This presentation will summarize the discovery, development, and use ofamyloid-beta (Ab) imaging agents in Alzheimer's disease (AD). Amyloids areproteins composed of different amino acids with variable molecular weights. Twodefining characteristics of amyloid proteins are their propensity to aggregate(both in vitro and in vivo in different tissues in the body) and to formextended beta-pleated sheet structures that avidly bind fluorescent dyes such asCongo red and thioflavin-S. Aggregated amyloids accumulate both peripherally andin the brain and are associated with a variety of diseases such as systemicamyloidosis, AD, frontal temporal dementias, Parkinson's disease, andHuntington's disease. Historically, the clinical diagnosis of many of theamyloid-related diseases was confirmed in post-mortem tissue samples andutilized the location and concentration of the amyloid aggregates as definitiveevidence of disease presence at the time of death.
A goal of researchers throughout the world was to develop methods capable of thedefinitive diagnosis of amyloid-related diseases prior to death in living humansubjects. This would permit the identification of early stage disease cases inwhom the natural history of disease progression could be defined, as well asassist in the evaluation of the efficacy of anti-amyloid therapeutics atdifferent stages of disease. Non-invasive imaging methods, such as positronemission tomography (PET), could be utilized pre-mortem for this purpose, ifsuitable radiotracers for the different amyloids were developed and applied. Thefirst reported amyloid imaging radiotracers were non-selective and bound withhigh affinity to a variety of different amyloids. The development of PETradiotracers that bound selectively to one type of aggregated amyloid over allother types is desirable, particularly in AD where both aggregated Ab (in theform of Ab plaques) and tau (in the form of neurofibrillary tangles) can bepresent. In the early 2000's, our research groups at the University ofPittsburgh identified the first selective amyloid PET radiotracer,carbon-11-labeled Pittsburgh Compound B (PiB), for imaging aggregated Ab.Thousands of research PiB PET imaging studies in normal elderly subjects,subjects diagnosed with mild cognitive impairment, and AD have been conductedworldwide, and the results of those studies will summarized. Subsequentdevelopment of longer-lived fluorine-18-labeled Ab PET radiotracers permittedtheir wider distribution and clinical use at a variety of PET imaging centers.Currently, Ab PET imaging is being used to understand the natural history of Abdeposition, to help define and identify pre-AD subjects, to assist drugdevelopment efforts in clinical trials, and to add an etiological component intothe pre-mortem diagnosis of probable AD in research settings. The future usesand expansion of Ab PET imaging will likely be affected by the degree of successof plasma-based AD biomarkers, such as Ab40/42 and phospho-tau species.
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