NOVEL PET TRACERS FOR IMAGING OF ALZHEIMER'S DISEASE

 描述(申请人提供)：多项研究表明，淀粉样蛋白的形成早在神经退化期开始前几十年，而非痴呆老年人中老年斑(SPS)和神经原纤维缠结(NFT)的存在可能代表AD在临床表现之前的早期表现。为了进一步美化诊断核医学成像资源，18F-AVID-45，18F-氟美他莫和18F-弗洛贝塔本最近获得了美国食品和药物管理局的Aβ成像批准。在这些药物中，11C-PIB仍然是用于Aβ成像的最彻底的正电子发射计算机断层扫描放射性药物。最近的报道表明，11C-PIB不能在经临床、认知和脑脊液生物标志物确诊的AD患者中检测到大脑Aβ，这进一步引起了人们对PIB和其他试剂检测以弥漫性Aβ斑块为主要特征的AD变异体的敏感性的关注。此外，最近对PD患者的临床病理研究也表明，尽管PD患者脑中存在丰富的Aβ，但PIB成像在区分PD患者是否患有痴呆方面存在局限性。为了进一步补充有前景的PET Aβ显像剂的设备，一种易于获得、高效、高度特异的18F-PET显像剂可能能够与纤维斑块和弥漫性斑块结合，包括Aβ上更致密的部位，以实现高灵敏度的检测(在疾病的前驱阶段) 非常令人向往，而且仍然是一个未实现的目标。为了实现这一目标，我们合理地设计了新型杂环荧光分子(18F-AI-187)，从一种全新的分子出发，它表现出浓度依赖的可饱和结合，Kd值分别为1.4±0.35 nM和2.9±1.35 nM，分别与AD匀浆和预先形成的Aβ1-42纤维结合，未标记的荧光对应物既能检测到纤维斑块，又能在体外显示APPsw+/-/PS1小鼠脑组织海马区的淀粉样血管病变，也能检测到人类组织中的弥漫斑块、致密斑块和血管沉积。此外，PET示踪剂18F-AI-187在FVB小鼠的大脑中显示出极高的第一次通过提取(8.86±0.32%ID/g%ID/g；尾静脉注射后2分钟)，然后在没有靶向斑块的情况下进行清洗(比18F-AVID 45快25%)。与在体内易代谢的11C-PIB、18F-氟替他莫(比11C-PIB代谢快)和18F-Avid45相比，18F-AI-187在人血清中仍未被代谢。因此，进入大脑的高首次通过提取，加上从血池中更快地清除和缺乏代谢物，提供了关键特征，可以提高总体信号与背景比和目标特异性，以辅助图像分析。在APPsw+/-/PS1(15个月大)小鼠身上的初步多光子显微镜显示，F-AI-187穿过血脑屏障，瞬时标记脑实质和血管(CAA)中的斑块，并在研究的时间点保持标记斑块。初步，microPET/CT成像显示，与年龄匹配的BL6小鼠相比，转基因小鼠的大脑对放射性示踪剂的摄取(尾静脉注射后30分钟)以及其在皮质中的滞留更高，这与示踪剂与Aβ斑块的结合一致。最后，也是重要的一点，该试剂对AD具有高度特异性(与其他神经退行性疾病的生物标记物没有交叉反应)；同时还在PIB-Aβ+AD病例中检测到弥漫和致密的斑块。有了这些关于我们的首席特工的极具挑衅性的数据，现在我们建议： 1)对18F-AI-187或第二代先导剂进行完整的药代动力学分析，以确定它们在年龄匹配的APPsw+/-转基因小鼠(靶点特异性)、WT对应物(对照)和非人灵长类动物(基线信噪比分析)中作为非侵入性Aβ靶向探针的翻译潜力，使用microPET/CT或microPET/MR或PET/MR成像进行时间-活动曲线(TAC)的评估，进行动脉代谢物分析和剂量学研究，以确定高度特异性的显像剂，以将翻译线索推进到非GMP毒理学研究中用于eIND归档；2)开展有针对性的合成孔径雷达研究，开发第二代新型杂环分子，能够在AD的临床表现之前检测到AD早期的Aβ斑块。3)通过多个结合和竞争置换分析来评估Aβ上的靶点，对第二代Aβ靶向试剂进行完整的生化表征，通过生物分布研究和体外双光子成像、磷成像研究来评估血脑屏障通透性和标记实质中斑块的能力， AD脑匀浆和人类AD脑组织切片的体外结合研究，包括Aβ与其他神经退行性疾病中普遍存在的其他生物标记物蛋白(tau、prion、TDP43和α-synclein)的特异性比较，以确定第二代药物的靶点选择性。经过进一步的生化验证，这些新型分子显像剂可以实现对AD先兆期患者Aβ的无创正电子发射计算机断层扫描，更好地指导AD患者与其他神经退行性疾病患者的分层，并辅助分析新的分子靶向疾病修饰疗法的疗效，从而全面辅助AD的治疗。

 DESCRIPTION (provided by applicant): Several lines of investigations indicate that amyloid formation precedes decades prior to beginning of neurodegeneration phase, and the presence of senile plaques (SPs) and neurofibrillary tangles (NFTs) in nondemented older adults could represent an earlier manifestation of AD prior to its clinical expression. To further embellish diagnostic nuclear medicine imaging resources, 18F-Avid-45,18F-Flutemetamol, and 18F-Florbetaben have recently gained FDA approval for Aβ imaging. Among these agents, 11C-PIB continues to be the most thoroughly investigated PET radiopharmaceutical for Aβ imaging. Recent reports indicate that 11C-PIB could not detect cerebral Aβ in a patient confirmed via clinical, cognitive, and cerebrospinal fluid biomarkers of AD, thus raising further concerns for sensitivity of PIB and other agents to detect AD variants characterized predominantly by diffuse Aβ plaques. Additionally, recent clinicopathological studies of PD patients have also indicated limitations of PIB imaging to differentiate PD patients with or without dementia, despite the presence of abundant Aβ in brains of those patients. To further supplement armamentarium of promising PET Aβ imaging agents, an easily accessible, efficient, highly specific 18F-PET agent and potentially capable of binding to both fibrillar and diffuse plaques, including the more dense sites on Aβ to enable high sensitivity for detection (at prodromal stages of the disease) would be highly desirable, and continues to be an unmet goal. To accomplish this objective, we have rationally designed a novel heterocyclic fluorescent molecule (18F-AI-187) from an entirely a new class of molecules that shows concentration dependent and saturable binding, with Kd values of 1.4±0.35nM and 2.9 ±1.35nM, to AD homogenates and preformed Aβ1-42 fibrils, respectively, the unlabeled fluorescent counterpart detects both fibrillar plaques and displays cerebral amyloid angiopathy (CAA) ex vivo in the hippocampus regions of brain sections in APPsw+/-/PS1 mice and also detects diffuse plaques, compact plaques, and vascular deposits (CAA) in human tissues. Further, the PET tracer 18F-AI-187 demonstrates an extremely high first pass extraction in brains (8.86 ± 0.32 %ID/g %ID/g; 2 min post tail-vein injection) of FVB mice, and followed by a washout (25% faster than 18F-Avid 45) in absence of targeted plaques. Compared with11C-PIB, 18F-Flutemetamol (metabolizes faster than 11C-PIB), and 18F-Avid45 that undergo facile metabolism in vivo, 18F-AI-187 remains non-metabolized in human serum. Therefore, the high first pass extraction into brains coupled with faster clearance from the blood pool and lack of metabolites offer critical characteristics that could enhance overall signal to background ratios and target specificity to assist image analysis. Preliminary multiphoton microscopy in live APPsw+/-/PS1 (15 months old) mice demonstrates that F-AI- 187 traverses blood brain barrier to instantaneously labels plaques in brain parenchyma and blood vessels (CAA), and plaques remain labeled for investigated time points. Preliminary, microPET/CT imaging shows higher brain uptake of the radiotracer (30 min post-tail-vein injection), and its retention in the cortex of transgenic mice compared with their age-matched Bl6 counterparts, consistent with the binding of the tracer to Aβ plaques. Finally and importantly, the agent is als highly specific for AD (displays no cross-reactivity with biomarkers of other neurodegenerative diseases); while also detecting diffuse and compact plaques in a PIB- Aβ+ AD case. Armed with this highly provocative data on our lead agent, now we propose to: 1) Perform complete pharmacokinetic analysis of 18F-AI-187 or the second generation of lead agents to determine their translational potential to serve as noninvasive Aβ-targeted probes in age-matched APPsw+/- transgenic mice (target specificity), WT counterparts (controls), and nonhuman primates (baseline SNR analysis) via evaluation of time-activity curves (TACs), using either microPET/CT or microPET/MR or PET/MR imaging, perform arterial metabolite analysis and dosimetry studies to identify highly specific imaging agents to advance translational leads into non-GMP-toxicology studies for eIND filing; 2) Perform focused SAR studies to develop second generation of novel heterocyclic molecules capable of detecting Aβ plaques in early stages of AD prior to its clinical expression. 3) Perform complete biochemical characterization of second generation Aβ-targeted agents via multiple binding and competitive displacement assays for evaluation of targeted sites on Aβ, assess BBB permeability and ability to label plaques in parenchyma via biodistribution studies and 2-photon imaging, phosphorimaging studies in vitro, perform ex vivo binding studies of AD brain homogenates and human AD brain tissue sections, including specificity for Aβ compared with other biomarker proteins (tau, prion, TDP43,and α-synclein) prevalent in other neurodegenerative diseases for determining target selectivity of second generation agents. Upon further biochemical validation, these novel molecular imaging agents could enable noninvasive PET interrogation of Aβ in patients at prodromal stages of AD, better guide stratification of AD patients from those of other neurodegenerative diseases, and assist analysis of the efficacy for new molecular-targeted disease-modifying therapies, thus overall assisting management of AD.