Yam, Einav Nili-Gal

Abstract Introduction: Estrogen receptor-positive (ER+)/HER2-negative (HER2-) breast cancers commonly acquire constitutively active mutations in the ERα-encoding gene (ESR1), resulting in endocrine therapy (ET) resistance. In the phase 2, ELAINE 2 study (NCT04432454), LAS (a breast ER antagonist) combined with Abema provided a median progression-free survival (PFS) of ~13 months (mos) and clinical benefit rate (CBR) of 66% in patients with endocrine-resistant, ER+/HER2- mBC and ESR1 mutations (mESR1; Damodaran. J Clin Oncol 2023;41:suppl16;1057). Our objective was to describe baseline genomic alterations co-occuring with mESR1 and treatment responses to LAS plus Abema in patients with these concurrent genomic alterations. Methods: Women (age ≥18 yrs) with ER+/HER2- mBC that progressed after prior ET and CDK4/6 inhibitor (CDK4/6i) with an mESR1 detected in blood using the Sysmex-Inostics SafeSeq circulating tumor DNA (ctDNA) test were enrolled in ELAINE 2. Oral LAS 5 mg/day and Abema (provided by Eli Lilly and Co) 150 mg BID were taken until disease progression, death, unacceptable toxicity, or withdrawal. The Guardant360 CDx test was used to identify and describe baseline genomic alterations in ctDNA (including mESR1). Median PFS and CBR were assessed in patients with mESR1 and co-existing baseline genomic alterations. For patients who withdrew before disease progression, their last observation time was used as the progression time for PFS estimation. Data were summarized descriptively with no formal hypothesis testing. Results: Of the 29 patients (median age 60 yrs) enrolled, mutations in ESR1 were identified in 26 by Guardant360; all these 26 patients had received prior CDK4/6i, 21 (81%) prior fulvestrant, and 12 (46%) prior chemotherapy for mBC. Alterations in 39 genes were concurrently identified with mESR1, including TP53 mutations (n=11), PIK3CA mutations (n=8), CCND1 amplifications (n=6), and FGFR1 amplifications (n=5); co-amplification of CCND1 and FGFR1 was identified in 3 patients. LAS plus Abema treatment was associated with a CBR of 73% and a median PFS of 12.9 mos in the 26 patients with mESR1 (Table). CBRs were 63% in the co-occurring PIK3CA mutation subgroup and 64% in the TP53 mutation subgroup; median PFS times were 7.8 and 8.3 mos, respectively. All patients in the CCND1 and FGFR1 amplification subgroups achieved clinical benefit (CBR 100%) and the median PFS was 16.6 mos for both subgroups. In patients with mESR1 and co-alterations in ≥2 other genes of interest (PIK3CA, FGFR1, CCND1, TP53, ERBB2, CCNE1, or RB1), CBR was 83% and median PFS was 16.6 mos. Conclusion: Using Guardant360 ctDNA profiling from patients in ELAINE 2, we demonstrate that other baseline genomic alterations are frequently detected concurrently with mESR1 in the endocrine resistant setting, but without apparent compromise on the efficacy of LAS plus Abema. These results should be interpreted with caution considering the small numbers of patients and the exploratory nature of the analysis. The ELAINE 2 study suggests the potential of LAS plus Abema for treating ESR1-mutated, ER+/HER2- mBC in the post-CDK4/6i setting. This will be further evaluated in the larger, registrational, phase 3, ELAINE 3 study (NCT05696626) that compares LAS plus Abema with fulvestrant plus Abema. Table. Treatment response by genomic alterations concurrent with ESR1 mutations (n=26). aFor patients who withdrew before disease progression, the last observation time was used as the progression time for PFS estimation. bOther genes of interest include PIK3CA, FGFR1, CCND1, TP53, ERBB2, CCNE1, or RB1. CBR, clinical benefit rate; mPFS, median progression-free survival. Citation Format: Senthil Damodaran, Massimo Cristofanilli, Matthew Goetz, Ciara O'Sullivan, Gary Riordan, Hope Rugo, Daniel Stover, Seth Wander, Dominic Carroll, Paul V. Plourde, David J Portman, Einav Nili-Gal Yam. Baseline genomic alterations and the activity of lasofoxifene (LAS) plus abemaciclib (Abema) in patients with ER+/HER2- metastatic breast cancer (mBC): the ELAINE 2 study [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO2-14-09.

Abstract Background: Patients with estrogen receptor-positive (ER+) metastatic breast cancer (BC) may develop resistance to endocrine therapy (ET), particularly following treatment with a cyclin-dependent kinase 4/6 inhibitor (CDK4/6i), potentially driven by a mutation in the ERa-coding gene, ESR1. Lasofoxifene, an oral, next-generation ET and ER breast antagonist, had potent activity in mBC xenograft models expressing ESR1 mutations, either as a single agent or combined with a CDK4/6i. Subsequently, two phase 2 studies evaluated lasofoxifene in women with ER+/HER2- metastatic BC and an ESR1 mutation who had disease progression on previous ET and CDK4/6i. The ELAINE 1 trial showed numerically greater progression-free survival (PFS, median ~6 vs 4 months; P=0.138), objective response rate (ORR, 13% vs 3%; P=0.124), and clinical benefit rate (CBR, 37% vs 22%; P=0.117) with lasofoxifene monotherapy versus the ER degrader fulvestrant, with a favorable safety profile (Goetz MP, et al. Ann Oncol. 2022;33:S1387). The single-arm, ELAINE 2 trial demonstrated that lasofoxifene combined with abemaciclib was well tolerated with a median PFS of ~13 months, ORR of 56%, and CBR of 66% (Damodaran S, et al. J Clin Oncol. 2023;41:suppl 16; abstr 1057). Based on these data, the phase 3, registrational, ELAINE 3 trial was initiated. Methods: ELAINE 3 (NCT05696626) is an open-label, phase 3, multicenter study evaluating the efficacy, safety, and tolerability of lasofoxifene plus abemaciclib versus fulvestrant plus abemaciclib. Key inclusion criteria are pre- and postmenopausal women and men aged ≥18 years; ER+/HER2-, locally advanced and/or metastatic BC (measurable and/or non-measurable disease); ≥1 acquired ESR1 mutation; progression on an aromatase inhibitor plus palbociclib or ribociclib as their first hormonal treatment for advanced/metastatic BC; and ≤1 line of chemotherapy in the advanced/metastatic setting. Patients will be randomized 1:1 to receive lasofoxifene 5 mg/day plus abemaciclib 150 mg BID, or fulvestrant 500 mg IM on days 1, 15, and 29, then 4 weekly plus abemaciclib 150 mg BID. Treatment will continue until progression, death, unacceptable toxicity, or withdrawal from the study. The primary endpoint is PFS; key secondary endpoints are ORR, overall survival, CBR, duration of response, and time to response. Time to cytotoxic chemotherapy, quality of life, and safety will also be evaluated. Blood samples for circulating tumor DNA (ctDNA) will be collected at screening, at weeks 4 and 8 and every 8 weeks thereafter, and at the final visit for genomic analyses. Outcomes with lasofoxifene/abemaciclib and fulvestrant/abemaciclib will be compared using a stratified Cox proportional hazards model and stratified logrank test. Expected PFS is ≥10.3 months for lasofoxifene/abemaciclib and 7 months for fulvestrant/abemaciclib (PFS hazard ratio of 0.68 at final analysis). The target sample size is 400, to achieve 90% power with a one-sided type I error rate of 0.025 after 285 PFS events. Full recruitment is expected to occur over 18 months at 125 global sites. Citation Format: Matthew Goetz, Seth Wander, Thomas Bachelot, Gerald Batist, Javier Cortés, Massimo Cristofanilli, Giuseppe Curigliano, Alexandre de Nonneville, Einav Nili-Gal Yam, Komal Jhaveri, Cynthia Ma, Heather Parsons, Hope Rugo, Sarah Sammons, Daniel Stover, Chris Twelves, Aditya Bardia, Paul V. Plourde, David J Portman, Senthil Damodaran. Trial in progress: Open-label, randomized, multicenter, phase 3, ELAINE 3 study of the efficacy and safety of lasofoxifene plus abemaciclib for treating locally advanced or ER+/HER2- metastatic breast cancer with an ESR1 mutation [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-18-12.