Anticancer Antibiotics & Plant Alkaloids
Pharmacology · Chemotherapy · lean revision notes
Anticancer Antibiotics & Plant Alkaloids
A favourite NEET PG zone where one fact wins one mark: every drug here has a signature dose-limiting toxicity (DLT) and a named antidote/rescue. Master the "drug → mechanism → organ-specific toxicity → antidote" chain and you have covered 90% of the questions.
These two cytotoxic families are grouped because both are natural-product derivatives that act largely in a cell-cycle–dependent (phase-specific or phase-non-specific) manner. Anticancer antibiotics (anthracyclines, bleomycin, actinomycin D, mitomycin) are soil-microbe (chiefly Streptomyces) products; plant alkaloids (vinca alkaloids, taxanes, podophyllotoxins, camptothecins) are derived from periwinkle, yew, mandrake and the happy tree.
Classification
| Class | Prototype drugs | Source | Mechanism (one line) |
|---|---|---|---|
| Anthracyclines | Doxorubicin, Daunorubicin, Epirubicin, Idarubicin | Streptomyces peucetius | Topoisomerase II poison + intercalation + free radicals |
| Non-anthracycline antibiotics | Bleomycin, Actinomycin-D (Dactinomycin), Mitomycin-C, Mithramycin (Plicamycin) | Streptomyces spp. | DNA strand breaks / intercalation / alkylation |
| Vinca alkaloids | Vincristine, Vinblastine, Vinorelbine | Catharanthus roseus (Vinca/periwinkle) | Bind β-tubulin → inhibit microtubule polymerisation (M-phase) |
| Taxanes | Paclitaxel, Docetaxel, Cabazitaxel | Taxus (Pacific yew) | Stabilise microtubules → block depolymerisation (M-phase) |
| Epipodophyllotoxins | Etoposide (VP-16), Teniposide | Podophyllum peltatum (mandrake) | Topoisomerase II poison (G2/late S) |
| Camptothecins | Irinotecan, Topotecan | Camptotheca acuminata | Topoisomerase I inhibitors (S-phase) |
High-yield: Vinca alkaloids depolymerise (inhibit assembly); taxanes stabilise/hyperpolymerise (inhibit disassembly). Both freeze the mitotic spindle in metaphase — opposite mechanism, same net M-phase arrest.
High-yield: Topoisomerase II poisons = anthracyclines + etoposide; Topoisomerase I inhibitors = the "-tecans" (irinotecan, topotecan).
Anthracyclines (Doxorubicin / Adriamycin)
Mechanism — the "triple hit"
- Intercalation between DNA base pairs → distorts helix.
- Topoisomerase II poisoning → stabilises the cleavable complex → permanent double-strand breaks (the dominant cytotoxic mechanism).
- Free-radical generation via quinone redox cycling and an iron-anthracycline complex → lipid peroxidation. The heart, poor in catalase and detoxifying enzymes, is uniquely vulnerable — basis of cardiotoxicity.
Cell-cycle: non-specific (active in all phases, max in S/G2).
Cardiotoxicity — the examiner's darling
| Type | Onset | Mechanism | Reversible? |
|---|---|---|---|
| Acute | Within hours–days | Arrhythmias, transient ECG changes, pericarditis-myocarditis | Usually reversible |
| Chronic / cumulative | Months–year | Dilated cardiomyopathy → CHF via iron-mediated free radicals | Often irreversible |
| Late | Years (esp. childhood survivors) | Progressive systolic dysfunction | Irreversible |
- Cumulative dose limit: keep total doxorubicin < 450–550 mg/m². Risk rises steeply beyond this.
- Monitoring: baseline + serial LVEF (echo or MUGA radionuclide scan); stop if LVEF falls below ~45% or drops >10–15 points. Endomyocardial biopsy is the most sensitive/specific but invasive marker.
- Antidote/cardioprotectant: Dexrazoxane — an EDTA-like iron chelator that prevents the iron–anthracycline free-radical complex.
- Liposomal doxorubicin reduces cardiotoxicity (and causes palmar-plantar erythrodysaesthesia/hand-foot syndrome instead).
High-yield: Dose-limiting toxicity of doxorubicin = cardiotoxicity (cumulative); cardioprotectant = dexrazoxane. This is among the most repeated single facts in pharmacology.
Other adverse effects: myelosuppression (acute DLT in many regimens), severe vesicant extravasation injury, mucositis, alopecia, and a harmless red discolouration of urine. Doxorubicin is a strong radiosensitiser → "radiation recall" dermatitis.
Bleomycin
Mechanism
A mixture of glycopeptides that chelates Fe²⁺, forms an oxygen complex, and generates free radicals causing single- and double-strand DNA breaks. Acts in G2/M. Unusually, it causes minimal myelosuppression — a defining MCQ point.
Toxicity
- Pulmonary fibrosis / interstitial pneumonitis = the dose-limiting toxicity. Dose-related (risk rises >400 units cumulative), age-related (>70 yr), worsened by high FiO₂ (supplemental oxygen — caution in surgery/diving) and chest irradiation. Presents with dry cough, dyspnoea, basal crackles; restrictive pattern with reduced DLCO (earliest change).
- Mucocutaneous: hyperpigmentation, flagellate dermatitis/erythema, hyperkeratosis, Raynaud phenomenon, nail changes.
- Fever/chills, anaphylactoid reaction (test dose advised in lymphoma).
- Inactivated by bleomycin hydrolase, an enzyme deficient in lung and skin — explaining why these two organs bear the brunt of toxicity.
High-yield: Bleomycin → pulmonary fibrosis (DLT) + skin pigmentation; NO myelosuppression. It is a key component of ABVD (Hodgkin lymphoma) and BEP (testicular germ-cell tumour).
Other anticancer antibiotics (quick facts)
- Actinomycin-D (Dactinomycin): intercalates and blocks RNA polymerase → inhibits transcription. Used in Wilms tumour, rhabdomyosarcoma, gestational trophoblastic neoplasia, Ewing sarcoma. Vesicant; radiosensitiser causing radiation recall.
- Mitomycin-C: bioreductive alkylating agent, especially active in hypoxic tumour cells. Toxicities — delayed cumulative myelosuppression and haemolytic-uraemic syndrome (HUS). Topical use in glaucoma surgery/pterygium.
- Mithramycin (Plicamycin): lowers calcium (osteoclast inhibition) — historically for hypercalcaemia/Paget disease.
Vinca Alkaloids (Vincristine, Vinblastine)
Mechanism
Bind to β-tubulin at the vinca domain → prevent tubulin polymerisation → microtubule disassembly → mitotic spindle cannot form → metaphase (M-phase) arrest. Phase-specific.
Why the toxicity differs (high-yield pair)
| Feature | Vincristine | Vinblastine |
|---|---|---|
| Dose-limiting toxicity | Peripheral neuropathy | Myelosuppression (bone marrow) |
| Marrow effect | Relatively spared | Marked |
| Neuropathy | Prominent | Mild |
| Classic regimen | CHOP, MOPP, ALL induction | ABVD |
| Memory hook | Vincristine → neurotoxic | Vinblastine → bone marrow |
- Vincristine neuropathy: axonal, dose-dependent. Earliest sign = loss of ankle jerk and paraesthesiae of fingertips; later foot drop, wrist drop, autonomic neuropathy → constipation/paralytic ileus, urinary retention. SIADH can occur.
- Both are powerful vesicants (extravasation → tissue necrosis; treat with hyaluronidase + warm compress).
High-yield (lethal exam point): Vinca alkaloids are FATAL if given intrathecally (causes ascending myeloencephalopathy and death). They must be given IV only — a never-event medication-safety topic.
Mnemonic for vincristine neuro effects — "VINCRISTINE": Vesicant, Ileus, Neuropathy, Constipation, Reflex loss, Inappropriate ADH, Spares marrow, Tubulin, Intrathecal-NEVER, Necrosis-extravasation, Eyes (ptosis).
Taxanes (Paclitaxel, Docetaxel)
Mechanism
Bind the N-terminal β-tubulin → promote and stabilise microtubule assembly → block depolymerisation → non-functional, frozen spindle → M-phase arrest + abnormal mitosis. Opposite biochemical action to vinca alkaloids.
Toxicity
- Dose-limiting: neutropenia (paclitaxel) and neutropenia with fluid retention/oedema (docetaxel).
- Peripheral sensory neuropathy (stocking-glove).
- Hypersensitivity reactions — largely due to the solvent Cremophor EL (polyoxyethylated castor oil) in paclitaxel; mandates premedication with corticosteroid + H1 + H2 blocker. Nab-paclitaxel (albumin-bound) avoids the solvent and its reactions.
- Bradycardia/heart block, alopecia, arthralgia-myalgia.
High-yield: Paclitaxel = microtubule stabiliser needing steroid/antihistamine premedication for Cremophor-related hypersensitivity. Used in ovarian, breast, NSCLC, Kaposi sarcoma.
Etoposide (VP-16) & Topoisomerase inhibitors
- Etoposide / Teniposide: Topoisomerase II poison → double-strand breaks; acts in late S–G2. DLT = myelosuppression. Hypotension if infused rapidly. Causes secondary acute myeloid leukaemia with characteristic 11q23 (MLL/KMT2A) translocation — a buzzword. Used in testicular cancer (BEP), small-cell lung cancer, lymphoma.
- Irinotecan (Topo I): prodrug → active SN-38. Early diarrhoea (cholinergic → treat with atropine) and late, severe diarrhoea (treat with high-dose loperamide). DLT = diarrhoea + neutropenia. UGT1A1 polymorphism (Gilbert) increases toxicity.
- Topotecan (Topo I): myelosuppression; used in ovarian and small-cell lung cancer.
High-yield: Irinotecan early diarrhoea = cholinergic → atropine; late diarrhoea → loperamide. Etoposide → secondary AML with t(11q23).
Clinical features / when these drugs surface
Questions usually embed the drug inside a regimen + a toxicity vignette:
- ABVD (Adriamycin, Bleomycin, Vinblastine, Dacarbazine) → Hodgkin lymphoma; watch lung (bleomycin) + heart (doxorubicin).
- BEP (Bleomycin, Etoposide, Platinum/cisplatin) → testicular germ-cell tumour.
- CHOP (Cyclophosphamide, Hydroxydaunorubicin = doxorubicin, Oncovin = vincristine, Prednisone) → non-Hodgkin lymphoma.
- MOPP (Mechlorethamine, Oncovin, Procarbazine, Prednisone) → older Hodgkin regimen.
Diagnosis & monitoring of toxicity (investigation of choice)
Cardiotoxicity workflow: Baseline LVEF → serial echo/MUGA scan during therapy → falling LVEF or symptoms → stop drug + start dexrazoxane consideration → endomyocardial biopsy = gold-standard confirmation if doubtful.
Bleomycin lung workflow: Baseline PFT with DLCO → monitor DLCO as the earliest marker (falls before symptoms/X-ray) → dry cough/dyspnoea → HRCT (basal reticular fibrosis) → stop drug + corticosteroids + avoid high FiO₂.
| Drug | Best monitoring test | Earliest abnormality |
|---|---|---|
| Doxorubicin | Serial LVEF (Echo/MUGA) | Fall in LVEF |
| Bleomycin | PFT — DLCO | ↓ DLCO |
| Vincristine | Clinical neuro exam | Loss of ankle jerk |
| Irinotecan | Stool chart / UGT1A1 | Diarrhoea |
Management / drug-of-choice antidotes (must memorise)
| Toxicity | Offending drug | Antidote / rescue |
|---|---|---|
| Cardiotoxicity (free radical) | Doxorubicin | Dexrazoxane (iron chelator) |
| Vesicant extravasation | Anthracyclines, vinca | Dexrazoxane (anthracycline) / hyaluronidase + warmth (vinca); cold compress for anthracyclines |
| Cholinergic (early) diarrhoea | Irinotecan | Atropine |
| Late diarrhoea | Irinotecan | High-dose loperamide |
| Hypersensitivity | Paclitaxel | Premed: steroid + H1 + H2 blocker |
| Haemorrhagic cystitis (companion fact) | Cyclophosphamide/ifosfamide | MESNA + hydration |
High-yield: Anthracycline extravasation → cold compress; vinca extravasation → warm compress + hyaluronidase. Reversed answers are a classic trap.
Complications (organ-specific summary)
| Drug | Dose-limiting toxicity | Signature/unique toxicity |
|---|---|---|
| Doxorubicin | Myelosuppression (acute) | Cumulative cardiomyopathy, red urine, radiation recall |
| Bleomycin | Pulmonary fibrosis | Skin pigmentation/flagellate dermatitis; no marrow suppression |
| Vincristine | Peripheral neuropathy | Ileus/constipation, SIADH, intrathecal-fatal |
| Vinblastine | Myelosuppression | Mucositis |
| Paclitaxel | Neutropenia | Cremophor hypersensitivity, neuropathy, bradycardia |
| Etoposide | Myelosuppression | Secondary AML t(11q23), hypotension on rapid infusion |
| Irinotecan | Diarrhoea + neutropenia | Cholinergic early diarrhoea |
| Mitomycin-C | Cumulative myelosuppression | HUS, active in hypoxic cells |
| Actinomycin-D | Myelosuppression | Radiation recall, used in paediatric tumours |
Key differentials (mechanism contrasts to disambiguate MCQs)
- Microtubule assembly: Vinca inhibits vs Taxane stabilises — both arrest mitosis but biochemically opposite. (Colchicine and griseofulvin also inhibit assembly — appear as distractors.)
- Topoisomerase target: -tecans → Topo I; anthracyclines/etoposide → Topo II.
- Causes pulmonary fibrosis (chemo): Bleomycin, busulfan, methotrexate, carmustine (BCNU), cyclophosphamide — bleomycin is the classic answer.
- Cardiotoxic chemo: Anthracyclines (cumulative CHF), trastuzumab (reversible, no cumulative limit), 5-FU/capecitabine (coronary vasospasm), cyclophosphamide (high-dose haemorrhagic myopericarditis).
- No/minimal myelosuppression chemo: Bleomycin, vincristine, asparaginase — the standard MCQ trio.
Recently asked / exam angle
- "Dose-limiting toxicity of doxorubicin?" → Cardiotoxicity (cumulative cardiomyopathy); protected by dexrazoxane.
- "Anticancer drug causing pulmonary fibrosis with no marrow suppression?" → Bleomycin; monitor with DLCO; avoid high O₂.
- "Which drug must NEVER be given intrathecally?" → Vincristine (fatal).
- "Vincristine vs vinblastine — which is neurotoxic / which is myelosuppressive?" → Vincristine = neuro; vinblastine = marrow.
- "Mechanism of paclitaxel?" → Stabilises microtubules, prevents depolymerisation.
- "Chemo drug causing secondary AML with 11q23/MLL translocation?" → Etoposide.
- "Early cholinergic diarrhoea drug + antidote?" → Irinotecan + atropine.
- Image/assertion-reason on flagellate dermatitis → Bleomycin.
- "Topoisomerase I inhibitor among the following?" → Topotecan/Irinotecan.
- "Cardioprotective iron chelator in oncology?" → Dexrazoxane.
Rapid revision
- Doxorubicin — Topo II poison + free radicals → cumulative cardiomyopathy; limit <450–550 mg/m²; antidote dexrazoxane; red urine.
- Bleomycin — DLT pulmonary fibrosis, skin pigmentation, flagellate dermatitis; no myelosuppression; monitor DLCO; avoid high FiO₂.
- Vincristine — inhibits microtubule assembly; DLT peripheral neuropathy; spares marrow; intrathecal = death; causes ileus/SIADH.
- Vinblastine — same mechanism but DLT = myelosuppression (Vinblastine → bone marrow).
- Paclitaxel — stabilises microtubules; DLT neutropenia; Cremophor hypersensitivity → steroid + antihistamine premed.
- Etoposide — Topo II poison; DLT myelosuppression; secondary AML t(11q23); hypotension on rapid infusion.
- Irinotecan/Topotecan — Topo I inhibitors; irinotecan → early diarrhoea (atropine), late diarrhoea (loperamide); UGT1A1.
- Actinomycin-D — blocks RNA polymerase; for Wilms, rhabdomyosarcoma, GTN, Ewing; radiation recall.
- Mitomycin-C — bioreductive alkylator active in hypoxia; causes HUS.
- Extravasation: anthracycline → cold compress; vinca → warm + hyaluronidase. Both are vesicants.
- Topo II poisons = anthracyclines + etoposide; Topo I = "-tecans". Vinca depolymerises, taxane stabilises.
- No-marrow-suppression trio: bleomycin, vincristine, L-asparaginase.