Development Of New Pet And Spect Radiotracers

利用正电子发射断层扫描（PET）和单光子发射计算机断层扫描（SPECT）对A-85380和A-84543类似物作为烟碱乙酰胆碱受体（nAChRs）显像剂进行了评价。根据在动物身上进行的辐射剂量学和毒理学研究，证明5-[I-125]碘- a -85380 （5IA）和2-氟- a -85380 （2FA）用于人类的安全性，提交了允许向人类施用这些放射性配体的INDs。与耶鲁大学合作，首次使用5IA和SPECT进行了人体研究，并证实了足够剂量的安全性，可以在人脑中可视化alpha4beta2 nachr。小鼠研究表明，给予细胞色素P450的非选择性抑制剂西咪替丁，大大减缓了5IA在体内的快速代谢，这表明使用这种或类似的化合物可以降低成功成像人类志愿者nachr所需的放射性剂量。在非人类灵长类动物（NHPs）的丘脑动力学研究中，研究人员开发了方法并收集了数据，用于估计nachr的密度，并与密歇根大学的研究人员合作，用2FA和PET可视化了单侧mptp损伤NHPs的纹状体中这些受体的损失。2FA和PET在NHPs中的动力学研究表明，2FA在脑内的积累相对缓慢，部分原因是其低亲脂性减慢了其血脑屏障的渗透。因此，为了收集足够的数据来量化2FA给药后受体与PET的结合，需要长达4 - 6小时的扫描。如此长时间的扫描可能会限制2FA在人体研究中的实际应用，特别是由于[F-18]的半衰期为1.8小时。努力的方向是开发具有更快动力学的放射配体，用于在体内成像nachr。为此，我们合成了A-85380和A-84543的6种新型类似物，其计算的log P （Clog P）值在1.2 ~ 3之间，并进行了放射性标记，并在体外和体内进行了评价。这些化合物的分子结构模型揭示了它们的构象分布和电子性质的差异，为深入了解它们与nachr的构效关系提供了依据。分子建模是与药物发现研究处药物化学科的工作人员合作进行的。提高Clog P = 0.6 （2FA）的亲脂性，可以使配体更快地穿透血脑屏障，增加非特异性结合。虽然[11C]methyl-PVC是该系列中最好的放射配体，在体内的特异性受体结合是2FA的一半，但[11C]methyl-PVC可能有助于研究内源性或外源性配体在体内对nAChRs的占用。通过对这些放射性配体的体外和体内研究（37℃下Kd值：9 ~ 600 pM， Clog P值：0.6 ~ 3），开发了一种预测结合电位（BP）的算法。BP是放射性配体最重要的特征之一，它表征了放射性配体受体结合的特异性，反映了其在体内定性和定量成像的适用性。通过37℃下的Kds、Clog Ps和各脑区α 4beta2 nachr的密度计算得到6个放射性配体的BP值，该算法与在恒河猴体内观察到的结果一致，表明该算法可用于筛选具有改进特性的α 4beta2 nachr放射性配体的潜在化合物。作为开发用于大麻素受体体内成像的放射配体的新努力的一部分，我们合成并在体外评估了12个新的SR141716类似物，SR141716是大麻素受体CB1亚型的选择性配体。其中两种化合物可能适合用作体内显像剂。该系列中的另外三种化合物逆转了大麻素受体激动剂WIN 55,212-2对兴奋传递的抑制作用，这表明它们可能对大麻和可卡因成瘾的治疗感兴趣。这项工作是与细胞神经生物学研究分部的细胞神经生理学部门的工作人员合作完成的。

Analogs of A-85380 and A-84543 have been evaluated as imaging agents for nicotinic acetylcholine receptors (nAChRs) with positron emission tomography (PET) and single photon emission computed tomography (SPECT). Based on radiation dosimetry and toxicological studies in animals that demonstrated the safety of 5-[I-125]iodo-A-85380 (5IA) and 2-fluoro-A-85380 (2FA) for use in humans, INDs allowing the administration of these radioligands to humans were submitted. In collaboration with Yale University, the first human studies with 5IA and SPECT were performed and confirmed the safety of doses sufficient to visualize alpha4beta2 nAChRs in the human brain. Mice studies showed that the administration of the non-selective inhibitor of cytochrome P450, cimetidine, substantially slowed the rapid in vivo metabolism of 5IA, suggesting that the use of this or similar compounds could reduce the dose of radioactivity needed to successfully image nAChRs in human volunteers. Methodology was developed and data were collected in kinetic studies of the thalamus in non-human primates (NHPs) for estimating the densities of nAChRs and, in collaboration with researchers from the University of Michigan, visualizing the loss of these receptors in the striatum of unilaterally MPTP-lesioned NHPs with 2FA and PET. Kinetic studies with 2FA and PET in NHPs revealed that 2FA accumulates relatively slowly in brain, partially because its low lipophilicity slows its blood-brain-barrier penetration. Therefore, to collect sufficient data to quantify receptor binding with PET after the bolus administration of 2FA, up to 4 - 6 hours of scanning are required. Such prolonged scanning could limit the practical use of 2FA in human studies, especially due to the 1.8 h half-life of [F-18]. Efforts were directed at developing radioligands with faster kinetics for imaging nAChRs in vivo. To that end, six novel analogs of A-85380 and A-84543 with calculated log P (Clog P) values in the range of 1.2 to 3 were synthesized, radiolabeled and evaluated in vitro and in vivo. Molecular modeling of the structures of these compounds revealed differences in their conformational profiles and the electronic properties, providing insight into structure-activity relationships with nAChRs. The molecular modeling was performed in collaboration with staff in the Medicinal Chemistry Section of the Medications Discovery Research Branch. Increasing the lipophilicity from Clog P = 0.6 (2FA) yielded ligands with faster blood brain barrier penetration and increased nonspecific binding. Although [11C]methyl-PVC, the best radioligand from the series, exhibits specific in vivo receptor binding that is half that of 2FA, [11C]methyl-PVC may be useful for studying the in vivo occupancy of nAChRs by endogenous or exogenous ligands. From in vitro and in vivo studies with these radioligands (Kd values at 37 degrees C: 9 to 600 pM, Clog P values: 0.6 to 3), an algorithm was developed for predicting binding potential (BP). BP is the one of the most important characteristic of a radioligand as it characterizes the specificity of radioligand receptor binding and reflects its suitability for qualitive and quantitative in vivo imaging. BP values for six radioligands calculated from their Kds at 37 oC, their Clog Ps, and the density of alpha4beta2 nAChRs in each brain region of interest and this algorithm were in good agreement with those observed in in vivo in Rhesus monkeys, suggesting that this algorithm might be useful for screening potential compounds for a new radioligand for alpha4beta2 nAChRs with improved characteristics. As part of a new effort to develop radioligands for in vivo imaging of cannabinoid receptors, we synthesized and evaluated in vitro twelve new analogs of SR141716, a selective ligand for CB1 subtype of cannabinoid receptors. Two of these compounds are potentially suitable for use as in vivo imaging agents. Three additional compounds in this series reversed the inhibition of excitatory transmission produced by WIN 55,212-2, a cannabinoid receptor agonist, suggesting that they may be of interest for the treatment of marijuana and cocaine addictions. This work was performed in collaboration with staff in the Cellular Neurophysiology Section of the Cellular Neurobiology Research Branch.