A targeted therapy is a therapy targeted to a specific biomarker to kill breast cancer. A targeted therapy can be a monoclonal antibody, or what we used to call a magic or silver bullet, directed against a specific target on a breast cancer cell. Targeted therapies are akin to a sniper with a high-powered rifle, whereas chemotherapy is akin to a hunter with a shotgun. Both can be very effective, but the sniper has a much greater chance of hitting its target and making a kill.
The body regulates itself by sending out signals via hormones to tell different parts of the body to do different things. The thyroid gland sends out thyroid hormone to regulate your activity level. The adrenal glands send out adrenaline to help you face a challenge. The ovaries send out estrogen and progesterone to tell the uterus and breasts to get ready for a baby.
Healthy cells in the body have an instruction book, what is known as it genes or DNA. This instruction book, in addition to thousand or so other functions, directs the cells of the body to produce proteins on its cell surface to act as a receptor for those hormones or signals from the body. When the instruction book is damaged, like a page is removed or added, the healthy cells are transformed into cancer cells that do not obey the physiologic laws of the body. These cancer cells grow uncontrollably. They grow large. They grow through things. They travel far and set up secondary homes. They are bad and cause trouble.
If a breast cell has its instruction book changed so that it produces an abnormally high number of receptors to estrogen and/or progesterone, this formerly healthy breast cells turns cancerous and grows uncontrollably. These receptors to estrogen and progesterone, i.e. the estrogen receptor (ER) and the progesterone receptor (PR), are known as biomarkers. They are biomarkers because they mark are the biologic target we can use to kill cancer. If the ER and the PR become unusable, then the breast cancer does not receive a message to grow and the breast cancer dies, as if you were to never drive your car and so it rusted in your driveway. The therapies we use to target these biomarkers are known as targeted therapies. Tamoxifen, anastrozole, and letrozole are some of the targeted therapies used against ER-positive and/or PR-positive breast cancers.
Another biomarker is the Her2 receptor, formerly known as her2/neu. Her2 is a potent growth signal for many cells. When Her2 is activated, the cell knows it needs to grow in a hurry. This can be a good thing. But when the instruction book of a breast cell becomes ruined and starts instructing the breast cell to produce millions of Her2 receptors, then the breast cell becomes cancerous. This breast cancer will grow fast and travel throughout the body. These Her2-positive breast cancers are very aggressive cancers and formerly claimed the lives of many women.
But today, we have a one-two punch against the Her2 receptor. Trastuzumab and pertuzumab are two drugs that act in different ways to break the Her2 receptor. It is almost as if a boxer were throwing a hook followed by an uppercut to knock out the Her2 receptor and kill the breast cancer. These two drugs are truly miracle medications that target the Her2 receptor. We have two outstanding targeted therapies against a Her2-positive breast cancer.
Biomarkers are very important because they inform us as to which growth pathway is activated so that we can then administer targeted therapies against that growth pathway and kill breast cancer.