What Is Targeted Therapy? Biomarkers, Genetic Testing and Smart Drugs

Chemotherapy attacks every fast-dividing cell it meets. Targeted therapy does something different: it blocks one specific molecule that a particular tumour depends on to grow. When the tumour carries that molecule, the results can be remarkable. When it does not, the same drug does nothing at all — which is why the test comes before the treatment.

How targeted drugs work

Cancers grow because a signal that tells cells to divide is stuck in the on position. Often the culprit is a single altered protein — a receptor on the cell surface or an enzyme inside it. Targeted drugs bind that protein and switch the signal off. Some are small molecules taken as a daily tablet; others are antibodies given by infusion.

The testing that comes first

• Immunohistochemistry (IHC) stains the tumour tissue for a protein such as HER2, ER or PD-L1. Fast and inexpensive.
• FISH looks for gene amplification or fusion when the stain is borderline.
• Next-generation sequencing (NGS) reads dozens to hundreds of genes at once from the tumour block and finds mutations no single test would look for.
• Liquid biopsy sequences tumour DNA circulating in a blood sample — useful when tissue is scarce or when checking for resistance later.

Ask one question at your next appointment: has my tumour had full biomarker testing, and can I see the report? In several cancers a treatable mutation is present in a substantial minority of patients, and it is only found if somebody orders the test.

Which targets are treatable today

Side effects are different, not absent

Because the drug follows a molecule rather than cell division, you are much less likely to lose your hair or your white cells. Instead the typical problems are an acne-like rash, diarrhoea, mouth sores, high blood pressure, or changes in liver, thyroid and heart function. Most are manageable with dose adjustment — but only if reported early, so keep a simple symptom diary.

Resistance, and what comes next

Tumours adapt. After months or years on a targeted drug, a subgroup of cells usually finds a way around the block and the disease starts moving again. This is expected, not a failure of the treatment. A repeat biopsy or a liquid biopsy at that moment often identifies the new mutation and points to a second-generation drug, a different combination, or a clinical trial.

This is also why targeted therapy is a long conversation rather than a single decision. Sequencing at diagnosis, again at progression, and access to the newer agents matter as much as the first prescription.

Frequently asked questions

Is targeted therapy the same as immunotherapy?

No. Targeted therapy blocks a molecule inside or on the cancer cell. Immunotherapy does not touch the tumour directly at all — it removes the brake that stops your immune system from attacking it. They are sometimes combined, but they are different mechanisms with different side effects.

My tumour has no mutation. Does that mean bad news?

It means one route is closed, not that treatment is limited. Chemotherapy, immunotherapy, radiotherapy and surgery all remain fully effective options, and panels are re-run at progression because a newer test may cover targets the first one did not.

Is biomarker testing available in Türkiye?

Yes. Accredited hospitals in İstanbul and Ankara run IHC, FISH, comprehensive NGS panels and liquid biopsy, usually at a fraction of U.S. list prices, and the report is issued in English so your own oncologist can use it.