Real-world (RW) elacestrant use patterns and therapeutic outcomes in patients (pts) with hormone receptor-positive (HR+)/HER2-negative advanced breast cancer (aBC).

1071 Background: Elacestrant was the first novel endocrine therapy (ET) to receive FDA approval in over two decades. In the EMERALD study, pts with ESR1-mutant ( ESR1m) HR+/HER2-negative aBC had a median progression free survival (PFS) of 3.8 months (mos). Here, we use a large clinical-genomic database to assess the RW use of elacestrant and examine the impact of line of therapy (LOT) and number of ERS1m on pt outcomes. Methods: Pts with HR+/HER2- aBC were identified via the GuardantINFORM database; all pts with ESR1m detected on circulating tumor DNA (ctDNA) testing done in routine clinical care and treated with elacestrant after the FDA approval date were included. Real-world time to treatment discontinuation (TTD) and time to next treatment (TTNT) were used as proxies for PFS (measured in mos). For TTD/TTNT, Kaplan-Meier curves and median time-to-event with 95% confidence intervals (CI) were used. For outcomes specific to single versus >1 ESR1m, only pts with an ESR1m detected within the 6 months prior to elacestrant start were included. Results: 418 aBC pts were treated with elacestrant, with a median age of 62 years. Elacestrant was used across metastatic LOT (2 nd : 12%, 3 rd : 18%, 4 tH : 17%, 5 th +: 53%). Considering all prior therapies, pts commonly received prior aromatase inhibitors (AI) (89%), CDK4/6i (73%), fulvestrant (63%), and chemotherapy (49%). Of 312 pts with testing within 6 months of elacestrant start, 56% harbored a single ESR1m, with a maximum of 11 ESR1m detected in a single patient. TTD was 5.4 mos (95% CI: 4.2 - not reached NR) and TTNT was 6.2 mos (95% CI: 5.7 - NR); most pts were censored at the time of analysis (65% for TTD, 71% for TTNT). There were no significant differences in TTD or TTNT based on number of prior LOT or number of ESR1m mutations (Table 1). Data was not yet mature to assess differences in outcomes based on prior ET use, duration of prior CDK4/6i exposure, specific ESR1m, or co-occurring alterations. Conclusions: Using a clinical-genomic database, we present early RW insights of aBC pts treated with elacestrant. Initial outcomes, as assessed via TTD and TTNT, exceed the PFS reported in the EMERALD trial, perhaps due to multiple factors impacting patient selection and restaging protocols, which can differ in prospective trials versus routine clinical care. Factors such as prior LOT and number of ESR1m did not impact patient outcomes; additional factors such as prior ET use, duration of CDK4/6i treatment, specific ESR1m, and co-occurring mutations will be assessed as data matures. Table: see text