Anthracyclines and Topoisomerase Inhibitors: DNA-Cutting Drugs and the Heart
These drugs work by cutting DNA — jamming the enzymes that untangle the double helix and littering the genome with breaks the cancer cell cannot survive. They are among our most powerful and most-used cytotoxics. But the same knife that cures a lymphoma can, dose by dose, weaken a heart or scar a lung. This chapter is about the double edge: how anthracyclines and topoisomerase poisons kill tumours, and the signature late toxicities you must count from the very first dose.
She was nineteen when the Hodgkin lymphoma was found, and twenty-one when the oncologist told her she was cured. The regimen that did it leaned on doxorubicin (the 'A' in ABVD). Years pass. She is well, working, planning a family — and still, every so often, she lies on an echocardiography table while a probe watches her heart squeeze. Not because the cancer came back. Because the drug that saved her carries a debt written in a single number: her lifetime cumulative dose. Below a threshold the heart usually forgives; above it, a slow, sometimes irreversible heart failure can surface years later. The cure and its late price are captured in that one running total — which is why someone wrote it down every single cycle.
How anthracyclines kill: three attacks on DNA
An anthracycline is not a subtle drug — it is a chemical wrecking crew. The anthracyclines (doxorubicin, daunorubicin, epirubicin, idarubicin) attack the tumour cell on three fronts at once. First, they intercalate — slide flat between the base pairs of the DNA double helix and distort it. Second, and most important, they poison topoisomerase II: the enzyme that cuts both strands of DNA to relieve tension during replication, then reseals them. The drug freezes it mid-cut, so the breaks are never repaired. Third, their iron-complexed quinone structure spins off reactive oxygen species (free radicals) that shred DNA and membranes. A cell hit this hard triggers apoptosis. This same free-radical talent, unfortunately, is what the heart pays for.
Because they work at every phase of the cycle (broadly cell-cycle non-specific), anthracyclines are backbone agents across an enormous range of tumours: breast cancer, the lymphomas (Hodgkin and non-Hodgkin), the sarcomas, and the acute leukaemias. Wherever you see the letters 'A' or 'H' in a regimen — the A of ABVD, the H (hydroxydaunorubicin = doxorubicin) of CHOP — an anthracycline is usually doing the heavy lifting.
The signature toxicity: cumulative cardiotoxicity
Everything else these drugs do is negotiable. The heart is not. The defining, exam-critical toxicity of the anthracyclines is dose-dependent, cumulative cardiotoxicity — a dilated cardiomyopathy that can progress to congestive heart failure. It is driven by free-radical injury to cardiac muscle, which is poorly equipped to mop up oxidative stress. Crucially, the risk is tied not to any single dose but to the total lifetime cumulative dose (for doxorubicin, risk climbs steeply beyond roughly 450–550 mg/m²). The early acute effects (transient arrhythmias) are usually benign; the feared form is the late, chronic cardiomyopathy that can appear months to years after treatment and may be irreversible. This is why the cumulative dose is tracked and written down at every cycle, and why cardiac function (ejection fraction) is checked before and during therapy.
Two shields exist. Dexrazoxane, an iron-chelator, mops up the iron that anthracyclines use to generate free radicals and can protect the heart in patients heading toward high cumulative doses. And liposomal doxorubicin — the drug wrapped in a fat bubble — concentrates in tumour tissue and spares the myocardium, meaningfully reducing cardiotoxicity. Neither abolishes the risk; both buy dose. Note too that concurrent trastuzumab (an anti-HER2 antibody) sharply amplifies anthracycline cardiotoxicity — the two are generally not given together.
In HER2-positive breast cancer, an anthracycline (doxorubicin) and trastuzumab are both cardiotoxic. Rather than stack them, oncologists sequence them and monitor ejection fraction throughout — one reason regimens are increasingly built to give the anthracycline first and the antibody after, or to choose anthracycline-free protocols in higher-cardiac-risk patients. The cross-link is straight to the Cardiovascular section: the same left-ventricular function you learned to protect in heart failure is what oncology is guarding here.
The rest of the anthracycline toxicity profile rounds out the picture. They turn the urine red-orange — alarming but entirely harmless, and worth warning the patient about. They are potent vesicants: extravasation from the vein into surrounding tissue causes severe local necrosis, so they are given with great care into a well-running line. And like most cytotoxics they cause myelosuppression (a drop in white cells, red cells and platelets) plus nausea, vomiting and mucositis. But the red urine washes out; the marrow recovers. It is the heart that keeps the ledger.
- Anthracyclines: doxorubicin, daunorubicin, epirubicin, idarubicin — intercalate DNA, poison topoisomerase II, and generate free radicals.
- Signature toxicity = dose-dependent, cumulative cardiotoxicity (heart failure); track the lifetime cumulative dose.
- Monitor cardiac function (ejection fraction) before and during therapy.
- Dexrazoxane (iron-chelator) protects the heart; liposomal doxorubicin reduces cardiotoxicity.
- Also: red urine (harmless), vesicant on extravasation, and myelosuppression.
- Very widely used: breast, lymphoma, sarcoma, leukaemia.
Topoisomerase II inhibitors: etoposide and the second cancer
Etoposide targets the same enzyme the anthracyclines poison but without the intercalation or the iron chemistry. It stabilizes the complex where topoisomerase II has cut both DNA strands, blocking the reseal and leaving permanent double-strand breaks — lethal to a dividing cell. Etoposide is a workhorse in testicular cancer, small-cell lung cancer, and the lymphomas and leukaemias. Its dose-limiting toxicity is myelosuppression.
Etoposide (and, to a degree, the anthracyclines) carries a characteristic long-term hazard: a treatment-related, secondary acute myeloid leukaemia (AML). The double-strand breaks these drugs cause can produce specific chromosomal translocations (classically involving the MLL/KMT2A gene at 11q23) that seed a new leukaemia years down the line. It is the sobering cross-link to the Hematology section: a drug used to cure one blood cancer can, rarely, plant another. This is why cumulative exposure and long-term follow-up matter even in survivors declared cured.
Topoisomerase I inhibitors: irinotecan and the two diarrhoeas
One enzyme down, one to go — and a drug with two entirely different diarrhoeas. The camptothecins — irinotecan and topotecan — poison topoisomerase I, the enzyme that nicks a single DNA strand to relieve supercoiling. The drug traps the enzyme on the DNA, and when the replication fork collides with the trapped complex, a lethal break results. Irinotecan is a mainstay of colorectal cancer (the cross-link to the Colorectal chapter); topotecan is used in ovarian and small-cell lung cancer.
Irinotecan causes TWO distinct diarrhoeas, and confusing them is a classic error. The EARLY diarrhoea (within hours, often during infusion) is a cholinergic effect — irinotecan inhibits acetylcholinesterase, producing a cramping, sweating, watery-stool syndrome that is treated with atropine (an anticholinergic). The DELAYED diarrhoea (days later) is mucosal toxicity from the active metabolite SN-38 and is treated with loperamide, often at high dose, plus hydration. Early = atropine; late = loperamide. Two mechanisms, two antidotes.
There is also a pharmacogenetic twist worth knowing. SN-38 is detoxified by the liver enzyme UGT1A1 (by glucuronidation). Patients with the UGT1A1*28 polymorphism (the same enzyme variant behind Gilbert syndrome) clear SN-38 slowly and are at far higher risk of severe neutropenia and diarrhoea — so some are dose-reduced based on genotype. That is a clean cross-link to the Pharmacodynamics chapter on individual variation and pharmacogenetics: a single inherited enzyme difference reshaping a drug's whole safety margin.
- Etoposide poisons topoisomerase II (double-strand breaks); dose-limiting myelosuppression; risk of secondary treatment-related AML.
- Irinotecan and topotecan poison topoisomerase I (single-strand nicks turned lethal).
- Irinotecan — early cholinergic diarrhoea → atropine; delayed diarrhoea (SN-38) → loperamide.
- UGT1A1*28 polymorphism slows SN-38 clearance → severe irinotecan toxicity (genotype-guided dosing).
- Irinotecan → colorectal cancer; topotecan → ovarian and small-cell lung cancer.
Bleomycin: the antibiotic that scars the lung
Every cytotoxic has a price. Bleomycin's is paid by the lungs, not the marrow. Bleomycin is an antitumour antibiotic that kills by a different route: it binds DNA and iron, generating free radicals that cause single- and double-strand breaks. Its two most useful features are linked. First, it causes minimal myelosuppression — a rarity among cytotoxics — which lets it be combined with marrow-suppressing partners without stacking the bone-marrow hit. That is exactly why it earns the 'B' in ABVD for Hodgkin lymphoma and features in regimens for testicular (germ-cell) cancer. The cross-links run to the Lymphoma (Hematology) and Genitourinary chapters.
The second feature is the catch. Bleomycin's dose-limiting and signature toxicity is pulmonary fibrosis — a progressive, dose-related scarring of the lungs that presents as a dry cough and worsening breathlessness and can be fatal. Like the anthracycline's heart, the lung's risk climbs with cumulative dose (and with age and renal impairment). A dangerous, well-documented trigger is high inspired-oxygen exposure: patients who have received bleomycin can flare their lung injury when given high-concentration oxygen — which is why the anaesthetist must know about prior bleomycin before surgery and keep the FiO₂ as low as safely possible.
A neat way to remember the two 'organ-specific' antibiotics: the Anthracycline is the heart-breaker (cardiomyopathy), and Bleomycin blows the lungs (pulmonary fibrosis, low marrow toxicity). Both are cumulative-dose diseases you prevent by counting, not by treating after the fact.
- Bleomycin — antitumour antibiotic; DNA strand breaks via free radicals.
- Signature toxicity = dose-related pulmonary fibrosis; minimal myelosuppression.
- Low marrow toxicity is why it fits ABVD (Hodgkin) and testicular (germ-cell) regimens.
- High-concentration oxygen can trigger/worsen bleomycin lung injury — warn the anaesthetist.
- Exceeding the cumulative anthracycline dose — or not tracking it at all. Cardiotoxicity is a lifetime-total disease; the running number must be recorded and respected every cycle.
- Forgetting bleomycin's pulmonary fibrosis — and its oxygen caution. Always flag prior bleomycin before anaesthesia and keep inspired oxygen as low as safe.
- Mismanaging irinotecan diarrhoea. Early (cholinergic) diarrhoea needs atropine; delayed diarrhoea needs loperamide — using the wrong one wastes the window.
- Treating red urine after doxorubicin as bleeding. It is a harmless dye effect — reassure the patient rather than investigate.
A 20-year-old cured of Hodgkin lymphoma with ABVD is followed for years with periodic echocardiograms. Which drug in her regimen justifies this cardiac surveillance, and by what mechanism?
- Anthracyclines (doxorubicin & family) intercalate DNA, poison topo II, and make free radicals — signature toxicity is dose-cumulative cardiotoxicity; track the total, monitor EF, consider dexrazoxane/liposomal forms.
- Etoposide poisons topo II and carries a risk of secondary treatment-related AML.
- Irinotecan/topotecan poison topo I; irinotecan = early diarrhoea (atropine) + delayed diarrhoea (loperamide), with UGT1A1 pharmacogenetics.
- Bleomycin causes DNA breaks with minimal myelosuppression (hence ABVD/testicular) but signature pulmonary fibrosis — beware high oxygen.
- Katzung BG. Basic & Clinical Pharmacology — Cancer Chemotherapy: anthracyclines, topoisomerase inhibitors & antitumour antibiotics.
- Brunton LL, et al. Goodman & Gilman's The Pharmacological Basis of Therapeutics — Antineoplastic agents: natural products, topoisomerase inhibitors & cytotoxic antibiotics.
- Chabner BA, Longo DL. Cancer Chemotherapy, Immunotherapy and Biotherapy — Anthracyclines, etoposide, camptothecins & bleomycin.
- Whalen K. Lippincott Illustrated Reviews: Pharmacology — Antineoplastic drugs: doxorubicin cardiotoxicity, irinotecan & bleomycin.
- DeVita, Hellman, and Rosenberg's Cancer: Principles & Practice of Oncology — Pharmacology of topoisomerase-interactive agents & anthracycline cardiotoxicity.
