Estrogen Receptor-positive Advanced Breast Cancer Research Articles

36P Megestrol acetate resistance in estrogen receptor-positive advanced breast cancer in the MEGA trial

Megestrol-Acetate (MA) has activity after progressive disease (PD) on aromatase inhibitors, providing a disease control rate of 40% and a duration of clinical benefit of 10 months. Ligand-bound progesterone receptor modulates estrogen receptor (ER) activity and reprograms ER-regulated transcription in breast cancer (BC), leading to regression of tumour xenografts. MEGA trial [NCT03024580] aimed to evaluate modulation of steroid receptor activity in advanced BC. Here we present the clinical and preliminary translational data. Eligible patients (pts) had metastatic ER+ HER2- BC, and were randomized to receive MA or another HT agent (AHT), Tamoxifen or Exemestane, with 10 pts planned for each arm. The primary endpoint was PFS, secondary were clinical benefit rate and OS. Tumour biopsies and blood for translational analysis (TA) were collected before day 1 of treatment and at PD. Paired samples were analysed by RNA-seq and by chromatin immunoprecipitation and sequencing (ChIPseq) for histone acetylation (H3K27-Ac). From Aug/17 to Jan/20, a total of 20 pts were recruited and evaluable for efficacy. 16 pts evaluable for TA. No difference was observed between both arms in mPFS, MA = 121 vs AHT = 137 days (p=0.66), or mOS, MA=26 vs AHT=30 months, (p=0.35). The best clinical response at 12 weeks (w) was stable disease for MA was (50%) and AHT (58%) and the duration of response >24w for both was 25%. 10 pts in the MA arm had samples evaluable for TA. RNA-seq (n=7) analysis revealed enrichment for estrogen early and late response pathways in MA resistant samples. There was a global increase in H3K27 acetylation (n=10) at ESR1 motifs in MA resistant samples at PD, although a subgroup of pts (n=3 of 10) showed decreased H3K27Ac at these loci globally. Numerically higher median PFS and OS was observed among pts with decreased (N=3) vs increased (N=7) H3K27Ac at ESR1 motif at PD, respectively mPFS 223 (84-337) vs 137 days (46-291), (p=0.25) and mOS 30 (26-35) vs 14 (7-63) months (p=0.42). Translational analysis find evidence of increased transcriptional activity at ER regulated genes in most resistant samples. This could indicate that MA resistant tumours will retain sensitivity to further HT, requiring future research to proof this hypothesis.

Abstract PD13-12: PD13-12 Imlunestrant, an oral selective estrogen receptor degrader, in combination with abemaciclib with or without an aromatase inhibitor, in estrogen receptor-positive advanced breast cancer: Results from the phase 1a/b EMBER 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. Preclinically, imlunestrant has favorable efficacy and pharmacokinetic (PK) properties, including antitumor activity in ESR1-mutant models, along with enhanced efficacy when combined with abemaciclib. In dose escalation (Phase 1a) and dose expansion (Phase 1b) in the EMBER study, imlunestrant monotherapy was well tolerated with favorable safety, PK and encouraging antitumor activity in heavily pre-treated ER+, HER2- advanced breast cancer (aBC) patients (Jhaveri, ASCO 2022); imlunestrant recommended phase 2 dose (RP2D) was determined as 400mg QD. Here, we present the phase 1b dose expansion of imlunestrant with abemaciclib ± aromatase inhibitor (AI) in EMBER (NCT04188548). Methods: Phase 1b enrolled patients with ER-positive (ER+), HER2-negative (HER2-) aBC [shown prior endocrine therapy (ET) sensitivity or untreated de novo aBC; ≤1 prior therapies for aBC but must not have received a prior CDK4/6 inhibitor]. Patients were randomized, based on menopausal status and presence of visceral metastases, to receive imlunestrant + abemaciclib OR imlunestrant + abemaciclib + AI. Men and premenopausal women received a concomitant GnRH agonist. Serial plasma samples were obtained for PK and ctDNA analysis. Key endpoints included safety and tolerability, PK, objective response rate (ORR) per RECIST v1.1 (ORR: complete response [CR] or partial response [PR]) in patients with measurable disease), 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 26 May 2022, 85 patients have received imlunestrant [n=80 at 400 mg (RP2D); n=5 at 800 mg] in combination with abemaciclib (150mg twice daily) ± AI. Forty-eight (56%) patients had visceral disease and 9% had at least 1 ESR1 mutation detected in ctDNA at baseline. Patients were predominantly (75%) ET pre-treated, 51% with an AI; and 8% and 5%, respectively, had received prior chemotherapy or fulvestrant, for aBC. The most common treatment-emergent adverse events were diarrhea (87%), nausea (58%), fatigue (45%), neutropenia (39%) and abdominal pain (34%). The majority of treatment-related AEs (TRAEs) were Grade 1 or 2, with Grade ≥3 TRAEs occurring in 36% of patients. Most common TRAEs at RP2D (400mg) were diarrhea (81%), nausea (45%), fatigue (33%) and neutropenia (35%). No patient discontinued treatment due to an AE. Dose reductions were required of both imlunestrant and abemaciclib in 6 (7%) patients and of either imlunestrant in 3 (4%) or abemaciclib in 22 (26%) patients. Preliminary efficacy is presented in Table 1. Conclusion: Imlunestrant in combination with abemaciclib ± AI showed acceptable safety and tolerability, comparable to the MONARCH 2 trial of fulvestrant + abemaciclib, along with evidence of clinical activity in ER+, HER2- aBC patients. These data suggest no additive toxicity of imlunestrant when administered in combination with abemaciclib, along with comparable clinical benefit to that observed in MONARCH 2. Further data will be presented at the meeting. The phase 3, EMBER-3 study is ongoing; evaluating imlunestrant, investigator’s choice ET, and imlunestrant + abemaciclib in ET pre-treated ER+, HER2- aBC patients (NCT04975308). Table 1. Preliminary efficacy in combination therapies in EMBER Citation Format: Komal Jhaveri, Hwei-Chung Wang, Cynthia Ma, Elgene Lim, Jessica J. Tao, Luis Manso, Jean-Yves Pierga, Ritesh Parajuli, Yolanda Jerez Gilarranz, Yen-Shen Lu, Muralidhar Beeram, Tim Larson, Ajay Dhakal, Roohi Ismail-Khan, Claudia Karacsonyi, Shanshan Cao, Cynthia Osborne, Shawn T. Estrem, Bastien Nguyen, Yujia Li, Eunice Yuen. PD13-12 Imlunestrant, an oral selective estrogen receptor degrader, in combination with abemaciclib with or without an aromatase inhibitor, in estrogen receptor-positive advanced breast cancer: Results from the phase 1a/b EMBER 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 PD13-12.

High p16 expression and heterozygous RB1 loss are biomarkers for CDK4/6 inhibitor resistance in ER+ breast cancer

CDK4/6 inhibitors combined with endocrine therapy have demonstrated higher antitumor activity than endocrine therapy alone for the treatment of advanced estrogen receptor-positive breast cancer. Some of these tumors are de novo resistant to CDK4/6 inhibitors and others develop acquired resistance. Here, we show that p16 overexpression is associated with reduced antitumor activity of CDK4/6 inhibitors in patient-derived xenografts (n = 37) and estrogen receptor-positive breast cancer cell lines, as well as reduced response of early and advanced breast cancer patients to CDK4/6 inhibitors (n = 89). We also identified heterozygous RB1 loss as biomarker of acquired resistance and poor clinical outcome. Combination of the CDK4/6 inhibitor ribociclib with the PI3K inhibitor alpelisib showed antitumor activity in estrogen receptor-positive non-basal-like breast cancer patient-derived xenografts, independently of PIK3CA, ESR1 or RB1 mutation, also in drug de-escalation experiments or omitting endocrine therapy. Our results offer insights into predicting primary/acquired resistance to CDK4/6 inhibitors and post-progression therapeutic strategies.

Oral Selective Estrogen Receptor Degraders (SERDs) in Breast Cancer: Advances, Challenges, and Current Status.

Several endocrine therapies are currently available for the treatment of estrogen receptor (ER) positive breast cancer, but the clinical benefit of these agents is limited by endocrine therapy drug resistance. A common mechanism of endocrine therapy resistance is ESR1 mutations. The first-generation selective estrogen receptor degrader (SERD) fulvestrant has activity against ESR1 mutant tumors but requires intramuscular injection and has poor bioavailability that precludes optimal drug dosing. This led to the development of second-generation SERDs which are potent and have improved oral bioavailability and pharmacokinetics. Several of these oral SERDs are now in phase III trials in both the early and advanced ER positive breast cancer settings. This review summarizes the background of oral SERD development, the current status and future perspectives.

Factors associated with overall survival after recurrence in patients with ER-positive/HER2-negative postmenopausal breast cancer: an ad hoc analysis of the JBCRG-C06 Safari study.

The Safari study (UMIN000015168) was a retrospective, multicenter study in which 1072 consecutive cases of estrogen receptor-positive advanced breast cancer treated using 500mg fulvestrant were registered. We previously reported the relationship between the patient factors and overall survival after the diagnosis using the same cases and the same factors for the analysis of time to treatment failure in patients with estrogen receptor-positive advanced breast cancer. The current study is an ad hoc analysis that focused on the relationship between the patient factors and overall survival after recurrence by adding factors generally associated with overall survival after recurrence. The overall survival after recurrence in patients with estrogen receptor-positive human epidermal growth factor receptor 2 negative recurrent breast cancer was analyzed via univariate and multivariate analyses with a Cox proportional hazards model. A total of 598 cases were used for the analysis of overall survival after recurrence. Multivariate analysis revealed that favorable overall survival (median, 6.4years) was significantly correlated with long time from recurrence to fulvestrant use (≥3years), low nuclear or histological grade (G3 vs. G1), long time to treatment failure of initial palliative endocrine therapy (≥12months) and long time to initial palliative chemotherapy (≥2years). The results of this study indicate that sequential endocrine monotherapy may be a useful treatment option for patients with estrogen receptor-positive/human epidermal growth factor receptor 2 negative recurrent breast cancer who have been successfully treated with initial long-term palliative endocrine therapy.

Resistance Mechanisms to Combined CDK4/6 Inhibitors and Endocrine Therapy in ER+/HER2- Advanced Breast Cancer: Biomarkers and Potential Novel Treatment Strategies.

Simple SummaryAdvanced stage breast cancer is incurable, and a leading cause of cancer-related death in women. Recently, novel treatment options have improved clinical outcomes of breast cancer patients. Combined CDK4/6 inhibitor and endocrine therapy have been shown to be highly effective, improving both progression-free and overall survival, but resistance inevitably develops and disease progression occurs. Identification of clinically useful biomarkers predicting therapeutic response/failure and resistance mechanisms are essential for optimal patient stratification and the development of novel therapeutic strategies for patients who have progressed on combined CDK4/6 inhibitor and endocrine therapy. However, finding a universal single predictive biomarker is unlikely mainly due to the heterogenicity of the cancers and the associated resistance mechanisms. Here, we review the current state of knowledge on resistance mechanisms to combined CDK4/6 inhibitor and endocrine therapy and associated predictive molecular biomarkers, including the use of liquid biopsy.The introduction of CDK4/6 inhibitors (CDK4/6i) in combination with endocrine therapy (ET) has revolutionized the treatment landscape for patients with estrogen receptor-positive (ER+) advanced breast cancer (ABC) and has become the new standard treatment. However, resistance to this combined therapy inevitably develops and represents a major clinical challenge in the management of ER+ ABC. Currently, elucidation of the resistance mechanisms, identification of predictive biomarkers, and development of novel effective combined targeted treatments to overcome the resistance are active areas of research. Given the heterogeneity of the resistance mechanisms towards combined CDK4/6i and ET, identification of a single universal predictive biomarker of resistance is unlikely. Novel approaches are being explored, including examination of multiple genetic alterations in circulating cell-free tumor DNA in liquid biopsies from ABC patients with disease progression on combined CDK4/6i and ET treatment. Here, we review the molecular basis of the main known resistance mechanisms towards combined CDK4/6i and ET and associated potential biomarkers. As inhibiting key molecules in the pathways driving resistance may play an important role in the selection of therapeutic strategies for patients who experience disease progression on combined CDK4/6i and ET, we also review preclinical and early phase clinical data on novel combination therapies for these patients.

247P Efficacy and safety of ribociclib (RIB) in combination with letrozole (LET) in patients with estrogen receptor–positive advanced breast cancer (ABC): Secondary and exploratory results of phase 3b RIBECCA study

RIBECCA was a phase 3b, multicentre, open-label study of RIB + LET in patients (pts) of any menopausal status with HR+, HER2–ABC, who were not amenable to curative treatment (tx). The study investigated RIB + LET in pt settings broader than those considered in MONALEESA (ML) studies. Data on primary endpoint, clinical benefit rate (CBR) at week 24 in cohorts A (tx naive postmenopausal pts) and B (comprising cohort B1 [tx naive pre- and perimenopausal pts] and B2 [pre-, peri- and postmenopausal pts pretreated with ≤1 prior chemotherapy and ≤2 lines of endocrine therapy]) were reported previously.

Co-targeting CDK4/6 and AKT with endocrine therapy prevents progression in CDK4/6 inhibitor and endocrine therapy-resistant breast cancer

CDK4/6 inhibitors (CDK4/6i) combined with endocrine therapy have shown impressive efficacy in estrogen receptor-positive advanced breast cancer. However, most patients will eventually experience disease progression on this combination, underscoring the need for effective subsequent treatments or better initial therapies. Here, we show that triple inhibition with fulvestrant, CDK4/6i and AKT inhibitor (AKTi) durably impairs growth of breast cancer cells, prevents progression and reduces metastasis of tumor xenografts resistant to CDK4/6i-fulvestrant combination or fulvestrant alone. Importantly, switching from combined fulvestrant and CDK4/6i upon resistance to dual combination with AKTi and fulvestrant does not prevent tumor progression. Furthermore, triple combination with AKTi significantly inhibits growth of patient-derived xenografts resistant to combined CDK4/6i and fulvestrant. Finally, high phospho-AKT levels in metastasis of breast cancer patients treated with a combination of CDK4/6i and endocrine therapy correlates with shorter progression-free survival. Our findings support the clinical development of ER, CDK4/6 and AKT co-targeting strategies following progression on CDK4/6i and endocrine therapy combination, and in tumors exhibiting high phospho-AKT levels, which are associated with worse clinical outcome.

Survival rates of advanced estrogen-receptor positive breast cancer. Analysis of 211 cases.

About 80% of breast cancer (BC) cases express estrogen receptor (ER), which has been correlated with good prognosis and response to estrogen deprivation Aim: To characterize ER positive advanced BC (ABC) patients treated at our institution assessing the impact of clinical pre-sentation (stage IV, de novo disease at diagnosis versus systemic recurrence) and BC subtype on survival rates. We evaluated 211 ER+ advanced BC (ABC) patients, treated between 1997 and 2017. The median overall survival (OS) was 37 months. Median OS for the period 1997/2006 and 2007/2017 were 33 and 42 months, respectively (p = 0.47). Luminal A, ABC stage IV disease at diagnosis displayed better OS rates than Luminal B stage IV tumors (100 and 32 months respectively, p < 0.01). Clinical presentation (stage IV vs. systemic recurrence) and tumor subtype are key determinants of OS in ABC.

Monitoring of therapeutic efficacy to CDK4/6 inhibitors and early detection of metastatic relapse in breast cancer by ultra-deep sequencing of plasma cell-free DNA.

e15544 Background: CDK4/6 inhibition substantially improves progression-free survival for women with advanced estrogen receptor-positive breast cancer. Despite this, most patients experience acquired resistance. The analyses of cancer specific genomic aberrations in plasma cell free DNA, or commonly referred to as ‘liquid biopsy’, has generated immense interest. The role of liquid biopsy enables monitoring of therapeutic efficacy to detect relapse in a minimally invasive manner. It would drastically impact risk stratification, treatment selection and patient clinical outcome in advanced stage breast cancer. Methods: We have recruited consecutive patients with metastatic breast cancer who received CDK4/6 inhibitor treatment in combination with endocrine therapy between April 2018 and December 2019. Somatic mutations identified by the Oncomine Pan-Cancer cell free assay were used to assess the mutation profiles of circulating tumor DNA (ctDNA) by ultra-deep sequencing (~56,000 depth) of the plasma DNA. Plasma samples (N = 146) were collected before and during treatment (2 years) or until disease progression. Paired white blood cells (WBC) were also sequenced to exclude clonal hematopoiesis-derived mutations. Results: Twenty-eight patients were enrolled. Based on RECIST criteria, 1 patient showed complete response (CR) within 18 months of treatment, 5 patients showed partial response (PR) 6-18 months from start of treatment, 9 patients showed stable disease (SD) condition within 3-18 months and 13 patients developed progressive disease (PD) within 1-12 months. In 22 of 28 patients (79%), ctDNA was detected prior to start of treatment (12 responders; 10 PD) and used as a monitoring marker. Majority of mutations detected in pre-treatment samples were TP53, PIK3CA and ESR1. Among the 12 ctDNA positive responders, ctDNA levels of 9 patients were undetectable within 2 weeks to 6 months from start of treatment (median 3 months). ctDNA profiles were more reflective of clinical response compared to the tumor markers CEA and CA15-3. In 7 of the 10 PD patients with ctDNA monitoring markers, ctDNA showed higher sensitivity to predict disease progression compared to tumor markers that remained stagnant or below normal threshold. ctDNA also predicted PD earlier than radiological images with a median lead time of 3 months. Conclusions: ctDNA assessment for therapy monitoring and early relapse detection in advanced breast cancer is feasible and provides a basis to evaluate early therapeutic interventions.

Abstract PD7-09: A phase 1/1b study of LSZ102, an oral selective estrogen receptor degrader (SERD), in combination with ribociclib in patients with estrogen receptor-positive (ER+) advanced breast cancer (ABC) who had progressed after endocrine therapy (ET)

Abstract Background: The combination of ribociclib (an oral CDK4/6 inhibitor) and ET improved survival outcomes in patients (pts) with ER+ ABC in comparison to single-agent ET. In preclinical models, LSZ102 (an experimental oral SERD) demonstrated synergistic activity with ribociclib. This phase 1/1b, open-label study evaluates single-agent LSZ102 (Arm A) and LSZ102 in combination with ribociclib (Arm B) or alpelisib (Arm C) in pts with ER+ ABC who had progressed after ET. Here we present preliminary data from dose escalation of Arm B. Methods: Dose-escalation evaluated daily (QD) LSZ102 at different doses plus ribociclib starting at 300 mg QD either in a 3-week on/1-week (3w/1w) off or in continuous regimen. Pts (aged ≥18 years; ECOG PS 0-1) with histologically confirmed ER+ ABC and progression after ET were enrolled. Prior fulvestrant and/or CDK4/6 inhibitor therapy were permitted. The primary objective was to characterize the safety and tolerability of LSZ102 and ribociclib in combination and identify a recommended dose for expansion (RDE). Secondary objectives included preliminary antitumor activity and characterization of pharmacokinetics. Results: As of 30-Nov-2018, 76 pts were enrolled. The median age of pts was 59 years, 78% (n=59) were Caucasian, 75% (n=57) had an ECOG PS of 0, 63% (n=48) had received prior fulvestrant, and 43% (n=33) a prior CDK4/6 inhibitor; the median number of prior lines of therapy (metastatic setting) was 4 (range 0-10). Pts received LSZ102 QD in combination with ribociclib QD either in a 3w/1w (n=42) or continuous (n=24) regimen, fasted, fed or without regard to food. LSZ102 twice daily (BID) was also explored with a ribociclib BID continuous regimen (n=10). Doses of LSZ102 ranged from 200-600 mg QD and 200-300 mg BID. Doses of ribociclib ranged from 200-600 mg and 200-400 mg in the 3w/1w and continuous regimens, respectively. At data cut-off, 47 pts had discontinued treatment, 42 due to disease progression. Three dose-limiting toxicities were reported: decreased appetite (1 pt) and febrile neutropenia and sepsis (1 pt), all in the 3w/1w regimen. The most common treatment-related adverse events (AEs) were nausea (48.7%), diarrhea (31.6%), fatigue (26.3%), and neutropenia (25.0%). The most common treatment-related grade 3 or 4 AE was neutropenia (10.5%). Four pts (5.3%) required dose reduction due to an AE. Median PFS was 6.0 months. Of 71 evaluable pts, 11% (n=8) had a PR, 39% (n=28) had a best overall response of SD and 16% (n=11) had non-CR/non-PD. ORR, DCR and CBR ≥6 months (includes non-CR/non-PD) were 11% (n=8), 66% (n=47) and 21% (n=15), respectively. The RDE is 450 mg QD LSZ102 with 400 mg QD ribociclib for both ribociclib regimens. Preliminary PK showed the exposure of LSZ102 and ribociclib in combination at the RDE were largely comparable with exposure of the corresponding single agents. Conclusion: The combination of LSZ102 and ribociclib was well-tolerated and demonstrated antitumor activity in heavily pre-treated pts with ER+ ABC many of whom had progressed after fulvestrant and/or CDK4/6 inhibitor therapy. Citation Format: Komal Jhaveri, Dejan Juric, Sara Cresta, Yoon-Sim Yap, Francois P Duhoux, Catherine Terret, Rachel Layman, Nicole Kundamal, Serena Liao, Yan Ji, Adam Crystal, Giuseppe Curigliano. A phase 1/1b study of LSZ102, an oral selective estrogen receptor degrader (SERD), in combination with ribociclib in patients with estrogen receptor-positive (ER+) advanced breast cancer (ABC) who had progressed after endocrine therapy (ET) [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 PD7-09.

Abstract P2-11-08: ESR1 mutations are not a mechanism of primary resistance to aromatase inhibitors in ER-positive breast cancer treated with neoadjuvant endocrine therapy

Abstract Introduction: Mutations in the ESR1 gene (ESR1m) are important mechanisms of resistance to endocrine therapy (ET) in estrogen receptor-positive (ER+) advanced breast cancer and have been recognized as a prognostic and predictive biomarker as well as a potential therapeutic target. ESR1m prevalence has been described as 9-45% in cohorts of ET-resistant metastatic tumors in a variety of publications. We recently reported a prevalence of 25% in a cohort of ER+ breast cancer patients with visceral metastasis. However, the role of ESR1m as a mechanism of resistance to ET used in early-stage disease is not well studied. Neoadjuvant endocrine therapy (NET) is being increasingly explored, not only to allow less extensive surgery but also as a scientific tool, exploring biomarkers to predict outcomes. The preoperative endocrine prognostic index (PEPI) combining Ki67 score, ER Allred score, tumor size, and nodal status after NET is a surrogate of endocrine sensitivity and can identify a subgroup of patients with primary resistance to ET. In this study, we evaluated the prevalence of ESR1m in tumor samples from patients with ER+ breast cancer that were primarily resistant to neoadjuvant aromatase inhibitor (AI) therapy. Methods: We followed a prospective cohort of postmenopausal patients with ER+ HER2- stages II-III breast cancer treated with neoadjuvant endocrine therapy (NET). Patients were treated with anastrozole for a recommended period of at least three months. Tumor samples from patients with a pattern of primary endocrine-resistant tumors (defined as a PEPI Score higher than 3) were selected and analyzed for the presence of ESR1m. ESR1m were evaluated in formalin-fixed paraffin-embedded (FFPE) breast cancer tissue using real-time quantitative polymerase chain reaction (RT-qPCR). Mutations in codons 380, 537, and 538 of the ESR1 gene were analyzed. Results: 127 patients were included in the cohort, of which 100 (79%) had completed NET and had surgery. Among these patients, the PEPI Score ranged from 0 to 3 in 70% (70/100), and 30% (30/100) had a PEPI Score of 4 or more and were selected. 23 patients were included in the analysis (6 did not consent or were lost to follow-up, and one was HER2-positive). Patients characteristics are summarized in Table 1. The median duration of NET was 22 weeks. All samples of tumor tissue from the surgical specimens after NET were evaluable. Quantification of DNA extraction and reference gene cycle threshold values confirmed that the material was adequate for the analysis. We compared these findings with a study from our group using the same methodology in patients with advanced disease where ESR1 mutations were detected in 25% (n=32) of patients with visceral metastasis of ER+ breast cancer resistant to endocrine therapy. ESR1 mutations were not identified in any of the 23 patients with early-stage ER+ breast cancer resistant to NET (p 0.01, Fisher´s exact test) Discussion: Growing evidence supports the notion that there are different mechanisms of primary and secondary endocrine resistance. Our study suggests that ESR1 mutations do not evolve rapidly and do not represent a common mechanism of primary endocrine resistance in the neoadjuvant setting. Therefore, ESR1m should be considered a mechanism of acquired endocrine resistance in the context of advanced disease. Further research should be conducted to identify factors associated with intrinsic resistance to ET. Citation Format: Tomas Reinert, Susana Ramalho, Vivian CA Vasconcelos, Leonardo R Silva, Ana Elisa R Silva, Camila A Andrade, Maria Beatriz PL Kraft, Guilherme P Coelho, Jovana Mandelli, Monique Binotto, Cesar Cabello, Geisilene RP Silva, Jose Bines, Carlos H Barrios, Matthew J Ellis, Marcia S Graudenz. ESR1 mutations are not a mechanism of primary resistance to aromatase inhibitors in ER-positive breast cancer treated with neoadjuvant endocrine therapy [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 P2-11-08.

Abstract P5-11-11: Influence of the adjuvant hormonal therapy on hormone sensitivity and survival outcomes in ER+ and HER2−advanced breast cancer: A subgroup analysis of the JBCRG-C06 Safari study

Abstract Background: There are now many treatment options for estrogen receptor-positive (ER+) and human epidermal growth factor receptor 2-negative (HER2−) advanced breast cancer (ABC). However, there are few reports indicating the optimal treatment sequence for this disease. Information to predict response to treatment is vital for personalized therapy and determining the most beneficial approach for individual patients. In this study, we attempted to determine predictive factors of response to hormonal therapy and survival outcomes using the large-scale databases constructed in the Safari study (UMIN000015168), a retrospective, multicenter cohort study involving 1,072 Japanese patients receiving fulvestrant 500mg for ER+ ABC. We examined the association between clinicopathological factors and time to failure (TTF) of fulvestrant in Japanese ABC patients. Methods: Among 1072 patients, 247 patients were selected for this study. Inclusion criteria was as follows: 1) patients treated with either SERM or AI in the adjuvant setting (AS) with known starting and finishing date of the treatment (patients relapsed on adjuvant treatment were excluded), 2) patients treated with SERM, AI and SERD in the metastatic setting (MS) as the first and the second line treatment. Influence of the adjuvant hormonal therapy on the TTF of the first line and the second line treatment and overall survival (OS) was assessed. Cox proportional hazards model was used for this analysis. Results: Patients treated with SERM in AS had significantly longer TTF of 1st line and 1st+2nd line of hormonal therapy in MS (1st line: HR 1.519, 95% CI 1.04-2.218, p=0.0307, 1st+2nd line: HR 2.372, 95% CI 1.584-3.551, p&amp;lt;0.001) compared with patients treated with AI in AS. Same trend was observed in OS. SERM treated patients in AS had significantly longer OS (OS: HR 2.748, 95% CI 1.574-4.796, p=0.0004) compared with AI treated patients in AS. Since these results described above are the results analyzed by multivariate analysis, they are all statistically significant independent of other factors such as disease free interval in this cohort. Conclusions: Interestingly, once AI-treated patients in AS have relapsed, their hormone sensitivity is lower and survival outcomes are worse compared with SERM-treated patients in AS in this cohort. Our results have consistency with the results of ABCSG-12 trial showing a pronounced higher risk of death for anastrozole-treated patients (Annals of Oncology 26: 313–320, 2015). Since it is a cohort of selected patients who were successfully treated with fulvestrant, prospective analysis would be therefore warranted. Citation Format: Takahiro Nakayama, Hidetoshi Kawaguchi, Norikazu Masuda, Kenjiro Aogi, Keisei Anan, Yoshinori Ito, Shoichiro Ohtani, Nobuaki Sato, Shigehira Saji, Toshimi Takano, Eriko Tokunaga, Seigo Nakamura, Yoshie Hasegawa, Masaya Hattori, Tomomi Fujisawa, Satoshi Morita, Miki Yamaguchi, Hiroko Yamashita, Toshinari Yamashita, Yutaka Yamamoto, Daisuke Yotsumoto, Masakazu Toi, Shinji Ohno. Influence of the adjuvant hormonal therapy on hormone sensitivity and survival outcomes in ER+ and HER2−advanced 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 P5-11-11.

CDK4/6 Inhibitor Biomarker Research: Are We Barking Up the Wrong Tree?

CDK4/6 inhibitors have emerged as a significant advance for the treatment of patients with advanced estrogen receptor-positive breast cancer. However, the identification of predictive markers that optimize their use is proving harder than expected. In this commentary we advocate for unbiased discovery and a collaborative approach across trials.See related article by Finn et al., p. 110.

Factors associated with prolonged overall survival in patients with postmenopausal estrogen receptor-positive advanced breast cancer using real-world data: a follow-up analysis of the JBCRG-C06 Safari study

BackgroundAssessing survival risk is important for discussing treatment options with estrogen receptor-positive (ER+) advanced breast cancer (ABC) patients. However, there are few reports from large-scale databases on the survival risk factors in ER+ ABC. The Safari study (UMIN000015168) was a retrospective, multicenter cohort study involving 1072 Japanese patients receiving fulvestrant 500 mg mostly as a second- or later-line endocrine therapy for ER+ ABC. The follow-up data after the Safari study were examined, focusing on any relationship between clinicopathological factors and overall survival (OS) in ER+ ABC patients.MethodsOS in patients with ER+ ABC was analyzed by univariate and multivariate analyses with a Cox proportional hazards model in this study.ResultsA total of 1031 cases were evaluable for OS analysis. Multivariate analysis showed that younger age (< 60 years), longer time from ABC diagnosis to fulvestrant use (≥ 3 years), no prior palliative chemotherapy before fulvestrant use, and progesterone receptor (PgR) negativity (PgR−) were significantly correlated with prolonged OS (median 7.0 years). For cases with histological or nuclear grade data, lower histological or nuclear grades were also correlated with longer OS. In recurrent metastatic cases, long disease-free interval (DFI) was not correlated with longer OS.ConclusionsIn ER+ ABC patients whose treatment history included fulvestrant, younger age, longer time from ABC diagnosis to fulvestrant use, no prior palliative chemotherapy use, PgR−, and lower histological or nuclear grade correlated positively with prolonged OS.

Association of ESR1 Mutations and Visceral Metastasis in Patients with Estrogen Receptor-Positive Advanced Breast Cancer from Brazil.

Mutations in the ESR1 gene (ESR1m) are important mechanisms of resistance to endocrine therapy in estrogen receptor-positive advanced breast cancer and have been recognized as a prognostic and predictive biomarker as well as a potential therapeutic target. However, the prevalence of ESR1m in real-world patients has not been adequately described. Therefore, we sought to evaluate the prevalence of ESR1m in metastatic samples from Brazilian patients with estrogen receptor-positive (ER+) advanced breast cancer previously treated with endocrine therapy. The presence of ESR1m was evaluated in formalin-fixed paraffin-embedded (FFPE) breast cancer tissue using real-time quantitative polymerase chain reaction (RT-qPCR). Mutations in codons 380, 537, and 538 of the ESR1 gene were analyzed. Out of 77 breast cancer samples, 11 (14.3%) showed mutations in the ESR1 gene. ESR1m were detected in a variety of organs, and the D538G substitution was the most common mutation. In visceral metastasis, ESR1m were detected in 25% (8/32) of the samples, whereas in nonvisceral metastasis, ESR1m were detected in 6.7% (3/45) of the samples. The odds of a sample with visceral metastasis having an ESR1 mutation is 4.66 times the odds of a sample of nonvisceral metastasis having an ESR1 mutation (95% CI: 1.13–19.27; p value = 0.0333). Our study indicates that the prevalence of ESR1m in samples from Brazilian patients with metastatic ER+ breast cancer is similar to that described in patients included in clinical trials. We observed an association of ESR1m with visceral metastasis.

Multicenter Observational Study of Fulvestrant 500 mg in Postmenopausal Japanese Women with Estrogen Receptor-Positive Advanced or Recurrent Breast Cancer after Prior Endocrine Treatment (SBCCSG29 Study).

Fulvestrant 500 mg has been an option for endocrine therapy for advanced or recurrent breast cancer after prior endocrine treatment since November 2011 in Japan. This study aimed to clarify the effectiveness and safety of fulvestrant 500 mg in clinical settings. This was a multicenter, both prospective and retrospective, observational study of 132 postmenopausal women (median age 66) with locally advanced or metastatic breast cancer, who had been treated with fulvestrant. Information from medical records was retrospectively obtained from 9 hospitals (Saitama Breast Cancer Clinical Study Group: SBCCSG) in Saitama prefecture, Japan, from October 2012 to April 2014. The primary end point was time to treatment failure (TTF). The secondary end points were overall survival (OS), objective response rate (ORR), clinical benefit rate (CBR), and adverse events (AE) (CTCAE ver. 4). The choice of subsequent therapy after fulvestrant was also evaluated. The median TTF was 6.1 months. Median OS was 28.5 months (the starting date was the first day of fulvestrant). ORR was 12.9% and CBR was 45.5%. The most common AEs were injection site reactions (9.1%). The rate of grade 3 AE was only 2.3% (3/132). The number of patients who underwent subsequent therapy after fulvestrant were 54 (55.7%) receiving chemotherapy, 42 (43.3%) receiving non-fulvestrant endocrine therapy, and 1 (1%) receiving mammalian target of rapamycin inhibitor (mTORi) + endocrine therapy (ET). Fulvestrant 500 mg is an effective and safe treatment for patients with advanced or recurrent breast cancer after prior endocrine treatment.

Tracking endocrine resistance in estrogen-receptor positive breast cancer in ctDNA.

e14550 Background: Endocrine therapy is the standard of care treatment for patients with estrogen-receptor positive advanced breast cancer, owing to improved tolerability and comparable efficacy to that of cytotoxic chemotherapy. Half of such cancers will progress through first line therapy (primary endocrine resistance) and half will progress after an initial period of disease control (secondary or acquired endocrine resistance). A significant challenge is to test for and identify biomarkers which can guide the likely success of endocrine therapy as a single agent or in combination with small molecule inhibitors.This is particularly challenging where metastatic deposits reside at sites where biopsy is difficult. Potential biomarkers indicative of resistance to endocrine therapy have been identified and can be detected in circulating tumour DNA (ctDNA). CtDNA is shed from tumours and is detectable in a cancer patient’s bloodstream. Information on mutational profiles can guide an oncologist in the selection of targeted therapy in addition to hormonal therapy. Methods: We have analysed formalin-fixed paraffin-embedded(FFPE) tumour samples and longitudinal liquid biopsies from 19 patients who were treated with fulvestrant in combination with a novel inhibitor of the PIK3CA/AKT pathway with next-generation sequencing using a targeted 44-gene panel. Mutations identified using this technique were tracked during the course of treatment using droplet-digital PCR(ddPCR). Results: 57 samples were tested using a 44-gene panel; 19 FFPE tumour samples and matched ctDNA samples were obtained prior therapy and ctDNA samples at disease progression. Mutations detected in PIK3CA, AKT1, ESR1and TP53 genes were trackable in longitudinal ctDNA samples using ddPCR. The association between ctDNA dynamics and outcome will be presented. Conclusions: Multiple mutations that enable the early detection of treatment failure and resistance can be tracked in ctDNA. Investigating the clinical utility of ctDNA is paramount.

Early ctDNA dynamics as a surrogate for progression-free survival in advanced breast cancer in the BEECH trial.

BackgroundDynamic changes in circulating tumour DNA (ctDNA) levels may predict long-term outcome. We utilised samples from a phase I/II randomised trial (BEECH) to assess ctDNA dynamics as a surrogate for progression-free survival (PFS) and early predictor of drug efficacy.Patients and methodsPatients with estrogen receptor-positive advanced metastatic breast cancer (ER+ mBC) in the BEECH study, paclitaxel plus placebo versus paclitaxel plus AKT inhibitor capivasertib, had plasma samples collected for ctDNA analysis at baseline and at multiple time points in the development cohort (safety run-in, part A) and validation cohort (randomised, part B). Baseline sample ctDNA sequencing identified mutations for longitudinal analysis and mutation-specific digital droplet PCR (ddPCR) assays were utilised to assess change in ctDNA abundance (allele fraction) between baseline and 872 on-treatment samples. Primary objective was to assess whether early suppression of ctDNA, based on pre-defined criteria from the development cohort, independently predicted outcome in the validation cohort.ResultsIn the development cohort, suppression of ctDNA was apparent after 8 days of treatment (P = 0.014), with cycle 2 day 1 (4 weeks) identified as the optimal time point to predict PFS from early ctDNA dynamics. In the validation cohort, median PFS was 11.1 months in patients with suppressed ctDNA at 4 weeks and 6.4 months in patients with high ctDNA (hazard ratio = 0.20, 95% confidence interval 0.083–0.50, P < 0.0001). There was no difference in the level of ctDNA suppression between patients randomised to capivasertib or placebo overall (P = 0.904) nor in the PIK3CA mutant subpopulation (P = 0.071). Clonal haematopoiesis of indeterminate potential (CHIP) was evident in 30% (18/59) baseline samples, although CHIP had no effect on tolerance of chemotherapy nor on PFS.ConclusionEarly on-treatment ctDNA dynamics are a surrogate for PFS. Dynamic ctDNA assessment has the potential to substantially enhance early drug development.Clinical registration numberNCT01625286.

Abstract PD1-08: Phase 1/1b study of novel oral selective estrogen receptor degrader (SERD) LSZ102 for estrogen receptor-positive (ER+) advanced breast cancer (ABC) with progression on endocrine therapy (ET)

Abstract Background: LSZ102 is an orally bioavailable SERD that inhibits ER gene transcription, induces receptor degradation, and blocks ER-dependent cell growth in preclinical models. This study is evaluating LSZ102 as a single agent and in combination with the CDK 4/6 inhibitor ribociclib (LEE011) or the PI3K inhibitor alpelisib (BYL719) in patients (pts) with ER+ ABC. The LSZ102 single agent data are presented below; combination data are not discussed. Methods: In the dose-escalation phase evaluating single-agent LSZ102 (Arm A), pts (age ≥18 years; ECOG PS 0-1) with histologically confirmed ER+ ABC and progression on endocrine therapy (ET) received LSZ102. The starting dose was 200 mg once daily. The primary objective of Arm A was to characterize the safety and tolerability of LSZ102 and identify a recommended dose for expansion (RDE). Secondary objectives included preliminary antitumor activity and pharmacokinetics (PK). Results: As of January 22, 2018, 57 pts were enrolled to Arm A (LSZ102 200 mg, n=4; 400 mg, n=6; 450 mg fasted, n=15; 450 mg with food, n=6; 600 mg, n=20; 900 mg, n=6). Median age was 60 years, 75% (n=43) of pts had an ECOG PS of 0, 56% (n=32) had received prior fulvestrant, and 58% (n=33) had received prior CDK4/6 inhibitors; median number of prior lines of therapy (all settings) was 6. At data cut-off, 48 pts had discontinued treatment, most (n=45, 94%) due to disease progression. Dose-limiting toxicities across treatment groups included diarrhea (2 pts in the 900-mg group), vomiting (1 pt in the 600-mg group), and AST and ALT elevation (1 pt in the 450-mg with food group). The most common treatment-related adverse events (AEs) in the treatment period were diarrhea (60%), nausea (56%), and vomiting (30%). In the treatment period, treatment-related grade 3 AEs (12%) were infrequent, and there were no such grade 4 events. Six pts (11%) required dose reduction due to AEs (nausea, vomiting or diarrhea); 4/6 of the dose reductions occurred at 900 mg. Preliminary PK assessment showed rapid absorption and dose-proportional increases in LSZ102 exposure; trough concentrations were above the predicted tumorostatic concentrations at doses of ≥400 mg. Based on PK results for the 450-mg fasted and fed cohorts, LSZ102 exposure does not appear to be affected by dosing with a regular meal. Evidence of ER modulation by immunohistochemistry was observed in paired baseline and on-treatment biopsies. 18F-fluoroestradiol positron emission tomography (FES-PET) analysis (n=6) demonstrated abrogation of FES-PET signal for pts in the 450-mg and 600-mg dose groups. Seventeen pts (29.8%) had a best response of stable disease, and 1 pt, who happened to be in the 600-mg group, achieved a partial response. Conclusion: In heavily pretreated pts, LSZ102 was well tolerated, demonstrated antitumor activity, and achieved effective exposure levels based on PK and pharmacodynamics. Food intake did not appear to significantly alter the PK profile of LSZ102. Dose escalation for LSZ102 in combination with ribociclib or alpelisib is ongoing and will be reported in a future analysis. An update on the recommended single agent dose and schedule will be presented. Citation Format: Jhaveri K, Curigliano G, Yap Y-S, Cresta S, Duhoux FP, Terret C, Takahashi S, Ulaner GA, Kundamal N, Baldoni D, Liao S, Crystal A, Juric D. Phase 1/1b study of novel oral selective estrogen receptor degrader (SERD) LSZ102 for estrogen receptor-positive (ER+) advanced breast cancer (ABC) with progression on endocrine therapy (ET) [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr PD1-08.