What is an important characteristic of partial agonists?

• A. They can have high affinity for receptors, but most receptors are not activated, preventing 100% maximal response.
• B. They always produce full maximal response at high doses.
• C. They cannot bind receptors effectively.
• D. They only act as antagonists.

Answer: (A)

Partial agonists are defined by high affinity for a receptor but lower intrinsic efficacy than a full agonist. They bind the receptor readily, but the amount of activation they produce is only partial, so even with strong receptor occupancy the maximal response is below 100%. This can also allow them to dampen the effect of a full agonist by competing for the same receptors. For example, buprenorphine binds tightly to mu receptors but yields only a limited maximal effect. The other statements don’t fit because a partial agonist does not guarantee a full maximal response, it can bind effectively, and it can act as an antagonist in the presence of a full agonist (though it can also produce a partial agonist effect on its own).