A physician is reviewing pharmacodynamic principles to better understand drug-receptor int… — USMLE Step 3

A physician is reviewing pharmacodynamic principles to better understand drug-receptor interactions and their clinical implications. Match each receptor modulator on the left with its characteristic effect on a full agonist's log dose-response curve on the right.

Competitive antagonistShifts the agonist curve to the right (increases $EC_{50}$ ) without changing maximal efficacy ( $E_{max}$ ).
Noncompetitive antagonistDecreases maximal efficacy ( $E_{max}$ ) without significantly changing the potency ( $EC_{50}$ ) of the remaining receptors.
Partial agonistAchieves submaximal efficacy alone and acts as a net antagonist in the presence of a full agonist.
Positive allosteric modulatorBinds to a secondary site to increase the agonist's affinity, shifting the dose-response curve to the left (decreases $EC_{50}$ ).

Answer

Competitive antagonist pairs with rightward curve shift (increased EC50, unchanged Emax). Noncompetitive antagonist pairs with decreased Emax (unchanged EC50). Partial agonist pairs with submaximal efficacy and antagonism of full agonists. Positive allosteric modulator pairs with leftward curve shift (decreased EC50).

Each modulator has a distinct footprint on the dose-response curve. Competitive antagonists alter potency (

EC_{50}

) without changing efficacy (

E_{max}

). Noncompetitive antagonists alter efficacy (

E_{max}

) without changing potency (

EC_{50}

). Partial agonists display low intrinsic efficacy and compete with full agonists. Positive allosteric modulators increase agonist affinity, improving potency (

EC_{50}

Step-by-Step Solution

Determine the effect of a competitive antagonist on the dose-response curve.

It shifts the curve to the right, meaning a higher

EC_{50}

is required, but

E_{max}

is unchanged because the antagonism is surmountable.

Competitive antagonists bind reversibly to the orthosteric site, allowing high concentrations of agonist to outcompete them.

Determine the effect of a noncompetitive antagonist on the dose-response curve.

It lowers the peak of the curve, meaning

E_{max}

is decreased, while the

EC_{50}

of the remaining receptors is unchanged.

Noncompetitive antagonists bind irreversibly or to allosteric sites, preventing a maximal response regardless of how much agonist is added.

Determine the effect of a partial agonist.

It cannot reach the

E_{max}

of a full agonist on its own and acts as a competitive antagonist when a full agonist is present.

Partial agonists possess intrinsic efficacy that is greater than 0 but less than 1.

Determine the effect of a positive allosteric modulator.

It shifts the full agonist dose-response curve to the left, decreasing the

EC_{50}

Allosteric modulators do not bind the primary active site but enhance the receptor's affinity or response to the endogenous/primary agonist.

Key Concept

The characteristic changes in drug potency (

EC_{50}

) and efficacy (

E_{max}

) induced by different classes of receptor modulators.

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Answer

Step-by-Step Solution

Key Concept

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