FDA grants accelerated approval to camizestrant for ESR1-mutated advanced breast cancer

The US Food and Drug Administration has granted accelerated approval to camizestrant in combination with a CDK4/6 inhibitor for a biomarker-defined group of patients with advanced breast cancer.

The approval applies to adults with hormone receptor-positive, HER2-negative locally advanced or metastatic breast cancer when an ESR1 mutation is detected during treatment with an aromatase inhibitor and a CDK4/6 inhibitor, using an FDA-authorized test.

The important idea is not simply that another endocrine drug has been added to oncology practice. The treatment strategy changes therapy when molecular evidence of resistance appears — before conventional imaging necessarily shows progression.

What ESR1 mutations mean

Many hormone receptor-positive breast cancers depend on signalling through the estrogen receptor.

Aromatase inhibitors reduce estrogen production and can suppress that pathway effectively. But tumour cells can evolve under treatment pressure. One important resistance mechanism is mutation of ESR1, the gene that encodes estrogen receptor alpha.

Some ESR1 mutations allow the receptor to remain active even when estrogen levels are very low. That can make aromatase inhibition progressively less effective.

The mutation can be detected in circulating tumour DNA, or ctDNA — small fragments of tumour-derived DNA released into the bloodstream.

Why detecting resistance early matters

Traditional treatment changes are often made after radiological progression is visible.

The strategy underlying camizestrant is different: repeatedly test blood for emerging ESR1 mutations and change endocrine therapy once molecular resistance is detected, while continuing CDK4/6 inhibition.

That approach was tested in the SERENA-6 trial.

In the randomized study, patients whose ESR1 mutation emerged during first-line treatment but who had not yet shown radiological progression were assigned either to switch from the aromatase inhibitor to camizestrant or to continue the aromatase inhibitor. Both groups continued their existing CDK4/6 inhibitor.

What the trial found

In the published SERENA-6 analysis, median progression-free survival was 16.0 months in the camizestrant group and 9.2 months in the group that continued aromatase-inhibitor therapy.

The hazard ratio for progression or death was 0.44.

The study also reported a longer time before deterioration in patient-reported global health status and quality of life.

The trial was funded by the drug's manufacturer, which should be considered when interpreting the evidence, although the study design was randomized and the results were published in the New England Journal of Medicine.

What camizestrant does

Camizestrant is a selective estrogen-receptor degrader and complete estrogen-receptor antagonist.

Rather than simply lowering estrogen production, it binds the receptor and promotes its degradation while blocking receptor signalling.

That is particularly relevant when the tumour's resistance mechanism resides in the receptor itself.

What accelerated approval means

Accelerated approval is not the same as saying every uncertainty has been resolved.

The FDA pathway allows earlier approval in serious diseases when evidence indicates meaningful benefit based on an endpoint considered reasonably likely to predict clinical benefit. Confirmatory evidence may still be required to verify the benefit.

Patients and clinicians therefore need to distinguish the regulatory status from a claim that long-term survival benefit has already been fully established.

Why this is a broader oncology story

The approval illustrates a shift from static tumour classification to longitudinal molecular monitoring.

A cancer's biology can change during treatment. Blood-based ctDNA testing provides a way to observe some of that evolution before a tumour becomes obviously larger on imaging.

If changing treatment at the moment a resistance mutation appears improves outcomes, oncology moves closer to treating evolutionary change itself rather than waiting for its visible consequence.

What to watch next

Longer follow-up will be important for overall survival, durability of benefit and understanding how the strategy performs across different CDK4/6 inhibitors and patient subgroups.

The practical performance of repeated ctDNA testing outside clinical trials will also matter, because the treatment decision depends on detecting the mutation reliably and at the right time.

Primary sources

• US Food and Drug Administration. FDA Grants Accelerated Approval to a New Breast Cancer Treatment. 4 September 2026. https://www.fda.gov/news-events/press-announcements/fda-grants-accelerated-approval-new-breast-cancer-treatment
• Bidard F-C, Mayer EL, Park YH, et al. First-Line Camizestrant for Emerging ESR1-Mutated Advanced Breast Cancer. N Engl J Med. 2025;393:569-580. DOI: 10.1056/NEJMoa2502929.