๐Ÿ“– Mechanisms of Drug Action

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By Dr. Sk Sabir Rahaman, MBBS, MD (Pharmacology), DFM(Family Medicine), FCFM, CCEBDM, CCLSD 

๐Ÿ“ Specialist Family Physician | Consultant Pharmacologist | Lifestyle & Diabetes Expert 



Drugs influence the body through two main mechanisms:

  1. Non-receptor mediated actions

  2. Receptor-mediated actions

๐Ÿ”น I. Non–Receptor Mediated Mechanisms

These don’t rely on binding to specific receptors. Instead, drugs act via physical, chemical, enzymatic, or structural properties.

1. Physical Actions

  • Osmosis: Mannitol ↑ plasma osmolality → draws water out of brain/eye → used in cerebral edema, glaucoma.

  • Adsorption: Activated charcoal binds toxins in the gut → prevents absorption in poisoning.

  • Demulcents: Glycerin/syrup coat mucosa → soothe sore throat, pharyngitis.

  • Radioactivity: Radioactive iodine (¹³¹I) destroys thyroid tissue in Graves’ disease, thyroid cancer.

2. Chemical Reactions

  • Neutralization: Antacids (Al(OH)₃, Mg(OH)₂) neutralize gastric acid.

  • Chelation: Chelating agents bind toxic metals → enhance excretion.

    • BAL → arsenic, mercury

    • Desferrioxamine → iron

    • Penicillamine → copper (Wilson’s disease)

3. Enzyme Inhibition

  • ACE inhibitors (captopril, enalapril): ↓ Ang II → vasodilation → hypertension, heart failure.

  • Allopurinol: Inhibits xanthine oxidase → ↓ uric acid → gout.

4. Ion Channel Modulation

  • Lidocaine: Blocks Na⁺ channels in nerves → no impulse → local anesthesia.

5. Stimulation of Antibody Production

  • Vaccines (BCG, OPV) → trigger adaptive immunity.

6. Transporter Modulation

  • SSRIs (fluoxetine): Block serotonin reuptake → ↑ 5-HT in synapse → antidepressant effect.

7. Structural Interference

  • Colchicine: Binds tubulin → blocks microtubule formation → inhibits neutrophil migration → acute gout.

๐Ÿ”น II. Receptor-Mediated Mechanisms

Most drugs act by binding receptors (cell membrane, cytoplasm, or nucleus).

๐Ÿ‘‰ Drug + Receptor ⇌ Complex → Biological Response

Key Terms

  • Affinity: How well a drug binds the receptor.

  • Intrinsic Activity (Efficacy): Ability to produce a response once bound.

Classification

  • Agonist: High affinity + high activity → full response (adrenaline, morphine).

  • Antagonist: High affinity, no activity → blocks response (atropine, naloxone).

  • Partial agonist: High affinity, moderate activity → submaximal effect, blocks full agonist (buprenorphine, pindolol).

  • Inverse agonist: High affinity, negative activity → reduces basal receptor activity (ฮฒ-carbolines at BZD receptor).

Clinical Relevance (examples of receptor subtypes)

  • ฮฒ₁ adrenergic: ↑ HR, contractility (target for ฮฒ-blockers).

  • Muscarinic M3: Bronchoconstriction, secretions (blocked by atropine).

  • ฮผ-opioid: Analgesia, euphoria (morphine, naloxone).

๐Ÿงญ Summary & Study Tips

  • Not all drugs act on receptors (think: mannitol, antacids, charcoal).

  • Enzyme inhibitors, transport blockers, and vaccines are major non-receptor mechanisms.

  • For receptor drugs, remember:

    • Agonist = stimulate

    • Antagonist = block

    • Partial agonist = weaker stimulate + block stronger drugs

    • Inverse agonist = suppress basal activity

  • Always ask in practice:

    1. What is the target?

    2. What is the desired effect?

    3. Which mechanism explains side effects?

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