High-Resolution Mapping of Tumor Redox Status by Magnetic Resonance Imaging Using Nitroxides as Redox-Sensitive Contrast Agents

Abstract

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. Magnetic resonance imaging (MRI) provides images withuseful spatial and temporal resolutions and aids in the diagnosis of pathologic conditions in soft tissue. In addition to detailed anatomic information from such scans, suitable contrast agents provide important functional information pertaining to blood flow, perfusion, etc. (1). Most contrast agents used for T 1 -contrast enhancement contain paramagnetic entities such as the Gd 3+ complexes and Mn 2+ complexes. More recently, superparamagnetic iron oxide particles are being used as T 2 * contrast agents in MRI especially in cell tracking (2). Nitroxide radicals are organic molecules that have a single unpaired electron and therefore have the potential to provide T 1 contrast similar to gadolinium complexes. Feasibility of nitroxide radicals as T 1 contrast agents in MRI has been examined (3 -5) before their use for in vivo EPR imaging as probes (6). However, nitroxide spin probes were reported to be not optimal as MRI contrast agents due to their rapid in vivo reduction to the corresponding diamagnetic products (7). In the living body, paramagnetic nitroxide radicals are chemically and/or enzymatically reduced to the diamagnetic hydroxylamine (8 -11). In addition to their biological inst...

The chemistry and biology of nitroxide compounds

“…Studies showed that nitroxides provide significant contrast enhancement in phantom objects and that the rate of decrease in intensity measured by T1-weighted MRI scans can be used as an indicator of metabolism of nitroxides as a function of time [5]. In vivo experiments using the nitroxide 3CP were carried out in tumor bearing mice to examine the differences in nitroxide metabolism in tumor and normal tissue [6].…”

“…This ability to participate in redox reactions enables nitroxides to protect against oxidative damage in several models ranging from cell systems to isolated organs to whole animal models including humans [4]. More recently, they have been recognized as redox-sensitive paramagnetic contrast agents in Magnetic Resonance Imaging (MRI) [5,6]. In this review the chemical basis for the protective effects of nitroxides as well as the cellular and in vivo studies will be summarized.…”

Utility Decay Rates of T1-Weighted Magnetic Resonance Imaging Contrast Based on Redox-Sensitive Paramagnetic Nitroxyl Contrast Agents

“…Whether this surrogate decay rate calculated based on decreasing T 1 -weighted MRI contrast can be used as a fallback position was discussed to ensure in vivo redox mapping based on T 1 -weighted MRI, which was reported elsewhere. [27][28][29] In addition, the suitability of conventional nitroxyl paramagnetic contrast agents, such as 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPOL) and 3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-N-oxyl (CmP), as redox probes for MR redox imaging based on T 1 -contrast was also discussed. ; 4-9-1 Anagawa, Inage-ku, Chiba, Chiba 263-8555, Japan: and b Radiation Biology Branch, Center for Cancer Research, National Cancer Institute, NIH; Bethesda, MD 20892-1002, U.S.A.…”

“…Recently, high-power magnet MRI instrumentation, such as 4.7 T, with a T 1 -weighted spoiled gradient echo (SPGR) pulse sequence has enabled us to take a fresh look at the feasibility of employing nitroxyl radicals as MRI contrast agents. [27][28][29] The redox status of the tumor and normal tissue can be reflected in the time course of T 1 contrast after the injection of a paramagnetic nitroxyl contrast agent to an experimental animal.Gradient echo (GE)-based T 1 -weighted contrast imaging is fast and is suitable for dynamic imaging, although T 1 -weighted MRI contrast can not be utilized to quantify the amount of contrast agent directly. The pseudo decay rate calculated based on decreasing T 1 -weighted MRI contrast can show a similar value to the decay rate obtained by EPR measurement when an identical phantom sample is measured by both methods.…”

“…Recently, high-power magnet MRI instrumentation, such as 4.7 T, with a T 1 -weighted spoiled gradient echo (SPGR) pulse sequence has enabled us to take a fresh look at the feasibility of employing nitroxyl radicals as MRI contrast agents. [27][28][29] The redox status of the tumor and normal tissue can be reflected in the time course of T 1 contrast after the injection of a paramagnetic nitroxyl contrast agent to an experimental animal.…”