Mucolytic Drugs in Cough

1. Mucoregulators / Mucolytics

a) N-Acetylcysteine (NAC)

• Acts by breaking disulfide bonds in mucus glycoproteins, reducing viscosity
• Can be inhaled (nebulized) or given orally/IV
• Also has antioxidant properties (replenishes glutathione)
• Used in COPD, cystic fibrosis, and as a mucolytic in bronchiectasis
• Note: in the emergency setting, IV acetylcysteine is the mainstay for acetaminophen toxicity, not cough - but the mucolytic use is distinct

b) Carbocisteine

• A cysteine derivative that acts on the mucous glands to alter the composition (rather than just viscosity) of mucus
• Reduces sialomucin and increases fucomucin production, normalizing gel/sol layer

c) Erdosteine

• Similar mechanism to carbocisteine; used mainly in COPD and acute exacerbations
• Has antioxidant and antibiofilm properties in addition to mucolytic effects

d) Dornase alfa (rhDNase)

• Recombinant human DNase that cleaves extracellular DNA (from neutrophils) in mucus
• Used specifically in cystic fibrosis to reduce sputum viscosity and improve lung function
• Given by inhalation

e) Hypertonic Saline

• Inhaled hypertonic saline (3%-7%) draws water into the airway lumen by osmosis, hydrating mucus and stimulating mucociliary clearance
• Used in cystic fibrosis and bronchiectasis

2. Expectorants

Guaifenesin

• The only FDA-approved expectorant in the United States
• Mechanism: thought to increase the volume and reduce the viscosity of respiratory secretions, facilitating expectoration
• Despite widespread use, objective evidence for efficacy is limited
• Interestingly, it is significantly better than placebo for reducing acute viral cough and inhibits cough-reflex sensitivity in upper respiratory tract infections
• Most other historical expectorants (potassium iodide, ammonium chloride, ipecac) have been removed from the market by the FDA due to lack of evidence

"With the exception of guaifenesin, no agents are approved as expectorants in the U.S. For patients who find it difficult to clear mucus, adequate hydration and inhalation of steam may be of some benefit." - Goodman & Gilman's The Pharmacological Basis of Therapeutics

3. Antitussives (Cough Suppressants)

a) Opioids (Central mechanism)

• Act on mu opioid receptors in the medullary cough center
• May also act peripherally on airway cough receptors
• Codeine - most commonly used; limited evidence for post-viral cough; causes sedation and constipation
• Pholcodine - similar to codeine; not available in the US
• Morphine / Methadone - effective but reserved for intractable cough in bronchial carcinoma

b) Dextromethorphan

• Centrally active NMDA receptor antagonist; may also antagonize opioid receptors
• Found in many OTC cough preparations
• Evidence is poor - not significantly different from placebo in children with acute nocturnal cough
• Risk of hallucinations at high doses and significant abuse potential

c) Local Anesthetics - Benzonatate

• Acts peripherally by anesthetizing stretch receptors in the respiratory passages, lungs, and pleura
• Reduces afferent signaling driving the cough reflex
• Dose: 100 mg three times daily, up to 600 mg/day
• Caution: severe allergic reactions in patients sensitive to para-aminobenzoic acid (a metabolite)

4. Neuromodulators (for Chronic/Refractory Cough)

Gabapentin and Pregabalin

• GABA analogues that reduce neuronal excitability
• Benefit demonstrated in chronic idiopathic cough
• Work by dampening sensory nerve hypersensitivity driving the cough
• Side effects: somnolence, dizziness (start at low doses)

5. Novel/Emerging Antitussives

a) TRP Channel Antagonists

• TRPV1 (vanilloid receptor) is activated by capsaicin, H+, and bradykinin - all potent tussive triggers
• TRPV1 inhibitors show some protection against capsaicin-induced cough but failed to improve chronic idiopathic cough clinically; development limited by hyperthermia side effects
• TRPA1 antagonists are more promising - activated by oxidants and irritants, sensitized by cytokines; several in development
• TRPV4 may also contribute and is activated by ATP

b) ATP Receptor (P2X) Antagonists

• ATP is a potent tussive agent activating P2X receptors on afferent nerves
• Gefapixant (a P2X3 antagonist) reduces chronic idiopathic cough; main side effect is dysgeusia (taste disturbance)
• Named after the late Professor Geoffrey Burnstock, pioneer of purinergic signaling

c) Other Agents

• Moguisteine (not available in US) - acts peripherally, opens ATP-sensitive K+ channels
• Theobromine - a methylxanthine (naturally in cocoa) that reduces cough induced by tussive agents

Summary Table