Targeted Therapy for Cancer: Who It Suits and Why Testing Matters

Targeted therapy means drugs that act on a specific “fault” in cancer cells: an altered gene or protein that the tumour depends on to grow. So first that fault has to be found — this is what molecular (biomarker) testing is for. No target, no point in a targeted drug, however advanced it is.

How targeted therapy differs from chemotherapy

Chemotherapy acts on all rapidly dividing cells. Cancer cells divide actively, so they are hit harder, but blood cells, the lining of the mouth and gut, and hair follicles are affected too — hence the typical side effects of chemo.

A targeted drug is aimed at a specific protein that helps the cancer grow and spread. There are two main types:

• Small molecules — get inside the cell. Most often these are tablets or capsules taken at home every day.
• Monoclonal antibodies — laboratory-made proteins that bind to a target on the surface of the cell. They are usually given as an intravenous drip.

Targeted therapy is often given not instead of chemotherapy but together with it or after it. I describe what chemo involves in detail in my article on what to expect from chemotherapy.

Why molecular testing is needed

The test answers one question: does the tumour have a change that one of the existing drugs acts on? If it does, you gain a treatment option you simply wouldn’t have had without the test. If it doesn’t, that’s a useful result too: it spares you treatment that won’t help.

Here are the targets patients ask about most often:

This is not a complete list: in lung cancer, for example, doctors also look for changes in MET, RET and NTRK. Which genes are tested depends on the type of cancer and the stage — this is set out in clinical guidelines.

A separate note on HER2: in breast cancer it is more often assessed by immunohistochemistry (IHC) than by a genetic test. I explain how to read that result in my article on IHC in breast cancer.

What the test is done on

A tissue block. Most often the test is done on the same material used for the diagnosis: the paraffin block from the biopsy or surgery. That’s why blocks and slides should be kept and not lost — they may be needed for new tests even years later. If there is too little tissue, or it isn’t of suitable quality, a repeat biopsy may be needed.

Liquid biopsy. This is a blood test that looks for fragments of tumour DNA. It is used, for example, when tissue can’t be taken safely — the tumour is hard to reach with a needle. Liquid biopsy usually complements rather than replaces tissue testing: a negative blood result doesn’t always mean there is no mutation.

Tumour changes and inherited changes are not the same thing. Most mutations in a tumour arose during a person’s lifetime and cannot be passed on to relatives. But sometimes tumour testing finds a change that may turn out to be inherited — for example, in the BRCA1/2 genes. This is checked with a separate test on blood, saliva or skin and discussed with a geneticist: the result determines whether your relatives should be tested.

Ask your oncologist for a copy of the molecular test report and keep it with your pathology report. It is one of the most important documents in your medical history.

What treatment involves

If the targeted drug comes as tablets, treatment happens mostly at home. That’s convenient, but it brings more responsibility: you make sure you take it regularly and you are the first to notice side effects.

What I usually ask patients to stick to:

• Take the drug exactly as your doctor prescribed — at the same time each day, and on an empty stomach or with food, as instructed. Don’t double up after a missed dose — check with your doctor what to do.
• Check any new medicines, herbs and supplements with your doctor. Targeted tablets often interact with other products, including over-the-counter ones: some raise the level of the drug in the blood, others lower it.
• Have your follow-up blood tests on time — above all liver tests and any others your doctor has ordered for your drug.
• Keep a diary: blood pressure, bowel movements, skin, temperature. It makes appointments shorter and more precise.

Targeted therapy is usually continued for as long as it works and is tolerated. As with chemotherapy, its effect is assessed with follow-up CT scans or other imaging.

What side effects can occur

When targeted therapy first appeared, it was expected to have almost no side effects. In practice that’s not the case: the side effects are different, but they exist, and some can be serious.

The most common are:

• diarrhoea;
• changes in liver tests — which is why they are checked regularly;
• skin and nails: rash, dryness, itching, nail changes;
• raised blood pressure;
• problems with blood clotting and wound healing — important before any planned surgery or tooth extraction;
• tiredness, mouth sores.

Some drugs have their own specifics. Anti-HER2 antibodies, for example, sometimes affect heart function, so an echocardiogram (heart ultrasound) is done before and during treatment. Tell your doctor if you develop shortness of breath, swollen legs or severe tiredness.

Many side effects can be prevented or treated, and most go away after treatment ends. I usually advise patients not to put up with a rash or diarrhoea in silence: the sooner your doctor knows, the easier it is to find the right support and the less often treatment has to be interrupted.

Call your doctor the same day if diarrhoea won’t stop and you can’t keep up with fluids, if your skin or eyes turn yellow, or if you have bleeding or sudden shortness of breath. If you are short of breath at rest, have chest pain, or weakness in an arm or leg, call your local emergency number (911 in the US, 999 in the UK, 112 in the EU).

Why targeted therapy isn’t for everyone

This is one of the most common and most painful questions. There are different reasons:

• The tumour has no target for which a drug exists. In many tumours no suitable changes are found.
• There is a target, but no drug for your type of cancer — either it isn’t approved or it isn’t available.
• There wasn’t enough tissue for the test, and a repeat biopsy isn’t safe.
• Not all cancer cells are the same. A drug may act on some cells but not all of them.
• The tumour changes over time. A test is a snapshot of one moment. Over time the cancer may become resistant to the drug: for example, the target changes or the cells find another way to grow. In that case a repeat test is sometimes done to see what has changed.

And even with a suitable target, the drug may not work for a particular person. That is nobody’s fault; it is how the biology of the tumour works.

Questions to ask your oncologist

• Has my tumour had molecular testing, and for which genes?
• If not, is it needed for my diagnosis and stage?
• Is there enough material in the blocks, or is a repeat biopsy needed? Is a liquid biopsy possible?
• If a mutation was found, could it be inherited, and do I need to see a geneticist?
• Which side effects are most likely with my drug, and which tests should be monitored?
• What will we do if the cancer becomes resistant?

If you were diagnosed recently and aren’t sure whether all the necessary tests have been done, I’ll go through your documents in a consultation and draw up a list of questions for your oncologist. I also have a separate article on another type of drug treatment — immunotherapy.

Sources

1. Targeted Therapy to Treat Cancer — National Cancer Institute (NCI), accessed 2026
2. Biomarker Testing for Cancer Treatment — National Cancer Institute (NCI), accessed 2026
3. Targeted Drug Therapy for Non-Small Cell Lung Cancer — American Cancer Society, accessed 2026
4. Targeted Therapy for HER2-positive Breast Cancer — American Cancer Society, accessed 2026
5. PARP inhibitors: Overview and indications — The Jackson Laboratory, accessed 2026
6. Targeted Therapy for Cancer (booklet) — American Cancer Society, accessed 2026