Alkylating Agents

Alkylating agents are among the oldest and most heavily tested classes of cytotoxic anticancer drugs. They act by transferring alkyl groups to nucleophilic sites (especially N-7 of guanine) in DNA, producing cross-links that block replication. For NEET PG, the examiner rarely asks mechanism in isolation — the high-yield gold is the organ-specific toxicity of individual drugs and their antidotes/rescue agents.

Definition and place in chemotherapy

An alkylating agent is a cell-cycle non-specific (CCNS) cytotoxic that covalently binds electrophilic alkyl groups to DNA bases. Because they damage DNA regardless of cell-cycle phase, they kill both dividing and resting cells, though dividing cells are more susceptible. Their efficacy follows first-order (log) kill kinetics and a dose-dependent (often linear) cytotoxic curve, which is why high-dose alkylator regimens are used in transplant conditioning.

High-yield: Alkylating agents are the prototype cell-cycle non-specific drugs. The reactive intermediate alkylates N-7 of guanine, causing intra- and inter-strand DNA cross-links.

Classification

Alkylating agents are best grouped by chemical family. Memorise this table — sub-classification questions are common.

Sub-class	Key drugs	Signature point
Nitrogen mustards	Cyclophosphamide, Ifosfamide, Chlorambucil, Melphalan, Mechlorethamine, Bendamustine	Cyclophosphamide is a prodrug activated in liver
Nitrosoureas	Carmustine (BCNU), Lomustine (CCNU), Streptozocin	Lipophilic → cross blood–brain barrier, used in brain tumours
Alkyl sulfonates	Busulfan	Selective for myeloid series; pulmonary fibrosis
Ethylenimines / Aziridines	Thiotepa, Mitomycin C	Thiotepa crosses BBB; used in bladder instillation
Triazenes / hydrazines	Dacarbazine, Temozolomide, Procarbazine	Temozolomide for glioblastoma; Procarbazine → MAO inhibition, disulfiram-like
Platinum compounds*	Cisplatin, Carboplatin, Oxaliplatin	"Alkylating-like" — form DNA adducts, not true alkylators

*Platinum compounds are classically taught alongside alkylators because they also cross-link DNA, but strictly they are alkylating-like agents (no alkyl group transferred).

Mechanism and pharmacology of key agents

Cyclophosphamide is the single most tested drug. It is an inactive prodrug converted by hepatic CYP450 (CYP2B6) to 4-hydroxycyclophosphamide → aldophosphamide, which splits into the active phosphoramide mustard (cytotoxic) and acrolein (toxic to urothelium → haemorrhagic cystitis).

Flow of cyclophosphamide activation:

Cyclophosphamide → (CYP450, liver) → 4-hydroxycyclophosphamide ⇌ aldophosphamide → phosphoramide mustard (kills tumour) + acrolein (damages bladder)

High-yield: Acrolein causes haemorrhagic cystitis. Mesna (2-mercaptoethane sulfonate Na) is the uroprotectant — it binds and inactivates acrolein in the urine.

Ifosfamide is structurally similar; it causes more haemorrhagic cystitis (always co-administered with mesna) and a characteristic neurotoxicity / encephalopathy due to the metabolite chloroacetaldehyde. Ifosfamide encephalopathy is treated with methylene blue. Ifosfamide is also more nephrotoxic, causing a Fanconi-like proximal tubulopathy especially in children.

Busulfan is selective for the myeloid lineage and was historically used in chronic myeloid leukaemia (CML) and now mainly in bone-marrow transplant conditioning. Its toxicities are classic exam fodder.

Cisplatin enters cells, loses chloride in the low intracellular chloride environment, and forms a reactive aquated species that cross-links DNA (mainly intra-strand guanine adducts). Resistance involves increased glutathione/metallothionein and enhanced DNA repair (ERCC1).

Organ toxicities — the core exam content

This table is the heart of the topic; nearly every alkylator question maps to one row.

Drug	Signature toxicity	Antidote / prevention
Cyclophosphamide	Haemorrhagic cystitis (acrolein); SIADH at high dose	Mesna + vigorous hydration
Ifosfamide	Haemorrhagic cystitis (worse); encephalopathy; Fanconi syndrome	Mesna; methylene blue for encephalopathy
Cisplatin	Nephrotoxicity, ototoxicity (high-frequency hearing loss), severe vomiting, peripheral neuropathy, hypomagnesaemia	Aggressive hydration + mannitol/forced diuresis; amifostine; antiemetics
Carboplatin	Myelosuppression (thrombocytopenia) — less nephro/oto/emetogenic	Dose by Calvert formula (AUC based)
Oxaliplatin	Cold-induced peripheral neuropathy (laryngopharyngeal dysaesthesia)	Avoid cold; Ca/Mg infusions
Busulfan	Pulmonary fibrosis ("busulfan lung"); hyperpigmentation; veno-occlusive disease; seizures	Prophylactic phenytoin for seizures
Carmustine/Lomustine (nitrosoureas)	Delayed, cumulative, prolonged myelosuppression (4–6 wks); pulmonary fibrosis	Monitor counts; cumulative dose limit
Chlorambucil	Slow myelosuppression; seizures	Used in CLL
Procarbazine	Disulfiram-like reaction; MAO inhibition (tyramine crisis); secondary leukaemia	Avoid alcohol & tyramine
Mechlorethamine	Severe vesicant — tissue necrosis on extravasation	Sodium thiosulfate locally
Streptozocin	Diabetogenic (β-cell toxic), nephrotoxic	Used for insulinoma

High-yield: Cisplatin = nephrotoxic + ototoxic + most emetogenic + neurotoxic. Carboplatin = myelosuppressive (thrombocytopenia), spares the kidney and ear. This contrast is a perennial favourite.

High-yield: Busulfan → pulmonary fibrosis ("busulfan lung") and skin hyperpigmentation. Bleomycin also causes pulmonary fibrosis — but bleomycin is an antibiotic, not an alkylator; don't confuse them.

High-yield: Alkylating agents (especially melphalan, procarbazine, nitrosoureas, cyclophosphamide) carry the highest risk of secondary acute myeloid leukaemia (t-AML), typically 5–7 years later with deletions of chromosomes 5 and 7. This is distinct from topoisomerase-II inhibitor (etoposide) t-AML which is earlier (2–3 yr) with 11q23/MLL translocation.

Cisplatin toxicity management — stepwise approach

1. Pre-hydration with normal saline (e.g. 1–2 L) to maintain high urine output and dilute the drug in tubules.
2. Add mannitol or forced diuresis to reduce tubular contact time.
3. Replace magnesium and potassium (cisplatin causes renal Mg wasting → hypomagnesaemia, hypokalaemia, hypocalcaemia).
4. Give amifostine (a thiol cytoprotectant, free-radical scavenger) to reduce nephro- and neurotoxicity.
5. Audiometry baseline + monitoring — ototoxicity is high-frequency, bilateral, often irreversible.
6. Aggressive 5-HT3 antagonist + NK1 antagonist (aprepitant) + dexamethasone antiemetic regimen — cisplatin is highly emetogenic.

High-yield: Amifostine is the organ-protective agent for cisplatin nephrotoxicity and for radiation-induced xerostomia. Its main side effect is hypotension.

Clinical uses (drug-of-choice flavour)

• Cyclophosphamide: Lymphomas, breast cancer, also non-malignant — lupus nephritis, granulomatosis with polyangiitis (Wegener's), nephrotic syndrome. A common "immunosuppressant" MCQ.
• Chlorambucil: Chronic lymphocytic leukaemia (CLL), least toxic mustard.
• Melphalan: Multiple myeloma; high-dose melphalan is the conditioning agent for autologous stem-cell transplant in myeloma.
• Busulfan: CML (historical) and transplant conditioning.
• Nitrosoureas (carmustine, lomustine), Temozolomide: Brain tumours / glioblastoma because they cross the BBB.
• Cisplatin: Testicular (BEP regimen), ovarian, bladder, lung, head & neck cancers.
• Carboplatin: Ovarian and lung cancer when renal sparing is needed.
• Oxaliplatin: Colorectal cancer (FOLFOX regimen).
• Streptozocin: Pancreatic islet cell tumours (insulinoma).
• Dacarbazine: Hodgkin lymphoma (ABVD regimen) and melanoma.

Mnemonics and eponyms

• "Cyclophosphamide → Cystitis → Cured by mesna; Acrolein is the Assailant."
• Platinum nephro/oto vs marrow: "CisPlatin Pesters Pee & Perception (kidney & ear); Carboplatin Cuts Cells (counts)."
• Nitrosoureas cross the Brain — BCNU, loMustine, all lipophilic.
• Secondary leukaemia: "Alkylators give A-AML" — Alkylators → AML, late, chromosome 5/7.
• Calvert formula (eponymous): Carboplatin dose (mg) = AUC × (GFR + 25).

Complications (summary)

• Myelosuppression — dose-limiting for almost all (nitrosoureas: delayed and cumulative; carboplatin: thrombocytopenia predominant).
• Mucositis, nausea, alopecia — general.
• Gonadal toxicity / infertility — alkylators are notorious for azoospermia and premature ovarian failure; counsel for sperm banking.
• Teratogenicity — avoid in pregnancy, especially first trimester.
• Secondary malignancy — t-AML and bladder cancer (chronic cyclophosphamide).
• SIADH — with high-dose cyclophosphamide (worsens fluid retention, increasing cystitis risk).
• Pulmonary fibrosis — busulfan, nitrosoureas.

High-yield: Chronic cyclophosphamide exposure causes both haemorrhagic cystitis (acute, acrolein) and bladder transitional-cell carcinoma (chronic) — two separate, frequently-tested bladder effects.

Key differentials / confusables

Confusion	Correct association
Pulmonary fibrosis drug	Busulfan & nitrosoureas (alkylators); Bleomycin & Methotrexate & Amiodarone (non-alkylators)
Haemorrhagic cystitis	Cyclophosphamide & Ifosfamide only
Ototoxic anticancer drug	Cisplatin (also aminoglycosides, loop diuretics among non-cancer drugs)
Cold-triggered neuropathy	Oxaliplatin (unique)
Cardiotoxic chemo	Doxorubicin/anthracyclines, trastuzumab — NOT alkylators (high-dose cyclophosphamide is an exception → haemorrhagic myocarditis)
t-AML 5/7 vs 11q23	Alkylators = late, del 5/7; Etoposide (topo-II) = early, MLL/11q23

High-yield: High-dose cyclophosphamide (transplant conditioning) is itself cardiotoxic — causing acute haemorrhagic myocarditis/pericarditis. A subtle exception to "alkylators are not cardiotoxic."

Recently asked / exam angle

• "Antidote for cyclophosphamide-induced haemorrhagic cystitis" → Mesna (single most repeated alkylator MCQ).
• "Which platinum compound is least nephrotoxic / most myelosuppressive?" → Carboplatin (thrombocytopenia).
• "Cisplatin toxicity prevented by hydration with?" → Normal saline + mannitol/forced diuresis ± amifostine.
• "Drug causing pulmonary fibrosis + skin pigmentation used in CML/transplant" → Busulfan.
• "Ifosfamide encephalopathy is treated with" → Methylene blue.
• "Alkylating agent causing the highest secondary AML risk" → recall melphalan/procarbazine, late del 5/7.
• "Cell-cycle non-specific drug among options" → alkylators (also nitrosoureas, antibiotics like doxorubicin, cisplatin).
• "Oxaliplatin neuropathy precipitated by" → cold exposure.
• "Carboplatin dose by Calvert formula needs" → AUC and GFR.
• "Prodrug among anticancer agents" → Cyclophosphamide (also capecitabine, 6-MP).

Rapid revision

1. Alkylating agents are cell-cycle non-specific; alkylate N-7 of guanine → DNA cross-links.
2. Cyclophosphamide is a prodrug activated by hepatic CYP450 to phosphoramide mustard + acrolein.
3. Acrolein → haemorrhagic cystitis; antidote = Mesna + hydration.
4. Ifosfamide: more cystitis + encephalopathy (treat with methylene blue) + Fanconi syndrome.
5. Cisplatin: nephrotoxic, ototoxic (high-frequency), most emetogenic, neuropathy, hypomagnesaemia.
6. Carboplatin: myelosuppressive (thrombocytopenia), renal/ear sparing; dosed by Calvert (AUC) formula.
7. Oxaliplatin: cold-induced peripheral neuropathy; FOLFOX for colorectal cancer.
8. Busulfan: pulmonary fibrosis ("busulfan lung"), hyperpigmentation, seizures; CML/transplant conditioning.
9. Nitrosoureas (BCNU/CCNU): lipophilic, cross BBB → brain tumours; delayed cumulative myelosuppression.
10. Amifostine = cytoprotectant against cisplatin nephrotoxicity; main adverse effect = hypotension.
11. Alkylators → late secondary AML with del 5/7; etoposide → early t-AML with 11q23/MLL.
12. Cyclophosphamide also used for lupus nephritis & Wegener's; high-dose causes SIADH and haemorrhagic myocarditis.