The FDA granted full approval on September 18 to imlunestrant, sold as Inluriyo, in combination with abemaciclib, sold as Verzenio, for adults with estrogen receptor-positive, HER2-negative, ESR1-mutated advanced or metastatic breast cancer whose disease progressed after at least one line of endocrine therapy. The agency approved imlunestrant in combination with abemaciclib only for patients whose ESR1 mutation is identified by an FDA-authorized test.
The agency also approved the Guardant360 CDx assay as the companion diagnostic for selecting patients. Both drugs are made by Eli Lilly and Company.
For patients, the practical meaning is narrow and specific. This is not a new first treatment. It applies at the point where hormone-blocking therapy has stopped working and a blood test shows the tumor has acquired a particular resistance mutation. In about half of people with ER-positive, HER2-negative metastatic breast cancer, tumors develop an ESR1 mutation during or after treatment with an aromatase inhibitor.
The Trial Numbers Behind the Decision
Approval rests on EMBER-3, a randomized, open-label trial that enrolled 874 adults previously treated with an aromatase inhibitor, with or without a CDK4/6 inhibitor. Participants were assigned to imlunestrant alone, an investigator’s choice of endocrine therapy, or imlunestrant plus abemaciclib.
The figure driving the approval comes from an exploratory subgroup analysis of 159 patients whose tumors carried an ESR1 mutation. Median progression-free survival was 11.1 months for the combination against 5.5 months for imlunestrant alone, a hazard ratio of 0.53. Objective response rate was 35% versus 15%.
Two limitations belong near the top rather than buried. First, overall survival data were immature at interim analysis, with deaths recorded in 35% of the ESR1-mutated group, so the trial has not shown whether people live longer. Progression-free survival measures how long the cancer is held in check, which is not the same thing. Second, the FDA noted that imlunestrant alone did not show a progression-free survival improvement over standard endocrine therapy in either the overall population or the group without an ESR1 mutation, indicating the benefit was confined to patients with the mutation.
Dr. Komal Jhaveri of Memorial Sloan Kettering Cancer Center, principal investigator for EMBER-3, described the rationale in Lilly’s announcement of the approval. Combining therapies that work on two distinct drivers of tumor growth, the estrogen receptor and CDK4/6, is “an important strategy to help address treatment resistance,” she said. Memorial Sloan Kettering discloses that Jhaveri has financial interests related to Eli Lilly, and the trial was sponsored by the company.
A Blood Test Decides Who Qualifies
ESR1 mutation status in the trial was determined through circulating tumor DNA analysis of a blood sample rather than a tissue biopsy. That detail matters for access. A liquid biopsy is easier to repeat than a surgical sample, which is relevant because the mutation is typically acquired over time rather than present at diagnosis.
Patients whose disease is progressing on endocrine therapy may reasonably ask their oncologist whether ESR1 testing has been done, when it was last done, and whether a repeat test is warranted. That is a question about eligibility rather than a request for a specific drug, and it is the step that determines whether this option is even on the table.
Coverage remains an open question for most households. Both drugs are oral and typically fall under a plan’s pharmacy benefit, where specialty tier cost-sharing can be substantial. Patients facing denials can ask their care team about prior authorization, appeals, and manufacturer patient-assistance programs. The approval itself does not set a price or guarantee coverage.
Side Effects That Shape the Daily Reality of Treatment
The combination carries a real tolerability cost. In EMBER-3, diarrhea occurred in 86% of patients receiving the combination, with grade 3 or 4 cases in 9%. Neutrophil counts dropped in 86%, with grade 3 or 4 decreases in 21%. Interstitial lung disease or pneumonitis occurred in 2.9%, and venous thromboembolic events in 4.8%.
Serious adverse reactions occurred in 21% of patients on the combination, and fatal adverse reactions in 3.8%, including pneumonia, heart attack, interstitial lung disease, and sepsis. Most adverse events were graded mild to moderate, and permanent discontinuation rates were low, at 1% for imlunestrant and 3.4% for abemaciclib.
Monitoring is part of the regimen. Blood counts and liver function tests are checked before starting, every two weeks for the first two months, monthly for the next two months, and as clinically indicated. Both drugs carry warnings for embryo-fetal toxicity, and effective contraception is advised during treatment and for a period afterward.
Imlunestrant is taken once daily on an empty stomach and abemaciclib twice daily, until the disease progresses or side effects become unacceptable. Patients should report loose stools at the first sign, since starting antidiarrheal treatment, increasing fluids, and notifying the care team is the standard response.
This is the second FDA approval for imlunestrant in less than a year. The drug was cleared as a single agent in September 2025. It follows a separate decision two weeks earlier, when the FDA granted accelerated approval to camizestrant with a CDK4/6 inhibitor, but at a different point in care: when a blood test detects an emerging ESR1 mutation during first-line therapy, before imaging shows progression. Results from EMBER-4, a trial of more than 8,000 patients testing imlunestrant after surgery in early breast cancer, are anticipated in 2027.
Key Questions Answered
Who is this approval for? Adults with ER-positive, HER2-negative, ESR1-mutated advanced or metastatic breast cancer whose disease progressed after at least one line of endocrine therapy.
How is eligibility determined? Through an FDA-authorized test. The agency approved the Guardant360 CDx blood-based assay as the companion diagnostic.
What did the trial show? Median progression-free survival of 11.1 months with the combination versus 5.5 months with imlunestrant alone, in a subgroup of 159 patients with ESR1 mutations.
Does it help people live longer? Not established. Overall survival data were immature at interim analysis, so the trial has not answered that question.
What are the most common side effects? Diarrhea and reduced neutrophil counts, each reported in 86% of patients on the combination, along with reduced hemoglobin, nausea, fatigue, and other laboratory abnormalities.
Is this a first-line treatment? No. It is indicated after progression on at least one prior endocrine therapy.
What should patients ask their oncologist? Whether ESR1 testing has been performed, how recently, and whether repeat testing is appropriate, given that the mutation is usually acquired during treatment.
