Which statement correctly describes the relationship between efficacy and D2 occupancy thresholds?

• A. Efficacy correlates with binding above 60%
• B. Efficacy correlates with occupancy above 78%
• C. Efficacy correlates with occupancy below 50%
• D. Efficacy is independent of D2 occupancy

Answer: (A)

The main idea is that how well these drugs work depends on how much D2 receptor blockade they produce. You need to reach a minimum level of occupancy to see meaningful antipsychotic effects, and the commonly cited threshold is around 60%. Crossing that level means there’s enough blockage in the dopaminergic pathways to reduce psychotic symptoms. Pushing occupancy higher than this can raise the risk of motor side effects without guaranteeing additional efficacy, so the general takeaway is that efficacy tends to appear once binding exceeds about 60%. The other statements are less consistent with how receptor occupancy relates to response: too low occupancy (<50%) is usually not enough for full efficacy, the 78% figure is more specific than necessary for a general rule, and claiming independence from occupancy ignores the pharmacodynamic link between receptor blockade and therapeutic effect.