Elacestrant in Women with Estrogen Receptor-Positive and HER2-Negative Early Breast Cancer: Results from the Preoperative Window-of-Opportunity ELIPSE Trial.

Purpose: Elacestrant has shown significantly prolonged progression-free survival compared with standard-of-care endocrine therapy in estrogen receptor-positive (ER-positive), HER2-negative metastatic breast cancer, whereas 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 breast cancer.

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

Results: Overall, 22 patients were evaluable for the primary endpoint. Elacestrant was associated with a complete cell cycle arrest rate of 27.3% and a statistically significant Ki67 geometric mean change of -52.9% (P = 0.007; 95% confidence interval, -67.4 to -32.1). Notably, 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, and CD8A) and suppressed proliferation and estrogen-signaling genes (MKI67, ESR1, and AR). These biological changes were independent of the levels of Ki67 suppression on day 28. The most common adverse events were grade 1 anemia (21.7%), hot flushes (8.7%), constipation (8.7%), and abdominal pain (8.7%). 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 breast cancer demonstrated relevant biological and molecular responses and exhibited a manageable safety profile. These findings support further investigation of elacestrant in the early setting.