Beta-2 Adrenergic Sympathomimetics in the Treatment of Asthma: Bronchodilators Notes, Lecture and MCQ with Answer
Beta-2 Selective Sympathomimetics in the Treatment of Asthma
Beta-2 selective sympathomimetics are a class of drugs primarily used in the treatment of asthma due to their ability to cause bronchodilation by selectively stimulating beta-2 adrenergic receptors in the lungs. These drugs are essential in managing acute asthma attacks and in long-term asthma control.
Mechanism of Action
Beta-2 agonists bind to beta-2 adrenergic receptors on bronchial smooth muscle, leading to activation of adenylate cyclase. This increases cyclic AMP (cAMP) levels, which in turn activates protein kinase A (PKA). PKA phosphorylates target proteins that mediate smooth muscle relaxation, resulting in bronchodilation. Additionally, these drugs reduce airway inflammation and inhibit the release of bronchoconstrictor mediators from mast cells.
Classification and Pharmacokinetics
| Drug | Mechanism of Action | Pharmacokinetics | Therapeutic Uses | Side Effects | Drug-Drug Interactions |
|---|---|---|---|---|---|
| Short-Acting Beta-2 Agonists (SABAs) | |||||
| Albuterol (Salbutamol) | Rapidly binds to beta-2 receptors, causing bronchodilation | Onset: 5-15 min; Duration: 4-6 hrs; Metabolized in liver, excreted in urine | Acute asthma attack, exercise-induced bronchospasm | Tachycardia, tremors, hypokalemia | Enhanced effects with MAO inhibitors, increased risk of hypokalemia with diuretics |
| Levalbuterol | More selective beta-2 activation, causing bronchodilation | Onset: 5-10 min; Duration: 6-8 hrs | Acute asthma, COPD | Similar to albuterol but with fewer cardiac effects | Similar to albuterol |
| Terbutaline | Selective beta-2 activation, used in bronchospasm | Onset: 5-15 min; Duration: 4-6 hrs | Acute asthma, COPD, tocolytic agent in preterm labor | Tachycardia, tremor, dizziness | Interacts with beta-blockers and enhances hypokalemia with corticosteroids |
| Long-Acting Beta-2 Agonists (LABAs) | |||||
| Salmeterol | Binds to beta-2 receptors with prolonged action | Onset: 10-20 min; Duration: ~12 hrs | Chronic asthma, COPD | Arrhythmias, muscle cramps | Contraindicated as monotherapy in asthma |
| Formoterol | Full agonist of beta-2 receptors, longer action | Onset: 3-5 min; Duration: ~12 hrs | Asthma maintenance, COPD | Tachycardia, paradoxical bronchospasm | Potentiates effects of beta-blockers |
| Indacaterol | Ultra-LABA with prolonged bronchodilation | Onset: ~5 min; Duration: ~24 hrs | COPD only | Headache, throat irritation | Interacts with CYP3A4 inhibitors |
| Vilanterol | Ultra-long acting beta-2 agonist | Onset: ~5 min; Duration: ~24 hrs | Asthma (in combination with corticosteroids), COPD | QT prolongation, tremors | Metabolized via CYP3A4, interacts with antifungals and antivirals |
Multiple Choice Questions (MCQs)
- What is the primary mechanism of action of beta-2 agonists? A) Blocking beta-2 receptors
B) Increasing cAMP levels leading to bronchodilation
C) Decreasing norepinephrine release
D) Inhibiting adenylate cyclase
Answer: B - Which of the following is a short-acting beta-2 agonist (SABA)? A) Salmeterol
B) Formoterol
C) Albuterol
D) Indacaterol
Answer: C - Which beta-2 agonist has the longest duration of action? A) Albuterol
B) Salmeterol
C) Indacaterol
D) Terbutaline
Answer: C - Which of the following is a common side effect of beta-2 agonists? A) Bradycardia
B) Hyperkalemia
C) Tachycardia
D) Hypotension
Answer: C - Which LABA should NOT be used as monotherapy for asthma? A) Indacaterol
B) Formoterol
C) Salmeterol
D) Vilanterol
Answer: C - What is the major contraindication of LABAs in asthma treatment? A) Used alone without inhaled corticosteroids
B) Used in children under 5 years old
C) Used in patients with hypertension
D) Used in acute bronchospasm
Answer: A - Which beta-2 agonist is also used as a tocolytic agent? A) Albuterol
B) Terbutaline
C) Formoterol
D) Vilanterol
Answer: B - The onset of action of salmeterol is approximately: A) 2 minutes
B) 10-20 minutes
C) 1 hour
D) 24 hours
Answer: B - Which of the following beta-2 agonists has the highest selectivity? A) Levalbuterol
B) Albuterol
C) Terbutaline
D) Salmeterol
Answer: A - What is the primary metabolic pathway of beta-2 agonists? A) Renal excretion
B) Hepatic metabolism
C) Plasma esterases
D) Direct lung metabolism
Answer: B - Which enzyme primarily metabolizes vilanterol? A) CYP2D6
B) CYP3A4
C) MAO
D) CYP1A2
Answer: B - Which beta-2 agonist has the fastest onset of action? A) Indacaterol
B) Salmeterol
C) Formoterol
D) Terbutaline
Answer: C - What is the primary route of elimination of beta-2 agonists? A) Biliary excretion
B) Renal excretion
C) Pulmonary clearance
D) Direct metabolism in muscles
Answer: B - Which of the following can increase the risk of hypokalemia when used with beta-2 agonists? A) Beta-blockers
B) Corticosteroids
C) Antihistamines
D) NSAIDs
Answer: B - Which LABA is specifically indicated for COPD but not for asthma? A) Salmeterol
B) Formoterol
C) Indacaterol
D) Albuterol
Answer: C