Receptor Theories

Receptor Theories

Introduction

  • Receptor Theories: Theoretical frameworks to understand drug action through interaction with receptors.

  • Key Contributors:

    • John Langley (1878): Proposed the term "receptive substance"; laid the groundwork for receptor theory in drug action.

    • Paul Ehrlich (1854-1915): Extended receptor theory through his work on cellular interactions and won a Nobel Prize in Physiology or Medicine in 1908.

Side-Chain Theory by Paul Ehrlich

  • Core Concept:

    • Described as the "side-chain theory" of cellular interaction.

    • Illustrated at the Royal Society of London in 1900.

  • Mechanism:

    1. Immune cells possess many receptors specific to different substances.

    2. Upon interaction with a toxin, the corresponding receptor activates the cell.

    3. Activated cells produce more receptors, which are released into the bloodstream as antibodies to neutralize the toxin.

Classical Theories of Drug-Receptor Binding Interactions

  • Overview: Various theoretical frameworks explaining how drugs bind to receptors and elicit responses.

Occupation Theory

  • Definition: Drugs act on independent binding sites, activating them.

  • Key Points:

    • Biological response is proportional to the drug-receptor complex formed.

    • Response ceases when the complex dissociates.

    • The drug effect is directly proportional to the number of receptors occupied.

Lock and Key Concept

  • Origin: Introduced by Emil Fischer in 1894.

  • Analogy:

    • Lock: Enzyme

    • Key: Substrate

  • Function: Enzymes and substrates must fit together perfectly to exert a chemical reaction.

    • Illustration:

    • Correct fit results in a reaction.

    • Incorrect substrates do not react.

  • Quote by Emil Fischer:

    • “To use a picture, I would like to say that enzyme and glucoside have to fit like a lock and key, in order to exert a chemical action on each other.”

Rate Theory

  • Definition: Response is proportional to the rate of drug-receptor complex formation rather than the number of receptors occupied.

  • Key Points:

    • Pharmacological activity depends on the rates of dissociation and association.

    • The duration of receptor occupation determines whether the molecule acts as:

    • Agonist: Fast association and fast dissociation.

    • Partial Agonist: Intermediate association and intermediate dissociation.

    • Antagonist: Fast association and slow dissociation.

Induced-Fit Theory

  • Definition: The receptor's binding site may not be fully complementary to the ligand initially.

  • Mechanism:

    • Binding causes both ligand and receptor to change shape (dynamic process).

    • This conformational change leads to biological effects, moving away from the rigid "key and lock" concept.

Macromolecular Perturbation Theory

  • Core Concept: Drug-receptor interactions can cause two types of perturbations:

    1. Conformational Perturbation: Specific changes that lead to a biological response (agonist).

    2. Non-specific Conformational Perturbation: Changes that do not lead to any response (antagonist).

Activation-Aggregation Theory

  • Overview: Extends the macromolecular perturbation theory.

  • Key Points:

    • Receptors exist in equilibrium between activated (bioactive) and inactivated (bio-inactive) states; agonists bind to activated states, antagonists to inactivated states.

    • Receptor Signaling Dependency:

    • Activation: Conformational change.

    • Aggregation: Dimerization/oligomerization/clustering enhanced by ligand binding.

    • Particularly relevant for receptor tyrosine kinases (RTKs), immune receptors (e.g., Fc receptors), and cytokine receptors.

Molecular-Level Conceptual Models of Receptors

  • Core Concept: Receptors are not mere macromolecules but are dynamic structures.

  • Characteristics:

    • Many receptors are families of low-energy conformers existing in equilibrium.

    • Some receptors have complex multi-unit structures allowing for facilitatory and inhibitory interactions that can change drug-receptor dynamics.

    • Receptors may also be mobile, drifting within the membrane.

Two-State Receptor Model (Classical Pharmacology)

  • Definition: Receptors exist in two states, the resting state (R) and activated state (R*).

    • The equilibrium lies towards R in the absence of ligand; few are in R* state (except constitutively active receptors).

  • Agonists/Antagonists:

    • Agonist: Exhibits a high affinity for R*, shifting equilibrium rightward.

    • Antagonist: Equal affinity for both R and R*, does not alter the equilibrium.

    • Inverse Agonist: Has a higher affinity for R, shifting equilibrium leftward.

  • Contrast to Occupation Theory: Agonists do not directly activate receptors but instead shift the equilibrium toward R*.