Kinetic modeling of amyloid binding in humans using PET imaging and Pittsburgh Compound-B

Comparison between brain CT and MRI for voxel-based morphometry of Alzheimer's disease

The voxel-based morphometry (VBM) technique using brain magnetic resonance imaging (MRI) objectively maps gray matter loss on a voxel-by-voxel basis after anatomic standardization. In patients with Alzheimer's disease (AD), reductions of gray matter volume, mainly in the medial temporal structures, have been reported; however, inhomogeneity and geometric distortion of the field intensity hampers the reproducibility of MRI. In the present study, we developed a novel computed tomography (CT)-based VBM method and used this technique to detect volume loss in AD patients as compared with normal controls. The results were compared with MRI-based VBM using the same subjects. Pittsburgh Compound B ((11)C-PIB) positron emission tomography (PET)/CT was performed and two experts in neuro-nuclear medicine judged whether regional amyloid β load was consistent with a diagnosis of AD. Before the injection of (11)C-PIB, high-quality CT scans were obtained using the same PET/CT equipment. MRI was performed within a mean interval of 25.1 ± 8.2 days before the PET/CT scan. Using statistical parametric mapping 8 (SPM8), the extracted gray matter images from CT and MRI were spatially normalized using a gray matter template and smoothed using a Gaussian kernel. Group comparisons were performed using SPM8 between five (11)C-PIB-positive patients with probable AD and seven (11)C-PIB-negative age-matched controls with normal cognition. Gray matter volumes in the bilateral medial temporal areas were reduced in the AD group as compared with the cognitively normal group in both CT-based VBM (in the left; P < 0.0001, cluster size 2776 and in the right; P < 0.0001, cluster size 630) and MRI-based VBM (in the left; P < 0.0001, cluster size 381 and in the right, P < 0.0001, cluster size 421). This newly developed CT-based VBM technique can detect significant atrophy in the entorhinal cortex in probable AD patients as previously reported using MRI-based VBM. However, CT-VBM was more sensitive and revealed larger areas of significant atrophy than MR-VBM.

In vivo radioligand binding to translocator protein correlates with severity of Alzheimer's disease

Neuroinflammation is a pathological hallmark of Alzheimer's disease, but its role in cognitive impairment and its course of development during the disease are largely unknown. To address these unknowns, we used positron emission tomography with (11)C-PBR28 to measure translocator protein 18 kDa (TSPO), a putative biomarker for inflammation. Patients with Alzheimer's disease, patients with mild cognitive impairment and older control subjects were also scanned with (11)C-Pittsburgh Compound B to measure amyloid burden. Twenty-nine amyloid-positive patients (19 Alzheimer's, 10 mild cognitive impairment) and 13 amyloid-negative control subjects were studied. The primary goal of this study was to determine whether TSPO binding is elevated in patients with Alzheimer's disease, and the secondary goal was to determine whether TSPO binding correlates with neuropsychological measures, grey matter volume, (11)C-Pittsburgh Compound B binding, or age of onset. Patients with Alzheimer's disease, but not those with mild cognitive impairment, had greater (11)C-PBR28 binding in cortical brain regions than controls. The largest differences were seen in the parietal and temporal cortices, with no difference in subcortical regions or cerebellum. (11)C-PBR28 binding inversely correlated with performance on Folstein Mini-Mental State Examination, Clinical Dementia Rating Scale Sum of Boxes, Logical Memory Immediate (Wechsler Memory Scale Third Edition), Trail Making part B and Block Design (Wechsler Adult Intelligence Scale Third Edition) tasks, with the largest correlations observed in the inferior parietal lobule. (11)C-PBR28 binding also inversely correlated with grey matter volume. Early-onset (<65 years) patients had greater (11)C-PBR28 binding than late-onset patients, and in parietal cortex and striatum (11)C-PBR28 binding correlated with lower age of onset. Partial volume corrected and uncorrected results were generally in agreement; however, the correlation between (11)C-PBR28 and (11)C-Pittsburgh Compound B binding was seen only after partial volume correction. The results suggest that neuroinflammation, indicated by increased (11)C-PBR28 binding to TSPO, occurs after conversion of mild cognitive impairment to Alzheimer's disease and worsens with disease progression. Greater inflammation may contribute to the precipitous disease course typically seen in early-onset patients. (11)C-PBR28 may be useful in longitudinal studies to mark the conversion from mild cognitive impairment or to assess response to experimental treatments of Alzheimer's disease.

Associations between white matter hyperintensities and β amyloid on integrity of projection, association, and limbic fiber tracts measured with diffusion tensor MRI

The goal of this study was to assess the relationship between Aβ deposition and white matter pathology (i.e., white matter hyperintensities, WMH) on microstructural integrity of the white matter. Fifty-seven participants (mean age: 78±7 years) from an ongoing multi-site research program who spanned the spectrum of normal to mild cognitive impairment (Clinical dementia rating 0–0.5) and low to high risk factors for arteriosclerosis and WMH pathology (defined as WMH volume >0.5% total intracranial volume) were assessed with positron emission tomography (PET) with Pittsburg compound B (PiB) and magnetic resonance and diffusion tensor imaging (DTI). Multivariate analysis of covariance were used to investigate the relationship between Aβ deposition and WMH pathology on fractional anisotropy (FA) from 9 tracts of interest (i.e., corona radiata, internal capsule, cingulum, parahippocampal white matter, corpus callosum, superior longitudinal, superior and inferior front-occipital fasciculi, and fornix). WMH pathology was associated with reduced FA in projection (i.e., internal capsule and corona radiate) and association (i.e., superior longitudinal, superior and inferior fronto-occipital fasciculi) fiber tracts. Aβ deposition (i.e., PiB positivity) was associated with reduced FA in the fornix and splenium of the corpus callosum. There were interactions between PiB and WMH pathology in the internal capsule and parahippocampal white matter, where Aβ deposition reduced FA more among subjects with WMH pathology than those without. However, accounting for apoE ε4 genotype rendered these interactions insignificant. Although this finding suggests that apoE4 may increase amyloid deposition, both in the parenchyma (resulting in PiB positivity) and in blood vessels (resulting in amyloid angiopathy and WMH pathology), and that these two factors together may be associated with compromised white matter microstructural integrity in multiple brain regions, additional studies with a longitudinal design will be necessary to resolve this issue.

In vivo assessment of amyloid-β deposition in nondemented very elderly subjects

OBJECTIVE

This study examined amyloid-β (Aβ) deposition in 190 nondemented subjects aged ≥82 years to determine the proportion of Aβ-positive scans and associations with cognition, apolipoprotein E (APOE) status, brain volume, and Ginkgo biloba (Gb) treatment.

METHODS

Subjects who agreed to participate had a brain magnetic resonance imaging and positron emission tomography scan with (11) C-labeled Pittsburgh compound B (PiB) following completion of a Gb treatment clinical trial. The youngest subject in this imaging study was 82 years, and the mean age of the subjects was 85.5 years at the time of the scans; 152 (80%) were cognitively normal, and 38 (20%) were diagnosed with mild cognitive impairment (MCI) at the time of the PiB study.

RESULTS

A high proportion of the cognitively normal subjects (51%) and MCI subjects (68%) were PiB-positive. The APOE*4 allele was more prevalent in PiB-positive than in PiB-negative subjects (30% vs 6%). Measures of memory, language, and attentional functions were worse in PiB-positive than in PiB-negative subjects, when both normal and MCI cases were analyzed together; however, no significant associations were observed within either normal or MCI subject groups alone. There was no relationship between Gb treatment and Aβ deposition as determined by PiB.

INTERPRETATION

The data revealed a 55% prevalence of PiB positivity in nondemented subjects age >80 years and 85% PiB positivity in the APOE*4 nondemented elderly subjects. The findings also showed that long-term exposure to Gb did not affect the prevalence of cerebral Aβ deposition.

Reference region approaches in PET: a comparative study on multiple radioligands

Reference region (RR) approaches (RRAs) for positron emission tomography (PET) brain studies aim to obtain full quantification without arterial input function (IF) sampling, an invasive and costly procedure. Although they need a reliable RR and are only able to estimate the nondisplaceable binding potential (BPND), RRAs are widely used. If quantitatively reliable, then RRAs can greatly benefit PET investigations, but their suitability can vary widely from radioligand to radioligand. This study compares estimates of BPND both using IF data and several common RRAs on an extensive data set for each of several radioligands. In addition, two new methods, likelihood estimation in graphical analysis with RR and a bootstrapping algorithm for estimating precision, are presented here for the first time. Although many factors contribute to the performance of each RRA, our results suggest that the kinetics in the RR have a role. In particular, RRAs tend to be good when (1) the RR distribution volume is high; (2) the transfer rate constant from plasma to free compartment in the RR is high; and (3) the transfer rate constant from free to plasma compartment in the RR is low.

The aging brain and cognition: contribution of vascular injury and aβ to mild cognitive dysfunction

IMPORTANCE

β-Amyloid (Aβ) deposition and vascular brain injury (VBI) frequently co-occur and are both associated with cognitive decline in aging. Determining whether a direct relationship exists between them has been challenging. We sought to understand VBI's influence on cognition and clinical impairment, separate from and in conjunction with pathologic changes associated with Alzheimer disease (AD).

OBJECTIVE

To examine the relationship between neuroimaging measures of VBI and brain Aβ deposition and their associations with cognition.

DESIGN AND SETTING

A cross-sectional study in a community- and clinic-based sample recruited for elevated vascular disease risk factors.

PARTICIPANTS

Clinically normal (mean age, 77.1 years [N = 30]), cognitively impaired (mean age, 78.0 years [N = 24]), and mildly demented (mean age, 79.8 years [N = 7]) participants.

INTERVENTIONS

Magnetic resonance imaging, Aβ (Pittsburgh Compound B-positron emission tomographic [PiB-PET]) imaging, and cognitive testing.

MAIN OUTCOME MEASURES

Magnetic resonance images were rated for the presence and location of infarct (34 infarct-positive participants, 27 infarct-negative participants) and were used to quantify white matter lesion volume. The PiB-PET uptake ratios were used to create a PiB index by averaging uptake across regions vulnerable to early Aβ deposition; PiB positivity (29 PiB-positive participants, 32 PiB-negative participants) was determined from a data-derived threshold. Standardized composite cognitive measures included executive function and verbal and nonverbal memory.

RESULTS

Vascular brain injury and Aβ were independent in both cognitively normal and impaired participants. Infarction, particularly in cortical and subcortical gray matter, was associated with lower cognitive performance in all domains (P < .05 for all comparisons). Pittsburgh Compound B positivity was neither a significant predictor of cognition nor interacted with VBI.

CONCLUSIONS AND RELEVANCE

In this elderly sample with normal cognition to mild dementia, enriched for vascular disease, VBI was more influential than Aβ in contemporaneous cognitive function and remained predictive after including the possible influence of Aβ. There was no evidence that VBI increases the likelihood of Aβ deposition. This finding highlights the importance of VBI in mild cognitive impairment and suggests that the impact of cerebrovascular disease should be considered with respect to defining the etiology of mild cognitive impairment.

CD33 Alzheimer's disease locus: altered monocyte function and amyloid biology

In our functional dissection of the CD33 Alzheimer's disease susceptibility locus, we found that the rs3865444(C) risk allele was associated with greater cell surface expression of CD33 in the monocytes (t50 = 10.06, P(joint) = 1.3 × 10(-13)) of young and older individuals. It was also associated with diminished internalization of amyloid-β 42 peptide, accumulation of neuritic amyloid pathology and fibrillar amyloid on in vivo imaging, and increased numbers of activated human microglia.

Alzheimer's disease neurodegenerative biomarkers are associated with decreased cognitive function but not β-amyloid in cognitively normal older individuals

β-Amyloid (Aβ) plaque deposition and neurodegeneration within temporoparietal and hippocampal regions may indicate increased risk of Alzheimer's disease (AD). This study examined relationships between AD biomarkers of Aβ and neurodegeneration as well as cognitive performance in cognitively normal older individuals. Aβ burden was quantified in 72 normal older human subjects from the Berkeley Aging Cohort (BAC) using [(11)C] Pittsburgh compound B (PIB) positron emission tomography. In the same individuals, we measured hippocampal volume, as well as glucose metabolism and cortical thickness, which were extracted from a template of cortical AD-affected regions. The three functional and structural biomarkers were merged into a highly AD-sensitive multimodality biomarker reflecting neural integrity. In the normal older individuals, there was no association between elevated PIB uptake and either the single-modality or the multimodality neurodegenerative biomarkers. Lower neural integrity within the AD-affected regions and a control area (the visual cortex) was related to lower scores on memory and executive function tests; the same association was not found with PIB retention. The relationship between cognition and the multimodality AD biomarker was stronger in individuals with the highest PIB uptake. The findings indicate that neurodegeneration occurs within AD regions regardless of Aβ deposition and accounts for worse cognition in cognitively normal older people. The impact of neural integrity on cognitive functions is, however, enhanced in the presence of high Aβ burden for brain regions that are most affected in AD.

Imaging brain amyloid in nondemented young adults with Down syndrome using Pittsburgh compound B

Down syndrome (DS) is one of the most common causes of intellectual disability. Although DS accounts for only 15% of all individuals with intellectual disabilities, adults with DS account for approximately 60% of individuals with intellectual disabilities and Alzheimer's disease. This is thought to be because of overproduction of the β-amyloid (Aβ) protein due to trisomy for the Aβ precursor protein gene on chromosome 21. Pittsburgh compound B (PiB) is a noninvasive in vivo positron emission tomography tracer used to image amyloid deposition in living humans. Studies using PiB have shown an age-dependent asymptomatic amyloid deposition in more than 20% of the cognitively normal elderly population. Presymptomatic carriers of presenilin (PS-1) and Aβ precursor protein gene mutations who are destined to develop Alzheimer's disease also show preclinical amyloid deposition. This report describes a pilot study involving the use of PiB in seven adults with DS (age: 20-44 years). Compared with objective cutoffs for amyloid positivity in older non-DS cognitively normal control subjects, only two of the seven DS subjects (age: 38 and 44 years) showed increased PiB retention. The remaining five subjects aged between 20 and 35 years showed no detectable increase in PiB retention. Interestingly, the two subjects who showed elevated PiB retention showed a striatal-predominant pattern similar to that previously reported for PS-1 mutation carriers. These results demonstrate the feasibility of conducting PiB positron emission tomography scanning in this special population, and suggest a link between Aβ overproduction and early striatal deposition of fibrillar Aβ.

Amyloid imaging with PET in early Alzheimer disease diagnosis

In vivo imaging of amyloid-β (Aβ) with positron emission tomography has moved from the research arena into clinical practice. Clinicians working with cognitive decline and dementia must become familiar with its benefits and limitations. Amyloid imaging allows earlier diagnosis of Alzheimer disease and better differential diagnosis of dementia and provides prognostic information for mild cognitive impairment. It also has an increasingly important role in therapeutic trial recruitment and for evaluation of anti-Aβ treatments. Longitudinal observations are required to elucidate the role of Aβ deposition in the course of Alzheimer disease and provide information needed to fully use the prognostic power of this investigation.

White matter hyperintensities and cerebral amyloidosis: necessary and sufficient for clinical expression of Alzheimer disease?

IMPORTANCE

Current hypothetical models emphasize the importance of β-amyloid in Alzheimer disease (AD) pathogenesis, although amyloid alone is not sufficient to account for the dementia syndrome. The impact of small-vessel cerebrovascular disease, visualized as white matter hyperintensities (WMHs) on magnetic resonance imaging scans, may be a key factor that contributes independently to AD presentation.

OBJECTIVE

To determine the impact of WMHs and Pittsburgh Compound B (PIB) positron-emission tomography-derived amyloid positivity on the clinical expression of AD.

DESIGN

Baseline PIB-positron-emission tomography values were downloaded from the Alzheimer's Disease Neuroimaging Initiative database. Total WMH volume was derived on accompanying structural magnetic resonance imaging data. We examined whether PIB positivity and total WMHs predicted diagnostic classification of patients with AD (n = 20) and control subjects (n = 21). A second analysis determined whether WMHs discriminated between those with and without the clinical diagnosis of AD among those who were classified as PIB positive (n = 28). A third analysis examined whether WMHs, in addition to PIB status, could be used to predict future risk for AD among subjects with mild cognitive impairment (n = 59).

SETTING

The Alzheimer's Disease Neuroimaging Initiative public database.

PARTICIPANTS

The study involved data from 21 normal control subjects, 59 subjects with mild cognitive impairment, and 20 participants with clinically defined AD from the Alzheimer Disease's Neuroimaging Initiative database.

MAIN OUTCOME MEASURES

Clinical AD diagnosis and WMH volume.

RESULTS

Pittsburgh Compound B positivity and increased total WMH volume independently predicted AD diagnosis. Among PIB-positive subjects, those diagnosed as having AD had greater WMH volume than normal control subjects. Among subjects with mild cognitive impairment, both WMH and PIB status at baseline conferred risk for future diagnosis of AD.

CONCLUSIONS AND RELEVANCE

White matter hyperintensities contribute to the presentation of AD and, in the context of significant amyloid deposition, may provide a second hit necessary for the clinical manifestation of the disease. As risk factors for the development of WMHs are modifiable, these findings suggest intervention and prevention strategies for the clinical syndrome of AD.

Is verbal episodic memory in elderly with amyloid deposits preserved through altered neuronal function?

A potential mechanism that enables intellectual preservation in cognitively normal elderly that harbor beta-amyloid (Aβ) pathology is heightened cerebral glucose metabolism. To investigate cross-sectional inter-relationships between Aβ, glucose metabolism, and cognition, 81 subjects (mean age: 75 ± 7 years) underwent [(11)C]Pittsburgh Compound-B and [(18)F]fluorodeoxyglucose positron emission tomography scans and neuropsychological testing. They were divided into low-Aβ (n = 53), intermediate-Aβ (n = 13) and high-Aβ (n = 15) groups as defined by their global cortical [(11)C]PIB retention. Glucose metabolism was assessed using a MetaROI mask that covers metabolically critical regions in Alzheimer's disease (AD) (i.e., posterior cingulate and bilateral angular and inferior temporal gyri). Previously validated factor scores for verbal and visual episodic memory, semantic memory, working memory, and executive functioning were used to evaluate cognitive performances. Greater Aβ deposition in the precuneus was associated with higher metabolic activity (at trend level) and lower visual episodic memory scores. Glucose metabolism did not correlate with cognition across all subjects. However, heightened metabolic activity was associated with better verbal episodic memory performance in subjects with elevated amyloid levels. This preliminary study suggests that neural compensation, as a manifestation of brain reserve, enables elderly supposedly on the path to AD, at least temporarily, to preserve cognitive function.

Using Pittsburgh Compound B for in vivo PET imaging of fibrillar amyloid-beta

The development of Aβ-PET imaging agents has allowed for detection of fibrillar Aβ deposition in vivo and marks a major advancement in understanding the role of Aβ in Alzheimer's disease (AD). Imaging Aβ thus has many potential clinical benefits: early or perhaps preclinical detection of disease and accurately distinguishing AD from dementias of other non-Aβ causes in patients presenting with mild or atypical symptoms or confounding comorbidities (in which the distinction is difficult to make clinically). From a research perspective, imaging Aβ allows us to study relationships between amyloid pathology and changes in cognition, brain structure, and function across the continuum from normal aging to mild cognitive impairment (MCI) to AD; and to monitor the effectiveness of anti-Aβ drugs and relate them to neurodegeneration and clinical symptoms. Here, we will discuss the application of one of the most broadly studied and widely used Aβ imaging agents, Pittsburgh Compound-B (PiB).

PET quantification of 18F-florbetaben binding to β-amyloid deposits in human brains

(18)F-florbetaben is a novel (18)F-labeled tracer for PET imaging of β-amyloid deposits in the human brain. We evaluated the kinetic model-based approaches to the quantification of β-amyloid binding in the brain from dynamic PET data. The validity of the practically useful tissue ratio was also evaluated against the model-based parameters.

METHODS

(18)F-florbetaben PET imaging was performed with concurrent multiple arterial sampling after tracer injection (300 MBq) in 10 Alzheimer disease (AD) patients and 10 age-matched healthy controls. Regional brain-tissue time-activity curves for 90 min were analyzed by a 1-tissue-compartment model and a 2-tissue-compartment model (2TCM) with metabolite-corrected plasma data estimating the specific distribution volume (VS) and distribution volume ratio (DVR [2TCM]) and a multilinear reference tissue model estimating DVR (DVR [MRTM]) using the cerebellar cortex as the reference tissue. Target-to-reference tissue standardized uptake value ratios (SUVRs) at 70-90 min were also calculated.

RESULTS

All brain regions required 2TCM to describe the time-activity curves. All β-amyloid binding parameters in the cerebral cortex (VS, DVR [2TCM], DVR [MRTM], and SUVR) were significantly increased in AD patients (P < 0.05), and there were significant linear correlations among these parameters (r(2) > 0.83). Effect sizes in group discrimination between 8 β-amyloid-positive AD scans and 9 β-amyloid-negative healthy control scans for all binding parameters were excellent, being largest for DVR (2TCM) (4.22) and smallest for VS (3.25) and intermediate and the same for DVR (MRTM) and SUVR (4.03).

CONCLUSION

These results suggest that compartment kinetic model-based quantification of β-amyloid binding from (18)F-florbetaben PET data is feasible and that all β-amyloid binding parameters including SUVR are excellent in discriminating between β-amyloid-positive and -negative scans.

The effect of amyloid β on cognitive decline is modulated by neural integrity in cognitively normal elderly

OBJECTIVE

Alzheimer's disease (AD) pathology of amyloid β (Aβ) accumulation and neurodegeneration may be relevant to preclinical cognitive decline. The objective of this study was to relate AD-sensitive biomarkers of Aβ and neurodegeneration and their interaction to longitudinal cognitive change in cognitively normal elderly.

METHODS

Thirty-eight older people completed at least three consecutive neuropsychological examinations. Using positron emission tomography (PET), Aβ plaque burden was measured with [(11)C]Pittsburgh compound B (PiB). PiB retention was dichotomized into a positive (n = 13) and negative (n = 25) PiB status. Neurodegenerative biomarkers were extracted within AD-vulnerable regions of interest (ROIs)-namely, the hippocampus and temporoparietal cortical areas. Within each ROI, metabolism was quantified with [(18)F] fluorodeoxyglucose (FDG) PET, and the gray matter structure was evaluated using volume (hippocampus) or thickness (cortical regions). ROI-specific functional and structural biomarkers were combined further into cross-modality neurodegenerative composite measures. Using hierarchical regression models, PiB and the neurodegenerative biomarkers were related to cognitive trajectories.

RESULTS

PiB positivity was associated with memory and nonmemory worsening. The neurodegenerative biomarkers modified these relationships. Longitudinal cognitive decline was accelerated in those individuals who exhibited both PiB positivity and lower neurodegenerative biomarker scores, although the two measures appeared to be independent. PiB retention interacted predominantly with the cortical neurodegenerative composite for nonmemory change. Memory decline was best explained by the interaction between PiB and the hippocampal neurodegenerative composite, suggesting regional specificity of the neurodegenerative modulations.

CONCLUSIONS

Our findings indicate that cognitive trajectories deteriorate at a faster rate in cognitively normal individuals expressing Aβ burden and neurodegeneration within specific AD-sensitive regions.

Effect of complement CR1 on brain amyloid burden during aging and its modification by APOE genotype

BACKGROUND

The rs3818361 single nucleotide polymorphism in complement component (3b/4b) receptor-1 (CR1) is associated with increased risk of Alzheimer's disease (AD). Although this novel variant is associated with a small effect size and is unlikely to be useful as a predictor of AD risk, it might provide insights into AD pathogenesis. We examined the association between rs3818361 and brain amyloid deposition in nondemented older individuals.

METHODS

We used (11)C-Pittsburgh Compound-B positron emission tomography to quantify brain amyloid burden in 57 nondemented older individuals (mean age 78.5 years) in the neuroimaging substudy of the Baltimore Longitudinal Study of Aging. In a replication study, we analyzed (11)C-Pittsburgh Compound-B positron emission tomography data from 22 cognitively normal older individuals (mean age 77.1 years) in the Alzheimer's Disease Neuroimaging Initiative dataset.

RESULTS

Risk allele carriers of rs3818361 have lower brain amyloid burden relative to noncarriers. There is a strikingly greater variability in brain amyloid deposition in the noncarrier group relative to risk carriers, an effect explained partly by APOE genotype. In noncarriers of the CR1 risk allele, APOE ε4 individuals showed significantly higher brain amyloid burden relative to APOE ε4 noncarriers. We also independently replicate our observation of lower brain amyloid burden in risk allele carriers of rs3818361 in the Alzheimer's Disease Neuroimaging Initiative sample.

CONCLUSIONS

Our findings suggest complex mechanisms underlying the interaction of CR1, APOE, and brain amyloid pathways in AD. Our results are relevant to treatments targeting brain Aβ in nondemented individuals at risk for AD and suggest that clinical outcomes of such treatments might be influenced by complex gene-gene interactions.

Brain injury biomarkers are not dependent on β-amyloid in normal elderly

OBJECTIVE

The new criteria for preclinical Alzheimer disease (AD) proposed 3 stages: abnormal levels of β-amyloid (stage 1), stage 1 plus evidence of brain injury (stage 2), and stage 2 plus subtle cognitive changes (stage 3). However, a large group of subjects with normal β-amyloid biomarkers have evidence of brain injury; we labeled them as the "suspected non-Alzheimer pathophysiology" (sNAP) group. The characteristics of the sNAP group are poorly understood.

METHODS

Using the preclinical AD classification, 430 cognitively normal subjects from the Mayo Clinic Study of Aging who underwent brain magnetic resonance (MR), (18)fluorodeoxyglucose (FDG), and Pittsburgh compound B positron emission tomography (PET) were evaluated for FDG PET regional volumetrics, MR regional brain volumetrics, white matter hyperintensity volume, and number of infarcts. We examined cross-sectional associations across AD preclinical stages, those with all biomarkers normal, and the sNAP group.

RESULTS

The sNAP group had a lower proportion (14%) with apolipoprotein E ε4 genotype than the preclinical AD stages 2 + 3. The sNAP group did not show any group differences compared to stages 2 + 3 of the preclinical AD group on measures of FDG PET regional hypometabolism, MR regional brain volume loss, cerebrovascular imaging lesions, vascular risk factors, imaging changes associated with α-synucleinopathy, or physical findings of parkinsonism.

INTERPRETATION

Cognitively normal persons with brain injury biomarker abnormalities, with or without abnormal levels of β-amyloid, were indistinguishable on a variety of imaging markers, clinical features, and risk factors. The initial appearance of brain injury biomarkers that occurs in cognitively normal persons with preclinical AD may not depend on β-amyloidosis.

Profiling of hepatic clearance pathways of Pittsburgh compound B and human liver cytochrome p450 phenotyping

Background

¹¹C-PiB has been developed as a positron-emission tomography (PET) ligand for evaluating fibrillar β-amyloid (Aβ) in the human brain. The ligand is rapidly metabolized, with approximately 10% of intact tracer remaining 30 min after injection. When ¹¹C-PiB is used as a treatment endpoint in intervention studies for Alzheimer’s disease (AD), a concern is whether the clearance of the tracer changes from one scan to the next, increasing within subject variability in the PET signal. Subjects enrolled in AD trials may start or stop medications that inhibit or induce xenobiotic metabolizing enzymes such as the cytochrome P450 (CYP) isozymes.

Findings

We conducted CYP phenotyping in recombinantly expressed systems, and in human liver microsomes, to evaluate CYP isozyme contributions to the metabolism of PiB (carrier) and profiled microsomal and hepatocyte incubations for metabolites. The metabolism of PiB appears to be polyzymic, with direct conjugation via UDP-glucuronosyltransferases (UGTs) also occurring.

Conclusion

It is unlikely that CYP inhibition or induction will significantly influence the clearance of ¹¹C-PiB.

Brain amyloid imaging

Imaging of brain β-amyloid plaques with (18)F-labeled tracers for PET will likely be available in clinical practice to assist the diagnosis of Alzheimer disease (AD). With the rapidly growing prevalence of AD as the population ages, and the increasing emphasis on early diagnosis and treatment, brain amyloid imaging is set to become a widely performed investigation. All physicians reading PET scans will need to know the complex relationship between amyloid and cognitive decline, how to best acquire and display images for detection of amyloid, and how to recognize the patterns of tracer binding in AD and other causes of dementia. This article will provide nuclear medicine physicians with the background knowledge required for understanding this emerging investigation, including its appropriate use, and prepare them for practical training in scan interpretation.

Association of gray matter atrophy with age, β-amyloid, and cognition in aging

Both cognitive aging and β-amyloid (Aβ) deposition, a pathological hallmark of Alzheimer's disease, are associated with structural and cognitive changes in cognitively normal older people. To examine independent effects of age and Aβ deposition on cognition and brain structure in aging, 83 cognitively normal older adults underwent structural magnetic resonance imaging scans and neuropsychological tests and were classified as negative (PIB-) or positive (PIB+) for Aβ deposition using the radiotracer Pittsburgh compound B (PIB). Weighted composite discriminant scores represented subjects' cognition. Older adults showed age-related gray matter (GM) atrophy across the whole brain regardless of Aβ deposition. Amyloid burden within PIB+ subjects, however, was associated with GM atrophy in the frontal, parietal, and temporal cortices. Associations between cognition and volume in PIB- subjects were primarily seen throughout frontal regions and the striatum, while, in PIB+ subjects, these associations were seen in orbital-frontal and hippocampal regions. Furthermore, in PIB- subjects, cognition was related to putaminal volume, but not to hippocampus, while, in PIB+ subjects, cognition was related to hippocampal volume, but not to putamen. These findings highlight differential age and Aβ effects on brain structure, indicating effects of age and Aβ that operate somewhat independently to affect frontostriatal and medial temporal brain systems.

Positron emission tomography imaging in neurological disorders

Positron emission tomography (PET) is a powerful tool for in vivo imaging investigations of human brain function. It provides non-invasive quantification of brain metabolism, receptor binding of various neurotransmitter systems, and alterations in regional blood flow. The use of PET in a clinical setting is still limited due to the high costs of cyclotrons and radiochemical laboratories. However, once these limitations can be bypassed, PET could aid clinical practice by providing a useful imaging technique for the diagnosis, the planning of treatment, and the prediction outcome in various neurological diseases.This review aims to explain the PET imaging technique and its applications in neurological disorders such as Parkinson’s disease, Huntington’s disease, multiple sclerosis, and dementias.

Diverging patterns of amyloid deposition and hypometabolism in clinical variants of probable Alzheimer's disease

The factors driving clinical heterogeneity in Alzheimer's disease are not well understood. This study assessed the relationship between amyloid deposition, glucose metabolism and clinical phenotype in Alzheimer's disease, and investigated how these relate to the involvement of functional networks. The study included 17 patients with early-onset Alzheimer's disease (age at onset <65 years), 12 patients with logopenic variant primary progressive aphasia and 13 patients with posterior cortical atrophy [whole Alzheimer's disease group: age = 61.5 years (standard deviation 6.5 years), 55% male]. Thirty healthy control subjects [age = 70.8 (3.3) years, 47% male] were also included. Subjects underwent positron emission tomography with (11)C-labelled Pittsburgh compound B and (18)F-labelled fluorodeoxyglucose. All patients met National Institute on Ageing-Alzheimer's Association criteria for probable Alzheimer's disease and showed evidence of amyloid deposition on (11)C-labelled Pittsburgh compound B positron emission tomography. We hypothesized that hypometabolism patterns would differ across variants, reflecting involvement of specific functional networks, whereas amyloid patterns would be diffuse and similar across variants. We tested these hypotheses using three complimentary approaches: (i) mass-univariate voxel-wise group comparison of (18)F-labelled fluorodeoxyglucose and (11)C-labelled Pittsburgh compound B; (ii) generation of covariance maps across all subjects with Alzheimer's disease from seed regions of interest specifically atrophied in each variant, and comparison of these maps to functional network templates; and (iii) extraction of (11)C-labelled Pittsburgh compound B and (18)F-labelled fluorodeoxyglucose values from functional network templates. Alzheimer's disease clinical groups showed syndrome-specific (18)F-labelled fluorodeoxyglucose patterns, with greater parieto-occipital involvement in posterior cortical atrophy, and asymmetric involvement of left temporoparietal regions in logopenic variant primary progressive aphasia. In contrast, all Alzheimer's disease variants showed diffuse patterns of (11)C-labelled Pittsburgh compound B binding, with posterior cortical atrophy additionally showing elevated uptake in occipital cortex compared with early-onset Alzheimer's disease. The seed region of interest covariance analysis revealed distinct (18)F-labelled fluorodeoxyglucose correlation patterns that greatly overlapped with the right executive-control network for the early-onset Alzheimer's disease region of interest, the left language network for the logopenic variant primary progressive aphasia region of interest, and the higher visual network for the posterior cortical atrophy region of interest. In contrast, (11)C-labelled Pittsburgh compound B covariance maps for each region of interest were diffuse. Finally, (18)F-labelled fluorodeoxyglucose was similarly reduced in all Alzheimer's disease variants in the dorsal and left ventral default mode network, whereas significant differences were found in the right ventral default mode, right executive-control (both lower in early-onset Alzheimer's disease and posterior cortical atrophy than logopenic variant primary progressive aphasia) and higher-order visual network (lower in posterior cortical atrophy than in early-onset Alzheimer's disease and logopenic variant primary progressive aphasia), with a trend towards lower (18)F-labelled fluorodeoxyglucose also found in the left language network in logopenic variant primary progressive aphasia. There were no differences in (11)C-labelled Pittsburgh compound B binding between syndromes in any of the networks. Our data suggest that Alzheimer's disease syndromes are associated with degeneration of specific functional networks, and that fibrillar amyloid-β deposition explains at most a small amount of the clinico-anatomic heterogeneity in Alzheimer's disease.

Classification of amyloid-positivity in controls: comparison of visual read and quantitative approaches

An important research application of amyloid imaging with positron emission tomography (PET) is detection of the earliest evidence of fibrillar amyloid-beta (Aβ) deposition. Use of amyloid PET for this purpose, requires a reproducible method for defining a cutoff that separates individuals with no significant Aβ deposition from those in which Aβ deposition has begun. We previously reported the iterative outlier approach (IO) for the analysis of Pittsburgh Compound-B (PiB) PET data. Developments in amyloid imaging since the initial report of IO have led us to re-examine the generalizability of this method. IO was developed using full-dynamic atrophy-corrected PiB PET data obtained from a group of control subjects with a fairly distinct separation between PiB-positive [PiB(+)] and PiB-negative [PiB(-)] subjects.

METHODS

We tested the performance of IO using late-summed tissue ratio data with atrophy correction or with an automated template method without atrophy correction and tested the robustness of the method when applied to a cohort of older subjects in which separation between PiB(+) and PiB(-) subjects was not so distinct.

RESULTS

The IO method did not perform consistently across analyses and performed particularly poorly when separation was less clear. We found that a sparse k-means (SKM) cluster analysis approach performed significantly better; performing more consistently across methods and subject cohorts. We also compared SKM to a consensus visual read approach and found very good correspondence.

CONCLUSION

The visual read and SKM methods, applied together, may optimize the identification of early Aβ deposition. These methods have the potential to provide a standard approach to the detection of PiB-positivity that is generalizable across centers.

Low background and high contrast PET imaging of amyloid-β with [11C]AZD2995 and [11C]AZD2184 in Alzheimer's disease patients

PURPOSE

The aim of this study was to evaluate AZD2995 side by side with AZD2184 as novel PET radioligands for imaging of amyloid-β in Alzheimer's disease (AD).

METHODS

In vitro binding of tritium-labelled AZD2995 and AZD2184 was studied and compared with that of the established amyloid-β PET radioligand PIB. Subsequently, a first-in-human in vivo PET study was performed using [(11)C]AZD2995 and [(11)C]AZD2184 in three healthy control subjects and seven AD patients.

RESULTS

AZD2995, AZD2184 and PIB were found to share the same binding site to amyloid-β. [(3)H]AZD2995 had the highest signal-to-background ratio in brain tissue from patients with AD as well as in transgenic mice. However, [(11)C]AZD2184 had superior imaging properties in PET, as shown by larger effect sizes comparing binding potential values in cortical regions of AD patients and healthy controls. Nevertheless, probably due to a lower amount of nonspecific binding, the group separation of the distribution volume ratio values of [(11)C]AZD2995 was greater in areas with lower amyloid-β load, e.g. the hippocampus.

CONCLUSION

Both AZD2995 and AZD2184 detect amyloid-β with high affinity and specificity and also display a lower degree of nonspecific binding than that reported for PIB. Overall [(11)C]AZD2184 seems to be an amyloid-β radioligand with higher uptake and better group separation when compared to [(11)C]AZD2995. However, the very low nonspecific binding of [(11)C]AZD2995 makes this radioligand potentially interesting as a tool to study minute levels of amyloid-β. This sensitivity may be important in investigating, for example, early prodromal stages of AD or in the longitudinal study of a disease modifying therapy.

Alzheimer’s disease, cerebrovascular disease, and the β-amyloid cascade

Alzheimer’s disease (AD), considered the commonest neurodegenerative cause of dementia, is associated with hallmark pathologies including extracellular amyloid-β protein (Aβ) deposition in extracellular senile plaques and vessels, and intraneuronal tau deposition as neurofibrillary tangles. Although AD is usually categorized as neurodegeneration distinct from cerebrovascular disease (CVD), studies have shown strong links between AD and CVD. There is evidence that vascular risk factors and CVD may accelerate Aβ 40-42 production/ aggregation/deposition and contribute to the pathology and symptomatology of AD. Aβ deposited along vessels also causes cerebral amyloid angiopathy. Amyloid imaging allows in vivo detection of AD pathology, opening the way for prevention and early treatment, if disease-modifying therapies in the pipeline show safety and efficacy. In this review, we review the role of vascular factors and Aβ, underlining that vascular risk factor management may be important for AD prevention and treatment.

Apathy is associated with increased amyloid burden in mild cognitive impairment

Apathy is the most common neuropsychiatric symptom in mild cognitive impairment (MCI) and Alzheimer's disease (AD) dementia. The authors sought to determine whether apathy is associated with cortical amyloid burden, as measured by Pittsburgh Compound B (PiB) positron emission tomography (PET), and regional hypometabolism, measured by 18F-fluorodeoxyglocuse (FDG) PET in MCI. The authors found a significant association between increased apathy (lower Apathy Evaluation Scale score) and greater cortical PiB retention independent of age, but no significant association between apathy and regional FDG metabolism. These results suggest that increased apathy is associated with greater amyloid burden but not regional hypometabolism in MCI.

Comparison of MRI based and PET template based approaches in the quantitative analysis of amyloid imaging with PIB-PET

RATIONALE

[(11)C]Pittsburgh compound-B (PIB) has been the most widely used positron emission tomography (PET) imaging agent for brain amyloid. Several longitudinal studies evaluating the progression of Alzheimer's disease (AD), and numerous therapeutic intervention studies are underway using [(11)C]PIB PET as an AD biomarker. Quantitative analysis of [(11)C]PIB data requires the definition of regional volumes of interest. This investigation systematically compared two data analysis routes both using a probabilistic brain atlas with 11 bilateral regions. Route 1 used individually segmented structural magnetic resonance images (MRI) for each subject while Route 2 used a standardised [(11)C]PIB PET template.

METHODS

A total of 54 subjects, 20 with probable Alzheimer's disease (AD), 14 with amnestic Mild Cognitive Impairment (MCI) and 20 age-matched healthy controls, were scanned at two imaging centres either in London (UK) or in Turku (Finland). For all subjects structural volumetric MRI and [(11)C]PIB PET scans were acquired. Target-to-cerebellum ratios 40 min to 60 min post injection were used as outcome measures. Regional read outs for grey matter target regions were generated for both routes. Based on a composite neocortical, frontal, posterior cingulate, combined posterior cingulate and frontal cortical regions, scans were categorised into either 'PIB negative' (PIB-) or 'PIB positive' (PIB+) using previously reported cut-off target-to-cerebellar ratios of 1.41, 1.5 and 1.6, respectively.

RESULTS

Target-to-cerebellum ratios were greater when defined with a [(11)C]PIB PET template than with individual MRIs for all cortical regions regardless of diagnosis. This difference was highly significant for controls (p<0.001, paired samples t-test), less significant for MCIs and borderline for ADs. Assignment of subjects to raised or normal categories was the same with both routes with a 1.6 cut-off while with lower cut off using frontal cortex, and combined frontal cortex and posterior cingulate demonstrated similar results, while posterior cingulate alone demonstrated significantly higher proportion of controls as amyloid positive by Route 2.

CONCLUSIONS

Definition of cortical grey matter regions is more accurate when individually segmented MRIs (Route 1) were used rather than a population-based PET template (Route 2). The impact of this difference depends on the grey-to-white matter contrast in the PET images; specifically seen in healthy controls with high white matter and low grey matter uptake. When classifying AD, MCI and control subjects as normal or abnormal using large cortical regions; discordance was found between the MRI and template approach for those few subjects who presented with cortex-to-cerebellum ratios very close to the pre-assigned cut-off. However, posterior cingulate alone demonstrated significant discordance in healthy controls using template based approach. This study, therefore, demonstrates that the use of a [(11)C]PIB PET template (Route 2) is adequate for clinical diagnostic purposes, while MRI based analysis (Route 1) remains more appropriate for clinical research.

β-Amyloid in Lewy body disease is related to Alzheimer's disease-like atrophy

The aim of this study was to investigate whether amyloid deposition is associated with Alzheimer's disease (AD)-like cortical atrophy in Lewy body (LB) disease (LBD). Participants included 15 LBD with dementia patients (8 with dementia with Lewy bodies [DLB] and 7 with Parkinson's disease [PD] with dementia [PDD]), 13 AD patients, and 17 healthy controls. Age, gender, and Mini-Mental State Examination scores were matched between patient groups. All subjects underwent PET scans with [(11)C]Pittsburgh Compound B to measure brain amyloid deposition as well as three-dimensional T1-weighted MRI. Gray-matter volumes (GMVs) were estimated by voxel-based morphometry. Volumes-of-interest analyses were also performed. Forty percent of the 15 DLB/PDD patients were amyloid positive, whereas all AD patients and none of the healthy controls were amyloid positive. Amyloid-positive DLB/PDD and AD patients showed very similar patterns of cortical atrophy in the parahippocampal area and lateral temporal and parietal cortices, with 95.2% of cortical atrophy distribution being overlapped. In contrast, amyloid-negative DLB/PDD patients had no significant cortical atrophy. Compared to healthy controls, parahippocampal GMVs were reduced by 26% in both the amyloid-positive DLB/PDD and AD groups and by 10% in the amyloid-negative DLB/PDD group. The results suggest that amyloid deposition is associated with AD-like atrophy in DLB/PDD patients. Early intervention against amyloid may prevent or delay AD-like atrophy in DLB/PDD patients with amyloid deposition.

Correspondence between in vivo (11)C-PiB-PET amyloid imaging and postmortem, region-matched assessment of plaques

The definitive Alzheimer's disease (AD) diagnosis requires postmortem confirmation of neuropathological hallmarks-amyloid-β (Aβ) plaques and neurofibrillary tangles (NFTs). The advent of radiotracers for amyloid imaging presents an opportunity to investigate amyloid deposition in vivo. The (11)C-Pittsburgh compound-B (PiB)-PET ligand remains the most widely studied to date; however, regional variations in (11)C-PiB binding and the extent of agreement with neuropathological assessment have not been thoroughly investigated. Sojkova and colleagues [35] reported variable agreement between CERAD-based neuropathologic diagnosis of AD lesions and mean cortical PiB, suggesting the need for a more direct quantification of regional Aβ in relation to in vivo imaging. In the present study, we extend these findings by examining the correspondence among regional (11)C-PiB load, region-matched quantitative immunohistological assessments of Aβ and NFTs, and brain atrophy (MRI) in six older Baltimore Longitudinal Study of Aging participants who came to autopsy (imaging-autopsy interval range 0.2-2.4 years). The total number of Aβ plaques (6E10) and NFTs (PHF1) in paraffin sections from hippocampus, orbito-frontal cortex, anterior and posterior cingulate gyrus, precuneus and cerebellum was quantified using a technique guided by unbiased stereological principles. We report a general agreement between the regional measures of amyloid obtained via stereological assessment and imaging, with significant relationships evident for the anterior (r = 0.83; p = 0.04) and posterior (r = 0.94; p = 0.005) cingulate gyri, and the precuneus (r = 0.94; p = 0.005). No associations were observed between (11)C-PiB load and NFT count for any of the regions examined (p > 0.2 in all regions), or between regional Aβ or NFT counts and corresponding brain volumes. The strong associations of PiB retention with region-matched, quantitative analyses of Aβ in postmortem tissue offer support for the validity of (11)C-PiB-PET imaging as a method for evaluation of plaque burden in vivo.

Association of lifetime cognitive engagement and low β-amyloid deposition

OBJECTIVE

To assess the association between lifestyle practices (cognitive and physical activity) and β-amyloid deposition, measured with positron emission tomography using carbon 11-labeled Pittsburgh Compound B ([(11)C]PiB), in healthy older individuals.

DESIGN

Cross-sectional clinical study.

SETTING

Berkeley, California.

PARTICIPANTS

Volunteer sample of 65 healthy older individuals (mean age, 76.1 years), 10 patients with Alzheimer disease (AD) (mean age, 74.8 years), and 11 young controls (mean age, 24.5 years) were studied from October 31, 2005, to February 22, 2011.

MAIN OUTCOME MEASURES

Cortical [(11)C]PiB average (frontal, parietal, lateral temporal, and cingulate regions) and retrospective, self-report scales assessing participation in cognitive activities (eg, reading, writing, and playing games) and physical exercise.

RESULTS

Greater participation in cognitively stimulating activities across the lifespan, but particularly in early and middle life, was associated with reduced [(11)C]PiB uptake (P<.001, accounting for age, sex, and years of education). Older participants in the highest cognitive activity tertile had [(11)C]PiB uptake comparable to young controls, whereas those in the lowest cognitive activity tertile had [(11)C]PiB uptake comparable to patients with AD. Although greater cognitive activity was associated with greater physical exercise, exercise was not associated with [(11)C]PiB uptake.

CONCLUSIONS

Individuals with greater early- and middle-life cognitive activity had lower [(11)C]PiB uptake. The tendency to participate in cognitively stimulating activities is likely related to engagement in a variety of lifestyle practices that have been implicated in other studies showing reduced risk of AD-related pathology. We report a direct association between cognitive activity and [(11)C]PiB uptake, suggesting that lifestyle factors found in individuals with high cognitive engagement may prevent or slow deposition of β-amyloid, perhaps influencing the onset and progression of AD.

Shapes of the trajectories of 5 major biomarkers of Alzheimer disease

OBJECTIVE

To characterize the shape of the trajectories of Alzheimer disease biomarkers as a function of Mini-Mental State Examination (MMSE) score.

DESIGN AND SETTING

Longitudinal registries from the Mayo Clinic and the Alzheimer's Disease Neuroimaging Initiative.

PATIENTS

Two different samples (n = 343 and n = 598) were created that spanned the cognitive spectrum from normal to Alzheimer disease dementia. Subgroup analyses were performed in members of both cohorts (n = 243 and n = 328) who were amyloid positive at baseline.

MAIN OUTCOME MEASURES

The shape of biomarker trajectories as a function of MMSE score, adjusted for age, was modeled and described as baseline (cross-sectional) and within-subject longitudinal effects. Biomarkers evaluated were cerebrospinal fluid (CSF) Aβ42 and tau levels, amyloid and fluorodeoxyglucose positron emission tomography imaging, and structural magnetic resonance imaging.

RESULTS

Baseline biomarker values generally worsened (ie, nonzero slope) with lower baseline MMSE score. Baseline hippocampal volume, amyloid positron emission tomography, and fluorodeoxyglucose positron emission tomography values plateaued (ie, nonlinear slope) with lower MMSE score in 1 or more analyses. Longitudinally, within-subject rates of biomarker change were associated with worsening MMSE score. Nonconstant within-subject rates (deceleration) of biomarker change were found in only 1 model.

CONCLUSIONS

Biomarker trajectory shapes by MMSE score were complex and were affected by interactions with age and APOE status. Nonlinearity was found in several baseline effects models. Nonconstant within-subject rates of biomarker change were found in only 1 model, likely owing to limited within-subject longitudinal follow-up. Creating reliable models that describe the full trajectories of Alzheimer disease biomarkers will require significant additional longitudinal data in individual participants.

Different clinical phenotypes in siblings with a presenilin-1 P264L mutation

BACKGROUND

Mutations in the presenilin-1 gene (PSEN1) have been identified in autosomal dominant early-onset cases of Alzheimer's disease (AD).

AIMS

To investigate different clinical phenotypes of siblings possessing the same heterozygous P264L mutation in the PSEN1 gene.

METHODS

We evaluated clinical features, neuroimaging results, and neuropsychological examinations. The PSEN1 gene and other dementia-related gene mutations were screened.

RESULTS

We clinically diagnosed the proband as atypical AD with frontotemporal dementia features and diagnosed the elder brother of the proband as typical AD, based on neuropsychological symptoms and a brain imaging examination including amyloid imaging data. A heterozygous P264L mutation in the PSEN1 gene was identified in both siblings.

CONCLUSION

This study is one of few reports of AD siblings possessing the same mutation but exhibiting different clinical phenotypes in a Japanese family possessing a P264L mutation in the PSEN1 gene. The current results suggest that unknown modifiers, including both genetic and epigenetic factors, may alter the pathological and clinical phenotypes of a genetically predetermined disease.

Cognitive profile of amyloid burden and white matter hyperintensities in cognitively normal older adults

Amyloid burden and white matter hyperintensities (WMH) are two common markers of neurodegeneration present in advanced aging. Each represents a potential early indicator of an age-related neurological disorder that impacts cognition. The presence of amyloid is observed in a substantial subset of cognitively normal older adults, but the literature remains equivocal regarding whether amyloid in nondemented populations is deleterious to cognition. Similarly, WMH are detected in many nondemented older adults and there is a body of evidence indicating that WMH are associated with decreased executive function and other cognitive domains. The current study investigated amyloid burden and WMH in clinically normal older adult humans aged 65-86 (N = 168) and examined each biomarker's relation with cognitive domains of episodic memory, executive function, and speed of processing. Factors for each domain were derived from a neuropsychological battery on a theoretical basis without reference to the relation between cognition and the biomarkers. Amyloid burden and WMH were not correlated with one another. Age was associated with lower performance in all cognitive domains, while higher estimated verbal intelligence was associated with higher performance in all domains. Hypothesis-driven tests revealed that amyloid burden and WMH had distinct cognitive profiles, with amyloid burden having a specific influence on episodic memory and WMH primarily associated with executive function but having broad (but lesser) effects on the other domains. These findings suggest that even before clinical impairment, amyloid burden and WMH likely represent neuropathological cascades with distinct etiologies and dissociable influences on cognition.

Florbetapir PET analysis of amyloid-β deposition in the presenilin 1 E280A autosomal dominant Alzheimer's disease kindred: a cross-sectional study

BACKGROUND

Fibrillar amyloid-β (Aβ) is thought to begin accumulating in the brain many years before the onset of clinical impairment in patients with Alzheimer's disease. By assessing the accumulation of Aβ in people at risk of genetic forms of Alzheimer's disease, we can identify how early preclinical changes start in individuals certain to develop dementia later in life. We sought to characterise the age-related accumulation of Aβ deposition in presenilin 1 (PSEN1) E280A mutation carriers across the spectrum of preclinical disease.

METHODS

Between Aug 1 and Dec 6, 2011, members of the familial Alzheimer's disease Colombian kindred aged 18-60 years were recruited from the Alzheimer's Prevention Initiative's registry at the University of Antioquia, Medellín, Colombia. Cross-sectional assessment using florbetapir PET was done in symptomatic mutation carriers with mild cognitive impairment or mild dementia, asymptomatic carriers, and asymptomatic non-carriers. These assessments were done at the Banner Alzheimer's Institute in Phoenix, AZ, USA. A cortical grey matter mask consisting of six predefined regions.was used to measure mean cortical florbetapir PET binding. Cortical-to-pontine standard-uptake value ratios were used to characterise the cross-sectional accumulation of fibrillar Aβ deposition in carriers and non-carriers with regression analysis and to estimate the trajectories of fibrillar Aβ deposition.

FINDINGS

We enrolled a cohort of 11 symptomatic individuals, 19 presymptomatic mutation carriers, and 20 asymptomatic non-carriers, ranging in age from 20 to 56 years. There was greater florbetapir binding in asymptomatic PSEN1 E280A mutation carriers than in age matched non-carriers. Fibrillar Aβ began to accumulate in PSEN 1E280A mutation carriers at a mean age of 28·2 years (95% CI 27·3-33·4), about 16 years and 21 years before the predicted median ages at mild cognitive impairment and dementia onset, respectively. (18)F florbetapir binding rose steeply over the next 9·4 years and plateaued at a mean age of 37·6 years (95% CI 35·3-40·2), about 6 and 11 years before the expected respective median ages at mild cognitive impairment and dementia onset. Prominent florbetapir binding was seen in the anterior and posterior cingulate, precuneus, and parietotemporal and frontal grey matter, as well as in the basal ganglia. Binding in the basal ganglia was not seen earlier or more prominently than in other regions.

INTERPRETATION

These findings contribute to the understanding of preclinical familial Alzheimer's disease and help set the stage for assessment of amyloid-modifying treatments in the prevention of familial Alzheimer's disease.

FUNDING

Avid Radiopharmaceuticals, Banner Alzheimer's Foundation, Nomis Foundation, Anonymous Foundation, Forget Me Not Initiative, Colciencias, National Institute on Aging, and the State of Arizona.

A non-linear mixed effect modelling approach for metabolite correction of the arterial input function in PET studies

Quantitative PET studies with arterial blood sampling usually require the correction of the measured total plasma activity for the presence of metabolites. In particular, if labelled metabolites are found in the plasma in significant amounts their presence has to be accounted for, because it is the concentration of the parent tracer which is required for data quantification. This is achieved by fitting a Parent Plasma fraction (PPf) model to discrete metabolite measurements. The commonly used method is based on an individual approach, i.e. for each subject the PPf model parameters are estimated from its own metabolite samples, which are, in general, sparse and noisy. This fact can compromise the quality of the reconstructed arterial input functions, and, consequently, affect the quantification of tissue kinetic parameters. In this study, we proposed a Non-Linear Mixed Effect Modelling (NLMEM) approach to describe metabolite kinetics. Since NLMEM has been developed to provide robust parameter estimates in the case of sparse and/or noisy data, it has the potential to be a reliable method for plasma metabolite correction. Three different PET datasets were considered: [11C]-(+)-PHNO (54 scans), [11C]-PIB (22 scans) and [11C]-DASB (30 scans). For each tracer both simulated and measured data were considered and NLMEM performance was compared with that provided by individual analysis. Results showed that NLMEM provided improved estimates of the plasma parent input function over the individual approach when the metabolite data were sparse or contained outliers.

Indicators of amyloid burden in a population-based study of cognitively normal elderly

OBJECTIVES

Secondary prevention trials in subjects with preclinical Alzheimer disease may require documentation of brain amyloidosis. The identification of inexpensive and noninvasive screening variables that can identify individuals who have significant amyloid accumulation would reduce screening costs.

METHODS

A total of 483 cognitively normal (CN) individuals, aged 70-92 years, from the population-based Mayo Clinic Study of Aging, underwent Pittsburgh compound B (PiB)-PET imaging. Logistic regression determined whether age, sex, APOE genotype, family history, or cognitive performance was associated with odds of a PiB retention ratio >1.4 and >1.5. Area under the receiver operating characteristic curve (AUROC) evaluated the discrimination between PiB-positive and -negative subjects. For each characteristic, we determined the number needed to screen in each age group (70-79 and 80-89) to identify 100 participants with PiB >1.4 or >1.5.

RESULTS

A total of 211 (44%) individuals had PiB >1.4 and 151 (31%) >1.5. In univariate and multivariate models, discrimination was modest (AUROC ∼0.6-0.7). Multivariately, age and APOE best predicted odds of PiB >1.4 and >1.5. Subjective memory complaints were similar to cognitive test performance in predicting PiB >1.5. Indicators of PiB positivity varied with age. Screening APOE ε4 carriers alone reduced the number needed to screen to enroll 100 subjects with PIB >1.5 by 48% in persons aged 70-79 and 33% in those aged 80-89.

CONCLUSIONS

Age and APOE genotype are useful predictors of the likelihood of significant amyloid accumulation, but discrimination is modest. Nonetheless, these results suggest that inexpensive and noninvasive measures could significantly reduce the number of CN individuals needed to screen to enroll a given number of amyloid-positive subjects.

Covarying alterations in Aβ deposition, glucose metabolism, and gray matter volume in cognitively normal elderly

β-Amyloid (Aβ), a feature of Alzheimer's disease (AD) pathology, may precede reduced glucose metabolism and gray matter (GM) volume and cognitive decline in patients with AD. Accumulation of Aβ, however, has been also reported in cognitively intact older people, although it remains unresolved whether and how Aβ deposition, glucose metabolism, and GM volume relate to one another in cognitively normal elderly. Fifty-two cognitively normal older adults underwent Pittsburgh Compound B-positron emission tomography (PIB-PET), [(18)F]fluorodeoxyglucose-PET, and structural magnetic resonance imaging to measure whole-brain amyloid deposition, glucose metabolism, and GM volume, respectively. Covariance patterns of these measures in association with global amyloid burden measured by PIB index were extracted using principal component analysis-based multivariate methods. Higher global amyloid burden was associated with relative increases of amyloid deposition and glucose metabolism and relative decreases of GM volume in brain regions collectively known as the default mode network including the posterior cingulate/precuneus, lateral parietal cortices, and medial frontal cortex. Relative increases of amyloid deposition and glucose metabolism were also noted in the lateral prefrontal cortices, and relative decreases of GM volume were pronounced in hippocampus. The degree of expression of the topographical patterns of the PIB data was further associated with visual memory performance when controlling for age, sex, and education. The present findings suggest that cognitively normal older adults with greater amyloid deposition are relatively hypermetabolic in frontal and parietal brain regions while undergoing GM volume loss in overlapping brain regions.

Application of copulas to improve covariance estimation for partial least squares

Dimension reduction techniques, such as partial least squares, are useful for computing summary measures and examining relationships in complex settings. Partial least squares requires an estimate of the covariance matrix as a first step in the analysis, making this estimate critical to the results. In addition, the covariance matrix also forms the basis for other techniques in multivariate analysis, such as principal component analysis and independent component analysis. This paper has been motivated by an example from an imaging study in Alzheimer's disease where there is complete separation between Alzheimer's and control subjects for one of the imaging modalities. This separation occurs in one block of variables and does not occur with the second block of variables resulting in inaccurate estimates of the covariance. We propose the use of a copula to obtain estimates of the covariance in this setting, where one set of variables comes from a mixture distribution. Simulation studies show that the proposed estimator is an improvement over the standard estimators of covariance. We illustrate the methods from the motivating example from a study in the area of Alzheimer's disease.

Subjective cognitive complaints and amyloid burden in cognitively normal older individuals

Accumulating evidence suggests that subjective cognitive complaints (SCC) may indicate subtle cognitive decline characteristic of individuals with preclinical Alzheimer's disease (AD). In this study, we sought to build upon previous studies by associating SCC and amyloid-β deposition using positron emission tomography with Pittsburgh Compound B (PiB-PET) in cognitively normal older individuals. One-hundred thirty one subjects (mean age 73.5±6) were administered three subjective cognitive questionnaires and a brief neuropsychological battery. A relationship between a subjective memory complaints composite score and cortical PiB binding was found to be significant, even after controlling for depressive symptoms. By contrast, there were no significant relationships between objective cognitive measures of memory and executive functions and cortical PiB binding. Our study suggests that SCC may be an early indicator of AD pathology detectable prior to significant objective impairment.

Improved kinetic analysis of dynamic PET data with optimized HYPR-LR

PURPOSE

Highly constrained backprojection-local reconstruction (HYPR-LR) has made a dramatic impact on magnetic resonance angiography (MRA) and shows promise for positron emission tomography (PET) because of the improvements in the signal-to-noise ratio (SNR) it provides dynamic images. For PET in particular, HYPR-LR could improve kinetic analysis methods that are sensitive to noise. In this work, the authors closely examine the performance of HYPR-LR in the context of kinetic analysis, they develop an implementation of the algorithm that can be tailored to specific PET imaging tasks to minimize bias and maximize improvement in variance, and they provide a framework for validating the use of HYPR-LR processing for a particular imaging task.

METHODS

HYPR-LR can introduce errors into non sparse PET studies that might bias kinetic parameter estimates. An implementation of HYPR-LR is proposed that uses multiple temporally summed composite images that are formed based on the kinetics of the tracer being studied (HYPR-LR-MC). The effects of HYPR-LR-MC and of HYPR-LR using a full composite formed with all the frames in the study (HYPR-LR-FC) on the kinetic analysis of Pittsburgh compound-B ([11C]-PIB) are studied. HYPR-LR processing is compared to spatial smoothing. HYPR-LR processing was evaluated using both simulated and human studies. Nondisplaceable binding potential (BP(ND)) parametric images were generated from fifty noise realizations of the same numerical phantom and eight [(11)C]-PIB positive human scans before and after HYPR-LR processing or smoothing using the reference region Logan graphical method and receptor parametric mapping (RPM2). The bias and coefficient of variation in the frontal and parietal cortex in the simulated parametric images were calculated to evaluate the absolute performance of HYPR-LR processing. Bias in the human data was evaluated by comparing parametric image BP(ND) values averaged over large regions of interest (ROIs) to Logan estimates of the BP(ND) from TACs averaged over the same ROIs. Variance was assessed qualitatively in the parametric images and semiquantitatively by studying the correlation between voxel BP(ND) estimates from Logan analysis and RPM2.

RESULTS

Both the simulated and human data show that HYPR-LR-FC overestimates BP(ND) values in regions of high [(11)C]-PIB uptake. HYPR-LR-MC virtually eliminates this bias. Both implementations of HYPR-LR reduce variance in the parametric images generated with both Logan analysis and RPM2, and HYPR-LR-FC provides a greater reduction in variance. This reduction in variance nearly eliminates the noise-dependent Logan bias. The variance reduction is greater for the Logan method, particularly for HYPR-LR-MC, and the variance in the resulting Logan images is comparable to that in the RPM2 images. HYPR-LR processing compares favorably with spatial smoothing, particularly when the data are analyzed with the Logan method, as it provides a reduction in variance with no loss of spatial resolution.

CONCLUSIONS

HYPR-LR processing shows significant potential for reducing variance in parametric images, and can eliminate the noise-dependent Logan bias. HYPR-LR-FC processing provides the greatest reduction in variance but introduces a positive bias into the BP(ND) of high-uptake border regions. The proposed method for forming HYPR composite images, HYPR-LR-MC, eliminates this bias at the cost of less variance reduction.

Molecular brain imaging in the multimodality era

Multimodality molecular brain imaging encompasses in vivo visualization, evaluation, and measurement of cellular/molecular processes. Instrumentation and software developments over the past 30 years have fueled advancements in multimodality imaging platforms that enable acquisition of multiple complementary imaging outcomes by either combined sequential or simultaneous acquisition. This article provides a general overview of multimodality neuroimaging in the context of positron emission tomography as a molecular imaging tool and magnetic resonance imaging as a structural and functional imaging tool. Several image examples are provided and general challenges are discussed to exemplify complementary features of the modalities, as well as important strengths and weaknesses of combined assessments. Alzheimer's disease is highlighted, as this clinical area has been strongly impacted by multimodality neuroimaging findings that have improved understanding of the natural history of disease progression, early disease detection, and informed therapy evaluation.

Pd(II)-catalyzed ortho arylation of 2-arylbenzothiazoles with aryl iodides via benzothiazole-directed C-H activation

A novel and efficient method for the arylation of 2-arylbenzothiazoles is described via C-H activation. The desired C(Ar)-C(Ar) bond formation proceeded efficiently with good functional-group tolerance and high regioselectivity. Proposed mechanism for the arylation of 2-arylbenzothiazole is depicted.

Alzheimer's disease: biological aspects, therapeutic perspectives and diagnostic tools

Alzheimer's disease (AD) is the most common form of dementia among older people. Dementia is an irreversible brain disorder that seriously affects a person's ability to carry out daily activities. It is characterized by loss of cognitive functioning and behavioral abilities, to such an extent that it interferes with the daily life and activities of the affected patients. Although it is still unknown how the disease process begins, it seems that brain damage starts a decade or more before problems become evident. Scientific data seem to indicate that changes in the generation or the degradation of the amyloid-b peptide (Aβ) lead to the formation of aggregated structures that are the triggering molecular events in the pathogenic cascade of AD. This review summarizes some characteristic features of Aβ misfolding and aggregation and how cell damage and death mechanisms are induced by these supramolecular and toxic structures. Further, some interventions for the early diagnosis of AD are described and in the last part the potential therapeutic strategies adoptable to slow down, or better block, the progression of the pathology are reported.

The ups and downs of the posteromedial cortex: age- and amyloid-related functional alterations of the encoding/retrieval flip in cognitively normal older adults

Neural networks supporting memory function decline with increasing age. Accumulation of amyloid-β, a histopathological finding in Alzheimer's disease, is a likely contributor. Posteromedial cortices (PMCs) are particularly vulnerable to early amyloid pathology and play a role in both encoding and retrieval processes. The extent to which aging and amyloid influence the ability to modulate activity between these processes within the PMC was investigated by combining positron emission tomography-amyloid imaging with functional magnetic resonance imaging in cognitively normal older and young adults. Young subjects exhibited a marked decrease in activity during encoding and an increase during retrieval (also known as encoding/retrieval "flip"). Impaired ability to modulate activity was associated with increasing age, greater amyloid burden, and worse memory performance. In contrast, the hippocampus showed increased activity during both encoding and retrieval, which was not related to these variables. These findings support a specific link between amyloid pathology and neural dysfunction in PMC and elucidate the underpinnings of age-related memory dysfunction.

Preventive immunization of aged and juvenile non-human primates to β-amyloid

Background

Immunization against beta-amyloid (Aβ) is a promising approach for the treatment of Alzheimer’s disease, but the optimal timing for the vaccination remains to be determined. Preventive immunization approaches may be more efficacious and associated with fewer side-effects; however, there is only limited information available from primate models about the effects of preclinical vaccination on brain amyloid composition and the neuroinflammatory milieu.

Methods

Ten non-human primates (NHP) of advanced age (18–26 years) and eight 2-year-old juvenile NHPs were immunized at 0, 2, 6, 10 and 14 weeks with aggregated Aβ₄₂ admixed with monophosphoryl lipid A as adjuvant, and monitored for up to 6 months. Anti-Aβ antibody levels and immune activation markers were assessed in plasma and cerebrospinal fluid samples before and at several time-points after immunization. Microglial activity was determined by [¹¹C]PK11195 PET scans acquired before and after immunization, and by post-mortem immunohistochemical and real-time PCR evaluation. Aβ oligomer composition was assessed by immunoblot analysis in the frontal cortex of aged immunized and non-immunized control animals.

Results

All juvenile animals developed a strong and sustained serum anti-Aβ IgG antibody response, whereas only 80 % of aged animals developed detectable antibodies. The immune response in aged monkeys was more delayed and significantly weaker, and was also more variable between animals. Pre- and post-immunization [¹¹C]PK11195 PET scans showed no evidence of vaccine-related microglial activation. Post-mortem brain tissue analysis indicated a low overall amyloid burden, but revealed a significant shift in oligomer size with an increase in the dimer:pentamer ratio in aged immunized animals compared with non-immunized controls (P < 0.01). No differences were seen in microglial density or expression of classical and alternative microglial activation markers between immunized and control animals.

Conclusions

Our results indicate that preventive Aβ immunization is a safe therapeutic approach lacking adverse CNS immune system activation or other serious side-effects in both aged and juvenile NHP cohorts. A significant shift in the composition of soluble oligomers towards smaller species might facilitate removal of toxic Aβ species from the brain.

Short-term clinical outcomes for stages of NIA-AA preclinical Alzheimer disease

OBJECTIVE

Recommendations for the diagnosis of preclinical Alzheimer disease (AD) have been formulated by a workgroup of the National Institute on Aging and Alzheimer's Association. Three stages of preclinical AD were described. Stage 1 is characterized by abnormal levels of β-amyloid. Stage 2 represents abnormal levels of β-amyloid and evidence of brain neurodegeneration. Stage 3 includes the features of stage 2 plus subtle cognitive changes. Stage 0, not explicitly defined in the criteria, represents subjects with normal biomarkers and normal cognition. The ability of the recommended criteria to predict progression to cognitive impairment is the crux of their validity.

METHODS

Using previously developed operational definitions of the 3 stages of preclinical AD, we examined the outcomes of subjects from the Mayo Clinic Study of Aging diagnosed as cognitively normal who underwent brain MRI or [(18)F]fluorodeoxyglucose and Pittsburgh compound B PET, had global cognitive test scores, and were followed for at least 1 year.

RESULTS

Of the 296 initially normal subjects, 31 (10%) progressed to a diagnosis of mild cognitive impairment (MCI) or dementia (27 amnestic MCI, 2 nonamnestic MCI, and 2 non-AD dementias) within 1 year. The proportion of subjects who progressed to MCI or dementia increased with advancing stage (stage 0, 5%; stage 1, 11%; stage 2, 21%; stage 3, 43%; test for trend, p < 0.001).

CONCLUSIONS

Despite the short follow-up period, our operationalization of the new preclinical AD recommendations confirmed that advancing preclinical stage led to higher proportions of subjects who progressed to MCI or dementia.

Failure to modulate attentional control in advanced aging linked to white matter pathology

Advanced aging is associated with reduced attentional control and less flexible information processing. Here, the origins of these cognitive effects were explored using a functional magnetic resonance imaging task that systematically varied demands to shift attention and inhibit irrelevant information across task blocks. Prefrontal and parietal regions previously implicated in attentional control were recruited by the task and most so for the most demanding task configurations. A subset of older individuals did not modulate activity in frontal and parietal regions in response to changing task requirements. Older adults who did not dynamically modulate activity underperformed their peers and scored more poorly on neuropsychological measures of executive function and speed of processing. Examining 2 markers of preclinical pathology in older adults revealed that white matter hyperintensities (WMHs), but not high amyloid burden, were associated with failure to modulate activity in response to changing task demands. In contrast, high amyloid burden was associated with alterations in default network activity. These results suggest failure to modulate frontal and parietal activity reflects a disruptive process in advanced aging associated with specific neuropathologic processes.