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放射性示踪剂来量化囊泡乙酰胆碱转运体（VAChT）。VAChT是一种表达于突触前胆碱能神经末梢的新型蛋白。该蛋白是研究胆碱能神经元和突触丧失的关键生物标志物，在痴呆症的认知功能障碍中起重要作用，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损失，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，研究发现，在药物滥用和精神病治疗期间，大脑中VAChT的水平发生了改变。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新的工具。目前，由于缺乏合适的PET示踪剂，PET测量脑内VAChT受到阻碍。因此，开发用于VAChT的体内PET显像剂是非常重要的。为了开发用于VAChT成像的PET放射性示踪剂，我们建议探索一系列新的含羰基的VAChT抑制剂，并通过体外生物活性测定鉴定具有纳米摩尔亲和力（Ki < 10 nM）和高VAChT选择性的先导化合物的更有效的异构体。鉴定的异构体将与C-11或F-18放射合成，然后将在体内大鼠和非人灵长类动物中进行验证。此外，(-)-[18F]FBBV作为这类新型VAChT抑制剂中的一种先导放射性药物，将被进一步评估，以确定其是否适合作为VAChT临床影像学研究的PET示踪剂。（-）-[18F]FBBV在体内的初步验证表明，它在富含VAChT的大脑纹状体中具有很高的特异性结合亲和力。在目前的项目中，我们建议进一步评价（-）[18F]FBBV和几个结构同系物。在此拨款申请中有五个具体目标：具体目标1，合成和放射合成。我们将首先解析5种新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，使用体外结合亲和筛选试验更具选择性；更强效的对映体将与C-11或F-18进行放射性合成。特异性目的2：大鼠生物学评价。将进行大鼠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。特异性目标3，非人类灵长类动物的MicroPET成像和血液代谢物分析。将在猕猴中进行18F或11c标记放射性示踪剂的基线研究、重测变异性研究、特异性研究和可逆性/不可逆性研究，包括动脉血样本的代谢物分析。具体目标4，剂量学研究。将对猕猴进行全身微pet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。将对最有前途的PET放射性示踪剂的未标记形式进行毒性研究。具体目标4和5是未来人类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.