will be objective response rate (ORR) and progression-free survival (PFS) per RECIST v1.1 (both investigator-assessed). Secondary and exploratory efficacy endpoints will include duration of response, overall survival, clinical benefit rate, and patient-reported outcomes. Non-efficacy endpoints will be pharmacokinetics, immunogenicity, biomarkers, and safety. Statistical methods In Phase II, the primary efficacy populations will include all randomized pts according to their assigned treatment arm (intention-to-treat). A point estimate and 95% CI for ORR and the difference in ORR between treatment groups will be calculated using the normal approximation to the binomial distribution. PFS will be defined as time from randomization to the first occurrence of disease progression or death from any cause. Kaplan-Meier methodology will be used to estimate median PFS. An interim analysis is planned when ~56 PFS events have occurred. Primary efficacy analysis will occur when 161 pts have had a PFS event. Cox proportional-hazards models, stratified by the stratification factors, will be used to estimate the hazard ratio with 95% CI. Safety will be analyzed per treatment received in pts who received any study treatment (safety population). Accrual Target accrual is ~226-284 pts at 145 sites globally (Phase Ib dose escalation: 6-24 pts; Phase Ib expansion cohorts: ~20-40 pts; Phase II: 220 pts). Accrual is ongoing. Contact information For more information or to refer a patient, email global-roche-genentech-trials@gene.com or call 1-888-662-6728 (USA only). Citation Format: Geoffrey J Lindeman, Erika Hamilton, Ian Krop, Bora Lim, Shanu Modi, Cristina Saura, Rupal Desai, Bradford J Danner, Tharu M Fernando, Shengchun Kong, Fatema A Legrand, Federico Nasroulah. VICKI: A Phase Ib/II, randomized, placebo-controlled, study of venetoclax plus ado-trastuzumab emtansine (T-DM1) in patients (pts) with previously treated HER2-positive locally advanced (LA) or metastatic breast cancer (MBC) [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr OT-28-03.