Alkylating Agents Pharmacology
Alkylating agents form covalent bonds with DNA, crosslinking strands and causing cell death regardless of cell cycle phase.
## Overview
Alkylating agents were the first non-hormonal anticancer drugs, derived from mustard gas observations in World War II. They exert cytotoxic effects by transferring alkyl groups to nucleophilic sites on DNA, particularly the N7 position of guanine, causing interstrand crosslinks, intrastrand crosslinks, and strand breaks that trigger apoptosis.
## Mechanism of Action
Alkylating agents are electrophilic compounds that react with nucleophilic groups in DNA. Bifunctional agents (cyclophosphamide, mechlorethamine) form crosslinks between two DNA strands, preventing replication and transcription. Monofunctional agents cause single-point alkylation. These are cell cycle-nonspecific agents, meaning they kill both dividing and resting cells, making them useful for slow-growing tumors.
## Major Drug Classes
**Nitrogen mustards**: Cyclophosphamide is a prodrug activated by CYP2B6 and CYP3A4 to phosphoramide mustard (active) and acrolein (causes hemorrhagic cystitis). Ifosfamide is similar but has higher CNS toxicity risk. Mesna is co-administered to prevent bladder toxicity by binding acrolein.
**Nitrosoureas**: Carmustine (BCNU) and lomustine (CCNU) are highly lipophilic, cross the blood-brain barrier, and are used for brain tumors. They also cause delayed myelosuppression (nadir at 4-6 weeks).
**Alkyl sulfonates**: Busulfan alkylates DNA and is used in conditioning regimens before bone marrow transplant. High doses cause hepatic veno-occlusive disease and pulmonary fibrosis.
**Temozolomide**: An oral alkylating agent that methylates O6-guanine. Active against glioblastoma, especially in tumors with MGMT promoter methylation (MGMT repairs O6-guanine damage; its silencing increases sensitivity).
## Resistance Mechanisms
Resistance arises from increased DNA repair (MGMT overexpression), glutathione-mediated drug inactivation, reduced drug uptake, and accelerated DNA damage tolerance.
## Toxicities
- **Myelosuppression**: Universal; nadir at 10-14 days (except nitrosoureas at 4-6 weeks)
- **Hemorrhagic cystitis**: Cyclophosphamide/ifosfamide (acrolein); prevented by mesna + hydration
- **Gonadal toxicity**: Dose-dependent; risk of infertility
- **Secondary malignancies**: AML risk increases with cumulative alkylator exposure (latency 5-10 years)
## Key Takeaways
- Alkylating agents are cell cycle-nonspecific; they work on dividing and non-dividing cells
- Cyclophosphamide requires hepatic activation; mesna prevents bladder toxicity
- MGMT promoter methylation predicts temozolomide benefit in glioblastoma
- Secondary leukemia is a long-term complication of alkylator therapy