RECEPTOR THEORIES

Lock and Key Hypothesis

states that the drug molecules must fit into a receptor like a key fit into a lock

Induced-fit Theory (koshland)

• Postulated a complementary relationship between the drug molecules and its active site

• Provides for mutual conformational changes between the drug and its receptor

Hypothesis of Clark / occupation theory (1926)

This is the classic, foundational model of pharmacodynamics.

• Percentage of receptors occupied

• Maximum effectiveness / effect of a drug can be obtained if all the receptors are occupied

• The Flaw: Clark's original theory could not explain how partial agonists existed (drugs that occupy 100% of receptors but still only produce a 50% response) or how some drugs could achieve a 100% response while only occupying 10% of the receptors.

Hypothesis of Paton

• Rate Theory

• Effectiveness does not depend on binding to a receptor, but upon obtaining the proper stimulus

• Agonists: Have a fast association rate and a fast dissociation rate. They hit the receptor, trigger it, and bounce off rapidly, creating a high frequency of "hits."

• Antagonists: Have a fast association rate but a very slow dissociation rate. They bind to the receptor and just sit there, blocking other molecules from creating new "hits."

Modification A: Ariëns (Intrinsic Activity)

• Affinity: The ability of the drug to bind to the receptor.

• Intrinsic Activity (alpha): The ability of the drug to actually trigger a response once bound.

Full agonist: a = 1

Antagonist: a = 0

Partial agonist: 0 < a < 1

Modification B: Stephenson (Efficacy & Spare Receptors)

_{^{Stephenson introduced the concept of efficacy and spare receptors.}}

• He proved that a drug does not need to occupy all receptors to produce a maximum response.

• Spare Receptors (Reserve Receptors): The extra receptors that are not required to be bound to achieve 100% effect. This explains why your heart can still reach its maximum contractile force even if a small percentage of its beta-receptors are irreversibly blocked by a poison.

Two-State Receptor Model (Highly Tested)

_{^{This is the most modern and widely accepted model because it perfectly explains the existence of inverse agonists.}}

• The Core Concept: Receptors spontaneously flip back and forth between two states in a constant equilibrium:

  • $R$ = Resting (Inactive) state

  • $R^*$ = Active state

  • Equation: R ⇌ R^*

• Even without a drug present, a small fraction of receptors are in the $R^*$ state, creating a low-level baseline biological activity (basal tone).

Full Agonists

Have a massive affinity only for the R^* state. They pull the equilibrium heavily to the right, locking receptors in the active state.

Antagonists

Have an equal affinity for both R$ and R^*. They lock the equilibrium exactly where it is. They don't do anything on their own; they just prevent agonists from shifting the balance.

Inverse Agonists (Board Pearl)

Have a high affinity only for the R$ (Inactive) state. They pull the equilibrium to the left, which completely shuts down the receptor's baseline basal tone, causing an effect opposite to that of an agonist.

Macromolecular Perturbation Theory (Belleau)

_{^{A slightly less tested but good-to-know theory that blends thermodynamics and induced fit.}}

• The Core Concept: When a drug binds to a receptor, it causes a "perturbation" (a disturbance or change in the receptor's structure).

• Specific Perturbation: Creates a biologically active receptor shape (Agonist).

• Non-Specific Perturbation: Creates a biologically inactive receptor shape (Antagonist).

• If a drug causes a mixture of both specific and non-specific perturbations, it acts as a Partial Agonist.

Occupation

Lead Scientist: Clark

Core Principle: Response = % of receptors bound.

Key Drug Concept Explained: The foundation of Affinity.

Occupation (Modified)

Lead Scientist: Ariëns & Stephenson

Core Principle: Introduced Intrinsic Activity and Efficacy.

Key Drug Concept Explained: Spare Receptors, Partial Agonists.

Rate

Lead Scientist: Paton

Core Principle: Response = Speed of association/dissociation.

Key Drug Concept Explained: Fast dissociation = Agonist.

Induced Fit

Lead Scientist: Koshland

Core Principle: Drug alters the physical shape of the receptor.

Key Drug Concept Explained: Conformational change activation.

Two-State

Lead Scientist:

Core Principle: R ⇌ R^* equilibrium.

Key Drug Concept Explained: Inverse Agonists & Basal Tone.