What must agonists have regarding their action?

• A. Both great affinity and intrinsic activity
• B. High selectivity only
• C. High potency but no receptor binding
• D. No receptor occupancy

Answer: (A)

An agonist must both bind to a receptor and activate it. Binding is provided by affinity—the molecule has to fit and stick to the receptor. Activation comes from intrinsic activity (efficacy)—the bound molecule must induce a conformational change that produces a cellular response. If a compound binds but doesn’t activate, it acts as an antagonist or neutral ligand; if it doesn’t bind at all, it can’t have an effect. So the best description is that an agonist needs both strong binding (affinity) and the ability to produce a response (intrinsic activity). High selectivity alone isn’t enough to guarantee agonism, and having potency without receptor binding, or no receptor occupancy, would not produce an effect.