Design,Synthesis And Evaluation Of Near-infrared Fluorescent Probe For Aβ Plaques Based On Donor-acceptor Conjugated Structure

Alzheimer’s disease(AD)is a neurodegenerative disease.AD mainly occurs among the elderly.Its manifestations include memory disorders,cognitive difficulties,and changes in personality and behavior and so on.AD is the most common type of dementia.With the aging of the population,the incidence of the disease is increasing year by year.However,there is no effective treatment for AD currently.The drugs approved by FDA cannot cure or reverse AD.These drugs can only be used to alleviate the symptoms.Therefore,early diagnosis and prevention are still critical to control AD.The etiology of Alzheimer’s disease is not yet fully understood.Among the hypothesis that was put forward by researchers,the β-amyloid deposition hypothesis is widely recognized.According to the hypothesis,in the early stage of AD,β-amyloid protein is continuously deposited,forming β-amyloid plaques(Aβ plaques).The plaques will cause oxidative stress,inflammation,etc.,leading to neuronal degeneration and loss.Since Aβ plaques forms in the brain 20-30 years before appearance of the symptoms,it is considered to be hallmark of AD.Therefore,development of probes for Aβ plaques imaging is of great sense for the early diagnosis and prevention of disease.At the same time,it can also provide tools for the further study of the pathological process of AD.Currently,there are several molecular imaging approaches for Aβ plaques: positron emission tomography(PET),magnetic resonance imaging(MRI),single-photon emission computed tomography(SPECT),and near-infrared fluorescence imaging(NIRF).MRI and PET/SPECT have been applied clinically for the diagnosis of AD,but compared with MRI and PET/SPECT,there are several advantages of NIRF imaging:(1)high sensitivity;(2)No time-consuming;(3)No need of expensive equipment and skilled operator;(4)deep penetration of in biological tissues and low interference of background.The above advantages make the near-infrared fluorescence imaging very suitable for the detection of Aβ plaques in vivo to realize early diagnosis.Therefore,it is of great significance to design and synthesize NIRF probes for Aβ plaques that can be used in vivo to realize early diagnosis of AD.Three series of near-infrared fluorescent probes for Aβ plaques were designed and synthesized,and were evaluated through in vitro and in vivo studies.The main work of the thesis includes:(1)Seven merocyanine-based probes were designed and synthesized through structural modification of NIR merocyanine fluorescent dyes.Their chemical structures were characterized and their bioactivity was evaluated.The fluorescence emission of all probes reaches the near-infrared region(650-900 nm),which is one of the advantages to modify merocyanine dyes.Among these probes,a near-infrared fluorescent probe MC-1 specifically responding to Aβ plaques was screened with the emission wavelength of 695 nm.Fluorescence was enhanced when MC-1 was bound to Aβ plaques(28 fold).Also,MC-1 had no respond or weak respond toward other common proteins in vivo,which indicated high specificity.In vitro and in vivo experiments showed that the fluorescent signals and clearance rate are different in AD mice and normal mice respectively.According to in vitro and in vivo evaluation of MC-1,this probe had great potential to be applied in the Aβ plaques imaging.Also,this was the first example in the development of merocyanine dyes as Aβ plaques near infrared fluorescent probes,which paved the way for the future research of merocyanine-based near infrared probes.(2)Three dicyanoisophorone-based probes were designed and synthesized based on dicyanoisophorone structure.Their chemical structures were characterized and bioactivity was evaluated.Among these three probes,one near-infrared fluorescent probe Z1,which exhibited specific responses to Aβ protein,was obtained.Its emission wavelength was 675 nm,which fell in the NIR range(650-900 nm).In vitro and in vivo experiments,the probe showed high specificity,effective BBB penetration ability,fluorescent enhancement bound to Aβ plaques and suitable clearance rate in the brain.These results indicated that Z1 had potential to be applied in vivo imaging.(3)Two benzoxadiazole-based probes were synthesized and their chemical structures were characterized.However,these two probes couldn’t achieve desired goal.Further modification and optimization of these probes are under process.