Guardant360 CDx Wins FDA Nod as Companion Diagnostic for Camizestrant

Guardant Health (GH ) announced on September 4, 2026 that the U.S. Food and Drug Administration has approved its Guardant360 CDx blood test as a companion diagnostic for Etcamah (camizestrant), AstraZeneca’s (AZN ) oral selective estrogen receptor degrader, in advanced breast cancer. The FDA granted accelerated approval to camizestrant in combination with a CDK4/6 inhibitor for adults with hormone receptor-positive, HER2-negative locally advanced or metastatic breast cancer whose tumors develop an ESR1 mutation during aromatase inhibitor and CDK4/6 inhibitor therapy, with the mutation detected by an FDA-authorized test.

According to the Guardant Health announcement, Guardant360 CDx is the first FDA-approved liquid biopsy companion diagnostic for longitudinal testing in breast cancer, and the approval recommends repeat testing every three months during therapy to detect emerging ESR1 mutations before clinical or radiographic disease progression. The indication covers identifying patients who may benefit from camizestrant before radiographic progression occurs.

ESR1 mutations are acquired resistance mutations that a tumor may develop during treatment with an aromatase inhibitor, a common front-line endocrine therapy for locally advanced or metastatic breast cancer. According to the FDA’s approval announcement, fewer than 5 percent of patients carry the mutation at diagnosis of HR-positive metastatic breast cancer, while nearly 40 percent carry it after disease progression on an aromatase inhibitor. The agency authorized the Guardant360 CDx assay as the companion diagnostic to identify patients with ESR1-mutated breast cancer for camizestrant treatment.

The approval rests on the pivotal SERENA-6 Phase III trial, the first global, double-blind, registrational trial to use a circulating tumor DNA-guided approach to detect emerging endocrine resistance and inform a therapy switch before disease progression. Circulating tumor DNA consists of fragments of tumor DNA released into the blood, allowing molecular detection of resistance mutations earlier than imaging.

Patients enrolled in SERENA-6 underwent serial Guardant360 CDx testing every three months while receiving an aromatase inhibitor with a CDK4/6 inhibitor. Upon detection of an ESR1 mutation before radiographic progression, eligible patients switched from the aromatase inhibitor to camizestrant while continuing the same CDK4/6 inhibitor. Patients who switched experienced a 56% reduction in the risk of disease progression or death, with median progression-free survival of 16.0 months versus 9.2 months for those continuing aromatase inhibitor therapy.

The FDA stated that the accelerated approval program allows earlier approval of drugs treating serious conditions based on surrogate or intermediate endpoints, and that camizestrant’s approval was based on how long patients lived without their disease worsening, measured from the point the resistance mutation was first detected in blood. Because it is not yet confirmed whether intervening at that point, rather than at confirmed disease progression, yields a clinically meaningful benefit, the agency required confirmatory studies to verify and describe clinical benefit. The agency also noted that camizestrant’s prescribing information carries a boxed warning for irregular heart rhythm when taken with certain other medications, plus warnings for abnormally slow heart rate and potential harm to an unborn baby. The FDA convened the Oncologic Drugs Advisory Committee for the application on April 30, 2026.

Multi-Year Collaboration and Broader Approvals

Guardant Health said the approval reflects a multi-year collaboration with AstraZeneca in which Guardant supported SERENA-6 through the Guardant360 CDx platform, providing ESR1 mutation detection across serial testing timepoints. According to AstraZeneca, the trial’s safety profile for the camizestrant combination was consistent with the known profiles of each medicine, with no new safety concerns identified and low discontinuation rates similar across both arms.

The camizestrant approval marks the 29th companion diagnostic indication for Guardant360 CDx across multiple tumor types globally, according to Guardant Health, which said the platform holds coverage from Medicare and commercial payers representing more than 300 million covered lives. The company describes Guardant360 CDx as the first FDA-approved liquid biopsy for comprehensive genomic profiling, detecting genomic alterations across all solid tumors, and it is approved as a companion diagnostic for therapies in non-small cell lung cancer, breast cancer, and colorectal cancer.

Camizestrant is now approved in more than 30 countries, including the European Union, Japan, Canada, and the United Kingdom, based on SERENA-6 results, according to AstraZeneca. The recommended dose in combination with a CDK4/6 inhibitor is 75 milligrams once daily. AstraZeneca estimated that approximately 37,000 U.S. patients with HR-positive metastatic breast cancer receive first-line treatment, most often endocrine therapies paired with CDK4/6 inhibitors, and that about 30% of patients with endocrine-sensitive disease develop ESR1 mutations during first-line treatment before progression. Guardant Health said approximately 37,000 women in the U.S. with advanced HR-positive breast cancer on an aromatase inhibitor could potentially benefit from the new approach.

“This approval represents a landmark shift in how we define disease progression and reinforces the transformative role of liquid biopsy in guiding therapy decisions earlier at a critical moment in a breast cancer patient’s treatment journey,” said Helmy Eltoukhy, Guardant Health chairman and co-CEO.

AstraZeneca said the SERENA-6 trial will continue to assess overall survival as a key secondary endpoint, while the FDA’s confirmatory-study requirement remains in place under the accelerated approval pathway.