SUMMARY: The FDA on September 25, 2025, approved Imlunestrant (INLURIYO®), an Estrogen Receptor antagonist, for adults with Estrogen Receptor (ER)-positive, Human Epidermal growth factor Receptor 2 (HER2)-negative, ESR1-mutated advanced or metastatic breast cancer with disease progression following at least one line of endocrine therapy. FDA also approved the Guardant360 CDx assay as a companion diagnostic device to identify patients with breast cancer with ESR1 mutations for treatment with Imlunestrant.
Breast cancer is the most common cancer among women in the US and about 1 in 8 women (12%) will develop invasive breast cancer during their lifetime. It is estimated that in the US, approximately 316,950 new cases of female breast cancer will be diagnosed in 2025, and about 42,170 women will die of the disease, largely due to metastatic recurrence.
Approximately 70% of breast tumors express Estrogen Receptors and/or Progesterone Receptors. The most common subtype of metastatic breast cancer is Hormone Receptor-positive (HR-positive), HER2-negative breast cancer (65% of all metastatic breast tumors), and these patients are often treated with anti-estrogen therapy as first line treatment. However, resistance to hormonal therapy occurs in a majority of the patients, with a median Overall Survival (OS) of 36 months. With the development of Cyclin Dependent Kinases (CDK) 4/6 inhibitors, endocrine therapy plus a CDK4/6 inhibitor is the mainstay, for the management of ER+/HER2-negative metastatic breast cancer, as first line therapy. Even with this therapeutic combination, most patients will eventually experience disease progression, with up to 50% of patients acquiring ESR1 (Estrogen Receptor gene alpha) mutations after exposure to prior endocrine therapy in combination with CDK4/6 inhibitors. These mutations enable constitutive activation of the estrogen receptor, rendering tumors less responsive to traditional endocrine agents. Although Selective Estrogen Receptor Degraders (SERDs) such as Fulvestrant are often used in this setting, their clinical activity is modest and limited by pharmacokinetic and mechanistic constraints, especially in heavily pretreated, endocrine-resistant disease.
Imlunestrant: A Next-Generation ER Antagonist
Imlunestrant is an oral selective estrogen receptor antagonist and degrader designed to provide continuous ER inhibition, including in ESR1-mutated cancers. By binding, blocking, and promoting degradation of the receptor, Imlunestrant aims to suppress ER-driven tumor growth beyond the limits of standard endocrine therapy. Further, Imlunestrant crosses the blood-brain barrier.
The EMBER-3 Trial: Pivotal Data Supporting Approval
The efficacy and safety of Imlunestrant were evaluated in the Phase 3 EMBER-3 trial (NCT04975308), an open-label randomized study that enrolled 874 patients with ER-positive, HER2-negative locally advanced or metastatic breast cancer. All participants had received prior treatment with an aromatase inhibitor, either as monotherapy or in combination with a CDK4/6 inhibitor, but were ineligible for PARP inhibitor therapy.
Patients were randomized (1:1:1) to one of three arms:
- Arm A: Imlunestrant monotherapy 400 mg orally once daily (N=331)
- Arm B: Investigators choice of Fulvestrant or Exemestane (N=330)
- Arm C: Imlunestrant plus Abemaciclib (N=213, investigational)
Randomization was stratified by prior CDK4/6 inhibitor exposure, visceral disease status, and geographic region. ESR1 mutation status was determined via ctDNA analysis using the Guardant360 CDx assay, restricted to defined ligand-binding domain mutations.
The FDA approval was specifically based on results in the ESR1-mutated cohort (N=256). In this subgroup, 21% received therapy as first-line treatment for metastatic breast cancer (following recurrence on adjuvant Aromatase Inhibitor-AI) and 79% as second-line treatment (post-progression on AI, with or without prior CDK4/6 inhibitor).
Efficacy Outcomes
- Primary endpoint (PFS): Median Progression-Free Survival was 5.5 months with Imlunestrant vs. 3.8 months with standard endocrine therapy (HR 0.62; 95% CI: 0.46–0.82; P=0.0008).
- Objective Response Rate (ORR): 14.3% with Imlunestrant vs. 7.7% with investigator’s choice.
- Overall Survival (OS): Data remain immature, with 31% of deaths reported at the time of analysis.
These findings demonstrate a statistically and clinically meaningful improvement in PFS for patients with ESR1-mutant disease, a group with limited therapeutic options following resistance to aromatase inhibitors.
Safety Profile
The safety profile of Imlunestrant was consistent with ER-targeting strategies. Common adverse events (≥10%) included hematologic abnormalities (decreased hemoglobin, neutrophils, platelets), musculoskeletal pain, fatigue, gastrointestinal effects (diarrhea, nausea, constipation, abdominal pain), and laboratory changes such as elevated liver enzymes, triglycerides, or cholesterol.
Looking Ahead: Ongoing EMBER Program
Beyond metastatic disease, Imlunestrant is being studied in earlier disease settings. The EMBER-4 trial is enrolling about 8,000 patients worldwide to evaluate Imlunestrant in the adjuvant treatment of ER-positive, HER2-negative early breast cancer, at elevated risk of recurrence. Combination strategies, including Imlunestrant plus Abemaciclib, are also under active investigation to further enhance ER pathway blockade.
Clinical Perspective
The approval of Imlunestran marks an important advance in precision endocrine therapy, particularly for patients with ESR1-mutated metastatic breast cancer, a population historically limited to suboptimal options after progression on aromatase inhibitors. By offering a targeted, oral agent with meaningful PFS benefit, Imlunestran provides oncologists with a new tool to extend disease control in a challenging clinical context.
Imlunestrant with or without Abemaciclib in Advanced Breast Cancer. Jhaveri KL, Neven P, Casalnuovo ML, et al. for the EMBER-3 Study Group. N Engl J Med 2025;392:1189-1202
