Acetylcysteine is a derivative of cysteine; an acetyl group that is attached to the nitrogen atom. This compound is sold as a dietary supplement commonly claiming antioxidant and liver protecting effects. It is used as a cough medicine because it breaks disulfide bonds in mucus and liquefies it, making it easier to cough up. It is also this action of breaking disulfide bonds that makes it useful in thinning the abnormally thick mucus in cystic and pulmonary fibrosis patients.
Intravenous and oral formulations of acetylcysteine are available for the treatment of paracetamol (acetaminophen) overdose. When paracetamol is taken in large quantities, a minor metabolite called N-acetyl-p-benzoquinone imine (NAPQI) accumulates within the body. It is normally conjugated by glutathione, but when taken in excess, the body's glutathione reserves are not sufficient to inactivate the toxic NAPQI. This metabolite is then free to react with key hepatic enzymes, thereby damaging hepatocytes. This may lead to severe liver damage and even death by acute liver failure.
In the treatment of acetaminophen overdose, acetylcysteine acts to maintain or replenish depleted glutathione reserves in the liver and enhance non-toxic metabolism of acetaminophen. These actions serve to protect hepatocytes in the liver from NAPQI toxicity. It is most effective in preventing or lessening hepatic injury when administered within 8–10 hours after overdose. Research suggests that the rate of hepatotoxicity is approximately 3% when acetylcysteine is administered within 10 hours of overdose.
Although both IV and oral acetylcysteine are equally effective for this indication, oral administration is poorly tolerated because high oral doses are required due to low oral bioavailability, because of its very unpleasant taste and odour, and because of adverse effects, particularly nausea and vomiting. Prior pharmacokinetic studies of acetylcysteine did not consider acetylation as a reason for the low bioavailability of acetylcysteine. Oral acetylcysteine is identical in bioavailability to cysteine precursors. However, 3% to 6% of people given intravenous acetylcysteine show a severe, anaphylaxis-like allergic reaction, which may include extreme breathing difficulty (due to bronchospasm), a decrease in blood pressure, rash, angioedema, and sometimes also nausea and vomiting. Repeated doses of intravenous acetylcysteine will cause these allergic reactions to progressively worsen in these people.
Several studies have found this anaphylaxis-like reaction to occur more often in people given IV acetylcysteine despite serum levels of paracetamol not high enough to be considered toxic. In some countries, a specific intravenous formulation does not exist to treat paracetamol overdose. In these cases, the formulation used for inhalation may be used intravenously.
For this indication, acetylcysteine acts to reduce mucus viscosity by splitting disulfide bonds linking proteins present in the mucus (mucoproteins). Furthermore, with respect to its use as a mucolytic agent in patients with COPD, it is hypothesized that acetylcysteine may exert additional beneficial effects through its anti-inflammatory and antioxidant properties.
Oral acetylcysteine is used for the prevention of radiocontrast-induced nephropathy (a form of acute renal failure). Some studies show that prior administration of acetylcysteine markedly decreases radiocontrast nephropathy, whereas others appear to cast doubt on its efficacy. It has been concluded that
"Intravenous and oral N-acetylcysteine may prevent contrast-medium–induced nephropathy with a dose-dependent effect in patients treated with primary angioplasty and may improve hospital outcome."
"Acetylcysteine protects patients with moderate chronic renal insufficiency from contrast-induced deterioration in renal function after coronary angiographic procedures, with minimal adverse effects and at a low cost"
A clinical trial from 2010, however, found that acetylcysteine is ineffective for the prevention of contrast-induced nephropathy. This trial, involving 2,308 patients, found that acetylcysteine was no better than placebo; whether acetylcysteine or placebo was used, the incidence of nephropathy was the same — 13%.
Despite the conflicting research outcomes, the 2012 Kidney Disease: Improving Global Outcomes Guidelines suggest the use of oral acetylcysteine for the prevention of contrast-induced nephropathy in high-risk individuals, given its potential for benefit, low likelihood of adverse effects, and low cost.
Acetylcysteine has been used for cyclophosphamide-induced hemorrhagic cystitis, although mesna is generally preferred due to the ability of acetylcysteine to diminish the effectiveness of cyclophosphamide.
Acetylcysteine can be used in Petroff's method i.e. liquefaction and decontamination of sputum, in preparation for recovery of mycobacterium. It also displays significant antiviral activity against the influenza A viruses.
Acetylcysteine has also been hypothesized to exert beneficial effects through its modulation of glutamate and dopamine neurotransmission as well as its antioxidant properties. NAC has also been trialled with some efficacy in people with Alzheimer disease.
The most commonly reported adverse effects for IV formulations of acetylcysteine are rash, urticaria, and pruritis. Up to 18% of patients have been reported to experience anaphylactoid reactions, which are defined as rash, hypotension, wheezing, and/or shortness of breath. Lower rates of anaphylactoid reactions have been reported with slower rates of infusion.
Adverse effects for inhalational formulations of acetylcysteine include nausea, vomiting, stomatitis, fever, rhinorrhea, drowsiness, clamminess, chest tightness, and bronchoconstriction. Through infrequent, bronchospasm has been reported to occur unpredictably in some patients.
Adverse effects for oral formulations of acetylcysteine have been reported to include nausea, vomiting, rash, and fever.
Antioxidants are widely used to protect cells from damage induced by reactive oxygen species (ROS). The concept that antioxidants can help fight cancer is deeply rooted in the general population, promoted by the food supplement industry, and supported by some scientific studies. However, clinical trials have reported inconsistent results. Supplementing the diet with the antioxidants N-acetylcysteine (NAC) and vitamin E markedly increased tumor progression and reduced survival in mouse models of B-RAF and K-RAS induced lung cancer. RNA sequencing revealed that NAC and vitamin E, which are structurally unrelated, produce highly coordinated changes in tumor transcriptome profiles, dominated by reduced expression of endogenous antioxidant genes. NAC and vitamin E increase tumor cell proliferation by reducing ROS, DNA damage, and p53 expression in mouse and human lung tumor cells. High levels of ROS or prolonged stress upregulates p53 and provokes a pro-oxidant response to further increase ROS, which subsequently elicits the p53-dependent apoptotic processes to eliminate damaged cells. Thus, antioxidants can accelerate tumor growth by disrupting the ROS-p53 axis apoptosis, and autophagy, processes. Because somatic mutations in p53 occur late in tumor progression, antioxidants may accelerate the growth of early tumors or precancerous lesions in high-risk populations such as smokers and patients with chronic obstructive pulmonary disease who receive NAC to relieve mucus production. It is not clear what dose(s) induced these effects. Additionally, it is important to reiterate that NAC does not cause cancer, it counteracts ROS accumulation caused by p53 and down-regulates p53, which in turn prevents p53-induced apoptosis and promotes autophagy. in all cells; it is a dose dependent response, and the ability to manipulate cellular apoptosis and autophagy has many therapeutic benefits. Large doses in a mouse model that acetylcysteine could potentially cause damage to the heart and lungs. They found that acetylcysteine was metabolized to S-nitroso-N-acetylcysteine (SNOAC), which increased blood pressure in the lungs and right ventricle of the heart (pulmonary artery hypertension) in mice treated with acetylcysteine. The effect was similar to that observed following a 3-week exposure to an oxygen-deprived environment (chronic hypoxia). The authors also found that SNOAC induced a hypoxia-like response in the expression of several important genes both in vitro and in vivo.
The implications of these findings for long-term treatment with acetylcysteine have not yet been investigated. The dose used by Palmer and colleagues was dramatically higher than that used in humans, the equivalent of about 20 grams per day. Nonetheless, positive effects on age-diminished control of respiration (the hypoxic ventilatory response) have been observed previously in human subjects at more moderate doses.
Although N-acetylcysteine prevented liver damage when taken before alcohol, when taken 4 hours after alcohol it actually made liver damage worse in a dose-dependent fashion.
Mechanism of action
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. Acetylcysteine also possesses some anti-inflammatory effects possibly via inhibiting NF-κB and modulating cytokine synthesis. It may also facilitate dopamine release in certain brain areas.
Acetylcysteine has been used to complexpalladium, to help it dissolve in water. This helps to remove palladium from drugs or precursors synthesized by palladium-catalyzed coupling reactions.
The IV injection and inhalation preparations are, in general, prescription only, whereas the oral solution and the effervescent tablets are available over the counter in many countries. N-Acetyl L-Cysteine is available as a health supplement in the United States that are normally produced in capsule form. Dosage rates have been stated to be 1800 to 2000 milligrams per day in three divided doses. Dr. William LaValley from Austin, Texas has done extensive research in the area and feels that, "200-500mg once a day is probably ok in most non-cancer cases. NAC is an important consideration for inclusion, at some reasonable dose, in anti-aging formulas."
There is some evidence that acetylcysteine may be useful in traumatic brain injury. Trials in humans; however, have not been reported as of 2014.
It has been suggested that acetylcysteine may help sufferers of Samter's triad by increasing levels of glutathione allowing faster breakdown of salicylates, though there is no evidence that it is of benefit.
It has been shown effective in the treatment of Unverricht-Lundborg disease in an open trial in 4 patients. A marked decrease in myoclonus and some normalization of somatosensory evoked potentials with acetylcysteine treatment has been documented.
The effect of acetylcysteine in combination with glucocorticoids (combination group) for patients suffering from severe alcoholic hepatitis was examined and showed that the combination of acetylcysteine with prednisolone decreased mortality significantly at one month compared to the prednisolone-only group (8% vs 24%, P=0.006). However, the improvement was not as significant at 3 months or 6 months (22% vs 34%, P=0.06) and (27% vs 38%, P=0.07). Factors that were associated with increased 6-month survival included younger age, shorter prothrombin time, lower levels of bilirubin in baseline studies, and decrease in bilirubin on day 14, all (P<0.001). Death due to hepatorenal syndrome occurred less frequently for the combination group at 6 months (9% vs 22%, P=0.02) and infections were also less frequent in the combination group as well (P=0.001). Six-month survival, the primary outcome, was not improved in conclusion.
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