The accumulation of β-amyloid in the brain is an early event in Alzheimer’s disease. This study presents the first patient with Alzheimer’s disease who underwent positron emission tomography imaging with the amyloid tracer, Pittsburgh Compound B to visualize fibrillar β-amyloid in the brain. Here we relate the clinical progression, amyloid and functional brain positron emission tomography imaging with molecular neuropathological alterations at autopsy to gain new insight into the relationship between β-amyloid accumulation, inflammatory processes and the cholinergic neurotransmitter system in Alzheimer’s disease brain. The patient underwent positron emission tomography studies with 18F-fluorodeoxyglucose three times (at ages 53, 56 and 58 years) and twice with Pittsburgh Compound B (at ages 56 and 58 years), prior to death at 61 years of age. The patient showed a pronounced decline in cerebral glucose metabolism and cognition during disease progression, while Pittsburgh Compound B retention remained high and stable at follow-up. Neuropathological examination of the brain at autopsy confirmed the clinical diagnosis of pure Alzheimer’s disease. A comprehensive neuropathological investigation was performed in nine brain regions to measure the regional distribution of β-amyloid, neurofibrillary tangles and the levels of binding of 3H-nicotine and 125I-α-bungarotoxin to neuronal nicotinic acetylcholine receptor subtypes, 3H-L-deprenyl to activated astrocytes and 3H-PK11195 to microglia, as well as butyrylcholinesterase activity. Regional in vivo 11C-Pittsburgh Compound B-positron emission tomography retention positively correlated with 3H-Pittsburgh Compound B binding, total insoluble β-amyloid, and β-amyloid plaque distribution, but not with the number of neurofibrillary tangles measured at autopsy. There was a negative correlation between regional fibrillar β-amyloid and levels of 3H-nicotine binding. In addition, a positive correlation was found between regional 11C-Pittsburgh Compound B positron emission tomography retention and 3H-Pittsburgh Compound B binding with the number of glial fibrillary acidic protein immunoreactive cells, but not with 3H-L-deprenyl and 3H-PK-11195 binding. In summary, high 11C-Pittsburgh Compound B positron emission tomography retention significantly correlates with both fibrillar β-amyloid and losses of neuronal nicotinic acetylcholine receptor subtypes at autopsy, suggesting a closer involvement of β-amyloid pathology with neuronal nicotinic acetylcholine receptor subtypes than with inflammatory processes.
Alzheimer’s disease; autopsy brain; 11C-PIB positron emission tomography; inflammation; nicotinic acetylcholine receptors
To assess whether family history (FH) of Alzheimer’s disease (AD) alone influences AD biomarker abnormalities.
Adult Children Study (ACS).
Washington University's Knight Alzheimer's Disease Research Center.
Cognitively normal middle to older age individuals with and without a FH for AD (n=269).
Main Outcome Measures
Clinical and cognitive measures, magnetic resonance imaging (MRI)-based brain volumes, diffusion tensor imaging (DTI)-based white matter microstructure, cerebrospinal fluid (CSF) biomarkers, and molecular imaging of cerebral fibrillar amyloid with positron emission tomography (PET) using the [11C] benzothiazole tracer, Pittsburgh Compound-B (PIB).
A positive FH for AD was associated with an age-related decrease of CSF Aβ42; the ε4 allele of apolipoprotein E (APOE4) did not alter this effect. Age-adjusted CSF Aβ42 was decreased for individuals with APOE4 compared with those without, and the decrease was larger for individuals with a positive FH compared with those without. The variation of CSF tau and PIB mean cortical binding potential (MCBP) increased by age. For individuals younger than 55, an age-related increase in MCBP was associated with APOE4, but not FH. For individuals older than 55, a positive FH and a positive APOE4 implied the fastest age-related increase in MCBP. A positive FH was associated with decreased fractional anisotropy from DTI in the genu and splenium of the corpus callosum.
Independent of APOE4, FH is associated with age-related change of several CSF, PIB and DTI biomarkers in cognitively normal middle to older age individuals, suggesting that non-APOE susceptibility genes for AD influence AD biomarkers.
Amyloid imaging represents a major advance in neuroscience, enabling the detection and quantification of pathologic protein aggregations in the brain. In this review we survey current amyloid imaging techniques, focusing on positron emission tomography (PET) with 11carbon-labelled Pittsburgh Compound-B (11C-PIB), the most extensively studied and best validated tracer. PIB binds specifically to fibrillar beta-amyloid (Aβ) deposits, and is a sensitive marker for Aβ pathology in cognitively normal older individuals and patients with mild cognitive impairment (MCI) and Alzheimer's disease (AD). PIB-PET provides us with a powerful tool to examine in vivo the relationship between amyloid deposition, clinical symptoms, and structural and functional brain changes in the continuum between normal aging and AD. Amyloid imaging studies support a model in which amyloid deposition is an early event on the path to dementia, beginning insidiously in cognitively normal individuals, and accompanied by subtle cognitive decline and functional and structural brain changes suggestive of incipient AD. As patients progress to dementia, clinical decline and neurodegeneration accelerate and proceed independently of amyloid accumulation. In the future, amyloid imaging is likely to supplement clinical evaluation in selecting patients for anti-amyloid therapies, while MRI and FDG-PET may be more appropriate markers of clinical progression.
Amyloid imaging; PET; PIB; beta-amyloid; brain aging; MCI; Alzheimer's disease
Clinical use of positron emission tomography (PET) is now well established in neurodegenerative disorders, especially in the diagnosis of dementia. Measurement of cerebral glucose metabolism is of significant value, and it facilitates early diagnosis, appropriate differential diagnosis, and the evaluation of drug treatment in patients with dementia. In addition, tracers offer new perspectives for studying the neuropathology of underlying dementia, such as the accumulation of amyloid proteins, tau-proteins, or the presence of neuroinflammation. Finally, PET tracer studies of different neurotransmitter systems in dementia may not only increase the understanding of pathophysiologic mechanisms of the different disorders, but also improve diagnostic accuracy. In conclusion, PET imaging with different tracers offers reliable biomarkers in dementia, which can assist clinicians in the diagnosis of different dementing disorders, especially in the situation of overlapping phenotypes.
PET/CT; dementia; Alzheimer’s disease
Amyloid PET tracers have been developed for in vivo detection of brain fibrillar amyloid deposition in Alzheimer’s disease (AD). To serve as an early biomarker in AD the amyloid PET tracers need to be analysed in multicentre clinical studies.
In this study 238 [11C]Pittsburgh compound-B (PIB) datasets from five different European centres were pooled. Of these 238 datasets, 18 were excluded, leaving [11C]PIB datasets from 97 patients with clinically diagnosed AD (mean age 69 ± 8 years), 72 patients with mild cognitive impairment (MCI; mean age 67.5 ± 8 years) and 51 healthy controls (mean age 67.4 ± 6 years) available for analysis. Of the MCI patients, 64 were longitudinally followed for 28 ± 15 months. Most participants (175 out of 220) were also tested for apolipoprotein E (ApoE) genotype.
[11C]PIB retention in the neocortical and subcortical brain regions was significantly higher in AD patients than in age-matched controls. Intermediate [11C]PIB retention was observed in MCI patients, with a bimodal distribution (64 % MCI PIB-positive and 36 % MCI PIB-negative), which was significantly different the pattern in both the AD patients and controls. Higher [11C]PIB retention was observed in MCI ApoE ε4 carriers compared to non-ApoE ε4 carriers (p < 0.005). Of the MCI PIB-positive patients, 67 % had converted to AD at follow-up while none of the MCI PIB-negative patients converted.
This study demonstrated the robustness of [11C]PIB PET as a marker of neocortical fibrillar amyloid deposition in brain when assessed in a multicentre setting. MCI PIB-positive patients showed more severe memory impairment than MCI PIB-negative patients and progressed to AD at an estimated rate of 25 % per year. None of the MCI PIB-negative patients converted to AD, and thus PIB negativity had a 100 % negative predictive value for progression to AD. This supports the notion that PIB-positive scans in MCI patients are an indicator of prodromal AD.
Electronic supplementary material
The online version of this article (doi:10.1007/s00259-012-2237-2) contains supplementary material, which is available to authorized users.
Amyloid; Multicentre PET; PIB; MCI; Alzheimer’s disease; Mild cognitive impairment; Cognition
Over the last 20 years, there has been extraordinary progress in brain imaging research and its application to the study of Alzheimer's disease (AD). Brain imaging researchers have contributed to the scientific understanding, early detection and tracking of AD. They have set the stage for imaging techniques to play growing roles in the clinical setting, the evaluation of disease-modifying treatments, and the identification of demonstrably effective prevention therapies. They have developed ground-breaking methods, including positron emission tomography (PET) ligands to measure fibrillar amyloid-β (Aβ) deposition, new magnetic resonance imaging (MRI) pulse sequences, and powerful image analysis techniques, to help in these endeavors. Additional work is needed to develop even more powerful imaging methods, to further clarify the relationship and time course of Aβ and other disease processes in the predisposition to AD, to establish the role of brain imaging methods in the clinical setting, and to provide the scientific means and regulatory approval pathway needed to evaluate the range of promising disease-modifying and prevention therapies as quickly as possible. Twenty years from now, AD may not yet be a distant memory, but the best is yet to come.
Alzheimer's disease; dementia; mild cognitive impairment; MRI; PET; amyloid; diagnosis; prevention
Imaging has played a variety of roles in the study of Alzheimer disease (AD) over the past four decades. Initially, computed tomography (CT) and then magnetic resonance imaging (MRI) were used diagnostically to rule out other causes of dementia. More recently, a variety of imaging modalities including structural and functional MRI and positron emission tomography (PET) studies of cerebral metabolism with fluoro-deoxy-d-glucose (FDG) and amyloid tracers such as Pittsburgh Compound-B (PiB) have shown characteristic changes in the brains of patients with AD, and in prodromal and even presymptomatic states that can help rule-in the AD pathophysiological process. No one imaging modality can serve all purposes as each have unique strengths and weaknesses. These modalities and their particular utilities are discussed in this article. The challenge for the future will be to combine imaging biomarkers to most efficiently facilitate diagnosis, disease staging, and, most importantly, development of effective disease-modifying therapies.
Various neuroimaging modalities (e.g., MRI and PET) have shown characteristic changes in the brains of patients with Alzheimer disease. Identifying imaging biomarkers will facilitate diagnosis, disease staging, and drug development.
The development of prevention therapies for Alzheimer’s disease (AD) would greatly benefit from biomarkers that are sensitive to subtle brain changes occurring prior to the onset of clinical symptoms, when the potential for preservation of function is at the greatest. In vivo brain imaging is a promising tool for the early detection of AD through visualization of abnormalities in brain structure, function and histopathology. Currently, positron emission tomography (PET) imaging with amyloid-beta (Aβ) tracers and 2-[18F] fluoro-2-Deoxy-D-glucose (FDG) is largely utilized in the diagnosis of AD. This paper reviews brain Aβ- and FDG-PET studies in AD patients as well as in non-demented individuals at risk for AD. We then discuss the potential of combining symptoms-sensitive FDG-PET measures with pathology-specific Aβ-PET to improve the early detection of AD.
Alzherimer disease; Positron-emission tomography; Cell aging
The secretase BACE1 is fundamentally involved in the development of cerebral amyloid pathology in Alzheimer's disease (AD). It has not been studied so far to what extent BACE1 activity in cerebrospinal fluid (CSF) mirrors in vivo amyloid load in AD. We explored associations between CSF BACE1 activity and fibrillar amyloid pathology as measured by carbon-11-labelled Pittsburgh Compound B positron emission tomography ([11C]PIB PET). [11C]PIB and CSF studies were performed in 31 patients with AD. Voxel-based linear regression analysis revealed significant associations between CSF BACE1 activity and [11C]PIB tracer uptake in the bilateral parahippocampal region, the thalamus, and the pons. Our study provides evidence for a brain region-specific correlation between CSF BACE1 activity and in-vivo fibrillar amyloid pathology in AD. Associations were found in areas close to the brain ventricles, which may have important implications for the use of BACE1 in CSF as a marker for AD pathology and for antiamyloid treatment monitoring.
Alterations in cerebrospinal fluid (CSF) tau and β–amyloid peptide 1–42 (Aβ42) levels and rates of cerebral glucose (CMRglu) on fluorodeoxyglucose positron emission tomography (FDG PET) occur years before clinical symptoms of Alzheimer’s disease (AD) become manifest, but their relationship remains unclear.
To determine whether CSF AD biomarker levels and CMRglu in healthy individuals correlate in brain structures affected early in AD.
Alzheimer’s disease research center.
Twenty individuals without dementia, aged 46 to 83 years.
Lumbar CSF sampling and FDG-PET imaging of CMRglu. The CSF Aβ42, tau, and tau phosphorylated at threonine 181 (p–tau181) levels were measured using immunobead–based multiplex assays.
Main Outcome Measures
Correlations between CMRglu and CSF biomarker levels were analyzed via voxel–based and volume–of–interest approaches.
Voxel–based analyses demonstrated significant negative correlations between CSF tau and p–tau181 levels and CMRglu in the posterior cingulate, precuneus, and parahippocampal regions. In contrast, a limited positive correlation was found between CSF Aβ42 levels and CMRglu in the inferior temporal cortex. Volume–of–interest analyses confirmed negative associations between CSF tau and p–tau181 levels and CMRglu in the parietal and medial parietal lobes and a positive association between CSF Aβ42 levels and CMRglu in the parahippocampal gyrus.
In healthy individuals, higher CSF tau and p–tau181 concentrations were associated with more severe hypometabolism in several brain regions affected very early in AD, whereas lower CSF Aβ42 concentrations were associated with hypometabolism only in the medial temporal lobe. This suggests that early tau and Aβ abnormalities may be associated with subtle synaptic changes in brain regions vulnerable to AD. A longitudinal assessment of CSF and FDG–PET biomarkers is needed to determine whether these changes predict cognitive impairment and incipient AD.
The development of prevention therapies for Alzheimer’s disease (AD) would greatly benefit from biomarkers that are sensitive to subtle brain changes occurring in the preclinical stage of the disease. Early diagnostics is necessary to identify and treat at risk individuals before irreversible neuronal loss occurs. In vivo imaging has long been used to evaluate brain structural and functional abnormalities as predictors of future AD in non-demented persons. Prior to development of amyloid-β (Aβ) tracers for positron emission tomography (PET), the most widely utilized PET tracer in AD was 2-[P18PF]fluoro-2-Deoxy-D-glucose (PFDG) PET. For over 20 years, FDG-PET has been used to measure cerebral metabolic rates of glucose (CMRglc), a proxy for neuronal activity, in AD. Many studies have shown that CMRglc reductions occur early in AD, correlate with disease progression, and predict histopathological diagnosis. This paper reviews reports of clinical and preclinical CMRglc reductions observed in association with genetic and non-genetic risk factors for AD. We then briefly review brain Aβ PET imaging studies in AD and discuss the potential of combining symptoms-sensitive FDG-PET measures with pathology-specific Aβ-PET to improve the early detection of AD.
amyloid-β; cerebral metabolic rate of glucose (CMRglc); normal aging; positron emission tomography; preclinical detection
Pittsburgh compound-B (PiB), an amyloid-binding positron emission tomography (PET) tracer, is widely used for imaging amyloid-β in those with and at risk for Alzheimer disease. Here, we report on an otherwise normal 68-year-old female with abnormally high and very focal PiB retention. Coregistered T1-weighted magnetic resonance imaging and dynamic 2-fluoro-2-deoxy-D-glucose (FDG) images confirmed that the focal PiB enhancement was in the superior sagittal and transverse sinuses, outside of the adjacent cortex. Flow through the venous vasculature was normal as assessed by dynamic FDG PET imaging. These features supported the conclusion that PiB retention was not simply due to a hemodynamic abnormality, but may have represented PiB binding to fibrillar deposits of a β-sheet protein (ie, amyloid), whose nature is currently unclear.
amyloid-β; PET imaging; Alzheimer disease; PIB; FDG
A workgroup commissioned by the Alzheimer’s Association (AA) and the National Institute on Aging (NIA) recently published research criteria for preclinical Alzheimer’s disease (AD). We performed a preliminary assessment of these guidelines.
We employed Pittsburgh compound B positron emission tomography (PET) imaging as our biomarker of cerebral amyloidosis and 18fluorodeoxyglucose PET imaging and hippocampal volume as biomarkers of neurodegeneration. A group of 42 clinically diagnosed AD subjects was used to create imaging biomarker cut-points. A group of 450 cognitively normal (CN) subjects from a population based sample was used to develop cognitive cut-points and to assess population frequencies of the different preclinical AD stages using different cut-point criteria.
The new criteria subdivide the preclinical phase of AD into stages 1–3. To classify our CN subjects, two additional categories were needed. Stage 0 denotes subjects with normal AD biomarkers and no evidence of subtle cognitive impairment. Suspected Non-AD Pathophysiology (SNAP) denotes subjects with normal amyloid PET imaging, but abnormal neurodegeneration biomarker studies. At fixed cut-points corresponding to 90% sensitivity for diagnosing AD and the 10th percentile of CN cognitive scores, 43% of our sample was classified as stage 0; 16% stage 1; 12 % stage 2; 3% stage 3; and 23% SNAP.
This cross-sectional evaluation of the NIA-AA criteria for preclinical AD indicates that the 1–3 staging criteria coupled with stage 0 and SNAP categories classify 97% of CN subjects from a population-based sample, leaving just 3% unclassified. Future longitudinal validation of the criteria will be important.
Cerebral amyloid angiopathy (CAA) is characterized by deposition of fibrillar amyloid β (Aβ) within cerebral vessels. It is commonly seen in the elderly and almost universally present in patients with Alzheimer's Disease (AD). In both patient populations, CAA is an independent risk factor for lobar hemorrhage, ischemic stroke, and dementia. To date, definitive diagnosis of CAA requires obtaining pathological tissues via brain biopsy (which is rarely clinically indicated) or at autopsy. Though amyloid tracers labeled with positron-emitting radioligands such as [11C]PIB have shown promise for non-invasive amyloid imaging in AD patients, to date they have been unable to clarify whether the observed amyloid load represents neuritic plaques versus CAA due in large part to the low resolution of PET imaging and the almost equal affinity of these tracers for both vascular and parenchymal amyloid. Therefore, the development of a precise and specific non-invasive technique for diagnosing CAA in live patients is desired.
We found that the phenoxazine derivative resorufin preferentially bound cerebrovascular amyloid deposits over neuritic plaques in the aged Tg2576 transgenic mouse model of AD/CAA, whereas the congophilic amyloid dye methoxy-X34 bound both cerebrovascular amyloid deposits and neuritic plaques. Similarly, resorufin-positive staining was predominantly noted in fibrillar Aβ-laden vessels in postmortem AD brain tissues. Fluorescent labeling and multi-photon microscopy further revealed that both resorufin- and methoxy-X34-positive staining is colocalized to the vascular smooth muscle (VSMC) layer of vessel segments that have severe disruption of VSMC arrangement, a characteristic feature of CAA. Resorufin also selectively visualized vascular amyloid deposits in live Tg2576 mice when administered topically, though not systemically. Resorufin derivatives with chemical modification at the 7-OH position of resorufin also displayed a marked preferential binding affinity for CAA, but with enhanced lipid solubility that indicates their use as a non-invasive imaging tracer for CAA is feasible.
To our knowledge, resorufin analogs are the fist class of amyloid dye that can discriminate between cerebrovascular and neuritic forms of amyloid. This unique binding selectivity suggests that this class of dye has great potential as a CAA-specific amyloid tracer that will permit non-invasive detection and quantification of CAA in live patients.
Cerebral amyloid angiopathy; Alzheimer's disease; dementia; diagnosis; amyloid beta; positron emission tomography; amyloid imaging; tracer; resorufin; phenoxazines
The availability of new PET ligands offers the potential to measure fibrillar β-amyloid in the brain. Nevertheless, physiological information in the form of perfusion or metabolism may still be useful in differentiating causes of dementia during life. In this study we investigated whether early 11C-PIB PET frames (perfusion, pPIB) can provide information equivalent to blood flow and metabolism by assessing the similarity of pPIB and 18F-FDG PET images first in a test cohort with various clinical diagnoses (N=10) and then validating the results on a cohort of Alzheimer’s disease (AD, N=42, age 66.6±10.6, MMSE 22.2±6.0) and frontotemporal lobar degeneration (FTLD, N=31, age 63.9±7.1, MMSE 23.8±6.7) patients.
To identify the 11C-PIB frames best representing perfusion, an iterative algorithm was run on the test cohort. This included: (1) generating normalized (cerebellar reference) perfusion pPIB images across variable frame ranges, and (2) calculating Pearson’s R values of the sum of these pPIB frames with the sum of all 18F-FDG frames (cerebellar normalized) for all brain tissue voxels. Once this perfusion frame range was determined on the test cohort, it was then validated on an extended cohort and the power of pPIB in differential diagnosis was compared to 18F-FDG by performing a logistic regression of ROI tracer measure (pPIB or 18F-FDG) versus diagnosis.
A seven-minute window, corresponding to minutes 1–8 (frame 5–15) produced the highest voxel-wise correlation between 18F-FDG and pPIB (R=0.78±0.05). This pPIB frame range was further validated on the extended AD and FTLD cohort across 12 ROIs (R=0.91±0.09). A logistic model using pPIB was able to classify 90.5% of the AD and 83.9% of the FTLD patients correctly. Using 18F-FDG, 88.1% of AD and 83.9% of FTLD patients were classified correctly. The temporal pole and the temporal neocortex were significant discriminators (p<0.05) in both models, whereas in the model with pPIB the frontal region was also significant.
The high correlation between pPIB and 18F-FDG measures and their comparable performance in differential diagnosis is promising in providing functional information using 11C-PIB PET data. This could be a useful approach, obviating the need for 18F-FDG scans when longer-lived amyloid imaging agents become available
Pittsburgh compound-B (11C-PIB); perfusion; 18F-Fluorodeoxyglucose (18F-FDG); Aβ-amyloid plaques; cerebral glucose metabolism
A decreased concentration of beta amyloid (1–42) (Aβ42) has consistently been found in the cerebrospinal fluid (CSF) of patients with Alzheimer’s disease (AD) and is considered a diagnostic biomarker. However, it is not clear to which extent CSF Aβ42 levels are reflective of cerebral pathology in AD. The aim of the study was to determine the association between cerebral amyloid plaque load, as measured by means of the positron emission tomography (PET) tracer carbon-11-labeled Pittsburgh Compound B ([11C]PiB) and CSF Aβ42 in AD.
A group of 30 patients with probable AD, as defined by established clinical criteria and by an AD-typical pattern of tracer uptake in fluorine-18-labeled fluorodeoxyglucose ([18F]FDG) PET, were included. In all patients, [11C]PiB PET and CSF analysis were performed. The association between amyloid load and CSF Aβ42 levels was examined in three different ways: by linear regression analysis using an overall [11C]PiB value for the entire cerebrum, by correlation analyses using [11C]PiB measurements in anatomically defined regions of interest, and by voxel-based regression analyses.
All patients showed a positive [11C]PiB scan demonstrating amyloid deposition. Linear regression analysis revealed a significant inverse correlation between the overall [11C]PiB uptake and CSF Aβ42 levels. Voxel-based regression and regional correlation analyses did not attain statistical significance after correction for multiple comparisons. Numerically, correlation coefficients were higher in brain regions adjacent to CSF spaces.
The study demonstrates a moderate linear negative association between cerebral amyloid plaque load and CSF Aβ42 levels in AD patients in vivo and suggests possible regional differences of the association.
Alzheimer’s disease; Aβ42; [11C]PiB; CSF; [18F]FDG; positron emission tomography; Pittsburgh Compound B
Positron emission tomography (PET) provides a powerful means to non-invasively image and quantify protein expression and biochemical changes in living subjects at nano- and picomolar levels. As the field of molecular imaging develops, and as advances in the biochemistry, pharmacology, therapeutics, and molecular biology of disease are made, there is a corresponding increase in the number of clinically relevant, novel disease-associated biomarkers that are brought to the attention of those developing imaging probes for PET. In addition, due to the high specificity of the PET radiotracers being developed, there is a demand for PET cameras with higher sensitivity and resolution. This manuscript reviews advances over the past five years in clinical and pre-clinical PET instrumentation and in new PET probes and imaging methods associated with the latest trends in the molecular imaging of cancer. Included in the PET tracer review is a description of new radioligands for steroid receptors, growth factor receptors, receptor tyrosine kinases, sigma receptors, tumor-associated enzymes, gene reporter probes, markers for tumor hypoxia and metabolism, and sites associated with angiogenesis and cellular proliferation. The use of PET imaging in drug development, including the monitoring of cancer chemotherapy, also is discussed.
Currently available evidence strongly supports the position that the initiating event in Alzheimer’s disease (AD) is related to abnormal processing of β-amyloid (Aβ) peptide, ultimately leading to formation of Aβ plaques in the brain. This process occurs while individuals are still cognitively normal. Biomarkers of brain β-amyloidosis are reductions in CSF Aβ42 and increased amyloid PET tracer retention. After a lag period, which varies from patient to patient, neuronal dysfunction and neurodegeneration become the dominant pathological processes. Biomarkers of neuronal injury and neurodegeneration are increased CSF tau and structural MRI measures of cerebral atrophy. Neurodegeneration is accompanied by synaptic dysfunction, which is indicated by decreased fluorodeoxyglucose uptake on PET. We propose a model that relates disease stage to AD biomarkers in which Aβ biomarkers become abnormal first, before neurodegenerative biomarkers and cognitive symptoms, and neurodegenerative biomarkers become abnormal later, and correlate with clinical symptom severity.
Alzheimer’s disease is hypothesized to be caused by an over-production or reduced clearance of amyloid-beta (Aβ) peptide. Autosomal Dominant Alzheimer’s Disease (ADAD) caused by mutations in the presenilin (PSEN) gene have been postulated to result from increased production of Aβ42 compared to Aβ40 in the central nervous system (CNS). This has been demonstrated in rodent models of ADAD but not in human mutation carriers We used compartmental modeling of stable isotope labeling kinetic (SILK) studies in human carriers of PSEN mutations and related non-carriers to evaluate the pathophysiological effects of PSEN1 and PSEN2 mutations on the production and turnover of Aβ isoforms. We compared these findings by mutation status and amount of fibrillar amyloid deposition as measured by positron emission tomography (PET) using the amyloid tracer, Pittsburgh compound B (PiB). CNS Aβ42 to Aβ40 production rates were 24% higher in mutation carriers compared to non-carriers and this was independent of fibrillar amyloid deposits quantified by PET PiB imaging. The fractional turnover rate of soluble Aβ42 relative to Aβ40 was 65% faster in mutation carriers and correlated with amyloid deposition, consistent with increased deposition of Aβ42 into plaques leading to reduced recovery of Aβ42 in cerebrospinal fluid (CSF). Reversible exchange of Aβ42 peptides with pre-existing unlabeled peptide was observed in the presence of plaques. These findings support the hypothesis that Aβ42 is overproduced in the CNS of humans with presenilin mutations that cause AD, and demonstrate that soluble Aβ42 turnover and exchange processes are altered in the presence of amyloid plaques, causing a reduction in Aβ42 concentrations in the CSF.
The growing understanding of the use of biomarkers in Alzheimer's disease (AD) may enable physicians to make more accurate and timely diagnoses. Florbetaben, a beta-amyloid tracer used with positron emission tomography (PET), is one of these diagnostic biomarkers. This analysis was undertaken to explore the potential value of florbetaben PET in the diagnosis of AD among patients with suspected dementia and to identify key data that are needed to further substantiate its value. A discrete event simulation was developed to conduct exploratory analyses from both US payer and societal perspectives. The model simulates the lifetime course of disease progression for individuals, evaluating the impact of their patient management from initial diagnostic work-up to final diagnosis. Model inputs were obtained from specific analyses of a large longitudinal dataset from the New England Veterans Healthcare System and supplemented with data from public data sources and assumptions. The analyses indicate that florbetaben PET has the potential to improve patient outcomes and reduce costs under certain scenarios. Key data on the use of florbetaben PET, such as its influence on time to confirmation of final diagnosis, treatment uptake, and treatment persistency, are unavailable and would be required to confirm its value.
The positron emission tomography (PET) radiotracer Pittsburgh Compound-B (PiB) binds with high affinity to β-pleated sheet aggregates of the amyloid-β (Aβ) peptide in vitro. The in vivo retention of PiB in brains of people with Alzheimer's disease shows a regional distribution that is very similar to distribution of Aβ deposits observed post-mortem. However, the basis for regional variations in PiB binding in vivo, and the extent to which it binds to different types of Aβ-containing plaques and tau-containing neurofibrillary tangles (NFT), has not been thoroughly investigated. The present study examined 28 clinically diagnosed and autopsy-confirmed Alzheimer's disease subjects, including one Alzheimer's disease subject who had undergone PiB-PET imaging 10 months prior to death, to evaluate region- and substrate-specific binding of the highly fluorescent PiB derivative 6-CN-PiB. These data were then correlated with region-matched Aβ plaque load and peptide levels, [3H]PiB binding in vitro, and in vivo PET retention levels. We found that in Alzheimer's disease brain tissue sections, the preponderance of 6-CN-PiB binding is in plaques immunoreactive to either Aβ42 or Aβ40, and to vascular Aβ deposits. 6-CN-PiB labelling was most robust in compact/cored plaques in the prefrontal and temporal cortices. While diffuse plaques, including those in caudate nucleus and presubiculum, were less prominently labelled, amorphous Aβ plaques in the cerebellum were not detectable with 6-CN-PiB. Only a small subset of NFT were 6-CN-PiB positive; these resembled extracellular ‘ghost’ NFT. In Alzheimer's disease brain tissue homogenates, there was a direct correlation between [3H]PiB binding and insoluble Aβ peptide levels. In the Alzheimer's disease subject who underwent PiB-PET prior to death, in vivo PiB retention levels correlated directly with region-matched post-mortem measures of [3H]PiB binding, insoluble Aβ peptide levels, 6-CN-PiB- and Aβ plaque load, but not with measures of NFT. These results demonstrate, in a typical Alzheimer's disease brain, that PiB binding is highly selective for insoluble (fibrillar) Aβ deposits, and not for neurofibrillary pathology. The strong direct correlation of in vivo PiB retention with region-matched quantitative analyses of Aβ plaques in the same subject supports the validity of PiB-PET imaging as a method for in vivo evaluation of Aβ plaque burden.
Pittsburgh Compound-B; PiB; amyloid imaging; plaques; PET imaging
The Alzheimer’s Disease Neuroimaging Initiative (ADNI) beginning in October 2004, is a 6-year re-search project that studies changes of cognition, function, brain structure and function, and biomarkers in elderly controls, subjects with mild cognitive impairment, and subjects with Alzheimer’s disease (AD). A major goal is to determine and validate MRI, PET images, and cerebrospinal fluid (CSF)/blood biomarkers as predictors and outcomes for use in clinical trials of AD treatments. Structural MRI, FDG PET, C-11 Pittsburgh compound B (PIB) PET, CSF measurements of amyloid β (Aβ) and species of tau, with clinical/cognitive measurements were performed on elderly controls, subjects with mild cognitive impairment, and subjects with AD. Structural MRI shows high rates of brain atrophy, and has high statistical power for determining treatment effects. FDG PET, C-11 Pittsburgh compound B PET, and CSF measurements of Aβ and tau were significant predictors of cognitive decline and brain atrophy. All data are available at UCLA/LONI/ADNI, without embargo. ADNI-like projects started in Australia, Europe, Japan, and Korea. ADNI provides significant new information concerning the progression of AD.
ADNI; Alzheimer’s disease; MRI; PET; Amyloid; Memory; Tau
Drug candidates targeting amyloid-β (Aβ) pathology in Alzheimer's disease are in different phases of clinical trials. These treatments will probably be most effective in the earlier stages of the disease, before neurodegeneration is too severe, but at the same time symptoms are vague and the clinical diagnosis is difficult. Recent research advances have resulted in promising biomarkers, including cerebrospinal fluid analyses for tau and Aβ, magnetic resonance imaging measurement of atrophy, and positron emission tomography imaging of glucose metabolism and Aβ pathology, which allow identification of prodromal Alzheimer's disease. More details are needed, however, on how these biomarkers can be standardized, to allow a general implementation in the clinical routine diagnostic work-up of patients with cognitive disturbances.
PET imaging using [18F]fluorodeoxyglucose (FDG) and [11C]Pittsburgh compound B (PIB) have been proposed as biomarkers of Alzheimer disease (AD), as have CSF measures of the 42 amino acid β-amyloid protein (Aβ1-42) and total and phosphorylated tau (t-tau and p-tau). Relationships between biomarkers and with disease severity are incompletely understood.
Ten subjects with AD, 11 control subjects, and 34 subjects with mild cognitive impairment from the Alzheimer’s Disease Neuroimaging Initiative underwent clinical evaluation; CSF measurement of Aβ1-42, t-tau, and p-tau; and PIB-PET and FDG-PET scanning. Data were analyzed using continuous regression and dichotomous outcomes with subjects classified as “positive” or “negative” for AD based on cutoffs established in patients with AD and controls from other cohorts.
Dichotomous categorization showed substantial agreement between PIB-PET and CSF Aβ1-42 measures (91% agreement, κ = 0.74), modest agreement between PIB-PET and p-tau (76% agreement, κ = 0.50), and minimal agreement for other comparisons (κ <0.3). Mini-Mental State Examination score was significantly correlated with FDG-PET but not with PIB-PET or CSF Aβ1-42. Regression models adjusted for diagnosis showed that PIB-PET was significantly correlated with Aβ1-42, t-tau, and p-tau181p, whereas FDG-PET was correlated only with Aβ1-42.
PET and CSF biomarkers of Aβ agree with one another but are not related to cognitive impairment. [18F]fluorodeoxyglucose-PET is modestly related to other biomarkers but is better related to cognition. Different biomarkers for Alzheimer disease provide different information from one another that is likely to be complementary.
β1-42 = 42 amino acid β-amyloid protein;
= Alzheimer disease;
= Alzheimer’s Disease Neuroimaging Initiative;
= Clinical Dementia Rating;
= confidence interval;
= mild cognitive impairment;
= Mini-Mental State Examination;
= magnetic resonance;
= [11C]Pittsburgh compound B;
= phosphorylated tau;
= receiver operating characteristic;
= region of interest;
= standardized uptake value ratio;
= total tau;
= Wechsler Memory Scale–Revised.
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.
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.
It is unlikely that CYP inhibition or induction will significantly influence the clearance of 11C-PiB.
Amyloid; Pittsburgh compound B; Drug metabolism; Hepatic clearance; P450 phenotyping