Current drug targets for Alzheimer's disease treatment

Alzheimer's disease (AD), the most common form of dementia among the elderly, is a progressive, degenerative disorder of the brain with a loss of memory and cognition. A defining characteristic of AD is the deposition of amyloid fibrils and neurofibrillary tangles in the brain of afflicted individuals. Biochemically, they are mainly composed of beta-amyloid protein (AD) and phosphorylated tau proteins, respectively. There is also a loss of the presynaptic markers of the cholinergic system, such as acetylcholine, in the brain areas related to memory and learning. The biochemical pathways leading to AD are presently unknown and are a subject of intensive study with current theories favoring a hypothesis where AD aggregates to toxic forms that induce tau phosphorylation and aggregation. It is believed that this ultimately leads to dysfunction and death of cholinergic neurons, and compensation for this loss had been the primary focus of first generation therapeutic agents. The amyloid and tau hypotheses have lead to a focus on amyloid and tau as therapeutic targets. The current therapeutic goals are to reduce amyloid levels, prevention of amyloid aggregation/toxicity and tau phosphorylation/aggregation. AD has a heterogeneous etiology with a large percentage termed sporadic AD arising from unknown causes and a smaller fraction of early onset familial AD (FAD) caused by mutations in several genes, such as the beta-amyloid precursor protein (APP) and presenilins (PS1, PS2). Other genes, such as apolipoprotein E (APOE), are considered to be risk factors for AD. Several proteins, such as APP, APOE, BACE (P-amyloid cleaving enzyme), PS1/2, secretases, and tau play important roles in the pathology of AD. Therefore, attempts are being made to develop new inhibitors for BACE, PS-1 and gamma-secretase for treatment of AD. There is also a significant advancement in understanding the function of cholinesterase (ChE) in the brain and the use of ChE inhibitors in AD. The mechanism of a new generation of acetyl- and butyrylChE inhibitors is being studied and tested in human clinical trials for AD. Other strategies, such as vaccination, anti-inflammatory agents, cholesterol-lowering agents, anti-oxidants and hormone therapy, are also being studied for treating or slowing the progression of AD. Developments of early diagnostic tools based on quantitative biochemical markers will be useful to better follow the course of the disease and to evaluate different therapeutic strategies. In the present review, we attempt to critically examine recent trends in AD research from neurochemical to clinical areas. We analyze various neurobiological mechanisms that provide the basis of new targets for AD drug development. These current research efforts should lead to a deeper understanding of the pathobiochemical processes that occur in the AD brain to effectively diagnose and prevent their occurrence.