In clozapine-treated patients, which receptor-binding profile is most responsible for its superior efficacy against treatment-resistant negative symptoms and low extrapyramidal side effect burden compared with haloperidol?
- A Low D2 occupancy with high 5-HT2A/D2 ratio and muscarinic receptor antagonism ✓
- B High D2 receptor occupancy (>80%) with fast dissociation kinetics
- C Selective D4 blockade without D2 activity
- D Partial D2 agonism with 5-HT1A agonism
Correct answer: A. Low D2 occupancy with high 5-HT2A/D2 ratio and muscarinic receptor antagonism
Explanation
Clozapine has relatively low D2 occupancy (~40–60%), rapid dissociation from D2 receptors ('hit-and-run' kinetics per Kapur & Seeman), and a high 5-HT2A/D2 antagonism ratio. This profile spares striatal D2 receptors sufficiently to prevent extrapyramidal side effects and hyperprolactinaemia. Its broad receptor antagonism (5-HT2A, 5-HT2C, α1, H1, M1) contributes to its unique efficacy in treatment-resistant schizophrenia. Partial D2 agonism with 5-HT1A agonism describes aripiprazole, not clozapine.
Reference: Kaplan & Sadock's Synopsis of Psychiatry, 11th ed.
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