High-resolution mapping of tumor redox status by magnetic resonance imaging using nitroxides as redox-sensitive contrast agents

Purpose: There is considerable research directed toward the identification and development of functional contrast agents for medical imaging that superimpose tissue biochemical/molecular information with anatomical structures. Nitroxide radicals were identified as in vivo radioprotectors. Being paramagnetic, they can provide image contrast in magnetic resonance imaging (MRI) and electron paramagnetic resonance imaging (EPRI). The present study sought to determine the efficacy of nitroxide radioprotectors as functional image contrast agents. Experimental Design: Nitroxide radioprotectors, which act as contrast agents, were tested by EPRI and MRI to provide tissue redox status information noninvasively. Results: Phantom studies showed that the nitroxide, 3-carbamoyl-PROXYL (3CP), undergoes time-dependent reduction to the corresponding diamagnetic hydroxylamine only in the presence of reducing agents. The reduction rates of 3CP obtained by EPRI and MRI were in agreement suggesting the feasibility of using MRI to monitor nitroxide levels in tissues. The levels of 3CP were examined by EPRI and MRI for differences in reduction between muscle and tumor (squamous cell carcinoma) implanted in the hind leg of C3H mice simultaneously. In vivo experiments showed a T1-dependent image intensity enhancement afforded by 3CP which decreased in a time-dependent manner. Reduction of 3CP was found to be the dominant mechanism of contrast loss. The tumor regions exhibited a faster decay rate of the nitroxide compared to muscle (0.097 min^-1 versus 0.067 min^-1, respectively). Conclusions: This study shows that MRI can be successfully used to co-register tissue redox status along with anatomic images, thus providing potentially valuable biochemical information from the region of interest.