Elacestrant in women with estrogen receptor-positive and HER2-negative early breast cancer: results from the preoperative window-of-opportunity ELIPSE trial.

Introduction: Elacestrant has shown significantly prolonged progression-free survival compared to standard-of-care endocrine therapy in estrogen receptor-positive (ER-positive), HER2-negative metastatic breast cancer (BC), while potential benefit in early-stage disease requires further exploration. The SOLTI-ELIPSE window-of-opportunity trial investigated the biological changes induced by a short course of preoperative elacestrant in postmenopausal women with early BC.

Methods: Eligible patients with untreated T1c (≥1.5 cm)-T3, N0, ER-positive/HER2-negative BC with locally assessed Ki67≥10% received elacestrant at a daily dose of 345mg for 4 weeks. The primary efficacy endpoint was complete cell cycle arrest (CCCA), defined as Ki67≤2.7%, at day 28.

Results: Overall, 22 patients were evaluable for the primary endpoint. Elacestrant was associated with a CCCA rate of 27.3% and a statistically significant geometric mean change of -52.9% (p=0.007; 95%CI, -67.4 to -32.1). The treatment with elacestrant led to a shift toward a more endocrine-sensitive and less proliferative tumor phenotype based on PAM50-based gene signatures. Elacestrant increased the expression of immune-response genes (GZMB, CD4, CD8A) and suppressed proliferation and estrogen-signaling (MKI67, ESR1, and AR). These biological changes were independent of the levels of Ki67 suppression at day 28. Most common adverse events were grade 1: anemia (21.7%), hot flushes, constipation and abdominal pain (8.7%, each). One patient experienced a grade 3 cutaneous rash leading to treatment discontinuation. No other serious adverse events were reported.

Conclusions: Preoperative treatment with elacestrant in early BC demonstrated relevant biological and molecular responses and exhibited a manageable safety profile. These findings support further investigation of elacestrant in the early setting.