Acetylcysteine

Acetylcysteine (rINN; pronounced /ˌæsɛtəlˈsɪstiːn, əˌsɛtəl-/), also known as N-acetylcysteine (abbreviated NAC), is a pharmacological agent used mainly as a mucolytic and in the management of paracetamol (acetaminophen) overdose. For these indications, it is available under the trade names ACC (Hexal AG), Mucomyst (Bristol-Myers Squibb), Acetadote (Cumberland Pharmaceuticals), Fluimucil (Zambon) and Parvolex (GSK), Lysox (Menarini).

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Dosage forms

Acetylcysteine is available in different dosage forms for different indications:

• Solution for inhalation (Mucomyst, Mucosil) – inhaled for mucolytic therapy or ingested for nephroprotective effect (to protect the kidneys)
• IV injection (Parvolex, Acetadote) – treatment of paracetamol/acetaminophen overdose
• Oral solution – various indications

The IV injection and inhalation preparations are, in general, prescription only, whereas the oral solution is available over the counter in many countries.

Clinical use

Mucolytic therapy

Inhaled acetylcysteine is indicated for mucolytic ("mucus-dissolving") therapy as an adjuvant in respiratory conditions with excessive and/or thick mucus production. Such conditions include emphysema, bronchitis, tuberculosis, bronchiectasis, amyloidosis, pneumonia. It is also used post-operatively, as a diagnostic aid, and in tracheostomy care. It may be considered ineffective in cystic fibrosis (Rossi, 2006). However, a recent paper in the Proceedings of the National Academy of Sciences reports that high-dose oral N-acetylcysteine modulates inflammation in cystic fibrosis and has the potential to counter the intertwined redox and inflammatory imbalances in CF (Tirouvanziam et al., 2006). Oral acetylcysteine may also be used as a mucolytic in less serious cases.

For this indication, acetylcysteine acts to reduce mucus viscosity by splitting disulfide bonds linking proteins present in the mucus (mucoproteins).

Paracetamol overdose

Intravenous acetylcysteine is indicated for the treatment of paracetamol (acetaminophen) overdose. Although both IV and oral acetylcysteine are equally effective for this indication, oral administration is uncommon, as it is poorly tolerated, owing to the high doses required (due to low oral bioavailability), very unpleasant taste and odour, and adverse effects (particularly nausea and vomiting). 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 (Kanter, 2006). Repeated overdoses will cause the allergic reaction to get worse and worse. 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 (Dawson et al., 1989; Bailey & McGuigan, 1998; Schmidt & Dalhoff, 2001; Lynch & Robertson, 2004).

For this indication, acetylcysteine acts to augment glutathione reserves (depleted by toxic paracetamol metabolites) in the body and, together with glutathione to directly bind to toxic metabolites. These actions serve to protect hepatocytes in the liver from toxicity due to paracetamol overdose.

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.

Nephroprotective agent

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 (90%) radiocontrast nephropathy (Tepel et al 2000), whereas others appear to cast doubt on its efficacy (Hoffman et al., 2004; Miner et al., 2004). Worth considering is the newest data published in two papers in the New England Journal of Medicine and the Journal of the American Medical Association. The authors' conclusions in those papers were: 1) "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." (Marenzi et al, 2006) 2) "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" (Kay et al., 2003).

Acetylcysteine continues to be commonly used in individuals with renal impairment to prevent the precipitation of acute renal failure.

Investigational

The following uses have not been well-established or investigated:

• NAC is undergoing clinical trials in the United States for the treatment of obsessive-compulsive disorder.^[1] It is thought to counteract the glutamate hyperactivity in OCD.
• NAC has been shown to reduce cravings associated with chronic cocaine use in a study conducted at the Medical University of South Carolina (Mardikian et al, 2007; LaRowe et al, 2007)
• It may reduce the incidence of chronic obstructive pulmonary disease (COPD) exacerbations (Pela et al., 1999)
• In the treatment of AIDS, NAC has been shown to cause a "marked increase in immunological functions and plasma albumin concentrations" (Breitkreutz & al, 2000). Albumin concentration are inversely correlated with muscle wasting (cachexia), a condition associated with AIDS.
• An animal study indicates that acetylcysteine may decrease mortality associated with influenza (Ungheri et al., 2000)
• Animal studies suggest that NAC may help prevent noise-induced hearing loss (Kopke et al., 2005). A clinical trial to determine efficacy in preventing noise-induced sensorineural hearing loss in humans is currently (2006) being jointly conducted by the US Army and US Navy.
• It has been suggested that NAC 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 (Bachert et al., 2003).
• There are claims that acetylcysteine taken together with vitamin C and B1 can be used to prevent and relieve symptoms of veisalgia (hangover following ethanol (alcohol) consumption). The claimed mechanism is through scavenging of acetaldehyde, a toxic intermediate in the metabolism of ethanol.
• It has been shown to help women with PCOS (polycystic ovary syndrome) to reduce insulin problems and possibly improve fertility. (Fulghesu, et al, 2002)

Chemistry

Acetylcysteine is the N-acetyl derivative of the amino acid L-cysteine, and is a precursor in the formation of the antioxidant glutathione in the body. The thiol (sulfhydryl) group confers antioxidant effects and is able to reduce free radicals.

Possible toxicity

Researchers at the University of Virginia recently reported that acetylcysteine, which is found in many bodybuilding supplements, could potentially cause damage to the heart and lungs (Palmer et al., 2007). 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 (2007) was dramatically higher than that used in humans; nonetheless, the drug's effects on the hypoxic ventilatory response have been observed previously in human subjects at more moderate doses (Hildebrandt et al., 2002).

References

See also

• Glutathione
• Cysteine