Acetylcysteine protects against acetaminophen overdose-induced hepatotoxicity by maintaining or restoring hepatic concentrations of glutathione. It does this by producing the glutathione precursor L-cysteine. Glutathione is required to inactivate an intermediate metabolite (N-acetyl-p-benzoquinoneimine or NAPQI) of acetaminophen that is thought to be hepatotoxic. In acetaminophen overdose cases, 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 also directly inactivate the metabolite. The mechanisms of action for acetylcysteine’s well-known mucolytic effects are different.
In particular, when inhaled, acetylcysteine (and its metabolic byproduct cysteine) exerts its mucolytic action through its free sulfhydryl group, which reduces the disulfide bonds in the mucus matrix and lowers mucus viscosity. 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 is also an antioxidant and reduces oxidative stress.
Acetylcysteine serves as a prodrug to L-cysteine which is a precursor to the biologic antioxidant, glutathione and hence administration of acetylcysteine replenishes glutathione stores. L-cysteine also serves as a precursor to cystine which in turn serves as a substrate for the cystine-glutamate antiporter on astrocytes hence increasing glutamate release into the extracellular space. This glutamate in turn acts on mGluR2/3 receptors, and at higher doses of acetylcysteine, mGluR5. Glutathione also modulates the NMDA receptor by acting at the redox site. These effects on glutamate and NMDA signaling appear to explain some of the positive neuropsychotropic effects associated with NAC.
Acetylcysteine also possesses some anti-inflammatory effects possibly via inhibiting NF-κB through redox activation of the nuclear factor kappa kinases thereby modulating cytokine synthesis.
Acetylcysteine (also known as N-acetylcysteine or N-acetyl-L-cysteine or NAC) is primarily used as a mucolytic agent and in the management of acetaminophen poisoning. It is a derivative of cysteine with an acetyl group attached to the amino group of cysteine. NAC is essentially a prodrug that is converted to cysteine (in the intestine by the enzyme aminoacylase and absorbed in the intestine into the blood stream. Cysteine is a key constituent to glutathione and hence administration of acetylcysteine replenishes glutathione stores.
Acetylcysteine can also be used as a general antioxidant which can help mitigate symptoms for a variety of diseases exacerbated by reactive oxygen species (ROS). For instance, acetylcysteine is commonly used in individuals with renal impairment to prevent the precipitation of acute renal failure. Acetylcysteine has been shown to have efficacy in treating mild to moderate traumatic brain injury including ischemic brain injury, particularly in reducing neuronal losses, and also reducing cognitive and neurological symptoms when administered promptly after injury. N-acetylcysteine is now widely used in the treatment of HIV, and it has reported efficacy in chronic obstructive pulmonary disease and contrast-induced nephropathy.
Acetylcysteine is also being successfully used to treat a variety of neuropsychiatric and neurodegenerative disorders including cocaine, cannabis, and smoking addictions, Alzheimer's and Parkinson's diseases, autism, compulsive and grooming disorders, schizophrenia, depression, and bipolar disorder. Recent data also shows that N-acetylcysteine inhibits muscle fatigue and can be used to enhance performance in endurance events and in exercise and endurance training.
Acetylcysteine is also undergoing clinical trials as RK-0202, an oral rinse for the prevention and treatment of mucositis. It is comprised of acetylcysteine in a polymer matrix.