The Breast Cancer Test Many Patients Don’t Know to Ask For

Adding Keytruda to Chemotherapy Slows HR-Positive Breast Cancer Growth

Adding Keytruda (pembrolizumab) to chemotherapy helped delay disease progression in some patients with hormone receptor (HR)-positive, HER2-negative metastatic breast cancer, according to findings from the phase 3 KEYNOTE-B49 trial presented at the 2026 ESMO Congress.

The trial included 342 patients whose tumors expressed PD-L1, a protein that can help predict response to immunotherapy, and whose cancer had progressed after hormone therapy. Patients who received Keytruda with chemotherapy went a median of 6.7 months without their cancer growing, compared with 6.0 months for those who received chemotherapy alone. However, adding Keytruda did not help patients live longer, and immune-related side effects, such as thyroid problems, were more common.

In an interview with CURE, Dr. Hope S. Rugo, City of Hope, discussed what these results mean for patients.

What do the KEYNOTE-B49 results mean for patients?

Rugo: Our study was underpowered to really fully understand the impact of immunotherapy. But what we’ve really found is that in a small subset of patients in KEYNOTE-B49 who had a very high immune marker — the immune marker is PD-L1, and it’s evaluated by a combined positive score — if that score was 10 or more, it appeared that the benefit of adding pembrolizumab to chemotherapy was really pretty high.

So that’s a tiny number of patients. But for patients whose cancer is not responding to endocrine therapy quite as well as we think it should with targeted agents, these are patients where testing the tissue for PD-L1 would make a difference.

It’s important to know that you can’t test PD-L1 right now in the blood, where we can look for mutations. You have to test it in tissue.

Immunotherapy causes immune side effects. We want to know: Are you a patient whose cancer will respond to this treatment? And if so, we want to do it with the very first chemo, not later when the chances of response are less.

Transcript edited for clarity and conciseness.

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