A Review on the Natural Components Applied as Lead Compounds for Potential Multi-target Anti-AD Theranostic Agents

Alzheimer's disease (AD) is a neurodegenerative disease that seriously affects the health and quality of life of the elderly. Its pathogenesis is very complex and there is still a lack of effective clinical drugs to treat or control the development of AD. Studies have shown that β-amyloid (Aβ) deposition, tau protein hyperphosphorylation, reduced levels of brain cholinergic transmitters, and oxidative stress are the main causes of AD. Furthermore, recent studies showed that metal dyshomeostasis could relate to all the above pathogenesis of AD and was a key factor in the development of AD. Natural compounds and their derivatives have multi-target therapeutic effects on AD, and they also have the advantages of low toxicity, and low cost, which are important directions for anti- AD drugs. Meanwhile, early detection may play an important role in preventing the development of AD. The concept of "theranostic agent" combining molecular imaging probes and therapeutic drugs has emerged in recent years. Fluorescence imaging has been widely studied and applied because of its non-invasive, high resolution, high sensitivity, rapid imaging, and low cost. However, at present, most of the research methods in this field use individual therapeutic or diagnostic reagents, which is not conducive to exploring the optimal treatment time window and drug efficacy. Therefore, this work reviewed the natural compounds and their derivatives which all have been studied for both the in vitro and in vivo therapeutic and diagnostic anti-AD activities. At last, structure and activity relationship (SAR) was discussed and potential AD theranostic natural agents were put forwarded to provide a more detailed theoretical basis for the further development of drugs with diagnostic and therapeutic effects in AD.