What Is Targeted Therapy?

Targeted therapy is a type of cancer treatment that targets proteins that control how cancer cells grow, divide, and spread. It is the foundation of precision medicine. As researchers learn more about the DNA changes and proteins that drive cancer, they are better able to design treatments that target these proteins.

Targeted therapy goes after the specific proteins that help a cancer grow and spread.

Most targeted therapies are either small-molecule drugs or monoclonal antibodies.

• Small-molecule drugs are small enough to enter cells easily, so they are used for targets that are inside cells.
• Monoclonal antibodies, also known as therapeutic antibodies, are proteins produced in the lab. They are designed to attach to specific targets found on cancer cells. Some mark cancer cells so the immune system can better see and destroy them. Others directly stop cancer cells from growing or cause them to self-destruct. Still others carry toxins to cancer cells.

For some types of cancer, such as chronic myelogenous leukemia (CML), most people will have a target for a certain drug, so they can be treated with it. But most of the time, your tumor will need to be tested to see if it contains targets for which there is a drug. This is called biomarker testing.

You may need a biopsy for biomarker testing — a procedure in which your doctor removes a piece of the tumor for testing. There are some risks to a biopsy, which vary depending on the size and location of the tumor. Your doctor will explain the risks for your type of tumor.

Testing your tumor helps show whether a targeted drug is an option for you.

Most types of targeted therapy interfere with specific proteins that help tumors grow and spread. This is different from chemotherapy, which often kills all cells that grow and divide quickly. Targeted therapy can work in several ways:

• Help the immune system destroy cancer cells. Some targeted therapies mark cancer cells so the immune system can find and destroy them, or help boost the immune system.
• Stop signals that tell cancer cells to grow and divide. Some cancer cells have changes in surface proteins that tell them to divide even without signals. Some targeted therapies interfere with these proteins to slow the cancer's growth.
• Stop signals that help form blood vessels. To grow beyond a certain size, tumors need new blood vessels (angiogenesis). Angiogenesis inhibitors interfere with these signals, so tumors stay small or shrink.
• Deliver cell-killing substances to cancer cells. Some monoclonal antibodies are combined with toxins, chemotherapy, or radiation. Once they attach to cancer cells, the cells take up the substance and die. Cells without the target are not harmed.
• Cause cancer cell death. Some targeted therapies push cancer cells through the natural process of cell death (apoptosis) that they normally avoid.
• Starve cancer of hormones it needs to grow. Some breast and prostate cancers need certain hormones. Hormone therapies are a type of targeted therapy that either prevent the body from making hormones or stop hormones from acting on cells.

Targeted therapy has some drawbacks:

• Resistance. Cancer cells can become resistant to targeted therapy. This can happen when the target itself changes so the drug cannot interact with it, or when cancer cells find new ways to grow that do not depend on the target. Because of resistance, targeted therapy may work best when used with other targeted therapies or treatments such as chemotherapy and radiation.
• Hard-to-develop drugs. Drugs for some targets are hard to develop, because of the target's structure, its function in the cell, or both.

When targeted therapy was first developed, scientists thought it would be less toxic than chemotherapy. But they have learned it can also cause serious side effects. The most common are diarrhea and liver problems. Others may include problems with blood clotting and wound healing, high blood pressure, fatigue, mouth sores, nail changes, loss of hair color, and skin problems such as rash or dry skin.

There are medicines for many of these side effects that may prevent them or treat them once they occur. Most side effects of targeted therapy go away after treatment ends.

How it is given. Small-molecule drugs are pills or capsules you swallow. Monoclonal antibodies are usually given through a needle in a blood vein. If yours is a pill you take at home, ask exactly when to take it, whether food matters, and what to do if you miss a dose.

Where you go. You may take targeted therapy at home, or receive it in a doctor's office, clinic, or outpatient unit in a hospital.

How often. This depends on your type of cancer and how advanced it is, the type of targeted therapy, and how your body reacts. You may have treatment every day, week, or month. Some targeted therapies are given in cycles — treatment followed by a rest period.

How you will know if it is working. You will see your doctor often for physical exams and to be asked how you feel. You will have medical tests, such as blood tests, x-rays, and scans, which help your doctor know whether the treatment is working.

• Call 911 or go to an emergency department if you have trouble breathing, chest pain, or bleeding that does not stop. Some of these drugs interfere with blood clotting.
• Call 911 or go to an emergency department if you get sudden severe belly pain, a belly that feels hard or tender to touch, or belly pain with fever or vomiting. Very rarely these drugs let a hole form through the wall of the gut, and that needs surgical care at once.
• Call your care team the same day if diarrhea will not settle, or you cannot keep fluids down.
• Call your care team the same day if your skin or the whites of your eyes turn yellow, or your urine darkens. Those can point to a liver problem, which is one of the two most common serious effects of this class.
• Call your care team within a day or two if a rash, dry or sore skin, mouth sores, nail changes, or a rise in your blood pressure readings show up. There are medicines for many of these, and starting them early works better than waiting.