Drug-Receptor Interactions and SAR

Pharmacodynamics 312: This module focuses on the interactions between drugs and organs, specifically analyzing how drugs interact with receptors at a molecular level.
Molecular Nature: Both drugs and receptors are three-dimensional (3D) structures.
The Induced-Fit Model: * Drug-receptor interactions are described using the "induced fit model." * Initial state: Receptor $[R]$ and Drug $[D]$ . * Mechanism: The receptor adjusts its ligand-binding domain due to intermolecular forces (IMF) upon the approach of the drug. * Result: Formation of the Drug-Receptor complex $[DR]$ . * Interaction Basis: These interactions occur via intermolecular forces (IMF) of attraction between the drug molecule and the amino acid residues located on the ligand-binding site of the receptors.

Covalent Bonds: * Characterized as irreversible bonds.
Non-Covalent Bonds: * Characterized as reversible and relatively weak interactions. * These interactions are heavily dependent on the distance between the drug $[D]$ and the receptor $[R]$ . * Hierarchy of Non-Covalent Strength: The strength of IMFs increases in the following order: 1. $Van\,der\,Waals\,forces$ (Weakest) 2. $Pi-pi\,interactions$ 3. $Dipole-dipole\,interactions$ 4. $Hydrogen\,bonding$ 5. $Ionic\,attraction$ (Strongest non-covalent)

Defining SAR: Structure-Activity Relationship is the systematic understanding of how the chemical structure of drugs and receptors interact to influence biological effects.
Analytical Goals: SAR aims to qualitatively and quantitatively understand IMFs that directly affect binding affinity.
Determinants of Binding Affinity: * Functional groups found on the drugs and within the ligand-binding domain. * Physicochemical properties, including size, shape, and lipophilicity. * Spatial arrangement, such as the orientation of stereoisomers.
Visualizing Structure: The 3D structure of proteins and their binding sites are typically obtained through X-ray crystallography.
Recall Mnemonic: The transcript references "DDDP" as a point of recall for these concepts.

Precise Fit: A precise fit between the drug and the receptor is a prerequisite for effective drug activity.
Endogenous Ligand Relationship: Receptors typically only bind to drugs with a chemical structure that is closely related to the endogenous ligand that naturally interacts with the ligand-binding domain.
Binding Characteristics: * Drug-Receptor $[DR]$ interactions are usually highly selective and specific. * If the IMFs in the $[DR]$ complex are weak, it results in weak binding, which limits the overall effectiveness of the drug.
Application: SAR provides the chemical information necessary to design drugs that are both specific and selective.

Receptor States: Receptors exist in a dynamic equilibrium between two states: * Resting State $[R]$ . * Activated State $[R^*]$ .
Equilibrium and Reversibility: The two states are reversible and naturally work towards a state of equilibrium.
Constitutive Receptor Activity: Without the presence of an endogenous ligand (EL), the receptor remains in the resting state $[R]$ but still exhibits some appreciable baseline activity, known as constitutive receptor activity.
Response to Endogenous Ligands (EL): * When an EL binds to the receptor, it transitions to the activated state $[R^<em>]$ . This state produces a higher cellular response, reaching toward the Maximum Cellular Response (MCR).
Equilibrium Shifts: * Too little activation: When there is insufficient activation, the equilibrium will push towards the activated state $[R^<em>]$ . Excessive activation: When there is too much $[R^*]$ , the equilibrium is "stressed" and will ease itself by pushing back towards the resting state $[R]$ .

Interaction Outcomes: When a drug binds to a receptor through IMF, it follows one of two functional paths:
Agonists: * Mechanism: They bind and activate the receptor ( $R \rightarrow R^<em>$ ). Effect: They elicit a tissue response. * Clinical Application: Used in conditions where there is too little signaling, causing disease. * Example: Parkinson’s disease, which is caused by limited levels of Dopamine.
Antagonists: * Mechanism: They bind to the receptor but serve as a blockade to prevent agonists or endogenous ligands from binding. * Effect: They keep the receptor in the resting state $[R]$ . * Clinical Application: Used in conditions where excessive signaling results in disease. * Example: High blood pressure (Hypertension).