Dual-phase 18F-florbetapir positron emission tomography in patients with primary progressive aphasia, Alzheimer's disease, and healthy controls: A preliminary study.

BACKGROUND/PURPOSE: To determine whether dual-phase 18F-florbetapir positron emission tomography imaging with perfusion-like and amyloid deposition information can distinguish among primary progressive aphasia (PPA), Alzheimer's disease (AD), and healthy controls (HCs). METHODS: Patients diagnosed with PPA, including four semantic dementia (SD) and two progressive nonfluent aphasia (PNFA), as well as one logopenic variant (LV) of PPA, were studied. All PPA patients, and age-/sex-matched patients with probable AD (n=8) and HCs (n=8) were subjected to dual-phase 18F-florbetapir imaging. Atlas-based quantitative volumes of interest (VOIs) analysis for six cortical areas and whole cerebellum was performed. The standardized uptake value ratios were calculated by normalizing the dual-phase-integrated activities of the six VOIs to whole cerebellum counts. RESULTS: Early phase 18F-florbetapir image showed significantly lower global perfusion index in six PPA patients as compared with HCs. According to VOI analysis, the hypoperfusion lesions were identified in the frontal, anterior cingulate, parietal, precuneus, and temporal regions. Similar findings were confirmed by voxel-base image comparison. 18F-florbetapir late-phase image showed significantly increased amyloid burden in the global cortex index and all six brain regions of eight AD and LV patients when compared with the other six PPA patients and eight HCs. There was no apparent uptake of amyloid tracer in both six PPA patients and eight HCs. CONCLUSION: Dual-phase 18F-florbetapir images of six PPA (SD and PNFA) patients showed hypoperfusion in the frontotemporal cortex, and little global amyloid uptake, which may be a distinct image pattern for differentiation among HC, AD, and PPA patients.

Perfusion-like template and standardized normalization-based brain image analysis using 18F-florbetapir (AV-45/Amyvid) PET.

PURPOSE: Amyloid positron emission tomography (PET) is an important noninvasive method for detecting amyloid burden in Alzheimer's disease (AD) patients. As amyloid PET images have limited anatomical information, magnetic resonance (MR) imaging is usually acquired to perform reliable spatial normalization needed for large-scale analysis. This work proposed and evaluated the performance of new MR-free spatial normalization methods using a perfusion-like template for amyloid PET imaging. METHODS: Amyloid PET and MR images were collected in 35 subjects (cohort 1: 8 AD patients and 6 controls; cohort 2: 15 AD patients and 6 controls). Three ligand-related templates (AD, control, mixed group) and a perfusion-like template (pAV-45) from early time frames of amyloid PET images were constructed from cohort 1. The variations of (18)F-AV-45 standardized uptake value ratios (SUVRs) among AD patients, controls, and all subjects were tested with repeated two-way (template × brain region) analysis of variance (ANOVA) in cohort 2. (18)F-AV-45 SUVRs by region of interest analysis and voxelwise analysis between MR-based and MR-free approaches were compared and correlated to clinical and image parameters. Effect size (group mean SUVR difference between AD and control/standard deviation) was also evaluated for each template method. RESULTS: Significantly different (18)F-AV-45 SUVRs between MR-free spatial normalization and MR-based reference images were found among AD patients, controls, and all subjects by the effect of template and brain regions. The highest correlation (r=0.991) of (18)F-AV-45 SUVR to MR-based reference was found in the pAV-45 group. The SUVR percentage difference to MR-based reference showed the least variation and bias (control: -1.31±3.47 %; AD: -0.36±2.50 %) in the pAV-45 group as well. The voxelwise analysis showed the smallest t statistic value in pAV-45 followed by mixed, control, and AD groups when compared to MR-based reference images. Moreover, an overall larger effect size but compatible to that of MR-based reference result was observed in the pAV-45 group as compared to those of the other MR-free template. CONCLUSION: The novel MR-free template based on the early-phase perfusion images pAV-45 approach for amyloid imaging showed significantly better performance in quantitation accuracy, effect size, and stability when compared with other MR-free PET templates and thus has potential for large-scale clinical applications.

Correlation of early-phase 18F-florbetapir (AV-45/Amyvid) PET images to FDG images: preliminary studies.

PURPOSE: (18)F-Florbetapir (AV-45/Amyvid) is a novel positron emission tomography (PET) tracer for imaging plaque pathology in Alzheimer's disease (AD), while PET images of fluorodeoxyglucose (FDG) for cerebral glucose metabolism can provide complementary information to amyloid plaque images for diagnosis of AD. The goal of this preliminary study was to investigate the perfusion-like property of relative cerebral blood flow estimates (R(1)) and summed early-phase AV-45 images [perfusion AV-45 (pAV-45)] and optimize the early time frame for pAV-45. METHODS: Dynamic AV-45 PET scans (0-180 min) were performed in seven subjects. pAV-45, late-phase AV-45, and FDG images were spatially normalized to the Montreal Neurological Institute template aided by individual MRI images, and the corresponding standardized uptake value ratio (SUVR) was computed. The R(1) images were derived from a simplified reference tissue model. Correlations between regional and voxelwise R(1) and the corresponding FDG images were calculated. An optimization of time frames of pAV-45 was conducted in terms of correlation to FDG images. The optimal early time frame was validated in a separate cohort. RESULTS: The regional distribution in the R(1) images correlated well (R = 0.91) to that of the FDG within subjects. Consistently high correlation was noted across a long range of time frames. The maximal correlation of pAV-45 to FDG SUVR of R = 0.95 was observed at the time frame of 1-6 min, while the peak correlation of R = 0.99 happened at 0-2 min between pAV-45 and R(1). A similar result was achieved in the validation cohort. CONCLUSION: Preliminary results showed that the distribution patterns of R(1) and pAV-45 images are highly correlated with normalized FDG images, and the initial 5-min early time frame of 1-6 min is potentially useful in providing complementary FDG-like information to the amyloid plaque density by late-phase AV-45 images.

Quantitative analysis of binding sites for 9-fluoropropyl-(+)-dihydrotetrabenazine ([¹8F]AV-133) in a MPTP-lesioned PD mouse model.

UNLABELLED: [¹8F]AV-133 is a novel PET tracer for targeting the vesicular monoamine transporter 2 (VMAT2). The aim of this study is to characterize and quantify the loss of monoamine neurons with [¹8F]AV-133 in the MPTP-lesioned PD mouse model using animal PET imaging and ex vivo quantitative autoradiography (QARG). METHODS: Optimal imaging time window of [¹8F]AV-133 was first determined in normal C57BL/6 mice (n = 3) with a 90-min dynamic scan. The reproducibility of [¹8F]AV-133 PET imaging was evaluated by performing a test-retest study within 1 week for the normal group (n = 6). For MPTP-lesioned studies, normal, and MPTP-treated [25 mg mg/kg once (Group A) and twice (Group B), respectively, daily for 5 days, i.p., groups of four normal and MPTP-treated] mice were used. PET imaging studies at baseline and at Day 4 post-MPTP injections were performed at the optimal time window after injection of 11.1 MBq [¹8F]AV-133. Specific uptake ratio (SUr) of [¹8F]AV-133 was calculated by [(target uptake-cerebellar uptake)/cerebellar uptake] with cerebellum as the reference region. Ex vitro QARG and immunohistochemistry (IHC) studies with tyrosine hydroxylase antibody were carried out to confirm the abundance of dopaminergic neurons. RESULTS: The variability between [¹8F]AV-133 test-retest striatal SUr was 6.60 ± 3.61% with less than 5% standard deviation between animals (intervariability). The percentages of MPTP lesions were Group A 0.94 ± 0.29, -42.1% and Group B 0.65 ± 0.09, -60.4%. By QARG, specific binding of [¹8F]AV-133 was reduced relative to the control groups by 50.6% and 60.7% in striatum and by 30.6% and 46.4% in substantia nigra (Groups A and B, respectively). Relatively small [¹8F]AV-133 SUr decline was noted in the serotonin and norepinephrine-enriched regions (7.9% and 9.4% in mid-brain). Results obtained from IHC consistently confirmed the sensitivity and selectivity of dopaminergic neuron loss after MPTP treatment. CONCLUSIONS: [¹8F]AV-133 PET SUr displayed a high test-retest stability. The SUr significantly declined in the caudate putamen but not in the hypothalamus and midbrain regions after MPTP treatment in the mouse brain. The results obtained for QARG and IHC were consistent and correlated well with the PET imaging studies. On the basis of these concordant results, we find that [¹8F]AV-133 should serve as a useful and reliable PET tracer for evaluating nigrostriatal degeneration.

Iodophenyl tagged sphingosine derivatives: synthesis and preliminary biological evaluation.

A facile synthesis of six 4-iodophenyl tagged sphingosine (SP) derivatives bearing alkyl chain lengths from 6 to 13 is described. The key steps for the assembly of these molecules, 5a-f, are Suzuki-Miyaura cross-coupling and cross-metathesis reactions. The feasibility of radiolabeling was demonstrated by synthesizing two (125)I labeled compounds, [(125)I]5c and [(125)I]5e. In vitro enzyme assays indicated that the molecules, 5c-e, are potent inhibitors. Thus, they deserve further evaluation as potential radioactive probes for tumor imaging.

Congo red and thioflavin-T analogs detect Abeta oligomers.

Several small molecule ligands for amyloid-beta (Abeta) fibrils deposited in brain have been developed to facilitate radiological diagnosis of Alzheimer's disease (AD). Recently, the build-up of Abeta oligomers (AbetaO) in brain has been recognized as an additional hallmark of AD and may play a more significant role in early stages. Evidence suggests that quantitative assessment of AbetaO would provide a more accurate index of therapeutic effect of drug trials. Therefore, there is an urgent need to develop methods for efficient identification as well as structural analysis of AbetaO. We found that some well established amyloid ligands, analogs of Congo red and thioflavin-T (ThT), bind AbetaO with high affinity and detect AbetaO in vitro and in vivo. Binding studies revealed the presence of binding sites for Congo red- and thioflavin-T-analogs on AbetaO. Furthermore, these ligands can be used for imaging intracellular AbetaO in living cells and animals and as positive contrast agent for ultrastructural imaging of AbetaO, two applications useful for structural analysis of AbetaO in cells. We propose that by improving the binding affinity of current ligands, in vivo imaging of AbetaO is feasible by a 'signal subtraction' procedure. This approach may facilitate the identification of individuals with early AD.

Imaging of amyloid beta in Alzheimer's disease with 18F-BAY94-9172, a novel PET tracer: proof of mechanism.

BACKGROUND: Amyloid-beta (Abeta) plaque formation is a hallmark of Alzheimer's disease (AD) and precedes the onset of dementia. Abeta imaging should allow earlier diagnosis, but clinical application is hindered by the short decay half-life of current Abeta-specific ligands. (18)F-BAY94-9172 is an Abeta ligand that, due to the half-life of (18)F, is suitable for clinical use. We thus studied the effectiveness of this ligand in identifying patients with AD. METHODS: 15 patients with mild AD, 15 healthy elderly controls, and five individuals with frontotemporal lobar degeneration (FTLD) were studied. (18)F-BAY94-9172 binding was quantified by use of the standardised uptake value ratio (SUVR), which was calculated for the neocortex by use of the cerebellum as reference region. SUVR images were visually rated as normal or AD. FINDINGS: (18)F-BAY94-9172 binding matched the reported post-mortem distribution of Abeta plaques. All AD patients showed widespread neocortical binding, which was greater in the precuneus/posterior cingulate and frontal cortex than in the lateral temporal and parietal cortex. There was relative sparing of sensorimotor, occipital, and medial temporal cortex. Healthy controls and FTLD patients showed only white-matter binding, although three controls and one FTLD patient had mild uptake in frontal and precuneus cortex. At 90-120 min after injection, higher neocortical SUVR was observed in AD patients (2.0 [SD 0.3]) than in healthy controls (1.3 [SD 0.2]; p<0.0001) or FTLD patients (1.2 [SD 0.2]; p=0.009). Visual interpretation was 100% sensitive and 90% specific for detection of AD. INTERPRETATION: (18)F-BAY94-9172 PET discriminates between AD and FTLD or healthy controls and might facilitate integration of Abeta imaging into clinical practice.

In vivo imaging of beta-cell mass in rats using 18F-FP-(+)-DTBZ: a potential PET ligand for studying diabetes mellitus.

UNLABELLED: Recent studies on gene expression of beta-cell mass (BCM) in the pancreas showed that vesicular monoamine transporter 2 (VMAT2) is highly expressed in the BCM (mainly in the islets of Langerhans). Imaging pancreatic BCM may provide an important tool for understanding the relationship between loss of insulin-secreting beta-cells and onset of diabetes mellitus. In this article, 9-fluoropropyl-(+)-dihydrotetrabenazine (FP-(+)-DTBZ), which is a VMAT2 imaging agent, was evaluated as a PET agent for estimating BCM in vivo. METHODS: Organ biodistribution after an intravenous injection of (18)F-FP-(+)-DTBZ (active isomer) and (18)F-FP-(-)-DTBZ (inactive isomer) was evaluated in normal rats. The specificity of uptake of (18)F-FP-(+)-DTBZ was assessed by a pretreatment (3.8 mg of (+)-DTBZ per kilogram and 3.5 mg of FP-(+)-DTBZ per kilogram, intravenously, 5 min prior) or coadministration (2 mg of (+)-DTBZ per kilogram). PET studies were performed in normal rats. RESULTS: The in vivo biodistribution of (18)F-FP-(+)-DTBZ in rats showed the highest uptake in the pancreas (5% dose/g at 30 min after injection), whereas (18)F-FP-(-)-DTBZ showed a very low pancreas uptake. Rats pretreated with FP-(+)-DTBZ displayed a 78% blockade of pancreas uptake. PET studies in normal rats demonstrated an avid pancreas uptake of (18)F-FP-(+)-DTBZ. CONCLUSION: The preliminary data obtained with (18)F-FP-(+)-DTBZ suggest that this fluorinated derivative of DTBZ shows good pancreas specificity and has the potential to be useful for quantitative measurement of VMAT2 binding sites reflecting BCM in the pancreas.

Synthesis and screening of a library of Re/Tc-based amyloid probes derived from beta-breaker peptides.

Through the development and application of a unique approach for producing Re-metallopeptides, a new class of peptide-derived probes that are designed to target beta-amyloid plaques was developed. Derivatives of a class of beta-breaker peptides having the core sequence lvffa or affvl (lower case letters represent D-amino acids) and the single amino acid chelate quinoline (SAACQ) ligand which can bind Re and (99m)Tc were prepared on an automated peptide synthesizer. Both monomeric and dimeric peptides were synthesized in modest to good yields where in select examples a biotin-containing amino acid derivative was included to act as a linker point for further conjugation to carrier proteins. The Re complexes for all reported peptides were prepared similarly and screened for their ability to inhibit fibrillogenesis. Two of the reported compounds showed excellent inhibitory properties (8a: 40 +/- 5% amyloid formation versus control; 16: 40 +/- 4%) and warrant further investigation. For one of these leads, the (99m)Tc analogue was synthesized and the product showed high stability toward histidine and cysteine challenges, making it a viable candidate for in vivo biodistribution studies.

2-(2'-((Dimethylamino)methyl)-4'-(3-[(18)F]fluoropropoxy)-phenylthio)benzenamine for positron emission tomography imaging of serotonin transporters.

INTRODUCTION: A new (18)F ligand, 2-(2'-((dimethylamino)methyl)-4'-(3-[(18)F]fluoropropoxy)-phenylthio)benzenamine ([(18)F]1), for positron emission tomography (PET) imaging of serotonin transporters (SERT) was evaluated. METHODS: Binding affinity was determined through in vitro binding assays with LLC-PK1 cells overexpressing SERT, NET or DAT (LLC-SERT, LLC-NET and LLC-DAT) and with rat cortical homogenates. Localization and selectivity of [(18)F]1 binding in vivo were evaluated by biodistribution, autoradiography and A-PET imaging studies in rats. RESULTS: This compound displayed excellent binding affinity for SERT in vitro with K(i)=0.33 and 0.24 nM in LLC-SERT and rat cortical homogenates, respectively. Biodistribution studies with [(18)F]1 showed good brain uptake (1.61% dose/g at 2 min postinjection), high uptake into the hypothalamus (1.22% dose/g at 30 min) and a high target-to-nontarget (hypothalamus to cerebellum) ratio of 9.66 at 180 min postinjection. Pretreatment with a SERT selective inhibitor considerably inhibited [(18)F]1 binding in biodistribution studies. Ex vivo autoradiography reveals [(18)F]1 localization to brain regions with high SERT density, and this binding was blocked by pretreatment with SERT selective inhibitors. Small animal PET (A-PET) imaging in rats provided clear images of tracer localization in the thalamus, midbrain and striatum. In A-PET chasing experiments, injecting a SERT selective inhibitor 75 min post-tracer injection causes a dramatic reduction in regional radioactivity and the target-to-nontarget ratio. CONCLUSION: The results of the biological studies and the ease of radiosynthesis with moderately good radiochemical yield (RCY=10-35%) make [(18)F]1 an excellent candidate for SERT PET imaging.

(R)-N-Methyl-3-(3-(125)I-pyridin-2-yloxy)-3-phenylpropan-1-amine: a novel probe for norepinephrine transporters.

Alterations in serotonin and norepinephrine neuronal functions have been observed in patients with major depression. Several antidepressants bind to both serotonin transporters and norepinephrine transporters (NET). The ability to image NET in the human brain would be a useful step toward understanding how alterations in NET relate to disease. In this study, we report the synthesis and characterization of a new series of derivatives of iodonisoxetine, a known radioiodinated probe. The most promising, (R)-N-methyl-3-(3-iodopyridin-2-yloxy)-3-phenylpropylamine (PYINXT), displayed a high and saturable binding to NET, with a K(d) value of 0.53+/-0.03 nM. Biodistribution studies of (R)-N-methyl-3-(3-(125)I-pyridin-2-yloxy)-3-phenylpropan-1-amine in rats showed moderate initial brain uptake (0.54% dose/organ at 2 min) with a relatively fast washout from the brain (0.16% dose/organ at 2 h) as compared to [(125)I]INXT. The hypothalamus (a NET-rich region)-to-striatum (a region devoid of NET) ratio was found to be 2.14 at 4 h after intravenous injection. Preliminary results suggest that this improved iodinated ligand, when labeled with (123)I, may be useful for mapping NET-binding sites with single photon emission computed tomography in the living human brain.

A novel gallium bisaminothiolate complex as a myocardial perfusion imaging agent.

The development of new myocardial perfusion imaging agents for positron emission tomography (PET) may improve the resolution and quantitation of changes in regional myocardial perfusion measurement. It is known that a (68)Ge/(68)Ga generator can provide a convenient source of PET tracers because of the long physical half-life of (68)Ge (271 days). A new ligand, 7,8-dithia-16,24-diaza-trispiro[5.2.5.2.5.3]pentacosa-15,24-diene, which consists of a N(2)S(2)-chelating core incorporated into three cyclohexyl rings, was prepared. To test feasibility and potential utility, the N(2)S(2) ligand was successfully labeled and tested with (67)Ga (half-life=3.26 day; gamma=93.3, 184.6 and 300.2 keV), which showed >92% radiochemical purity. The corresponding "cold" Ga complex was synthesized, and its structure containing a pyramidal N(2)S(2) chloride core was elucidated with X-ray crystallography. In vivo biodistribution of this novel (67)Ga complex, evaluated in normal rats, exhibited excellent heart uptake and retention, with 2.1% and 0.9% initial dose/organ at 2 and 60 min, respectively, after an intravenous injection. Autoradiography was performed in normal rats and in rats that had the left anterior descending coronary artery permanently ligated surgically. Autoradiography showed an even uptake of activity in the normal heart, and there was a distinctively lower uptake in the damaged side of the surgically modified heart. In conclusion, the new N(2)S(2) ligand was readily prepared and labeled with radioactive (67)Ga. Biodistribution in rats revealed high initial heart uptake and relatively high retention reflecting regional myocardial perfusion.

In vivo imaging of vesicular monoamine transporter 2 in pancreas using an (18)F epoxide derivative of tetrabenazine.

OBJECTIVES: Development of imaging agents for pancreatic beta cell mass may provide tools for studying insulin-secreting beta cells and their relationship with diabetes mellitus. In this paper, a new imaging agent, [(18)F](+)-2-oxiranyl-3-isobutyl-9-(3-fluoropropoxy)-10-methoxy-2,3,4,6,7,11b-hexahydro-1H-pyrido[2,1-a]isoquinoline [(18)F](+)4, which displays properties targeting vesicular monoamine transporter 2 (VMAT2) binding sites of beta cells in the pancreas, was evaluated as a positron emission tomography (PET) agent for estimating beta cell mass in vivo. The hydrolyzable epoxide group of (+)4 may provide a mechanism for shifting biodistribution from liver to kidney, thus reducing the background signal. METHODS: Both (18)F- and (19)F-labeled (+) and (-) isomers of 4 were synthesized and evaluated. Organ distribution was carried out in normal rats. Uptake of [(18)F](+)4 in pancreas of normal rats was measured and correlated with blocking studies using competing drugs, (+)dihydrotetrabenazine [(+)-DTBZ] or 9-fluoropropyl-(+)dihydro tetrabenazine [FP-(+)-DTBZ, (+)2]. RESULTS: In vitro binding study of VMAT2 using rat brain striatum showed a K(i) value of 0.08 and 0.15 nM for the (+)4 and (+/-)4, respectively. The in vivo biodistribution of [(18)F](+)4 in rats showed the highest uptake in the pancreas (2.68 %ID/g at 60 min postinjection). In vivo competition experiments with cold FP-(+)-DTBZ, (+)2, (3.5 mg/kg, 5 min iv pretreatment) led to a significant reduction of pancreas uptake (85% blockade at 60 min). The inactive isomer [(18)F](-)4 showed significantly lower pancreas uptake (0.22 %ID/g at 30 min postinjection). Animal PET imaging studies of [(18)F](+)4 in normal rats demonstrated an avid pancreatic uptake in rats. CONCLUSION: The preliminary results suggest that the epoxide, [(18)F](+)4, is highly selective in binding to VMAT2 and it has an excellent uptake in the pancreas of rats. The liver uptake was significantly reduced through the use of the epoxide group. Therefore, it may be potentially useful for imaging beta cell mass in the pancreas.

IMPY, a potential beta-amyloid imaging probe for detection of prion deposits in scrapie-infected mice.

INTRODUCTION: A potential single-photon emission computed tomography imaging agent for labeling of A beta plaques of Alzheimer's disease, IMPY (2-(4'-dimethylaminophenyl)-6-iodo-imidazo[1,2-a]pyridine), would be effective in detection of prion amyloid deposits in transmissible spongiform encephalopathies (TSEs). METHODS: In vitro autoradiographic studies were carried out with [125 I]IMPY on brain sections from scrapie-infected mice and age-matched controls. Competition study was performed to evaluate the prion deposit binding specificity with nonradioactive IMPY. RESULTS: Binding of [125 I]IMPY was observed in infected brain sections, while on age-matched control brain sections, there was no or very low labeling. Prion deposit binding was confirmed by histoblots with prion protein-specific monoclonal antibody 2D6. In the presence of nonradioactive IMPY, the binding of [125 I]IMPY was significantly inhibited in all regions studied. CONCLUSIONS: These findings indicate that IMPY can detect the prion deposits in vitro in scrapie-infected mice. Labeled with 123 I, this ligand may be useful to quantitate prion deposit burdens in TSEs by in vivo imaging.

Synthesis and evaluation of indolinyl- and indolylphenylacetylenes as PET imaging agents for beta-amyloid plaques.

Two new phenylacetylene derivatives, 5-((4-(2-(2-(2-fluoroethoxy)ethoxy)ethoxy)phenyl)ethynyl)indoline 8 and 5-((4-(2-(2-(2-fluoroethoxy)ethoxy)ethoxy)phenyl)ethynyl)-1H-indole 14, targeting beta-amyloid (Abeta) plaques have been prepared. In vitro binding carried out in tissue homogenates prepared from postmortem AD brains with [(125)I]IMPY (6-iodo-2-(4'-dimethylamino-)phenyl-imidazo[1,2-a]pyridine) as the radioligand indicated good binding affinities (K(i)=4.0 and 1.5nM for 8 and 14, respectively). Brain penetration of the corresponding radiofluorinated ligands, evaluated in the normal mice, showed good initial brain penetration (4.50 and 2.43% ID/g (injected dose/gram) for [(18)F]8 and [(18)F]14 at 2min after injection) with moderate to low washout rates from the brain (1.71% ID/g at 2h and 2.10% ID/g at 3h, respectively). Autoradiography and homogenate binding studies demonstrated the high specific binding of [(18)F]14 to the Abeta plaques; however, [(18)F]8 showed low specific binding. These preliminary results identified that indolylphenylacetylene, 14, may be a good lead for further structural modification to develop a useful Abeta plaque imaging agent.

Synthesis of Fluorescent Probes based on Stilbenes and Diphenylacetylenes targeting ß-Amyloid Plaques.

Three fluorescent probes were synthesized aiming for optical imaging to detect amyloid plaques present in patients with Alzheimer's disease (AD). These compounds were prepared via Sonogashira coupling of a well-defined fluorophore (4-bora-3a, 4a-diaza s-indacene, BODIPY) with the pharmacophore possessing either a stilbene or a diphenylacetylene moiety. Different polyethylene glycol chain lengths were used as linkers between the fluorophore and the pharmacophore to adjust the lipophilicity of these probes. These compounds exhibit strong fluorescence emission between 665-680 nm and have very high extinction coefficients comparable to the parent fluorophore, BODIPY dye.

Quick assembly of 1,4-diphenyltriazoles as probes targeting beta-amyloid aggregates in Alzheimer's disease.

Accumulation of beta-amyloid aggregates (Abeta) in the brain is linked to the pathogenesis of Alzheimer's disease (AD). We report a novel approach for producing 1,4-diphenyltriazoles as probes for targeting Abeta aggregates in the brain. The imaging probes, a series of substituted tricyclic 1,4-diphenyltriazoles showing excellent binding affinities to Abeta aggregates (Ki = 4-30 nM), were conveniently assembled by "click chemistry." Two radioiodinated probes, [125I]10a and [125I]10b, and two radiofluorinated probes, [18F]17a and [18F]17b, exhibited moderate lipophilicities and showed excellent initial brain penetrations and fast washouts from the normal mouse brain. In vitro autoradiography of postmortem AD brain sections and homogenates showed that these triazoles were binding to Abeta plaques. Preliminary results strongly suggest that use of click chemistry, which led to a 1,4-diphenyltriazole-based core, is a highly convenient and flexible approach for assembling novel imaging agents for targeting Abeta aggregates in senile plaques in the living human brain.

Novel styrylpyridines as probes for SPECT imaging of amyloid plaques.

We report a series of radioiodinated styrylpyridines as single photon emission computed tomography probes for imaging Abeta plaques in the brain of patients with Alzheimer's disease (AD). In vitro binding showed that all of the styrylpyridines displayed very good binding affinities in postmortem AD brain homogenates (Ki = 3.6 to 15.5 nM). No-carrier-added samples of 13a, 13b, 16a, 16b, and 16e (radioiodinated with 125I) were successfully prepared. The in vivo biodistribution in normal mice, at 2 min after injection, showed excellent initial brain penetrations (4.03, 6.22, 5.43, and 8.04% dose/g for [125I]13a, 13b, 16a, and 16b, respectively). Furthermore, in vitro autoradiography of AD brain sections showed that the high binding signal was specifically due to the presence of Abeta plaques. Taken together, these results strongly suggest that these styrylpyridines are useful for imaging Abeta plaques in the living human brain.

New diphenylacetylenes as probes for positron emission tomographic imaging of amyloid plaques.

A series of 18F fluoropegylated diphenylacetylenes as probes for binding to Abeta plaques were successfully prepared. These relatively rigid acetylenes, 12a, 12b, 14a, and 14b, displayed high binding affinities in postmortem AD brain homogenates (Ki ranging from 1.2 to 2.9 nM). In vivo biodistribution in normal mice exhibited excellent initial brain penetrations (4.42, 4.55, 5.41, and 6.78% dose/g at 2 min for [18F]12a, 12b, 14a, and 14b, respectively). [18F]12b and [18F]14b, with a longer fluoropegylated unit, that is, n=3, showed faster brain washout at 30 min postinjection (0.42 and 1.57% dose/g) as compared to the shorter fluoropegylated chain ligands, that is, [18F]12a and [18F]14a (1.03 and 3.69% dose/g). Autoradiography and homogenate binding confirmed the high binding signal due to Abeta plaques. These preliminary results suggest that the novel diphenylacetylenes may be potentially useful for imaging of Abeta plaques in the brain of patients with Alzheimer's disease.

2-(2'-((dimethylamino)methyl)-4'-(fluoroalkoxy)-phenylthio)benzenamine derivatives as serotonin transporter imaging agents.

A novel series of ligands with substitutions at the 5-position on phenyl ring A and at the 4'-position on phenyl ring B of 2-(2'-((dimethylamino)methyl)-4'-(fluoroalkoxy)phenylthio)benzenamine (4'-2-fluoroethoxy derivatives 28-31 and 4'-3-fluoropropoxy derivatives 40-42) were prepared and tested as serotonin transporter (SERT) imaging agents. The new ligands displayed high binding affinities to SERT (Ki ranging from 0.03 to 1.4 nM). The corresponding 18F labeled compounds, which can be prepared readily, showed excellent brain uptake and retention after iv injection in rats. The hypothalamus region showed high uptake values between 0.74% and 2.2% dose/g at 120 min after iv injection. Significantly, the hypothalamus to cerebellum ratios (target to nontarget ratios) at 120 min were 7.8 and 7.7 for [18F]28 and [18F]40, respectively. The selective uptake and retention in the hypothalamus, which has a high concentration of SERT binding sites, demonstrated that [18F]28 and [18F]40 are promising positron emission computed tomography imaging agents for mapping SERT binding sites in the brain.