Metastatic Breast Cancer: Understanding Targeted Therapy

Learn how some cancer treatments stop the growth and spread of cancer by targeting specific molecules in cancer cells.

A 3D rendering of a breast cancer cell.

Targeted therapies are a type of cancer treatment that work by blocking or inhibiting the function of specific molecules found in cancer cells, which disrupts the cancer’s ability to grow and spread. Targeted therapies are one treatment option for certain types of metastatic breast cancer. Metastatic breast cancer is cancer that began in the breasts, but has spread to other locations in the body.

Before getting into how targeted therapies work and the potential benefits of targeted therapies, it is important to understand that not every type of metastatic breast cancer can be treated with a targeted therapy, and targeted therapies may not be the best option for every patient.

Molecular targets

Before deciding on a treatment, a patient will have numerous diagnostic tests to determine the size and locations of the tumors, as well as information about the biology of the cancer cells, including the presence of specific enzymes and proteins that can be used as molecular targets.

Sometimes called biological targets, molecular targets are the specific molecules present in cancer cells that targeted therapies are designed to identify and attack. Specifically, with metastatic breast cancer, there are three potential molecular targets that will determine whether a cancer can be treated with a targeted therapy.

HER2 (human epidermal growth factor receptor 2)

Certain cancers use the protein HER2 to grow, and certain drugs can target cancer cells by identifying this protein. A biopsy will be performed on a tumor site, and a sample of the tumor will be tested to determine whether the cancer is HER2-positive. If yes, then the cancer may be treated with a targeted therapy. HER2-positive cancers tend to be more aggressive, and roughly 20 percent of patients with MBC are HER2-positive.

Hormone receptors

Some types of breast cancer can use the hormones estrogen (ER) or progesterone (PR) to fuel growth. Approximately two out of three breast cancers are hormone receptor-positive. Cancer cells that are ER- or PR-positive, can be treated with drugs called CDK4/6 inhibitors, which can stop cancer cells from dividing and slow the growth of the cancer. ER/PR positive cancers that are HER2-negative may also be treated with drugs that block mTOR, an enzyme that signals some cancers to grow, though these are only prescribed to post-menopausal women.

BRCA gene mutations

Oncologists can also test for mutations in the BRCA genes. Under normal circumstances, BRCA genes help repair damaged DNA. In some cancer cells, these genes do not function normally—and the cancer cells cannot repair their damaged DNA. Cancer cells that have this BRCA gene mutation are susceptible to a type of drug called a PARP inhibitor. This type of drug further reduces the cancer cells’ ability to repair damaged DNA, which causes the cancer cells to die.

How are targeted therapies used

Some targeted therapies are administered as an infusion, while others are taken as pills. Targeted therapies are often used in combination with other treatments, such as chemotherapy or hormonal therapy. Some targeted therapies can also be used in combination with one another. Because they specifically target cancer cells, targeted therapies can help minimize damage to the body’s healthy cells. However, like other cancer treatments such as chemotherapy and radiation therapy, targeted cancer therapies can also cause side effects and potentially serious reactions.

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