Abstract PS1-10-06: Impact of prior fulvestrant use and endocrine therapy duration on real-world outcomes of elacestrant treatment in ER+/HER2- metastatic breast cancer

Abstract Background: Endocrine therapy (ET) combined with a CDK4/6 inhibitor (CDK4/6i) is the standard first-line treatment for patients (pts) with estrogen receptor-positive (ER+), HER2-negative (HER2-) metastatic breast cancer (mBC). However, many pts eventually develop endocrine resistance, often driven by acquired ESR1 mutations (ESR1m). Elacestrant is the first oral selective estrogen receptor degrader approved for pts with ESR1-mutated ER+/HER2- mBC. Approval was based on the phase III EMERALD trial, which demonstrated significantly improved progression-free survival (PFS) compared to standard-of-care ET in pts with ESR1m. Notably, ∼29% of pts received prior fulvestrant, and subgroup analyses showed a PFS benefit with elacestrant even in this subgroup. Exploratory analyses suggest that pts with longer disease control on prior ET + CDK4/6i may derive greater benefit from elacestrant. Despite these findings, real-world evidence on how prior ET exposure, including fulvestrant use and ET duration, impacts elacestrant outcomes is limited. This study assessed the association of prior fulvestrant use and prior ET duration with time to next treatment (TTNT) and real-world overall survival (rwOS) among pts who received elacestrant monotherapy. Methods: The IntegraConnect PrecisionQ de-identified electronic health record database, (>3 million cancer pts, >500 care sites) was utilized to identify pts treated with elacestrant monotherapy between 1/27/2023 and 1/31/2025. All pts received ≥1 ET before elacestrant initiation. ESR1m was not assessed but assumed present. Pts were stratified by prior use of fulvestrant monotherapy, prior use of fulvestrant + CDK4/6i, and duration of prior ET (