Redox Imaging using Nitroxides and MRI

虽然辐射生物学分支的其他研究表明氮氧化物是有效的抗氧化剂和辐射保护剂，但最近的研究表明它们可以用作功能性MRI对比探针。因为氮氧化物是顺磁性的，它们在组织中的存在可以通过核磁共振成像进行无创监测。此外，组织中氮氧化物诱导的MR强度增强的消失是细胞内氮氧化物还原为羟胺的结果。通过跟踪组织中氮氧化物的还原速率，可以确定氧化还原速率。这一特性将氮氧化物区分为功能性磁共振造影剂，揭示了细胞/组织细胞内氧化还原能力的信息。氮氧化物介导的MRI最近被用于对氮氧化物的放射防护特性提供额外的见解，特别是关于Tempol对正常组织和肿瘤的差异放射防护。分次放射治疗前10分钟给予Tempol不能保护SCC和HT-29肿瘤的生长；然而，以同样的方式施用Tempol可以防止辐射引起的唾液腺损伤。我们的实验室最近取得了重大进展，可以用来解释这种差异。我们已经表明，Tempol在肿瘤组织中还原为非辐射防护羟胺的速度比唾液腺组织快约2倍。因此，在受辐射时，天波尔的辐射防护形式仍然存在于唾液腺中，提供保护。因此，基于氮氧化物的MRI研究有可能指导适当的氮氧化物给药时间，从而在不保护肿瘤的情况下对正常组织产生最大的放射保护。与放射肿瘤学分支合作，已提交I期临床试验（首次在人体中进行），以评估Tempol作为功能性MRI造影剂。进一步的研究表明，6元环氮氧化物在组织中的还原速度比5元环氮氧化物更快。我们还表明，氮氧化物的还原率在不同的肿瘤类型和小鼠的各种正常组织中是不同的。由于氮氧化物很容易穿透细胞膜，是有效的抗氧化剂，它们可能用于其他医学研究领域，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评估可能在临床上用于确定上述情况。

Summary While other studies in the Radiation Biology Branch have shown nitroxides to be efficient antioxidants and radiation protectors, recent studies have shown that they can be used as functional MRI contrast probes. Because nitroxides are paramagnetic their presence in tissue can be monitored non-invasively by MRI. Further the disappearance of nitroxide induced MR intensity enhancement in tissue is a result of intracellular reduction of the nitroxides to the hydroxylamine. By following the rate of reduction of the nitroxide in tissue the redox rate can be determined. This property distinguishes nitroxides as functional MR contrast agents revealing information about the intracellular redox capacity of cells/tissues. Nitroxide-mediated MRI was recently used to provide additional insight into the radio-protective properties of nitroxides, particularly with respect to the differential radioprotection of Tempol toward normal tissues as opposed to tumor. Tempol administered 10 min prior to fractionated radiation treatment does not protect SCC and HT-29 tumor growth; whereas, Tempol administered in the same fashion protects against radiation-induced salivary gland damage. Our lab has made a recent major advancement that can be used to explain this difference. We have shown that Tempol is reduced in tumor tissue to the non-radioprotective hydroxylamine approximately 2-fold faster than in salivary gland tissue. Thus, the radioprotective form of Tempol is still present in salivary glands at the time of radiation to provide protection. Nitroxide based MRI studies thus have the potential to guide to appropriate timing of nitroxide administration to yield maximal radioprotection of normal tissues without protection of tumor. In collaboration with the Radiation Oncology Branch a Phase I clinical trail (first in humans) has been submitted to evaluate Tempol as a functional MRI contrast probe. Further studies have shown that 6-membered ring nitroxides are reduced faster in tissue than 5-membered ring nitroxides. We have also shown that the reduction rate of nitroxides differs among tumor types and among various normal tissues in mice. 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 clinical utility in defining the above-mentioned conditions.