If a drug has high binding affinity, which statement is true?

• A. It requires a higher concentration to form receptor complexes
• B. It has no effect on the required concentration
• C. It typically requires a lower concentration to form receptor complexes
• D. It prevents all receptor interactions

Answer: (C)

High binding affinity means the drug binds the receptor very tightly, so you can achieve receptor occupancy at a lower concentration. This is because a strong interaction corresponds to a low dissociation constant (Kd), and the fraction of receptors bound is [D]/([D]+Kd). With a small Kd, even a small drug concentration leads to many receptors being occupied, so it typically requires a lower concentration to form receptor complexes. The idea that you’d need a higher concentration contradicts this relationship, and saying affinity has no effect on required concentration ignores how binding strength determines occupancy. Also, high affinity doesn’t by itself prevent all receptor interactions; it just promotes binding, not universal blockade.