What are the biomarkers for metastatic breast cancer?

Biomarkers are “biological markers” and refer to many pieces of measurable data that can help assess a person’s health. Heart rate is a biomarker. So are blood pressure, blood glucose levels, and cholesterol levels.

When talking about cancer, biomarkers refer to molecules that are found in fluid or tissue samples that indicate the presence of cancer. Biomarkers provide information about the biology of the cancer and help you and your healthcare providers decide what treatment might be effective against the cancer.

There are two major biomarkers that healthcare providers look for when treating metastatic breast cancer—hormone receptor status and HER2 status.

Below, we look at how those biomarkers can influence MBC, why knowing the status of these biomarkers is important, and how this information can help inform treatment options.

Hormone receptor status

Hormone receptors are proteins found on the surface of some breast cancer cells that allow the hormones estrogen or progesterone to attach to those cells. Cancer cells can use these hormones to promote growth and division.

Breast cancer is considered hormone-receptor positive if it is estrogen-receptor-positive (ER-positive), progesterone-receptor-positive (PR-positive), or both. Hormone receptor status can be determined with a biopsy and lab tests.

Breast cancer that is hormone-receptor-positive may respond to treatment with endocrine therapy. Also called hormone therapy, endocrine therapy works by preventing cancer cells from using these hormones to grow. The choice of what endocrine therapy to use will depend on a number of factors including whether the person being treated for cancer has been through menopause.

HER2 status

HER2 refers to “human epidermal growth factor receptor 2,” a protein that healthy breast tissue uses to grow, repair, and divide. In breast cancers that are HER2-positive, breast cancer cells produce excess amounts of HER2 protein. These proteins exist on the surface of the cancer cells and attract growth hormones, which attach to the cells and promote growth and division.

Your healthcare providers may order tests to determine if breast cancer is HER2-positive. Testing involves taking a tissue sample from a breast tumor. The sample is then examined in a lab. Depending on the test being used, it will look for HER2 proteins on the surface of cancer cells or the genes that contain instructions for making HER2 proteins.

If breast cancer is HER2-positive, it may respond to targeted therapies. Targeted therapies block or inhibit the function of those proteins, interfering with the cancer cells’ ability to grow, divide, and spread. There are also targeted therapies that are conjugates—medications that contain a targeted therapy drug, plus another anti-cancer drug such as a chemotherapy agent or a topoisomerase inhibitor. Conjugate targeted therapies are able to deliver these anti-cancer drugs directly to HER2-positive cancer cells.

Status and treatment decisions

There are numerous therapy options for MBC, and different types of cancer therapies can be used in combination with one another. Some examples:

• If MBC is both hormone-receptor positive and HER2-positive, it might be treated with a combination of endocrine therapy and targeted therapy—and chemotherapy may also be included in a treatment plan.
• To treat MBC that is HER2-positive but hormone receptor negative, targeted therapy might be paired with chemotherapy.
• Endocrine therapy might be used to treat MBC that is hormone-receptor-positive but HER2-negative. It might be paired with other anti-cancer drugs that disrupt cell division in cancer cells.
• Breast cancer that is HER2-negative and hormone-receptor-negative is referred to as triple-negative breast cancer. Triple negative breast cancer might be treated with chemotherapy and immunotherapy.

These are only a few very general examples, and MBC treatment is different for every person. In every case, the therapies used and the dosing schedules will depend on a number of factors, including the burden of the disease, the risk of side effects, and patient preferences. Remember, there is no best treatment for MBC, only a treatment that is best for a particular person with MBC at a particular time. The best way to make the treatment decision that is right for you is to work with your healthcare team and learn about your treatment options.