## Distinguishing Nitrogen Mustards from Nitrosoureas ### Key Structural and Pharmacological Differences **Key Point:** Nitrosoureas are lipophilic compounds that readily cross the blood–brain barrier (BBB), making them the alkylating agents of choice for CNS malignancies. Nitrogen mustards are hydrophilic and cannot effectively penetrate the BBB. ### Comparative Table | Feature | Nitrogen Mustards | Nitrosoureas | | --- | --- | --- | | **BBB penetration** | Poor (hydrophilic) | Excellent (lipophilic) ✓ | | **Clinical use in CNS tumors** | Not preferred | Preferred (BCNU, CCNU) | | **Activation requirement** | Direct alkylators | Require metabolic activation | | **Crosslinking capacity** | Yes (bifunctional) | Yes (bifunctional) | | **Cardiotoxicity** | Minimal | Minimal | | **Pulmonary toxicity** | Uncommon | Common (nitrosourea-specific) | ### Mechanism of BBB Penetration Nitrosoureas (e.g., BCNU, CCNU) are small, lipophilic molecules that dissolve in the lipid bilayer of the BBB and achieve CSF concentrations of 20–30% of plasma levels. Nitrogen mustards (e.g., mechlorethamine, cyclophosphamide) are ionized and hydrophilic, requiring active transport or disruption of the BBB — neither of which occurs reliably in intact CNS. **High-Yield:** Nitrosoureas are the alkylating agents used for glioblastoma, medulloblastoma, and CNS lymphoma precisely because of their BBB penetration. **Clinical Pearl:** Although both classes are bifunctional alkylators capable of DNA crosslinking, only nitrosoureas are suitable for primary CNS malignancies.
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