Abstract PS17-02: Elacestrant vs standard-of-care in ER+/HER2- advanced or metastatic breast cancer (mBC) with ESR1 mutation: key biomarkers and clinical subgroup analyses from the phase 3 EMERALD trial

Abstract

Abstract Background: The EMERALD trial reported significantly prolonged progression-free survival (PFS) and a manageable safety profile with elacestrant vs standard of care (SOC) endocrine therapy in patients (pts) with ER+/HER2− ESR1 mutated (ESR1-mut) mBC following progression on prior endocrine therapy (ET). EMERALD is the only pivotal oral SERD clinical trial where prior CDK4/6i usage was mandated. Duration of prior CDK4/6i was shown to be a predictor of efficacy in patients with ESR1 mutations (ESR1-mut) receiving elacestrant, with median PFS (mPFS) for patients with ESR1-mut receiving at least 12 months of CDK4/6i achieved 8.61 months (elacestrant) vs 1.91 months (SOC) (SABCS 2022). This analysis evaluates the clinical benefit of single-agent elacestrant in key clinically relevant subgroups, including biomarkers, usually associated with poorer prognosis. Methods: Patients with ER+/HER2- advanced or mBC who previously had 1-2 lines of endocrine therapy, and prior CDK4/6i, were randomized 1:1 to receive elacestrant or SOC (aromatase inhibitor or fulvestrant). A subgroup analysis was performed on patients with ESR1-mut, by prior duration of CDK4/6i plus ET with at least 12 months in the advanced or metastatic setting, with concomitant presence of liver and/or lung metastases, PIK3CA mutations, TP53 mutation, and HER2 low expression. Results: Overall, 478 patients were randomized to elacestrant (n=239) or SOC (n=239), 228 pts (47.7%) had ESR1-mut, and 159 pts (71.6%) received at least 12 months of prior CDK4/6i. Out of these 159 pts, 113 pts (71%) had liver and/or lung metastases, 62 pts (39%) had a PIK3CA-mut, 61 pts (38%) had TP53m, and 77 pts (48%) had HER2 low expression. A clinically meaningful improvement in PFS favoring elacestrant compared to SOC was consistent across all relevant subgroups in pts with ESR1-mut. Conclusions: Elacestrant showed significantly greater PFS when prior treatment duration with CDK4/6i was at least 12 months, suggesting prior exposure to CDK4/6i is a surrogate marker for endocrine sensitivity. In this population, elacestrant demonstrated superior efficacy, compared to SOC, even in patients with concomitant PIK3CA or TP53 mutations, expression of HER2 low, or presence of liver and/or lung metastases. These results suggest an active ER-driven pathway for this group despite the presence of other resistance mechanisms, where single-agent oral elacestrant could be an attractive option compared to combination therapies or intravenous HER2 low-targeted ADCs. NCT number: NCT03778931 Table. a. Includes E545K, H1047R, E542K amongst others b. HER2 IHC 1+, and 2+ with no ISH amplification Citation Format: Aditya Bardia, Joyce O'Shaughnessy, Francois-Clement Bidard, Patrick Neven, José Ángel García-Sáenz, Philippe Aftimos, Javier Cortés, Janice Lu, Giulia Tonini, Kathy Puvana Theall, Alessandro Paoli, Virginia Kaklamani. Elacestrant vs standard-of-care in ER+/HER2- advanced or metastatic breast cancer (mBC) with ESR1 mutation: key biomarkers and clinical subgroup analyses from the phase 3 EMERALD trial [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 PS17-02.