Amyloid imaging in cognitively normal individuals, at-risk populations and preclinical Alzheimer's disease

Advances in the development of tau PET radiotracers and their clinical applications

Alzheimer's disease and other neurodegenerative dementias belong to the family of tauopathies. These diseases are characterized by the deposition of insoluble tau aggregates possessing an enriched β-sheet structure. In vivo imaging of the tau deposits by positron emission tomography (PET) will facilitate the early and accurate diagnosis of these diseases, tracking of disease progression, assessment of disease severity, and prediction of disease prognosis. Furthermore, this technology is expected to play a vital role in the monitoring of treatment outcomes and in the selection of patients for the therapeutic trials of anti-dementia drugs. Recently, several tau PET tracers have been successfully developed and demonstrated as having high binding affinity and selectivity to tau protein deposits. Recent clinical studies using these tracers have demonstrated significant tracer retention in sites susceptible to tau deposition in Alzheimer's disease, as well as correlations with the disease severity and cognitive impairment in cases with dementia. These tracers, thus, have the potential to effectively diagnose the tauopathies. Further longitudinal assessment will clarify the effect of the tau deposition on the neurodegenerative process and cognitive decline and the interaction of tau with amyloid-β in the human brain.

Neuroimaging biomarkers in Alzheimer's disease and other dementias

In vivo imaging of β-amyloid (Aβ) has transformed the assessment of Aβ pathology and its changes over time, extending our insight into Aβ deposition in the brain by providing highly accurate, reliable, and reproducible quantitative statements of regional or global Aβ burden in the brain. This knowledge is essential for therapeutic trial recruitment and for the evaluation of anti-Aβ treatments. Although cross sectional evaluation of Aβ burden does not strongly correlate with cognitive impairment, it does correlate with cognitive (especially memory) decline and with a higher risk for conversion to AD in the aging population and MCI subjects. This suggests that Aβ deposition is a protracted pathological process starting well before the onset of symptoms. Longitudinal observations, coupled with different disease-specific biomarkers to assess potential downstream effects of Aβ are required to confirm this hypothesis and further elucidate the role of Aβ deposition in the course of Alzheimer's disease.

Multimodality Imaging of Neurodegenerative Processes: Part 1, The Basics and Common Dementias

OBJECTIVE

Multimodality imaging plays an important role in the structural and functional characterization of neurodegenerative conditions. This article illustrates the basic concepts of anatomic, metabolic, and amyloid imaging and describes the application of a multimodality approach in the evaluation of patients with the more common neurodegenerative dementia processes. Proper utilization of clinically available imaging techniques allows greater insight into these common disease processes.

CONCLUSION

Recognizing the strength of combined anatomic, metabolic, and amyloid imaging can allow a more complete and confident assessment of patients with common degenerative dementias. This added knowledge can improve clinical care, allow initiation of appropriate therapies and counseling, and improve prognostication.

Assessing risk for preclinical β-amyloid pathology with APOE, cognitive, and demographic information

INTRODUCTION

Clinical trials in Alzheimer's disease are aimed at early stages of disease, including preclinical Alzheimer's disease. The high cost and time required to screen large numbers of participants for Aβ pathology impede the development of novel drugs. This study's objective was to evaluate the extent to which inexpensive and easily obtainable information can reduce the number of screen failures by increasing the proportion of Aβ+ participants identified for screening.

METHODS

We used random forest models to evaluate the positive predictive value of demographics, APOE, and longitudinal cognitive rates in the prediction of amyloid pathology, measured by florbetapir PET or cerebrospinal fluid.

RESULTS

Predicting Aβ positivity with demographic, APOE, and cognitive information yielded a positive predictive value estimate of 0.65 (95% CI, 0.50-0.96), nearly a 60% increase over the reference Aβ+ prevalence in the cohort of 0.41.

CONCLUSIONS

By incorporating this procedure, clinical trial screening costs of 7500 USD per participant may be reduced by nearly 7 million USD total.

Attention and Working Memory-Related EEG Markers of Subtle Cognitive Deterioration in Healthy Elderly Individuals

Future treatments of Alzheimer's disease need the identification of cases at high risk at the preclinical stage of the disease before the development of irreversible structural damage. We investigated here whether subtle cognitive deterioration in a population of healthy elderly individuals could be predicted by EEG signals at baseline under cognitive activation. Continuous EEG was recorded in 97 elderly control subjects and 45 age-matched mild cognitive impairment (MCI) cases during a simple attentional and a 2-back working memory task. Upon 18-month neuropsychological follow-up, the final sample included 55 stable (sCON) and 42 deteriorated (dCON) controls. We examined the P1, N1, P3, and PNwm event-related components as well as the oscillatory activities in the theta (4-7 Hz), alpha (8-13 Hz), and beta (14-25 Hz) frequency ranges (ERD/ERS: event-related desynchronization/synchronization, and ITC: inter-trial coherence). Behavioral performance, P1, and N1 components were comparable in all groups. The P3, PNwm, and all oscillatory activity indices were altered in MCI cases compared to controls. Only three EEG indices distinguished the two control groups: alpha and beta ERD (dCON >  sCON) and beta ITC (dCON <  sCON). These findings show that subtle cognitive deterioration has no impact on EEG indices associated with perception, discrimination, and working memory processes but mostly affects attention, resulting in an enhanced recruitment of attentional resources. In addition, cognitive decline alters neural firing synchronization at high frequencies (14-25 Hz) at early stages, and possibly affects lower frequencies (4-13 Hz) only at more severe stages.

Evolving Evidence for the Value of Neuroimaging Methods and Biological Markers in Subjects Categorized with Subjective Cognitive Decline

There is evolving evidence that individuals categorized with subjective cognitive decline (SCD) are potentially at higher risk for developing objective and progressive cognitive impairment compared to cognitively healthy individuals without apparent subjective complaints. Interestingly, SCD, during advancing preclinical Alzheimer's disease (AD), may denote very early, subtle cognitive decline that cannot be identified using established standardized tests of cognitive performance. The substantial heterogeneity of existing SCD-related research data has led the Subjective Cognitive Decline Initiative (SCD-I) to accomplish an international consensus on the definition of a conceptual research framework on SCD in preclinical AD. In the area of biological markers, the cerebrospinal fluid signature of AD has been reported to be more prevalent in subjects with SCD compared to healthy controls; moreover, there is a pronounced atrophy, as demonstrated by magnetic resonance imaging, and an increased hypometabolism, as revealed by positron emission tomography, in characteristic brain regions affected by AD. In addition, SCD individuals carrying an apolipoprotein ɛ4 allele are more likely to display AD-phenotypic alterations. The urgent requirement to detect and diagnose AD as early as possible has led to the critical examination of the diagnostic power of biological markers, neurophysiology, and neuroimaging methods for AD-related risk and clinical progression in individuals defined with SCD. Observational studies on the predictive value of SCD for developing AD may potentially be of practical value, and an evidence-based, validated, qualified, and fully operationalized concept may inform clinical diagnostic practice and guide earlier designs in future therapy trials.

The effects of normal aging on amyloid-β deposition in nondemented adults with Down syndrome as imaged by carbon 11-labeled Pittsburgh compound B

INTRODUCTION

In Down syndrome (DS), the overproduction of amyloid precursor protein is hypothesized to predispose young adults to early expression of Alzheimer-like neuropathology.

METHODS

PET imaging with carbon 11-labeled Pittsburgh compound B examined the pattern of amyloid-β deposition in 68 nondemented adults with DS (30-53 years) to determine the relationship between deposition and normal aging. Standard uptake value ratio (SUVR) images were created with cerebellar gray matter as the reference region.

RESULTS

Multiple linear regression revealed slight but highly significant (corrected P < .05) positive correlations between SUVR and age. The striatum showed the strongest correlation, followed by precuneus, parietal cortex, anterior cingulate, frontal cortex, and temporal cortex.

CONCLUSION

There is an age-related amyloid-β deposition in the DS population, but as a pattern of elevated cortical retention becomes apparent, the correlation of SUVR with age ceases to be significant. Factors unrelated to aging may drive an increase in deposition during early Alzheimer's disease pathogenesis.

Synthesis and Evaluation of [(18)F]RAGER: A First Generation Small-Molecule PET Radioligand Targeting the Receptor for Advanced Glycation Endproducts

The receptor for advanced glycation endproducts (RAGE) is a 35 kDa transmembrane receptor that belongs to the immunoglobulin superfamily of cell surface molecules. Its role in Alzheimer's disease (AD) is complex, but it is thought to mediate influx of circulating amyloid-β into the brain as well as amplify Aβ-induced pathogenic responses. RAGE is therefore of considerable interest as both a diagnostic and a therapeutic target in AD. Herein we report the synthesis and preliminary preclinical evaluation of [(18)F]RAGER, the first small molecule PET radiotracer for RAGE (Kd = 15 nM). Docking studies proposed a likely binding interaction between RAGE and RAGER, [(18)F]RAGER autoradiography showed colocalization with RAGE identified by immunohistochemistry in AD brain samples, and [(18)F]RAGER microPET confirmed CNS penetration and increased uptake in areas of the brain known to express RAGE. This first generation radiotracer represents initial proof-of-concept and a promising first step toward quantifying CNS RAGE activity using PET. However, there were high levels of nonspecific [(18)F]RAGER binding in vitro, likely due to its high log P (experimental log P = 3.5), and rapid metabolism of [(18)F]RAGER in rat liver microsome studies. Therefore, development of second generation ligands with improved imaging properties would be advantageous prior to anticipated translation into clinical PET imaging studies.

Prefrontal Lobe Brain Reserve Capacity with Resistance to Higher Global Amyloid Load and White Matter Hyperintensity Burden in Mild Stage Alzheimer's Disease

BACKGROUND

Amyloid deposition and white matter lesions (WMLs) in Alzheimer's disease (AD) are both considered clinically significant while a larger brain volume is thought to provide greater brain reserve (BR) against these pathological effects. This study identified the topography showing BR in patients with mild AD and explored the clinical balances among BR, amyloid, and WMLs burden.

METHODS

Thirty patients with AD were enrolled, and AV-45 positron emission tomography was conducted to measure the regional standardized uptake value ratio (SUVr) in 8 cortical volumes-of- interests (VOIs). The quantitative WMLs burden was measured from magnetic resonance imaging while the normalized VOIs volumes represented BR in this study. The cognitive test represented major clinical correlates.

RESULTS

Significant correlations between the prefrontal volume and global (r = 0.470, p = 0.024), but not regional (r = 0.264, p = 0.223) AV-45 SUVr were found. AD patients having larger regional volume in the superior- (r = 0.572, p = 0.004), superior medial- (r = 0.443, p = 0.034), and middle-prefrontal (r = 0.448, p = 0.032) regions had higher global AV-45 SUVr. For global WML loads, the prefrontal (r = -0.458, p = 0.019) and hippocampal volume (r = -0.469, p = 0.016) showed significant correlations while the prefrontal (r = -0.417, p = 0.043) or hippocampal volume (r = -0.422, p = 0.04) also predicted better composite memory scores. There were no interactions between amyloid SUVr and WML loads on the prefrontal volume.

CONCLUSIONS

BR of the prefrontal region might modulate the adverse global pathological burden caused by amyloid deposition. While prefrontal volume positively associated with hippocampal volume, WMLs had an adverse impact on the hippocampal volume that predicts memory performance in mild stage AD.

Association of Elevated Amyloid Levels With Cognition and Biomarkers in Cognitively Normal People From the Community

IMPORTANCE

The role of amyloid in the progression of Alzheimer disease (AD) pathophysiology is of central interest to the design of randomized clinical trials. The presence of amyloid has become a prerequisite for enrollment in several secondary prevention trials for AD, yet the precise effect of elevated amyloid levels on subsequent clinical and biomarker events is less certain.

OBJECTIVE

To explore the effect of elevated amyloid levels on subsequent changes in cognition and biomarkers.

DESIGN, SETTING, AND PARTICIPANTS

A total of 564 cognitively normal individuals (median age, 78 years) from the Mayo Clinic Study of Aging, a population-based longitudinal study in Olmsted County, Minnesota, with serial cognitive data were selected for this study. The data used in this study were collected from January 12, 2006, to January 9, 2014. Individuals included in this study had undergone magnetic resonance imaging, fluorodeoxyglucose positron emission tomography (FDG-PET), and Pittsburgh Compound B (PiB) PET at baseline were not cognitively impaired at baseline and had at least 1 clinical follow-up. A subset of 286 individuals also underwent serial imaging. Elevated amyloid level was defined as a standardized uptake value ratio of greater than 1.5 on PiB PET. Associations with baseline amyloid status and baseline and longitudinal change in clinical and imaging measures were evaluated after adjusting for age and hippocampal volume. APOE4 effects were also evaluated.

MAIN OUTCOMES AND MEASURES

Cognitive measures of memory, language, attention/executive function, visuospatial skills, PiB levels, hippocampal and ventricular volumes, and FDG-PET measures.

RESULTS

At baseline, 179 (31.7%) individuals with elevated amyloid levels had poorer cognition in all domains measured, reduced hippocampal volume, and greater FDG-PET hypometabolism. Elevated amyloid levels at baseline were associated with a greater rate of cognitive decline in all domains (0.04 to 0.09 z score units per year) except language and a greater rate of amyloid accumulation (1.6% per year), hippocampal atrophy (30 mm3 per year), and ventricular enlargement (565 mm3 per year). Elevated amyloid levels were also associated with an increased risk of mild cognitive impairment (hazard ratio, 2.9; 95% CI, 1.7-5.0, and hazard ratio, 1.6; 95% CI, 0.9-2.8, for PiB+ APOE4 carriers and PiB+ noncarriers, respectively, compared with PiB- noncarriers). These associations were largely independent of APOE4.

CONCLUSIONS AND RELEVANCE

In persons selected from a population-based study, elevated amyloid levels at baseline were associated with worse cognition and imaging biomarkers at baseline and with greater clinical decline and neurodegeneration. These results have implications for the design of randomized clinical trials for AD.

Cognitive and Brain Profiles Associated with Current Neuroimaging Biomarkers of Preclinical Alzheimer's Disease

Neuroimaging biomarkers, namely hippocampal volume loss, temporoparietal hypometabolism, and neocortical β-amyloid (Aβ) deposition, are included in the recent research criteria for preclinical Alzheimer's disease (AD). However, how to use these biomarkers is still being debated, especially regarding their sequence. Our aim was to characterize the cognitive and brain profiles of elders classified as positive or negative for each biomarker to further our understanding of their use in the preclinical diagnosis of AD. Fifty-four cognitively normal individuals (age = 65.8 ± 8.3 years) underwent neuropsychological tests (structural MRI, FDG-PET, and Florbetapir-PET) and were dichotomized into positive or negative independently for each neuroimaging biomarker. Demographic, neuropsychological, and neuroimaging data were compared between positive and negative subgroups. The MRI-positive subgroup had lower executive performances and mixed patterns of lower volume and metabolism in AD-characteristic regions and in the prefrontal cortex. The FDG-positive subgroup showed only hypometabolism, predominantly in AD-sensitive areas extending to the whole neocortex, compared with the FDG-negative subgroup. The amyloid-positive subgroup was older and included more APOE ε4 carriers compared with the amyloid-negative subgroup. When considering MRI and/or FDG biomarkers together (i.e., the neurodegeneration-positive), there was a trend for an inverse relationship with Aβ deposition such that those with neurodegeneration tended to show less Aβ deposition and the reverse was true as well. Our findings suggest that: (1) MRI and FDG biomarkers provide complementary rather than redundant information and (2) relatively young cognitively normal elders tend to have either neurodegeneration or Aβ deposition, but not both, suggesting additive rather than sequential/causative links between AD neuroimaging biomarkers at this age. Significance statement: Neuroimaging biomarkers are included in the recent research criteria for preclinical Alzheimer's disease (AD). However, how to use these biomarkers is still being debated, especially regarding their sequence. Our findings suggest that MRI and FDG-PET biomarkers should be used in combination, offering an additive contribution instead of reflecting the same process of neurodegeneration. Moreover, the present study also challenges the hierarchical use of the neuroimaging biomarkers in preclinical AD because it suggests that the neurodegeneration observed in this population is not due to β-amyloid deposition. Rather, our results suggest that β-amyloid- and tau-related pathological processes may interact but not necessarily appear in a systematic sequence.

Early-Phase 11C-PiB PET in Amyloid Angiopathy-Related Symptomatic Cerebral Hemorrhage: Potential Diagnostic Value?

Although late-phase (>35min post-administration) 11C-PiB-PET has good sensitivity in cerebral amyloid angiopathy (CAA), its specificity is poor due to frequently high uptake in healthy aged subjects. By detecting perfusion-like abnormalities, early-phase 11C-PiB-PET might add diagnostic value. Early-frame (1-6min) 11C-PiB-PET was obtained in 11 non-demented patients with probable CAA-related symptomatic lobar intracerebral haemorrhage (70±7yrs), 9 age-matched healthy controls (HCs) and 10 HCs <55yrs. There was a significant decrease in early-phase atrophy-corrected whole-cortex SUV relative to cerebellar vermis (SUVR) in the CAA vs age-matched HC group. None of the age-matched controls fell below the lower 95% confidence limit derived from the young HCs, while 6/11 CAA patients did (sensitivity = 55%, specificity = 100%). Combining both early- and late-phase 11C-PiB data did not change the sensitivity and specificity of late-phase PiB, but combined early- and late-phase positivity entails a very high suspicion of underlying Aβ-related clinical disorder, i.e., CAA or Alzheimer disease (AD). In order to clarify this ambiguity, we then show that the occipital/posterior cingulate ratio is markedly lower in CAA than in AD (N = 7). These pilot data suggest that early-phase 11C-PiB-PET may not only add to late-phase PiB-PET with respect to the unclear situation of late-phase positivity, but also help differentiate CAA from AD.

Regional brain hypometabolism is unrelated to regional amyloid plaque burden

In its original form, the amyloid cascade hypothesis of Alzheimer's disease holds that fibrillar deposits of amyloid are an early, driving force in pathological events leading ultimately to neuronal death. Early clinicopathological investigations highlighted a number of inconsistencies leading to an updated hypothesis in which amyloid plaques give way to amyloid oligomers as the driving force in pathogenesis. Rather than focusing on the inconsistencies, amyloid imaging studies have tended to highlight the overlap between regions that show early amyloid plaque signal on positron emission tomography and that also happen to be affected early in Alzheimer's disease. Recent imaging studies investigating the regional dependency between metabolism and amyloid plaque deposition have arrived at conflicting results, with some showing regional associations and other not. We extracted multimodal neuroimaging data from the Alzheimer's disease neuroimaging database for 227 healthy controls and 434 subjects with mild cognitive impairment. We analysed regional patterns of amyloid deposition, regional glucose metabolism and regional atrophy using florbetapir ((18)F) positron emission tomography, (18)F-fluordeoxyglucose positron emission tomography and T1-weighted magnetic resonance imaging, respectively. Specifically, we derived grey matter density and standardized uptake value ratios for both positron emission tomography tracers in 404 functionally defined regions of interest. We examined the relation between regional glucose metabolism and amyloid plaques using linear models. For each region of interest, correcting for regional grey matter density, age, education and disease status, we tested the association of regional glucose metabolism with (i) cortex-wide florbetapir uptake; (ii) regional (i.e. in the same region of interest) florbetapir uptake; and (iii) regional florbetapir uptake while correcting in addition for cortex-wide florbetapir uptake. P-values for each setting were Bonferroni corrected for 404 tests. Regions showing significant hypometabolism with increasing cortex-wide amyloid burden were classic Alzheimer's disease-related regions: the medial and lateral parietal cortices. The associations between regional amyloid burden and regional metabolism were more heterogeneous: there were significant hypometabolic effects in posterior cingulate, precuneus, and parietal regions but also significant positive associations in bilateral hippocampus and entorhinal cortex. However, after correcting for global amyloid burden, few of the negative associations remained and the number of positive associations increased. Given the wide-spread distribution of amyloid plaques, if the canonical cascade hypothesis were true, we would expect wide-spread, cortical hypometabolism. Instead, cortical hypometabolism appears to be linked to global amyloid burden. Thus we conclude that regional fibrillar amyloid deposition has little to no association with regional hypometabolism.

Striatal and Cortical β-Amyloidopathy and Cognition in Parkinson's Disease

INTRODUCTION

Although most previous cognitive studies of β-amyloidopathy in PD focused on cortical plaque deposition, recent postmortem studies point to an important role of striatal β-amyloid plaque deposition. The aim of this study was to investigate the relative contributions of striatal and cortical β-amyloidopathy to cognitive impairment in PD.

METHODS

Patients with PD (n = 62; age, 68.9 ± 6.4 years; H & Y stage: 2.7 ± 0.5; MoCA score: 25.2 ± 3.0) underwent [(11) C]Pittsburgh compound B β-amyloid, [(11) C]dihydrotetrabenazine monoaminergic, and [(11) C]methyl-4-piperidinyl propionate acetylcholinesterase brain PET imaging and neuropsychological assessment. [(11) C]Pittsburgh compound B β-amyloid data from young to middle-aged healthy subjects were used to define elevated [(11) C]Pittsburgh compound B binding in patients.

RESULTS

Elevated cortical and striatal β-amyloid deposition were present in 37% and 16%, respectively, of this predominantly nondemented cohort of patients with PD. Increased striatal β-amyloid deposition occurred in half of all subjects with increased cortical β-amyloid deposition. In contrast, increased striatal β-amyloid deposition did not occur in the absence of increased cortical β-amyloid deposition. Analysis of covariance using global composite cognitive z scores as the outcome parameter showed significant regressor effects for combined striatal and cortical β-amyloidopathy (F = 4.18; P = 0.02) after adjusting for covariate effects of cortical cholinergic activity (F = 5.67; P = 0.02), caudate nucleus monoaminergic binding, duration of disease, and age (total model: F = 3.55; P = 0.0048). Post-hoc analysis showed significantly lower cognitive z score for combined striatal and cortical β-amyloidopathy, compared to cortical-only β-amyloidopathy and non-β-amyloidopathy subgroups.

CONCLUSIONS

The combined presence of striatal and cortical β-amyloidopathy is associated with greater cognitive impairment than cortical β-amyloidopathy alone in PD.

Combination of dynamic (11)C-PIB PET and structural MRI improves diagnosis of Alzheimer's disease

Structural magnetic resonance imaging (sMRI) is an established technique for measuring brain atrophy, and dynamic positron emission tomography with (11)C-Pittsburgh compound B ((11)C-PIB PET) has the potential to provide both perfusion and amyloid deposition information. It remains unclear, however, how to better combine perfusion, amyloid deposition and morphological information extracted from dynamic (11)C-PIB PET and sMRI with the goal of improving the diagnosis of Alzheimer's disease (AD) and mild cognitive impairment (MCI). We adopted a linear sparse support vector machine to build classifiers for distinguishing AD and MCI subjects from cognitively normal (CN) subjects based on different combinations of regional measures extracted from imaging data, including perfusion and amyloid deposition information extracted from early and late frames of (11)C-PIB separately, and gray matter volumetric information extracted from sMRI data. The experimental results demonstrated that the classifier built upon the combination of imaging measures extracted from early and late frames of (11)C-PIB as well as sMRI achieved the highest classification accuracy in both classification studies of AD (100%) and MCI (85%), indicating that multimodality information could aid in the diagnosis of AD and MCI.

Multimodal imaging in Alzheimer's disease: validity and usefulness for early detection

Alzheimer's disease is a progressive neurodegenerative disease that typically manifests clinically as an isolated amnestic deficit that progresses to a characteristic dementia syndrome. Advances in neuroimaging research have enabled mapping of diverse molecular, functional, and structural aspects of Alzheimer's disease pathology in ever increasing temporal and regional detail. Accumulating evidence suggests that distinct types of imaging abnormalities related to Alzheimer's disease follow a consistent trajectory during pathogenesis of the disease, and that the first changes can be detected years before the disease manifests clinically. These findings have fuelled clinical interest in the use of specific imaging markers for Alzheimer's disease to predict future development of dementia in patients who are at risk. The potential clinical usefulness of single or multimodal imaging markers is being investigated in selected patient samples from clinical expert centres, but additional research is needed before these promising imaging markers can be successfully translated from research into clinical practice in routine care.

Synthesis optimization of 2-(4-N-[11C]methylaminophenyl)-6-hydroxybenzothiazole ([11C]PIB), β-amyloid PET imaging tracer for Alzheimer's disease diagnosis

[11C]PIB is the most used amyloid plaques-specific positron-emitting radiotracers. The radiosynthesis of this compound, carried out by methylation of its precursor with [11C]methyl triflate in 2-butanone, has been improved optimizing the initial concentration and the purification method. Two HPLC methods were compared: good radiochemical yields, specific activities, and chemical purity above 98% were achieved by using as eluant acetonitrile/citrate and formulation in 10% ethanol.

Treatment of Mild Cognitive Impairment

OPINION STATEMENT

It is increasingly evident that early identification of cognitive impairment in older adults presents opportunities for interventions that aim to mitigate the impact of cognitive symptoms on daily function and that attempt to delay (or ultimately prevent) progression from mild cognitive impairment (MCI) to dementia. To date, no intervention has proven protective in ultimately preventing conversion to dementia. However, several lifestyle, dietary, and pharmacologic interventions have suggested symptomatic benefit for those having MCI. A number of diet and lifestyle recommendations have been associated with decreased risk of dementia both in cognitively intact older adults and in those having mild cognitive impairment. Thus, these recommendations may be appropriate for both people presenting with subjective cognitive concerns and for those having objective evidence of memory problems. It remains less certain whether adopting these lifestyle habits in later life confers the benefits seen in epidemiological cohorts (where people have likely practiced them for many years). Discussion of starting on a cholinesterase inhibitor is appropriate for those having MCI, particularly those in whom the MCI is thought to have a vascular etiology or to represent the prodromal stage of a neurodegenerative disease. Recent meta-analyses exploring the use of cholinesterase inhibitors in patients having MCI have concluded that there is no evidence to support this practice. Although meta-analytic techniques seemingly strengthen the confidence in a recommendation via the incorporation of a large number of subjects analyzed, the technique is not capable of overcoming any inherent weaknesses of the individual studies included in the analysis. It is arguable whether studies in MCI may have employed endpoints poorly adapted to investigating effect of cholinesterase inhibitors. Most studies have used cognitive screening examinations, all of which stretch their detection ability to identify subjects with MCI, let alone discriminate subtle differences between them. Some have used conversion from MCI to dementia as an endpoint, which may not be the best measure for a symptomatic treatment. Further, once conversion to dementia has occurred, a cholinesterase inhibitor would be started in most (if not all) clinical settings, a reality not well reflected in most study designs. Additionally, several large studies have not permitted subject stratification by APOE carrier status, another important defect in assessing outcome. In clinical practice, our center typically does recommend cholinesterase inhibitors for patients having MCI. Despite the modest effect size, many patients do wish to start on treatment. It appears that this is a generally accepted practice and experience, as most clinical trials for prodromal Alzheimer's disease specify that participants should be taking a cholinesterase inhibitor.

Nonlinear cerebral atrophy patterns across the Alzheimer's disease continuum: impact of APOE4 genotype

The progression of Alzheimer's disease (AD) is characterized by complex trajectories of cerebral atrophy that are affected by interactions with age and apolipoprotein E allele ε4 (APOE4) status. In this article, we report the nonlinear volumetric changes in gray matter across the full biological spectrum of the disease, represented by the AD-cerebrospinal fluid (CSF) index. This index reflects the subject's level of pathology and position along the AD continuum. We also evaluated the associated impact of the APOE4 genotype. The atrophy pattern associated with the AD-CSF index was highly symmetrical and corresponded with the typical AD signature. Medial temporal structures showed different atrophy dynamics along the progression of the disease. The bilateral parahippocampal cortices and a parietotemporal region extending from the middle temporal to the supramarginal gyrus presented an initial increase in volume which later reverted. Similarly, a portion of the precuneus presented a rather linear inverse association with the AD-CSF index whereas some other clusters did not show significant atrophy until index values corresponded to positive CSF tau values. APOE4 carriers showed steeper hippocampal volume reductions with AD progression. Overall, the reported atrophy patterns are in close agreement with those mentioned in previous findings. However, the detected nonlinearities suggest that there may be different pathological processes taking place at specific moments during AD progression and reveal the impact of the APOE4 allele.

[(18)F]flutemetamol amyloid positron emission tomography in preclinical and symptomatic Alzheimer's disease: specific detection of advanced phases of amyloid-β pathology

BACKGROUND

Amyloid positron emission tomography (PET) has become an important tool to identify amyloid-β (Aβ) pathology in Alzheimer's disease (AD) patients. Here, we determined the diagnostic value of the amyloid PET tracer [(18)F]flutemetamol in relation to Aβ pathology at autopsy.

METHODS

[(18)F]flutemetamol PET was carried out in a cohort of 68 patients included in a [(18)F]flutemetamol amyloid PET imaging end-of-life study (GE067-007). At autopsy, AD pathology was determined and Aβ plaque pathology was classified into phases of its regional distribution (0-5).

RESULTS

[(18)F]flutemetamol PET was universally positive in cases with advanced stage postmortem Aβ pathology (Aβ phases 4 and 5). Negative amyloid PET was universally observed in nondemented or non-AD dementia cases with initial Aβ phases 1 and 2, whereas 33.3% of the phase 3 cases were positive.

CONCLUSIONS

[(18)F]flutemetamol amyloid PET detects primarily advanced stages of Aβ pathology in preclinical and symptomatic AD cases.

Can insulin signaling pathways be targeted to transport Aβ out of the brain?

Although the causal role of Amyloid-β (Aβ) in Alzheimer’s disease (AD) is unclear, it is still reasonable to expect that lowering concentrations of Aβ in the brain may decrease the risk of developing the neurocognitive symptoms of the disease. Brain capillary endothelial cells forming the blood-brain barrier (BBB) express transporters regulating the efflux of Aβ out of the cerebral tissue. Age-related BBB dysfunctions, that have been identified in AD patients, might impair Aβ clearance from the brain. Thus, targeting BBB outward transport systems has been suggested as a way to stimulate the clearance of Aβ from the brain. Recent data indicate that the increase in soluble brain Aβ and behavioral impairments in 3×Tg-AD mice generated by months of intake of a high-fat diet can be acutely reversed by the administration of a single dose of insulin. A concomitant increase in plasma Aβ suggests that clearance from the brain through the BBB is a likely mechanism for this rapid effect of insulin. Here, we review how BBB insulin response pathways could be stimulated to decrease brain Aβ concentrations and improve cognitive performance, at least on the short term.

Multi-resolution statistical analysis of brain connectivity graphs in preclinical Alzheimer's disease

There is significant interest, both from basic and applied research perspectives, in understanding how structural/functional connectivity changes can explain behavioral symptoms and predict decline in neurodegenerative diseases such as Alzheimer's disease (AD). The first step in most such analyses is to encode the connectivity information as a graph; then, one may perform statistical inference on various 'global' graph theoretic summary measures (e.g., modularity, graph diameter) and/or at the level of individual edges (or connections). For AD in particular, clear differences in connectivity at the dementia stage of the disease (relative to healthy controls) have been identified. Despite such findings, AD-related connectivity changes in preclinical disease remain poorly characterized. Such preclinical datasets are typically smaller and group differences are weaker. In this paper, we propose a new multi-resolution method for performing statistical analysis of connectivity networks/graphs derived from neuroimaging data. At the high level, the method occupies the middle ground between the two contrasts - that is, to analyze global graph summary measures (global) or connectivity strengths or correlations for individual edges similar to voxel based analysis (local). Instead, our strategy derives a Wavelet representation at each primitive (connection edge) which captures the graph context at multiple resolutions. We provide extensive empirical evidence of how this framework offers improved statistical power by analyzing two distinct AD datasets. Here, connectivity is derived from diffusion tensor magnetic resonance images by running a tractography routine. We first present results showing significant connectivity differences between AD patients and controls that were not evident using standard approaches. Later, we show results on populations that are not diagnosed with AD but have a positive family history risk of AD where our algorithm helps in identifying potentially subtle differences between patient groups. We also give an easy to deploy open source implementation of the algorithm for use within studies of connectivity in AD and other neurodegenerative disorders.

Type 2 diabetes as a protein misfolding disease

Type 2 diabetes (T2D) is a highly prevalent and chronic metabolic disorder. Recent evidence suggests that formation of toxic aggregates of the islet amyloid polypeptide (IAPP) might contribute to β-cell dysfunction and disease. However, the mechanism of protein aggregation and associated toxicity remains unclear. Misfolding, aggregation, and accumulation of diverse proteins in various organs is the hallmark of the group of protein misfolding disorders (PMDs), including highly prevalent illnesses affecting the central nervous system (CNS) such as Alzheimer's disease (AD) and Parkinson's disease (PD). In this review we discuss the current understanding of the mechanisms implicated in the formation of protein aggregates in the endocrine pancreas and associated toxicity in the light of the long-standing knowledge from neurodegenerative disorders associated with protein misfolding.

The relationship between amyloid deposition, neurodegeneration, and cognitive decline in dementia

Amyloid imaging has been clinically approved for measuring β amyloid plaque load in patients being evaluated for Alzheimer's disease or other causes of cognitive decline. Here we explore a multidimensional approach to cognitive decline, where we situate amyloid plaque burden among a number of other relevant dimensions, such as aging, volume loss, other proteinopathies such as TDP43 and Lewy bodies, and functional reorganisation of cognitive brain systems. The multidimensional model incorporates a 'pure AD' trajectory, corresponding to e.g. monogenic Alzheimer's disease, but leaves room for other combinations of biomarker abnormalities (e.g. volume loss without amyloid positivity) and other trajectories. More tools will become available in the future that allow one to carve out a causal-mechanistic space for explaing cognitive decline in a personalized manner, enhancing progress towards more efficacious interventions.

Development of a process to disclose amyloid imaging results to cognitively normal older adult research participants

INTRODUCTION

The objective of this study was to develop a process to maximize the safety and effectiveness of disclosing Positron Emission Tomography (PET) amyloid imaging results to cognitively normal older adults participating in Alzheimer's disease secondary prevention studies such as the Anti-Amyloid Treatment in Asymptomatic Alzheimer's Disease (A4) Study.

METHODS

Using a modified Delphi Method to develop consensus on best practices, we gathered and analyzed data over three rounds from experts in two relevant fields: informed consent for genetic testing or human amyloid imaging.

RESULTS

Experts reached consensus on (1) text for a brochure that describes amyloid imaging to a person who is considering whether to undergo such imaging in the context of a clinical trial, and (2) a process for amyloid PET result disclosure within such trials. Recommendations included: During consent, potential participants should complete an educational session, where they receive verbal and written information covering what is known and unknown about amyloid imaging, including possible results and their meaning, implications of results for risk of future cognitive decline, and information about Alzheimer's and risk factors. Participants should be screened for anxiety and depression to determine suitability to receive amyloid imaging information. The person conducting the sessions should check comprehension and be skilled in communication and recognizing distress. Imaging should occur on a separate day from consent, and disclosure on a separate day from imaging. Disclosure should occur in person, with time for questions. At disclosure, investigators should assess mood and willingness to receive results, and provide a written results report. Telephone follow-up within a few days should assess the impact of disclosure, and periodic scheduled assessments of depression and anxiety, with additional monitoring and follow-up for participants showing distress, should be performed.

CONCLUSIONS

We developed a document for use with potential study participants to describe the process of amyloid imaging and the implications of amyloid imaging results; and a disclosure process with attention to ongoing monitoring of both mood and safety to receive this information. This document and process will be used in the A4 Study and can be adapted for other research settings.

Amyloid burden, cortical thickness, and cognitive function in the Wisconsin Registry for Alzheimer's Prevention

There is a growing interest in understanding how amyloid β (Aβ) accumulation in preclinical Alzheimer's disease relates to brain morphometric measures and cognition. Existing investigations in this area have been primarily conducted in older cognitively normal (CN) individuals. Therefore, not much is known about the associations between Aβ burden, cortical thickness, and cognition in midlife. We examined this question in 109, CN, late to middle-aged adults (mean age = 60.72 ± 5.65 years) from the Wisconsin Registry for Alzheimer's Prevention. They underwent Pittsburgh Compound B (PiB) and anatomical magnetic resonance (MR) imaging, and a comprehensive cognitive examination. Blinded visual rating of the PiB scans was used to classify the participants as Aβ+ or Aβ−. Cortical thickness measurements were derived from the MR images. The Aβ+ group exhibited significant thinning of the entorhinal cortex and accelerated age-associated thinning of the parahippocampal gyrus compared with the Aβ− group. The Aβ+ group also had numerically lower, but nonsignificant, test scores on all cognitive measures, and significantly faster age-associated cognitive decline on measures of Speed & Flexibility, Verbal Ability, and Visuospatial Ability. Our findings suggest that early Aβ aggregation is associated with deleterious changes in brain structure and cognitive function, even in midlife, and that the temporal lag between Aβ deposition and the inception of neurodegenerative/cognitive changes might be narrower than currently thought.

Alzheimer's disease and vascular dementia: one potentially preventable and modifiable disease. Part I: Pathology, diagnosis and screening

Alzheimer's disease (AD) and vascular dementia (VaD) frequently coexist and their separation may well be a false dichotomy. The neurodegenerative and cerebrovascular changes vary from the extremes of 'pure AD' to 'pure VaD' and a combination of changes is the most frequent. Both changes have a preclinical phase of decades. The clinical presentation is often out of proportion to the neuropathology to a lesser or greater degree. The diagnosis of dementia or mild cognitive impairment is based on clinical assessment and diagnostic criteria have been published. Neuropsychological testing and specialist evaluation may be required. The UK and US governments propose screening of all older adults. The value of routine screening has been questioned. Consent for testing and counseling are essential.

Targeting Metal-Aβ Aggregates with Bifunctional Radioligand [11C]L2-b and a Fluorine-18 Analogue [18F]FL2-b

Interest in quantifying metal-Aβ species in vivo led to the synthesis and evaluation of [¹¹C]L2-b and [¹⁸F]FL2-b as radiopharmaceuticals for studying the metallobiology of Alzheimer’s disease (AD) using positron emission tomography (PET) imaging. [¹¹C]L2-b was synthesized in 3.6% radiochemical yield (nondecay corrected, n = 3), >95% radiochemical purity, from the corresponding desmethyl precursor. [¹⁸F]FL2-b was synthesized in 1.0% radiochemical yield (nondecay corrected, n = 3), >99% radiochemical purity, from a 6-chloro pyridine precursor. Autoradiography experiments with AD positive and healthy control brain samples were used to determine the specificity of binding for the radioligands compared to [¹¹C]PiB, a known imaging agent for β-amyloid (Aβ) aggregates. The K_d for [¹¹C]L2-b and [¹⁸F]FL2-b were found to be 3.5 and 9.4 nM, respectively, from those tissue studies. Displacement studies of [¹¹C]L2-b and [¹⁸F]FL2-b with PiB and AV-45 determined that L2-b binds to Aβ aggregates differently from known radiopharmaceuticals. Finally, brain uptake of [¹¹C]L2-b was examined through microPET imaging in healthy rhesus macaque, which revealed a maximum uptake at 2.5 min (peak SUV = 2.0) followed by rapid egress (n = 2).

FDG-PET and amyloid-PET imaging: the diverging paths

PURPOSE OF REVIEW

The availability of PET neuroimaging tools for the in-vivo assessment of metabolic dysfunction and amyloid burden in Alzheimer's disease has opened important methodological and practical issues in the diagnostic design and the conduct of new clinical trials. This review, addressing the different molecular information that the amyloid-PET and fluorodeoxyglucose-PET (FDG-PET) tools can provide, highlights their diverging paths in Alzheimer's disease and possible new perspectives in research and clinical applications.

RECENT FINDINGS

Senile plaques and neurofibrillary tangles are prominent neuropathological hallmarks in Alzheimer's disease and are considered to be targets for therapeutic intervention and biomarkers for diagnostic in-vivo imaging agents. Alzheimer's disease is a slowly progressing disorder, in which pathophysiological abnormalities, detectable in vivo by PET biomarkers, precede clinical symptoms by many years to decades. The unitary view of Alzheimer's disease as a sequential pathological pathway, with beta-amyloid (Aβ) as the only initial and causal event (the 'amyloid cascade hypothesis'), is likely to be progressively replaced by a more complex picture, also on the basis of recent PET imaging findings showing that neuronal injury biomarkers and tau pathology can be independent of β-amyloid deposition.

SUMMARY

The different molecular paths that PET in-vivo biomarkers can reveal in the timeframe of Alzheimer's disease progression reflect the events leading to deposition of Aβ and phosphorylated tau, neuronal injury and neurodegeneration, which can run in parallel instead of in a sequential manner. The amyloid and neuronal injury paths may diverge along the Alzheimer's disease cascade and bear separate relationships with Alzheimer's disease symptoms and clinical phenotypes. All these evidences are crucial for the diagnosis and the development of new drugs aimed at slowing or preventing dementia.

Neuropathology and biochemistry of Aβ and its aggregates in Alzheimer's disease

Alzheimer's disease (AD) is characterized by β-amyloid plaques and intraneuronal τ aggregation usually associated with cerebral amyloid angiopathy (CAA). Both β-amyloid plaques and CAA deposits contain fibrillar aggregates of the amyloid β-peptide (Aβ). Aβ plaques and CAA develop first in neocortical areas of preclinical AD patients and, then, expand in a characteristic sequence into further brain regions with end-stage pathology in symptomatic AD patients. Aβ aggregates are not restricted to amyloid plaques and CAA. Soluble and several types of insoluble non-plaque- and non-CAA-associated Aβ aggregates have been described. Amyloid fibrils are products of a complex self-assembly process that involves different types of transient intermediates. Amongst these intermediate species are protofibrils and oligomers. Different variants of Aβ peptides may result from alternative processing or from mutations that lead to rare forms of familial AD. These variants can exhibit different self-assembly and aggregation properties. In addition, several post-translational modifications of Aβ have been described that result, for example, in the production of N-terminal truncated Aβ with pyroglutamate modification at position 3 (AβN3pE) or of Aβ phosphorylated at serine 8 (pSer8Aβ). Both AβN3pE and pSer8Aβ show enhanced aggregation into oligomers and fibrils. However, the earliest detectable soluble and insoluble Aβ aggregates in the human brain exhibit non-modified Aβ, whereas AβN3pE and pSer8Aβ are detected in later stages. This finding indicates the existence of different biochemical stages of Aβ aggregate maturation with pSer8Aβ being related mainly to cases with symptomatic AD. The conversion from preclinical to symptomatic AD could thereby be related to combined effects of increased Aβ concentration, maturation of aggregates and spread of deposits into additional brain regions. Thus, the inhibition of Aβ aggregation and maturation before entering the symptomatic stage of the disease as indicated by the accumulation of pSer8Aβ may represent an attractive treatment strategy for preventing disease progression.

Amyloid PET imaging: applications beyond Alzheimer's disease

As a biomarker of beta-amyloid, positron emission tomography (PET) amyloid imaging offers a unique opportunity to detect the presence of this protein in the human body during life. Besides Alzheimer's disease (AD), deposits of beta-amyloid in the brain are also present in other neurodegenerative diseases associated to dementia, such as Parkinson's disease and dementia with Lewy bodies, as well as in other processes affecting brain function, such as cerebral amyloid angiopathy, brain trauma, Down's syndrome and meningiomas, as shown by post-mortem pathology studies. Furthermore, in systemic amyloidosis other organs besides the brain are affected, and amyloid PET imaging may be suitable for the identification of these extra-cerebral amyloid depositions. Finally, the potential use of amyloid PET tracer accumulation in cerebral white matter (WM) as a marker of myelin is being investigated, leading to some promising results in patients with WM lesions and multiple sclerosis. In this article, a review of the ongoing research pointing to a broader application of amyloid PET imaging in clinical practice beyond AD is provided.

Advances in the therapy of Alzheimer's disease: targeting amyloid beta and tau and perspectives for the future

Worldwide multidisciplinary translational research has led to a growing knowledge of the genetics and molecular pathogenesis of Alzheimer's disease (AD) indicating that pathophysiological brain alterations occur decades before clinical signs and symptoms of cognitive decline can be diagnosed. Consequently, therapeutic concepts and targets have been increasingly focused on early-stage illness before the onset of dementia; and distinct classes of compounds are now being tested in clinical trials. At present, there is a growing consensus that therapeutic progress in AD delaying disease progression would significantly decrease the expanding global burden. The evolving hypothesis- and evidence-based generation of new diagnostic research criteria for early-stage AD has positively impacted the development of clinical trial designs and the characterization of earlier and more specific target populations for trials in prodromal as well as in pre- and asymptomatic at-risk stages of AD.

Functional Changes in the Language Network in Response to Increased Amyloid β Deposition in Cognitively Intact Older Adults

Word finding symptoms are frequent early in the course of Alzheimer's disease and relate principally to functional changes in left posterior temporal cortex. In cognitively intact older adults, we examined whether amyloid load affects the network for language and associative-semantic processing. Fifty-six community-recruited subjects (52-74 years), stratified for apolipoprotein E and brain-derived neurotrophic factor genotype, received a neurolinguistic assessment, (18)F-flutemetamol positron emission tomography, and a functional MRI of the associative-semantic system. The primary measure of amyloid load was the cerebral-to-cerebellar gray matter standardized uptake value ratio in a composite cortical volume of interest (SUVR(comp)). The primary outcome analysis consisted of a whole-brain voxelwise linear regression between SUVR(comp) and fMRI response during associative-semantic versus visuoperceptual processing. Higher activity in one region, the posterior left middle temporal gyrus, correlated positively with increased amyloid load. The correlation remained significant when only the word conditions were contrasted but not for pictures. According to a stepwise linear regression analysis, offline naming reaction times correlated positively with SUVR(comp). A binary classification into amyloid-positive and amyloid-negative cases confirmed our findings. The left posterior temporal activity increase may reflect higher demands for semantic control in the presence of a higher amyloid burden.

Biomarkers in dementia: clinical utility and new directions

Imaging, cerebrospinal fluid (CSF) and blood-based biomarkers have the potential to improve the accuracy by which specific causes of dementia can be diagnosed in vivo, provide insights into the underlying pathophysiology, and may be used as inclusion criteria and outcome measures for clinical trials. While a number of imaging and CSF biomarkers are currently used for each of these purposes, this is an evolving field, with numerous potential biomarkers in varying stages of research and development. We review the currently available biomarkers for the three most common forms of neurodegenerative dementia, and give an overview of research techniques that may in due course make their way into the clinic.

Theoretical impact of Florbetapir (18F) amyloid imaging on diagnosis of alzheimer dementia and detection of preclinical cortical amyloid

In 2012, florbetapir (F) (Amyvid) received US Food and Drug Administration approval as a diagnostic agent for detecting neuritic (β-amyloid) plaques in living patients. Although such approval is specifically not extended to the use of florbetapir as a single definitive diagnostic test for Alzheimer disease dementia (ADD), it is of considerable importance to examine its potential in this regard. To estimate the ability of florbetapir amyloid imaging to detect specified densities of postmortem-identified neuritic plaques, we used the data of Clark et al [Clark CM, Pontecorvo MJ, Beach TG, et al. Cerebral PET with florbetapir compared with neuropathology at autopsy for detection of neuritic amyloid-beta plaques: A prospective cohort study. Lancet Neurol 2012;11:669-78]. We then used the data of Beach et al [Beach TG, Monsell SE, Phillips LE, et al. Accuracy of the clinical diagnosis of Alzheimer disease at National Institute on Aging Alzheimer Disease Centers, 2005-2010. J Neuropathol Exp Neurol 2012;71:266-73], derived from the National Alzheimer's Coordinating Center, to estimate the fraction of subjects who would have been called florbetapir-positive and, among these, the fraction of subjects who would also meet neuropathologic criteria for the presence of ADD. The accuracy of a positive florbetapir β-amyloid scan for the detection of neuropathologically defined ADD is estimated at between 69% and 95% sensitivity and between 83% and 89% specificity. From the same National Alzheimer's Coordinating Center data set, 144 subjects were recorded as having normal cognition. Among these, 84 (58%) had at least sparse neuritic plaques at autopsy and, among these, florbetapir imaging was estimated to detect 47 (56%). These findings suggest that amyloid imaging may significantly improve the clinical identification of ADD.

Decreased Default Mode Network connectivity correlates with age-associated structural and cognitive changes

Ageing entails cognitive and motor decline as well as brain changes such as loss of gray (GM) and white matter (WM) integrity, neurovascular and functional connectivity alterations. Regarding connectivity, reduced resting-state fMRI connectivity between anterior and posterior nodes of the Default Mode Network (DMN) relates to cognitive function and has been postulated to be a hallmark of ageing. However, the relationship between age-related connectivity changes and other neuroimaging-based measures in ageing is fragmentarily investigated. In a sample of 116 healthy elders we aimed to study the relationship between antero-posterior DMN connectivity and measures of WM integrity, GM integrity and cerebral blood flow (CBF), assessed with an arterial spin labeling sequence. First, we replicated previous findings demonstrating DMN connectivity decreases in ageing and an association between antero-posterior DMN connectivity and memory scores. The results showed that the functional connectivity between posterior midline structures and the medial prefrontal cortex was related to measures of WM and GM integrity but not to CBF. Gray and WM correlates of anterio-posterior DMN connectivity included, but were not limited to, DMN areas and cingulum bundle. These results resembled patterns of age-related vulnerability which was studied by comparing the correlates of antero-posterior DMN with age-effect maps. These age-effect maps were obtained after performing an independent analysis with a second sample including both young and old subjects. We argue that antero-posterior connectivity might be a sensitive measure of brain ageing over the brain. By using a comprehensive approach, the results provide valuable knowledge that may shed further light on DMN connectivity dysfunctions in ageing.

Inconsistencies and controversies surrounding the amyloid hypothesis of Alzheimer's disease

The amyloid hypothesis has driven drug development strategies for Alzheimer's disease for over 20 years. We review why accumulation of amyloid-beta (Aβ) oligomers is generally considered causal for synaptic loss and neurodegeneration in AD. We elaborate on and update arguments for and against the amyloid hypothesis with new data and interpretations, and consider why the amyloid hypothesis may be failing therapeutically. We note several unresolved issues in the field including the presence of Aβ deposition in cognitively normal individuals, the weak correlation between plaque load and cognition, questions regarding the biochemical nature, presence and role of Aβ oligomeric assemblies in vivo, the bias of pre-clinical AD models toward the amyloid hypothesis and the poorly explained pathological heterogeneity and comorbidities associated with AD. We also illustrate how extensive data cited in support of the amyloid hypothesis, including genetic links to disease, can be interpreted independently of a role for Aβ in AD. We conclude it is essential to expand our view of pathogenesis beyond Aβ and tau pathology and suggest several future directions for AD research, which we argue will be critical to understanding AD pathogenesis.

Automated synthesis of 18F-fluoropropoxytryptophan for amino acid transporter system imaging

Objective. This study was to develop a cGMP grade of [¹⁸F]fluoropropoxytryptophan (¹⁸F-FTP) to assess tryptophan transporters using an automated synthesizer. Methods. Tosylpropoxytryptophan (Ts-TP) was reacted with K¹⁸F/kryptofix complex. After column purification, solvent evaporation, and hydrolysis, the identity and purity of the product were validated by radio-TLC (1M-ammonium acetate : methanol = 4 : 1) and HPLC (C-18 column, methanol : water = 7 : 3) analyses. In vitro cellular uptake of ¹⁸F-FTP and ¹⁸F-FDG was performed in human prostate cancer cells. PET imaging studies were performed with ¹⁸F-FTP and ¹⁸F-FDG in prostate and small cell lung tumor-bearing mice (3.7 MBq/mouse, iv). Results. Radio-TLC and HPLC analyses of ¹⁸F-FTP showed that the Rf and Rt values were 0.9 and 9 min, respectively. Radiochemical purity was >99%. The radiochemical yield was 37.7% (EOS 90 min, decay corrected). Cellular uptake of ¹⁸F-FTP and ¹⁸F-FDG showed enhanced uptake as a function of incubation time. PET imaging studies showed that ¹⁸F-FTP had less tumor uptake than ¹⁸F-FDG in prostate cancer model. However, ¹⁸F-FTP had more uptake than ¹⁸F-FDG in small cell lung cancer model. Conclusion. ¹⁸F-FTP could be synthesized with high radiochemical yield. Assessment of upregulated transporters activity by ¹⁸F-FTP may provide potential applications in differential diagnosis and prediction of early treatment response.

The interest of amyloid PET imaging in the diagnosis of Alzheimer's disease

PURPOSE OF REVIEW

This review evaluates the potential clinical utility of amyloid imaging.

RECENT FINDINGS

Amyloid PET is a valid in-vivo marker of neuritic plaque load and correlates with amyloid plaque surface area. Abundant diffuse plaques, however, with scant neuritic plaques can also give rise to a positive scan, most often reported in association with Lewy body disease. Specificity of amyloid PET for discriminating Alzheimer's disease from healthy controls is higher than that of structural MRI. Sensitivity for discriminating Alzheimer's disease from healthy controls or from frontotemporal lobar degeneration is also higher than that of fluorodeoxyglucose-PET, with higher interreader reliability. Within a same center there is high concordance between dichotomization of cases based on amyloid PET versus cerebrospinal fluid Aβ42. In a tentative algorithm, we restrict clinical-diagnostic use to dementia with age of onset before 60 years, primary progressive aphasia and corticobasal syndrome, cases with objective cognitive deficits that could be due to a neurodegenerative cause but also have significant cerebrovascular or psychiatric comorbidity, and rapidly progressive dementia.

SUMMARY

Empirical studies that evaluate how amyloid PET can change clinical-diagnostic thinking are starting to emerge. Key questions to be resolved are its role compared with cerebrospinal fluid markers and its impact on patient outcome.

The earlier the better: Alzheimer's prevention, early detection, and the quest for pharmacological interventions

Although the risk factors, biomarkers, and medications for Alzheimer's disease appear to be almost identical in 1993 and 2013, profound changes can de detected throughout this time period. This article maps these recent changes in the conceptualization of Alzheimer's disease, especially the emerging trend toward prevention. While some preventive practices (e.g., brain training) and the search for early signs and biomarkers (such as APOEε4) have existed for a long time, the recent broadening of scope to include cardiovascular risk factors and their prevention, paired with pre-symptomatic detection of disease-specific biomarkers, has considerably impacted the conventional understanding of this syndrome and the possibilities for pharmacological and non-pharmacological interventions. The rationale for emphasizing multiple logics when explaining these changes is to avoid simplified argumentative pathways that exist among some scientists.

What is normal in normal aging? Effects of aging, amyloid and Alzheimer's disease on the cerebral cortex and the hippocampus

What can be expected in normal aging, and where does normal aging stop and pathological neurodegeneration begin? With the slow progression of age-related dementias such as Alzheimer's disease (AD), it is difficult to distinguish age-related changes from effects of undetected disease. We review recent research on changes of the cerebral cortex and the hippocampus in aging and the borders between normal aging and AD. We argue that prominent cortical reductions are evident in fronto-temporal regions in elderly even with low probability of AD, including regions overlapping the default mode network. Importantly, these regions show high levels of amyloid deposition in AD, and are both structurally and functionally vulnerable early in the disease. This normalcy-pathology homology is critical to understand, since aging itself is the major risk factor for sporadic AD. Thus, rather than necessarily reflecting early signs of disease, these changes may be part of normal aging, and may inform on why the aging brain is so much more susceptible to AD than is the younger brain. We suggest that regions characterized by a high degree of life-long plasticity are vulnerable to detrimental effects of normal aging, and that this age-vulnerability renders them more susceptible to additional, pathological AD-related changes. We conclude that it will be difficult to understand AD without understanding why it preferably affects older brains, and that we need a model that accounts for age-related changes in AD-vulnerable regions independently of AD-pathology.

Diagnostic utility of amyloid PET in cerebral amyloid angiopathy-related symptomatic intracerebral hemorrhage

By detecting β-amyloid (Aβ) in the wall of cortical arterioles, amyloid positron emission tomography (PET) imaging might help diagnose cerebral amyloid angiopathy (CAA) in patients with lobar intracerebral hemorrhage (l-ICH). No previous study has directly assessed the diagnostic value of (11)C-Pittsburgh compound B (PiB) PET in probable CAA-related l-ICH against healthy controls (HCs). (11)C-PiB-PET and magnetic resonance imaging (MRI) including T2* were obtained in 11 nondemented patients fulfilling the Boston criteria for probable CAA-related symptomatic l-ICH (sl-ICH) and 20 HCs without cognitive complaints or impairment. After optimal spatial normalization, cerebral spinal fluid (CSF)-corrected PiB distribution volume ratios (DVRs) were obtained. There was no significant difference in whole cortex or regional DVRs between CAA patients and age-matched HCs. The whole cortex DVR was above the 95% confidence limit in 4/9 HCs and 10/11 CAA patients (sensitivity=91%, specificity=55%). Region/frontal or occipital ratios did not have better discriminative value. Similar but less accurate results were found using visual analysis. In patients with sl-ICH, (11)C-PiB-PET has low specificity for CAA due to the frequent occurrence of high (11)C-PiB uptake in the healthy elderly reflecting incipient Alzheimer's disease (AD), which might also be present in suspected CAA. However, a negative PiB scan rules out CAA with excellent sensitivity, which has clinical implications for prognostication and selection of candidates for drug trials.

Amyloid burden, neuronal function, and cognitive decline in middle-aged adults at risk for Alzheimer's disease

The relative influence of amyloid burden, neuronal structure and function, and prior cognitive performance on prospective memory decline among asymptomatic late middle-aged individuals at risk for Alzheimer's disease (AD) is currently unknown. We investigated this using longitudinal cognitive data from 122 middle-aged adults (21 "Decliners" and 101 "Stables") enrolled in the Wisconsin Registry for Alzheimer's Prevention who underwent multimodality neuroimaging [11C-Pittsburgh Compound B (PiB), 18F-fluorodeoxyglucose (FDG), and structural/functional magnetic resonance imaging (fMRI)] 5.7 ± 1.4 years (range = 2.9-8.9) after their baseline cognitive assessment. Covariate-adjusted regression analyses revealed that the only imaging measure that significantly distinguished Decliners from Stables (p = .027) was a Neuronal Function composite derived from FDG and fMRI. In contrast, several cognitive measures, especially those that tap episodic memory, significantly distinguished the groups (p's<.05). Complementary receiver operating characteristic curve analyses identified the Brief Visuospatial Memory Test-Revised (BVMT-R) Total (.82 ± .05, p < .001), the BVMT-R Delayed Recall (.73 ± .06, p = .001), and the Reading subtest from the Wide-Range Achievement Test-III (.72 ± .06, p = .002) as the top three measures that best discriminated the groups. These findings suggest that early memory test performance might serve a more clinically pivotal role in forecasting future cognitive course than is currently presumed.