PET Probes for Imaging the Vesicular Acetylcholine Transporter

描述(由申请人提供)：本研究项目的目标是开发一种PET放射性示踪剂来定量囊泡乙酰胆碱转运体(Vacht)。Vacht是一种在突触前胆碱能神经末梢表达的新蛋白。这种蛋白是研究胆碱能神经元和突触丧失的关键生物标志物，胆碱能神经元和突触的丧失在痴呆症的认知功能障碍中起着重要作用，并与许多神经退行性疾病有关。Vacht的PET示踪剂将提供一种独特的工具来测量胆碱能神经元的丢失，并评估与神经退行性疾病相关的痴呆认知功能障碍的严重程度。此外，研究还发现，在药物滥用和精神病治疗期间，大脑中的Vacht水平会发生变化。因此，Vacht的PET示踪剂也将为监测药物滥用和精神病的治疗提供一种新的工具。目前，由于缺乏合适的PET示踪剂，对大脑中Vacht的PET测量一直受到阻碍。因此，研制VACHT活体PET显像剂具有十分重要的意义。为了开发一种用于Vacht成像的PET放射性示踪剂，我们建议探索一系列新的含有Vacht抑制剂的羰基化合物，并通过体外生物活性测定来鉴定具有纳摩尔亲和力(Ki&lt；10 nm)和对Vacht的高选择性的先导化合物的更有效的异构体。已鉴定的异构体将用C-11或F-18进行放射合成，然后将在活体大鼠和非人类灵长类动物中进行验证。此外，(-)-[18F]FBBV是属于这种新型Vacht抑制剂的一种主要放射性物质，将进一步进行评估，以确定它是否适合作为Vacht临床成像研究的PET示踪剂。初步的体内验证表明，(-)-[18F]FBBV在Vacht丰富的大脑纹状体内具有高度的特异性结合亲和力。在本项目中，我们建议进一步评估(-)[18F]FBBV和几个结构类似物。在这项赠款申请中有五个具体目标：具体目标1，合成和放射合成。我们将首先拆分5个新的先导化合物的外消旋混合物，以确定哪些对映异构体更有效和更具选择性，使用体外结合亲和筛选试验；更有效的对映体将与C-11或F-18进行放射合成。具体目标2，大鼠的生物学评价。将进行大鼠的生物分布和放射自显影研究，以确定放射性示踪剂是否与富含Vacht的大脑纹状体结合。特定目标3，非人灵长类动物的microPET成像和血液代谢物分析。将在猕猴身上进行18F或11C标记的放射性示踪剂的基线研究、测试-重测变异性研究、特异性研究和可逆性/不可逆性研究，包括对动脉血样的代谢物分析。具体目标4，剂量学研究。将在猕猴身上进行全身microPET成像研究，以估计人体不同器官吸收的辐射剂量。具体目标5，毒性研究。将对最有希望的PET放射性示踪剂的未标记形式进行毒性研究。特定的AIMS 4和5是未来在人体上进行PET成像研究的先决条件。

DESCRIPTION (provided by applicant): The goal of this research project is to develop a PET radiotracer to quantify the vesicular acetylcholine transporter (VAChT). VAChT is a novel protein expressed in presynaptic cholinergic nerve terminals. This protein is a key biomarker for studying the loss of cholinergic neurons and synapses, which plays a major role in the cognitive dysfunction of dementia and is associated with a number of neurodegenerative diseases. A PET tracer for VAChT will provide a unique tool to measure the loss of cholinergic neurons and assess the severity of cognitive dysfunction of dementia associated with neurodegenerative diseases. In addition, it has been found that the level of VAChT in the brain is altered during treatment of drug abuse and psychosis. Therefore, a PET tracer for VAChT also will provide a new tool to monitor treatment of drug abuse and psychosis. Currently, PET measurement of brain VAChT in the brain has been hampered due to the lack of a suitable PET tracer. Therefore, it is extremely important to develop in vivo PET imaging agents for VAChT. To develop a PET radiotracer for imaging VAChT, we propose to explore a series of new novel carbonyl containing VAChT inhibitors and identify the more potent isomers of the lead compounds with nanomolar affinity (Ki < 10 nM) and high selectivity for VAChT by in vitro bioactivity determination. The identified isomers will be radiosynthesized with C-11 or F-18 and then will be validated in vivo rats and non-human primates. In addition, (-)-[18F]FBBV, a lead radio agent that belongs in this new class of VAChT inhibitors will be further evaluated to determine if it will be a suitable PET tracer for clinical imaging studies of VAChT. The initial in vivo validation of (-)-[18F]FBBV demonstrated high specific binding affinity in the VAChT enriched striatum of the brain. In the current project, we propose to further evaluate (-)[18F]FBBV and several structural congeners. There are five specific aims in this grant application: Specific Aim 1, Synthesis and Radiosynthesis. We will first resolve the racemic mixtures of 5 new lead compounds to determine which enantiomeric isomers are more potent and more selective using in vitro binding affinity screening assays; the more potent enantiomers will then be radiosynthesized with C-11 or F-18. Specific Aim 2, Biological evaluation in rats. Rat biodistribution and autoradiography studies will be conducted to determine if the radiotracers bind to VAChT-enriched striatum of the brain. Specific Aim 3, MicroPET imaging and blood metabolite analysis in nonhuman primates. Baseline studies, test-retest variability studies, specificity studies and reversibility/irreversibility studies of the 18F or 11C-labeled radiotracers will be conducted in macaques including metabolite analysis of arterial blood samples. Specific Aim 4, Dosimetry Studies. Whole body microPET imaging studies will be conducted in macaques to estimate the dose of radiation absorbed by different organs in human. Specific Aim 5, Toxicity Studies. Toxicity studies of the unlabeled form of the most promising PET radiotracer will be conducted. Specific Aims 4 and 5 are prerequisite steps for future PET imaging studies in humans.