4.9
Dose-Response Relationship: Potency and Efficacy
Q1: What does EC50 tell you about a drug's potency?
EC50 represents the drug concentration producing half-maximal response and directly indicates potency. A lower EC50 value means higher potency—the drug achieves desired effects at lower doses. For example, candesartan has lower EC50 than irbesartan, making candesartan more potent despite both drugs producing similar maximum effects.
Q2: How do full agonists differ from partial agonists in terms of efficacy?
Full agonists completely activate receptors, producing maximal response and high efficacy. Partial agonists activate receptors to a lesser extent, generating lower efficacy and submaximal responses. Both drug types depend on receptor occupancy and activation extent, but full agonists trigger complete biological effects while partial agonists produce limited responses.
Q3: Why do antagonists have zero efficacy?
Antagonists do not activate receptors; instead, they block agonist binding through combined effects of drugs antagonism mechanisms. They reduce available receptors for agonist interaction or alter receptor conformation, preventing effective binding. Since antagonists produce no receptor activation, they exhibit zero efficacy regardless of concentration.
Q4: What happens when a drug reaches maximal efficacy?
Maximal efficacy occurs when all available receptors are occupied and fully activated. At this point, further increasing drug concentration produces no additional biological response because no receptors remain unbound. This plateau represents the ceiling effect, beyond which dose escalation cannot enhance the drug's therapeutic action.
Q5: Can two drugs have different potencies but equal efficacy?
Yes. Candesartan and irbesartan exemplify this relationship: candesartan is more potent (lower EC50) but both achieve identical maximum effects (similar Emax). Potency reflects the dose needed for response; efficacy reflects the maximum achievable effect. Two drugs can differ in potency while maintaining equal maximal therapeutic outcomes.
Q6: How does the number of drug-receptor complexes affect drug efficacy?
Efficacy depends directly on drug-receptor complex formation and the extent of receptor activation. More complexes formed and greater receptor activation produce higher efficacy. When all receptors form complexes and activate fully, maximal efficacy is achieved. Partial agonists form fewer activated complexes, resulting in lower efficacy than full agonists.
Q7: What is the relationship between potency and the dose-response curve?
Potency is visualized on the dose-response curve through the EC50 value's position on the x-axis. Drugs with higher potency show EC50 values shifted leftward, requiring lower concentrations for half-maximal effect. The curve's shape and position reflect both potency and efficacy, with leftward shifts indicating increased potency and vertical plateau height indicating efficacy.