Imlunestrant Is an Oral, Brain-Penetrant Selective Estrogen Receptor Degrader with Potent Antitumor Activity in ESR1 Wild-Type and Mutant Breast Cancer

Abstract

Targeting of the estrogen receptor (ER) by antiestrogens is the standard of care for patients with ER+ HER2- advanced/metastatic breast cancer. Although antiestrogens that degrade ERα (fulvestrant) or block estrogen production (aromatase inhibitors) have improved patient outcomes, clinically important challenges remain related to drug administration, limited bioavailability, lack of brain exposure, and acquired resistance due to ESR1 mutations. These limitations indicate a need for more robust ER-targeted therapies. Here, we discovered and characterized imlunestrant, a next-generation potent, brain-penetrant oral selective ER degrader. Imlunestrant degraded ERα and decreased ERα-mediated gene expression both in vitro and in vivo. Cell proliferation and tumor growth in ESR1 wild-type (WT) and mutant models were significantly inhibited by imlunestrant. Combining imlunestrant with abemaciclib (CDK4/6 inhibitor), alpelisib (PI3K inhibitor), or everolimus (mTOR inhibitor) further enhanced tumor growth inhibition, regardless of ESR1 mutational status. In an ER+ breast cancer intracranial tumor model, imlunestrant prolonged survival compared with vehicle or alternative selective ER degrader therapies. Together, these findings support the potential of imlunestrant to degrade ERα and suppress the growth of ESR1-WT and mutant breast cancer, including brain metastatic tumors. Significance: Imlunestrant, a next-generation, brain-penetrant oral ERα degrader, displays potent activity in ESR1 wild-type and mutant breast cancer, enhances combination activity with standard-of-care agents, and inhibits growth of ER+ intracranial tumors.

Figure 1.

Biochemical and in vitro characterization of imlunestrant. A, Structure of LY3484356. B, Imlunestrant binding kinetics for WT and mutant ERα and WT ERβ. C, ERα degradation assay (24 hours). D, Western blot showing imlunestrant-mediated ERα degradation in ESR1-WT cell lines and a PDX-derived ESR1-mutant cell line (ST941/C). E, MCF7 or ST941/C cells were treated for 24 hours with indicated concentrations of imlunestrant with or without 1 nmol/L E2, and PR transcript levels were measured. F, MCF7 cells were treated with increasing concentrations of imlunestrant, elacestrant, or fulvestrant for 24 hours and imaged to assess PR protein levels. G, MCF7 cells were treated with increasing concentrations of imlunestrant, elacestrant, or fulvestrant for 7 days, and cell proliferation assay was performed. H, A panel of breast cancer cells was treated with increasing concentrations of imlunestrant, and relative IC₅₀ values were determined. ER and HER2 status are indicated in the figure.

Figure 2.

Imlunestrant alone and in combination with various therapeutic partners is effective in vitro. A-C, Breast cancer cell line panels were treated with imlunestrant in combination with abemaciclib (A), alpelisib (B), and everolimus (C) and subjected to cell proliferation combination analysis (CI₅₀). D–G, MCF7 cells were treated with imlunestrant alone and in combination with abemaciclib at the indicated concentrations and subjected to antiproliferation (D), Ki67 (E), senescence (F), and apoptosis (G) assays. *, P < 0.05; ***, P < 0.001; ****, P < 0.0001 compared to the bracketed arm.

Figure 3.

Imlunestrant in vivo exposure and PK analysis. A and B, Tumor-bearing mice were treated with the indicated dose of imlunestrant, and drug plasma, tumor, and brain exposure (A), and ERα-mediated gene expression was measured at the indicated time points after last dose (B). C, Representative ERα and PR staining of MCF7 tumors at the indicated time points. D, Tumor-bearing mice were treated with the indicated dose of imlunestrant once daily for 3 days, and dose–response drug plasma, tumor, brain, duodenum, and spleen exposure were measured at 24 hours after last dose. E, Plasma and tumor concentrations of imlunestrant were measured in patients enrolled in the EMBER-2 preoperative window of opportunity study. Concentration units are indicated in the legend.

Figure 4.

Imlunestrant efficacy in ESR1-WT and mutant breast cancer xenograft and PDX models. Breast cancer cell lines (A–C) and PDX tumors (D–F) were implanted into the flank of immune-deficient mice. When tumors reached a prespecified volume, mice were randomized to each treatment group and then treated as described in Materials and Methods. Tumor volume was measured at the indicated time points. The black line represents the treatment period. Statistical analysis is represented in Supplementary Table S2. mpk, mg/kg.

Figure 5.

Imlunestrant combination efficacy with various therapeutic partners in ESR1-WT and mutant breast cancer xenograft and PDX models. Indicated cell-derived xenografts and PDX breast cancer models were implanted and then treated with imlunestrant alone and in combination with abemaciclib (A), imlunestrant and fulvestrant alone and in combination with abemaciclib (B–D), imlunestrant alone and in combination with alpelisib (E), or imlunestrant alone and in combination with everolimus (F). Tumor volume was measured at indicated time points. The black line represents the treatment period. Statistical analysis is represented in Supplementary Tables S3 and S4. mpk, mg/kg.

Figure 6.

Imlunestrant efficacy in an ER⁺ intracranial breast cancer tumor model. MCF7 cells expressing luciferase (MCF7-luc) were injected into the brains of female immune-deficient mice (n = 10) and treated with the indicated SERD. Mice were monitored and OS (A), individual animal images (collected at day 26, indicating tumor burden; B), and body weight changes (C) were assessed at indicated times. The black line represents the treatment period. D, Nontumor-bearing mice were treated with the indicated SERD orally once daily for 7 days, except for fulvestrant, which was dosed once by s.c. injection. Brains were harvested on day 8, and drug exposure levels were determined. ***, P < 0.001; ****, P < 0.0001 compared with imlunestrant arm.