Redox Imaging using Nitroxides and MRI

摘要氮氧化物已被证明是有效的抗氧化剂和辐射防护剂。氮氧化物是顺磁性的，它们在组织中的存在可以通过磁共振成像进行非侵入性监测。组织中氮氧化物诱导的MR强度增强(氮氧化物代谢)的消失是氮氧化物在细胞内还原为羟胺(非顺磁性)的结果。由于氧化应激，氮氧化物的还原速率可以增加或减少，这表明氮氧化物可以提供一种基于成像的组织氧化还原状态分析。此外，使用5元环氮氧化物(3-CP)和6元环氮氧化物对小鼠不同正常组织和不同类型肿瘤中氮氧化物还原速率的研究表明，在正常组织和选定的肿瘤类型之间，氮氧化物的还原速率可能有很大差异，一般情况下，6元氮氧化合物的还原速度快于5元氮氧化合物。小鼠体内可达到的最大氮氧化物组织水平接近8 mm，而在选定的啮齿动物肿瘤中，该值要小得多(0.6-0.8 mm)。这种浓度上的差异可能解释了坦普尔在正常组织而不是在肿瘤中的不同辐射防护。对于给定的组织，两种氮氧化物之间的最大氮氧化物浓度通常不会改变。虽然长期以来人们一直认为低氧肿瘤组织中的氮氧化物还原会更快，但一些正常组织被发现具有与低氧肿瘤相似的还原速率，这表明组织中的pO2不是体内氮氧化物还原速率的主要决定因素。除了肿瘤缺氧，氧化还原循环系统，如NADP/NADPH，可能有助于氮氧化物的减少。对于氧化还原成像的目的，3-CP被证明是基于体内观察到的可实现的浓度和生物还原的最佳选择。另一种5元环氮氧化合物(命名为23c)被发现在小鼠的脑和心肌中提供T1对比度。我们已经确定了一些可以穿越血脑屏障的氮氧化物，但到目前为止23c是最有效的。~(23)C不仅通过了血脑屏障，而且最大浓度约为3.6 mM。此外，研究还发现，23c在大脑腹侧的下降速度比在背侧更大，这表明23c可能在评估辐射诱导的神经认知损伤和其他脑损伤(包括缺血再灌注损伤)的研究中有用。这种氮氧化物也被发现是一种非常有效的保护，防止全身辐射引起的辐射致死。由于氮氧化物很容易穿透细胞膜，是有效的抗氧化剂，它们可能在其他医学研究领域中使用，如缺血/再灌注损伤研究、中风、预防白内障、炎症过程和衰老。基于氮氧化物的MRI评估可能在确定上述情况方面具有临床应用价值。

SummaryNitroxides have been shown to be efficient antioxidants and radiation protectors. Nitroxides are paramagnetic and their presence in tissue can be monitored non-invasively by MRI. The disappearance of nitroxide induced MR intensity enhancement (nitroxide metabolism) in tissue is a result of intracellular reduction of the nitroxides to the hydroxylamine (not paramagnetic). The rate of nitroxide reduction can increase or decrease due to oxidative stress, suggesting that nitroxides can provide an imaging-based assay of tissue redox status. In addition, the concentration of the nitroxide in tissue can be determined using MR technology.A study of nitroxide reduction rates in different normal tissues in mice and various types of tumors using a 5-membered ring nitroxide (3-CP) and a 6-membered ring nitroxide, Tempol has revealed that reduction rates can vary substantially among normal tissues and selected tumor types and that in general 6-membered nitroxide are reduced faster than 5-membered nitroxides. Maximum nitroxide tissue levels achievable in mouse approach 8 mM, while in selected rodent tumors the values were much less (0.6-0.8 mM). This differential in concentration may explain the differential radioprotection of Tempol in normal tissues and not tumor. For a given tissue, the maximum nitroxide concentration usually did not vary between the two nitroxides. While it has long been thought that nitroxide reduction would be faster in hypoxic tumor tissue, several normal tissues were found to have comparable reduction rates to hypoxic tumors, suggesting that tissue pO2 is not a major determinant of the nitroxide reduction rate in vivo. In addition to tumor hypoxia, redox-recycling systems such as NADP/NADPH may contribute to nitroxide reduction. For the purpose of redox imaging, 3-CP was shown to be an optimal choice based on the achievable concentrations and bioreduction observed in vivo.Another 5-membered ring nitroxide (designated 23c) was found to provide T1 contrast in the brain and myocardium of mice. We have identified a number of nitroxides that cross the blood brain barrier, but 23c to date is the most efficient. Not only did 23c cross the blood brain barrier, but also the maximal concentration obtained was approximately 3.6 mM. Further, it was found that the rate of 23c reduction was greater in the ventral as opposed to the dorsal brain region, suggesting that 23c may be useful in studies assessing radiation-induced neurocognitive damage and other damage to the brain including ischemia reperfusion injury. This nitroxide was also found to be a very potent protector against radiation-induced lethality by total body radiation. Since nitroxides readily penetrate cell membranes and are potent antioxidants, they may be of use in other areas of medical research such as ischemia/reperfusion injury studies, stroke, prevention of cataracts, inflammatory processes, and aging. Nitroxide based MRI evaluation may have clinical utility in defining the above-mentioned conditions.