Whatever happened to Pittsburgh Compound-A?

A History of Senile Plaques: From Alzheimer to Amyloid Imaging

Senile plaques have been studied in postmortem brains for more than 120 years and the resultant knowledge has not only helped us understand the etiology and pathogenesis of Alzheimer disease (AD), but has also pointed to possible modes of prevention and treatment. Within the last 15 years, it has become possible to image plaques in living subjects. This is arguably the single greatest advance in AD research since the identification of the Aβ peptide as the major plaque constituent. The limitations and potentialities of amyloid imaging are still not completely clear but are perhaps best glimpsed through the perspective gained from the accumulated postmortem histological studies. The basic morphological classification of plaques into neuritic, cored and diffuse has been supplemented by sophisticated immunohistochemical and biochemical analyses and increasingly detailed mapping of plaque brain distribution. Changes in plaque classification and staging have in turn contributed to changes in the definition and diagnostic criteria for AD. All of this information continues to be tested by clinicopathological correlations and it is through the insights thereby gained that we will best be able to employ the powerful tool of amyloid imaging.

PET Radiopharmaceuticals for Alzheimer's Disease and Parkinson's Disease Diagnosis, the Current and Future Landscape

Ironically, population aging which is considered a public health success has been accompanied by a myriad of new health challenges, which include neurodegenerative disorders (NDDs), the incidence of which increases proportionally to age. Among them, Alzheimer's disease (AD) and Parkinson's disease (PD) are the most common, with the misfolding and the aggregation of proteins being common and causal in the pathogenesis of both diseases. AD is characterized by the presence of hyperphosphorylated τ protein (tau), which is the main component of neurofibrillary tangles (NFTs), and senile plaques the main component of which is β-amyloid peptide aggregates (Aβ). The neuropathological hallmark of PD is α-synuclein aggregates (α-syn), which are present as insoluble fibrils, the primary structural component of Lewy body (LB) and neurites (LN). An increasing number of non-invasive PET examinations have been used for AD, to monitor the pathological progress (hallmarks) of disease. Notwithstanding, still the need for the development of novel detection tools for other proteinopathies still remains. This review, although not exhaustively, looks at the timeline of the development of existing tracers used in the imaging of Aβ and important moments that led to the development of these tracers.

Small-molecule PET Tracers for Imaging Proteinopathies

In this chapter, we provide a review of the challenges and advances in developing successful PET imaging agents for 3 major types of aggregated amyloid proteins: amyloid-beta (Aβ), tau, and alpha-synuclein (α-syn). These 3 amyloids are involved in the pathogenesis of a variety of neurodegenerative diseases, referred to as proteinopathies or proteopathies, that include Alzheimer disease, Lewy body dementias, multiple system atrophy, and frontotemporal dementias, among others. In the Introduction section, we briefly discuss the history of amyloid in neurodegenerative diseases and describe why progress in developing effective imaging agents has been hampered by the failure of crystallography to provide definitive ligand-protein interactions for rational radioligand design efforts. Instead, the field has relied on largely serendipitous, trial-and-error methods to achieve useful and specific PET amyloid imaging tracers for Aβ, tau, and α-syn deposits. Because many of the proteopathies involve more than 1 amyloid protein, it is important to develop selective PET tracers for the different amyloids to help assess the relative contribution of each to total amyloid burden. We use Pittsburgh compound B to illustrate some of the critical steps in developing a potent and selective Aβ PET imaging agent. Other selective Aβ and tau PET imaging compounds have followed similar pathways in their developmental processes. Success for selective α-syn PET imaging agents has not been realized yet, but work is ongoing in multiple laboratories throughout the world. In the tau sections, we provide background regarding 3-repeat (3R) and 4-repeat (4R) tau proteins and how they can affect the binding of tau radioligands in different tauopathies. We review the ongoing efforts to assess the properties of tau ligands, which are useful in 3R, 4R, or combined 3R-4R tauopathies. Finally, we describe in the α-syn sections recent attempts to develop selective tracers to image α-synucleinopathies.

Amyloid Proteins and Their Role in Multiple Sclerosis. Considerations in the Use of Amyloid-PET Imaging

Thioflavin T derivatives are used in positron-emission tomography (PET) studies to detect amyloid protein deposits in patients with Alzheimer disease. These tracers bind extensively to white matter, which suggests that they may be useful in studies of multiple sclerosis (MS), and that proteins resulting from proteolytic processing of the amyloid precursor protein (APP) may contribute to MS. This article reviews data from both clinical and preclinical studies addressing the role of these proteins, whether they are detected in CSF studies or using PET imaging. APP is widely expressed in demyelinated axons and may have a protective effect in MS and in experimental allergic encephalomyelitis in animals. Several mechanisms associated with this increased expression may affect the degree of remyelination in MS. Amyloid-PET imaging may help determine the degree of demyelination and provide information on the molecular changes linked to APP proteolytic processing experienced by patients with MS.

Development of positron emission tomography β-amyloid plaque imaging agents

For 100 years, β-amyloid (Aβ) plaques and neurofibrillary tangles (NFTs) have been recognized as the neuropathological hallmarks of Alzheimer's disease (AD), and their presence or absence could only be assessed postmortem using stains and dyes that identified these microscopic structures. Approximately 10 years ago, the first successful Aβ plaque-specific positron emission tomography (PET) imaging study was conducted in a living human subject clinically diagnosed with probable AD using the (11)C-labeled radiopharmaceutical Pittsburgh Compound B (PiB). Laboratory studies and preclinical evaluations to design PiB began a decade earlier than the first human PiB PET study and involved chemical modifications of different well-known dyes that bound specifically to the extended β-pleated sheets that comprise the fibrils of amyloid proteins such as Aβ plaques, NFTs, α-synuclein deposits, and prions. These preclinical studies were conducted in our laboratories at the University of Pittsburgh, starting with Congo red derivatives, followed by Chrysamine G derivatives, followed by X-series compounds, and finally with neutral derivatives of thioflavin-T. The in vitro and in vivo evaluations of the different derivatives as candidate PET radioligands for imaging Aβ plaques and neurofibrillary tangles in human brain are described in this review, along with the specific evaluation criteria by which the candidate radioligands were judged. Out of these studies came PiB, a PET radioligand that binds selectively and with high affinity to only fibrillar forms of Aβ. PiB has been used in many different human research protocols throughout the world and has demonstrated the usefulness of assessing the Aβ plaque status of subjects many years before the clinical diagnosis of probable AD. Recently, longer-lived (18)F-radiolabeled Aβ-selective radiopharmaceuticals have been developed. It is likely that the full clinical impact of these imaging agents will be realized by identifying presymptomatic subjects who would benefit from early drug treatments with future disease-modifying AD therapeutics.

Preclinical studies of potential amyloid binding PET/SPECT ligands in Alzheimer's disease

Visualizing the neuropathological hallmarks amyloid plaques and neurofibrillary tangles of Alzheimer's disease in vivo using positron emission tomography (PET) or single photon emission computed tomography will be of great value in diagnosing the individual patient and will also help in our understanding of the disease. The successful introduction of [(11)C]PIB as a PET tracer for the amyloid plaques less than 10 years ago started an intensive research, and numerous new compounds for use in molecular imaging of the amyloid plaques have been developed. The candidates are based on dyes like thioflavin T, Congo red and chrysamine G, but also on other types such as benzoxazoles, curcumin and stilbenes. In the present review, we present methods of the radiochemistry and preclinical evaluation as well as the main properties of some of these compounds.