Small-molecule BACE1 inhibitors: a patent literature review (2006 - 2011)

INTRODUCTION: Alzheimer's disease is a devastating neurodegenerative disorder for which no disease-modifying therapy exists. The amyloid hypothesis, which implicates Aβ as the toxin initiating a biological cascade leading to neurodegeneration, is the most prominent theory concerning the underlying cause of the disease. BACE1 is one of two aspartyl proteinases that generate Aβ, thus inhibition of BACE1 has the potential to ameliorate the progression of Alzheimer's disease by abating the production of Aβ. AREAS COVERED: This review chronicles small-molecule BACE1 inhibitors as described in the patent literature between 2006 and 2011 and their potential use as disease-modifying treatments for Alzheimer's disease. Over the past half a dozen years, numerous BACE1 inhibitors have been published in the patent applications, but often these contain a paltry amount of pertinent biological data (e.g. potency, selectivity, and efficacy). Fortunately, numerous relevant publications containing important data have appeared in the journal literature during this period. The goal in this effort was to create an amalgam of the two records to add value to this review. EXPERT OPINION: The pharmaceutical industry has made tremendous progress in the development of small-molecule BACE1 inhibitors that lower Aβ in the central nervous system. Assuming the amyloid hypothesis is veracious, we anticipate a disease-modifying therapy to combat Alzheimer's disease is near.     

Approaches to palliative therapies for Alzheimer's disease

Alzheimer's disease is a progressive neurodegenerative disease characterized by gradual and increasing loss of cognitive function and behavioral abnormalities. The formation of beta-amyloid plaques and neurofibrillary tangles are recognized as the key pathologies of the disease. Changes in the levels of various key neurotransmitters has been noted in patients with Alzheimer's disease and may represent the earliest biochemical casualty, preceding or signifying the onset of the disease. Over the last 20 years a number of approaches to the palliative treatment of Alzheimer's disease have been scrutinized. The majority of effort has been focused on cognitive dysfunction, as this is the initial and key debilitating symptom of the disease. The identification and commercial development of the acetylcholinesterase inhibitors has, until recently, virtually dominated the field, and although efficacy has been demonstrated, the clinical results suggest alternate approaches are warranted. This review will highlight those palliative approaches that have focused on the improvement of learning and memory and not on the disease-modifying strategies of the beta-amyloid, tau phosphorylation or other neurodegenerative hypotheses.     

Treatment strategies in Alzheimer's disease with a focus on early pharmacological interventions

Complex interactive effects of genetic predisposition, neurochemical changes and disease comorbidity have been elucidated in the genesis of dementia syndromes. Alzheimer's disease is the most prevalent type of dementia in developed Western countries. In Alzheimer's disease, pharmacological treatment aims at symptomatic relief, disease modification or disease prevention. Cholinesterase inhibitors are established for the treatment of mild-to-moderate Alzheimer's disease. In Europe and the US, memantine is approved for the treatment of moderate-to-severe Alzheimer's disease. To date, there are no drugs with a disease modifying action that have proven efficacy in randomised, double-blind, placebo-controlled clinical trials. In patients not fulfilling the diagnostic criteria for early Alzheimer's disease, e.g. mild cognitive impairment, the efficacy of several drugs, mainly cholinesterase inhibitors, is currently tested in prospective studies by determining the conversion rate to Alzheimer's disease. However, prevention and disease-modifying strategies raise ethical questions because interventions are focused on non-diseased elderly at risk, which means that emphasis should be not only on efficacy but also on long-term safety. No disease-modifying strategy can presently be offered to patients; however, given the pace of recent research there is optimism that slowing progression of Alzheimer's disease will soon be possible.     

Biomarkers of neurodegeneration for diagnosis and monitoring therapeutics

Rapid progress towards understanding the molecular underpinnings of neurodegenerative disorders such as Alzheimer's disease is revolutionizing drug discovery for these conditions. Furthermore, the development of models for these disorders is accelerating efforts to translate insights related to neurodegenerative mechanisms into disease-modifying therapies. However, there is an urgent need for biomarkers to diagnose neurodegenerative disorders early in their course, when therapy is likely to be most effective, and to monitor responses of patients to new therapies. As research related to this need is currently most advanced for Alzheimer's disease, this Review focuses on progress in the development and validation of biomarkers to improve the diagnosis and treatment of Alzheimer's disease and related disorders.     

Clinical practice with anti-dementia drugs: a revised (second) consensus statement from the British Association for Psychopharmacology

The British Association for Psychopharmacology (BAP) coordinated a meeting of experts to review and revise its first (2006) Guidelines for clinical practice with anti-dementia drugs. As before, levels of evidence were rated using accepted standards which were then translated into grades of recommendation A to D, with A having the strongest evidence base (from randomized controlled trials) and D the weakest (case studies or expert opinion). Current clinical diagnostic criteria for dementia have sufficient accuracy to be applied in clinical practice (B) and brain imaging can improve diagnostic accuracy (B). Cholinesterase inhibitors (donepezil, rivastigmine, and galantamine) are effective for mild to moderate Alzheimer's disease (A) and memantine for moderate to severe Alzheimer's disease (A). Until further evidence is available other drugs, including statins, anti-inflammatory drugs, vitamin E and Ginkgo biloba, cannot be recommended either for the treatment or prevention of Alzheimer's disease (A). Neither cholinesterase inhibitors nor memantine are effective in those with mild cognitive impairment (A). Cholinesterase inhibitors are not effective in frontotemporal dementia and may cause agitation (A), though selective serotonin reuptake inhibitors may help behavioural (but not cognitive) features (B). Cholinesterase inhibitors should be used for the treatment of people with Lewy body dementias (Parkinson's disease dementia and dementia with Lewy bodies (DLB)), especially for neuropsychiatric symptoms (A). Cholinesterase inhibitors and memantine can produce cognitive improvements in DLB (A). There is no clear evidence that any intervention can prevent or delay the onset of dementia. Although the consensus statement focuses on medication, psychological interventions can be effective in addition to pharmacotherapy, both for cognitive and non-cognitive symptoms. Many novel pharmacological approaches involving strategies to reduce amyloid and/or tau deposition are in progress. Although results of pivotal studies are awaited, results to date have been equivocal and no disease-modifying agents are either licensed or can be currently recommended for clinical use.     

Novel chelators targeting cell cycle arrest, acetylcholinesterase, and monoamine oxidase for Alzheimer's therapy

The recent finding that acetylcholinesterase (AChE) colocalizes with β-amyloid (Aβ), promotes and accelerates Aβ aggregation has renewed an intense interest in developing new multitarget AChE inhibitors as potential disease-modifying drugs for Alzheimer's therapy. In this review, we first briefly discuss the linkage and complex interplay among the three characteristic hallmarks of Alzheimer's disease (AD): amyloid (Aβ) plaques, neurofibrillary tangles (NFTs), and cholinergic hypofunction. We then review the recent studies on the four marketed cholinesterase inhibitors in term of their multiple activities, potential disease-modifying effects, and the underlying mechanisms of these actions. We finally focus on a new emerging strategy or multitarget AChE inhibitors as effective drugs for AD therapy. We explore some examples of multitarget ChE inhibitors developed in our own and other laboratories, which were purposely designed to address multiple AD etiological targets. These new AChE inhibitors hold great promise for improving cognitive functions in AD patients, slowing down the disease progression, as well as treating behavior problems related to AD.     

环氧合酶-2抑制剂防治阿尔茨海默氏病的进展

阿尔茨海默氏病(Alzheimer's disease,AD)是最常见的一种老年期痴呆综合症，它的发生与炎症密切相关。炎症过程在阿尔茨海默氏病的退行性变和认知功能障碍的发病机理中起了关键性的作用。据报道，环氧合酶—2(COX—2)抑制剂在延缓AD病的进程和改善认知能力上是很有效的。该领域的研究对于探讨AD病的发病机制以及研究其防治措施均有重要意义。     

Design, synthesis, and biological evaluation of dual binding site acetylcholinesterase inhibitors: new disease-modifying agents for Alzheimer's disease

New dual binding site acetylcholinesterase (AChE) inhibitors have been designed and synthesized as new potent drugs that may simultaneously alleviate cognitive deficits and behave as disease-modifying agents by inhibiting the beta-amyloid (A beta) peptide aggregation through binding to both catalytic and peripheral sites of the enzyme. Particularly, compounds 5 and 6 emerged as the most potent heterodimers reported so far, displaying IC50 values for AChE inhibition of 20 and 60 pM, respectively. More importantly, these dual AChE inhibitors inhibit the AChE-induced A beta peptide aggregation with IC50 values 1 order of magnitude lower than that of propidium, thus being the most potent derivatives with this activity reported up to date. We therefore conclude that these compounds are very promising disease-modifying agents for the treatment of Alzheimer's disease (AD).     

Selective phosphodiesterase (PDE)-4 inhibitors: a novel approach to treating memory deficit?

Phosphodiesterase-4 (PDE4) belongs to an important family of proteins that regulates the intracellular level of cyclic adenosine monophosphate (cAMP). Several lines of evidence indicate that targeting PDE4 with selective inhibitors may offer novel strategies in the treatment of age-related memory impairment and Alzheimer's disease. The rationale for such an approach stems from preclinical studies indicating that PDE4 inhibitors can counteract deficits in long-term memory caused by pharmacological agents, aging or overexpression of mutant forms of human amyloid precursor proteins. In addition to their pro-cognitive and pro-synaptic plasticity properties, PDE4 inhibitors are potent neuroprotective, neuroregenerative and anti-inflammatory agents. Based on the fact that Alzheimer's disease is a progressive neurodegenerative disorder that is characterised by cognitive impairment, and that neuroinflammation is now recognised as a prominent feature in Alzheimer's pathology, we have concluded that targeting PDE4 with selective inhibitors may offer a novel therapy aimed at slowing progression, prevention and, eventually, therapy of Alzheimer's disease.     

Predicting Alzheimer dementia in mild cognitive impairment patients. Are biomarkers useful?

A correct clinical diagnosis in the early stage of Alzheimer disease is not only of importance given the current available treatment with acetylcholine esterase inhibitors, but would be the basis for disease-modifying therapy slowing down or arresting the degenerative process. Moreover, in the last years, several efforts have been made to determine if a patient with mild cognitive impairment has incipient Alzheimer disease, i.e. will progress to Alzheimer disease with dementia, or have a benign form of mild cognitive impairment. In this review, the recent published reports regarding progress in early and preclinical Alzheimer disease diagnosis are discussed and the role of peripheral and cerebrospinal fluid biomarkers highlighted. Approaches combining panels of different biomarkers show promise for discovering profiles that are characteristic of Alzheimer disease, even in the pre-symptomatic stage. More work is needed but available novel perspectives offered by recent introduced technologies shed some lights in identifying incipient Alzheimer disease in mild cognitive impairment subjects.     

Rivastigmine. A pharmacoeconomic review of its use in Alzheimer's disease

Alzheimer's disease is associated with a large cost burden, of which institutionalised care constitutes a major component. Therefore, the decision to move a patient from the community to institutionalised care is associated with a significant increase in direct costs. About three-quarters of patients with Alzheimer's disease are admitted to a nursing home within 5 years of diagnosis. Unpaid or informal caregiver time is another large cost in Alzheimer's disease, especially for patients cared for in the community; informal care can account for up to three-quarters of healthcare costs in non-institutionalised patients. Several cholinesterase inhibitors, of which rivastigmine is one, are available for the treatment of patients with mild to moderate Alzheimer's disease. By improving cognitive function and slowing the rate of cognitive decline, cholinesterase inhibitor therapy may reduce a significant part of the economic burden of the disease by delaying the move to institutionalised care. In the absence of prospective long term data which focus on pharmacoeconomic end-points, modelling techniques have been used to extrapolate clinical data available for some cholinesterase inhibitors, including rivastigmine. Four economic analyses, based on a single model of cognitive decline, have been performed with rivastigmine from the perspective of the provider or society. All show that rivastigmine therapy (excluding drug-related costs) is associated with cost savings in patients with mild to moderate Alzheimer's disease by delaying the time to institutionalisation. If the acquisition cost of the drug was factored in, the cost savings completely or partially offset treatment costs. The magnitude of the cost savings increased as the time horizon increased (up to 2 years). The largest savings were realised in patients with mild disease over a 2-year time-frame, suggesting that treatment should be initiated early from an economic viewpoint. Pharmacoeonomic data comparing different cholinesterase inhibitors are, as yet, unavailable. CONCLUSION: Pharmacoeconomic analyses, based on modelled data excluding drug costs, indicate that rivastigmine completely or partially offsets the costs of treatment by delaying cognitive decline and the time to institutionalisation in patients with mild to moderate Alzheimer's disease. From a societal perspective, cost savings are realised if the drug is introduced early in the disease. Additional benefits offered by rivastigmine on behavioural symptoms, which may reduce caregiver burden, have yet to be investigated from a pharmacoeconomic perspective.     

Cholinergic therapies for Alzheimer's disease: progress and prospects

Cholinomimetic drugs are now available to treat the cognitive impairments associated with Alzheimer's disease (AD), although evidence indicates that the effects of acetylcholinesterase (AChE) inhibitors on measures of cognitive function may be secondary to an action on attentional mechanisms. There is also convincing evidence suggesting that cholinomimetic drugs (e.g., AChE inhibitors and agents acting on cholinergic receptors) will have utility in the treatment of other, non-cognitive changes in behavior that are often associated with AD. This review describes some of the approaches to cholinergic therapy and considers the symptoms that are best suited to these treatment modalities.     

Cholinesterase inhibitors for the treatment of Alzheimer's disease in the elderly

The treatment of Alzheimer's disease is of increasing importance as the population ages and the number of people with the disease increases. The aetiology of Alzheimer's disease is complex and therefore treatment strategies rely on generalised pathological findings. Cholinesterase inhibitors enhance a generalised deficit of central nervous system acetylcholine and are the first class of agents specifically approved for the treatment of Alzheimer's disease. The clinical efficacy of the different cholinesterase inhibitors is similar; however, differences in pharmacodynamic and pharmacokinetic parameters can influence tolerability and safety in the elderly population. Concomitant disease states, significant drug interactions and the altered kinetics and dynamics seen in elderly patients can also affect treatment outcome. Although cholinesterase inhibitors are not 'curative' for Alzheimer's disease, clinical evidence indicates that these drugs can significantly delay the progress of cognitive impairment. Consequently, they represent a useful treatment for the symptoms of Alzheimer's disease in the elderly.     

Acetylcholinesterase inhibitors for the treatment of dementia in Alzheimer's disease: do we need new inhibitors?

Acetylcholinesterase inhibitors (AChEIs) have been shown to produce a small, but significant, improvement in cognition in patients with Alzheimer's disease. However, not all patients respond equally, and cognitive benefit may be of limited duration. Although new AChEIs continue to be developed, more recent studies have been aimed at developing inhibitors that have additional actions separate from AChE inhibition. Importantly, new treatments that target the underlying pathogenic mechanism of Alzheimer's disease (statins, secretase inhibitors, vaccination) may eventually emerge. These new treatments could make AChEI therapy less relevant for treatment of Alzheimer's disease.     

The role of acetylcholinesterase in the pathogenesis of Alzheimer's disease

Treatment of Alzheimer's disease has been dominated by the use of acetylcholinesterase (AChE) inhibitors. These drugs compensate for the death of cholinergic neurons and offer symptomatic relief by inhibiting acetylcholine (ACh) turnover and restoring synaptic levels of this neurotransmitter. Recently, however, AChE itself has been implicated in the pathogenesis of Alzheimer's disease. In particular, it appears that AChE may directly interact with amyloid-beta in a manner that increases the deposition of this peptide into insoluble plaques. This new role suggests that properly designed AChE inhibitors might be able to act as disease-modifying agents rather than as mere palliatives. Additionally, numerous studies have suggested that cholinergic modulation and other functional consequences of AChE inhibition may affect amyloid precursor protein processing and protect neurons against a variety of insults. It therefore seems likely that new AChE inhibitors, which capitalize on all these strengths would be excellent candidates for future Alzheimer's disease therapy.     

Novel cholinesterase inhibitors as future effective drugs for the treatment of Alzheimer's disease

Current pharmacotherapy for Alzheimer's disease involves compounds that are aimed at increasing the levels of acetylcholine in the brain by facilitating cholinergic neurotransmission through inhibition of cholinesterase. These drugs, known as acetylcholinesterase inhibitors, have been shown to improve cognition and global functions but have little impact on improving the eventual progression of the disease; however, there is evidence that other cholinesterases such as butyrylcholinesterase can play an important role in cholinergic function in the brain, and the long-suspected non-cholinergic actions of acetylcholinesterase, mainly the interference with the beta-amyloid protein cascade, have recently driven a profound revolution in cholinesterase drug research. Several disease-modifying agents are under development that target these enzymes and have hope of becoming the next generation of effective drugs in the treatment of Alzheimer's disease.     

Second generation of hydroxyethylamine BACE-1 inhibitors: optimizing potency and oral bioavailability

BACE-1 inhibition has the potential to provide a disease-modifying therapy for the treatment of Alzheimer's disease. Optimization of a first generation of BACE-1 inhibitors led to the discovery of novel hydroxyethylamines (HEAs) bearing a tricyclic nonprime side. These derivatives have nanomolar cell potency and are orally bioavailable.     

Acetylcholinesterase inhibitors as disease-modifying therapies for Alzheimer's disease

The therapeutic arsenal for the treatment of Alzheimer's disease (AD) remains confined to a group of four inhibitors of AChE and one NMDA receptor antagonist, which are used to provide a relief of the very late symptoms of the dementia, i.e. the cognitive and functional decline. In line with the growing body of evidence of the pivotal role of the beta-amyloid peptide (Abeta) in the pathogenesis of AD, alternative classes of drugs targeting mainly the formation or the aggregation of Abeta are actively pursued by the pharmaceutical industry, as they could positively modify the course of AD, stopping or slowing down disease progression. While the first amyloid-directed disease-modifying drugs go ahead with their clinical development and could reach the market as soon as 2009, mounting preclinical and clinical evidences is pointing towards a disease-modifying role also for currently marketed anti-Alzheimer AChE inhibitors (AChEIs), particularly for donepezil. In this review, the neuroprotective effects exhibited by currently commercialized AChEIs will be briefly discussed, together with the secondary mechanisms through which they could exert such effects. This review will focus also on particular classes of AChEIs, namely dual binding site AChEIs, which are being purposely designed to target Abeta aggregation and / or other biological targets that contribute to AD pathogenesis, thus constituting very promising disease-modifying anti-Alzheimer drug candidates.     

Cholinergic nicotinic systems in Alzheimer's disease: prospects for pharmacological intervention

Within the last few years, research into the cause and progression of Alzheimer's disease has made significant advances. Although there is still no preventative treatment or cure for this neurodegenerative illness, the development of drugs that may alleviate some of the cognitive symptoms associated with it is advancing. Cholinesterase inhibitors are at present the most effective form of treatment and have shown significant overall response rates in clinical trials. However, although some patients show substantial improvement when treated with this class of drugs, there is considerable variability in the amount of benefit gained in different individuals in terms of their cognitive and behavioural functioning. Furthermore, unfortunately some patients gain little or no benefit from these drugs. It would therefore be of great advantage to explore alternative therapeutic possibilities. This article reviews the potential involvement of the nicotinic cholinergic system in Alzheimer's disease and discusses the possibility of nicotinic pharmacotherapy. Substantial evidence indicates the involvement of the nicotinic cholinergic system in the pathology of Alzheimer's disease. Drugs targeting these sites may not only have a positive effect on cognitive function, but also have additional therapeutic benefits in terms of restoring the hypoactivity in the excitatory amino acid pyramidal system and even slowing the emergence of Alzheimer's disease pathology. The conclusion of this review is that nicotinic treatments are an important potential source of new therapeutic interventions in Alzheimer's disease.     

Cholinesterase inhibitors used in the treatment of Alzheimer's disease: the relationship between pharmacological effects and clinical efficacy

The deficiency in cholinergic neurotransmission in Alzheimer's disease has led to the development of cholinesterase inhibitors as the first-line treatment for symptoms of this disease. The clinical benefits of these agents include improvements, stabilisation or less than expected decline in cognition, function and behaviour. The common mechanism of action underlying this class of agents is an increase in available acetylcholine through inhibition of the catabolic enzyme, acetylcholinesterase. There is substantial evidence that the cholinesterase inhibitors, including donepezil, galantamine and rivastigmine, decrease acetylcholinesterase activity in a number of brain regions in patients with Alzheimer's disease. There is also a significant correlation between acetylcholinesterase inhibition and observed cognitive improvement. However, the cholinesterase inhibitors are reported to have additional pharmacological actions. Rivastigmine inhibits butyrylcholinesterase with a similar affinity to acetylcholinesterase, although it is not clear whether the inhibition of butyrylcholinesterase contributes to the therapeutic effect of rivastigmine. Based on data from preclinical studies, it has been proposed that galantamine also potentiates the action of acetylcholine on nicotinic receptors via allosteric modulation; however, the effects appear to be highly dependent on the concentrations of agonist and galantamine. It is not yet clear whether these concentrations are related to those achieved in the brain of patients with Alzheimer's disease within therapeutic dose ranges. Preclinical studies have shown that donepezil and galantamine also significantly increase nicotinic receptor density, and increased receptor density may be associated with enhanced synaptic strengthening through long-term potentiation, which is related to cognitive function. Despite these differences in pharmacology, a review of clinical data, including head-to-head studies, has not demonstrated differences in efficacy, although they may have an impact on tolerability. It seems clear that whatever the subsidiary modes of action, clinical evidence supporting acetylcholinesterase inhibition as the mechanism by which cholinesterase inhibitors treat the symptoms of Alzheimer's disease is accumulating. Certainly, as a class, the currently approved cholinesterase inhibitors (donepezil, galantamine, rivastigmine and tacrine) provide important benefits in patients with Alzheimer's disease and these drugs offer a significant advance in the management of dementia.