Positron emission tomography radiopharmaceuticals for imaging brain Beta-amyloid

Is Cerebral Amyloid-β Deposition Related to Post-stroke Cognitive Impairment?

Approximately two-thirds of ischemic stroke patients suffer from different levels of post-stroke cognitive impairment (PSCI), but the underlying mechanisms of PSCI remain unclear. Cerebral amyloid-β (Aβ) deposition, a pathological hallmark of Alzheimer's disease, has been discovered in the brains of stroke patients in some autopsy studies. However, less is known about the role of Aβ pathology in the development of PSCI. It is hypothesized that cerebral ischemic injury may lead to neurotoxic Aβ accumulation in the brain, which further induces secondary neurodegeneration and progressive cognitive decline after stroke onset. In this review, we summarized available evidence from pre-clinical and clinical studies relevant to the aforementioned hypothesis. We found inconsistency in the results obtained from studies in rodents, nonhuman primates, and stroke patients. Moreover, the causal relationship between post-stroke cerebral Aβ deposition and PSCI has been uncertain and controversial. Taken together, evidence supporting the hypothesis that brain ischemia induces cerebral Aβ deposition has been insufficient so far. And, there is still no consensus regarding the contribution of cerebral amyloid pathology to PSCI. Other non-amyloid neurodegenerative mechanisms might be involved and remain to be fully elucidated.

Brain SPECT as a Biomarker of Neurodegeneration in Dementia in the Era of Molecular Imaging: Still a Valid Option?

Biomarkers are playing a progressively leading role in both clinical practice and scientific research in dementia. Although amyloid and tau biomarkers have gained ground in the clinical community in recent years, neurodegeneration biomarkers continue to play a key role due to their ability to identify different patterns of brain involvement that sign the transition between asymptomatic and symptomatic stages of the disease with high sensitivity and specificity. Both ¹⁸F-FDG positron emission tomography (PET) and perfusion single photon emission computed tomography (SPECT) have proved useful to reveal the functional alterations underlying various neurodegenerative diseases. Although the focus of nuclear neuroimaging has shifted to PET, the lower cost and wider availability of SPECT make it a still valid alternative for the study of patients with dementia. This review discusses the principles of both techniques, compares their diagnostic performance for the diagnosis of neurodegenerative diseases and highlights the role of SPECT to characterize patients from low- and middle-income countries, where special care of additional costs is particularly needed to meet the new recommendations for the diagnosis and characterization of patients with dementia.

An one-pot two-step automated synthesis of [18F]T807 injection, its biodistribution in mice and monkeys, and a preliminary study in humans

[18F]T807 is a potent tau protein imaging agent. In order to fulfill the demand from preclinical and clinical studies, we developed an automated one-pot two-step synthesis of this potent tau imaging agent and studied its stability, and dosimetry in mice and monkeys. We also conducted a preliminary study of this imaging agent in humans. Using this one-pot two-step method, the radiochemical yield (RCY) of [18F]T807 was 20.5 ± 6.1% (n = 15) at the end of bombardment (EOB) in a synthesis time of 70±5 min. The chemical and radiochemical purities were >90% and the specific activities were 151 ± 52 GBq/μmol. The quality of [18F]T807 synthesized by this method met the U.S. Pharmacopoeia (USP) criteria. The stability test showed that the [18F]T807 injection was stable at room temperature for up to 4 h after the end of synthesis (EOS). The estimated effective dose of the [18F]T807 injection extrapolated from monkeys was 19 μSv/MBq (n = 2), while the estimated effective doses of the [18F]T807 injection extrapolated from fasted and non-fasted mice were 123 ± 27 (n = 3) and 94 ± 19 (n = 4) μSv/MBq, respectively. This one-pot two-step automated method produced the [18F]T807 injection with high reproducibility and high quality. PET imaging and radiation dosimetry evaluation in mice and Formosan rock monkeys suggested that the [18F]T807 injection synthesized by this method is suitable for use in human PET imaging studies. Thus, this method could fulfill the demand for the [18F]T807 injection in both preclinical and clinical studies of tauopathies, especially for nearby study sites without cyclotrons.

A Systematic Review of Positron Emission Tomography of Tau, Amyloid Beta, and Neuroinflammation in Chronic Traumatic Encephalopathy: The Evidence To Date

Chronic traumatic encephalopathy (CTE) is associated with pathological changes, yet detecting these changes during life has proven elusive. Positron emission tomography (PET) offers the potential for identifying such pathology. Few studies have been completed to date and their approaches and results have been diverse. It was the objective of this review to systematically examine relevant research using ligands for PET that bind to identified pathology in CTE. We focused on identification of patterns of binding and addressing gaps in knowledge of PET imaging for CTE. A comprehensive literature search was conducted. Data used were published on or before May 22, 2017. As the extant literature is limited, any peer-reviewed article assessing military, contact sports athletes, or professional fighters was considered for inclusion. The main outcomes were regional binding to brain regions identified through control comparisons or through clinical metrics (e.g., standardized uptake volume ratios). A total of 1207 papers were identified for review, of which six met inclusion criteria. Meta-analyses were planned but were deemed inappropriate given the small number of studies identified. Methodological concerns in these initial papers included small sample sizes, lack of a control comparison, use of nonstandard statistical procedures to quantify data, and interpretation of potentially off-target binding areas. Across studies, the hippocampi, amygdalae, and midbrain had reasonably consistent increased uptake. Evidence for increased uptake in cortical regions was less consistent. The evidence suggests that the field of PET imaging in those at risk for CTE remains nascent. As the field evolves to include more stringent studies, ligands for PET may prove an important tool in identifying CTE in vivo.

Conversion of iodine to fluorine-18 based on iodinated chalcone and evaluation for β-amyloid PET imaging

In the amyloid cascade hypothesis, β-amyloid (Aβ) plaques is one of the major pathological biomarkers in the Alzheimer's disease (AD) brain. We report the synthesis and evaluation of novel radiofluorinated chalcones, [¹⁸F]4-dimethylamino-4'-fluoro-chalcone ([¹⁸F]DMFC) and [¹⁸F]4'-fluoro-4-methylamino-chalcone ([¹⁸F]FMC), as Aβ imaging probes. The conversion of iodine directly introduced to the chalcone backbone into fluorine was successfully carried out by ¹⁸F-labeling via the corresponding boronate precursors, achieving the direct introduction of fluorine-18 into the chalcone backbone to prepare [¹⁸F]DMFC and [¹⁸F]FMC. In a biodistribution study using normal mice, [¹⁸F]DMFC and [¹⁸F]FMC showed a higher initial uptake (4.43 and 5.47% ID/g at 2 min postinjection, respectively) into and more rapid clearance (0.52 and 0.66% ID/g at 30 min postinjection, respectively) from the brain than a Food and Drug Administration (FDA)-approved Aβ imaging agent ([¹⁸F]Florbetapir), meaning the improvement of the probability of detecting Aβ plaques and the reduction of non-specific binding in the brain. In the in vitro binding studies using aggregates of recombinant Aβ peptides, [¹⁸F]DMFC and [¹⁸F]FMC showed high binding affinity to recombinant Aβ aggregates at the K_d values of 4.47 and 6.50 nM, respectively. In the in vitro autoradiography (ARG) experiment with AD brain sections, [¹⁸F]DMFC and [¹⁸F]FMC markedly accumulated only in a region with abundant Aβ plaques, indicating that they clearly recognized human Aβ plaques in vitro. These encouraging results suggest that [¹⁸F]DMFC and [¹⁸F]FMC may be promising PET probes for the detection of an amyloid pathology and the early diagnosis of AD with marked accuracy.

Exploring Metabolism In Vivo Using Endogenous 11C Metabolic Tracers

Cancer and other diseases are increasingly understood in terms of their metabolic disturbances. This thinking has revolutionized the field of ex vivo metabolomics and motivated new approaches to detect metabolites in living systems, including proton magnetic resonance spectroscopy (1H-MRS), hyperpolarized 13C MRS, and PET. For PET, imaging abnormal metabolism in vivo is hardly new. Positron-labeled small-molecule metabolites have been used for decades in humans, including 18F-FDG, which is used frequently to detect upregulated glycolysis in tumors. Many current 18F metabolic tracers including 18F-FDG have evolved from their 11C counterparts, chemically identical to endogenous substrates and thus approximating intrinsic biochemical pathways. This mimicry has stimulated the development of new radiochemical methods to incorporate 11C and inspired the synthesis of a large number of 11C endogenous radiotracers. This is in spite of the 20-minute half-life of 11C, which generally limits its use in patients to centers with an on-site cyclotron. Innovation in 11C chemistry has persisted in the face of this limitation, because (1) the radiochemists involved are inspired, (2) the methods of 11C incorporation are diverse, and (3) 11C compounds often show more predictable in vivo behavior, thus representing an important first step in the validation of new tracer concepts. In this mini-review we will discuss some of the general motivations behind PET tracers, rationales for the use of 11C, and some of the special challenges encountered in the synthesis of 11C endogenous compounds. Most importantly, we will try to highlight the exceptional creativity used in early 11C tracer syntheses, which used enzyme-catalyzed and other "green" methods before these concepts were commonplace.

Alzheimer's disease and osteoporosis

Alzheimer's disease (AD) and osteoporosis are both common degenerative diseases in the elderly population. The incidence of both diseases increases with age and will be posing enormous societal burden worldwide. It may appear that AD and osteoporosis are two distinct diseases although many risk factors are shared. Previous observational studies have shown that patients with osteoporosis have higher risks of developing AD than those who do not have osteoporosis. Although osteoporosis, falls, and fractures are more often seen in patients with AD than other older adults, the association between these two diseases may be due to a pathophysiological link rather than one condition causing the other. Several in vitro and in vivo studies lend support to this notion. Patients with AD have excessive amyloid plaques in the brain, and the pathology may extend to peripheral organs and cause skeletal amyloid deposition, which would enhance receptor activator nuclear factor-kappa B ligand signaling and lead to greater osteoclast activities. Patients with osteoporosis may have Vitamin D deficiency or lower levels of Vitamin D binding protein, which protects against amyloid aggregation, thus linking Vitamin D deficiency and AD or osteoporosis and AD. Osteoporosis coexisting with AD provides a window to examine the amyloid hypothesis from peripheral tissues. Future studies are warranted to clarify the role of genetic background regarding Vitamin D levels, exposure to sunlight, estrogen replacement therapy, and physical activity in patients with both chronic diseases.

Automatic concentration and reformulation of PET tracers via microfluidic membrane distillation

Short-lived radiolabeled tracers for positron emission tomography (PET) must be rapidly synthesized, purified, and formulated into injectable solution just prior to imaging. Current radiosynthesizers are generally designed for clinical use, and the HPLC purification and SPE formulation processes often result in a final volume that is too large for preclinical and emerging in vitro applications. Conventional technologies and techniques for reducing this volume tend to be slow, resulting in radioactive decay of the product, and often require manual handling of the radioactive materials. We present a fully-automated microfluidic system based on sweeping gas membrane distillation to rapidly perform the concentration and formulation process. After detailed characterization of the system, we demonstrate fast and efficient concentration and formulation of several PET tracers, evaluate residual solvent content to establish the safety of the formulated tracers for injection, and show that the formulated tracer can be used for in vivo imaging.

PET Imaging for Early Detection of Alzheimer's Disease: From Pathologic to Physiologic Biomarkers

This article describes the application of various PET imaging agents in the investigation and diagnosis of Alzheimer's disease (AD), including radiotracers for pathologic biomarkers of AD such as β-amyloid deposits and tau protein aggregates, and the neuroinflammation biomarker 18 kDa translocator protein, as well as physiologic biomarkers, such as cholinergic receptors, glucose metabolism, and the synaptic density biomarker synaptic vesicle glycoprotein 2A. Potential of these biomarkers for early AD diagnosis is also assessed.

Behavioral Changes and Hippocampus Glucose Metabolism in APP/PS1 Transgenic Mice via Electro-acupuncture at Governor Vessel Acupoints

Objective: Investigating the effects of electro-acupuncture (EA) treatment on mice with Alzheimer’s disease (AD), using Morris water maze (MWM) for spatial learning and memory behavior tests combined with micro-positron emission tomography (micro-PET) imaging for glucose metabolism in hippocampus.

Methods: Thirty seven-month-old APP/PS1 mice were randomly divided into AD Model group (AD group), medicine group (M group) and EA group, C57BL/6 mice were used for Normal control group (N group), n = 10 in each group. Mice in M group received donepezil intervention by gavage with dose at 0.92 mg/kg. EA was applied at Baihui (GV20) and Yintang (GV29) acupoints for 20 min then pricked at Shuigou (GV26) acupoint, while mice in N, M and AD groups were received restriction for 20 min, with all treatment administrated once a day for 15 consecutive days. After the treatment, MWM was performed to observe behavioral changes in mice, then hippocampus glucose metabolism level was tested by micro-PET imaging.

Results: Compared with that of AD group, the escape latency of M and EA groups declined significantly (P < 0.01), while the proportion of the platform quadrant swimming distance in total swimming distance showed an obvious increase (P < 0.01), and EA group occupied a higher percentage than that in M group. The micro-PET imaging showed that mice in AD group performed a lower glucose metabolic rate in hippocampus compared with N group (P < 0.01). Both M and EA groups presented a significant higher injected dose compared with AD group (P < 0.01), and the uptake rate of EA group was higher than M group.

Conclusion: Both donepezil and EA have therapeutic effects on AD mice. To a certain extent, EA shows a better efficacy in treatment of AD by improving the spatial learning and memory ability, while also enhancing glucose metabolism in hippocampus.

Pittsburgh Compound-B (PiB) binds amyloid β-protein protofibrils

The neuropathology of Alzheimer's disease (AD) includes amyloid plaque formation by the amyloid β-protein (Aβ) and intracellular paired helical filament formation by tau protein. These neuropathogenetic features correlate with disease progression and have been revealed in brains of AD patients using positron emission tomography (PET). One of the most useful positron emission tomography imaging agents has been Pittsburgh Compound-B (PiB). However, since its introduction in 2002, substantial evidence has accumulated suggesting that Aβ oligomerization and protofibril formation, rather than fibril formation per se, may be the more important pathogenetic event in AD. Detecting protofibrils and oligomeric forms of Aβ thus may be of value. We report here the results of experiments to determine whether PiB binds to oligomers or protofibrils formed by Aβ40 and Aβ42. We observed strong binding to Aβ42 fibrils, significant binding to protofibrils, and weaker binding to Aβ42 oligomers. PiB also binds Aβ40 fibrils, but its binding to Aβ40 protofibrils and oligomers is substantially lower than for that observed for Aβ42.

18F-Labeled Benzyldiamine Derivatives as Novel Flexible Probes for Positron Emission Tomography of Cerebral β-Amyloid Plaques

Early noninvasive visualization of cerebral β-amyloid (Aβ) plaques with positron emission tomography (PET) is the most feasible way to diagnose Alzheimer's disease (AD). In this study, a series of flexible benzyldiamine derivatives (BDA) were proposed for binding to aggregated β-amyloid 1-42 (Aβ_1-42) with high adaptability, high binding affinity (6.8 ± 0.6 nM), and rapid body excretion. The methylthio (12) and ethoxyl (10) derivatives were further labeled with ¹⁸F directly on their benzene ring and examined as PET probes for Aβ plaque imaging. [¹⁸F]12 displayed 4.87 ± 0.52% ID/g initial uptake and prompt washout from normal brain in biodistribution studies. MicroPET-CT imaging indicated sufficient retention of [¹⁸F]12 but lower white matter uptake in the brain of an AD transgenic mouse model compared with that of commercial [¹⁸F]AV-45. Our experimental results provide new insights for developing targeting ligands possessing a flexible framework for use as efficient Aβ probes for PET imaging of AD brain.

PET Neurochemical Imaging Modes

PET has deep roots in neuroscience stemming from its first application in brain tumor and brain metabolism imaging. PET emerged over the past few decades and continues to play a prominent role in the study of neurochemistry in the living human brain. Over time, neurochemical imaging with PET has been expanded to address a host of research questions related to, among many others, protein density, drug occupancy, and endogenous neurochemical release. Each of these imaging modes has distinct design and analysis considerations that are critical for enabling quantitative measurements. The number of considerations required for a neurochemical PET study can make it unapproachable. This article aims to orient those interested in neurochemical PET imaging to three of the common imaging modes and to provide some perspective on needs that exist for expansion of neurochemical PET imaging.

Radiopharmaceuticals in positron emission tomography: present situation and future perspectives

Positron emission tomography (PET) is an imaging technique that has grown greatly in recent years. PET is considered a fundamental tool in oncology, and it also has indications in other fields such as neurology and cardiology. Although ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) is the radiopharmaceutical most widely used in PET, the availability of new radiotracers has been a key element in the expansion of the use of PET. These new radiopharmaceuticals have made it possible to study different biological targets that are essential for obtaining greater knowledge and better characterization of different diseases and have thus contributed to the research and development of different therapeutic agents. This article provides a description of different PET radiopharmaceutical, structured according to their areas of application. Some of these radiotracers are already commercially available, whereas others are still under research or pending approval by regulatory bodies.

Rethinking on the concept of biomarkers in preclinical Alzheimer's disease

The neuropathological processes eventually leading to Alzheimer's disease (AD) are thought to start decades before the appearance of clinical symptoms and the clinical diagnosis of AD dementia. The term "preclinical AD" has been recently introduced to identify this "silent stage" of AD, when the disease is already present, but symptoms are not yet clinically evident. Advances in AD biomarkers have dramatically improved the ability to detect AD pathological processes in vivo in cognitively intact subjects, thus demonstrating the presence of AD pathology in the preclinical phase. This review focuses on the recent advances in the field of neuroimaging and CSF AD biomarkers specifically in the preclinical phase of AD, and aims to discuss the significance that such biomarkers could have in cognitively intact subjects. Even though the use of such biomarkers in AD preclinical phase has contributed to improve our understanding of AD early pathological processes, it raised also a number of new challenges that still remain to be overcome, such as a better definition of the clinical and individual significance of currently known biomarkers in preclinical stages and the development of novel biomarkers of different early AD-related events.

Imaging Multimodalities for Dissecting Alzheimer's Disease: Advanced Technologies of Positron Emission Tomography and Fluorescence Imaging

The rapid progress in advanced imaging technologies has expanded our toolbox for monitoring a variety of biological aspects in living subjects including human. In vivo radiological imaging using small chemical tracers, such as with positron emission tomography, represents an especially vital breakthrough in the efforts to improve our understanding of the complicated cascade of neurodegenerative disorders including Alzheimer's disease (AD), and it has provided the most reliable visible biomarkers for enabling clinical diagnosis. At the same time, in combination with genetically modified animal model systems, the most recent innovation of fluorescence imaging is helping establish diverse applications in basic neuroscience research, from single-molecule analysis to animal behavior manipulation, suggesting the potential utility of fluorescence technology for dissecting the detailed molecular-based consequence of AD pathophysiology. In this review, our primary focus is on a current update of PET radiotracers and fluorescence indicators beneficial for understanding the AD cascade, and discussion of the utility and pitfalls of those imaging modalities for future translational research applications. We will also highlight current cutting-edge genetic approaches and discuss how to integrate individual technologies for further potential innovations.

18F-FDG and 18F-florbetapir PET in clinical practice: regional analysis in mild cognitive impairment and Alzheimer disease

OBJECTIVE

The aim of this study is to compare regional cerebral metabolic rate of glucose metabolism and amyloid-β density in patients with Alzheimer disease (AD), mild cognitive impairment (MCI), and healthy elderly subjects.

METHODS

Eighteen patients (including 6 AD, 5 amnestic MCI, and 7 controls) were enrolled at the University Hospital of Tours, France, and submitted to clinical, neuropsychological, and MRI examinations. PET images using F-florbetapir (266 MBq) and F-FDG (185 MBq) were acquired. SUV ratios in specific regions were defined using PMOD3.2 software.

RESULTS

The mean values of F-FDG SUV ratio were significantly lower in frontal, anterior cingulate, and temporal regions in MCI patients than in normal elderly (-15%, -22%, and -11%, respectively). Alzheimer disease patients showed global cerebral metabolic rate of glucose metabolism decrease, especially in parietal and precuneus regions (-15% and -13% compared with healthy control subjects). Only precuneus cortex showed an increased F-florbetapir uptake in AD. There was no other significant regional difference in the amyloid-β density.

CONCLUSIONS

In this study, we observed regional brain metabolic changes between MCI, AD, and controls, whereas only precuneus showed an increased amyloid-β density in AD. F-florbetapir PET analysis needs to be visual and global, whereas F-FDG analysis can be regional.

A review of β-amyloid neuroimaging in Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia worldwide. As advancing age is the greatest risk factor for developing AD, the number of those afflicted is expected to increase markedly with the aging of the world's population. The inability to definitively diagnose AD until autopsy remains an impediment to establishing effective targeted treatments. Neuroimaging has enabled in vivo visualization of pathological changes in the brain associated with the disease, providing a greater understanding of its pathophysiological development and progression. However, neuroimaging biomarkers do not yet offer clear advantages over current clinical diagnostic criteria for them to be accepted into routine clinical use. Nonetheless, current insights from neuroimaging combined with the elucidation of biochemical and molecular processes in AD are informing the ongoing development of new imaging techniques and their application. Much of this research has been greatly assisted by the availability of transgenic mouse models of AD. In this review we summarize the main efforts of neuroimaging in AD in humans and in mouse models, with a specific focus on β-amyloid, and discuss the potential of new applications and novel approaches.

Amyloid imaging with (11)C-PIB PET/CT and glucose metabolism with (18)F-FDG PET/CT in a study on cognitive impairment in the clinical setting

OBJECTIVE

The objective of this study was to evaluate the contribution of amyloid imaging with (11)C-Pittsburgh compound B ((11)C-PIB) and of glucose metabolism on F-fluorodeoxyglucose ((18)F-FDG) PET/CT to the study of cognitive impairment in the clinical setting.

PATIENTS AND METHODS

Thirty-four patients (15 male, 19 female) were enrolled in the study. They were classified according to their clinically presented symptoms. Six patients had subjective memory complaints, five had nonamnestic mild cognitive impairment (MCI), seven had amnestic MCI, seven had prodromal Alzheimer's disease (AD), five had frontotemporal dementia, two had dementia with Lewy bodies, and two had cortical degeneration. All the scans were conducted to determine the likelihood of AD or to differentiate between AD and other dementia. Static 30-min (11)C-PIB and 15-min (18)F-FDG PET/CT scans were obtained. A visual analysis of images was performed.

RESULTS

Three of the six patients with subjective memory complaints had positive (11)C-PIB scans and one of them also had (18)F-FDG hypometabolism. All five nonamnestic MCI patients had normal (11)C-PIB and (18)F-FDG. Four of the seven amnestic MCI patients showed (11)C-PIB cortical retention but only one had positive (18)F-FDG. Positive (11)C-PIB and (18)F-FDG were detected in five of the seven prodromal AD patients. All the five patients with FDT had positive (18)F-FDG scans, but only one of the five had (11)C-PIB cortical retention. Both dementia with Lewy bodies and cortical degeneration patients had positive (11)C-PIB and (18)F-FDG scans.

CONCLUSION

The combined use of (11)C-PIB and (18)F-FDG PET provides relevant information for the clinical management of cognitive impairment. The detection of positive (11)C-PIB cortical retention in patients may be an indicator of the need for further clinical assessment and monitoring.

Use of amyloid PET across the spectrum of Alzheimer's disease: clinical utility and associated ethical issues

Abstract Recent advances have made possible the in vivo detection of beta-amyloid (Aβ) pathology using positron emission tomography. While the gold standard for amyloid imaging, carbon-11 labeled Pittsburgh compound B is increasingly being replaced by fluorine-18 labeled radiopharmaceuticals, with three already approved for clinical use by US and European regulatory bodies. Appropriate use criteria proposed by an amyloid imaging taskforce convened by the Alzheimer's Association and the Society of Nuclear Medicine and Molecular Imaging recommend restricting use of this technology to the evaluation of patients with mild cognitive impairment or atypical dementia syndromes. While use among asymptomatic individuals is currently viewed as inappropriate due prognostic uncertainty, elevated levels of brain Aβ among asymptomatic individuals may represent preclinical Alzheimer's disease. Amyloid imaging is likewise expected to play a role in the design of clinical trials. Though preliminary results suggest amyloid imaging to possess clinical utility and cost-effectiveness, both domains have yet to be assessed systematically. As the field moves toward adoption of a pro-disclosure stance for amyloid imaging findings, it is imperative that a broad range of stakeholders be involved to ensure the appropriateness of emerging policies and protocols.

Molecular imaging as a guide for the treatment of central nervous system disorders

Molecular imaging techniques have a number of advantages for research into the pathophysiology and treatment of central nervous system (CNS) disorders. Firstly, they provide a noninvasive means of characterizing physiological processes in the living brain, enabling molecular alterations to be linked to clinical changes. Secondly, the pathophysiological target in a given CNS disorder can be measured in animal models and in experimental human models in the same way, which enables translational research. Moreover, as molecular imaging facilitates the detection of functional change which precedes gross pathology, it is particularly useful for the early diagnosis and treatment of CNS disorders.

This review considers the application of molecular imaging to CNS disorders focusing on its potential to inform the development and evaluation of treatments. We focus on schizophrenia, Parkinson's disease, depression, and dementia as major CNS disorders. We also review the potential of molecular imaging to guide new drug development for CNS disorders.

Imaging biomarkers for amyloid: a new generation of probes and what lies ahead

Since the original 1984 criteria for Alzheimer's disease (AD), put forth by a work group jointly established by the National Institute of Neurological and Communicative Disorders and Stroke (NINCDS) and the Alzheimer's Disease and Related Disorders Association (ADRDA) (McKhannet al., 1984), important advances have occurred in our ability to detect AD pathophysiology, with the incorporation of biomarkers – defined as anatomic, biochemical, or physiologic parameters that providein vivoevidence of AD neuropathology (Cummings, 2011) – that can improve the certainty of AD diagnosis. Use of imaging biomarkers such as positron emission tomography (PET) with amyloid ligands, particularly in asymptomatic and pre-dementia stages of AD, however, has been the subject of debate (Duboiset al., 2013), with arguments both for and against the biomarker driven diagnosis of AD.

A survey of FDG- and amyloid-PET imaging in dementia and GRADE analysis

PET based tools can improve the early diagnosis of Alzheimer's disease (AD) and differential diagnosis of dementia. The importance of identifying individuals at risk of developing dementia among people with subjective cognitive complaints or mild cognitive impairment has clinical, social, and therapeutic implications. Within the two major classes of AD biomarkers currently identified, that is, markers of pathology and neurodegeneration, amyloid- and FDG-PET imaging represent decisive tools for their measurement. As a consequence, the PET tools have been recognized to be of crucial value in the recent guidelines for the early diagnosis of AD and other dementia conditions. The references based recommendations, however, include large PET imaging literature based on visual methods that greatly reduces sensitivity and specificity and lacks a clear cut-off between normal and pathological findings. PET imaging can be assessed using parametric or voxel-wise analyses by comparing the subject's scan with a normative data set, significantly increasing the diagnostic accuracy. This paper is a survey of the relevant literature on FDG and amyloid-PET imaging aimed at providing the value of quantification for the early and differential diagnosis of AD. This allowed a meta-analysis and GRADE analysis revealing high values for PET imaging that might be useful in considering recommendations.

Potential role of antimicrobial peptides in the early onset of Alzheimer's disease

Cerebral aggregation of amyloid-β (Aβ) is thought to play a major role in the etiology of Alzheimer's disease. Environmental influences, including chronic bacterial or viral infections, are thought to alter the permeability of the blood-brain barrier (BBB) and thereby facilitate cerebral colonization by opportunistic pathogens. This may eventually trigger Aβ overproduction and aggregation. Host biomolecules that target and combat these pathogens, for instance, antimicrobial peptides (AMPs) such as Aβ itself, are an interesting option for the detection and diagnostic follow-up of such cerebral infections. As part of the innate immune system, AMPs are defensive peptides that efficiently penetrate infected cells and tissues beyond many endothelial barriers, most linings, including the BBB, and overall specifically target pathogens. Based on existing literature, we postulate a role for labeled AMPs as a marker to target pathogens that play a role in the aggregation of amyloid in the brain.

The complexities of the pathology-pathogenesis relationship in Alzheimer disease

Current pathogenic theories for Alzheimer disease (AD) and aging favor the notion that lesions and their constituent proteins are the initiators of disease due to toxicity. Whether this is because structural pathology is traditionally viewed as deleterious, and whether this, in turn, is a fundamental misinterpretation of the relationship between pathology and pathogenesis across the spectrum of chronic diseases, remains to be determined. As more and more detailed information about the biochemical constituents of AD lesions becomes available, it may also be argued that just as much knowledge of cellular physiology as pathophysiology has been gained. Indeed, essentially all major proteins in AD lesions are derived from molecular cascades, which are in turn highly conserved across cells, tissues, and species. Moreover, the lesions themselves are observed in the cognitively intact, and sometimes in large numbers, while major consensus criteria indicate that an extent of pathology is normal with advanced age. As the medical science community continues to pursue lesion targeting for therapeutic purposes, the notion that AD pathology is indicative of an active host response or environmental adaptation, and therefore a poor target, is becoming clearer.

Alternative approaches for PET radiotracer development in Alzheimer's disease: imaging beyond plaque

Alzheimer's disease (AD) and related dementias show increasing clinical prevalence, yet our understanding of the etiology and pathobiology of disease-related neurodegeneration remains limited. In this regard, noninvasive imaging with radiotracers for positron emission tomography (PET) presents a unique tool for quantifying spatial and temporal changes in characteristic biological markers of brain disease and for assessing potential drug efficacy. PET radiotracers targeting different protein markers are being developed to address questions pertaining to the molecular and/or genetic heterogeneity of AD and related dementias. For example, radiotracers including [(11) C]-PiB and [(18) F]-AV-45 (Florbetapir) are being used to measure the density of Aβ-plaques in AD patients and to interrogate the biological mechanisms of disease initiation and progression. Our focus is on the development of novel PET imaging agents, targeting proteins beyond Aβ-plaques, which can be used to investigate the broader mechanism of AD pathogenesis. Here, we present the chemical basis of various radiotracers which show promise in preclinical or clinical studies for use in evaluating the phenotypic or biochemical characteristics of AD. Radiotracers for PET imaging neuroinflammation, metal ion association with Aβ-plaques, tau protein, cholinergic and cannabinoid receptors, and enzymes including glycogen-synthase kinase-3β and monoamine oxidase B amongst others, and their connection to AD are highlighted.

Initial experience of MR/PET in a clinical cancer center

Magentic Resonance/positron emission tomography (PET) has been introduced recently for imaging of clinical patients. This hybrid imaging technology combines the inherent strengths of MRI with its high soft-tissue contrast and biological sequences with the inherent strengths of PET, enabling imaging of metabolism with a high sensitivity. In this article, we describe the initial experience of MR/PET in a clinical cancer center along with a review of the literature. For establishing MR/PET in a clinical setting, technical challenges, such as attenuation correction and organizational challenges, such as workflow and reimbursement, have to be overcome. The most promising initial results of MR/PET have been achieved in anatomical areas where high soft-tissue and contrast resolution is of benefit. Head and neck cancer and pelvic imaging are potential applications of this hybrid imaging technology. In the pediatric population, MR/PET can decrease the lifetime radiation dose. MR/PET protocols tailored to different types of malignancies need to be developed. After the initial exploration phase, large multicenter trials are warranted to determine clinical indications for this exciting hybrid imaging technology and thereby opening new horizons in molecular imaging.

Appropriate use criteria for amyloid PET: a report of the Amyloid Imaging Task Force, the Society of Nuclear Medicine and Molecular Imaging, and the Alzheimer's Association

Positron emission tomography (PET) of brain amyloid b is a technology that is becoming more available, but its clinical utility in medical practice requires careful definition. To provide guidance to dementia care practitioners, patients, and caregivers, the Alzheimer's Association and the Society of Nuclear Medicine and Molecular Imaging convened the Amyloid Imaging Taskforce (AIT). The AIT considered a broad range of specific clinical scenarios in which amyloid PET could potentially be used appropriately. Peer-reviewed, published literature was searched to ascertain available evidence relevant to these scenarios, and the AIT developed a consensus of expert opinion. Although empirical evidence of impact on clinical outcomes is not yet available, a set of specific appropriate use criteria (AUC) were agreed on that define the types of patients and clinical circumstances in which amyloid PET could be used. Both appropriate and inappropriate uses were considered and formulated,and are reported and discussed here. Because both dementia care and amyloid PET technology are in active development, these AUC will require periodic reassessment. Future research directions are also outlined, including diagnostic utility and patient-centered outcomes.

Recent advances in imaging Alzheimer's disease

Advances in brain imaging technology in the past five years have contributed greatly to the understanding of Alzheimer's disease (AD). Here, we review recent research related to amyloid imaging, new methods for magnetic resonance imaging analyses, and statistical methods. We also review research that evaluates AD risk factors and brain imaging, in the context of AD prediction and progression. We selected a variety of illustrative studies, describing how they advanced the field and are leading AD research in promising new directions.

Perspectives in molecular imaging using staging biomarkers and immunotherapies in Alzheimer's disease

Sporadic Alzheimer's disease (AD) is an emerging chronic illness characterized by a progressive pleiotropic pathophysiological mode of actions triggered during the senescence process and affecting the elderly worldwide. The complex molecular mechanisms of AD not only are supported by cholinergic, beta-amyloid, and tau theories but also have a genetic basis that accounts for the difference in symptomatology processes activation among human population which will evolve into divergent neuropathological features underlying cognitive and behaviour alterations. Distinct immune system tolerance could also influence divergent responses among AD patients treated by immunotherapy. The complexity in nature increases when taken together the genetic/immune tolerance with the patient's brain reserve and with neuropathological evolution from early till advance AD clinical stages. The most promising diagnostic strategies in today's world would consist in performing high diagnostic accuracy of combined modality imaging technologies using beta-amyloid 42 peptide-cerebrospinal fluid (CSF) positron emission tomography (PET), Pittsburgh compound B-PET, fluorodeoxyglucose-PET, total and phosphorylated tau-CSF, and volumetric magnetic resonance imaging hippocampus biomarkers for criteria evaluation and validation. Early diagnosis is the challenge task that needs to look first at plausible mechanisms of actions behind therapies, and combining them would allow for the development of efficient AD treatment in a near future.

Barriers to clinical translation with diagnostic drugs

Radioactive imaging agents, like diagnostic drugs generally, undergo a drug development process that parallels that of therapeutic agents, with similar development times but substantially lower development costs and substantially smaller postapproval markets. Although rapid advances in genetic and expression profiling are furthering the development of expensive pharmacotherapies targeted to small patient populations, the commercial development of imaging agents for small patient populations is blocked by the limited revenues available with current per-dose pricing and the relatively small numbers of imaging procedures that would be performed. A wide-ranging discussion on the best approaches to allow new diagnostic imaging agents to become part of the health-care system, and benefit the patient, is needed.

Influence of scan duration on the accuracy of β-amyloid PET with florbetaben in patients with Alzheimer's disease and healthy volunteers

PURPOSE

Florbetaben is a β-amyloid-targeted PET tracer with significant potential for augmenting the toolbox in the clinical diagnosis of Alzheimer's disease (AD). In dementia imaging, shortening of scan duration may simplify future clinical use. The aim of this retrospective study was to investigate the effect of scan duration on diagnostic accuracy.

METHODS

PET scans obtained from 25 AD patients and 25 healthy volunteers (HVs) were analysed. In each subject, scans of three different durations (5, 10 and 20 min; all starting 90 min after injection) were obtained, randomized, and visually assessed by three experts blinded to the subject's identity and group affiliation. Presence/absence of β-amyloid and diagnostic confidence (0-100 %) were scored, and 10 % of the scans were re-read. Further, randomly selected datasets of ten AD patients and ten HVs were quantified using an established VOI-based approach and using a voxel-based approach.

RESULTS

The sensitivity and specificity of the blinded read were 80 % and 96 %, respectively, for all scan durations. Diagnostic confidence was high (97 ± 6 %, 97 ± 6 % and 95 ± 8 % for the 20-min, 10-min and 5-min scans, respectively; n.s.), as was interreader agreement (kappa(20 min) = 0.94, kappa(10 min) = 0.94, kappa(5 min) = 0.89; n.s.). Intrareader agreement was highest for the 20-min scan (kappa = 1.00) and lower for the 10-min scan (kappa = 0.71) and 5-min scan (kappa = 0.80; p = 0.002 and 0.003 vs. the 20-min scan). For all scan durations, composite SUVRs (Cohen's d effect size 4.5, 3.9 and 4.8 for the 5-min, 10-min and 20-min scans; p < 0.0001 each) and individual brain volumes affected by β-amyloid (Cohen's d effect size 1.6, 1.8 and 2.0 for the 5-min, 10-min and 20-min scans; p < 0.005 each) were significantly higher in AD patients than in HVs.

CONCLUSION

Reduction in scan duration did not relevantly affect the accuracy of florbetaben PET scans in discriminating between AD patients and HVs. Thus, a reduction in scan duration seems conceivable for the future clinical use of florbetaben.

Butyrylcholinesterase radioligands to image Alzheimer's disease brain

Butyrylcholinesterase (BuChE) is found to have a brain distribution pattern that is distinct from that of acetylcholinesterase (AChE). Neurons containing BuChE are particularly located in the amygdala, hippocampal formation and the thalamus, structures involved in the normal functions of cognition and behavior that typically become compromised in Alzheimer's disease (AD). Progress of this disease is thought to result, at least in part, from the accumulation of β-amyloid (Aβ) plaques and neurofibrillary tangles (NFTs) in the brain. These structures characteristically become associated with cholinesterase activity, and are major determinants of AD diagnosis post-mortem. Early definitive AD diagnosis in the living brain could greatly facilitate specific timely treatment of the disorder and the search for novel drugs to preempt progress of this disease. Radioligands have been developed to detect deposition of Aβ plaques in the brain; however, since many cognitively normal individuals also exhibit Aβ plaque deposition, this approach has inherent disadvantages for definitive AD diagnosis during life. The association of BuChE with Aβ plaques appears to be a characteristic of AD. This has prompted the search for radioligands that target BuChE in association with Aβ plaques that accumulate in cortical grey matter, a region normally with very little of this enzyme activity. A number of BuChE radioligands have been synthesized and preliminary testing indicates that some such radioligands enter the brain and accumulate in regions known to contain BuChE. Radioligands targeting unusual BuChE activity in the brain may represent a means for early diagnosis and treatment monitoring of AD.

Task force on Alzheimer's disease trial methodology: recruitment and retention, data management and analysis. The use of imaging in recruitment: maintenance and methodological issues in Alzheimer's trials

Imaging has been used in clinical trials for decades and has proven a useful biomarker in many disease states. Although imaging is not widely used in clinical practice for AD patient care, many pharmaceutical and biotech companies have relied on this technique for eligibility, safety and efficacy roles in their trials. In this overview, issues with standardization, validation and regulations will be discussed in order to gain a better understanding of the benefits, risks and impact on study conduct of adding imaging to Alzheimer's disease clinical trials.

Relative equilibrium plot improves graphical analysis and allows bias correction of standardized uptake value ratio in quantitative 11C-PiB PET studies

Both the standardized uptake value ratio (SUVR) and the Logan plot result in biased distribution volume ratios (DVRs) in ligand-receptor dynamic PET studies. The objective of this study was to use a recently developed relative equilibrium-based graphical (RE) plot method to improve and simplify the 2 commonly used methods for quantification of (11)C-Pittsburgh compound B ((11)C-PiB) PET.

METHODS

The overestimation of DVR in SUVR was analyzed theoretically using the Logan and the RE plots. A bias-corrected SUVR (bcSUVR) was derived from the RE plot. Seventy-eight (11)C-PiB dynamic PET scans (66 from controls and 12 from participants with mild cognitive impaired [MCI] from the Baltimore Longitudinal Study of Aging) were acquired over 90 min. Regions of interest (ROIs) were defined on coregistered MR images. Both the ROI and the pixelwise time-activity curves were used to evaluate the estimates of DVR. DVRs obtained using the Logan plot applied to ROI time-activity curves were used as a reference for comparison of DVR estimates.

RESULTS

Results from the theoretic analysis were confirmed by human studies. ROI estimates from the RE plot and the bcSUVR were nearly identical to those from the Logan plot with ROI time-activity curves. In contrast, ROI estimates from DVR images in frontal, temporal, parietal, and cingulate regions and the striatum were underestimated by the Logan plot (controls, 4%-12%; MCI, 9%-16%) and overestimated by the SUVR (controls, 8%-16%; MCI, 16%-24%). This bias was higher in the MCI group than in controls (P < 0.01) but was not present when data were analyzed using either the RE plot or the bcSUVR.

CONCLUSION

The RE plot improves pixelwise quantification of (11)C-PiB dynamic PET, compared with the conventional Logan plot. The bcSUVR results in lower bias and higher consistency of DVR estimates than of SUVR. The RE plot and the bcSUVR are practical quantitative approaches that improve the analysis of (11)C-PiB studies.

Alzheimer's disease in the retina: imaging retinal aβ plaques for early diagnosis and therapy assessment

BACKGROUND

Definite Alzheimer's disease (AD) diagnosis at early stages is vital for targeting intervention, yet currently unavailable. Noninvasive detection of the pathological hallmark, amyloid-β protein (Aβ) plaques, is limited in the brain. However, the existence of Aβ plaques in the retina, possibly at presymptomatic stages, may improve early detection of AD.

OBJECTIVE

To summarize clinical and preclinical evidence showing that the retina, an accessible part of the central nervous system, displays abnormalities in AD, especially Aβ plaque pathology. The ability to monitor in vivo retinal plaque dynamics in response to immunotherapy is also assessed.

METHODS

Literature analysis of retinal AD pathology and imaging is provided. In our studies, systemic curcumin is administered to enable monitoring of retinal Aβ plaques in live APP(SWE)/PS1(Δ)(E9) transgenic mice by optical imaging.

RESULTS

Visual and retinal abnormalities, including early manifestation of retinal Aβ plaque pathology, have been documented in AD patients and animal models. In mouse models, retinal Aβ plaques accumulate with age and decrease in response to immunotherapy, consistent with brain pathology. Here, we demonstrate that retinal plaques can be individually monitored in real time following glatiramer acetate immunization.

CONCLUSION

Translation of noninvasive retinal-plaque imaging to humans could eventually facilitate early and accurate AD diagnosis and therapy assessment.

PET amyloid-beta imaging in preclinical Alzheimer's disease

Alzheimer's disease (AD) is the leading cause of dementia, accounting for 60-70% of all cases [Hebert et al., 2003, 1]. The need for effective therapies for AD is great. Current approaches, including cholinesterase inhibitors and N-methyl-d-aspartate (NMDA) receptor antagonists, are symptomatic treatments for AD but do not prevent disease progression. Many diagnostic and therapeutic approaches to AD are currently changing due to the knowledge that underlying pathology starts 10 to 20 years before clinical signs of dementia appear [Holtzman et al., 2011, 2]. New therapies which focus on prevention or delay of the onset or cognitive symptoms are needed. Recent advances in the identification of AD biomarkers now make it possible to detect AD pathology in the preclinical stage of the disease, in cognitively normal (CN) individuals; this biomarker data should be used in the selection of high-risk populations for clinical trials. In vivo visualization of AD neuropathology and biological, biochemical or physiological confirmation of the effects of treatment likely will substantially improve development of novel pharmaceuticals. Positron emission tomography (PET) is the leading neuroimaging tool to detect and provide quantitative measures of AD amyloid pathology in vivo at the early stages and follow its course longitudinally. This article is part of a Special Issue entitled: Imaging Brain Aging and Neurodegenerative disease.

Multidentate (18)F-polypegylated styrylpyridines as imaging agents for Aβ plaques in cerebral amyloid angiopathy (CAA)

β-Amyloid plaques (Aβ plaques) in the brain are associated with cerebral amyloid angiopathy (CAA). Imaging agents that could target the Aβ plaques in the living human brain would be potentially valuable as biomarkers in patients with CAA. A new series of (18)F styrylpyridine derivatives with high molecular weights for selectively targeting Aβ plaques in the blood vessels of the brain but excluded from the brain parenchyma is reported. The styrylpyridine derivatives, 8a-c, display high binding affinities and specificity to Aβ plaques (K(i) = 2.87, 3.24, and 7.71 nM, respectively). In vitro autoradiography of [(18)F]8a shows labeling of β-amyloid plaques associated with blood vessel walls in human brain sections of subjects with CAA and also in the tissue of AD brain sections. The results suggest that [(18)F]8a may be a useful PET imaging agent for selectively detecting Aβ plaques associated with cerebral vessels in the living human brain.