STUDY NOTES: INTRODUCTION TO PHARMACODYNAMICS

1. Definition of a Drug

• A drug is a chemical substance that, when administered to a living organism, produces a biological effect.

• Drugs can be:

  • Therapeutic: Used to treat, cure, or alleviate disease symptoms (e.g., antibiotics, analgesics).

  • Non-therapeutic: Recreational (e.g., caffeine, nicotine) or experimental (research tools).

• Medicine: A drug specifically formulated for therapeutic use; may contain stabilizers or solvents.

• Poison vs Drug:

  • The difference often lies in dose — small (therapeutic) doses can be beneficial, while large doses can be toxic.

  • “The dose makes the poison.”

2. Classification of Drugs

Drugs can be classified by:

• Chemical Structure: Based on molecular composition.

• Mechanism of Action (MOA): How the drug produces its biological effect.

• Therapeutic Use: What the drug is designed to do (e.g., antihypertensive, analgesic, antidepressant).

3. Drug Naming Systems

Drugs have three major types of names:

Type	Description	Example
Chemical Name	Describes molecular structure; used by chemists; complex	(RS)-2-(4-(2-methylpropyl)phenyl)propanoic acid
Generic Name	Approved by regulatory bodies; used universally	Ibuprofen
Proprietary (Brand) Name	Trademarked by pharmaceutical companies	Advil, Nurofen

• Example: Ibuprofen = Generic name; Advil and Nurofen = Brand names.

• Generic names prevent confusion and are standardized for global recognition.

4. What is Pharmacodynamics?

• Pharmacodynamics (PD) is the study of how drugs produce their effects on the body.

• It examines:

  • Drug–receptor interactions

  • Cellular responses

  • Organ-level and systemic effects

Importance:

• Helps in developing safe and effective drugs

• Predicts drug effects, dosing, and toxicity

• Reduces adverse reactions

5. How Drugs Exert Effects

• Drugs work by modifying existing physiological or biochemical processes.

• They may:

  • Activate or inhibit biological targets

  • Form covalent, electrostatic, or hydrophobic interactions with targets

Drug Targets Include:

1. Ion Channels

2. Carrier Proteins (Transporters)

3. Enzymes

4. Receptors

6. Types of Drug Targets

A. Ion Channels

• Ion channels are membrane proteins that control the flow of ions (Na⁺, K⁺, Ca²⁺) across cell membranes.

• Important in neurons, muscle cells, and secretory cells.

Types:

1. Voltage-Gated Ion Channels:

   • Open/close in response to changes in membrane potential.

   • Example: Sodium channels in neurons.

   • Drugs:

     • Tetrodotoxin — blocks Na⁺ channels (causes paralysis).

     • Lidocaine (Lignocaine) — blocks Na⁺ channels in active neurons (local anesthetic).

2. Ligand-Gated Ion Channels (Ionotropic Receptors):

   • Open when a chemical ligand (e.g., neurotransmitter) binds.

   • Example: Nicotinic acetylcholine receptor opens Na⁺ channels when ACh binds.

Other Types:

• Stretch-sensitive (respond to physical force)

• Temperature-sensitive channels

B. Carrier Proteins (Transporters)

• Carrier proteins move molecules across membranes via conformational change — no open pore.

• Can be:

  • Passive (facilitated diffusion) — no energy needed.

  • Active transport — requires ATP.

Drug Effects:

• Drugs can block carrier proteins to alter neurotransmitter reuptake.

Examples:

• Cocaine: Blocks dopamine and norepinephrine reuptake → euphoria, stimulation.

• Fluoxetine (Prozac): Selective serotonin reuptake inhibitor (SSRI) → increases serotonin levels → antidepressant effect.

C. Enzymes

• Enzymes are biological catalysts that speed up reactions.

• Drugs may inhibit or activate enzymes to alter biological pathways.

Example:

• Neostigmine: Reversibly inhibits acetylcholinesterase → prevents breakdown of acetylcholine → ↑ ACh at neuromuscular junction → treats myasthenia gravis.

• Organophosphates: Irreversible AChE inhibitors → toxic accumulation of acetylcholine.

D. Receptors

• Receptors are proteins that detect chemical signals (hormones, neurotransmitters) and trigger cellular responses.

Drug–Receptor Interactions:

• Agonist: Binds and activates receptor → mimics natural ligand.

• Antagonist: Binds but does not activate → blocks natural ligand.

Examples:

• Beta-Agonists: Stimulate β-adrenergic receptors → increase heart rate, dilate airways.

• Antihistamines: Block histamine receptors → relieve allergy symptoms.

7. Non-Selective Drug Actions

• Some drugs act via simple physical or chemical mechanisms, not specific targets.
  Examples:

• Antacids (e.g., CaCO₃): Neutralize stomach acid chemically.

• Osmotic Laxatives: Draw water into the intestines → promote bowel movement.

8. Summary Table

Drug Target	Example Drugs	Mechanism	Effect
Ion Channels	Lidocaine, Tetrodotoxin	Block Na⁺ channels	Nerve impulse inhibition
Carrier Proteins	Cocaine, Fluoxetine	Block neurotransmitter reuptake	Enhanced synaptic signaling
Enzymes	Neostigmine, Organophosphates	Inhibit AChE	↑ ACh levels
Receptors	Beta-Agonists, Antihistamines	Activate or block receptor	Alter physiological response
Non-Selective Agents	Antacids, Laxatives	Physical/chemical actions	Neutralize acid, draw water

9. Key Takeaways

• Pharmacodynamics = What the drug does to the body.

• Pharmacokinetics = What the body does to the drug.

• Drug action depends on target, dose, binding, and interaction type.

• Understanding PD is vital for:

  • Safe drug design

  • Dosing precision

  • Preventing side effects