苯并噻唑希夫碱类化合物的合成及其与β-淀粉样蛋白结合性能的研究

阿尔茨海默病(Alzheimer’s disease,AD)是一种神经退行性疾病。随着人口老龄化,发病率逐年增高,不仅严重危害着老年人的健康,而且给家庭、社会带来了巨大的经济和人力负担。在我国1.3亿老年人中,痴呆患者600～700万人,发病率在5%以上,约占世界总病例的1/4,成为世界老年痴呆症第一大国[1]。AD常隐匿起病,逐渐出现记忆力减退、认知     

Radioligand development for PET imaging of beta-amyloid (Abeta)--current status

Two of the main pathological hallmarks of Alzheimer's disease (AD) are neuritic plaques and neurofibrillary tangles. Significant evidence supports a critical and probable causative role of beta amyloid (Abeta) plaque formation. Since neuroprotective treatments are typically most effective at early stages of injury, the detection and measurement of Abeta load in living brain should be performed at early and perhaps even presymptomatic stages of AD. Two primary targets of molecular imaging research with positron emission tomography (PET) are to develop surrogate markers (radioligands) for assessing disease progression and for monitoring the efficacy of developmental therapeutics. Here, we review the current status of radioligand development for PET imaging of Abeta aggregates. General structure-activity relationships have emerged, including the identification of at least three different ligand binding sites in various Abeta aggregates and recognition of the general structural requirements for ligand binding at each site. Also a few radioligands applicable to imaging Abeta plaques in living human brain with positron emission tomography (PET) have emerged, including [(11)C]PIB, [(11)C]SB-13 and [(18)F]FDDNP.     

Novel benzofuran derivatives for PET imaging of beta-amyloid plaques in Alzheimer's disease brains

A novel series of benzofuran derivatives as potential positron emission tomography (PET) tracers targeting amyloid plaques in Alzheimer's disease (AD) were synthesized and evaluated. The syntheses of benzofurans were successfully achieved by an intramolecular Wittig reaction between triphenylphosphonium salt and 4-nitrobenzoyl chloride. When in vitro binding studies using AD brain gray matter homogenates were carried out with a series of benzofuran derivatives, all the derivatives examined displayed high binding affinities with K(i) values in the subnanomolar range. Among these benzofuran derivatives, compound 8, 5-hydroxy-2-(4-methyaminophenyl)benzofuran, showed the lowest K(i) value (0.7 nM). In vitro fluorescent labeling of AD sections with compound 8 intensely stained not only amyloid plaques, but also neurofibrillary tangles. The (11)C labeled compound 8, [(11)C]8, was prepared by reacting the normethyl precursor, 5-hydroxy-2-(4-aminophenyl)benzofuran, with [(11)C]methyl triflate. The [(11)C]8 displayed moderate lipophilicity (log P = 2.36), very good brain penetration (4.8%ID/g at 2 min after iv injection in mice), and rapid washout from normal brains (0.4 and 0.2%ID/g at 30 and 60 min, respectively). In addition, this PET tracer showed in vivo amyloid plaque labeling in APP transgenic mice. Taken together, the data suggest that a relatively simple benzofuran derivative, [(11)C]8, may be a useful candidate PET tracer for detecting amyloid plaques in the brains of patients with Alzheimer's disease.     

Radioiodinated styrylbenzenes and thioflavins as probes for amyloid aggregates

We report for the first time that small molecule-based radiodiodinated ligands, showing selective binding to Abeta aggregates, cross the intact blood-brain barrier by simple diffusion. Four novel ligands showing preferential labeling of amyloid aggregates of Abeta(1-40) and Abeta(1-42) peptides, commonly associated with plaques in the brain of people with Alzheimer's disease (AD), were developed. Two 125I-labeled styrylbenzenes, (E,E)-1-iodo-2,5-bis(3-hydroxycarbonyl-4-hydroxy)styrylbenzene, 12 (ISB), and (E,E)-1-iodo-2,5-bis(3-hydroxycarbonyl-4-methoxy)styrylbenzene, 13 (IMSB), and two 125I-labeled thioflavins, 2-[4'-(dimethylamino)phenyl]-6-iodobenzothiazole, 18a (TZDM), and 2-[4'-(4'-methylpiperazin-1-yl)phenyl]-6-iodobenzothiazole, 18b (TZPI), were prepared at a high specific activity (2200 Ci/mmol). In vitro binding studies of these ligands showed excellent binding affinities with Kd values of 0.08, 0.13, 0.06, and 0.13 nM for aggregates of Abeta(1-40) and 0.15, 0.73, 0.14, and 0.15 nM for aggregates of Abeta(1-42), respectively. Interestingly, under a competitive-binding assaying condition, different binding sites on Abeta(1-40) and Abeta(1-42) aggregates, which are mutually exclusive, were observed for styrylbenzenes and thioflavins. Autoradiography studies of postmortem brain sections of a patient with Down's syndrome known to contain primarily Abeta(1-42) aggregates in the brain showed that both [(125)I]18a and [125I]18b labeled these brain sections, but [125I]13, selective for Abeta(1-40) aggregates, exhibited very low labeling of the comparable brain section. Biodistribution studies in normal mice after an iv injection showed that [125I]18a and [(125)I]18b exhibited excellent brain uptake and retention, the levels of which were much higher than those of [125I]12 and [125I]13. These findings strongly suggest that the new radioiodinated ligands, [125I]12 (ISB), [125I]13 (IMSB), [125I]18a (TZDM), and [125I]18b (TZPI), may be useful as biomarkers for studying Abeta(1-40) as well as Abeta(1-42) aggregates of amyloidogenesis in AD patients.     

Recent advances in the development of amyloid imaging agents

Excessive amyloid-beta (Abeta) deposition in the brain is one of the most crucial events in the early pathological stage of Alzheimer's disease (AD). Therefore, Abeta deposits have enough potential to become a useful biomarker for not only an early diagnosis of AD, but also for the assessment of the clinical efficacy of anti-Abeta therapies, if they can be measured non-invasively and reliably in living patients. As a potent candidate technique to measure this biomarker, PET amyloid imaging using a radioligand for Abeta deposits has received much attention. A large number of Abeta ligands have been synthesized and evaluated as candidates for amyloid imaging agents. These can be classified into six categories of derivatives: Congo-red, Thioflavine T, stilbene, vinylbenzoxazole, DDNP, and miscellaneous. Many of these derivatives exhibit high binding affinities to Abeta fibrils (below 20 nM) and some of them also show excellent brain pharmacokinetic profiles. The concept of amyloid imaging is currently being tested in human PET studies using optimized amyloid imaging agents. Despite the small number of subjects, these studies have demonstrated sufficiently promising results. This review article provides an overview of recent advances in the development of amyloid imaging agents, and includes: a summary of the fundamental basis and clinical significance of amyloid imaging; lists of binding affinity data for 135 compounds classified into 12 molecular frameworks; a comprehensive discussion of the in vitro and in vivo features of representative Abeta ligands; and a discussion of the current state of clinical evaluation of these amyloid imaging agents (PIB, SB-13, BF-227, and FDDNP).     

Curcumin and dehydrozingerone derivatives: synthesis, radiolabeling, and evaluation for beta-amyloid plaque imaging

Alzheimer's disease (AD) is pathologically characterized by the accumulation of amyloid plaques and neurofibrillary tangles in the brain, and thus, the in vivo imaging of plaques and tangles would be beneficial for the early diagnosis of AD. It has been suggested that 5-hydroxy-1,7-bis(4-hydroxy-3-methoxyphenyl)-1,4,6-heptatrien-3-one (curcumin) may be responsible for low age-adjusted prevalence of AD in India. In the present study, eight novel derivatives of curcumin and 4-(4-hydroxy-3-methoxyphenyl)-3-buten-2-one (dehydrozingerone) were synthesized and their binding affinities for beta-amyloid (Abeta) aggregates were measured. Of these ligands, fluoropropyl-substituted curcumin (8) showed the highest binding affinity (Ki=0.07 nM), and therefore, 8 was radiolabeled and evaluated as a potential probe for Abeta plaque imaging. Partition coefficient measurement and biodistribution in normal mice demonstrated that [18F]8 has a suitable lipophilicity and reasonable initial brain uptake. Metabolism studies also indicated that [18F]8 is metabolically stable in the brain. These results suggest that [18F]8 is a suitable radioligand for Abeta plaque imaging.     

F-18 stilbenes as PET imaging agents for detecting beta-amyloid plaques in the brain

Imaging agents targeting beta-amyloid (Abeta) may be useful for diagnosis and treatment of patients with Alzheimer's disease (AD). Compounds 3e and 4e are fluorinated stilbene derivatives displaying high binding affinities for Abeta plaques in AD brain homogenates (Ki = 15 +/- 6 and 5.0 +/- 1.2 nM, respectively). In vivo biodistributions of [18F]3e and [18F]4e in normal mice exhibited excellent brain penetrations (5.55 and 9.75% dose/g at 2 min), and rapid brain washouts were observed, especially for [18F]4e (0.72% dose/g at 60 min). They also showed in vivo plaque labeling in APP/PS1 or Tg2576 transgenic mice, animal models for AD. Autoradiography of postmortem AD brain sections and AD homogenate binding studies confirmed the selective and specific binding properties to Abeta plaques. In conclusion, the preliminary results strongly suggest that these fluorinated stilbene derivatives, [18F]3e and [18F]4e, are suitable candidates as Abeta plaque imaging agents for studying patients with AD.     

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.     

Synthesis and preliminary evaluation of 2‐arylhydroxyquinoline derivatives for tau imaging

Alzheimer's disease (AD) is the most common cause of dementia. Senile plaques, consisting of β‐amyloid, and neurofibrillary tangles (NFTs), composed of tau protein, are representative pathological hallmarks of AD. It is believed that the accumulation of NFTs precedes the onset of clinical symptoms of AD and correlates with the progression of memory dysfunction. Thus, the use of noninvasive detection techniques including radiolabeled probes and positron emission tomography (PET) will facilitate early diagnosis or staging of AD. In this study, we synthesized and evaluated novel hydroxylated 2‐arylquinoline derivatives as tau imaging PET probes. The binding affinities of compounds for tau were evaluated by fluorescent staining of the AD hippocampal section and a competitive binding assay using [¹⁸F]THK‐523. THK‐951 showed high binding affinity for tau pathology in an AD brain section and K18Δ280K fibrils (K_i = 20.7 nM); thus, we radiosynthesized a ¹¹C‐labeled THK‐951 and further studied its potential as a tau PET probe. The [¹¹C]THK‐951 demonstrated excellent kinetics in a normal mouse brain (3.23% ID/g at 2 min postinjection and 0.15% ID/g at 30 min postinjection) and showed the labeling of NFTs in an AD brain section by autoradiography assay. These findings indicate the availability of [¹¹C]THK‐951 for in vivo PET imaging of tau pathology in AD. Copyright © 2013 John Wiley & Sons, Ltd.     

Synthesis and evaluation of novel radioligands for positron emission tomography imaging of metabotropic glutamate receptor subtype 1 (mGluR1) in rodent brain

We designed three novel positron emission tomography ligands, N-(4-(6-(isopropylamino)pyrimidin-4-yl)-1,3-thiazol-2-yl)-4-[(11)C]methoxy-N-methylbenzamide ([(11)C]6), 4-[(18)F]fluoroethoxy-N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methylbenzamide ([(18)F]7), and 4-[(18)F]fluoropropoxy-N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methylbenzamide ([(18)F]8), for imaging metabotropic glutamate receptor type 1 (mGluR1) in rodent brain. Unlabeled compound 6 was synthesized by benzoylation of 4-pyrimidinyl-2-methylaminothiazole 10, followed by reaction with isopropylamine. Removal of the methyl group in 6 gave phenol precursor 12 for radiosynthesis. Two fluoroalkoxy analogues 7 and 8 were prepared by reacting 12 with tosylates 13 and 14. Radioligands [(11)C]6, [(18)F]7, and [(18)F]8 were synthesized by O-[(11)C]methylation or [(18)F]fluoroalkylation of 12. Compound 6 showed high in vitro binding affinity for mGluR1, whereas 7 and 8 had weak affinity. Autoradiography using rat brain sections showed that [(11)C]6 binding is aligned with the reported distribution of mGluR1 with high specific binding in the cerebellum and thalamus. PET study with [(11)C]6 in rats showed high brain uptake and a similar distribution pattern to that in autoradiography, indicating the usefulness of [(11)C]6 for imaging brain mGluR1.     

Development of a new radioligand, N-(5-fluoro-2-phenoxyphenyl)-N-(2-[18F]fluoroethyl-5-methoxybenzyl)acetamide, for pet imaging of peripheral benzodiazepine receptor in primate brain

To develop a positron emission tomography (PET) ligand for imaging the 'peripheral benzodiazepine receptor' (PBR) in brain and elucidating the relationship between PBR and brain diseases, four analogues (4-7) of N-(2,5-dimethoxybenzyl)-N-(5-fluoro-2-phenoxyphenyl)acetamide (2) were synthesized and evaluated as ligands for PBR. Of these compounds, fluoromethyl (4) and fluoroethyl (5) analogues had similar or higher affinities for PBR than the parent compound 2 (K(i) = 0.16 nM for PBR in rat brain sections). Iodomethyl analogue 6 displayed a moderate affinity, whereas tosyloxyethyl analogue 7 had weak affinity. Radiolabeling was performed for the fluoroalkyl analogues 4 and 5 using fluorine-18 ((18)F, beta(+); 96.7%, T(1/2) = 109.8 min). Ligands [(18)F]4 and [(18)F]5 were respectively synthesized by the alkylation of desmethyl precursor 3 with [(18)F]fluoromethyl iodide ([(18)F]8) and 2-[(18)F]fluoroethyl bromide ([(18)F]9). The distribution patterns of [(18)F]4 and [(18)F]5 in mice were consistent with the known distribution of PBR. However, compared with [(18)F]5, [(18)F]4 displayed a high uptake in the bone of mice. The PET image of [(18)F]4 for monkey brain also showed significant radioactivity in the bone, suggesting that this ligand was unstable for in vivo defluorination and was not a useful PET ligand. Ligand [(18)F]5 displayed a high uptake in monkey brain especially in the occipital cortex, a region with richer PBR than the other regions in the brain. The radioactivity level of [(18)F]5 in monkey brain was 1.5 times higher than that of [(11)C]2, and 6 times higher than that of (R)-(1-(2-chlorophenyl)-N-[(11)C]methyl,N-(1-methylpropyl)isoquinoline ([(11)C]1). Moreover, the in vivo binding of [(18)F]5 was significantly inhibited by PBR-selective 2 or 1, indicating that the binding of [(18)F]5 in the monkey brain was mainly due to PBR. Metabolite analysis revealed that [(18)F]4 was rapidly metabolized by defluorination to [(18)F]F(-) in the plasma and brain of mice, whereas [(18)F]5 was metabolized by debenzylation to a polar product [(18)F]13 only in the plasma. No radioactive metabolite of [(18)F]5 was detected in the mouse brain. The biological data indicate that [(18)F]5 is a useful PET ligand for PBR and is currently used for imaging PBR in human brain.     

Isomerization of (Z,Z) to (E,E)1-bromo-2,5-bis-(3-hydroxycarbonyl-4-hydroxy)styrylbenzene in strong base: probes for amyloid plaques in the brain.

In developing probes for detecting beta-amyloid (Abeta) plaques in the brain of Alzheimer's disease (AD), we have synthesized 1-bromo-2,5-bis-(3-hydroxycarbonyl-4-hydroxy)styrylbenzene (5, BSB). Due to the presence of two double bonds, formation of four different isomers is possible. Four isomers, E,E-5, E,Z-5, Z,E-5, and Z,Z-5, were prepared. Surprisingly, all showed strong fluorescent labeling of Abeta plaques in the brain of postmortem brain sections of patients with confirmed AD. In vitro binding assay also showed that all four isomers of BSB (E,E-5, E,Z-5, Z,E-5, and Z,Z-5) displayed a similar high binding affinity inhibiting the binding of [(125)I]E,E-6, 1-iodo-2,5-bis-(3-hydroxycarbonyl-4-methoxy)styrylbenzene (IMSB) to Abeta(1-40) aggregates. The inhibition constants (K(i)) of E,E-5, E,Z-5, Z,E-5, and Z,Z-5 were 0.11 +/- 0.01, 0.19 +/- 0.03, 0.27 +/- 0.06, and 0.13 +/- 0.02 nM, respectively. Due to the fact that geometric stability of these styrylbenzenes is unknown, and the conversion of Z,Z-5 to E,E-5 may occur automatically in the binding or labeling assaying conditions, we have investigated the kinetics of conversion of Z,Z-5 to E,E-5 by NMR in D(2)O/NaOD at elevated temperatures (70, 95, and 115 degrees C). The activation energy was determined to be 14.15 kcal/mol. The results strongly suggest that the isomeric conversion at room temperature in aqueous buffer solution is unlikely. All of the styrylbenzene isomers clearly showed potential as useful tools for studying Abeta aggregates in the brain. The data suggest that, despite the rigidity of this series of styrylbenzenes, the binding sites on Abeta aggregates may have certain flexibility and the binding pockets could be adaptable for binding to other smaller ligands. Such information could be exploited to develop new ligands for detecting amyloid plaques in AD.     

Synthesis and evaluation of novel carbon-11 labeled oxopurine analogues for positron emission tomography imaging of translocator protein (18 kDa) in peripheral organs

To develop a PET ligand for imaging TSPO in peripheral organs, we designed three novel oxopurine analogues [(11)C]3a-c (LogD: 1.81-2.17) by introducing a pyridine ring in place of a benzene ring in the lead compound [(11)C]2 (LogD: 3.48). The desmethyl precursors 10 for radiosynthesis were synthesized by reacting glycine 7 with picolylamines 4, followed by hydrolysis and by Curtius rearrangement with diphenylphosphoryl azide. Methylation of 10a-c with methyl iodide produced unlabeled compounds 3a-c. The radiosynthesis of [(11)C]3a-c was performed by reacting 10a-c with [(11)C]methyl iodide. Compounds 3a-c displayed high or moderate in vitro binding affinities (K(i): 5-40 nM) for TSPO. PET with [(11)C]3a-c in rats showed high uptake in the lung, heart, and kidney, which are organs with high TSPO expression. Metabolite analysis with [(11)C]3a showed that radioactivity in these organs mainly corresponded with unchanged [(11)C]3a. PET with [(11)C]3a using a rat model of lung inflammation showed a significant signal in the lipopolysaccharide-treated lung.     

2-11C]isopropyl-, [1-11C]ethyl-, and [11C]methyl-labeled phenoxyphenyl acetamide derivatives as positron emission tomography ligands for the peripheral benzodiazepine receptor: radiosynthesis, uptake, and in vivo binding in brain

The peripheral benzodiazepine receptor (PBR) is widely expressed in peripheral tissues, blood cells, and in glia cells in the brain. We have previously developed two positron emission tomography (PET) ligands, N-(2-[(11)C],5-dimethoxybenzyl)-N-(5-fluoro-2-phenoxyphenyl)acetamide ([(11)C]2) and its [(18)F]fluoroethyl analogue ([(18)F]6), for the current investigation of PBR in the human brain. The aim of this study was to label the potent PBR agonist N-(4-chloro-2-phenoxyphenyl)-N-(isopropoxybenzyl)acetamide (3) and its ethyl (7) and methyl (8) homologues with (11)C and to evaluate them as PET ligands for PBR with mice, rats, and monkeys. Ligands [(11)C]3, [(11)C]7, and [(11)C]8 were synthesized by alkylation of phenol precursor 9 with 2-[2-(11)C]iodopropane ([(11)C]10), [1-(11)C]iodoethane ([(11)C]11), and [(11)C]iodomethane ([(11)C]12), respectively. The alkylating agent [(11)C]10 or [(11)C]11 was prepared by reacting CH(3)MgBr with [(11)C]CO(2), followed by reduction with LiAlH(4) and iodination with HI. In vitro quantitative autoradiography determined that 3, 7, and 8 had potent binding affinities (K(i) = 0.07-0.19 nM) for PBR in the rat brain. These [(11)C]ligands could pass across the blood-brain barrier and enter the rat brain (0.17-0.32% of injected dose per gram wet tissue). Ex vivo autoradiography showed that the [(11)C]ligands preferably distributed in the olfactory bulb and cerebellum, two regions with richer PBR density in the rat brain. The co-injection of PBR-selective 2 reduced the [(11)C]ligand binding in the two regions, suggesting that binding in the rat brain was specific to PBR. PET study determined that the [(11)C]ligands preferably accumulate in the occipital cortex of the monkey brain, a region with a high density of PBR in the primate brain. Moreover, in vivo binding of the methyl homologue [(11)C]8 in the monkey brain could be inhibited by PBR-selective 2 or 1, indicating that some of the [(11)C]8 binding was due to PBR. Metabolite analysis demonstrated that these [(11)C]ligands were metabolized by debenzylation to polar products mainly in the plasma.     

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.     

Aβ斑块显像剂[3H]PIB的合成及生物活性评价

目的：合成2-（4＇-甲基氨基苯）-6-羟基苯并噻唑（6-OH-BTA-1,也称PIB）,并进行3H标记得到[3H]6-CH3O-BTA-Me,评价其生物活性和应用价值.方法：对PIB前体6-OH-BTA-0的合成方法进行优化改进,使用氢碘酸经一步反应由化合物4得到6-OH-BTA-0,继续和碘甲烷反应得到6-OH-BTA-1,6-OH-BTA-1与氚标记的碘甲烷反应制备[3H]6-CH3O-BTA-Me,在体外测定6-OH-BTA-0和6-OH-BTA-1与AD患者脑匀浆组织的亲和性.结果：所有化合物结构得到确证,6-OH-BTA-0和6-OH-BTA-1的Ki值分别为33.5 nmol·L-1和8.8 nmol·L-1,表明与Aβ具有良好的亲和性.结论：自制的PIB可以用于AD的早期诊断实验研究.     

Structure-activity relationships at the monoamine transporters and sigma receptors for a novel series of 9-[3-(cis-3, 5-dimethyl-1-piperazinyl)propyl]carbazole (rimcazole) analogues.

9-[3-(cis-3,5-Dimethyl-1-piperazinyl)propyl]carbazole (rimcazole) has been characterized as a sigma receptor antagonist that binds to the dopamine transporter with moderate affinity (K(i) = 224 nM). Although the binding affinities at the dopamine transporter of rimcazole and cocaine are comparable, rimcazole only depressed locomotor activity in mice and antagonized the stimulant effects produced by cocaine. The neurochemical mechanisms underlying the attenuation of cocaine's effects are not understood, although interaction at a low affinity site/state of the dopamine transporter has been suggested. To explore further this class of compounds, a series of rimcazole analogues was designed and synthesized. Displacement of [(3)H]WIN 35,428 binding at the dopamine transporter in rat caudate-putamen revealed that aromatic substitutions on rimcazole were not well tolerated, generally, with significant reductions in affinity for the 3,6-dibromo (5; K(i) = 3890 nM), 1,3, 6-tribromo (6; K(i) = 30300 nM), 3-amino (8; K(i) = 2400 nM), and 3, 6-dinitro (9; K(i) = 174000 nM) analogues. The N-phenylpropyl group was the only terminal piperazine nitrogen substituent that retained moderate affinity at the dopamine transporter (11; K(i) = 263 nM). Analogues in which the carbazole ring was replaced with a freely rotating diphenylamine moiety were also prepared. Although the diphenylamino analogue in which the terminal piperazine nitrogen was unsubstituted, as in rimcazole, demonstrated relatively low binding affinity at the dopamine transporter (24; K(i) = 813 nM), the N-phenylpropyl analogue was found to have the highest affinity for the dopamine transporter within the series (25; K(i) = 61.0 nM). All of the analogues that had affinity for the dopamine transporter inhibited [(3)H]dopamine uptake in synaptosomes, and potencies for these two effects showed a positive correlation (r(2) = 0.7731, p = 0.0018). Several of the analogues displaced [(3)H]paroxetine from serotonin transporters with moderate to high affinity, with the N-phenylpropyl derivative (11) having the highest affinity (K(i) = 44.5 nM). In contrast, none of the analogues recognized the norepinephrine transporter with an affinity of <1.3 microM. Binding affinities for sigma(1) and sigma(2) receptors were also determined, and several of the compounds were more potent than rimcazole with affinities ranging from 97 nM to >6 microM at sigma(1) sites and 145 to 1990 nM at sigma(2) sites. The compound with the highest affinity (25) at sigma(1) sites was also the compound with highest affinity at the dopamine transporter. These novel rimcazole analogues may provide important tools with which to characterize the relationship between the low affinity site or state of the dopamine transporter, sigma receptors, and their potential roles in modulating cocaine's psychostimulant actions.     

Multidentate (18)F-polypegylated styrylpyridines as imaging agents for Aβ plaques in cerebral amyloid angiopathy (CAA)

β-Amyloid plaques (Aβ plaques) in the brain are associated with cerebral amyloid angiopathy (CAA). Imaging agents that could target the Aβ plaques in the living human brain would be potentially valuable as biomarkers in patients with CAA. A new series of (18)F styrylpyridine derivatives with high molecular weights for selectively targeting Aβ plaques in the blood vessels of the brain but excluded from the brain parenchyma is reported. The styrylpyridine derivatives, 8a-c, display high binding affinities and specificity to Aβ plaques (K(i) = 2.87, 3.24, and 7.71 nM, respectively). In vitro autoradiography of [(18)F]8a shows labeling of β-amyloid plaques associated with blood vessel walls in human brain sections of subjects with CAA and also in the tissue of AD brain sections. The results suggest that [(18)F]8a may be a useful PET imaging agent for selectively detecting Aβ plaques associated with cerebral vessels in the living human brain.     

Synthesis and in vivo validation of [O-methyl-11C]2-{4-[4-(7-methoxynaphthalen-1-yl)piperazin- 1-yl]butyl}-4-methyl-2H-[1,2,4]triazine-3,5-dione: a novel 5-HT1A receptor agonist positron emission tomography ligand

Antagonist 5-HT(1A) PET ligands are available, but an agonist ligand would give more information about signal transduction capacity. Synthesis and in vivo evaluation of [O-methyl-(11)C]2-{4-[4-(7-methoxynaphthalen-1-yl)piperazin-1-yl]butyl}-4-methyl-2H-[1,2,4]triazine-3,5-dione (10), a potential high affinity (K(i) = 1.36 nM) 5-HT(1A) agonist PET tracer is described. Piperazine 10 is a 5-HT(1A) agonist with an EC(50) comparable to serotonin, based on cAMP formation and GTP(gamma)S binding assays. Radiosynthesis of [(11)C]10 has been achieved by reacting 2-{4-[4-(7-hydroxynaphthalen-1-yl)piperazin-1-yl]butyl}-4-methyl-2H-[1,2,4]triazine-3,5-dione (9) and [(11)C]CH(3)OTf in 25 +/- 5% (n = 15) yield at the end of synthesis (EOS). The chemical and radiochemical purities of [(11)C]10 were >99% with a specific activity 1500 +/- 300 Ci/mmol (n =15). PET studies in anesthetized baboon demonstrate [(11)C]10 specific binding in brain regions rich in 5-HT(1A) receptors. Binding of [(11)C]10 was blocked by WAY100635 and 8-OH-DPAT. The regional brain volumes of distribution (V(T)) of [(11)C]10 in baboon correlate with [(11)C]WAY100635 V(T) in baboons. These data provide evidence that [(11)C]10 is the first promising agonist PET tracer for the 5-HT(1A) receptors.     

Synthesis and evaluation of [N-methyl-11C]N-desmethyl-loperamide as a new and improved PET radiotracer for imaging P-gp function

[(11)C]Loperamide has been proposed for imaging P-glycoprotein (P-gp) function with positron emission tomography (PET), but its metabolism to [N-methyl-(11)C] N-desmethyl-loperamide ([(11)C]dLop; [(11)C]3) precludes quantification. We considered that [(11)C]3 might itself be a superior radiotracer for imaging brain P-gp function and therefore aimed to prepare [(11)C]3 and characterize its efficacy. An amide precursor (2) was synthesized and methylated with [(11)C]iodomethane to give [(11)C]3. After administration of [(11)C]3 to wild-type mice, brain radioactivity uptake was very low. In P-gp (mdr-1a(-/-)) knockout mice, brain uptake of radioactivity at 30 min increased about 3.5-fold by PET measures, and over 7-fold by ex vivo measures. In knockout mice, brain radioactivity was predominantly (90%) unchanged radiotracer. In monkey PET experiments, brain radioactivity uptake was also very low but after P-gp blockade increased more than 7-fold. [(11)C]3 is an effective new radiotracer for imaging brain P-gp function and, in favor of future successful quantification, appears free of extensive brain-penetrant radiometabolites.