Probing the Intracellular Redox Status of Tumors with Magnetic Resonance Imaging and Redox-Sensitive Contrast Agents

Abstract

Nitroxide radicals are paramagnetic contrast agents, used in magnetic resonance imaging (MRI), that also exert antioxidant effects. Participating in cellular redox reactions, they lose their ability to provide contrast as a function of time after administration. In this study, the rate of contrast loss was correlated to the reducing power of the tissue or the ''redox status.'' The preferential reduction of nitroxides in tumors compared with normal tissue was observed by MRI. The influence of the structure of the nitroxide on the reduction rate was investigated by MRI using two cell-permeable nitroxides, 4-hydroxy-2,2,6,6,-tetramethyl-1-piperidynyloxyl (Tempol) and 3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl (3CP), and one cell-impermeable nitroxide, 3-carboxy-2, 2,5,5,5-tetramethylpyrrolidine-1-oxyl (3CxP). Pharmacokinetic images of these nitroxides in normal tissue, tumor, kidney, and artery regions in mice were simultaneously obtained using MRI. The decay of Tempol and 3CP in tumor tissue was significantly faster than in normal tissue. No significant change in the total nitroxide (oxidized + reduced forms) was noted from tissue extracts, suggesting that the loss in contrast as a function of time is a result of intracellular bioreduction. However, in the case of 3CxP (membrane impermeable), there was no difference in the reduction rates between normal and tumor tissue. The time course of T 1 enhancement by 3CxP and the total amount of 3CxP (oxidized + reduced) in the femoral region showed similar pharmacokinetics. These results show that the differential bioreduction of cell-permeable nitroxides in tumor and normal tissue is supported by intracellular processes and the reduction rates are a means by which the intracellular redox status can be assessed noninvasively.

The chemistry and biology of nitroxide compounds

“…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]. The intensity of T1-weighted MR images as a function of time after intravenous administration of 3CP was found to decrease more quickly in tumor compared to normal tissue ( Figure 9).…”

“…The distribution and reduction of the nitroxides Tempol, 3CP, and carboxy-Proxyl were examined in MRI experiments [6]. While 3CP and Tempol are known to enter cells, carboxy-Proxyl is membrane impermeable and is restricted to extracellular regions.…”

“…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

“…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.…”

“…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.…”