ER/estrogen receptor; selective estrogen receptor degrader (SERD)
Imlunestrant targets the estrogen receptor (ER), specifically estrogen receptor alpha (ERα), functioning as a selective estrogen receptor degrader (SERD) with pure antagonistic properties. It works by binding to ER and inducing a conformational change that results in ER degradation via the ubiquitin-proteasome pathway. Imlunestrant also acts as a P-glycoprotein inhibitor and breast cancer resistance protein inhibitor. It is able to cross the blood-brain barrier (BBB), making it particularly relevant for treating brain metastases. The compound targets both wild-type ERα and mutant forms such as Y537N.

In ESR1 mutants, LY3484356 exhibits favorable pharmacokinetic (PK) characteristics, including antitumor activity [1].
Estrogen receptor signalling has closely been tied to breast cancer progression and cancer cell proliferation. Estrogen receptor alpha (ERα) has been primarily implicated in breast cancer, and its activation promotes the expression of oncogenic factors that increase cancer cell proliferation, such as MYC, Cyclin D1, FOXM1, GREB1, BCL2 or amphiregulin, IGF-1 and CXCL12. Imlunestrant binds to ERα with high affinity and, in vitro, induces degradation of ERα: This leads to the inhibition of ER-dependent gene transcription and cellular proliferation in ER+ breast cancer cells. Imlunestrant is an estrogen receptor (ER) antagonist that induces degradation of ERα, leading to inhibition of ER-dependent gene transcription and cellular proliferation in ER+ breast cancer cells. Imlunestrant exposure-response relationships and the time course of pharmacodynamics have not been fully characterized.

---

In vitro, Imlunestrant demonstrates potent degradation of wild-type ERα and Y537N mutant ERα proteins in cells with IC₅₀ values of 3.0 nM and 9.6 nM, respectively. It functions as a pure antagonist of ERα, inhibiting ER-dependent gene transcription and cell proliferation in ER-positive breast cancer cell lines. The compound results in sustained inhibition of ER-dependent gene transcription and cell growth, leading to long-lasting suppression of estrogen-driven proliferation. Imlunestrant shows activity against both endocrine therapy-sensitive and -resistant ER-positive breast cancer models, including those with ESR1 mutations.

Imlunestrant demonstrated in vivo antitumour activity in ER+ breast cancer xenograft models, including models with ESR1 mutations. Imlunestrant is an orally available selective estrogen receptor degrader (SERD), with potential antineoplastic activity. Upon oral administration, imlunestrant specifically targets and binds to the estrogen receptor (ER) and induces a conformational change that results in ER degradation. This prevents ER-mediated signaling and inhibits both the growth and survival of ER-expressing cancer cells. Imlunestrant is able to cross the blood-brain barrier (BBB). IMLUNESTRANT is a small molecule drug with a maximum clinical trial phase of III (across all indications) and has 3 investigational indications.

---

In vivo, Imlunestrant has demonstrated significant antitumor activity in preclinical models of ER-positive breast cancer. The compound exhibits sustained inhibition of ER-dependent gene transcription and tumor growth in xenograft models. Its brain-penetrant properties allow for activity against intracranial tumors and brain metastases. In clinical trials, imlunestrant has been investigated as monotherapy and in combination with other drugs in breast cancer patients. The FDA approval was based on clinical data demonstrating efficacy in ER-positive, HER2-negative, ESR1-mutated advanced or metastatic breast cancer.

In vitro enzyme/receptor binding assays for Imlunestrant typically involve competitive binding experiments using ERα protein and radiolabeled estradiol or fluorescent tracers to assess binding affinity. The compound's degradation activity is evaluated in cell-based assays measuring ERα protein levels by Western blot or immunofluorescence after treatment. IC₅₀ values for ERα degradation (3.0 nM for wild-type, 9.6 nM for Y537N mutant) are determined through dose-response experiments. Assays for ER-dependent gene transcription inhibition use luciferase reporter constructs containing estrogen response elements (EREs) transfected into ER-positive cell lines. Typical incubation conditions are 37°C with 5% CO₂ for 24-72 hours.

In vitro cell-based assays for Imlunestrant utilize ER-positive breast cancer cell lines such as MCF-7 and T47D. Cells are treated with varying concentrations of the compound for 24-72 hours, followed by assessment of ERα protein degradation via Western blot or high-content imaging. Cell proliferation is measured using MTT, CCK-8, or BrdU incorporation assays. ER-dependent transcriptional activity is evaluated using reporter gene assays with ERE-luciferase constructs. The compound's pure antagonistic properties are confirmed by the absence of estrogenic activity in these assays. Standard cell culture conditions (37°C, 5% CO₂) with charcoal-stripped serum are employed to minimize background estrogenic activity.

In vivo animal studies with Imlunestrant typically use immunocompromised mice bearing ER-positive breast cancer xenografts, including models with wild-type or mutant ESR1. The compound is administered orally once daily at various dose levels. Endpoints include tumor volume measurements, assessment of ERα protein levels in tumor tissues by immunohistochemistry or Western blot, evaluation of ER-dependent gene expression by qPCR, and pharmacokinetic profiling. Brain penetration is assessed through measurement of compound concentrations in brain tissue and evaluation of activity against intracranial tumor models. All procedures comply with institutional animal care and use guidelines.

Absorption
The mean (%CV) maximum concentration (Cmax) of imrenlastane was 141 ng/mL (45%), and the area under the concentration-time curve (AUC) was 2,400 ng·h/mL (46%). Administered once daily at doses ranging from 200 mg to 1,200 mg (equivalent to 0.5 to 3 times the approved recommended dose), both Cmax and AUC of imrenlastane increased proportionally with the dose. Steady-state was reached in approximately 6 days, with a cumulative AUC of 2.3 times. The absolute oral bioavailability following a single oral dose of 400 mg imrenlastane was 10% (32%). The median time (min, maxima) to reach maximum plasma concentration (Tmax) of imrenlastane was 4 (2, 8) hours. Following co-administration with a low-fat meal (approximately 475 calories, of which 13% fat, 16% protein, and 71% carbohydrates), the AUC of imrulenstra increased 2-fold, and the Cmax increased 3.6-fold. The effect of a high-fat meal (approximately 800-1000 calories, of which 500-600 calories are from fat) on imrulenstra exposure is unclear.
Elimination Route
Following a single dose of 400 mg of radiolabeled imrulenstra in healthy subjects, 97% of the dose was excreted in feces (62% unchanged) and 0.3% in urine.
Volume of Distribution
The apparent (oral) volume of distribution was 8120 L (69%).
Clearance
The estimated apparent clearance was 166 L/h (51%).
Protein Binding
The protein binding of imrulenstra is >99% and is concentration-independent.
Metabolism/Metabolites
Immulustae is primarily metabolized via CYP3A4-mediated sulfation and direct glucuronidation catalyzed by UGT1A1, 1A3, 1A8, 1A9, and 1A10. In a drug metabolism and disposal study, the metabolite with the highest plasma radioactivity was M1. Other metabolites with relatively identifiable radioactivity include M2 and M12.
Biological Half-Life
The elimination half-life of imulustae is 30 hours.

---

Imlunestrant is orally bioavailable with favorable pharmacokinetic properties. It is soluble in DMSO at 100 mg/mL and in ethanol at 100 mg/mL, but is insoluble in water. For in vivo oral administration, formulations such as homogeneous suspension in CMC-Na at ≥5 mg/mL or clear solutions in 5% DMSO + 40% PEG300 + 5% Tween-80 + 50% ddH₂O can be used. The compound is brain-penetrant, capable of crossing the blood-brain barrier. It exhibits sustained inhibition of ER-dependent gene transcription, indicating a prolonged pharmacodynamic effect. Detailed PK parameters including half-life, Cmax, and AUC are available in preclinical and clinical study reports.

Efficacy assessment was conducted in the EMBER-3 (NCT04975308) study, a randomized, open-label, positive-controlled, multicenter trial that enrolled 874 patients with ER-positive, HER2-negative locally advanced or metastatic breast cancer who had previously received aromatase inhibitor monotherapy or combination therapy with CDK4/6 inhibitors. Patients eligible for PARP inhibitor therapy were excluded. Patients were randomized 1:1:1 to the imaxisome group, the investigator-selected endocrine therapy group (fulvestrant or exemestane), or an additional investigational combination therapy group. Randomization was stratified by prior CDK4/6 inhibitor therapy, presence of visceral metastases, and geographic region. ESR1 mutation status was determined by circulating tumor DNA (ctDNA) analysis using the Guardant360 CDx assay, limited to specific ESR1 mutations in the ligand-binding domain. The primary efficacy endpoint was investigator-assessed progression-free survival (PFS) (RECIST v1.1), comparing the efficacy of imirucept to investigator-selected endocrine therapy in patients with ESR1-mutant tumors. Other efficacy endpoints included overall survival (OS) and objective response rate (ORR). In the ESR1-mutant population (n=256), there was a statistically significant difference in investigator-assessed PFS between the imirucept group and the investigator-selected endocrine therapy group. The median PFS was 5.5 months in the imirucept group (95% CI: 3.9, 7.4) and 3.8 months in the investigator-selected endocrine therapy group (95% CI: 3.7, 5.5) (hazard ratio 0.62 [95% CI: 0.46, 0.82]; p = 0.0008). The objective response rate (ORR) was 14.3% in the imirucept group and 7.7% in the investigator-selected group. When performing progression-free survival (PFS) analysis, overall survival (OS) data were not yet mature, with a mortality rate of 31% in the ESR1 mutation population. The most common adverse events (≥10%) included abnormal laboratory findings, including decreased hemoglobin, musculoskeletal pain, decreased calcium, neutropenia, increased AST, fatigue, diarrhea, increased ALT, increased triglycerides, nausea, thrombocytopenia, constipation, increased cholesterol, and abdominal pain.

---

Toxicological data for Imlunestrant have been generated through preclinical safety studies and clinical trials supporting its FDA approval. As an approved therapeutic for breast cancer, the compound has undergone comprehensive toxicological evaluation including genotoxicity, reproductive toxicity, and organ-specific toxicity assessments. Common adverse effects observed in clinical trials are consistent with those expected from ER-targeting therapies. The compound is not recommended for use during pregnancy. Comprehensive safety data are available in the FDA-approved prescribing information. Patients should be monitored for potential side effects including those related to estrogen deprivation.

[1]. A first-in-human phase 1a/b trial of LY3484356, an oral selective estrogen receptor (ER) degrader (SERD) in ER+ advanced breast cancer (aBC) and endometrial endometrioid cancer (EEC): Results from the EMBER study. 2021 ASCO Annual Meeting I.

[2]. Oral Selective Estrogen Receptor Degraders (SERDs) as a Novel Breast Cancer Therapy: Present and Future from a Clinical Perspective. Int. J. Mol. Sci. 2021, 22(15), 7812.

Imlunestrant is a selective estrogen receptor antagonist and degrader that can cross the blood-brain barrier. On September 25, 2025, the U.S. Food and Drug Administration (FDA) approved Imlunestrant for the treatment of advanced or metastatic breast cancer with estrogen receptor (ER)-positive, HER2-negative, and estrogen receptor 1 (ESR1) mutations that has progressed after at least one line of endocrine therapy. Imlunestrant promotes ER degradation via the ubiquitin-proteasome pathway by antagonizing ER transcriptional activity and forming an unstable drug-receptor complex. In clinical trials, Imlunestrant has been used as monotherapy and in combination with other drugs in breast cancer patients. Imlunestrant is an estrogen receptor antagonist. Its mechanism of action includes acting as an estrogen receptor antagonist, a P-glycoprotein inhibitor, and an inhibitor of breast cancer resistance proteins. Background: Novel estrogen receptor (ER) degraders and antagonists are being evaluated in abreast cancer (aBC) to overcome ER-mediated resistance and the bioavailability and dose limitations of fulvestrant, the only currently approved selective estrogen receptor degrader (SERD). ER is overexpressed in approximately 80% of endometrial cancers (EEC), patients who are typically receiving endocrine therapy (ET). LY3484356 is a novel, orally bioavailable SERD with pure antagonistic properties that sustainably inhibits ER-dependent gene transcription and cell growth. Preclinical studies have demonstrated favorable efficacy and pharmacokinetic (PK) properties for LY3484356, including antitumor activity in ESR1 mutants. This article presents preliminary clinical data from the ongoing first-in-human Phase 1a/b clinical trial EMBER of this novel drug. Methods: This phase 1a clinical trial evaluated the dose-escalation (i3+3 design) of LY3484356 in women with ER+, HER2- advanced breast cancer (≤3 prior lines of advanced breast cancer therapy after regimen revision; prior sensitivity to endocrine therapy) and ER+ endometrial cancer (prior platinum-based therapy). Premenopausal women were concurrently receiving GnRH agonist therapy. The primary endpoints included determining the recommended dose in the phase 2 clinical trial, safety and tolerability, pharmacokinetics, and objective response rate and clinical benefit rate as assessed according to RECIST v1.1 criteria. Results: As of the data cutoff date (November 9, 2020), 28 patients were enrolled (n = 24 with advanced breast cancer, n = 4 with endometrial cancer), with doses ranging from 200 to 1200 mg once daily. The median age was 59 years (range 35–80 years). The median number of patients with prior treatment was 2 (range 1–8; 6 patients enrolled before the protocol revision had received ≥4 prior treatments), including those previously treated with fulvestrant (46%), CDK4/6 inhibitors (83%), and chemotherapy (33%). No dose-limiting toxicities were observed. Treatment-related adverse events (TEAEs) were predominantly grade 1–2, including nausea (32%), fatigue (25%), and diarrhea (18%). The only grade 3 treatment-related adverse event was diarrhea (n = 1). Despite intensive central ECG monitoring, no TEAEs such as bradycardia or QTc interval prolongation were observed. At all assessed doses, exposure to LY3484356 increased proportionally to the dose, with a half-life of 25–30 hours. At the starting dose level (200 mg, once daily), exposure to free LY3484356 exceeded that to fulvestrant. Of the 28 patients, 16 were evaluable for efficacy, while the remaining 12 were still on treatment prior to the initial scan. Of the 16 evaluable patients, 11 (8 aBC, 3 EEC) had stable disease (10 still on treatment), and 5 experienced disease progression. The RECIST efficacy assessment, conducted after the data cutoff, will be detailed at the conference. Plasma ctDNA analysis showed a reduced frequency of mutant alleles, including detected mutant ESR1 in 9 (75%) of the 12 evaluable patients at all dose levels. Conclusion: The once-daily dosing regimen of LY3484356 demonstrated favorable safety and pharmacokinetic properties, as well as preliminary efficacy in patients with ER+ aBC and EEC who had previously received extensive therapy. Updated data will be presented at the conference. Clinical trial information: NCT04188548.

---

Imlunestrant (LY-3484356) (CAS#: 2408840-26-4) has a molecular formula of C₂₉H₂₄F₄N₂O₃ and a molecular weight of 524.5 g/mol. Its IUPAC name is (5R)-5-[4-[2-[3-(fluoromethyl)azetidin-1-yl]ethoxy]phenyl]-8-(trifluoromethyl)-5H-chromeno[4,3-c]quinolin-2-ol. The FDA approved imlunestrant on September 25, 2025. The mechanism of action involves antagonizing ER transcriptional activity and promoting ER degradation via the ubiquitin-proteasome pathway. Clinical trials have investigated imlunestrant as monotherapy and in combination with other agents. It is indicated for ER-positive, HER2-negative, ESR1-mutated advanced or metastatic breast cancer.

C29H24F4N2O3

524.506081581116

524.172

C, 66.41; H, 4.61; F, 14.49; N, 5.34; O, 9.15

2408840-26-4

Imlunestrant tosylate;2408840-41-3;(S)-Imlunestrant tosylate;2408840-43-5

146603228

Light yellow to yellow solid powder

6

1

9

6

38

788

1

FCC1CN(CCOC2C=CC(=CC=2)[C@@H]2C3C4C=CC(=CC=4N=CC=3C3C=CC(C(F)(F)F)=CC=3O2)O)C1

UVBQMXOKKDCBJN-MUUNZHRXSA-N

InChI=1S/C29H24F4N2O3/c30-13-17-15-35(16-17)9-10-37-21-5-1-18(2-6-21)28-27-23-8-4-20(36)12-25(23)34-14-24(27)22-7-3-19(29(31,32)33)11-26(22)38-28/h1-8,11-12,14,17,28,36H,9-10,13,15-16H2/t28-/m1/s1

(5R)-5-[4-[2-[3-(fluoromethyl)azetidin-1-yl]ethoxy]phenyl]-8-(trifluoromethyl)-5H-chromeno[4,3-c]quinolin-2-ol

Imlunestrant; 2408840-26-4; LY3484356; 9CXQ3PF69U; LY-3484356; .

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Room temperature (This product is stable at ambient temperature for a few days during ordinary shipping and time spent in Customs)

DMSO: 250 mg/mL (476.64 mM)

Note: Listed below are some common formulations that may be used to formulate products with low water solubility (e.g. < 1 mg/mL), you may test these formulations using a minute amount of products to avoid loss of samples.

---

Injection Formulation 1: DMSO : Tween 80： Saline = 10 : 5 : 85 (i.e. 100 μL DMSO stock solution → 50 μL Tween 80 → 850 μL Saline)
*Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH ₂ O to obtain a clear solution.
Injection Formulation 2: DMSO : PEG300 ：Tween 80 : Saline = 10 : 40 : 5 : 45 (i.e. 100 μL DMSO → 400 μLPEG300 → 50 μL Tween 80 → 450 μL Saline)
Injection Formulation 3: DMSO : Corn oil = 10 : 90 (i.e. 100 μL DMSO → 900 μL Corn oil)
Example: Take the Injection Formulation 3 (DMSO : Corn oil = 10 : 90) as an example, if 1 mL of 2.5 mg/mL working solution is to be prepared, you can take 100 μL 25 mg/mL DMSO stock solution and add to 900 μL corn oil, mix well to obtain a clear or suspension solution (2.5 mg/mL, ready for use in animals).

View More

Injection Formulation 4: DMSO : 20% SBE-β-CD in saline = 10 : 90 [i.e. 100 μL DMSO → 900 μL (20% SBE-β-CD in saline)]
*Preparation of 20% SBE-β-CD in Saline (4°C，1 week): Dissolve 2 g SBE-β-CD in 10 mL saline to obtain a clear solution.
Injection Formulation 5: 2-Hydroxypropyl-β-cyclodextrin : Saline = 50 : 50 (i.e. 500 μL 2-Hydroxypropyl-β-cyclodextrin → 500 μL Saline)
Injection Formulation 6: DMSO : PEG300 : castor oil : Saline = 5 : 10 : 20 : 65 (i.e. 50 μL DMSO → 100 μLPEG300 → 200 μL castor oil → 650 μL Saline)
Injection Formulation 7: Ethanol : Cremophor : Saline = 10： 10 : 80 (i.e. 100 μL Ethanol → 100 μL Cremophor → 800 μL Saline)
Injection Formulation 8: Dissolve in Cremophor/Ethanol (50 : 50), then diluted by Saline
Injection Formulation 9: EtOH : Corn oil = 10 : 90 (i.e. 100 μL EtOH → 900 μL Corn oil)
Injection Formulation 10: EtOH : PEG300：Tween 80 : Saline = 10 : 40 : 5 : 45 (i.e. 100 μL EtOH → 400 μLPEG300 → 50 μL Tween 80 → 450 μL Saline)

---

Oral Formulation 1: Suspend in 0.5% CMC Na (carboxymethylcellulose sodium)
Oral Formulation 2: Suspend in 0.5% Carboxymethyl cellulose
Example: Take the Oral Formulation 1 (Suspend in 0.5% CMC Na) as an example, if 100 mL of 2.5 mg/mL working solution is to be prepared, you can first prepare 0.5% CMC Na solution by measuring 0.5 g CMC Na and dissolve it in 100 mL ddH2O to obtain a clear solution; then add 250 mg of the product to 100 mL 0.5% CMC Na solution, to make the suspension solution (2.5 mg/mL, ready for use in animals).

View More

Oral Formulation 3: Dissolved in PEG400
Oral Formulation 4: Suspend in 0.2% Carboxymethyl cellulose
Oral Formulation 5: Dissolve in 0.25% Tween 80 and 0.5% Carboxymethyl cellulose
Oral Formulation 6: Mixing with food powders

---

Note: Please be aware that the above formulations are for reference only. InvivoChem strongly recommends customers to read literature methods/protocols carefully before determining which formulation you should use for in vivo studies, as different compounds have different solubility properties and have to be formulated differently.

---

 (Please use freshly prepared in vivo formulations for optimal results.)