Clinical Outcomes and Molecular Biomarkers of Response to Pembrolizumab plus Fulvestrant in ER+/HER2− Metastatic Breast Cancer: Results from a Big Ten Cancer Research Consortium Multicenter Phase II Study

For decades, the standard approach to treating breast cancer that relies on estrogen to grow has been to block that hormone. Doctors use drugs to starve the tumor of its fuel, a strategy that works well for many years. However, when the cancer eventually returns or spreads to other parts of the body, it often becomes resistant to these hormone blockers. At that point, the usual next step is to switch to chemotherapy, which uses powerful toxins to kill rapidly dividing cells. While effective, chemotherapy is harsh on the body, causing severe fatigue, hair loss, and damage to the immune system. For patients who have already tried a few rounds of hormone therapy, the medical community has been searching for a gentler way to control the disease, one that might delay the need for chemotherapy. This search has led researchers to look at the immune system, the body's natural defense force, and whether it can be trained to recognize and attack these stubborn cancer cells without the heavy toll of traditional drugs.

A team of researchers from the Big Ten Cancer Research Consortium set out to test a specific combination of two drugs for women with this type of advanced breast cancer. They paired pembrolizumab, a medication that helps the immune system see and attack cancer, with fulvestrant, a drug that blocks estrogen receptors. The goal was to see if this duo could control the cancer's growth and keep patients healthy for longer, offering a treatment path that avoids the immediate use of chemotherapy. The study involved 47 patients from five different academic medical centers. These were individuals whose cancer had spread but who had not received more than two previous lines of treatment for their advanced disease. The researchers gave the patients the immune drug every three weeks and the hormone-blocking drug on a specific schedule, watching closely to see how long the cancer stayed under control and what side effects occurred.

The results showed that this combination worked for a meaningful group of patients. Out of the 44 people who could be properly evaluated, 16 saw their cancer shrink or stop growing for at least four months. This is known as a clinical benefit, and it happened in about 36 percent of the patients. For those who did respond, the benefit lasted a median of six months, and some remained on the treatment for more than two years without their disease getting worse. The researchers found that the treatment was generally safe and manageable. The most common side effects were mild to moderate, such as joint pain, fatigue, and changes in liver enzymes or thyroid function. While a few patients experienced more serious reactions, no one died from the treatment itself. This suggests that for a significant subset of patients, this approach offers a way to manage their cancer with a toxicity profile that is far easier to live with than chemotherapy.

One of the most important discoveries in this study was about who would benefit from the treatment. In many other types of cancer, doctors look for a specific marker on the surface of cancer cells to decide if immunotherapy will work. This marker is a protein called PD-L1. In this study, the researchers checked for this protein in the tumors of 40 patients. They found that the presence or absence of this protein did not predict who would get better. Some patients with high levels of the protein did not respond, while others with no detectable levels of the protein saw their cancer stop growing. This finding is crucial because it means doctors cannot rely on this single test to rule out immunotherapy for these patients. The study suggests that the immune system might be able to fight this type of breast cancer even when the usual "on switch" for the treatment is not visible.

The researchers also looked deep inside the genetic code of the tumors to find other clues. They examined the DNA of 26 patients to see if specific mutations, or changes in the genetic instructions, were linked to a positive response. They found that patients whose tumors had mutations in a gene called ESR1, which is directly involved in how the cancer responds to estrogen, were more likely to benefit from the treatment. They also saw a link between mutations in a gene called KMT2C and a better outcome. Interestingly, the total number of genetic errors in the tumor, a measure often used to predict immunotherapy success in other cancers, did not seem to matter here. Patients with very few genetic changes responded just as well as those with many. This tells us that the rules for how the immune system fights this specific type of breast cancer are different from the rules for other cancers, and that the presence of certain estrogen-related mutations might be a better guide for treatment than the overall amount of genetic damage.

The study did have its limits. It was a relatively small group of people, and because it was a single group without a comparison group taking a different treatment, the researchers cannot say for certain that the drug combination is better than other options available. They also noted that the landscape of breast cancer treatment has changed since they started the study, with new drugs now available that were not part of their plan. However, the data provides a clear picture of what is possible with this specific combination. It demonstrates that for some patients with estrogen-positive, HER2-negative metastatic breast cancer, adding an immune-boosting drug to hormone therapy can create a durable response. This offers a potential path to delay the use of chemotherapy, giving patients more time with a better quality of life. The work highlights that while the immune system is a powerful tool, understanding how to use it in breast cancer requires looking beyond the standard tests and paying close attention to the unique genetic features of each patient's tumor.