Introduction to Pharmacodynamics

Lecture Learning Outcomes

At the end of this lecture, students should be able to:

1. Define pharmacodynamics (PD) and distinguish it from pharmacokinetics (PK).

2. Describe Receptor Occupancy Theory, and how receptor binding (occupancy) translates into pharmacologic effect.

3. Evaluate dose-response curves to determine key drug properties (graded vs. quantal, Kd, EC₅₀, Emax, potency, efficacy, ED₅₀, LD₅₀).

4. Differentiate classes of drugs based on their mechanisms of action (orthosteric vs. allosteric, agonist vs. antagonist, reversible vs. irreversible) with representative examples.

5. Explain tolerance and tachyphylaxis, and why drug effects may decline over time.

6. Define therapeutic index (TI) and discuss its role in drug safety and selectivity.

7. Apply these concepts to real-life drug examples.

Pharmacology Overview

• Pharmacology: The study of chemical actions on biological systems.

  • Medical Pharmacology: Concerned with using chemicals for the prevention, diagnosis, and treatment of diseases.

  • Toxicology: Concerned with the undesirable effects of chemicals on biological systems.

• Pharmacokinetics (PK): Describes the effects of the body on drugs (Absorption, Distribution, Metabolism, Excretion - ADME).

• Pharmacodynamics (PD): Denotes the actions of the drug on the body, including the mechanisms of action and therapeutic and toxic effects.

PK vs. PD

• Pharmacokinetics (PK): Refers to what the body does to the drug.

• Pharmacodynamics (PD): Refers to what the drug does to the body.

Pharmacodynamics: What the Drug Does to the Body

Definition: The study of the biochemical and physiological effects of drugs and their mechanisms of action.

Receptor Occupancy Theory

Mechanism:

• Receptors: Specific molecules in biological systems that drugs interact with to produce functional changes.

• Receptor Binding (Occupancy): Directly correlates with pharmacologic effects.

Types of Receptors

• Binding Sites:

  • Receptor: Any binding site for a drug on a biological macromolecule.

  • Drug Targets: A broader term encompassing all molecular targets of drugs, including enzymes, ion channels, transporters, structural proteins, or DNA.

  • Specific Examples:

  • Ligand-gated ion channels: GABAA receptor, nACh receptor

  • Nuclear receptors: AR, ER, RAR

  • Receptor tyrosine kinases: EGFR, InsR, FGFR

Effectors in Pharmacodynamics

• Effectors: Molecules that translate drug-receptor interactions into changes in cellular activity.

  • Signal Detector: Receptor

  • Response Generator: Effector

  • Signal Transduction Pathway: Links drug binding to physiological effects.
    Example:

• Albuterol:

  • Binds to β2-adrenergic receptor → activates adenylyl cyclase → increases cAMP → activates PKA → downstream effects leading to smooth muscle relaxation and bronchodilation.

Drug Classification by Mechanism of Action

1. By Binding Site:

   • Orthosteric: Binds at the natural ligand site (e.g., Albuterol at the β₂ receptor).

   • Allosteric: Binds at a different site and modulates receptor function (e.g., benzodiazepines at GABA_A receptor).

2. By Functional Effect:

   • Agonist: Activates receptor → induces effects.

     • Full vs. Partial Agonist

   • Antagonist: Binds but does not activate → blocks agonist actions.

3. By Mode of Modulation:

   • Activator: Increases target activity (allosteric activators, channel openers).

   • Inhibitor: Decreases target activity (enzyme inhibitors, channel blockers).

4. By Chemical Interaction:

   • Non-covalent (reversible): E.g., hydrogen bonds, ionic, hydrophobic forces.

   • Covalent (irreversible): Permanent bond, e.g., aspirin inhibits COX enzyme.

Agonist and Antagonist Detailed

• Agonist:

  • Definition: Binds to the active (orthosteric) site and activates the receptor.

  • Example: β2-receptor agonist Albuterol for asthma.

• Antagonist:

  • Definition: Binds to the receptor and blocks agonist-mediated receptor activation.

  • Example: β1-receptor blocker Metoprolol for cardiovascular conditions.

  • Endogenous Ligands: Natural agonists e.g., epinephrine, estradiol, testosterone.

Allosteric Drugs

• Definition: Act away from the active site to modulate receptor activity.

  • Positive Allosteric Modulator (PAM):

  • Example: Alprazolam targets GABA-A receptor, enhances GABA effect → neuronal inhibition.

  • Applications: Anxiety, epilepsy, insomnia.

  • Negative Allosteric Modulator (NAM):

  • Example: Efavirenz targets HIV-1 reverse transcriptase, changes enzyme conformation → inhibits viral DNA synthesis.

  • Application: HIV/AIDS treatment.

Dose–Response Curves in Pharmacodynamics

• Definition: Link drug dose to effect; drugs produce effects that depend on dose/concentration.

• Importance:

  • Predict drug effects at different doses.

  • Compare drugs for strength and safety.

  • Basis for understanding agonists, antagonists, and modulators.

  • Clinically informs dosing regimens.

Types of Dose–Response Curves

1. Graded Dose-Response Curve:

   • Features: Continuous response in a single subject.

   • Parameters: Emax, EC₅₀.

     • Curve: Sigmoidal (log-dose vs response).

   • Application: Analyzing potency & efficacy.

   1. Quantal Dose-Response Curve:

   • Features: Binary responses in a population.

   • Measures: Median effective dose (ED₅₀), median toxic dose (TD₅₀), median lethal dose (LD₅₀).

   • Clinical Use: Evaluate therapeutic window.

Understanding Graded Dose–Response Curves

• Example: Change in heart rate with increasing drug dose.

• Parameters: Emax (maximum effect), EC₅₀ (50% of Emax).

• Curve Characteristics:

  • Graded responses give insights into drug effectiveness.

Drug-Receptor Interactions

Binding Affinities:

• Thermodynamics Equation: $ΔG = ΔH - TΔS$

  • Interpretation:

  • If ΔG < 0, the reaction is spontaneous.

  • If $ΔG = 0$, the system is in equilibrium.

• Equilibrium Expression: $ΔG = ΔG° + RT ext{ln}Q$

  • Where:

    • $ΔG°$ = standard Gibbs free energy change (under standard conditions)

    • $R$ = gas constant

    • $T$ = temperature (Kelvin)

    • $Q$ = reaction quotient

• Binding Affinity:

  • Directly related to standard Gibbs free energy change: $ΔG° = -RT ext{ln}K_a$

  • Or equivalently (using Kd): $ΔG° = RT ext{ln}Kd$

    • $K_a$ = association constant,

    • $K_d$ = dissociation constant.

Receptor Occupancy Theory: Mathematical Insights

• The binding ratio can be expressed as:
  $rac{(1-y)}{y}$

• Receptor–Ligand Binding Equation:
  Assumes receptor concentration is much smaller than drug concentration.
  Let:

• $x = D$ (free drug concentration)

• $y = ext{fraction of receptors bound}$

Graded Dose–Response Curve Explained

• Relationship: Increasing drug dose → greater % receptor occupancy → greater effect.

• Measurement Context: In cell/patient activity assays, this variation with increasing drug concentrations/doses provides insights into effectiveness.

Kd versus EC50 Examples

1. Example 1: Kd and EC50 are analogous; drug effect is correlated to receptor occupancy under the receptor occupancy theory.

2. Example 2: Spare Receptors: exist in excess of what is needed for maximal response (Emax).

   • E.g., Insulin receptors allow strong glucose uptake even at low concentrations of the hormone; Kd is higher than EC50.

Drug Efficacy vs. Potency

• Efficacy: Greatest effect (Emax) a drug can produce.

• Potency: Amount of drug needed to achieve a specified effect; lower EC50 or Kd indicates higher potency.

Quantal Dose–Response Curve Defined

• Definition: Measures all-or-none responses in a population.

• Axes:

  • X-axis: Drug dose

  • Y-axis: % of individuals with defined effect.

• Key Metrics: ED₅₀ (effective dose), TD₅₀ (toxic dose), LD₅₀ (lethal dose).

Summary of Key Concepts

1. Pharmacodynamics vs. Pharmacokinetics

   • PD: What the drug does to the body

   • PK: What the body does to the drug (ADME)

2. Different Classes of Drugs

   • Orthosteric vs. Allosteric

   • Agonists vs. Antagonists

3. Receptor Occupancy Theory

   • Drug binding initiates effects characterized by affinity (Kd).

4. Dose–Response Curves

   • Graded response: % effect vs. drug concentration; key terms include Kd, EC₅₀, Emax, potency, efficacy.

   • Quantal response: population-based, shows ED₅₀, LD₅₀.

Different Types of Agonists

• Agonist: Any molecule binding to a receptor to produce a biological response.

  • Examples:

  • Full Agonist: E.g., Morphine at μ-opioid receptor, Epinephrine at β-adrenergic receptors.

  • Partial Agonist: E.g., Buprenorphine at μ-opioid receptor.

  • Inverse Agonist: E.g., Antihistamine actions reducing basal activity at H₁ receptors.

Different Types of Antagonists

1. Competitive Antagonists: Binds reversibly to the active site, competes with agonists.

   • Results in rightward shift of agonist dose-response curve (↑EC₅₀), no change in Emax.

2. Non-competitive Antagonists: Binds to a different site, reduces receptor availability regardless of agonist concentration.

   • Causes ↓ Emax, slight or no change in EC₅₀.

3. Physiological Antagonists: Binds to different receptors to produce opposite effects.

4. Chemical Antagonists: Interacts chemically to prevent agonist binding.

5. Functional Antagonist: Reduces full agonist effects by occupying the receptor without altering its basal activity.

Partial Agonists

• Varenicline:

  • Acts as a partial agonist at nicotinic receptors, provides mild activation in non-smoking state, blocking nicotine's rewarding effects when smoking occurs.

Tolerance vs. Tachyphylaxis

Drug Selectivity

• Definition: How selectively a drug binds to and acts on its intended receptor.

• Importance: Increased selectivity reduces potential toxicity from off-target activity.

• Calculating Selectivity: Based on ratio of Kds or EC50s.

Drug-Target Selectivity Issues

• Between species (e.g., antibacterial and antiviral agents targeting pathogens).

• Within the body (selectivity between different enzymes and receptor types, subtypes, and isozymes).

Improving Drug Selectivity

• Critical step in drug discovery; involves optimizing lead compounds based on target interactions.

Therapeutic Index (TI)

• Definition: A measure of a drug's safety margin.

• Interpretation:

  • High TI = safer drug with a large margin between effective and toxic doses.

  • Low TI = narrow margin; requires careful monitoring.

• Formula: $TI = rac{TD<em>{50}}{ED</em>{50}}$

  • Where $ED₅₀$ = effective dose for 50% of subjects, $TD₅₀$ = toxic dose for 50% of subjects.

Examples of Therapeutic Index

• Wide TI: Penicillin, benzodiazepines

• Narrow TI: Digoxin, warfarin

Understanding TI with Schematic Curves

• Illustrate differences in TI; a wider index indicates a safer drug.

Dose-Response Curve Variability

• Low Slope: Greater variability in population response.

• High Slope: Uniform sensitivity; predictability in dosing.