Pharmacodynamics: Dose-Response Curves, Agonism, and Antagonism

Vocabulary flashcards covering core pharmacodynamic principles including receptor affinity, potency, efficacy, agonist and antagonist classifications, and therapeutic index concepts.

Affinity

The attraction of a ligand (drug) for a receptor, or the strength of the interaction between a drug molecule and its target receptor. Higher affinity allows a drug to produce an effect at lower concentrations.

Biological Response

The reaction of a living organism or tissue to a drug, represented by changes in physiology or behavior (such as a rise in blood pressure, enzyme activation, or hormone release) that can be plotted as a concentration-response or dose-response curve.

Dose

The actual amount of drug administered.

Concentration

The amount of drug present in the bloodstream per unit volume from a given dose (e.g., $\text{mg/l}$).

Potency

The amount of a drug needed to produce a specific effect, reflecting the strength or concentration required to elicit a defined biological response.

$$\text{EC}_{50}$$

The effective concentration of an agonist that produces $50\%$ of the maximal response; used as the standard measure of agonist potency, where a lower value indicates higher potency.

Efficacy (Intrinsic Activity)

The ability of a drug to bind to a receptor and cause a change in the receptor's action, measured by $\text{E}_{\max}$ on a scale where maximum effect equals $1$ and no effect equals $0$.

Agonist

A drug with positive efficacy that activates receptors to promote a cellular response, possessing both affinity and efficacy.

Full Agonist

A drug that elicits the maximum possible tissue response ($\text{E}_{\max} = 1$).

Partial Agonist

A drug that produces less than the maximal response and cannot produce a maximal response even at $100\%$ receptor occupancy.

Inverse Agonist

A drug with negative efficacy that binds to receptors to decrease their basal (constitutive) receptor activity in the absence of ligand.

Antagonist

A drug with no efficacy ($0$) that binds to receptors without activating or inactivating them, preventing agonist actions; possesses affinity but no intrinsic activity.

Competitive Antagonist

An antagonist that binds reversibly to the receptor and competes directly with agonist binding (e.g., metoprolol, naloxone, losartan).

Non-Competitive Antagonist

An antagonist that covalently binds to its receptor irreversibly, reducing the total number of receptors available to the agonist (e.g., clopidogrel at $\text{P2Y}_{12}$ receptors).

$$\text{ED}_{50}$$

The dose that exerts a therapeutic or effective response in $50\%$ of the population.

$$\text{TD}_{50}$$

The dose that exerts toxicity in $50\%$ of the population.

Therapeutic Index (TI)

The dose range where medication efficacy is optimized while side-effects are minimized, defined mathematically as $\text{TI} = \frac{\text{TD}_{50}}{\text{ED}_{50}}$.

High Therapeutic Index

A broad safety margin (typically $\text{TI} > 10$) where a significantly higher dose is required to produce toxicity compared to the effective dose.

Narrow Therapeutic Index (NTI) Drugs

Medications where a very small change in dose or blood concentration can cause serious adverse drug reactions and therapeutic failure (e.g., digoxin, warfarin, heparin, levothyroxine, lithium, methotrexate, clozapine, vancomycin, theophylline, phenytoin, valproate, and aminoglycosides).