分子影像对中枢神经髓鞘的表征与定量

The objective of this project is to develop myelin-targeting contrast agents for early and accurate assessment of ischemic stroke. Stroke is an acute neurologic dysfunction of vascular origin. Patients admitted with stroke often show sudden occurrence of symptoms and signs corresponding to multiple areas of damage to myelinated nerve fibers in the brain. Clinical diagnosis, initial assessment, and prognosis of recovery intimately depend on the capability to selectively detect and monitor myelin damage. At present, magnetic resonance imaging (MRI) is the primary imaging modality widely used to detect brain lesions. However, conventional MRI techniques cannot distinguish between demyelinated lesions from other inflammatory lesions, which often render MR results disconnected from disease progression and clinical outcome. Because no contrast agents have been approved for clinical use that exhibit any affinity or specificity for myelin, use of contrast MRI can only provide lesion enhancement that is solely indicative of disruption of the blood-brain barrier (BBB). Thus, it is imperative to develop myelin-targeting contrast agents for accurate diagnosis and efficacy evaluation of new therapeutic interventions. To meet this long-standing challenge, we have developed a series of molecular probes with high affinity and specificity for myelin, which have been successfully used for longitudinal studies of myelination based on positron emission tomography (PET) and near-infrared optical imaging. In this project, we plan to exploit the myelin-specific probes that we have previously identified in our laboratory and develop myelin-targeting MR contrast agents through conjugation with paramagnetic Gd-complexes while retaining high binding affinity and specificity for myelin. Since the BBB in lesion regions is often disrupted in patients afflicted with stroke, the newly developed Gd-based contrast agents will be brain permeable for MR imaging similarly to other clinically used contrast agents. We hypothesize that the myelin-binding properties, pharmacokinetic profiles, thermodynamic and kinetic stabilities, and relaxometric properties of these contrast agents can be optimized through combination of Gd-complexes and myelin-binding agents. The ideal contrast agent will exhibit characteristic MR properties so that the regional distribution and relaxation of these contrast agents will correlate with the myelin content in brain lesions. To test this hypothesis, we have developed a prototypical myelin-targeting contrast agent, termed MIC, which exhibits promising myelin-binding specificity and MR relaxometric properties. These preliminary studies form the basis for us to address the following specific aims: 1) Rationally design and synthesize a selected array of MR contrast agents by conjugating different Gd-complexes to myelin-binding agents that we have previously developed; 2) Characterize in vitro MR relaxometric properties and transmetallation stability of selected agents; 3) Conduct in vivo MR studies to identify contrast agents that selectively enhance demyelinated lesions in animal models of ischemic stroke. Completion of these aims will allow us to determine the pharmacokinetics and define the optimal MR parameters of selected contrast agents for future clinical studies.

缺血性脑梗死是起源于血管的急性神经功能障碍。会在很短的时间内，造成大脑中多个区域中髓鞘神经的损伤。在临床上，脑梗死的诊断，初始评估和恢复预后密切依赖于选择性检测髓鞘损伤的能力。目前，磁共振成像（MRI）是广泛用于检测脑损伤的主要成像模式。然而，常规MRI技术不能区分髓鞘损伤与其它炎性病变，这常常使MR结果与临床结果不符。因此，需要开发新的髓鞘靶向造影剂，以便准确诊断和评估患者在临床上的表现。在这个项目中，我们计划将与髓鞘有特异性结合的分子与顺磁性Gd复合物共轭，开发可以满足临床需要的髓鞘靶向MR造影剂。为此，我们将解决以下三个目标：1）通过将不同的Gd-复合物与髓鞘探针缀合，合理地设计，合成， 和筛选一系列MR造影剂; 2）检测所选造影剂的体外MR弛豫性质和稳定性; 3）在缺血性脑梗死动物模型中进行体内MR成像，确定药代动力学，并为未来的临床应用确定所选造影剂的最佳MR参数。

髓鞘损伤是影响缺血性脑卒中患者预后的关键因素之一。目前，现有的磁共振成像技术只能间接检测髓鞘变化，无法直观反映髓鞘的真实情况。因此，本研究旨在开发一系列用于直接检测髓鞘的靶向磁共振成像探针。我们采用连接靶向标记髓鞘的材料和磁共振对比剂Gd-DTPA或Gd-DOTA的方法，成功合成了11种具有荧光-磁共振成像功能的化合物。通过对这些化合物进行相关评价，我们筛选出了两种较为优异的化合物6a和18a。研究发现，合成的化合物与其他临床探针相比具有更高的稳定性和令人满意的弛豫率。细胞实验表明该探针对细胞的毒性较低，可被细胞中的髓鞘摄取。脑切片实验证实该探针能特异结合脑组织中髓鞘丰富的区域，并能够识别缺血性脑卒中大小鼠髓鞘损伤的区域。体内成像实验表明，该化合物能够与髓鞘特异性结合，并通过磁共振序列识别。相较检测髓鞘变化常用的扩散张量成像，该化合物具有更高的对比度和更好的灵敏度。.基于以上研究结果，我们合成的化合物有可能为诊断和监测缺血性脑卒中的髓鞘变化提供一系列新的髓鞘靶向探针。本研究的成功开发了用于磁共振成像的新的探针，并为增加髓鞘靶向磁共振探针的结构多样性，进一步优化体内的药代动力学，筛选出最适合临床应用的成像探针提供了理论参考和试验依据。