For some people living with metastatic breast cancer, a blood test could help open the door to a newly approved treatment. The U.S. Food and Drug Administration approved a new drug combination on Sept. 18 for adults with a common form of advanced breast cancer. The treatment combines imlunestrant, sold as Inluriyo, with abemaciclib, sold as Verzenio.

The treatment is not for everyone with breast cancer. It is approved for adults with estrogen receptor-positive, or ER-positive, and HER2-negative advanced or metastatic breast cancer. Their cancer must also have a change, or mutation, in a gene called ESR1. The cancer must have grown after at least one previous hormone therapy. The FDA also approved the Guardant360 CDx blood test to identify patients with ESR1 mutations who may qualify for the treatment.

ER-positive breast cancers use the hormone estrogen to help them grow. Hormone therapies can block that process and slow the cancer. But cancer can change during treatment. An ESR1 mutation is one change that can help cancer resist certain hormone therapies. These mutations can develop over time, which means testing done earlier in a patient’s cancer journey may not tell the full story later.

Doctors can look for these changes through biomarker testing. In this case, a blood test can search for small pieces of DNA released by cancer cells into the bloodstream. This is called circulating tumor DNA, or ctDNA. Finding an ESR1 mutation can give doctors information about why a cancer may be growing. It may also help determine which treatment to consider next.

Yet patients do not always understand when this testing should happen. Our social listening research found recurring questions from metastatic breast cancer patients and caregivers about biomarker testing, including tests for ESR1 and PIK3CA mutations. People expressed confusion about when testing should take place, what the results mean and how those results could change treatment. Patients also raised questions about blood tests compared with tissue biopsies, insurance coverage and access to advanced testing.

The conversations point to a growing challenge in cancer care. Developing a treatment for a specific mutation is only one part of precision medicine. Patients must also know about and have access to the testing needed to find that mutation.

What the New Metastatic Breast Cancer Treatment Could Mean for Patients

The FDA based its decision on results from a clinical trial called EMBER-3. The study included 874 adults with ER-positive, HER2-negative advanced or metastatic breast cancer. All had previously received an aromatase inhibitor, a type of hormone therapy. Some had also received a CDK4/6 inhibitor, another type of medicine commonly used to treat this form of breast cancer.

In an exploratory analysis of 159 patients with ESR1-mutated tumors, patients taking imlunestrant with abemaciclib went a median of 11.1 months before their cancer worsened. That compared with 5.5 months for patients taking imlunestrant alone. The FDA also reported that tumors responded to treatment in 35% of patients receiving the combination, compared with 15% receiving imlunestrant alone.

Those numbers need context. They do not mean the combination doubled how long patients lived. The FDA said overall survival data were still immature when it reviewed the results. Researchers need more time to determine whether the combination helps patients live longer.

Treatment can also cause serious side effects. Abemaciclib carries warnings for diarrhea, low white blood cell counts, liver problems, blood clots and serious lung problems. Both medicines also carry warnings about potential harm during pregnancy. Patients should discuss the possible benefits and risks with their cancer care team.

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The approval may be especially meaningful when viewed alongside what patients say happens when treatment stops working. Our social listening research found anxiety among people with metastatic breast cancer about what comes next after endocrine therapy no longer controls their disease. Patients sought information about new medicines, clinical trials and biomarker testing. They also wanted complicated medical information explained in clearer language. Cost, insurance coverage and delays in getting advanced testing were recurring concerns.

Those barriers could have an unequal impact. Black women already face a heavier burden from breast cancer. The American Cancer Society reports that Black women are 38% more likely to die from breast cancer than White women, even though their overall rate of developing the disease is slightly lower. Research also suggests differences in access to the testing that increasingly guides cancer treatment.

A 2025 study of more than 36,000 women with HR-positive, HER2-negative metastatic breast cancer found racial and ethnic differences in biomarker testing. Black, Asian and Hispanic patients were less likely than White patients to receive biomarker testing overall. Black patients were also less likely to receive ESR1 testing.

That gap becomes more important as treatments are increasingly matched to specific features of a person’s cancer. Consider a woman whose ER-positive metastatic breast cancer begins growing again after months or years of hormone therapy. Her next treatment may depend not only on where the cancer has spread, but also on what has changed inside the cancer itself. If an ESR1 mutation has developed, knowing that could affect the treatments available to her. If testing never happens, that option may never enter the conversation.

For people whose ER-positive, HER2-negative metastatic breast cancer has progressed, the new approval creates a practical question to bring to their oncology team: Has my cancer been tested for an ESR1 mutation, and could the result change my treatment options?

The FDA approval gives some patients another treatment choice, but it also highlights a larger challenge. Precision medicine depends on more than developing new drugs. Patients need access to the tests, information and conversations that allow new treatments to reach the people who may benefit from them.