The FDA approved Inluriyo (imlunestrant) in combination with Verzenio (abemaciclib) on September 18, 2026, 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 approval adds a new all-oral combination for a molecularly defined group of patients whose tumors have evolved resistance to earlier endocrine treatment.
The new approval expands Inluriyo from monotherapy to combination treatment
Inluriyo was first approved in September 2025 as a single-agent estrogen receptor antagonist for adults with ER-positive, HER2-negative, ESR1-mutated advanced or metastatic breast cancer after at least one line of endocrine therapy. The September 2026 action expands its use by adding abemaciclib, a CDK4/6 inhibitor, for the same biomarker-defined disease setting.
The approved regimen is imlunestrant 400 mg by mouth once daily on an empty stomach plus abemaciclib 150 mg by mouth twice daily, continued until disease progression or unacceptable toxicity.
Both drugs are manufactured by Eli Lilly. FDA also approved Guardant360 CDx as the companion diagnostic for identifying the ESR1 mutations used to select patients for the combination.
EMBER-3 provided the efficacy evidence
The approval is based on EMBER-3, a randomized, open-label, active-controlled phase 3 trial that enrolled 874 adults with ER-positive, HER2-negative locally advanced or metastatic breast cancer previously treated with an aromatase inhibitor, either alone or with a CDK4/6 inhibitor.
Participants were randomized 1:1:1 to imlunestrant, investigator’s choice of fulvestrant or exemestane, or imlunestrant plus abemaciclib. ESR1 mutation status was determined from circulating tumor DNA in blood using the Guardant360 CDx assay.
The original New England Journal of Medicine publication showed that adding abemaciclib to imlunestrant significantly improved progression-free survival across the concurrently randomized population. The investigators wrote that the combination “significantly improved progression-free survival” compared with imlunestrant alone.
FDA focused the approval on the ESR1-mutated subgroup
For the 2026 combination approval, FDA highlighted an exploratory subgroup of 159 patients whose tumors carried qualifying ESR1 mutations. In that subgroup, median progression-free survival was 11.1 months with imlunestrant plus abemaciclib compared with 5.5 months with imlunestrant alone.
The hazard ratio for progression or death was 0.53, with a 95% confidence interval of 0.35 to 0.80. Objective response rate was 35% with the combination versus 15% with imlunestrant alone.
Overall survival data were still immature at the interim analysis, with only 35% of expected deaths observed in the ESR1-mutated subgroup. That means the current evidence supports a progression-free survival benefit, but it is too early to conclude that the combination extends overall survival.
Updated EMBER-3 data support a durable progression-free survival signal
A 2026 update from EMBER-3 reported a median follow-up of 28.5 months. Across all patients in the combination comparison, median progression-free survival was 10.9 months with imlunestrant plus abemaciclib versus 5.5 months with imlunestrant alone, with a hazard ratio of 0.59.
In the original trial, the benefit of adding abemaciclib was observed regardless of ESR1 mutation status. The FDA indication, however, is specifically restricted to ESR1-mutated disease. That distinction matters because a trial can show efficacy in a broader population while the approved label targets a narrower group based on the total regulatory evidence.
Why ESR1 mutations matter in endocrine-resistant breast cancer
ESR1 encodes estrogen receptor alpha, the protein that drives growth in many ER-positive breast cancers. Mutations in the receptor’s ligand-binding domain can allow estrogen receptor signaling to remain active even when estrogen production is suppressed with an aromatase inhibitor.
These mutations are usually acquired during treatment rather than present at diagnosis. They are particularly relevant after exposure to aromatase inhibitors and are increasingly detected through circulating tumor DNA testing.
Our ESR1 mutations explainer covers how these mutations emerge and why they make aromatase inhibitors less effective. The approval also fits into a rapidly changing treatment landscape that includes the recent camizestrant approval for ESR1-mutated disease.
Imlunestrant and abemaciclib attack two different parts of the same growth program
Imlunestrant is an oral selective estrogen receptor degrader, or SERD. It binds the estrogen receptor and promotes receptor degradation, reducing estrogen-driven transcription even when ESR1 mutations alter the receptor.
Abemaciclib inhibits CDK4 and CDK6, kinases that help move cells from the G1 phase of the cell cycle into DNA synthesis. In ER-positive breast cancer, estrogen receptor signaling and the cyclin D-CDK4/6 pathway are closely linked, which is why combining endocrine therapy with CDK4/6 inhibition has become a central treatment strategy.
Our CDK4/6 inhibitor explainer reviews how abemaciclib and related drugs slow cell-cycle progression, while our ctDNA explainer explains how blood-based testing can identify emerging resistance mutations such as ESR1.
The companion diagnostic is part of the approval, not an optional add-on
FDA approved Guardant360 CDx as the companion diagnostic for this use. The assay analyzes circulating tumor DNA from blood and detects specific ESR1 mutations in the ligand-binding domain.
That means the treatment is not simply approved for any ER-positive, HER2-negative advanced breast cancer. Eligibility depends on demonstrating a qualifying ESR1 mutation with an FDA-authorized test.
Safety reflects both drugs in the combination
The imlunestrant prescribing information includes a warning for embryo-fetal toxicity. Abemaciclib carries additional warnings and precautions for diarrhea, neutropenia, interstitial lung disease or pneumonitis, hepatotoxicity, venous thromboembolism, and embryo-fetal toxicity.
In the original EMBER-3 publication, grade 3 or higher adverse events occurred in 48.6% of patients receiving imlunestrant plus abemaciclib, compared with 17.1% receiving imlunestrant alone. That difference reflects the added toxicity expected when a CDK4/6 inhibitor is layered onto endocrine therapy.
What the evidence cannot yet answer
The ESR1-mutated combination results cited by FDA come from an exploratory subgroup rather than a separately powered randomized trial confined to ESR1-mutated disease. The subgroup included 159 patients, which is substantially smaller than the full 874-patient EMBER-3 population.
The trial was open-label and funded by Eli Lilly, and several authors were Lilly employees. The randomized design and prespecified endpoints strengthen the evidence, but the subgroup analysis and immature overall survival data mean the long-term survival advantage remains uncertain.
The approval also does not establish that imlunestrant plus abemaciclib is superior to every other available ESR1-directed strategy. Cross-trial comparisons with other oral SERDs or targeted combinations are not reliable substitutes for direct randomized comparisons.
The approval makes ESR1 testing even more clinically relevant
The broader significance of this approval is that ESR1 mutation status is becoming a treatment-selection variable rather than simply a marker of endocrine resistance. The combination links a blood-based companion diagnostic directly to an all-oral endocrine plus CDK4/6 strategy.
For patients whose tumors have progressed after endocrine therapy, the key question is increasingly not only whether the cancer remains ER-positive, but which resistance mechanisms have emerged. Inluriyo plus Verzenio adds another option specifically for the ESR1-mutated branch of that decision tree.
References
- U.S. Food and Drug Administration. FDA approves imlunestrant in combination with abemaciclib for ER-positive, HER2-negative, ESR1-mutated advanced or metastatic breast cancer. September 18, 2026. FDA approval notice.
- Jhaveri KL, Neven P, Casalnuovo ML, et al. Imlunestrant with or without Abemaciclib in Advanced Breast Cancer. N Engl J Med. 2025;392:1189-1202. DOI: 10.1056/NEJMoa2410858.