This post has been updated to reflect new research. It was first published on February 21, 2019.

Dr. Norah Lynn Henry is an Associate Professor in the University of Utah’s Division of Oncology in the Department of Internal Medicine. She is Leader of the Women’s Cancers Disease Center at the Huntsman Cancer Institute. She is the Cancer.Net Associate Editor for Breast Cancer. Dr. Charles Loprinzi is the Regis Professor of Breast Cancer Research at the Mayo Clinic in Rochester, MN, where he is an emeritus chair of the Division of Medical Oncology and an emeritus vice-chair of the Department of Oncology. He is the Cancer.Net Associate Editor for Psychosocial Oncology. Dr. Lidia Schapira is Associate Professor of Medicine at Stanford University School of Medicine and Director of Cancer Survivorship at the Stanford Comprehensive Cancer Institute. Dr. Schapira is the Cancer.Net Editor in Chief.

Immunotherapy, also called biologic therapy, helps a person’s immune system attack cancer cells. This type of treatment has dramatically improved survival rates for many people with melanoma, lung cancer, bladder cancer, and head and neck cancer. Immunotherapy approaches to breast cancer are also being explored, but research results are still early.share on twitter Here’s what we know today. 

1. Is immunotherapy a proven treatment for breast cancer?

There are a relatively large number of treatments other than immunotherapy for most types and stages of breast cancer. These treatments have been shown to have more benefits and fewer side effects compared to what has been found so far with immunotherapy. As of 2019, there are 2 immunotherapies approved to treat breast cancer. The first is atezolizumab (Tecentriq) plus protein-bound paclitaxel (Abraxane) for locally advanced triple-negative breast cancer that cannot be removed by surgery and for metastatic triple-negative breast cancer. Atezolizumab is only approved to treat breast cancers that test positive for the PD-L1 protein. Pembrolizumab (Keytruda) as a single treatment is approved to treat metastatic cancer or cancer that cannot be removed by surgery that has a molecular alteration called microsatellite instability-high (MSI-H) or DNA mismatch repair deficiency (dMMR).

Immunotherapy is still being tested extensively in clinical trials, which are research studies that test new approaches to treatment. Results from a clinical trial that tested the combination of the immunotherapy pembrolizumab plus chemotherapy for treatment of breast cancer before surgery were presented in late 2019. This study showed that the addition of immunotherapy resulted in more people having their breast tumors shrink completely compared with chemotherapy alone, although it is not yet known whether that means the cancer will be less likely to return in the future. Other similar studies, however, did not show this type of benefit with the addition of immunotherapy to chemotherapy. Therefore, immunotherapy is not yet an established treatment for early-stage breast cancer, but it is still being studied.

2. Which people with breast cancer will be most likely to benefit from immunotherapy?

Researchers are starting to get some clues from clinical trials about who immunotherapy is most likely to help. For example, people with triple-negative breast cancer may get more benefit from immunotherapy than people with other subtypes of breast cancer. Immunotherapy may be more likely to work in people whose breast cancer has more gene mutations or whose tumor cells have higher levels of a protein called PD-L1. In addition, there is some information that suggests immunotherapy may work better if it is given early during treatment.

In late 2018, results from a breast cancer immunotherapy clinical trial were published in the New England Journal of Medicine. The study examined adding atezolizumab to a standard chemotherapy drug called protein-bound paclitaxel in people with metastatic, triple-negative breast cancer who had not previously received treatment for their cancer. Researchers found that adding immunotherapy to the chemotherapy drug may be beneficial for a subset of people, although it increases harmful side effects, also called toxicity. In addition, the findings support the clues found in other clinical trials about who may most benefit from immunotherapy. This treatment combination was approved by the U.S. Food and Drug Administration in March 2019. More research is needed to determine which cancers are most likely to respond to the combination.

3. Should immunotherapy be tried if a breast cancer has not responded to multiple standard treatments?

Unfortunately, studies of immunotherapy for breast cancer that is resistant to multiple prior treatments have not shown much benefit for the vast majority of people. However, there are hints that immunotherapy will eventually play a role in treating breast cancer. There are many clinical trials that are currently testing different types and combinations of immunotherapy in this situation.

4. What are the challenges of immunotherapy? 

One challenge of immunotherapy is not knowing who is likely to benefit from the treatment, as mentioned above. Second, immunotherapy can cause substantial side effects, including life-threatening ones. The most common immunotherapy side effects are skin reactions, such as redness and blistering, and flu-like symptoms, such as fever, nausea, weakness, and body aches. Different types of immunotherapy can cause different side effects. An important third challenge is the high cost of this treatment, which insurance companies may not cover.

5. Is breast cancer immunotherapy research continuing?

New information about breast cancer immunotherapy is expected in the coming months and years, as more clinical trials are completed. Clinical trials are being conducted across the United States and around the world to evaluate combinations of immunotherapy drugs for the disease. Some studies, like the one mentioned above, combine immunotherapy with chemotherapy or targeted therapy. People with breast cancer are encouraged to participate in immunotherapy clinical trials, whenever appropriate. Although immunotherapy is currently only a standard part of breast cancer treatment for a small number of women with metastatic disease, there is hope that this will change.