Regarding alkylating agents used in cancer chemotherapy, all of the following statements are TRUE EXCEPT:

Explanation

## Analysis of Alkylating Agent Mechanisms ### Correct Statements (Options 0, 1, 3) **Option 0 — Cyclophosphamide Activation:** **Key Point:** Cyclophosphamide is a prodrug that requires hepatic metabolism via cytochrome P450 enzymes to generate the active form, phosphoramide mustard. This metabolite is the true alkylating species that crosslinks DNA. **Option 1 — Nitrous Ureas (Carmustine/BCNU) CNS Penetration:** **High-Yield:** Nitrous ureas are lipophilic compounds with excellent blood-brain barrier (BBB) penetration, making them the agents of choice for CNS malignancies (glioblastoma, CNS lymphoma). This is a classic NEET PG fact. **Option 3 — Mechlorethamine Bifunctionality:** **Key Point:** Mechlorethamine (nitrogen mustard) is a bifunctional alkylating agent — it has two reactive chloroethyl groups that can each form one covalent bond with DNA bases. This allows it to create interstrand crosslinks, which are lethal to dividing cells. It is highly potent but also highly toxic. ### Incorrect Statement (Option 2 — Cisplatin) **Warning:** This is the trap. Cisplatin is **NOT** a classical alkylating agent in the traditional sense. Although it does form covalent DNA adducts, the mechanism is **NOT** electrophilic substitution by an alkyl group. Instead, cisplatin works via **coordinate covalent bonding** (platinum-DNA adduct formation), which is mechanistically distinct from the nucleophilic displacement mechanism of true alkylating agents like cyclophosphamide or mechlorethamine. **Clinical Pearl:** Cisplatin is classified as a **platinum compound** or **heavy metal alkylating agent**, not a classical organic alkylating agent. Its DNA damage mechanism (platinum adducts) is fundamentally different from electrophilic alkylation. ### Mechanism Comparison Table | Agent | Class | Activation | BBB Penetration | Mechanism | | --- | --- | --- | --- | --- | | Cyclophosphamide | Oxazaphosphorine | Hepatic (P450) | Poor | Electrophilic alkylation | | Carmustine (BCNU) | Nitrous urea | Spontaneous | Excellent | Electrophilic alkylation | | Mechlorethamine | Nitrogen mustard | None (active as-is) | Poor | Bifunctional crosslinking | | Cisplatin | Platinum compound | Spontaneous aquation | Moderate | Coordinate bonding (NOT alkylation) | **High-Yield:** The key distinguishing feature is that cisplatin's mechanism is **coordinate covalent bonding**, not electrophilic alkylation. This is why it is NOT classified as a true alkylating agent despite causing DNA damage. [cite:KD Tripathi 8e Ch 62]