What must generally be confirmed before starting a targeted therapy such as a tyrosine kinase inhibitor or monoclonal antibody?
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Clinical Pharmacology · Oncology Medications
Targeted Cancer Therapies
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Targeted cancer therapies attack a specific molecular abnormality driving a particular tumor's growth, rather than killing every rapidly dividing cell the way traditional chemotherapy does. Because the target must actually be present, patients are tested for a biomarker before treatment begins — this is precision oncology. The main classes are tyrosine kinase inhibitors (the "-nib" drugs) and monoclonal antibodies (the "-mab" drugs), joined by antibody-drug conjugates, PARP inhibitors, CDK4/6 inhibitors, and proteasome inhibitors. Side effects tend to track the target's normal biology, not the tissue where the tumor started.
The college version
The conceptual shift
Traditional chemotherapy poisons any fast-dividing cell, damaging bone marrow, gut lining, and hair follicles along with the tumor. Targeted therapy instead identifies a specific mutated protein, receptor, or signaling pathway a cancer depends on for survival — an "oncogenic driver" — and blocks it selectively. This only works if the tumor carries that abnormality, so genomic or biomarker testing of tumor tissue, or blood via liquid biopsy, precedes treatment selection. A drug aimed at a mutation the tumor lacks will not work, so testing is not paperwork; it is the therapeutic decision itself.
Tyrosine kinase inhibitors (-nib)
Tyrosine kinases are enzymes that relay growth signals inside the cell; many cancers carry a mutated, permanently "on" kinase. Imatinib is the landmark example: it blocks the BCR-ABL fusion protein driving chronic myeloid leukemia, converting a once-fatal disease into a manageable chronic condition for most patients. Others follow the same logic against different targets: osimertinib blocks mutated EGFR in a subset of lung cancers; ALK inhibitors block ALK rearrangements in another lung cancer subset; BRAF inhibitors like vemurafenib block mutated BRAF in melanoma, often paired with a MEK inhibitor to blunt resistance; and BTK inhibitors like ibrutinib block a signaling enzyme B lymphocytes need in lymphoid cancers. These oral drugs are heavily metabolized through CYP3A4, creating dense interactions with common medications and foods. Class-wide concerns include hepatotoxicity, QT prolongation, and interstitial lung disease.
Monoclonal antibodies (-mab)
These are large engineered proteins that bind a specific surface antigen, marking the cell for immune destruction or blocking a growth signal directly. Trastuzumab targets HER2, overexpressed in a breast cancer subset; its signature risk is cardiotoxicity, so cardiac function is monitored throughout treatment. Rituximab targets CD20 on B lymphocytes for B-cell lymphomas; it carries infusion-reaction risk and can reactivate dormant hepatitis B, so screening precedes therapy. Bevacizumab targets VEGF, the signal tumors use to build blood vessels; blocking angiogenesis causes hypertension, proteinuria, impaired wound healing, bleeding, and rarely gastrointestinal perforation. Cetuximab targets EGFR and produces an acneiform facial rash that, counterintuitively, correlates with better response — a target-driven toxicity doubling as a biological signal.
Other precision mechanisms
Antibody-drug conjugates combine an antibody's precision with a potent cytotoxic payload chemically attached to it, delivering the toxin directly to antigen-positive tumor cells while sparing much surrounding tissue. PARP inhibitors exploit "synthetic lethality": cells with a BRCA mutation already have impaired DNA repair, so blocking the PARP repair pathway too is enough to kill the cancer cell while sparing normal cells with one working pathway. CDK4/6 inhibitors block cell-cycle proteins driving proliferation in hormone receptor-positive breast cancer, usually combined with hormonal therapy. Proteasome inhibitors block the cell's protein-recycling machinery, to which myeloma cells are especially vulnerable given their heavy antibody production.
Shared themes
Adverse effects across this family follow the target's normal physiologic role rather than the organ of origin — EGFR inhibitors cause rash and diarrhea because EGFR is active in skin and gut; VEGF inhibitors cause hypertension because VEGF maintains vascular tone. Tumors can also develop acquired resistance, often via a new mutation in the target, which is why repeat tissue or liquid biopsy re-characterizes a cancer that stops responding. These agents also carry substantial cost, a real barrier to access.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Imagine a tumor is like a car with one broken part stuck on the gas pedal, making it speed out of control. Old chemotherapy is like flooding the whole street with something that stops every car, ambulances and school buses included — it works, but it hurts things you didn't mean to hurt. Targeted therapy is different: doctors first check under the hood to see exactly which part is broken, then send a tiny wrench shaped only for that one part. If a tumor doesn't have that broken part, the wrench does nothing, so doctors test first to make sure they're sending the right tool. Some wrenches are pills that jam the broken switch (the "-nib" drugs), and some are like guided drones (the "-mab" drugs) that latch onto one exact spot on the cancer cell and mark it for the body's defenders or block its growth signal. Because the wrench matches a part the body also uses elsewhere, it can cause side effects wherever that part normally works — that's why a wrench for a skin-related switch can cause a rash. Sometimes the cancer reshapes the broken part so the wrench no longer fits, which is why doctors sometimes need a fresh sample to check the shape again.
Check yourself
2 review questions from the chapter. Try each one, then open the answer.
A patient on cetuximab develops a noticeable facial rash a few weeks into treatment and asks whether this means the drug isn't working. What should the nurse explain about this rash? A short scenario answer is expected.
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The rash reflects the drug hitting its target, and it actually correlates with a better chance of response.
Because cetuximab blocks EGFR, and EGFR is also active in skin, the rash is an expected on-target effect rather than a sign of failure — patients who develop this rash tend to respond at least as well as, and often better than, those who don't.
A patient with chronic myeloid leukemia has been stable on imatinib for years but now shows rising disease markers. What is the most appropriate next step to guide further treatment, and why? A short scenario answer is expected.
Show answer
Repeat biopsy or liquid biopsy to look for a new resistance-causing mutation, since rising disease markers after a long stable response suggest the leukemia has changed and treatment may need adjusting accordingly.
Cancers can acquire a new mutation in the very target the drug was blocking, making the original drug less effective, so re-testing the tumor's molecular makeup tells the team whether a different or additional targeted drug is now needed.
Quick check
3 questions here. Answers stay hidden until you check.
Imatinib is the landmark targeted therapy for which condition, acting against the BCR-ABL fusion protein?
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Which adverse effect is most specifically associated with VEGF-targeting agents like bevacizumab?
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