Development Of New Approaches To Neuroimaging with PET a

成像和量化人脑中烟碱受体的能力可能对更好地理解吸烟成瘾的神经生物学至关重要。尼古丁与大脑中高亲和力烟碱乙酰胆碱受体（主要是α 4 β 2亚型）的相互作用被认为介导了烟草烟雾的奖励和成瘾特性。文献表明，在人类死后脑组织中的高亲和力烟碱乙酰胆碱受体的测定显示，吸烟者在几个脑区域中具有比不吸烟者或戒烟者更高的密度。本研究的目的是确定这些差异是否可以在体内用正电子发射断层扫描（PET）显示，并确定差异的神经解剖学范围。在本研究中，我们使用2[F-18]F-A-85380进行动态PET成像，以测量非吸烟者（n=10）和重度吸烟者（>14支烟/天;戒烟>36小时，n=7）的总分布容积（VD）。在推注2[F-18]F-A-85380后立即开始，在Siemens EXACT HR+扫描仪中获得8小时内的动态PET扫描。对动态PET图像进行运动和衰减校正，并对与从每名志愿者采集的MRI扫描共配准的建模PET图像进行解剖采样。获得总分布容积（VDtot）值。Logan图以及使用2[F-18]F-A-85380的代谢物校正动脉输入函数的1-和2-组织房室模型均产生了相似的结果。取样区域包括丘脑（TH）、中脑（MB）、脑桥（P）、小脑（CB）、额叶皮质（FC）、壳核（PUT）和胼胝体（CC）。吸烟者和不吸烟者的VDtot均以TH > MB > P > CB > FC > PUT最高，CC最低。吸烟者CB、FC、MB、P和PUT的VDtot显著高于非吸烟者（p<0.01）。使用VDtot的胼胝体在非吸烟者作为一个估计的不可替代的VD，我们发现30%以上的特定结合区室（VDsb）在吸烟者的值高出非吸烟者在这些大脑区域。使用2-[F-18]F-A-85380进行的烟碱受体PET成像显示，与对照组相比，可在体内观察到吸烟者中这些受体的密度更高，这表明使用2-[F-18]F-A-85380进行的PET研究可用于研究尼古丁诱导的烟碱受体上调在吸烟行为和戒烟中的作用。 已经开发了使用PET定量脑受体的各种方法。这些方法中的大多数需要动脉血液输入功能，因此在人类或小动物中是侵入性的和不方便的。参考组织方法需要大脑中令人满意的参考区域，这通常是不可用的。由于这些限制，我们探索了使用脑外区域定量中枢受体的潜力。如果感兴趣区域中不可置换隔室的分布容积（VD）（VDnd）与脑内或甚至脑外参考区域的VD之间的比值恒定且已知，则理论上可以使用参考区域和感兴趣区域中随时间的累积放射性来计算表观结合电位（BP*）的值。为了验证这种方法，我们在松鼠猴中用2[F-18]fluoro-A-85380对α 4 β 2 * 烟碱乙酰胆碱受体进行了定量PET研究。使用动脉血输入函数或使用简化的参考组织模型来自脑内部（小脑，Cb）和外部（肌肉）的参考区域的数据计算几个脑区域中的总VD和BP* 值。当肌肉用作参考区域时，BP* 值针对肌肉中的VD与VDnd之间的差异进行校正。VD总值和BP* 值在丘脑中最高，在小脑和肌肉中最低，与灵长类动物中α 4 β 2 * 烟碱乙酰胆碱受体的分布模式一致。有限（9？4%（Cb）或未观察到尼古丁对放射性配体的置换（肌肉），表明2[F-18]F-A-85380在这些区域的特异性结合有限。这些结果表明，定量中枢受体是可行的，使用脑外参考区域，提供了一种新的方法来定量脑受体时，没有合适的参考区域内的大脑存在。 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可显著减缓5[I-123]I-A-85380的快速体内代谢，表明使用该化合物或类似化合物可降低在人类志愿者中成功成像烟碱乙酰胆碱受体所需的放射性剂量。与密歇根大学的研究人员合作，证明了帕金森病动物模型（单侧MPTP损伤的非人灵长类动物）纹状体中烟碱乙酰胆碱受体的损失，使用2[F-18]F-A-85380和PET。 通过PET对中央CB 1大麻素受体进行成像将提高我们对这些受体在大脑中作用的理解。用于成像CB 1的现有PET放射性示踪剂是具有高非特异性与特异性结合比率的非常亲脂的化合物，因此不足以用于定量研究。1-[（N-甲基-哌啶-2-基）甲基]-3-萘甲酰基吲哚（CB 1激动剂）的14元系列类似物具有高结合亲和力，亲脂性降低。先导化合物显示Ki为0.9 nM，这代表了对CB 1受体配体观察到的最高亲和力之一;此外，它具有cLogD值，这应该会减少放射性配体的非特异性蓄积。

The ability to image and quantify nicotinic receptors in human brain may be crucial to better understanding the neurobiology of smoking addiction. The interaction of nicotine with high affinity nicotinic acetylcholine receptors in the brain (predominantly the alpha4beta2 subtype) is thought to mediate the rewarding and addictive properties of tobacco smoke. The literature shows that assays of high affinity nicotinic acetylcholine receptors in human postmortem brain tissue revealed that smokers have higher densities than non-smokers or exsmokers in several brain regions. The goal of the current study was to determine if these differences could be shown in vivo with positron emission tomography (PET) and to determine the neuroanatomical extent of the difference. In this study, we used dynamic PET imaging with 2[F-18]F-A-85380 to measure total volumes of distribution (VD) in non-smokers (n=10) and heavy smokers (>14 cigarettes/day; abstinent for >36 h, n=7). Dynamic PET scans were obtained in a Siemens EXACT HR+ scanner over 8 h commencing immediately after a bolus injection of 2[F-18]F-A-85380. Dynamic PET images were motion and decay corrected and anatomical sampling was performed on modeled PET images that were coregistered to MRI scans acquired from each volunteer. Values for total volumes of distribution (VDtot) were obtained. Logan plots, as well as 1- and 2-tissue compartment models using a metabolite-corrected arterial input function for 2[F-18]F-A-85380 all yielded similar results. Regions sampled included thalamus (TH), midbrain (MB), pons (P), cerebellum (CB), frontal cortex (FC), putamen (PUT) and corpus callosum (CC). For smokers and nonsmokers, VDtot was highest in TH > MB > P > CB > FC > PUT and lowest in CC. VDtot was significantly higher (p<0.01) in smokers than in nonsmokers in CB, FC, MB, P and PUT. Using VDtot for the corpus callosum in nonsmokers as an estimate of nondisplaceable VD, we found 30% to more than 200% higher values for the specific binding compartment (VDsb) in smokers compared to non-smokers in these brain regions. PET imaging of nicotinic receptors with 2-[F-18]F-A-85380 shows that the greater density of these receptors in smokers compared to controls can be observed in vivo and suggests that PET studies with 2[F-18]F-A-85380 can be used to study the role of nicotine-induced upregulation of nicotinic receptors in smoking behaviors and in smoking cessation. Various methods for quantifying cerebral receptors using PET have been developed. Most of these methods require an arterial blood input function and therefore are invasive and inconvenient in humans or in small animals. The reference tissue approach requires a satisfactory reference region in the brain, which is often unavailable. Due to these limitations, we explored the potential of using an extra-cerebral area for quantification of central receptors. If the ratio between the volume of distribution (VD) of non-displaceable compartment in the region of interest (VDnd) and the VD of the reference region inside or even outside of the brain is constant and known, then theoretically it is possible using the accumulation radioactivity in reference region and region of interest over time to calculate the value of apparent binding potential (BP*). To validate this approach, we performed quantitative PET studies of alpha4beta2* nicotinic acetylcholine receptors with 2[F-18]fluoro-A-85380 in squirrel monkeys. Total VD and BP* values in several brain areas were calculated using an arterial blood input function or data from a reference region inside (cerebellum, Cb) and outside (muscle) of the brain using the simplified reference tissue model. When muscle was used as the reference region, BP* values were corrected for the difference between VD in muscle and VDnd. VD total and BP* values were highest in the thalamus and lowest in cerebellum and muscle, consistent with the distribution pattern of alpha4beta2* nicotinic acetylcholine receptors in primates. Limited (9?4% for Cb) or no displacement (for muscle) of radioligand by nicotine was observed, indicating the limited specific binding of 2[F-18]F-A-85380 in these regions. These results suggest that quantifying central receptors is feasible using an extra-cerebral reference region, providing a novel approach for quantification of brain receptors when no suitable reference region inside the brain exists. Mice studies showed that the administration of the non-selective inhibitor of cytochrome P450, cimetidine, substantially slowed the rapid in vivo metabolism of 5[I-123]I-A-85380, suggesting that the use of this or similar compounds could reduce the dose of radioactivity needed to successfully image nicotinic acetylcholine receptors in human volunteers. In collaboration with researchers from the University of Michigan, the loss of nicotinic acetylcholine receptors in the striatum of an animal model for Parkinson's disease, unilaterally MPTP-lesioned nonhuman primates, with 2[F-18]F-A-85380 and PET was demonstrated. Imaging of the central CB1 cannabinoid receptors by PET will improve our understanding of the roles of these receptors in the brain. Existing PET radiotracers for imaging CB1 are very lipophilic compounds with high non-specific to specific binding ratios and as a result are not adequate for quantitative studies. A 14-member series of analogs of 1-[(N-methyl-piperidin-2-yl)methyl]-3-naphthoylindole, a CB1 agonist with high binding affinity, with reduced lipophilicities has been synthesized. The lead compound shows a Ki of 0.9 nM, which represents one of the highest affinities observed for CB1 receptor ligands; further, it has a cLogD value, which should reduce non specific accumulation of the radioligand.