Therapeutic Approaches to ModulatingGlutathione Levels as a Pharmacological strategy in Alzheimer's Disease

Accumulating evidence has suggested the involvement of oxidative stress in the pathogenesis of Alzheimer's disease (AD).The main endogenousantioxidant,glutathione (GSH),has been shown to decline with ageing and in several age-related degenerative diseases, including AD. Potential options for replenishing GSH levels as a therapeutic target to treat these conditions include the administration of GSH itself, and low toxicity forms of the limiting amino acid for GSH synthesis; cysteineHowever, passive GSH uptake is limited due to an unfavourable concentration gradient between the plasma and cytosol. Similarly, cysteine prodrugs have demonstrated limited efficacyto elevatedepleted GSH levels in several in vivo and in vitro models of disease.It has beensuggestedthat the decline in GSH levels in AD, may be associated with down regulation ofGSH homeostasis rather than substrate limitation. Cellular GSH homeostasis is regulatedby non-allosteric feedback inhibition exerted by GSH on glutamate cysteine ligase (GCL), which is responsible for the synthesis of the GSH precursor γ-glutamylcysteine (GGC). In conditions involving down regulated GSH homeostasis, GGC serves asa crucialrate-limiting substrate for GSH synthetase, the main enzyme responsible for condensing glycine with GGC to form the final thiol tripeptide, GSH. In this review, we focus on the therapeutic potential of GGC to elevate cellular GSH levels. We also discuss the efficacy of GGC prodrugs which would be taken up and converted by the unregulated GS to GSH,andthe administration of modified GSH compounds, such as GSH esters that could potentially overcome the concentration gradient that prohibits passive GSH uptake, in AD.