How Targeted Therapy Works

This page is for people with cancer, and their families, who want to understand targeted therapy. It explains how it works, who can get it, and which side effects to watch for.

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 aim at those proteins.

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

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. They are usually pills or capsules you swallow.
• Monoclonal antibodies are proteins produced in the lab, designed to attach to specific targets on cancer cells. Some mark cancer cells so the immune system destroys them. Some directly stop cancer cells from growing or cause them to self-destruct. Others carry toxins to cancer cells. They are usually given through a needle in a vein.

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 find and destroy cancer cells, or boost the immune system to work better.
• Stop cancer cells from growing by interrupting the signals that tell them to divide without order.
• Stop signals that form blood vessels. Tumors need new blood vessels (a process called angiogenesis) to grow. Angiogenesis inhibitors block these signals so tumors stay small or shrink.
• Deliver cell-killing substances to cancer cells. Some antibodies carry toxins, chemotherapy, or radiation directly to cancer cells that have the target.
• Cause cancer cell death (apoptosis), the orderly process healthy cells use when damaged, which cancer cells often avoid.
• Starve cancer of hormones it needs. Some breast and prostate cancers need certain hormones, and hormone therapies are a type of targeted therapy that block those hormones.

For some cancers, most people will have a target for a certain drug, so they can be treated with it. Chronic myelogenous leukemia (CML) is one example. But most of the time, your tumor needs to be tested to see if it contains targets for which there is a drug.

Testing your cancer for such targets is called biomarker testing. You may need a biopsy, a procedure in which your doctor removes a piece of the tumor for testing. There are some risks to a biopsy. They vary depending on the size and location of the tumor. Your doctor will explain them.

Targeted therapy does have drawbacks. Cancer cells can become resistant to it, which means they stop responding to it. This can happen when the target itself changes. It can also happen when the cells find new ways to grow that do not depend on the target.

Because of resistance, targeted therapy may work best when used with more than one type of targeted therapy. It may also work best with other treatments such as chemotherapy and radiation. Also, drugs for some targets are simply hard to develop.

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. Very rarely, a hole might form through the wall of the digestive tract. There are medicines for many of these side effects, and most side effects go away after treatment ends.

Most side effects of targeted therapy improve after treatment ends, and many can be managed with medicine.

Some side effects need attention quickly. Use this guide to decide how fast to call.

• Call 911 or go to an emergency department if severe belly pain comes on and your abdomen feels hard or tender all over. This is rare, but it can mean a hole has opened in the wall of the gut. Do not drive yourself.
• Call your care team the same day if loose stools jump well above your normal count, or you cannot hold fluids down. Call the same day too if the whites of your eyes or your skin look yellow, or your urine darkens.
• Call your care team right away, day or night, if you are also having chemotherapy and get chills, a sore throat or a new mouth sore. Chemotherapy lowers the white cells that fight infection.
• Call your care team within a day or two if a rash spreads or starts to blister, or home blood pressure readings keep running high. If you are not having chemotherapy, tell them too when mouth sores stop you eating.

Source: National Cancer Institute: targeted therapy to treat cancer.