Pharmacology Fundamentals: Drug Action and Receptor Dynamics

Core Branches of Pharmacology

• Pharmacodynamics: The study of biological effects produced by drugs and how those effects are generated.
• Pharmacokinetics: The study of drug movement through the body via ADME (Absorption, Distribution, Metabolism, and Excretion).
• Toxicology: The study of mechanisms and conditions by which chemicals produce harmful effects on living tissues or organisms.
• Specialized Fields: Systems pharmacology, Pharmacogenetics, Clinical pharmacology, and Molecular pharmacology.

Drug Nomenclature and Ideal Characteristics

Drugs are identified by their chemical name, generic name (e.g., paracetamol), and trade name (e.g., Panadol). An ideal drug should possess:

• Effectiveness, Safety, and Selectivity.
• Reversible action and Predictability (lack of predictability leads to side effects).
• Ease of administration and chemical stability.
• Freedom from drug-drug interactions.

Common Protein Targets

1. Receptors: Membrane proteins that translate extracellular information into intracellular transduction and cellular response.
2. Ion Channels: Proteins spanning the plasma membrane that allow the passage of charged ions.
3. Enzymes: Biological catalysts.
4. Transporters: Membrane proteins forming pores to allow the passage of ions, small molecules, and proteins.

Drug-Receptor Binding Factors

• Affinity: The strength of attraction between a ligand and a receptor. Measured by the dissociation constant (Kd) at 50% occupancy; a lower Kd indicates higher affinity.
• Intrinsic Activity: The ability of a drug to activate a receptor, measured by the Emax on a dose-response curve.
• Chemical Structure: Determines both affinity and intrinsic activity.
• Residence Time: The duration the drug-receptor complex persists. Longer residence times can lead to prolonged pharmacological effects but may also increase toxicity.

Potency and Agonist/Antagonist Classification

Potency is the amount of drug required to produce an effect, measured by the EC50. Selectivity is determined by structural specificity; lower specificity often leads to deleterious side effects.

Agonists

• Full Agonist: Exhibits maximum positive intrinsic activity and fully activates the receptor.
• Partial Agonist: Exhibits lower positive intrinsic activity, producing only a partial response.

Antagonists

• Active Site Binding:
  • Reversible: Decreases agonist potency without changing agonist efficacy.
  • Irreversible: Cannot be outcompeted by high agonist concentrations; limits the number of available receptors.
• Allosteric Binding: Prevents receptor activation even when an agonist binds to the active site, effectively decreasing the maximal efficacy of the agonist.