Guardant360 CDx wins FDA nod for ESR1 breast cancer, 29th indication

Doctors can now switch breast cancer therapy on a blood test that catches drug resistance before scans show progression, after the FDA cleared Guardant360 CDx as the first liquid-biopsy companion diagnostic for longitudinal testing.

"This approval represents a landmark shift in how we define disease progression," Helmy Eltoukhy, Guardant Health chairman and co-CEO, said.

The clearance rests on SERENA-6, a Phase III trial in which 315 patients whose ESR1 mutation was caught by serial Guardant360 CDx testing every three months were switched to camizestrant plus a CDK4/6 inhibitor. Those patients posted median progression-free survival of 16 months versus 9.2 months for patients who stayed on an aromatase inhibitor, a 56 percent reduction in the risk of progression or death.

Guardant said the nod is the 29th companion diagnostic indication for Guardant360 CDx globally, backed by Medicare and commercial coverage representing more than 300 million covered lives. The label recommends repeat testing every three months during therapy, a schedule that could turn a one-off test into recurring revenue.

A blood test that catches resistance before scans do

ESR1 mutations are acquired resistance mutations that emerge as HR-positive breast cancer adapts to aromatase inhibitors, a standard first-line endocrine therapy. Fewer than 5 percent of patients carry the mutation at diagnosis of metastatic disease, but nearly 40 percent develop it after progression on an aromatase inhibitor, according to the FDA. Historically, biomarker testing happened after clinical or radiographic progression; SERENA-6 tested patients every three months while they were on an aromatase inhibitor plus a CDK4/6 inhibitor and switched eligible patients to camizestrant the moment an ESR1 mutation appeared in circulating tumor DNA.

Camizestrant, sold by AstraZeneca as Etcamah, is an oral selective estrogen receptor degrader. The FDA granted it accelerated approval in combination with a CDK4/6 inhibitor — abemaciclib, palbociclib, or ribociclib — at a dose of 75 milligrams once daily. The approval is based on progression-free survival measured from the point the mutation was detected, and the agency requires confirmatory studies to verify clinical benefit. The prescribing information carries a boxed warning for irregular heart rhythm when taken with certain other drugs, plus warnings for slow heart rate and embryo-fetal toxicity.

What the nod means for Guardant's franchise

The clearance is the first FDA approval of a cancer therapy guided by detection of a resistance mutation in circulating tumor DNA before imaging shows progression, according to the agency's Oncology Center of Excellence. For Guardant, it extends a companion diagnostic franchise that already spans non-small cell lung cancer and colorectal cancer into a longitudinal breast cancer role, where repeat testing every three months could drive sustained volume.

Guardant estimated about 37,000 U.S. women with advanced HR-positive breast cancer on an aromatase inhibitor could benefit from the approach, while AstraZeneca said roughly 30 percent of patients with endocrine-sensitive disease develop ESR1 mutations during first-line treatment before progression. Camizestrant is now approved in more than 30 countries, including the European Union, Japan, Canada, and the United Kingdom.

The circulating tumor DNA market is crowded. Guardant competes with Natera's Signatera, Exact Sciences, and Roche's Foundation Medicine for testing share, though Guardant360 CDx holds the distinction of being the first FDA-approved liquid biopsy for genomic profiling. The FDA convened its Oncologic Drugs Advisory Committee on the camizestrant application on April 30, 2026, and the accelerated approval still hinges on a confirmatory trial that will track overall survival as a key secondary endpoint.

Guardant Health trades on Nasdaq under the ticker GH; AstraZeneca lists as AZN. The approval strengthens the case for Guardant's platform as companion diagnostics shift from single-timepoint selection to serial monitoring, a model that, if it spreads to other tumor types, could expand the addressable market for liquid biopsy well beyond the roughly 37,000 patients named in this indication.

This article is for informational purposes only and does not constitute investment advice.