Research Summary:
Estrogen receptor–positive (ER+) breast cancer is primarily driven by signaling through estrogen receptor α, a nuclear hormone receptor that regulates transcriptional pathways promoting tumor growth and survival. Endocrine therapies targeting ER, including selective estrogen receptor modulators (SERMs), aromatase inhibitors, and selective estrogen receptor degraders (SERDs), are central to the management of hormone receptor–positive (HR+) breast cancer. However, the development of resistance to endocrine therapy remains a significant clinical challenge. Mechanisms of resistance frequently involve ligand-independent ER activation or mutations within the ligand-binding domain of ESR1, such as Y537S, which confer constitutive receptor activity and reduce the efficacy of existing ER antagonists. Fulvestrant, the only SERD currently approved for advanced ER+ breast cancer, is limited by poor bioavailability and incomplete ER antagonism, highlighting the need for next-generation oral SERDs with improved pharmacokinetics and potency against mutant ER. Imlunestrant is a novel SERD designed to overcome ESR1-mediated resistance and achieve complete ER antagonism. Investigators sought to assess preclinical imlunestrant activity in ER+ breast cancer cell lines engineered with the Y537S mutation, alongside wild type (WT)-ER comparators and a triple-negative model.
Imlunestrant suppressed growth of ER+ cells in a dose-dependent manner and showed lower half maximal inhibitory concentration (IC50) values than fulvestrant, while showing no antiproliferative effect in triple-negative cells. In ERE-luciferase assays, imlunestrant exhibited pure antagonism without detectable agonist activity up to a concentration of 1,000nM and up to treatment for 24 hours. Immunoblotting demonstrated that ER degradation began at six hours and was sustained through 72 hours. Degradation of mutant Y537S ER was superior with imlunestrant compared with fulvestrant, and longer-term colony assays confirmed that higher imlunestrant concentrations overcame diminished antagonism in Y537S-expressing cells. Ribonucleic acid (RNA) sequencing showed that, under hormone-deprived conditions, imlunestrant and fulvestrant were transcriptionally inert in WT-ER cells, consistent with ER inactivity. In contrast, both compounds downregulated estrogen response and cell-cycle pathways in Y537S-expressing cells, with broadly comparable pathway signatures over 6 to 12 hours. These transcriptomic results confirmed that imlunestrant retained strict estrogen pathway specificity while exerting more complete suppression of ER-dependent proliferation pathways.
In the ESR1 Y537S patient-derived xenograft (PDX) model, fulvestrant-resistant tumors regressed rapidly with imlunestrant monotherapy by Day 10, with regression sustained to Day 28. Treatment was well tolerated without significant weight changes. At Day 10, ER, progesterone receptor (PR), and Ki-67 were significantly lower with imlunestrant versus vehicle and fulvestrant, and at Day 28, Ki-67 remained significantly reduced with imlunestrant. Immunofluorescence and multivariate proliferation index (MPI) revealed that imlunestrant significantly decreased the proliferating fraction and increased the nonproliferative fraction at both time points, without increasing the arrested fraction. Together, these in vivo data demonstrated that imlunestrant achieved deeper and more durable suppression of tumor proliferation compared with fulvestrant in an ESR1-mutant setting.
A CRISPR/Cas9-knockout screen was conducted to test on-target effects of imlunestrant and identify acquired vulnerabilities in the presence of imlunestrant that could act as therapeutic targets. After 31 days, ESR1 was not essential under imlunestrant (β-score: −0.88) but remained essential under vehicle (β-score: −2.05). Vulnerabilities in HR+ breast cancer (eg, CCND1, CDK4, CDK2, PIK3CA, MTOR1, AKTI1) persisted under imlunestrant, and CDK7, a key regulator of the cell cycle and transcription, remained essential.
Across complementary assays, imlunestrant demonstrated pure antagonism, potent ER degradation, and selective suppression of ER-driven pathways in WT and Y537S settings. The PDX experiment provided functional confirmation that improved ER targeting translated to deeper cell-cycle arrest and tumor regression in an ESR1 Y537S context where fulvestrant activity was limited, aligning with mechanistic data indicating stronger downregulation of proliferation pathways. The CRISPR screen indicated that ER signaling became nonessential, supporting combinations already considered in ER+ disease. In preclinical ER+ breast cancer models, including Y537S, imlunestrant acted as a selective ER antagonist without agonist activity, degraded WT and mutant ER, suppressed ER transcriptional pathways, and produced tumor regression in vivo. Analyses identified persistent pathway dependencies suitable for combination strategies (CDK and PI3K/AKT/mTOR).
Collectively, these data supported imlunestrant as a next-generation oral SERD capable of overcoming ESR1 Y537S–mediated resistance through on-target ER antagonism and degradation, with mechanistic and functional evidence across molecular, cellular, and PDX models.
Reference:
- Sherman S, Sandusky ZM, Russo D, et al. Imlunestrant a next-generation oral SERD overcomes ESR1 mutant resistance in estrogen receptor-positive breast cancer. JCI Insight. 2025;10(12):e188051.