Potential therapeutic agents against Alzheimer's disease from natural sources.

The average human life span in developed countries has increased to more than 80 years following rapid breakthrough and developments in modern medicine and science, resulting in prolonged life expectancy and increase in the population counts of the geriatric age group. This translates into a dramatic increase in disease burden of elderly patients suffering from senile disorders including neurodegenerative diseases, particularly Alzheimer's disease (AD). AD is characterized by the death of nerve cells in the cerebral cortex and is the most common subtype of dementia that affected 25 million people worldwide in 2000 and is expected to increase to 114 million by 2050. Despite the exponential growth in the number of AD patients, only acetylcholinesterase (AChE) inhibitors are being currently used to treat AD. It is well known that AChE inhibitors can alleviate the symptoms of AD but not halt the disease progression. Consequently, therapeutic agents against AD acting at various pathologic levels are needed. In the recent decade, natural products with anti-AD properties have attracted much attention. But very few natural products have been investigated in a scientifically justifiable method for these biological activities. Following a detailed research process, it is certain that natural products have a strong potential to develop biologically active compounds with new chemical structures. Many studies have been carried out to identify the naturally occurring anti-AD agents. This review article describes the molecular targets aiming at developing the anti-AD agents including the inhibition of AChE, inhibition of Aβ production by enhancing α-secretase (non-amyloidogenic pathway) or inhibiting β- and γ-secretases (amyloidogenic pathway), alleviating Aβ-induced neurotoxicity or reducing Aβ-induced neuroinflammation. In addition, this paper summarizes the potential of some of the natural products that might inhibit specific molecular targets and slow the progression of this disease.