How is targeted therapy different from chemotherapy?

Chemotherapy attacks cells for being fast-growing. Targeted therapy attacks cells for carrying one specific protein. That single difference drives everything else, including which patients get which drug and which side effects they get.

Blanket versus keyhole

Chemotherapy kills or slows cells that grow and divide quickly. Cancer cells qualify, but so do the cells lining your mouth and intestines and the cells that grow your hair. The drug cannot tell them apart. That is why mouth sores, nausea, and hair loss are classic chemotherapy problems, and why fatigue is the single most common one.

Targeted therapy works on proteins that control how cancer cells grow, divide, and spread. The National Cancer Institute calls it the foundation of precision medicine. It does its job in several distinct ways:

• Blocking growth signals. Some cancer cells carry altered surface proteins that shout "divide" whether or not the body has asked. Targeted drugs jam that shout.
• Cutting off blood supply. Tumors past a certain size have to grow new blood vessels, a process called angiogenesis. Angiogenesis inhibitors block the signals that start it, so the tumor stays small or shrinks.
• Flagging cancer for the immune system so it can be found and destroyed.
• Delivering poison to one address. Some monoclonal antibodies carry a toxin, a chemotherapy drug, or radiation. They stick to the target, the cell swallows the payload, and cells without the target are not harmed.
• Restarting apoptosis, the orderly cell death that healthy cells undergo and cancer cells learn to dodge.
• Starving cancer of hormones. Hormone therapies for breast and prostate cancer count as targeted therapy for this reason.

Most targeted drugs come in one of two forms. Small-molecule drugs are pills or capsules, small enough to slip inside a cell and reach targets there. Monoclonal antibodies are lab-made proteins, usually given through a vein, that latch onto targets on the cell surface.

The test that usually comes first

This is the practical difference patients feel. Chemotherapy is generally chosen by cancer type and stage. Targeted therapy usually requires proof that your tumor carries the target.

That proof comes from biomarker testing, which examines your cancer for changes a drug can act on. It may need a biopsy, and a biopsy carries risks that depend on the tumor's size and location. There are exceptions. In chronic myelogenous leukemia, nearly everyone with the disease has the target, so testing is not a gate in the same way.

The side effects are not milder, just different

When targeted therapy was new, researchers expected it to be gentler than chemotherapy. NCI says plainly that they have since learned it can also cause serious side effects.

The most common are diarrhea and liver problems. Others include high blood pressure, fatigue, mouth sores, nail changes, loss of hair color, rash and dry skin, and trouble with blood clotting and wound healing. That last one matters if surgery is planned, so tell your surgeon what you are taking.

Go to an emergency room for sudden, severe abdominal pain with a rigid or tender belly, fever, or vomiting. Very rarely, targeted therapy causes a hole through the wall of the esophagus, stomach, small intestine, colon, rectum, or gallbladder. That is a surgical emergency and hours matter.

Why you may end up on both

Cancer cells can become resistant to a targeted drug. The target itself can change shape so the drug no longer fits, or the cells find a new growth route that ignores the target entirely.

Because of that, targeted therapy often works best combined with a second targeted drug or with chemotherapy or radiation. These are not rival camps. Many modern regimens use both, and the antibody-drug conjugates blur the line completely by using a targeted antibody to carry a chemotherapy payload.

Sources

https://www.cancer.gov/about-cancer/treatment/types/targeted-therapies

https://www.cancer.gov/about-cancer/treatment/types/chemotherapy

https://www.cancer.gov/about-cancer/treatment/types/biomarker-testing-cancer-treatment