Associate editor: S. KennedyExisting and potential therapeutic uses for N-acetylcysteine: The need for conversion to intracellular glutathione for antioxidant benefits
Introduction
N-acetyl-l-cysteine (NAC) is a drug that was first reported to have clinical benefit in the early 1960s, when it was shown to be an effective mucolytic agent in patients with cystic fibrosis (CF; Hurst et al., 1967). The concept derived from the need to deliver reduced sulfhydryl moieties to effect the disruption of disulfide bridges within the glycoprotein matrix of mucus in CF patients. The amino acid residue, l-cysteine (Cys), represents an obvious candidate for such an agent, but unfortunately, it is susceptible to metabolism and undergoes rapid oxidation in solution, generating the inactive disulfide, cystine (Cys–Cys). Acetylation of the N-terminal of Cys was found to confer sufficient stability to the molecule to facilitate delivery of reduced sulfhydryl (thiol) moieties to work effectively as a mucolytic agent in this clinical setting.
During the 1970s, a substantial sequence of studies involving potential sulfhydryl donor candidates was conducted in paracetamol poisoning. However, other donors were either ineffective or provoked a significant number of adverse effects (Prescott et al., 1976). A new and important role for NAC emerged on account of subsequent studies investigating its therapeutic potential in the treatment of acetaminophen (paracetamol; N-acetyl-p-aminophenol) poisoning (Prescott et al., 1977, Prescott et al., 1979). The founding principle that underpinned the mechanism of action in this setting was similar to that for CF: delivery of sulfhydryl moieties. However, the mode of action of NAC in acetaminophen overdose was thought to rely not only on the ability of NAC to offer some protection against oxidation, but also through facilitation of rapid membrane permeability on account of the reduced polarity of the molecule compared to the parent amino acid, Cys. Cleavage of the acetyl group is thought to reveal free, reduced Cys, which is available for incorporation into the highly abundant intracellular antioxidant, glutathione (GSH). The benefit conveyed by NAC in the setting of acetaminophen overdose is to replenish hepatic GSH that has become depleted through the use of the tripeptide in the drug detoxification process. That NAC remains the treatment of choice for acetaminophen overdose more than 50 years after its first use is testament both to the importance of maintaining cellular GSH reserves and to the exceptional qualities of NAC in helping to replenish this key antioxidant when it has become acutely depleted. Despite this, the precise pharmacological mechanisms that underpin NAC activity are, as yet, not fully understood and may not even be entirely related to GSH repletion (Waring, 2012, Gosselin et al., 2013).
Since the 1980s, there has been a growing interest in the therapeutic potential of NAC in a range of diseases where oxidative stress is seen to be a driver and in which antioxidant effects might convey benefit. The original premise for believing that NAC might be effective as an antioxidant is unclear; perhaps its ability to drive synthesis of the powerful antioxidant, GSH, in hepatic cells rendered deficient through acetaminophen detoxification has been misconstrued as direct antioxidant capability, or maybe there is a general belief that all sulfhydryls will share the antioxidant power of GSH. However, the evidence regarding the antioxidant potential of NAC is that it is a relatively weak antioxidant: direct experiments to assess its antioxidant potential suggest that ~10-fold more NAC is required compared to GSH to facilitate equivalent oxygen-centred radical scavenging (Gibson et al., 2009), while the ability of NAC to scavenge one of the major biologically relevant radical species, superoxide, is not detectable (Aruoma et al., 1989). It is highly likely, therefore, that the vast majority of the antioxidant effects attributed directly to NAC are actually mediated by increased intracellular GSH. The distinction is important because it means that certain conditions might have to be satisfied in order for NAC to confer antioxidant activity: first, the enzymatic machinery necessary for GSH synthesis must be intact and expressed at sufficient levels and second, it is likely that GSH might have to be depleted for NAC to have any beneficial effect. Far from being impediments to the use of NAC in a range of clinical indications, these conditions for NAC antioxidant activity should be viewed as positive indicators for use and could open the way to stratified approaches for application of NAC only in those patients likely to benefit.
This review will provide a brief overview of the biochemistry and clinical pharmacology associated with NAC activity, followed by an evaluation of the licenced uses of NAC in clinical conditions worldwide and an appraisal of the potential of NAC in novel indications, with a bias towards those with an antioxidant component to the suggested mode of action.
Section snippets
Biochemistry
Irrespective of the clinical target, the role of NAC is to deliver sulfhydryl moieties for utilisation in biological processes. NAC has advantages over Cys in this respect because it is relatively resistant to oxidation to the disulfide and was originally believed to have the capability to cross cell membranes without the need for amino acid transporters on account of the reduced charge imparted by the acetyl moiety. However, residual polarity of the NAC molecule on account of the –SH and –COOH
Clinical pharmacology
While depletion of GSH is a feature of many disease states and its replenishment is desirable, administration of GSH per se is not considered optimal on account of its poor bioavailability and its limited ability to cross the phospholipid bilayer of cells. Likewise, delivery of Cys suffers from rapid oxidation to its disulfide, cystine, which has poor solubility and renders the crucial sulfhydryl functional group at least temporarily inaccessible. As a result, other means of sulfhydryl delivery
Cystic fibrosis and other lung diseases
Genetic mutation to the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR), which functions as a cAMP chloride channel in healthy lung cells, is responsible for cystic fibrosis (CF; Rowe & Clancy, 2006). This aberration leads to an alteration in trans-epithelial ion transport, electrolyte balance and fluid content in a range of tissues. The effect is most evident in the lung, where viscous mucus production is prolific, difficult to clear and susceptible to repeated
Discussion and conclusions
It is vital to think of NAC as a pro-drug, the actions of which are almost exclusively driven by, and dependent on, successful conversion to the powerful detoxifying agent and antioxidant, GSH. In those therapeutic targets where an antioxidant activity is the principal mode of activity, it is apparent that intracellular incorporation into GSH is vital for efficacy. The success of NAC in acetaminophen overdose is testament to this concept: there is an absolute requirement for de novo synthesis
Conflict of interest statement
The authors declare that there are no conflicts of interest.
Author declaration
The authors declare that this manuscript has not been published or submitted for publication to any other journals.
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