Bronchial asthma. Describe drug mechanism of ac-action of any one drug. Enumerate group of drug with one example and MOA. Classify drugs to adverse effect of any one of them.
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Bronchial Asthma - Drug Pharmacology
Classification of Drugs Used in Bronchial Asthma
Drugs are broadly classified into Bronchodilators (Relievers) and Anti-inflammatory Agents (Controllers).
Group 1: β2-Adrenoceptor Agonists (Sympathomimetics)
| Subclass | Example |
|---|---|
| Short-Acting (SABA) | Salbutamol (Albuterol) |
| Long-Acting (LABA) | Salmeterol, Formoterol |
Group 2: Methylxanthines
Example: Theophylline, Aminophylline
Group 3: Anticholinergics (Muscarinic Antagonists)
| Subclass | Example |
|---|---|
| Short-Acting (SAMA) | Ipratropium bromide |
| Long-Acting (LAMA) | Tiotropium |
Group 4: Inhaled Corticosteroids (ICS)
Examples: Beclomethasone, Budesonide, Fluticasone
Group 5: Leukotriene Modifiers
Examples: Montelukast (receptor antagonist), Zileuton (synthesis inhibitor)
Group 6: Mast Cell Stabilizers
Example: Sodium cromoglycate, Nedocromil
Group 7: Biologicals / Monoclonal Antibodies
Examples: Omalizumab (anti-IgE), Mepolizumab (anti-IL-5)
Drug Mechanism of Action (Detailed) - Salbutamol (β2-Agonist)
Salbutamol is a selective β2-adrenoceptor agonist and the prototype short-acting bronchodilator.
Step-by-step mechanism:
- Salbutamol binds to β2-adrenoceptors on airway smooth muscle cells (these receptors are abundant in bronchial smooth muscle).
- The receptor is coupled to Gs protein, which activates adenylyl cyclase (AC).
- Adenylyl cyclase converts ATP → cyclic AMP (cAMP).
- Elevated intracellular cAMP activates protein kinase A (PKA).
- PKA phosphorylates myosin light chain kinase (MLCK), inactivating it, preventing actin-myosin cross-bridge formation.
- PKA also opens large-conductance Ca²⁺-activated K⁺ channels, causing membrane hyperpolarization and reducing intracellular Ca²⁺.
- Net result: bronchial smooth muscle relaxation → bronchodilation.
Additional effects:
- Inhibits mediator release from mast cells
- Inhibits microvascular leakage
- Stimulates mucociliary clearance
MOA of One Drug from Each Group (Enumerated)
| Group | Example Drug | Mechanism of Action |
|---|---|---|
| β2-Agonist | Salbutamol | Binds β2-receptor → activates adenylyl cyclase → ↑cAMP → PKA activation → smooth muscle relaxation (bronchodilation) |
| Methylxanthines | Theophylline | Inhibits phosphodiesterase (PDE) → prevents cAMP breakdown → ↑cAMP → bronchodilation; also antagonizes adenosine receptors on bronchial smooth muscle |
| Anticholinergics | Ipratropium | Competitively blocks muscarinic (M3) receptors on bronchial smooth muscle → prevents acetylcholine-mediated bronchoconstriction |
| Corticosteroids (ICS) | Budesonide | Binds intracellular glucocorticoid receptors → translocates to nucleus → inhibits transcription of pro-inflammatory cytokines (IL-4, IL-5, TNF-α) → reduces airway inflammation, eosinophil/mast cell infiltration, and bronchial hyperreactivity |
| Leukotriene antagonist | Montelukast | Competitively antagonizes CysLT1 receptors → blocks action of leukotrienes (LTC4, LTD4, LTE4) → reduces bronchoconstriction, mucus secretion, and airway edema |
| Mast cell stabilizer | Sodium cromoglycate | Blocks Ca²⁺ channels on mast cells → prevents mast cell degranulation → inhibits release of histamine, leukotrienes, and other mediators |
| Anti-IgE | Omalizumab | Monoclonal antibody that binds free IgE → prevents IgE binding to FcεRI receptors on mast cells and basophils → reduces allergen-triggered degranulation |
Adverse Effects - Theophylline (Classified)
Theophylline has a narrow therapeutic index (therapeutic range: 10-20 mcg/mL); toxicity is common.
Dose-related / Concentration-dependent ADRs
Mild (serum level > 20 mcg/mL):
- Nausea, vomiting, anorexia
- Headache
- Restlessness, insomnia
Moderate (serum level > 25 mcg/mL):
- Tachycardia, palpitations
- Tremors
- Diarrhea
Severe (serum level > 35 mcg/mL):
- Cardiac arrhythmias (ventricular tachycardia, atrial fibrillation)
- Seizures (life-threatening, may be refractory to diazepam)
- Hypotension
System-based Classification of Theophylline ADRs
| System | Adverse Effect |
|---|---|
| GI | Nausea, vomiting, gastric irritation, peptic ulceration |
| CNS | Headache, insomnia, restlessness, tremors, seizures |
| CVS | Tachycardia, arrhythmias, hypotension |
| Metabolic | Hypokalemia, hyperglycemia |
| Renal | Diuresis (due to adenosine antagonism) |
Note: Theophylline toxicity is worsened by drugs that inhibit CYP1A2 (e.g., ciprofloxacin, erythromycin, cimetidine) - these raise serum theophylline levels. Smoking induces CYP1A2 and lowers levels.
Sources: Katzung's Basic and Clinical Pharmacology, 16th Edition (Ch. 20); Lippincott Illustrated Reviews: Pharmacology, 8th Edition (Ch. on Respiratory Pharmacology).
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