ER-positive Advanced Breast Cancer Research Articles

Abstract CT075: A Phase 1 trial evaluating AC699, an orally bioavailable chimeric estrogen receptor degrader, in patients with advanced or metastatic breast cancer

Abstract Background The role of estrogen receptor (ER) signaling in driving breast carcinogenesis is well established, and endocrine therapy has been a mainstay in the treatment of patients with ER-positive advanced or metastatic breast cancer for decades. Aromatase inhibitors, selective estrogen receptor modulators, and selective ER degraders (SERDs) such as fulvestrant, and more recently elacestrant, are integral components of the therapeutic strategy for patients with ER-positive breast cancer. However, unmet medical need persists in this patient population, due to suboptimal clinical outcomes and the development of resistance to therapy. E3 ligase-engaged chimeric degraders represent a technological advance with the potential to induce potent and deeper ER degradation. Utilizing Accutar's proprietary Protein-Protein Interaction Targeting Chimeras (PPI-TAC) platform, AC699 was designed as a novel chimeric degrader targeting ERα. By effectively linking an ER ligand to an E3-ligase recruiting ligand, AC699 brings ERα in proximity to an E3 ligase, resulting in ubiquitination and subsequent degradation of ERα. Chimeric ER degraders possess the unique advantage of degrading the ER protein without the inherent risk of activating an ER signal. Moreover, these molecules are not degraded alongside the target protein, allowing for their efficient recycling within the cell. This direct mechanism enables chimeric ER degraders to achieve potent ER degradation with increased specificity, thereby potentially providing a higher therapeutic index compared to SERDs. Study Description The AC699-001 trial is a first in human Phase 1 dose escalation study (NCT05654532) which will enroll up to 60 patients with locally advanced or metastatic ER-positive, human epidermal growth factor receptor 2-negative, breast cancer. Patients must have received at least two prior lines of endocrine treatment, or at least one prior line, if combined with a CDK4/6 inhibitor. Prior chemotherapy is not required but must not exceed three prior lines of cytotoxic treatment. Patients must have at least one measurable lesion or at least one predominantly lytic bone lesion. AC699 is administered orally, once daily, with doses ranging from 100 mg to 600 mg in a standard 3+3 dose escalation design. The primary objective is evaluation of the safety and tolerability of AC699. Secondary objectives include assessment of preliminary anti-tumor activity and characterization of the pharmacokinetic profile of single and multiple doses of AC699. Enrollment began in December 2022 and a total of 5 sites have been activated in the United States. Citation Format: Rachel M. Layman, Manish R. Patel, David Cosgrove, Michael Danso, Nancy Mota, Marjorie E. Zettler, Katherine C. Pehlivan, Erika Hamilton. A Phase 1 trial evaluating AC699, an orally bioavailable chimeric estrogen receptor degrader, in patients with advanced or metastatic breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr CT075.

Design, synthesis and biological evaluation of fluorinated selective estrogen receptor degraders (FSERDs) --- A promising strategy for advanced ER positive breast cancer

Although endocrine therapies involving pharmaceuticals, such as tamoxifen and aromatase inhibitors, had initially demonstrated good responses in patients with estrogen receptor-positive (ER+) breast cancer, they often led to drug resistance. ER plays a vital role in the progression of metastatic diseases. Fulvestrant, a first generation selective estrogen receptor degrader (SERD), can effectively downregulate the ER protein and inhibit its downstream signaling pathways. However, as the drug needs to be intramuscularly injected, its widespread use is limited owing to poor patient compliance. Herein, we described a novel class of orally bioavailable fluorine-substituted SERDs that exhibit improved pharmacokinetic profiles. We substituted the hydroxyl group of clinical SERD candidate 6 with a fluorine atom to diminish phase II metabolism. The subsequent structure–activity relationship (SAR) investigation identified 22h and 27b, which can effectively degrade ER in a dose-dependent manner and exhibit considerable antiproliferative potency and efficacy in vitro and in vivo. The excellent pharmacokinetic profiles of 27b render it promising candidate of clinically useful oral SERD.

SERD-NHC-Au(I) complexes for dual targeting ER and TrxR to induce ICD in breast cancer

The development of selective estrogen receptor degraders (SERDs) has brought new ideas for the clinical treatment of ER-positive advanced breast cancer. The successful application of combinational therapy inspired the exploration of other targets to prevent breast cancer progression. Thioredoxin reductase (TrxR) is an important enzyme that can regulate redox balance in cells and it was considered as a potential target for anticancer treatment. In this study, we firstly combine a clinical SERD candidate——G1T48 (NCT03455270), with a TrxR inhibitor——N-heterocyclic carbene gold(I) [NHC-Au(I)] to form dual targeting complexes that can regulate both signaling pathways. The most efficacious complex 23 exhibited significant antiproliferative profile through degrading ER and inhibiting TrxR activity. Interestingly, it can induce immunogenic cell death (ICD) caused by ROS. This is the first evidence to elucidate the role of ER/TrxR-ROS-ICD axis in ER positive breast cancer and this research may inspire new drug development with novel mechanisms. The in vivo xenograft study demonstrated that complex 23 had excellent antiproliferative activity toward MCF-7 cells in mice model.

Abstract P6-10-06: A preoperative window-of-opportunity study of imlunestrant in estrogen receptor-positive, HER2-negative early breast cancer: Results from the EMBER-2 study

Abstract Background: Imlunestrant is a novel, orally bioavailable selective estrogen receptor degrader (SERD) with pure antagonistic properties that result in sustained inhibition of estrogen receptor (ER)-dependent gene transcription and cell growth. In a phase 1 study, imlunestrant monotherapy showed favourable safety, pharmacokinetics (PK) and preliminary efficacy in heavily pre-treated ER-positive (ER+) advanced breast cancer patients (Jhaveri ASCO 2022). Here, we present pharmacodynamic (PD) data from the preoperative window of opportunity (WOO) study (EMBER-2, NCT04647487), evaluating the biological activity of imlunestrant monotherapy in ER+, HER2-negative (HER2-) early breast cancer (EBC). Methods: Post-menopausal women with stage I–III operable ER+ (>50%) or Allred score >5, HER2- untreated EBC ≥1 cm in diameter were randomized 1:1 to imlunestrant 400 mg once daily (QD) or imlunestrant 800 mg QD for 15 days (treatment window of -2 to +7 days) up to the surgery date. Pre- and on-treatment tumor samples were compared for changes in PD biomarkers. Primary study objective was change in ER expression (measured by IHC and quantified by H-score). Secondary objectives were change in progesterone receptor (PR) expression (measured by IHC and quantified by H-score) and Ki-67 (measured by IHC and expressed by percentage positive scoring) along with evaluation of safety and tolerability. Results: From Apr 28, 2021, to Mar 11, 2022, 58 patients were enrolled of which 54 were biomarker-evaluable for ER expression (400 mg: n = 28; 800 mg: n = 26). Patient demographics and tumor characteristics for all enrolled patients were similar across cohorts, with a median age of 64 years (50-83), 72% invasive ductal carcinoma (IDC), 28% invasive lobular carcinoma (ILC), 59% stage I, 36% stage II and 5% stage III disease. 91% of the patients had a compliance rate higher than 80%. Among biomarker evaluable patients, relative reduction in PD biomarkers after a median of 15 days (range 13 to 23 days) of treatment are presented in Table 1. There was no significant difference in PD biomarker modulation noted between the two imlunestrant doses (400 mg vs 800 mg) or based on tumor histology (IDC, ILC). Imlunestrant was well tolerated. There were no discontinuations due to adverse events (AEs). Treatment-related AEs (TRAEs) were mainly grade 1, most commonly: fatigue (10%), diarrhea (9%), hot flushes (7%), and nausea (5%). There were no TRAEs of diarrhea and nausea observed at the 400 mg dose. No grade 3 or higher TRAEs were reported. Conclusion: Imlunestrant demonstrated evidence of target engagement along with consistent biological activity across all evaluated dose levels and was well tolerated in an EBC population, further supporting continued adjuvant development in the ongoing EMBER-4 study. Additional biomarker analyses for the EMBER-2 study are also planned. Table 1. Relative reduction in PD biomarkers from Baseline to Day 15 Citation Format: Patrick Neven, Nicole Stahl, Maria Vidal, Miguel Martín, Nadia Harbeck, Peter A. Kaufman, Francois-Clement Bidard, Peter A. Fasching, Philippe Aftimos, Erika Hamilton, Stacey Carter, Peter Schmid, Duncan Wheatley, Manali Bhave, Kelly K. Hunt, Swati A. Kulkarni, Roohi Ismail-Khan, Claudia Karacsonyi, Shawn T. Estrem, Umut Ozbek, Bastien Nguyen, Eva Ciruelos. A preoperative window-of-opportunity study of imlunestrant in estrogen receptor-positive, HER2-negative early breast cancer: Results from the EMBER-2 study. [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 P6-10-06.

Research Progress on Bioactive Metal Complexes against ER-Positive Advanced Breast Cancer.

Breast cancer is the most prevalent cancer in women and represents a serious disease that is harmful to life and health. In 1977, with the approval of tamoxifen, endocrine therapy has become the main clinical treatment for ER-positive (ER+) breast cancer. Although patients initially respond well to endocrine therapies, drug resistance often emerges and side effects can be challenging. To overcome drug resistance, the exploration for new drugs is a priority. Metal complexes have demonstrated significant antitumor activities, and platinum complexes are widely used in the clinic against various cancers, including breast cancer. In this Perspective, the first section describes the classification and mechanism of endocrine therapy drugs for ER+ breast cancer, and the second section summarizes research since 2000 into metal complexes with activity toward ER+ breast cancer. Finally, we discuss the opportunities, challenges, and future directions for metal complexes in the treatment of ER+ breast cancer.

A phase 1a/b trial of imlunestrant (LY3484356), an oral selective estrogen receptor degrader (SERD) in ER-positive (ER+) advanced breast cancer (aBC) and endometrial endometrioid cancer (EEC): Monotherapy results from EMBER.

1021 Background: Imlunestrant is a novel, orally bioavailable SERD with pure antagonistic properties that result in sustained inhibition of ER-dependent gene transcription and cell growth. In dose escalation, imlunestrant showed favorable safety, pharmacokinetics (PK) and preliminary efficacy in patients with ER+, HER2- aBC and ER+ EEC (Phase 1a EMBER, Jhaveri 2021). Here we present updated data from the dose escalation (Phase 1a) and dose expansion (Phase 1b) of imlunestrant monotherapy in EMBER (NCT04188548). Methods: Phase 1a/1b enrolled patients with ER+ aBC (prior ET sensitivity; ≤3 prior therapies for aBC in Phase 1a following protocol amendment and ≤2 in Phase 1b) and ER+ EEC (prior platinum therapy; no prior fulvestrant or aromatase inhibitor). Premenopausal women received a concomitant GnRH agonist. Serial plasma samples were obtained for PK and ctDNA analysis. Key endpoints included recommended phase 2 dose (RP2D) determination, safety and tolerability, PK, objective response rate per RECIST v1.1 (ORR: complete response [CR] or partial response [PR]) in patients with measurable disease and ≥1 post-baseline tumor assessment or discontinued prior to tumor assessment, and clinical benefit rate (CBR: CR or PR, or stable disease ≥24 weeks) in patients enrolled ≥24 weeks prior to data cut. Results: As of January 14, 2022, 138 patients (n = 114 aBC, n = 24 EEC) received imlunestrant monotherapy at doses ranging from 200-1200 mg QD. Median age was 62.0 years (range 32-95). Median number of prior therapies for aBC and EEC was 2 (range 0-8) and 1 (0-5), respectively. aBC patients had received a prior ET (94.7%), CDK4/6 inhibitor (92.1%), fulvestrant (50.9%) and chemotherapy (26.3%). No dose-limiting toxicities were observed. Most treatment-emergent adverse events (TEAEs) were grade 1. At the RP2D (400 mg QD, n= 69), the most common all grade TEAE’s were nausea (33.3%), fatigue (27.5%), and diarrhea (23.2%). Across all doses, the incidence of treatment-related grade 3 AEs was low (3.6%). No patient discontinued due to a TEAE. In evaluable aBC patients, ORR was 8.0% (6/75) and CBR was 40.4% (42/104). In evaluable EEC patients, ORR was 5.0% (1/20 had a PR- ongoing pending confirmation) and CBR was 47.1% (8/17). Clinical benefit was observed regardless of baseline ESR1 mutation status as determined by ctDNA sequencing. Additional biomarker analyses will be presented at the meeting. Conclusions: Imlunestrant continues to demonstrate a favorable side effect profile, with no cardiac or opthalmic safety signals, and has continued evidence of clinical activity in heavily pre-treated ER+ aBC and EEC patients. Clinical trial information: NCT04188548.

A Novel CDK4/6 and PARP Dual Inhibitor ZC-22 Effectively Suppresses Tumor Growth and Improves the Response to Cisplatin Treatment in Breast and Ovarian Cancer.

In recent years, three PARP inhibitors and three CDK4/6 inhibitors have been approved by the FDA for the treatment of recurrent ovarian cancer and advanced ER-positive breast cancer, respectively. However, the clinical benefits of the PARPi or CDK4/6i monotherapy are not as satisfied as expected and benefit only a fraction of patients. Current studies have shown therapeutic synergy for combinations of PARPi and CDK4/6i in breast and ovarian cancers with homologous recombination (HR) proficiency, which represents a new synthetic lethal strategy for treatment of these cancers regardless HR status. Thus, any compounds or strategies that can combine PARP and CDK4/6 inhibition will likely have great potential in improving clinic outcomes and in benefiting more patients. In this study, we developed a novel compound, ZC-22, that effectively inhibited both PARP and CDK4/6. This dual-targeting compound significantly inhibited breast and ovarian cancer cells by inducing cell cycle arrest and severe DNA damage both in vitro and in vivo. Interestingly, the efficacy of ZC-22 is even higher than the combination of PARPi Olaparib and CDK4/6i Abemaciclib in most breast and ovarian cancer cells, suggesting that it may be an effective alternative for the PARPi and CDK4/6i combination therapy. Moreover, ZC-22 sensitized breast and ovarian cancer cells to cisplatin treatment, a widely used chemotherapeutic agent. Altogether, our study has demonstrated the potency of a novel CDK4/6 and PARP dual inhibitor, which can potentially be developed into a monotherapy or combinatorial therapy with cisplatin for breast and ovarian cancer patients with HR proficiency.

Role of PI3K/Akt/mTOR pathway in mediating endocrine resistance: concept to clinic.

The majority of breast cancers express the estrogen receptor (ER) and for this group of patients, endocrine therapy is the cornerstone of systemic treatment. However, drug resistance is common and a focus for breast cancer preclinical and clinical research. Over the past 2 decades, the PI3K/Akt/mTOR axis has emerged as an important driver of treatment failure, and inhibitors of mTOR and PI3K are now licensed for the treatment of women with advanced ER-positive breast cancer who have relapsed on first-line hormonal therapy. This review presents the preclinical and clinical data that led to this new treatment paradigm and discusses future directions.

Genomic profile of advanced breast cancer in circulating tumour DNA

The genomics of advanced breast cancer (ABC) has been described through tumour tissue biopsy sequencing, although these approaches are limited by geographical and temporal heterogeneity. Here we use plasma circulating tumour DNA sequencing to interrogate the genomic profile of ABC in 800 patients in the plasmaMATCH trial. We demonstrate diverse subclonal resistance mutations, including enrichment of HER2 mutations in HER2 positive disease, co-occurring ESR1 and MAP kinase pathway mutations in HR + HER2− disease that associate with poor overall survival (p = 0.0092), and multiple PIK3CA mutations in HR + disease that associate with short progression free survival on fulvestrant (p = 0.0036). The fraction of cancer with a mutation, the clonal dominance of a mutation, varied between genes, and within hotspot mutations of ESR1 and PIK3CA. In ER-positive breast cancer subclonal mutations were enriched in an APOBEC mutational signature, with second hit PIK3CA mutations acquired subclonally and at sites characteristic of APOBEC mutagenesis. This study utilises circulating tumour DNA analysis in a large clinical trial to demonstrate the subclonal diversification of pre-treated advanced breast cancer, identifying distinct mutational processes in advanced ER-positive breast cancer, and novel therapeutic opportunities.

Real-world data of fulvestrant as first-line treatment of postmenopausal women with estrogen receptor-positive metastatic breast cancer

Goals of endocrine therapy for advanced breast cancer (ABC) include prolonging survival rates, maintaining the quality of life, and delaying the initiation of chemotherapy. We evaluated the effectiveness of fulvestrant as first-line in patients with estrogen receptor (ER)-positive ABC with relapse during or after adjuvant anti-estrogenic therapy in real-world settings. Retrospective, observational study involving 171 postmenopausal women with ER-positive ABC who received fulvestrant as first-line between January 2011 and May 2018 in Spanish hospitals. With a median follow-up of 31.4 months, the progression-free survival (PFS) with fulvestrant was 14.6 months. No differences were seen in the visceral metastatic (14.3 months) versus non-visceral (14.6 months) metastatic subgroup for PFS. Overall response rate and clinical benefit rate were 35.2% and 82.8%. Overall survival was 43.1 months. The duration of the clinical benefit was 19.2 months. Patients with ECOG performance status 0 at the start of treatment showed a significant greater clinical benefit rate and overall survival than with ECOG 1–2. Results in real-world settings are in concordance with randomized clinical trials. Fulvestrant continues to demonstrate clinical benefits in real-world settings and appears be well tolerated as first-line for the treatment of postmenopausal women with ER-positive ABC.

18F-Fluoroestradiol PET in the evaluation of probable oligometastatic breast cancer.

Recent advances with specific PET tracer ligands, for example prostate-specific membrane antigen (PSMA)-based PET imaging, have substantially improved the sensitivity and specificity of PET imaging. 18F-Fluoroestradiol (FES) is a novel PET tracer that has been demonstrated to have high specificity for oestrogen receptor (ER) in breast cancer. This case describes the use of FES-PET to further characterise a solitary scapular lesion in a patient with locally advanced ER-positive breast cancer. In demonstrating FES avidity in the lesion, it was determined to represent metastatic disease, thus avoiding further biopsy with its associated morbidity. This case demonstrates the potential diagnostic utility of FES PET-CT in the management of ER-positive breast cancer patients with probable oligometastatic disease.

Abstract 1903: PTEN loss mediates clinical cross-resistance to CDK4/6 and PI3Ká inhibitors in breast cancer

Abstract The combination of CDK4/6 inhibitors with anti-estrogen therapies significantly improves clinical outcomes in ER-positive advanced breast cancer. To identify mechanisms of acquired resistance, we analyzed serial biopsies and rapid autopsies from patients treated with the combination of the CDK4/6 inhibitor ribociclib with letrozole. This study revealed that some resistant tumors acquired RB loss, whereas other tumors lost PTEN expression at the time of progression. In breast cancer cells ablation of PTEN, through increased AKT activation, was sufficient to promote resistance to CDK4/6 inhibition in vitro and in vivo. Mechanistically, PTEN loss resulted in exclusion of p27 from the nucleus, leading to increased activation of both CDK4 and CDK2. Since PTEN loss also causes resistance to PI3Kα-inhibitors, currently approved in the post-CDK4/6 setting, these findings provide critical insight into how this single genetic event may cause clinical cross-resistance to multiple targeted therapies in the same patient, with implications for optimal treatment sequencing strategies. Citation Format: Carlotta Costa, Ye Wang, Amy Ly, Yasuyuki Hosono, Ellen Murchie, Charlotte S. Walmsley, Tiffany Huynh, Christopher Healy, Rachel Peterson, Shogo Yanase, Charles T. Jakubik, Laura E. Henderson, Leah J. Damon, Daria Timonina, Ioannis Sanidas, Christopher J. Pinto, Mari Mino-Kenudson, James Stone, Nicholas J. Dyson, Leif W. Ellisen, Aditya Bardia, Hiromichi Ebi, Cyril H. Benes, Jeffrey A. Engelman, Dejan Juric. PTEN loss mediates clinical cross-resistance to CDK4/6 and PI3Ká inhibitors in breast cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1903.

The Dysregulated Pharmacology of Clinically Relevant ESR1 Mutants is Normalized by Ligand-activated WT Receptor.

The estrogen receptor (ER/ESR1) is expressed in a majority of breast cancers and drugs that inhibit ER signaling are the cornerstone of breast cancer pharmacotherapy. Currently, aromatase inhibitors are the frontline endocrine interventions of choice although their durability in metastatic disease is limited by activating point mutations within the ligand-binding domain of ESR1 that permit ligand-independent activation of the receptor. It has been suggested that the most commonly occurring ESR1 mutations would likely compromise the clinical activity of selective estrogen receptor downregulators and selective estrogen receptor modulators (SERMs) when used as second-line therapies. It was unclear, however, how these mutations, which are likely coexpressed in cells with ERWT, may impact response to ER ligands in a clinically meaningful manner. To address this issue, we dissected the molecular mechanism(s) underlying ESR1-mutant pharmacology in models relevant to metastatic disease. These studies revealed that the response of ESR1 mutations to ligands was dictated primarily by the relative coexpression of ERWT in cells. Specifically, dysregulated pharmacology was only evident in cells in which the mutants were overexpressed relative to ligand-activated ERWT; a finding that highlights the role of allelism in determining ER-mutant pharmacology. Importantly, we demonstrated that the antagonist activity of the SERM, lasofoxifene, was not impacted by mutant status; a finding that has led to its clinical evaluation as a treatment for patients with advanced ER-positive breast cancer whose tumors harbor ESR1 mutations.

Abstract P1-19-04: Results from plasmaMATCH trial treatment cohort A: A phase II trial of extended-dose fulvestrant in patients with an ESR1 mutation identified via ctDNA screening (CRUK/15/010)

Abstract Background: Ligand binding domain ESR1 mutations are acquired in ER positive cancers during prior aromatase inhibitor therapy for advanced ER positive breast cancer (BC). ESR1 mutant cancer models are sensitive to fulvestrant at high concentrations, however standard dose fulvestrant may not achieve the concentration required to fully inhibit mutant ESR1 in the clinic. The plasmaMATCH trial Cohort A assessed the efficacy of extended-dose fulvestrant (double the current standard dose achieved by doubling the frequency of administration) in patients with an ESR1 mutation identified via ctDNA testing. Methods: The plasmaMATCH trial was an open-label, multi-centre, multi-cohort platform trial, consisting of ctDNA testing in ~1000 patients with advanced BC. Patients with an ESR1 mutation identified in ctDNA testing were potentially eligible to enter Cohort A. Patients were treated with fulvestrant 500mg intramuscularly on Cycle 1 Days 1, 8 and 15 of a 28 day cycle, and from Cycle 2 onwards every 15 days. Pharmacokinetics samples were collected pre-dose on Day 1 of Cycles 2, 3 and 4. The primary endpoint for Cohort A was confirmed objective response rate as defined by RECIST v1.1. The original planned sample size was 40 patients, to detect a 25% response rate, assuming predominantly clonally dominant mutations. As the trial progressed it became apparent that ctDNA screening may also detect sub clonal ESR1 mutations, which were expected to have a lower response rate. Using a single-stage A’Hern design with a target response rate of 20%, unacceptable response rate of 10%, alpha=5%, power=80, the sample size was increased to 78 evaluable patients with 13 or more responses required to infer efficacy. Results: Following ctDNA testing, 84 patients enrolled in Cohort A (38% of patients with ESR1 mutations identified in ctDNA testing). All were ER positive, seven were HER2 amplified, 78 (93%) had visceral metastases. The most common ESR1 mutations detected in baseline plasma were D538G (52.4%), Y537S (35.7%), E380Q (33.3%). In the 74 evaluable patients, confirmed response rate was 8.1% (95%CI 3.0-16.8%, 6/74). One additional patient had an unconfirmed partial response. Median progression free survival was 2.2 months (IQR 1.7-5.3 months) and median duration of response was 7.0 months (IQR 3.7-8.3 months) with 4 patients continuing on treatment. In exploratory analysis, 39 patients had clonally dominant ESR1 mutations in baseline ctDNA analysis whilst 25 patients had subclonal mutations and 10 had unknown clonality. The response rate in those with clonally dominant ESR1 mutations was 10% (95%CI: 2.9-24.2%, 4/39) with no confirmed responses in those with subclonal mutations. The most common clinically significant grade 3 or 4 adverse event was hypertension (13%). Pharmacokinetic analysis was consistent with elevated fulvestrant exposure compared to approved 500mg PopPk model (pre-dose Cycle 3, 71% increase, and pre-dose Cycle 4 66% increase). Conclusion: In the pre-treated population studied, the response rate of extended-dose fulvestrant did not meet pre-specified criteria for efficacy in patients with ESR1 mutations identified in ctDNA testing. Extended-dose fulvestrant was well tolerated and enhanced exposure was observed. Assessment of clonal dominance of ESR1 mutations in ctDNA may identify patients who are more likely to benefit from extended-dose fulvestrant therapy. Citation Format: Iain Macpherson, Lucy Kilburn, Sarah Kernaghan, Andrew M Wardley, Richard D Baird, Rebecca Roylance, Peter Stephens, Olga Oikonomidou, Jeremy P Braybrooke, Mark Tuthill, Jacinta Abraham, Matthew C Winter, Belinda Kingston, Katie Wilkinson, Alistair Ring, Judith M Bliss, Nicholas Turner, on behalf of the plasmaMATCH Trial Management Group. Results from plasmaMATCH trial treatment cohort A: A phase II trial of extended-dose fulvestrant in patients with an ESR1 mutation identified via ctDNA screening (CRUK/15/010) [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P1-19-04.

Abstract OT2-02-09: Phase II study of trimebutine maleate and probiotics for abemaciclib-induced diarrhea in patients with ER-positive and HER2-negative advanced breast cancer (MERMAID) WJOG11318B

Abstract Background: The standard treatment for estrogen receptor-positive, HER2-negative inoperable and/or recurrent breast cancers is a combination of endocrine therapy and a CDK4/6 inhibitor (abemaciclib). When abemaciclib is administered, 80%-90% of patients experience diarrhea, resulting in the reduction of patient QOL and compliance. Loperamide is recommended for abemaciclib-induced diarrhea as well as chemotherapy induced diarrhea. However, previous reports showed that approximately 40% patients had constipation due to supportive treatment of loperamide. Moreover, the side effects of molecular targeted drugs, including abemaciclib, are not clear in the mechanism, and the effectiveness of coping therapy similar to chemotherapy and the treatment for side effects have not been clearly established. Thus developing new measures to effectively manage abemaciclib-induced diarrhea is an important clinical task, especially prevention of diarrhea without increasing adverse events such as constipation will be important to maintain QOL of patients receiving abemaciclib. Trial Design: This is a multicenter, randomized, open-label Phase II study to evaluate the efficacy and safety of probiotics (bifidobacterial formulation) and trimebutine maleate in treatment of abemaciclib-induced diarrhea. Both Arms A and B will receive hormone therapies (selected by physician in charge*) as directed along with abemaciclib 150 mg bid over the course of 28 days, and simultaneously to initiation of abemaciclib administration, will be given two pills of Biofermin® tid in Both Arms. Once patients experience diarrhea that is grade 6 or greater on the Bristol scale (higher grade than loose stool), salvage therapy using loperamide will be administered in Arm A. In contrast, participants from Arm B will be administered 100 mg trimebutine maleate. Thereafter, following meals, trimebutine maleate tid will be given throughout the trial period of 28 days. Eligibility Criteria: Key eligibility criteria for this trial are: Histologically diagnosed estrogen receptor-positive, HER2-negative breast cancer. Stage IIIB/IIIC breast cancer that is not subjected to surgery for curative intent, Stage IV breast cancer suitable for endocrine therapy. No prior treatment with CDK 4/6 inhibitors, and any type and number of previous treatments will be allowed in the selection criteria except CDK4/6 inhibitors. Specific Aims: The primary endpoint is the percentage of patients who experienced Grade 2 or greater diarrhea during the trial duration of 28 days and key secondary endpoints include safety, frequency and duration of all-grade diarrhea, frequency of emesis and constipation, comparative usage of loperamide in Arms A and B, and QOL/PRO in the 28-day study duration Statistical Design: By using the 40% threshold as the null hypothesis, appropriate statistical analysis will be performed based on a binomial distribution. The significance level will be 2.5%, determined using a one-tailed test. Interval estimation will be appropriately deduced from a binomial distribution. If either arm shows a significant reduction compared with the threshold value, as secondary validation, intergroup comparison of the primary endpoint percentage of diarrhea onset will be performed. In this case, the significance level will be 10%, determined using a two-tailed test. Target Accrual: A total of 50 patients will be enrolled and duration of enrollment will be 1 year. This trial will open to accrual in September 2019. Contact Information: Hiroko Masuda, Showa University Hospital, Tokyo, Japan, hmasuda@med.showa-u.ac.jp Citation Format: Hiroko Masuda, Tsutomu Iwasa, Toru Mukohara, Shinya Tokunaga, Koji Matsumoto, Naoki Niikura, Yasuaki Sagara, Yasuo Miyoshi, Akihiko Shimomura, Masato Takahashi, Takeshi Nagashima, Mihoko Doi, Manabu Futamura, Kenichi Yoshimura, Toshimi Takano, Junji Tsurutani. Phase II study of trimebutine maleate and probiotics for abemaciclib-induced diarrhea in patients with ER-positive and HER2-negative advanced breast cancer (MERMAID) WJOG11318B [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr OT2-02-09.

Abstract P5-11-13: Outcomes of fulvestrant therapy among Japanese women with ER-positive HER2-positive advanced or metastatic breast cancer: A subgroup analysis of the JBCRG-C06 safari study

Abstract Background: After the CONFIRM trial, fulvestrant 500 mg (F500) became a standard treatment for patients with estrogen receptor positive (ER+) advanced/metastatic breast cancer (AMBC). The JBCRG-C06 Safari study (UMIN 000015168) showed that earlier F500 use, a longer time from diagnosis to F500 use, and no prior chemotherapy were associated with significantly longer time to treatment failure (TTF) among Japanese patients with ER+ and HER2-negative AMBC (Kawaguchi H, et al. Breast Cancer Res Treat, 2017; Kawaguchi H, et al. Curr Med Res Opin, 2018). We had carried out subgroup analyses on real world data from the Safari study focusing on ER+ and HER2+ AMBC with the objective to examine further on potential interactions between TTF and the following: F500 single treatment or F500 with anti-HER2 therapy; presence or absence of visceral metastasis; progesterone receptor (PgR) status; and prior chemotherapy for AMBC. Material & methods: The Safari study was a retrospective, multicenter cohort study, conducted in 1072 patients at 16 sites in Japan for ER+ AMBC. Patients starting treatment with F500 between Nov 2011 and Dec 2014 were registered. After the data cleaning through the central review, 94 patients (9.6%) has been included in this sub-analysis among the 978 patients in which HER2 status was confirmed by the documentation. The relationship between baseline clinical/pathological factors, treatment line, and TTF (time from start to cessation of F500 treatment for any reason, including toxicity and death) were analyzed using Kaplan-Meier methods. Univariate analysis of TTF data was performed by Cox hazards model using: age, histological type, histological/nuclear grade, visceral-metastases, stage, PgR, period from AMBC diagnosis to F500 use, treatment line of F500, prior chemotherapy. Results: Median age at diagnosis of AMBC was 57.5yrs (33-85); 59(33-85) and 56.5(39-75) for F500 with/ without anti-HER2 therapy. Median age of fulvestrant start was 61yrs (35-96). 21.3% had de novo metastatic disease; 78.7% recurrent disease; 37.2% visceral metastases. All patients’ ER status was positive, and PgR status was positive in 65.9%, negative 25.5% respectively. HER2 IHC 3+ patients accounted for 44.7%, IHC 2+/1+ for 38.3%. In the case of IHC 2+/1+, HER2-positivity was confirmed by FISH. F500 alone was administered in 52 patients, while F500 with anti-HER2 therapy in 42 patients. At median follow up of 3.9 years, median TTF was 4.4m (95% CI: 3.3-5.7); 3.7m (95% CI: 2.9-5.9), 5.1m(95% CI: 3.6-6.4) for the F500 alone group, F500 with anti-HER2 therapy group respectively (p = 0.5973). F500 with/ without anti-HER2 therapy was administered in 1stor 2nd-line in 20 (21.3%), and ≥3rd-line in 74(78.7%) patients. Median TTF was 6.6m (95% CI: 4.6-8.5) and 3.7m (95% CI: 2.8-5.2) in the 1stor 2nd-line and ≥3rd-line respectively (p value:0.0145). Although no statistically significant difference was found, TTF tended to be shorter in the group receiving chemotherapy prior to F500 (median TTF: 3.7m vs 5.5m). No other clinical parameters including age, PgR status, metastatic sites were associated with the effectiveness. The median OS was 7.8-years (95% CI: 5.9-9.5), which had not been influenced by with/ without anti-HER2 therapy. The toxicity profile we observed was similar to that in the registration trial. Conclusions: Our findings suggest that F500 with/without anti-HER2 therapy appears to be effective also in the ER+ and HER2-positive ABMC setting which is consistent with the limited results previously published. Earlier line use of F500 use may be useful in HER2-positive patients as well as HER2-negative patients. Citation Format: Misato Hagi, Hidetoshi Kawaguchi, Norikazu Masuda, Shigehira Saji, Yutaka Yamamoto, Takahiro Nakayama, Kenjiro Aogi, Keisei Anan, Yoshinori Ito, Shoichiro Ohtani, Nobuaki Sato, Toshimi Takano, Eriko Tokunaga, Seigo Nakamura, Yoshie Hasegawa, Masaya Hattori, Tomomi Fujisawa, Miki Yamaguchi, Hiroko Yamashita, Toshinari Yamashita, Daisuke Yotsumoto, Satoshi Morita, Masakazu Toi, Shinji Ohno. Outcomes of fulvestrant therapy among Japanese women with ER-positive HER2-positive advanced or metastatic breast cancer: A subgroup analysis of the JBCRG-C06 safari study [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P5-11-13.

Abstract P1-19-44: Real-world efficacy and safety of palbociclib combined with fulvestrant in japanese patients with ER-positive HER2-negative advanced or metastatic breast cancer

Abstract Background: Targeting the molecular components of the cell cycle to interfere with cell cycle progression is a logical strategy for cancer treatment. Cyclin-dependent kinase (CDK) 4 and 6 promote cell cycle progression. Palbociclib is a selective inhibitor of CDK4 and CDK6 that obstructs progression from the G1 phase to the S phase and inhibits subsequent DNA synthesis. Palbociclib has been approved as a novel molecular targeting drug for hormone receptor-positive/HER2- advanced or metastatic breast cancer (MBC) combined with endocrine therapy. In a phase 3 trial, palbociclib-fulvestrant (PF) significantly improved progression-free survival (PFS) of patients with ER+/HER2- advanced or MBC. However, in Japanese patients, did not improve in the PF group. Moreover, neutropenia was higher in Japanese patients. We conducted a retrospective study to verify the efficacy and safety of PF in Japanese patients. Methods: Thirty-nine ER+/HER2- advanced or MBC patients treated with fulvestrant (F) alone and 31 patients treated with PF at the Saitama Medical Center from July 2012 to November 2018 were included. Fulvestrant 500 mg was administered intramuscularly on days 1 and 15 of cycle 1, and then every 28 days thereafter starting from day 1 of cycle 1. Palbociclib 125 mg/day was administered orally on days 1 to 21, followed by 7 days off treatment for every 28-day cycle. Premenopausal and perimenopausal women were also treated with LHRH agonist subcutaneously. Results: In the F and PF groups, the complete response were 0% and 3.2%, respectively, the partial response were 2.6% and 38.7%, respectively, and the rates of long stable disease were 20.5% and 22.6%, respectively. Therefore, the objective response rate in the F and PF groups were 2.6% and 41.9% (P < 0.001), respectively, and the clinical benefit rate (CBR) were 23.1% and 61.3% (P = 0.002), respectively. We observed significant differences in the CBR in patients with the following characteristics: age <70 years (F: 16.7% vs PF: 63.2%; P = 0.002), BMI ≥25 (F: 10% vs PF: 69.2%; P = 0.001), ≤1 previous endocrine therapy (F: 21.1% vs PF: 60%; P = 0.001), ≤1 previous chemotherapy (F: 21.1% vs PF: 61.5%; P = 0.002), no sensitivity to prior endocrine therapy (F: 22.5% vs PF: 60%; P = 0.0049), stage I-III at initial diagnosis (F: 23.3% vs PF: 59.1%; P = 0.011), DFI ≥24 months (F: 30% vs PF: 69.2%; P = 0.038), two or more metastatic sites (F: 21.7% vs PF: 65%; P = 0.006), and visceral metastasis (F: 22.7% vs PF: 73.3%; P = 0.006). The median PFS was significantly improved in the PF (PF vs F: 13.3 months vs 3.9 months, HR, 0.272, 95% CI, 0.128-0.574, p < 0.001). The most common adverse events reported for the PF group were leukopenia, neutropenia, anemia, and fatigue. Hematologic adverse events were more frequent in the PF group than in the F group. There was also no febrile neutropenia in either group. The most common nonhematologic adverse events were fatigue (41.9% in PF vs 5.2% in F). Fever without neutropenia occurred in two patients in the PF group. The only higher than Grade 3 nonhematologic adverse event was liver dysfunction (5.1%), which occurred in the F group. There were no serious adverse events, such as embolism, in either group. There was no dose discontinuation of palbociclib due to adverse events; however, 58.1% (18/31) of patients required dose interruption and 71% (18/31) required dose reduction due to Grade 3 or 4 neutropenia. Sixteen of 31 (51.6%) patients required one dose-level reduction and 6 of 31 (19.4%) required two dose-level reductions. The median number of courses for the first dose reduction was 2 (range: 1-5), and the median for the second dose reduction was 3 (range: 2-5). Conclusion: PF was tolerable and significantly improved clinical outcome in Japanese patients with ER+/HER2- MBC. Citation Format: Hirohito Seki, Takashi Sakurai, Yuka Maeda, Naohiko Oki, Mina Aoyama, Ryou Yamaguchi, Ken Shimizu, Kaori Higeta. Real-world efficacy and safety of palbociclib combined with fulvestrant in japanese patients with ER-positive HER2-negative advanced or metastatic breast cancer [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P1-19-44.

Abstract P6-04-12: STAT3 as a therapeutic target in estrogen receptor positive breast cancer patients refractory to CDK4/6 inhibition

Abstract Background: CDK4/6 inhibitors (i.e. palbociclib) in combination with endocrine therapy (ET) are currently standard of care for estrogen receptor (ER) positive breast cancer patients with advanced/metastatic disease. While this combination has proven successful in delaying progression, no improvement in long-term survival has been observed to date in the post-menopausal setting. Further, resistance is inevitable as 50-60% of patients’ metastatic lesions progress on CDK4/6 inhibitor therapy within 2-years of treatment. For patients experiencing resistance to CDK4/6 inhibitors, novel treatment strategies are needed to delay progression or to improve survival. Our recently published data shows that palbociclib-resistance is marked by significant upregulation of the IL6- STAT3 signaling pathway in ER-positive breast cancer cells. Knockdown of STAT3 in resistance cancer cells restored sensitivity to palbociclib. Additionally, matched biopsies from advanced ER-positive breast cancer patients who progressed on palbociclib showed upregulation in p-STAT3 as compared to their pre-treatment biopsy samples (14 out of 25 patients; p=0.042). Collectively, these data suggest that STAT3 is viable therapeutic target to overcome palbociclib resistance. Hence, the goal of this study is to translate these findings in vivo and provide pre-clinical rational to the efficacy of TTI-101 (an orally bioavailable STAT3 inhibitor) in PDX models of palbociclib resistance. Methods: To identify PDX models that recapitulate palbociclib resistance observed in cell line models, we correlated two published RNA-seq data sets of a panel of 64 PDX models from breast cancer patients to our multi-omics analysis of palbociclib resistant cell lines. Four PDXs that recapitulated the resistant-transcriptome expression pattern based on Spearman’s correlation, were narrowed down for future studies. In addition, we have also established PDX models (1st-4th passage) from two sets of patients (1) those who are intrinsically resistant to palbociclib (the patients progressed at 2-3 months while on palbociclib + ET) and (2) those who have developed resistance over time (i.e. acquired resistance-patients progress at 12-18 months while on palbociclib + ET). Results: We chose PDX model BCX94 for our preliminary studies as it showed baseline induction of p-STAT3 by immunoblot analysis. Using BCX94, we show that TTI-101 at 50mg/kg twice a day potently reduced tumor volume and improved survival of these mice. We also observed a significant reduction in tumor-derived IL-6 in circulation; as assayed by hIL-6 levels in the serum. Further, complete blood count analysis of whole blood collected at endpoint (i.e. 28 days on treatment) showed no drug toxicities. Treatment of intrinsic palbociclib resistant PDX models with TTI-101, on the other hand, showed only a brief delay in tumor progression, but no overall benefit. Conclusions: Collectively, these results indicate that TT1-101 is safely tolerated and efficacious at the dose examined in PDX models and that TTI-101 may be a suitable target for those tumors that are resistant to palbociclib. The differences in response to TTI-101 in the BCX94 versus our intrinsic PDX models, suggest that acquired vs. intrinsic resistance signatures of palbociclib may be mutually exclusive and that the IL-6-STAT3 signaling axis may be a driver in acquired palbociclib resistance, but not the intrinsic setting. Our ongoing studies are therefore geared towards testing the efficacy of IL-6-STAT3 inhibition in both settings (i.e. acquired and intrinsic) and identifying distinct therapeutic vulnerabilities of intrinsic palbociclib resistance that may be unresponsive to STAT3 inhibition. Citation Format: Nicole M Kettner, Tuyen Bui, Min Jin Ha, T Kris Eckols, David J Tweardy, Funda Meric-Bernstam, Kelly K Hunt, Debu Tripathy, Khandan Keyomarsi. STAT3 as a therapeutic target in estrogen receptor positive breast cancer patients refractory to CDK4/6 inhibition [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P6-04-12.

Fulvestrant in the fight against ER-positive advanced breast cancer; lone soldier or an important partner?

Fulvestrant in the fight against ER-positive advanced breast cancer; lone soldier or an important partner?

PTEN Loss Mediates Clinical Cross-Resistance to CDK4/6 and PI3Kα Inhibitors in Breast Cancer.

The combination of CDK4/6 inhibitors with antiestrogen therapies significantly improves clinical outcomes in ER-positive advanced breast cancer. To identify mechanisms of acquired resistance, we analyzed serial biopsies and rapid autopsies from patients treated with the combination of the CDK4/6 inhibitor ribociclib with letrozole. This study revealed that some resistant tumors acquired RB loss, whereas other tumors lost PTEN expression at the time of progression. In breast cancer cells, ablation of PTEN, through increased AKT activation, was sufficient to promote resistance to CDK4/6 inhibition in vitro and in vivo. Mechanistically, PTEN loss resulted in exclusion of p27 from the nucleus, leading to increased activation of both CDK4 and CDK2. Because PTEN loss also causes resistance to PI3Kα inhibitors, currently approved in the post-CDK4/6 setting, these findings provide critical insight into how this single genetic event may cause clinical cross-resistance to multiple targeted therapies in the same patient, with implications for optimal treatment-sequencing strategies. SIGNIFICANCE: Our analysis of serial biopsies uncovered RB and PTEN loss as mechanisms of acquired resistance to CDK4/6 inhibitors, utilized as first-line treatment for ER-positive advanced breast cancer. Importantly, these findings have near-term clinical relevance because PTEN loss also limits the efficacy of PI3Kα inhibitors currently approved in the post-CDK4/6 setting.This article is highlighted in the In This Issue feature, p. 1.