U01.05.007 Efficacy vs potency

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Learning Objectives

Master the pharmacodynamic concepts of Efficacy and Potency. Learn to interpret dose-response curves, understand the significance of E_{max} and EC_{50}, and distinguish between the effects of full and partial agonists for the USMLE Step 1.

1. Efficacy (E_{max})

Efficacy represents the maximal effect a drug can produce, also known as its intrinsic activity. It is determined by the nature of the drug-receptor interaction.

Feature Details
Graphical Marker Represented by the Y-axis value (E_{max}).
Relationship \uparrow Y-value = \uparrow Efficacy.
Comparison Partial agonists have lower efficacy than full agonists, regardless of how much drug is given.
Independence Efficacy is unrelated to potency. A drug can be highly efficacious but require a high dose (low potency).

2. Potency (EC_{50})

Potency refers to the amount of drug required to produce a specific effect (usually 50% of its own maximal effect).

Feature Details
Graphical Marker Represented by the X-axis value (EC_{50}).
Relationship \downarrow EC_{50} (left shift) = \uparrow Potency.
Definition EC = Effective Concentration.
Independence Potency is unrelated to efficacy. A very potent drug may only produce a small maximal effect.

3. Comparative Analysis Summary

Change in Curve Pharmacodynamic Meaning
Shift to the Left \uparrow Potency (\downarrow EC_{50})
Shift to the Right \downarrow Potency (\uparrow EC_{50})
Shift Upward \uparrow Efficacy (\uparrow E_{max})
Shift Downward \downarrow Efficacy (\downarrow E_{max})

Activity:

High-Yield Clinical Pearls:

  • Clinical Importance: Efficacy is generally more clinically important than potency. We care more about how well a drug works (maximal effect) than how small the pill is (potency).
  • Partial Agonists: These can act as antagonists in the presence of a full agonist because they compete for receptors but have lower intrinsic activity (efficacy).
  • Potency Calculation: If Drug A has a EC_{50} of 1 mg and Drug B has an EC_{50} of 10 mg, Drug A is 10 times more potent.

Activity: