Monoclonal Antibodies and Antibody–Drug Conjugates in Solid Tumours
Chemotherapy poisons every fast-dividing cell and hopes the tumour dies first. What if instead you could build a molecule that recognizes the cancer cell by name, docks onto a protein on its surface, and either switches off its growth signal or hand-delivers a chemotherapy warhead only to that cell? That is the promise of monoclonal antibodies — the big IV "-mab" drugs — and their sharpest new form, the antibody–drug conjugate.
For decades, HER2-positive breast cancer was a death sentence read out loud — one of the most aggressive subtypes, its cells studded with thousands of copies of a growth-driving protein called HER2, dividing faster than chemotherapy could keep up. Then came an antibody engineered to home onto that exact protein, latch on, and blunt the signal. Survival curves bent. Years later came the next leap: instead of just blocking HER2, chemists bolted a potent chemotherapy warhead onto that same antibody — so the poison rides the antibody like a guided missile and is released only where the antibody lands, inside the cancer cell. A subtype that once killed quickly became one of the great success stories of targeted medicine.
Two kinds of targeted drug: the big "-mab" and the small "-nib"
Targeted therapy comes in two body types. The small-molecule kinase inhibitors end in "-nib" (imatinib, erlotinib, sunitinib): they are tiny, taken as a daily oral pill, and slip inside the cell to jam an enzyme's engine. The monoclonal antibodies end in "-mab": they are large proteins, far too big to survive the gut, so they are given by intravenous (or subcutaneous) infusion, and they work from OUTSIDE the cell — grabbing a target on the surface or floating in the blood. This article is about the "-mab" family used in solid tumours. Read the name and you already know the route: "-nib" is a pill you swallow; "-mab" is a drip in a chair.
The suffix "-mab" literally stands for Monoclonal AntiBody. The letters before it once encoded the source: "-ximab" = chimeric (part mouse, e.g. cetuximab), "-zumab" = humanized (mostly human, e.g. bevacizumab, trastuzumab), "-umab" = fully human (e.g. panitumumab). The more human the antibody, the lower the risk of the patient's immune system reacting against it.
HER2: trastuzumab, the landmark antibody
About one in five breast cancers over-express HER2, a receptor that drives relentless proliferation. Trastuzumab (Herceptin) binds the external part of HER2, blocks its signalling and flags the cell for immune destruction. It is used in HER2-positive breast cancer and HER2-positive gastric cancer, and it turned an aggressive disease into a treatable one. Pertuzumab binds a different part of HER2 and is added on top of trastuzumab for a deeper, more complete blockade.
Trastuzumab's hallmark adverse effect is cardiotoxicity: a fall in the heart's pumping strength (a drop in left-ventricular ejection fraction), sometimes overt heart failure. Unlike anthracycline heart damage, it is usually REVERSIBLE if caught and the drug stopped — but the risk is much worse when trastuzumab is combined with, or follows, an anthracycline (like doxorubicin). So we monitor ejection fraction (echocardiogram or MUGA) before and during treatment, and we are wary of overlapping the two cardiotoxic drugs. Trastuzumab does NOT cause the usual chemo hair loss or marrow suppression — its problem is the heart.
- "-mab" = monoclonal antibody: large protein, given IV/SC, works from outside the cell.
- "-nib" = small oral kinase inhibitor, works inside the cell — the contrast to remember.
- Trastuzumab targets HER2 in HER2-positive breast and gastric cancer.
- Its signature toxicity is cardiotoxicity — usually reversible, worse with anthracyclines.
- Monitor ejection fraction before and during trastuzumab; pertuzumab adds to the blockade.
EGFR: cetuximab and panitumumab — and the RAS gatekeeper
Some tumours are driven by the EGFR receptor. Cetuximab and panitumumab are antibodies against the epidermal growth factor receptor (EGFR). They are used in metastatic colorectal cancer and in head-and-neck cancer. But there is a decisive catch in colorectal cancer: they only help if the tumour's RAS gene is normal ("wild-type"). RAS sits DOWNSTREAM of EGFR in the growth pathway. If RAS is mutated it is stuck permanently ON, so blocking the receptor upstream achieves nothing — the signal fires anyway. This is why RAS testing is mandatory before prescribing: it is a biomarker that predicts whether the drug can possibly work.
The characteristic toxicity of EGFR antibodies is an acneiform (acne-like) rash on the face and upper trunk, because normal skin cells also use EGFR. Counter-intuitively, a more prominent rash often correlates with a BETTER tumour response — it is a visible sign the drug is engaging its target. Hypomagnesaemia (low magnesium) is another class effect worth checking.
VEGF: bevacizumab starves the tumour's blood supply
A solid tumour cannot grow beyond a few millimetres without building its own blood vessels — a process called angiogenesis, driven by a signal the tumour secretes, vascular endothelial growth factor (VEGF). Bevacizumab is unusual: it does not target a cell at all. It binds the free VEGF floating in the blood, mopping it up before it can reach its receptor, so the tumour cannot summon new vessels and is starved of supply. It is used across colorectal, lung (non-squamous), ovarian and glioblastoma, always alongside chemotherapy.
Every side effect of bevacizumab flows from blocking blood-vessel biology: hypertension and proteinuria (the kidney's filter depends on VEGF), bleeding and haemorrhage, impaired wound healing, gastrointestinal perforation, and arterial/venous thrombosis. Because it cripples wound healing, bevacizumab must be stopped several weeks before elective surgery and not restarted until the wound is soundly healed — a classic perioperative trap.
- Cetuximab/panitumumab hit EGFR — only useful in RAS wild-type colorectal cancer.
- Test RAS FIRST; a RAS mutation predicts no benefit. EGFR toxicity = acneiform rash.
- Bevacizumab targets circulating VEGF to block angiogenesis — not a cell target.
- Bevacizumab toxicities: hypertension, proteinuria, bleeding, poor wound healing, GI perforation, thrombosis.
- Stop bevacizumab well before surgery — it impairs wound healing.
Antibody–drug conjugates: the guided missile
Now combine the two ideas. An antibody–drug conjugate (ADC) is three parts fused into one: a targeting antibody, a chemical linker, and a highly potent cytotoxic "warhead" — a chemotherapy so toxic it could never be given on its own. The antibody finds and binds the cancer cell's surface protein; the whole complex is drawn inside; the linker is cleaved and the warhead is released right where it can kill. The point is delivery: the poison travels harmlessly through the blood on the antibody and is only unleashed at the target. It is chemotherapy with a postcode.
On the HER2 antibody: trastuzumab emtansine (T-DM1) links trastuzumab to a microtubule poison; trastuzumab deruxtecan links it to a topoisomerase-I inhibitor and is even more potent, now used in HER2-positive and even HER2-"low" breast cancer. Beyond HER2: sacituzumab govitecan targets Trop-2 and carries an SN-38 (topoisomerase-I) warhead, used in triple-negative breast cancer — a subtype with no HER2 or hormone target to aim at otherwise. Each proves the same concept: antibody targeting plus a chemotherapy payload, delivered together.
Class themes that tie the family together
Four threads run through every drug here. They are large proteins given intravenously, never as a pill. They can trigger infusion reactions — fever, chills, rash, bronchospasm — usually during or just after the drip, managed by slowing the rate and premedicating. They are biomarker-dependent: you must prove the target is present (HER2 status by immunohistochemistry/FISH, RAS status by sequencing) before the drug can be expected to work, which is why targeted oncology and pathology are inseparable. And they carry a big price tag — these are among the most expensive drugs in medicine, which shapes who can access them.
The antibody idea did not start in solid tumours. In blood cancers, rituximab (anti-CD20) transformed the treatment of B-cell lymphomas years earlier — the proof of concept that a monoclonal antibody could reliably target and kill cancer cells. The lymphoma story is told in the Hematology chapter; here we follow the same idea into breast, colon, lung and beyond.
- Prescribing cetuximab or panitumumab in RAS-MUTANT colorectal cancer. There is no benefit — RAS is downstream and locked on. Always test RAS status first.
- Stacking trastuzumab on an anthracycline without cardiac monitoring. The combined cardiotoxicity is far higher — check ejection fraction before and during.
- Forgetting bevacizumab impairs wound healing and causes bleeding — never continue it around elective surgery.
- Treating an ADC as "just an antibody". It carries a cytotoxic payload, so it also causes chemotherapy-type toxicities (marrow suppression, nausea).
Before starting cetuximab in metastatic colorectal cancer, which test most determines whether the drug can work?
- "-mab" = large IV monoclonal antibodies (outside the cell); "-nib" = small oral kinase inhibitors (inside).
- Trastuzumab → HER2 (breast/gastric); watch cardiotoxicity, worse with anthracyclines.
- Cetuximab/panitumumab → EGFR (colorectal only if RAS wild-type, head & neck); acneiform rash.
- Bevacizumab → circulating VEGF to block angiogenesis; hypertension, bleeding, poor wound healing.
- ADCs (T-DM1, trastuzumab deruxtecan, sacituzumab govitecan) = antibody + chemo warhead delivered on target.
- DeVita VT, Lawrence TS, Rosenberg SA. DeVita, Hellman & Rosenberg's Cancer: Principles & Practice of Oncology — Monoclonal antibodies & antibody–drug conjugates.
- Katzung BG. Basic & Clinical Pharmacology — Targeted therapy: monoclonal antibodies (trastuzumab, cetuximab, bevacizumab).
- Brunton LL, et al. Goodman & Gilman's The Pharmacological Basis of Therapeutics — Antineoplastic agents: targeted antibodies.
- Chabner BA, Longo DL. Cancer Chemotherapy, Immunotherapy and Biotherapy — Antibody-based and antibody–drug conjugate therapeutics.
- Whalen K. Lippincott Illustrated Reviews: Pharmacology — Anticancer targeted biologic agents.
