Imlunestrant combination approved for ESR1-mutated breast cancer

The FDA has approved Eli Lilly's imlunestrant in combination with abemaciclib for adults with ER-positive, HER2-negative, ESR1-mutated advanced or metastatic breast cancer following disease progression on endocrine therapy. The approval, supported by EMBER-3 trial data showing a median progression-free survival of 11.1 months versus 5.5 months with imlunestrant alone, comes shortly after the FDA's accelerated approval of AstraZeneca's rival camizestrant regimen.

Imlunestrant-abemaciclib FDA approval

• ▪The FDA approved the selective estrogen receptor degrader imlunestrant in combination with abemaciclib for adults with ER-positive, HER2-negative, ESR1-mutated advanced or metastatic breast cancer.
• ▪Eli Lilly stated that switching endocrine therapy and CDK4/6 inhibitors at clinical progression improves patient outcomes more than switching therapy earlier.
• ▪The recommended dosage for imlunestrant is 400 mg orally once daily, and the recommended dosage for abemaciclib is 150 mg orally twice daily.
• ▪The FDA-approved imlunestrant and abemaciclib combination is indicated for patients whose breast cancer has progressed following at least one line of endocrine therapy.

ESR1 mutation detection methods

• ▪The FDA approved the Guardant360 Liquid CDx circulating tumor DNA assay to detect ESR1 mutations in breast cancer patients.
• ▪Approximately 50% of breast cancer patients develop ESR1 mutations during upfront aromatase inhibitor therapy, which causes resistance to the aromatase inhibitor.

Camizestrant approval comparison

• ▪The camizestrant regimen is initiated when the ESR1 mutation is detected while patients are still on an aromatase inhibitor, whereas the imlunestrant regimen is used after disease progression.
• ▪Two weeks prior to the imlunestrant approval, the FDA granted accelerated approval to camizestrant in combination with abemaciclib, palbociclib, or ribociclib for ESR1-mutated breast cancer.
• ▪The FDA's Oncologic Drugs Advisory Committee voted 6-3 against the approval of camizestrant in April 2026 due to a lack of evidence that clinical outcomes improve before disease progression.

EMBER-3 trial efficacy data

• ▪In the EMBER-3 trial subgroup, patients receiving the imlunestrant and abemaciclib combination achieved a median progression-free survival of 11.1 months compared to 5.5 months for those receiving imlunestrant alone.
• ▪The FDA approval of the imlunestrant and abemaciclib combination was based on a subgroup analysis of 159 patients with ESR1-mutated disease in the EMBER-3 trial.

Combination therapy safety profile

• ▪In the EMBER-3 trial, Grade 3 or 4 toxicities for the imlunestrant and abemaciclib combination included neutropenia (21%), diarrhea (9%), alanine aminotransferase elevation (5%), and aspartate aminotransferase elevation (2.5%).
• ▪In the EMBER-3 trial, approximately 3% of patients developed interstitial lung disease or pneumonitis, and just under 5% experienced venous thromboembolic events when treated with the imlunestrant and abemaciclib combination.

Debatable claims

• ▪The FDA should approve cancer treatment changes based on ctDNA testing
• ▪Clinicians should switch ESR1-mutated breast cancer therapies before clinical disease progression