Abstract GS2-02: Elacestrant, an oral selective estrogen receptor degrader (SERD), vs investigator’s choice of endocrine monotherapy for ER+/HER2- advanced/metastatic breast cancer (mBC) following progression on prior endocrine and CDK4/6 inhibitor therapy: Results of EMERALD phase 3 trial

Abstract Background: Endocrine therapy(ET) plus CDK4/6 inhibitor (i) is the mainstay for the management of estrogenreceptor-positive (ER+)/HER2- mBC. However, most patients (pts) with ER+ mBCeventually experience disease progression, including development of ESR1mutations (mESR1). Elacestrant, an oral SERD, demonstrated preclinical activity,and clinical activity in a phase 1 trial in ER+ mBC, including responses in ptswith prior fulvestrant, CDK4/6i, and mESR1tumors (Bardia JCO 2021).. Methods: EMERALD(NCT03778931), a multicenter, international, randomized, open-label, controlledphase 3 trial, enrolled postmenopausal pts with ER+/HER2- mBC who had received 1-2prior lines of ET and ≤1 line of chemotherapy in the mBC setting and had prior progressionon a ET plus CDK4/6i. Pts were randomized 1:1 to elacestrant (400 mg orallydaily) or standard of care (SOC; investigator’s choice of fulvestrant or anaromatase inhibitor). Stratification factors included mESR1 status (by central lab), priorfulvestrant exposure, and presence of visceral disease. The study had 2 primary endpoints of progression-free survival (PFS), by blindedindependent review committee, in pts withtumors harboring mESR1 and in all pts (mESR1 or mESR1 notdetected). Secondary endpoints included: overall survival (OS), safety, tolerability,and quality of life. An alpha-value of 0.0475 was used to determine statisticalsignificance (2-sided using the truncated Hochberg procedure).. Results: EMERALD enrolled 477 pts(228 with mESR1) between Feb 2019 - Oct 2020, with 239 pts randomized toreceive elacestrant vs 238 pts to SOC. Demographics and disease characteristicswere well-balanced across treatment arms [median age: 63 yrs vs 63.5 yrs; 2prior lines: 46% vs 40.8%; prior CDK4/6i: 100% in both arms]. The study met bothprimary endpoints. There was a 30% reduction in the risk of progression ordeath in the elacestrant arm in all pts (HR=0.697 [95% CI: 0.552, 0.88]; P=0.0018),and a 45% (HR=0.546 [95% CI: 0.387, 0.768]; P=0.0005) reduction in therisk of progression or death in pts with mESR1.For both endpoints, results in key prespecified subgroups, including visceral metastases,number of prior lines of therapy, pretreatment with fulvestrant, and geographicalregion, were consistent with the overall outcome. The PFS rate at 12 months was 22.32% (95% CI: 15.24%, 29.40%)with elacestrant vs 9.42% (95% CI: 4.02%, 14.81%) with SOC in all pts, and26.76% (95% CI: 16.17%, 37.36%) vs 8.19% (95% CI: 1.26%, 15.12%) in the mESR1 subgroup. The prespecified interim OS analysis plannedat the time of the final PFS analysis (allocated2-sided alpha level of 0.0001) demonstrated a trend in favor of elacestrant inall pts (HR=0.751 [95% CI: 0.542, 1.038]; P=0.0821) and in pts with mESR1(HR=0.592 [95% CI: 0.361, 0.958]; P=0.0325). The final OS analysisis expected next year. Common (>10%) treatment-related adverse events (AEs) withelacestrant vs SOC included: nausea (25.3% vs 8.7%), vomiting (11% vs 2.6%), and fatigue (11% vs 7.9%), mostlygrade 1/2. Treatment-emergent AEs leading to discontinuation of elacestrant orSOC were infrequent in both arms (6.3% and 4.4%). Grade ≥3 treatment-relatedAEs in the elacestrant arm vs SOC were 7.2% vs 3.1%, mainly driven by nausea(2.1% vs 0.9%). There were no treatment-related deaths in either group.. Conclusions:Elacestrant is the first oral SERD to demonstrate a statistically significantand clinically meaningful improvement of PFS vs SOC in a randomized phase 3study in pts with ER+/HER2- mBC in the 2nd/3rd-linesetting, including those whose tumors harbor mESR1. Elacestrant was well tolerated and hasthe potential to become the new standard of care for pts with ER+/HER2- mBC. Citation Format: Aditya Bardia, Patrick Neven, Guillermo Streich, Alberto J. Montero, Frédéric Forget, Marie-Ange Mouret-Reynier, Joo Hyuk Sohn, Peter Vuylsteke, Kathleen K. Harnden, Hung Khong, Judit Kocsis, Florence Dalenc, Virginia Kaklamani, Patrick Dillon, Sunil Babu, Simon Waters, Ines Deleu, José García-Sáenz, Emilio Bria, Marina Cazzaniga, Janice Lu, Philippe Aftimos, Javier Cortes, Shubin Liu, Dirk Laurent, Maureen G. Conlan, Francois-Clement Bidard. Elacestrant, an oral selective estrogen receptor degrader (SERD), vs investigator’s choice of endocrine monotherapy for ER+/HER2- advanced/metastatic breast cancer (mBC) following progression on prior endocrine and CDK4/6 inhibitor therapy: Results of EMERALD phase 3 trial [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr GS2-02.