Blood Cancer Drugs: How the Classes Work, From Chemo to Targeted Therapy
For a century we fought blood cancer with poisons — drugs that killed every dividing cell, cancerous or not, and made the patient nearly as sick as the disease. Then came a different idea: instead of poisoning everything, aim at the ONE broken molecule that makes the cancer a cancer. This chapter maps the whole armamentarium — the cytotoxic families and why they cause the toxicities they do, and the targeted and immune agents that changed the story — using one leukaemia that went from a death sentence to a cure as the emblem of the shift.
Picture two clinics a generation apart. In the first, a young woman with acute promyelocytic leukaemia (APL) is bleeding uncontrollably — her disease trips off a catastrophic clotting cascade, and the blunt cytotoxic drugs we throw at her poison her marrow, her gut, her hair, and often kill her before the leukaemia does. In the second clinic, another woman with the same disease swallows a vitamin-A derivative — all-trans retinoic acid (ATRA) — and a splash of arsenic. No poison, no bald head. Her leukaemic cells, instead of being slaughtered, are simply told to grow up and finish maturing. She is cured. Between those two rooms lies the entire story of this chapter: the move from carpet-bombing every dividing cell to a precision strike on the one broken molecule.
The old workhorses: cytotoxic chemotherapy families
Cancer is, at its core, uncontrolled cell division. So the oldest strategy is to sabotage the machinery of division itself. Cytotoxic chemotherapy is not one drug but several families, each hitting a different step of the cell cycle. Alkylating agents (e.g., cyclophosphamide) cross-link DNA so the strands cannot separate to copy. Antimetabolites masquerade as the raw materials of DNA/RNA and jam the assembly line — methotrexate blocks folate metabolism, cytarabine (Ara-C) is a fake nucleoside that poisons DNA synthesis, and 6-mercaptopurine sabotages purine building. Anthracyclines and topoisomerase poisons (doxorubicin, etoposide) snap or tangle DNA as it unwinds. Vinca alkaloids (vincristine) freeze the microtubule spindle so chromosomes cannot be pulled apart in mitosis.
Chemotherapy is almost never a single drug — resistance emerges fast, so we combine agents with different mechanisms and non-overlapping toxicities, named by acronym. The classic lymphoma backbone CHOP = Cyclophosphamide (alkylator) + Hydroxydaunorubicin (doxorubicin, anthracycline) + Oncovin (vincristine, vinca) + Prednisolone (steroid). Add the antibody rituximab and it becomes R-CHOP, the standard for diffuse large B-cell lymphoma. Notice how the acronym is literally the map of drug classes.
Corticosteroids (prednisolone, dexamethasone) are not just anti-nausea supportive drugs here — they are directly cytotoxic to LYMPHOID cells, triggering apoptosis in lymphocytes. That is why steroids are a genuine chemotherapy component in virtually every lymphoma, lymphoblastic leukaemia (ALL) and myeloma regimen, not an afterthought. In lymphoid malignancy, the humble steroid is a killing drug.
Why chemo makes patients so sick: the collateral damage
Here is the tragedy built into the design. Cytotoxic drugs target fast-dividing cells — but cancer holds no monopoly on dividing fast. The body's own rapidly renewing tissues are hit just as hard, and that predicts nearly every classic side effect. Bone marrow → myelosuppression: low neutrophils (life-threatening infection), low platelets (bleeding), low red cells (anaemia). Gut lining → mucositis and diarrhoea. Hair follicles → alopecia. Chemoreceptor trigger zone and gut → nausea and vomiting. Germ cells → infertility. The toxicity list is not random — it is simply the roster of the body's fastest-dividing tissues.
Some agents carry a signature toxicity beyond the general pattern. Doxorubicin (and the anthracyclines) cause a dose-dependent, potentially irreversible cardiomyopathy — heart failure — through free-radical injury to cardiac muscle, which is why a patient's LIFETIME cumulative anthracycline dose is tracked. Vincristine's hallmark is peripheral neuropathy (numb fingers, foot drop, constipation) because microtubules are essential to nerve axons; notably it spares the marrow relatively. Cyclophosphamide can cause haemorrhagic cystitis (bladder toxicity from its metabolite acrolein), prevented with hydration and mesna. Each drug's special toxicity is worth memorising as its 'signature'.
Tumour lysis syndrome: killing too well, too fast
There is a danger unique to blood cancers: sometimes the drugs work TOO well, too fast. When a large, rapidly proliferating tumour (bulky lymphoma, high-count leukaemia) is killed en masse, the dying cells spill their contents into the blood — a flood of potassium, phosphate, and nucleic acids that break down to uric acid. The result is tumour lysis syndrome (TLS): hyperkalaemia (arrhythmias), hyperphosphataemia, hypocalcaemia, and hyperuricaemia that can crystallise in the kidneys and cause acute kidney injury. Paradoxically, it is a complication of successful treatment.
Before treating bulky or high-turnover disease we give prophylaxis. Aggressive IV hydration flushes the kidneys. Allopurinol blocks xanthine oxidase, cutting NEW uric acid formation (start before chemo). For high-risk cases, rasburicase (recombinant urate oxidase) goes further — it enzymatically digests uric acid already present into soluble allantoin, dropping urate fast. (Rasburicase is contraindicated in G6PD deficiency, as its by-product hydrogen peroxide can trigger haemolysis.) These urate drugs link straight to the gout/urate chapter in the endocrine section.
- Cytotoxic families: alkylators, antimetabolites, anthracyclines/topoisomerase poisons, vinca alkaloids, plus lymphotoxic steroids.
- Drugs are combined (e.g., CHOP) for non-overlapping mechanisms and toxicities.
- Toxicity targets the body's own fast-dividing tissues: marrow, gut, hair, gonads.
- Signature toxicities: doxorubicin → heart, vincristine → nerves, cyclophosphamide → bladder.
- Bulky/high-turnover disease → tumour lysis syndrome; prevent with hydration + allopurinol/rasburicase.
The shift: targeted and immune therapy
Then the question changed from 'how do we kill dividing cells?' to 'what exactly is broken in THIS cancer?' As molecular biology exposed the specific lesions driving each malignancy, drugs could be aimed at them. Tyrosine kinase inhibitors (TKIs) block an overactive signalling enzyme — imatinib silences the BCR-ABL fusion kinase that drives chronic myeloid leukaemia (CML), the field's proof-of-concept (detailed in the CML chapter). Monoclonal antibodies flag a surface marker for immune destruction — rituximab binds CD20 on B cells, transforming B-cell lymphoma outcomes (detailed in the Lymphoma chapter). In multiple myeloma, proteasome inhibitors (bortezomib) and immunomodulatory drugs (lenalidomide) exploit the plasma cell's own biology (detailed in the Myeloma chapter). And the newest immune approaches — CAR-T cells, engineered from the patient's own T cells to hunt the tumour, and checkpoint inhibitors that release the brakes on anti-tumour immunity — round out the map.
Targeted does not mean toxicity-free — it means DIFFERENT toxicity. TKIs cause fluid retention, cytopenias and rashes; rituximab causes infusion reactions and reactivates hepatitis B; CAR-T can trigger cytokine release syndrome (a dangerous systemic inflammatory storm) and neurotoxicity; checkpoint inhibitors unleash autoimmune 'itis' anywhere (colitis, hepatitis, thyroiditis, pneumonitis). The lesson: every gain in precision brings its own new hazard profile, not zero risk.
The emblem of the revolution: APL, ATRA and arsenic
No story captures the shift better than acute promyelocytic leukaemia (APL). Its cells carry the PML-RARα fusion, which freezes immature promyelocytes so they cannot mature — and they were once among the deadliest of leukaemias, killing by haemorrhage within days. The breakthrough was not a stronger poison but differentiation therapy: all-trans retinoic acid (ATRA), a vitamin-A derivative, unlocks the maturation block and coaxes the leukaemic cells to grow up into normal neutrophils and die naturally. Add arsenic trioxide, which degrades the same fusion protein, and APL — once a death sentence — is now largely curable, often without traditional chemotherapy at all. It is the cleanest demonstration that understanding the molecular lesion can turn a killer into a cure.
The elegance has a catch. As ATRA (or arsenic) drives a mass of promyelocytes to mature at once, the newly differentiating cells can flood the circulation and release cytokines — differentiation syndrome (formerly 'retinoic acid syndrome'): fever, weight gain, breathlessness, pulmonary infiltrates and hypotension. It can be fatal if missed, but responds to prompt corticosteroids (dexamethasone) and holding the drug. So even the gentlest 'non-poison' therapy carries a hazard you must anticipate — the recurring theme of this chapter.
- Targeted therapy aims at the cancer's specific molecular lesion, not all dividing cells.
- TKIs (imatinib/CML), monoclonal antibodies (rituximab/CD20), proteasome & immunomodulators (myeloma) are the major targeted classes.
- Immune approaches: CAR-T cells and checkpoint inhibitors harness the patient's own immunity.
- APL is the emblem: ATRA + arsenic trioxide (differentiation therapy) made a lethal leukaemia largely curable.
- Targeted ≠ safe: each class has its own toxicities (infusion reactions, cytokine release, autoimmune effects, differentiation syndrome).
- Assuming chemotherapy is selective for cancer. It hits ALL fast-dividing cells — hence the marrow, gut, hair and fertility toxicity.
- Forgetting tumour lysis prophylaxis before treating bulky or rapidly proliferating disease. Hydrate and give allopurinol/rasburicase BEFORE, not after.
- Thinking targeted therapy has 'no toxicity'. Each has its own — infusion reactions, cytokine release syndrome, autoimmune inflammation, differentiation syndrome.
- Treating steroids as merely supportive in lymphoid cancers. They are directly cytotoxic to lymphocytes and a core part of the regimen.
A patient with bulky, rapidly proliferating lymphoma is about to start chemotherapy. Which measure most directly prevents tumour lysis syndrome?
- Cytotoxic chemo (alkylators, antimetabolites, anthracyclines, vinca, lymphotoxic steroids) sabotages cell division and is given in combination regimens like CHOP.
- Its toxicity is collateral damage to the body's own fast-dividing tissues — marrow, gut, hair, gonads — plus tumour lysis when big tumours die fast.
- Targeted/immune therapy (TKIs, monoclonal antibodies, proteasome inhibitors, CAR-T, checkpoint inhibitors) aims at the cancer's specific lesion — with its own new toxicities.
- APL treated with ATRA + arsenic trioxide is the emblem: a once-lethal leukaemia made largely curable by differentiation therapy (mind differentiation syndrome).
- Katzung BG. Basic & Clinical Pharmacology — Cancer Chemotherapy: cytotoxic drug classes, targeted agents & toxicities.
- Hoffbrand AV, et al. Hoffbrand's Essential Haematology — Management of haematological malignancy; chemotherapy, targeted therapy & supportive care.
- Chabner BA, Longo DL. Cancer Chemotherapy, Immunotherapy and Biotherapy: Principles and Practice.
- Brunton LL, et al. Goodman & Gilman's The Pharmacological Basis of Therapeutics — Antineoplastic agents.
- Hoffbrand AV, et al. Hoffbrand's Essential Haematology — Acute promyelocytic leukaemia: ATRA, arsenic trioxide & differentiation syndrome.
