Imaging brain amyloid in Alzheimer's disease with Pittsburgh Compound-B

Delineation of positron emission tomography imaging agent binding sites on beta-amyloid peptide fibrils

A range of imaging agents for use in the positron emission tomography of Alzheimer's disease is currently under development. Each of the main compound classes, derived from thioflavin T (PIB), Congo Red (BSB), and aminonaphthalene (FDDNP) are believed to bind to mutually exclusive sites on the beta-amyloid (Abeta) peptide fibrils. We recently reported the presence of three classes of binding sites (BS1, BS2, BS3) on the Abeta fibrils for thioflavin T derivatives and now extend these findings to demonstrate that these sites are also able to accommodate ligands from the other chemotype classes. The results from competition assays using [3H]Me-BTA-1 (BS3 probe) indicated that both PIB and FDDNP were able to displace the radioligand with Ki values of 25 and 42 nM, respectively. BSB was unable to displace the radioligand tracer from the Abeta fibrils. In contrast, each of the compounds examined were able to displace thioflavin T (BS1 probe) from the Abeta fibrils when evaluated in a fluorescence competition assay with Ki values for PIB, FDDNP, and BSB of 1865, 335, and 600 nM, respectively. Finally, the Kd values for FDDNP and BSB binding to Abeta fibrils were directly determined by monitoring the increases in the ligand intrinsic fluorescence, which were 290 and 104 nM, respectively. The results from these assays indicate that (i) the three classes of thioflavin T binding sites are able to accommodate a wide range of chemotype structures, (ii) BSB binds to two sites on the Abeta fibrils, one of which is BS2, and the other is distinct from the thioflavin T derivative binding sites, and (iii) there is no independent binding site on the fibrils for FDDNP, and the ligand binds to both the BS1 and BS3 sites with significantly lower affinities than previously reported.

In vivo detection of amyloid-β deposits by near-infrared imaging using an oxazine-derivative probe

As Alzheimer's disease pathogenesis is associated with the formation of insoluble aggregates of amyloid beta-peptide, approaches allowing the direct, noninvasive visualization of plaque growth in vivo would be beneficial for biomedical research. Here we describe the synthesis and characterization of the near-infrared fluorescence oxazine dye AOI987, which readily penetrates the intact blood-brain barrier and binds to amyloid plaques. Using near-infrared fluorescence imaging, we demonstrated specific interaction of AOI987 with amyloid plaques in APP23 transgenic mice in vivo, as confirmed by postmortem analysis of brain slices. Quantitative analysis revealed increasing fluorescence signal intensity with increasing plaque load of the animals, and significant binding of AOI987 was observed for APP23 transgenic mice aged 9 months and older. Thus, AOI987 is an attractive probe to noninvasively monitor disease progression in animal models of Alzheimer disease and to evaluate effects of potential Alzheimer disease drugs on the plaque load.

Efficient radiosynthesis of carbon-11 labelled uncharged Thioflavin T derivatives using [11C]methyl triflate for beta-amyloid imaging in Alzheimer's Disease with PET

The synthesis of carbon-11 amino function labelled uncharged Thioflavin T derivatives is known to be performed by reaction of the demethyl-precursors with [11C]methyl iodide but the labelling yields are only mediocre. The use of [11C]methyl triflate improved the radiochemical yield of three potential beta-amyloid imaging PET-radiotracers significantly. Performance of the labelling reaction by reacting the corresponding precursor molecules with [11C]methyl triflate for 1 min at 80 degrees C led to radiochemical yields of 44+/-10% (n=5) for [11C]6-Me-BTA-1, 68+/-4% (n=10) for [11C]BTA-1 and 58+/-2% (n=5) for [11C]6-OH-BTA-1 with respect to [11C]methyl triflate. In production runs (60 min, 50 microA) up to 6500 MBq (mean: 4000+/-1900 MBq) of [11C]6-Me-BTA-1, 7900 MBq (mean: 6000+/-1000 MBq) of [11C]BTA-1 and 7100 MBq (mean: 6300+/-600 MBq) of [11C]6-OH-BTA-1 could be obtained ready for intravenous injection. The radiochemical purity was >95% with specific activities in the range of 80-120 GBq/micromol (EOS) within a total synthesis time of less than 40 min after EOB.

Brain aging: reorganizing discoveries about the aging mind

New discoveries challenge the long-held view that aging is characterized by progressive loss and decline. Evidence for functional reorganization, compensation and effective interventions holds promise for a more optimistic view of neurocognitive status in later life. Complexities associated with assigning function to age-specific activation patterns must be considered relative to performance and in light of pathological aging. New biological and genetic markers, coupled with advances in imaging technologies, are enabling more precise characterization of healthy aging. This interdisciplinary, cognitive neuroscience approach reveals dynamic and optimizing processes in aging that might be harnessed to foster the successful aging of the mind.

PET imaging of brain with the β-amyloid probe, [11C]6-OH-BTA-1, in a transgenic mouse model of Alzheimer’s disease

PURPOSE

The purpose of this study was to evaluate the capacity of [11C]6-OH-BTA-1 and positron emission tomography (PET) to quantify beta-amyloid (Abeta) plaques in the Tg2576 mouse model of Alzheimer's disease (AD).

METHODS

PET imaging was performed with the NIH ATLAS small animal scanner in six elderly transgenic mice (Tg2576; age 22.0+/-1.8 months; 23.6+/-2.6 g) overexpressing a mutated form of human beta-amyloid precursor protein (APP) known to result in the production of Abeta plaques, and in six elderly wild-type litter mates (age 21.8+/-1.6 months; 29.5+/-4.7 g). Dynamic PET scans were performed for 30 min in each mouse under 1% isoflurane inhalation anesthesia after a bolus injection of 13-46 MBq of [11C]6-OH-BTA-1. PET data were reconstructed with 3D OSEM. On the coronal PET image, irregular regions of interest (ROIs) were placed on frontal cortex (FR), parietal cortex (PA), striatum (ST), thalamus (TH), pons (PO), and cerebellum (CE), guided by a mouse stereotaxic atlas. Time-activity curves (TACs) (expressed as percent injected dose per gram normalized to body weight: % ID-kg/g) were obtained for FR, PA, ST, TH, PO, and CE. ROI-to-CE radioactivity ratios were also calculated. Following PET scans, sections of mouse brain prepared from anesthetized and fixative-perfused mice were stained with thioflavin-S.

RESULTS

TACs for [11C]6-OH-BTA-1 in all ROIs peaked early (at 30-55 s), with radioactivity washing out quickly thereafter in both transgenic and wild-type mice. Peak uptake in all regions was significantly lower in transgenic mice than in wild-type mice. During the later part of the washout phase (12-30 min), the mean FR/CE and PA/CE ratios were higher in transgenic than in wild-type mice (1.06+/-0.04 vs 0.98+/-0.07, p=0.04; 1.06+/-0.09 vs 0.93+/-0.08 p=0.02) while ST/CE, TH/CE, and PO/CE ratios were not. Ex vivo staining revealed widespread Abeta plaques in cortex, but not in cerebellum of transgenic mice or in any brain regions of wild-type mice.

CONCLUSION

Marked reductions in brain uptake of this radioligand in transgenic mice may be due to reduced cerebral blood flow relative to that in wild-type mice. Specific [11C]6-OH-BTA-1 binding to Abeta plaques, if any, is probably very low, as reflected in the small FR/CE and PA/CE ratio differences. FR/CE and PA/CE ratios are considerably higher in AD patients while Abeta plaque densities in 22-month-old transgenic mice may be expected to show essentially the same density as is observed in the AD brain. This implies that the absence of tracer retention in 22-month-old transgenic mice may be due to the smaller number of Abeta plaque binding sites and/or to lower affinity of the binding sites for [11C]6-OH-BTA-1 as compared with AD patients. [11C]6-OH-BTA-1 shows excellent brain uptake in mice.

19F and 1H MRI detection of amyloid beta plaques in vivo

Formation of senile plaques composed of amyloid beta peptide, a pathological hallmark of Alzheimer disease, in human brains precedes disease onset by many years. Noninvasive detection of such plaques could be critical in presymptomatic diagnosis and could contribute to early preventive treatment strategies. Using amyloid precursor protein (APP) transgenic mice as a model of amyloid beta amyloidosis, we demonstrate here that an intravenously administered (19)F-containing amyloidophilic compound labels brain plaques and allows them to be visualized in living mice by magnetic resonance imaging (MRI) using (19)F and (1)H. Our findings provide a new direction for specific noninvasive amyloid imaging without the danger of exposure to radiation. This approach could be used in longitudinal studies in mouse models of Alzheimer disease to search for biomarkers associated with amyloid beta pathology as well as to track disease course after treatment with candidate medications.

In vivo visualization of Alzheimer's amyloid plaques by magnetic resonance imaging in transgenic mice without a contrast agent

One of the cardinal pathologic features of Alzheimer's disease (AD) is the formation of senile, or amyloid, plaques. Transgenic mice have been developed that express one or more of the genes responsible for familial AD in humans. Doubly transgenic mice develop "human-like" plaques, providing a mechanism to study amyloid plaque biology in a controlled manner. Imaging of labeled plaques has been accomplished with other modalities, but only MRI has sufficient spatial and contrast resolution to visualize individual plaques noninvasively. Methods to optimize visualization of plaques in vivo in transgenic mice at 9.4 T using a spin echo sequence based on adiabatic pulses are described. Preliminary results indicate that a spin echo acquisition more accurately reflects plaque size, while a T2* weighted gradient echo sequence reflects plaque iron content, not plaque size. In vivo MRI-ex vivo MRI-in vitro histologic correlations are provided. Histologically verified plaques as small as 50 microm in diameter were visualized in living animals. To our knowledge this work represents the first demonstration of noninvasive in vivo visualization of individual AD plaques without the use of a contrast agent.

Multiple ligand binding sites on A beta(1-40) fibrils

Although the structures of Thioflavin T and another benzothiazole, BTA-1, are similar, they bind to A beta non-competitively, probably to different sites on the A beta(1-40) fibrils. The amyloid fibril-induced fluorescence of ThT that corresponds to a fraction of total ThT binding is not displaced by high concentrations of (S)-naproxen or (R)-ibuprofen, which are reported to potently block high affinity binding of the radiolabeled malononitrile FDDNP and derivatives. The binding of the benzothiazole ligands is significantly substoichiometric with respect to A beta(1-40) monomer peptide, unlike Congo Red, which binds to A beta(1-40) fibrils on a 1:1 basis with monomer peptide. These results indicate that there are multiple domains for ligand binding to amyloid fibrils and suggest that it may be possible to design ligands that bind selectively to particular forms of fibrils that are connected with the pathogenesis of Alzheimer's disease and potentially other protein misfolding diseases.

Biphenyls labeled with technetium 99m for imaging β-amyloid plaques in the brain

Formation and accumulation of excess aggregates of beta-amyloid (Abeta) plaques in the brain are critical factors contributing to the development and progression of Alzheimer's disease (AD). There is an urgent need for in vivo imaging agents that can specifically demonstrate the location and density of Abeta plaques in the brain. The aim of this study was to develop potential technetium 99m (99mTc)-labeled diagnostic imaging agents specific for the detection of Abeta plaques. Based on previously obtained Abeta plaque-specific biphenyls containing a p-N, N-dimethylaminophenyl group, a series of 99mTc and Re-N2S2-biphenyl derivatives was prepared. The stable neutral and lipophilic 99mTc complexes, [99mTc]19 and [99mTc]23, A+B, were successfully obtained. As surrogates for the 99mTc complexes, the corresponding surrogates, Re complexes of 23, were also prepared. Surprisingly, it was found that the Re complexes showed distinctively different retention profiles as compared with the corresponding 99mTc complexes. Biodistribution studies indicated that [99mTc]23A readily passed through the blood-brain barrier (1.18% dose/brain at 2 min) in contrast to the low brain penetration of [99mTc]19 (0.29% dose/brain at 2 min). Initial results suggested that [99mTc]23A showed selective binding to the Abeta plaque-like structures in the brain sections from transgenic mice but not in the postmortem human brain tissue of patients with confirmed AD. The results provide encouraging evidence that development of a 99mTc-labeled agent for imaging Abeta plaques in the brain may be feasible. Caution should be taken when comparing these 99mTc complexes with the corresponding surrogates--the Re complexes.

A preclinical view of cholinesterase inhibitors in neuroprotection: do they provide more than symptomatic benefits in Alzheimer's disease?

The prevalence of Alzheimer's disease (AD), a neurodegenerative condition whose greatest risk factor is old age, is expected to rise dramatically during the next five decades, along with the trend for increased longevity. Early diagnosis and intervention with therapies that halt or slow disease progress are likely to represent an important component of effective treatment. Although much progress has been made in this area, there are currently no clinically approved interventions for AD that are classed as disease modifying or neuroprotective. Cholinesterase inhibitors are a drug class used for the symptomatic treatment of AD. Recent evidence from preclinical studies indicates that these agents can attenuate neuronal damage and death from cytotoxic insults, and therefore might affect AD pathogenesis. The mechanisms by which these actions are mediated might or might not be directly related to their primary mode of action.

Binding of two potential imaging agents targeting amyloid plaques in postmortem brain tissues of patients with Alzheimer's disease

In vivo imaging of amyloid plaques may be useful for evaluation and diagnosis of Alzheimer's disease (AD) patients. Towards that end, we have developed 6-iodo-2-(4'-dimethylamino-)phenyl-imidazo[1,2]pyridine (IMPY), and 4-N-methylamino-4'-hydroxystilbene (SB-13) as ligands for specifically targeting amyloid plaques. These ligands can be readily radiolabeled with I-123 or C-11, for in vivo imaging using single photon emission computerized tomography (SPECT) or positron emission tomography (PET), respectively. However, in order to be useful in vivo, probes must show selective high affinity binding to a sufficiently abundant binding site on amyloid plaques. Therefore, as a prelude to in vivo imaging studies, we evaluated the binding properties of these two potential imaging agents to amyloid plaques present in human brain tissues. In vitro binding studies were carried out with [(125)I]IMPY and [(3)H]SB-13 in homogenates prepared from postmortem samples of affected cortex and cerebellum of pathologically confirmed AD patients and age-matched controls. Binding parameters such as K(d) and B(max) were estimated. Competition study was designed to evaluate the amyloid plaque binding specificity using human brain tissues. Plaque binding was confirmed by thioflavin-S staining. Specific [(125)I]IMPY or [(3)H]SB-13 binding can be clearly measured in the cortical gray matter, but not in the white matter of AD cases. There was a very low specific binding in cortical tissue homogenates of control brains. Cerebellar homogenates prepared from either AD or control brains did not show any specific [(125)I]IMPY or [(3)H]SB-13 binding. The K(d) values of AD cortical homogenates were 5.3+/-1.0 and 2.4+/-0.2 nM for [(125)I]IMPY and [(3)H]SB-13, respectively. High binding capacity and comparable values were observed for both ligands (14-45 pmol/mg protein). The location and density of specific signal detected by [(125)I]IMPY or [(3)H]SB-13 correlated with the distribution of amyloid plaques in these brain specimens, as confirmed by thioflavin-S staining. Competition profiles of known ligands suggest that the binding is highly selective and comparable to that reported by using preformed Abeta peptide aggregates. [(125)I]IMPY and [(3)H]SB-13 show an abundant binding capacity with high binding affinities for amyloid plaques in affected cortical regions of AD brains. These properties suggest that when labeled with I-123 or C-11, these two ligands may be useful to quantitate amyloid plaque burdens in the living AD patients.

Development of an optical approach for noninvasive imaging of Alzheimer's disease pathology

Alzheimer's disease (AD) is characterized by the presence of aggregates of the amyloid-beta (A beta) peptide in the brain. These aggregates manifest themselves as senile plaques and cerebrovascular amyloid angiopathy (CAA). While traditional histochemical approaches can easily identify these deposits in postmortem tissue, only recently have specific ligands been developed to target A beta in living patients using positron emission tomography (PET). Successful detection of A beta pathology in patients will enable definitive preclinical diagnosis of AD, and enable quantitative evaluation of the efficacy of anti-A beta therapeutics developed to treat the disease. PET scanning, however, has several disadvantages including high cost, low availability, and the requirement for radioactive tracers. We describe recent progress in the development of techniques for imaging A beta deposits noninvasively using optical approaches. Successful development of an optical detection platform would enable inexpensive, accessible, nonradioactive detection of the A beta deposits found in AD.

A Computational Positron Emission Tomography Simulation Model for Imaging ?-Amyloid in Mice

PURPOSE

We aimed to develop a computational simulation model for beta-amyloid (Abeta) positron emission tomography (PET) imaging.

PROCEDURES

Model parameters were set to reproduce levels of Abeta within the PDAPP mouse. Pharmacokinetic curves of virtual tracers were computed and a PET detector simulator was configured for a commercially available preclinical PET-imaging system.

RESULTS

We modeled the effects of Abeta therapy and tracer affinity on the ability to differentiate Abeta levels by PET. Varying affinity had a significant effect on the ability to quantitate Abeta. Further, PET tracers for Abeta monomers were more sensitive to the therapeutic reduction in Abeta levels than total brain amyloid. Following therapy, the decrease in total brain Abeta corresponded to the slow rate of change in total amyloid load as expected.

CONCLUSIONS

We have developed a first proof-of-concept Abeta-PET simulation model that will be a useful tool in the interpretation of preclinical Abeta imaging data and tracer development.

Amyloid Imaging: From Benchtop to Bedside

Tremendous efforts have been made in the search for a cure or effective treatment of Alzheimer's disease (AD) to develop therapies aimed at halting or reversing amyloid plaque deposition in the brain. This necessitates in vivo detection and quantification of amyloid plaques in the brain for efficacy evaluation of anti-amyloid therapies. For this purpose, a wide array of amyloid-imaging probes has been developed, mainly for in vivo studies based on positron emission tomography and single photon emission computed tomography. This review provides a full account of the development of amyloid-imaging agents. The in vitro binding properties and in vivo pharmacokinetic profiles of all amyloid-imaging agents so far reported are comprehensively and uniquely surveyed. Emphasis is placed on the development of small-molecule probes based on amyloid dyes, such as Congo red and thioflavin T. Compared to large biomolecules, these small-molecule probes have been systematically investigated through extensive structure activity relationship studies. Many of the probes show favorable properties for in vivo studies. As a result, three lead compounds, termed PIB (Pittsburgh-Compound B, [(11)C]6-OH-BTA-1), FDDNP (2-(1-[6-[(2-[(18)F]fluoroethyl)(methyl)amino]-2-naphthyl]ethylidene)malononitrile), and SB-13 (4-N-methylamino-4'-hydroxystilbene), have been identified and evaluated in human subjects. Preliminary studies have indicated that these lead compounds exhibit a characteristic retention in AD subjects that is consistent with the AD pathology, thus proving the concept that amyloid deposits in the brain can be readily detected and quantified in vivo. The progress to date paves the way for further investigation in various aspects of AD research. Once developed, these amyloid-imaging agents could be used as biomarkers to aid in early and definitive diagnosis of AD, facilitate drug discovery and development, and allow pathophysiological studies of the disease mechanism. Furthermore, the success in the development of amyloid-imaging agents helps with the development of imaging agents for in vivo studies of other AD pathologies in particular and of neurodegenerative disorders in general.

Development of a PET/SPECT agent for amyloid imaging in Alzheimer's disease

In the search for a cure for Alzheimer's disease (AD), efforts have been focused on preventing or reversing amyloid deposition in the brain. Efficacy evaluation of these antiamyloid therapies would greatly benefit from development of a tool for the in vivo detection and quantitation of amyloid deposits in the brain. Toward this goal, we have developed a series of benzothiazole derivatives as amyloid-imaging agents for positron emission tomography (PET). To extend the potential of these amyloid-imaging agents for routine clinical studies, we also set out to develop iodinated benzothiazole derivatives that could be used as dual agents for either PET or the complementary single photon emission computed tomography (SPECT). Such dual agents would permit PET or SPECT studies using radiotracers with the same chemical identity but labeled with different radionuclides. This would facilitate the validation of clinical SPECT studies, based on quantitative PET studies. In this work we report the synthesis and biological evaluation of a potent, selective, and brain-permeable benzothiazole compound, 2-(3'-iodo-4'-methylaminophenyl)-6-hydroxy-benzothialzole, termed 6-OH-BTA-1-3'-I (4), which can be radiolabeled with either positron-emitting carbon-11 or single photon-emitting iodine-125/iodine-123. The synthesis and radiolabeling of [125I]4 or [11C]4 were achieved through direct iodination with sodium [125I]iodide in the presence of chloramine T or through radiomethylation with [11C]CH3I. In vitro amyloid binding assays indicated that [125I]4 bound to amyloid deposits in a saturable manner and exhibited affinities in the nanomolar concentration range. Binding studies of [125I]4 to postmortem human brain homogenates also showed preference of binding to frontal cortex in the AD homogenates relative to age-matched control homogenates or cerebellum from either AD or control. In vivo pharmacokinetic studies in normal mice following iv injection of [11C]4 indicated that the radioligand entered the brain readily at early time points and cleared from the brain rapidly at later time points with a 2- to 30-min ratio >3. These results suggest that the new radioiodinated benzothiazole ligand might be useful as a surrogate marker for the in vivo quantitation of amyloid deposition in human brain for use with either PET or SPECT.

Evidence for the presence of three distinct binding sites for the thioflavin T class of Alzheimer's disease PET imaging agents on beta-amyloid peptide fibrils

Imaging the progression of Alzheimer's disease would greatly facilitate the discovery of therapeutics, and a wide range of ligands are currently under development for the detection of beta-amyloid peptide (Abeta)-containing plaques by using positron emission tomography. Here we report an in-depth characterization of the binding of seven previously described ligands to in vitro generated Abeta-(1-40) polymers. All of the compounds were derived from the benzothiazole compound thioflavin T and include 2-[4'-(methylamino)phenyl]benzothiazole and 2-(4'-dimethylamino-)phenyl-imidazo[1,2-a]-pyridine derivatives, 2-[4'-(dimethylamino)phenyl]-6-iodobenzothiazole and 2-[4'-(4''-methylpiperazin-1-yl)phenyl]-6-iodobenzothiazole, and a benzofuran compound (5-bromo-2-(4-dimethylaminophenyl)benzofuran). By using a range of fluorescent and radioligand binding assays, we find that these compounds display a more complex binding pattern than described previously and are consistent with three classes of binding sites on the Abeta fibrils. All of the compounds bound with very high affinity (low nm K(d)) to a low capacity site (BS3) (1 ligand-binding site per approximately 300 Abeta-(1-40) monomers) consistent with the previously recognized binding site for these compounds on the fibrils. However, the compounds also bound with high affinity (K(d) approximately 100 nm) to either one of two additional binding sites on the Abeta-(1-40) polymer. The properties of these sites, BS1 and BS2, suggest they are adjacent or partially overlapping and have a higher capacity than BS3, occurring every approximately 35 or every approximately 4 monomers of Abeta-(1-40)-peptide, respectively. Compounds appear to display selectivity for BS2 based on the presence of a halogen substitution (2-[4'-(dimethylamino)phenyl]-6-iodobenzothiazole, 2-[4'-(4''-methylpiperazin-1-yl)phenyl]-6-iodobenzothiazole, and 5-bromo-2-(4-dimethylaminophenyl)benzofuran) on their aromatic ring system. The presence of additional ligand-binding sites presents potential new targets for ligand development and may allow a more complete modeling of the current positron emission tomography data.

Cerebrospinal fluid beta-amyloid1-42 and tau in control subjects at risk for Alzheimer's disease: the effect of APOE epsilon4 allele

BACKGROUND

Cerebrospinal fluid (CSF) measures of beta-amyloid(1-42) and tau are linked with the known neuropathology of Alzheimer's disease (AD). Numerous lines of evidence have also suggested that individuals with at least one APOE epsilon4 allele on chromosome 19 are at increased risk of developing AD. We tested these CSF markers in groups of subjects with AD and healthy older control subjects, using the absence or presence of the APOE epsilon4 allele as a predictive variable in the search for possible prognostic biomarkers of AD.

METHODS

We assessed the levels of beta-amyloid(1-42) and total tau in the CSF of 292 subjects (142 control subjects and 150 subjects with mild-to-moderate AD), who were research participants at the National Institute of Mental Health. The group of control subjects was enriched with a high percentage of subjects with a positive family history of AD. All subjects underwent extensive global cognitive testing.

RESULTS

When divided according to the absence or presence of the APOE epsilon4 allele, the control subjects with at least one epsilon4 allele had significantly lower CSF beta-amyloid(1-42) but not tau levels than control subjects without an APOE epsilon4 allele (p < .01). As expected, the AD patients had lower levels of CSF beta-amyloid(1-42) and higher CSF tau levels than the normal control group (p < .01).

CONCLUSIONS

The association of APOE epsilon4 allele and lower, more AD-like levels of CSF beta-amyloid(1-42) in older control subjects is consistent with previous studies showing possible neuroimaging and cognitive abnormalities with epsilon4 carriers and suggests that CSF beta-amyloid(1-42) decreases might represent an early biomarker of AD. Longitudinal follow-up is of course required to verify whether this biomarker is indeed predictive of clinical conversion to AD.

High-field magnetic resonance imaging of brain iron: birth of a biomarker?

The brain has an unusually high concentration of iron, which is distributed in an unusual pattern unlike that in any other organ. The physiological role of this iron and the reasons for this pattern of distribution are not yet understood. There is increasing evidence that several neurodegenerative diseases are associated with altered brain iron metabolism. Understanding these dysmetabolic conditions may provide important information for their diagnosis and treatment. For many years the iron distribution in the human brain could be studied effectively only under postmortem conditions. This situation was changed dramatically by the finding that T2-weighted MR imaging at high field strength (initially 1.5 T) appears to demonstrate the pattern of iron distribution in normal brains and that this imaging technique can detect changes in brain iron concentrations associated with disease states. Up to the present time this imaging capability has been utilized in many research applications but it has not yet been widely applied in the routine diagnosis and management of neurodegenerative disorders. However, recent advances in the basic science of brain iron metabolism, the clinical understanding of neurodegenerative diseases and in MRI technology, particularly in the availability of clinical scanners operating at the higher field strength of 3 T, suggest that iron-dependent MR imaging may soon provide biomarkers capable of characterizing the presence and progression of important neurological disorders. Such biomarkers may be of crucial assistance in the development and utilization of effective new therapies for Alzheimer's and Parkinson's diseases, multiple sclerosis and other iron-related CNS disorders which are difficult to diagnose and treat.

Update on PET radiopharmaceuticals: life beyond fluorodeoxyglucose

Twenty-eight years after its inception, 2-[18F]FDG- is still the most widely used radiopharmaceutical for PET studies, but numerous more specific radiotracers have been developed and applied in neuroscience and oncology. The advances in radiotracer chemistry, especially the nucleophilic substitution reaction, have played the pivotal role in synthesizing various no-carrier-added 18F-labeled radiotracers for PET studies of various receptor systems. This article lists some of the radiotracers that are available for PET studies in neuroscience and oncology. The prospects for developing other new radiotracers for imaging other organ diseases also seem to be promising.

A unified approach for morphometric and functional data analysis in young, old, and demented adults using automated atlas-based head size normalization: reliability and validation against manual measurement of total intracranial volume

Atlas normalization, as commonly used by functional data analysis, provides an automated solution to the widely encountered problem of correcting for head size variation in regional and whole-brain morphometric analyses, so long as an age- and population-appropriate target atlas is used. In the present article, we develop and validate an atlas normalization procedure for head size correction using manual total intracranial volume (TIV) measurement as a reference. The target image used for atlas transformation consisted of a merged young and old-adult template specifically created for cross age-span normalization. Automated atlas transformation generated the Atlas Scaling Factor (ASF) defined as the volume-scaling factor required to match each individual to the atlas target. Because atlas normalization equates head size, the ASF should be proportional to TIV. A validation analysis was performed on 147 subjects to evaluate ASF as a proxy for manual TIV measurement. In addition, 19 subjects were imaged on multiple days to assess test-retest reliability. Results indicated that the ASF was (1) equivalent to manual TIV normalization (r = 0.93), (2) reliable across multiple imaging sessions (r = 1.00; mean absolute percentage of difference = 0.51%), (3) able to connect between-gender head size differences, and (4) minimally biased in demented older adults with marked atrophy. Hippocampal volume differences between nondemented (n = 49) and demented (n = 50) older adults (measured manually) were equivalent whether corrected using manual TIV or automated ASF (effect sizes of 1.29 and 1.46, respectively). To provide normative values, ASF was used to automatically derive estimated TIV (eTIV) in 335 subjects aged 15-96 including both clinically characterized nondemented (n = 77) and demented (n = 90) older adults. Differences in eTIV between nondemented and demented groups were negligible, thus failing to support the hypothesis that large premorbid brain size moderates Alzheimer's disease. Gender was the only robust factor that influenced eTIV. Men showed an approximately approximately 12% larger eTIV than women. These results demonstrate that atlas normalization using appropriate template images provides a robust, automated method for head size correction that is equivalent to manual TIV correction in studies of aging and dementia. Thus, atlas normalization provides a common framework for both morphometric and functional data analysis.

PET imaging of amyloid in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia and is characterised by progressive impairment in cognitive function and behaviour. The pathological features of AD include neuritic plaques composed of amyloid-beta peptide (Abeta) fibrils, neurofibrillary tangles of hyperphosphorylated tau, and neurotransmitter deficits. Increases in the concentration of Abeta in the course of the disease with subtle effects on synaptic efficacy will lead to gradual increase in the load of amyloid plaques and progression in cognitive impairment. Direct imaging of amyloid load in patients with AD in vivo would be very useful for the early diagnosis of AD and the development and assessment of new treatment strategies. Three different strategies are being used to develop compounds suitable for in vivo imaging of amyloid deposits in human brains. Monoclonal antibodies against Abeta and peptide fragments have had limited uptake by the brain when tested in patients with AD. When putrescine-gadolinium-Abeta has been injected into transgenic mice overexpressing amyloid, labelling has been observed with MRI. The small molecular approach for amyloid imaging has so far been most successful. The binding of different derivatives of Congo red and thioflavin has been studied in human autopsy brain tissue and in transgenic mice. Two compounds, fluorine-18-labelled-FDDNP and carbon-11-labelled-PIB, both show more binding in the brains of patients with AD than in those of healthy people. Additional compounds will probably be developed that are suitable not only for PET but also for single photon emission CT (SPECT).

Amyloid Angiopathy-Related Vascular Cognitive Impairment

We review accumulating evidence that cerebrovascular amyloid deposition (cerebral amyloid angiopathy [CAA]) is an independent risk factor for cognitive dysfunction. The two population-based autopsy studies that have analyzed cognitive status during life as a function of CAA have each suggested deleterious effects of CAA on cognition even after controlling for age and Alzheimer disease pathology. We also review data from patients with CAA-related intracerebral hemorrhage (the one form of CAA that can be noninvasively recognized) suggesting associations of CAA with radiographic white matter abnormalities and cognitive impairment. These data highlight the importance of elucidating the effects of vascular amyloid on cerebrovascular function and of developing therapeutic strategies for this potentially widespread form of microvascular cognitive impairment.

Memory and Executive Function in Aging and AD

Memory decline in aging results from multiple factors that influence both executive function and the medial temporal lobe memory system. In advanced aging, frontal-striatal systems are preferentially vulnerable to white matter change, atrophy, and certain forms of neurotransmitter depletion. Frontal-striatal change may underlie mild memory difficulties in aging that are most apparent on tasks demanding high levels of attention and controlled processing. Through separate mechanisms, Alzheimer's disease preferentially affects the medial temporal lobe and cortical networks, including posterior cingulate and retrosplenial cortex early in its progression, often before clinical symptoms are recognized. Disruption of the medial temporal lobe memory system leads directly to memory impairment. Recent findings further suggest that age-associated change is not received passively. Reliance on reserve is emerging as an important factor that determines who ages gracefully and who declines rapidly. Functional imaging studies, in particular, suggest increased recruitment of brain areas in older adults that may reflect a form of compensation.

Targeting prion amyloid deposits in vivo

The diagnosis of prion diseases in humans is challenging due to a lack of specific and sensitive non-invasive tests. Many forms of human prion disease including variant Creutzfeldt-Jakob disease (vCJD), Gerstmann-Sträussler-Scheinker (GSS) syndrome, and 10% of sporadic CJD cases are associated with amyloid deposition. Several positron emission tomography (PET) ligands have recently been developed to directly image beta-amyloid associated with Alzheimer disease. One of them, methoxy-X04, is a fluorescent derivative of Congo red with high binding affinity toward amyloid fibrils and good blood-brain barrier permeability. Using methoxy-X04, we investigated whether amyloid-targeting ligands can be also employed for direct imaging of amyloid deposits associated with some prion diseases. Such a method could potentially become a novel diagnostic approach for these conditions. Studies were performed on MB mice infected with the 87V mouse-adapted scrapie strain. Labeling of PrP amyloid plaques in brains of presymptomatic and symptomatic mice was demonstrated using in vivo transcranial two-photon microscopy after systemic administration of methoxy-X04. During real-time imaging, PrP amyloid deposits could be clearly distinguished 15 min after intravenous administration of methoxy-X04. The ligand showed rapid clearance from brain areas that did not contain amyloid deposits. PrP amyloid deposits could also be detected by direct application of methoxy-X04 on cerebellar sections from GSS patients. These results suggest that methoxy-X04 or similar derivatives could be used as PET imaging agents to improve the diagnosis of human prion diseases associated with amyloid deposition.

Advances in the early detection of Alzheimer's disease

The combination of an aging population and the promise, possibly in the near future, of disease-modifying therapies have made the characterization of the early stages of Alzheimer's disease (AD) a topic of major research interest. In this article we review recent progress in our understanding of the evolution of early AD with particular reference to the symptomatic pre-dementia stage designated 'mild cognitive impairment', emphasizing work on the early cognitive profile and associated neuroimaging studies.