Oncology · Module 2 of 6

Alkylating Agents

Mechanisms, toxicity profiles, and key clinical pearls

Alkylating agents are cycle-nonspecific — they form covalent DNA adducts and interstrand cross-links that block replication regardless of cell cycle phase. The platinum compounds (cisplatin, carboplatin, oxaliplatin) act by a similar mechanism but are classified separately; cisplatin and carboplatin are completely cross-resistant with each other, while oxaliplatin shows only partial cross-resistance.
Cyclophosphamide and ifosfamide are prodrugs requiring hepatic CYP2B6 activation to phosphoramide mustard (cytotoxic) and acrolein (toxic). Acrolein accumulates in urine and causes hemorrhagic cystitis — prevented by mesna at all ifosfamide doses and at high cyclophosphamide doses. Ifosfamide additionally generates chloroacetaldehyde, causing encephalopathy treatable with methylene blue IV.
Nitrosoureas (carmustine, lomustine) penetrate the blood-brain barrier due to high lipophilicity and are cornerstone agents in glioblastoma. Their defining pharmacological feature is a delayed myelosuppression nadir at 4–6 weeks — applying a standard 3-week cycle interval produces overlapping, potentially fatal cumulative myelosuppression. Cycle intervals must be at least 6 weeks.
Temozolomide is 100% orally bioavailable and penetrates the CNS to ~30% of plasma levels. It methylates O6-guanine; MGMT promoter methylation silences the repair enzyme that reverses this lesion, predicting treatment benefit. The Stupp protocol — concurrent temozolomide plus radiation followed by 6 adjuvant cycles — is standard of care for glioblastoma. Pneumocystis jirovecii pneumonia prophylaxis with trimethoprim-sulfamethoxazole is mandatory during the concurrent phase.
Busulfan is used exclusively for myeloablative conditioning before hematopoietic stem cell transplantation. Intravenous formulation is preferred over oral due to erratic oral bioavailability (40–100%). Therapeutic drug monitoring targets an AUC of 900–1,350 µmol·min/day; overexposure risks hepatic sinusoidal obstruction syndrome, treated with defibrotide.

AUC = area under the curve · BEP = bleomycin + etoposide + cisplatin · CNS = central nervous system · CYP = cytochrome P450 · DNA = deoxyribonucleic acid · GFR = glomerular filtration rate · HSCT = hematopoietic stem cell transplantation · MGMT = O6-methylguanine-DNA methyltransferase · SOS = sinusoidal obstruction syndrome · TMP-SMX = trimethoprim-sulfamethoxazole · TMZ = temozolomide

Platinum Compounds — Key Differences

Feature	Cisplatin	Carboplatin	Oxaliplatin
Dose-limiting toxicity	Nephrotoxicity, ototoxicity	Thrombocytopenia	Cumulative sensory neuropathy
Dosing method	Body surface area (mg/m²)	Calvert formula: target AUC × (GFR + 25)	Body surface area (mg/m²)
Cross-resistance	Complete with carboplatin	Complete with cisplatin	Partial only
Key indication	Testicular cancer (BEP), chemoradiation (cervical, head and neck)	Ovarian cancer, lung cancer when cisplatin not tolerated	Colorectal cancer (FOLFOX)
Special precaution	Aggressive IV hydration required; monitor Mg²⁺ and creatinine	Reduce dose by Calvert formula if GFR declines	Cold-triggered acute neuropathy; avoid cold exposure after infusion

Cyclophosphamide & Ifosfamide — Activation and Toxicity

Prodrug Activation

Hepatic CYP2B6 Pathway

Both drugs require hepatic CYP2B6 activation → 4-hydroxycyclophosphamide → spontaneous decomposition → phosphoramide mustard (active alkylator) + acrolein (urotoxic)
Acrolein is excreted in urine → direct urothelial injury → hemorrhagic cystitis
Prevention: mesna (sodium 2-mercaptoethane sulfonate) binds acrolein in bladder lumen, rendering it non-toxic; mandatory for all ifosfamide doses and high-dose cyclophosphamide
Rifampin (CYP2B6 inducer) increases active metabolite formation → excess toxicity; azole antifungals (CYP3A4 inhibitors) reduce activation → reduced efficacy

Ifosfamide-Specific Toxicities

Encephalopathy & Fanconi Syndrome

Side-chain oxidation generates chloroacetaldehyde → CNS toxicity: confusion, ataxia, somnolence, seizures; onset 12–48 h after infusion start
Treatment: methylene blue 50 mg IV every 4–8 h (inhibits chloroacetaldehyde-mediated mitochondrial electron transport disruption)
Renal tubular toxicity → Fanconi syndrome: phosphate, bicarbonate, glucose, and amino acid wasting; may be irreversible with cumulative exposure
Mesna is mandatory at all ifosfamide doses — hemorrhagic cystitis risk is higher than with cyclophosphamide

CNS-Penetrating Alkylating Agents

Nitrosoureas

Carmustine & Lomustine

High lipophilicity → blood-brain barrier penetration; indicated for glioblastoma and CNS lymphoma
Myelosuppression nadir delayed to 4–6 weeks (vs 10–14 days for most cytotoxics) — cycle interval must be at least 6 weeks
Carmustine cumulative pulmonary fibrosis risk above 1,200 mg/m²
Carmustine also available as biodegradable wafer (Gliadel) for local delivery after surgical resection

Oral CNS Agent

Temozolomide

100% oral bioavailability; CNS penetration ~30% of plasma concentration
Methylates O6-guanine → mismatch repair-mediated apoptosis
MGMT promoter methylated → repair enzyme silenced → sensitized to TMZ; MGMT unmethylated → repair intact → reduced benefit
Stupp protocol: concurrent TMZ + radiation, then adjuvant TMZ × 6 cycles
TMP-SMX prophylaxis mandatory during concurrent chemoradiation due to lymphodepletion

Transplant Conditioning

Busulfan

Myeloablative conditioning before HSCT; IV formulation preferred (oral bioavailability 40–100%)
Therapeutic drug monitoring required: target AUC 900–1,350 µmol·min/day
Overexposure → hepatic SOS (tender hepatomegaly, jaundice, ascites, thrombocytopenia)
Established severe SOS: treat with defibrotide

Clinical Safety Rule

Nitrosourea Scheduling — 6-Week Minimum Interval

Lomustine and carmustine produce myelosuppression with a nadir at 4 to 6 weeks — not the 10 to 14 days typical of most cytotoxic agents. Applying a standard 3-week or 4-week cycle interval means the next dose is given before the nadir of the first has resolved, producing overlapping suppressions that can cause life-threatening aplasia. Always verify the 6-week minimum cycle interval before prescribing any nitrosourea.

References

Author / Source	Title	Publication
Katzung BG (ed)	Basic & Clinical Pharmacology, 15th ed.	McGraw-Hill, 2021
Brunton LL, Knollmann BC (eds)	Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed.	McGraw-Hill, 2023
Colvin OM	An overview of cyclophosphamide development and clinical applications	Curr Pharm Des. 1999;5(8):555–560
Fu D, Calvo JA, Samson LD	Balancing repair and tolerance of DNA damage caused by alkylating agents	Nat Rev Cancer. 2012;12(2):104–120
Emadi A, Jones RJ, Brodsky RA	Cyclophosphamide and cancer: golden anniversary	Nat Rev Clin Oncol. 2009;6(11):638–647
Sweiss KI, Beri R, Shord SS	Encephalopathy after high-dose ifosfamide: a retrospective cohort study and review of the literature	Drug Saf. 2008;31(11):989–996
Dasari S, Tchounwou PB	Cisplatin in cancer therapy: molecular mechanisms of action	Eur J Pharmacol. 2014;740:364–378
Calvert AH, Newell DR, Gumbrell LA, et al	Carboplatin dosage: prospective evaluation of a simple formula based on renal function	J Clin Oncol. 1989;7(11):1748–1756
Mohty M, Malard F, Abecassis M, et al	Sinusoidal obstruction syndrome/veno-occlusive disease: current situation and perspectives — a position statement from the European Society for Blood and Marrow Transplantation	Bone Marrow Transplant. 2015;50(6):781–789
Stupp R, Mason WP, van den Bent MJ, et al	Radiotherapy plus concomitant and adjuvant temozolomide for glioblastoma	N Engl J Med. 2005;352(10):987–996
Hegi ME, Diserens AC, Gorlia T, et al	MGMT gene silencing and benefit from temozolomide in glioblastoma	N Engl J Med. 2005;352(10):997–1003
Galluzzi L, Senovilla L, Vitale I, et al	Molecular mechanisms of cisplatin resistance	Oncogene. 2012;31(15):1869–1883
Kelland L	The resurgence of platinum-based cancer chemotherapy	Nat Rev Cancer. 2007;7(8):573–584