For almost 90 years following the original description of Alois Alzheimer's patient and the identification of Alzheimer's disease (AD) (Alzheimer, 1907), physicians faced the bleak prospect of observing the inexorable and relentless decline in cognition, function, and behavior with little or no opportunity for therapeutic intervention. In the last 5 years clinicians have finally been provided with a class of medications, the cholinesterase (ChE) inhibitors, which have passed the test of efficacy and safety in the symptomatic management of AD and related dementias. With the arrival of donepezil, rivastigmine, and galantamine as the second generation of ChE inhibitors, a renewed and sustained interest in the diagnosis and care of AD patients might have been anticipated. However, there remains residual therapeutic nihilism and skepticism over the utility of these treatments in some quarters of the medical community and among some paying authorities. In moving forward and addressing these concerns, we must reflect carefully on the question, “What have we learned about the ChE inhibitors so far?”

Alzheimer's disease (AD) is characterized by deterioration in the ability to perform activities of daily living (ADL) in addition to loss of cognitive function and behavioral changes. This decline in day-to-day functioning is increasingly recognized as a source of considerable social, health, and economic costs. Inability to perform ADL results in growing caregiver burden and may lead to the eventual need for alternative care or nursing home placement. The measurement of ADL, which enables monitoring of the effectiveness of therapeutic interventions, can be performed using a number of inventories including the Progressive Deterioration Scale (PDS), the Disability Assessment for Dementia (DAD), and the Alzheimer Disease Cooperative Study ADL (ADCS/ADL) assessment scale. Clinical studies using these and other scales have indicated that cholinesterase (ChE) inhibitors offer an effective approach to treating the functional decline of AD. Donepezil, rivastigmine, and galantamine have been shown in some studies to prevent or slow decline in ADL over treatment periods of one to two years. For instance, in a 24-week study in subjects with moderate to severe AD, donepezil-treated patients remained stable compared with the placebo-treated patients. Rivastigmine has shown improvement or stabilization of PDS scores in patients with mild to moderate disease following 26 weeks of treatment and slowed deterioration in patients with more severe disease. Evidence to date suggests that these agents may not be equally effective at slowing or stabilizing loss in ADL over time and that these differences may reflect differences in pharmacology. In addition to inhibition of acetylcholinesterase (AChE), these compounds have other putative differences in mechanisms of action. Galantamine allosterically modulates the nicotinic receptor and may prevent the loss of ADL. Rivastigmine robustly inhibits butyrylcholinesterase in addition to AChE and therefore acts as a dual ChE inhibitor. Comparative studies evaluating the differential effects of these ChE inhibitors on ADL are awaited.

Behavioral and psychological symptoms of dementia (BPSD) are a common manifestation of Alzheimer's disease (AD) and other dementia syndromes. Patients experience prominent and multiple symptoms, which are both distressing and a source of considerable social, health, and economic cost. Development of symptoms is in part related to progressive neurodegeneration and cholinergic deficiency in brain regions important in the regulation of behavioral and emotional responses including the cortex, hippocampus, and limbic system. Cholinesterase (ChE) inhibitors offer a mechanism-based approach to therapy to enhance endogenous cholinergic neurotransmission. Studies using ChE inhibitors have demonstrated their clear potential to improve or stabilize existing BPSD. Differences have been noted between selective acetylcholinesterase (AChE) inhibitors (donepezil and galantamine) and dual ChE inhibitors (rivastigmine) in terms of treatment response. While donepezil has shown efficacy in moderate to severe noninstitutionalized AD patients, conflicting results have been obtained in mild to moderate patients and in nursing home patients. Galantamine has been shown to delay the onset of BPSD during a five-month study but has been otherwise poorly studied to-date. Both donepezil and galantamine have not as yet demonstrated efficacy in reducing psychotic symptoms or in reducing levels of concomitant psychotropic medication use. Studies with the dual ChE inhibitor rivastigmine in mild to moderately severe AD and in Lewy body dementia (LBD) have shown improvements in behavioral symptoms including psychosis. Improvements have been maintained over a period of up to two years. In addition, institutionalized patients with severe AD have shown symptomatic benefits with a reduction in the requirement for additional psychotropic drugs following treatment with rivastigmine. The psychotropic properties associated with rivastigmine may in part be mediated through effects on butyrylcholinesterase. Current treatment options are limited for patients with dementia syndromes other than AD. However, data concerning rivastigmine in patients with LBD and preliminary studies in Parkinson's disease dementia and vascular dementia suggest a role for ChE inhibitors across the spectrum of dementia syndromes. Finally, studies that incorporated a delayed start design demonstrate that ChE inhibitors may delay the progression of BPSD.

Cognitive decline, commonly first recognized as memory impairment, is a typical feature of Alzheimer's disease (AD). Neuropathological changes in the cerebral cortex and limbic system lead to deficits in learning, memory, language, and visuospatial skills. The precise nature of cognitive dysfunction reflects the distribution of pathological changes in AD. These will vary along the disease severity continuum and may also depend on where the disease sits in the spectrum of dementia. For example, AD-related disorders such as Lewy body dementia (LBD) and Parkinson's disease dementia (PDD) also show symptoms of cognitive decline and share several pathological features, including degeneration of cortical cholinergic and striatal dopaminergic neurons. In vascular dementia (VaD), there is often an unequal distribution of cognitive deficit, with severe impairment in some functions and relative sparing of others. Cholinesterase (ChE) inhibitors, which help restore acetylcholine levels in the brain, are licensed for the symptomatic treatment of AD and have shown additional benefit in related dementias. Physiological correlates of cholinergic function/dysfunction in the brain include regional cerebral blood flow, glucose metabolism, and cerebrospinal fluid levels of ChE enzymes. These variables represent valuable markers of the clinical efficacy of ChE inhibitors. However, direct assessment of cognitive improvement, stabilization or decline is usually considered the key efficacy parameter in clinical studies of ChE inhibitors in AD and related dementias. Large-scale, placebo-controlled clinical studies of ChE inhibitors have demonstrated efficacy in treating the cognitive impairments associated with AD. Randomized comparative studies of ChE inhibitors are now under way to directly compare symptomatic efficacy and effects on disease progression. Clinical trial data of the cognitive effects of ChE inhibitors in AD, LBD, PDD, and VaD are discussed in detail in this article. The benefits of long-term treatment on symptomatic improvement in cognition and further potential disease-modifying effects are highlighted.

Acetylcholinesterase (AChE) predominates in the healthy brain, with butyrylcholinesterase (BuChE) considered to play a minor role in regulating brain acetylcholine (ACh) levels. However, BuChE activity progressively increases in patients with Alzheimer's disease (AD), while AChE activity remains unchanged or declines. Both enzymes therefore represent legitimate therapeutic targets for ameliorating the cholinergic deficit considered to be responsible for the declines in cognitive, behavioral and global functioning characteristic of AD. The two enzymes differ in substrate specificity, kinetics and activity in different brain regions. Experimental evidence from the use of agents with enhanced selectivity for BuChE (cymserine analogues, MF-8622) and the dual inhibitor of both AChE and BuChE, rivastigmine, indicates potential therapeutic benefits of inhibiting both AChE and BuChE in AD and related dementias. Recent evidence suggests that both AChE and BuChE may have roles in the aetiology and progression of AD beyond regulation of synaptic ACh levels. The development of specific BuChE inhibitors and further experience with the dual enzyme inhibitor rivastigmine will improve understanding of the aetiology of AD and should lead to a wider variety of potent treatment options.

This review provides an overview of the three most widely used cholinesterase (ChE) inhibitors: donepezil, rivastigmine, and galantamine. Differences in pharmacologic profiles will be discussed, and consideration will be given to how such differences may relate to and influence the clinical efficacy and tolerability of the various agents. In addition to providing cognitive benefits in patients with Alzheimer's disease (AD), growing clinical evidence also suggests that ChE inhibitors can produce favorable and clinically relevant effects on neuropsychiatric/behavioral disturbances and activities of daily living. Furthermore, recent data indicate that these agents may be effective at all levels of disease severity and for all rates of disease progression. The clinical utility of ChE inhibitors in a wider spectrum of dementias which share a common cholinergic deficit, such as Lewy body dementia, Parkinson's disease dementia, and vascular dementia, is currently under investigation. Beyond symptomatic relief, data suggest that ChE inhibitors may also slow the underlying disease process. As clinical and research experience with these agents continues to accumulate, the differences in their effects will become more apparent and will help physicians tailor ChE inhibition treatment to the needs of the individual patient.