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Indications

• Chronic obstructive pulmonary disease (COPD)
• Cystic fibrosis
• Acetaminophen overdose
• Mucolytic therapy for respiratory conditions

Dosage

Adults: For acetaminophen overdose, intravenous administration is typically 150 mg/kg loading dose, followed by 50 mg/kg over the next 4 hours, then 100 mg/kg over the next

Mechanism of action

N-acetylcysteine exerts its mucolytic action through its free sulfhydryl group, which hydrolyzes disulfide bonds within mucin, resulting in decreased viscosity of mucus. It also reduces mucin secretion and has antioxidant properties by being deacetylated to cysteine, which is critical for glutathione synthesis. In cases of acetaminophen overdose, NAC conjugates with the toxic metabolite N-acetyl-p-benzoquinone imine (NAPQI), thus preventing hepatotoxicity and supporting glutathione restoration.

Pharmacodynamics

N-acetylcysteine is indicated for both mucolytic therapy and management of acetaminophen overdose, showing a short duration of action. It is administered every 1 to 8 hours, depending on the route. The drug has a wide therapeutic window, and patients should be advised about potential hypersensitivity reactions and gastrointestinal risks when taking the oral formulation, which may be diluted in cola to improve palatability.

Pharmacokinetics

N-acetylcysteine is rapidly absorbed and has a bioavailability that varies based on the method of administration. It is metabolized predominantly in the liver, with its effects lasting a few hours due to its short half-life. The excretion of metabolites occurs through the kidneys, and its pharmacokinetics can be influenced by factors such as pH, which affects its mucolytic efficacy.

Contra-indications

• Known hypersensitivity to acetylcysteine or any of its excipients
• Severe asthma or bronchospasm

Adverse effects

• Nausea
• Vomiting
• Diarrhea
• Rash
• Fever
• Anaphylactoid reactions
• Chest tightness

Interactions

• Nitroglycerin: May enhance the vasodilatory effects of nitroglycerin
• Activated charcoal: May reduce the effectiveness of acetylcysteine when administered concomitantly

Precautions

• Use with caution in patients with a history of asthma or bronchospasm
• Monitor for signs of hypersensitivity reactions during administration
• Ensure adequate hydration during treatment

Pregnancy

Acetylcysteine is categorized as a pregnancy category B drug. It should be used in pregnancy only if clearly needed, and the benefits outweigh the risks.

Breast-feeding

Acetylcysteine is excreted in breast milk. Caution should be exercised when administering to nursing mothers.

Storage

Store at room temperature, away from moisture and heat. Protect from light.

Formulations

• Effervescent tablets
• Oral solution
• Injectable solution

Mechanism of action

A number of possible mechanisms for the mucolytic activity of acetylcysteine have been proposed. Acetylcysteine's sulfhydryl groups may hydrolize disulfide bonds within mucin, breaking down the oligomers, and making the mucin less viscous. Acetylcysteine has also been shown to reduce mucin secretion in rat models. It is an antioxidant in its own right but is also deacetylated to cysteine, which participates in the synthesis of the antioxidant glutathione. The antioxidant activity may also alter intracellular redox reactions, decreasing phosphorylation of EGFR and MAPK, which decrease transcription of the gene MUC5AC which produces mucin. In the case of acetaminophen overdoses, a portion of the drug is metabolized by CYP2E1 to form the potentially toxic metabolite N-acetyl-p-benzoquinone imine (NAPQI). The amount of NAPQI produced in an overdose saturates and depletes glutathione stores. The free NAPQI promiscuously binds to proteins in hepatocytes, leading to cellular necrosis. Acetylcysteine can directly conjugate NAPQI or provide cysteine for glutathione production and NAPQI conjugation. Acetylcysteine exerts its mucolytic action through its free sulfhydryl group, which opens the disulfide bonds and lower the viscosity of the mucus. This action increases with increasing pH and is most significant at pH 7 to 9. The mucolytic action of acetylcysteine is not affected by the presence of DNA. Acetylcysteine may protect against acetaminophen overdose-induced hepatotoxicity by maintaining or restoring hepatic concentrations of glutathione. Glutathione is required to inactivate an intermediate metabolite of acetaminophen that is thought to be hepatotoxic. In acetaminophen overdose, excessive quantities of this metabolite are formed because the primary metabolic (glucuronide and sulfate conjugation) pathways become saturated. Acetylcysteine may act by reducing the metabolite to the parent compound and/or by providing sulfhydryl for conjugation of the metabolite. Experimental evidence also suggests that a sulfhydryl-containing compound such as acetylcysteine may directly inactivate the metabolite.

Pharmacodynamics

Acetylcysteine is indicated for mucolytic therapy and in the management of acetaminophen overdose. It has a short duration of action as it is given every 1-8 hours depending on route of administration, and has a wide therapeutic window. Patients should be counselled regarding diluting oral solutions in cola for taste masking, the risk of hypersensitivity, and the risk of upper gastrointestinal hemorrhage.