Clozapine's unique antipsychotic efficacy in treatment-resistant schizophrenia is attributed to its distinct receptor pharmacology. Which profile best explains its superiority over typical antipsychotics?

• A High D2 receptor occupancy (>80%) with no serotonergic activity
• B Low D2 affinity with high 5-HT2A antagonism and fast D2 receptor dissociation (hit-and-run kinetics) ✓
• C Selective D4 blockade with full D2 agonism
• D Inverse agonism at D1 receptors in the prefrontal cortex

Explanation

Clozapine exemplifies the 'atypical' antipsychotic profile: it has relatively low D2 receptor affinity, rapid dissociation from D2 receptors (loose binding/hit-and-run kinetics), and robust 5-HT2A antagonism. This combination reduces extrapyramidal side effects and is theorised to enhance dopamine release in the prefrontal cortex (improving negative symptoms) while still controlling mesolimbic dopamine excess. High D2 occupancy (>80%) is characteristic of typical antipsychotics and correlates with EPS risk. Clozapine is not a D4 selective or D1 inverse agonist.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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