Nitroxides as Protectors against Oxidative StressExtensive studies in our laboratory have demonstrated that nitroxides (such as Tempol) are effective antioxidants and protectors against ionizing radiation damage. We have previously demonstrated that the potential mechanism(s) for nitroxide-mediated protection against oxidative stress include superoxide dismutase- and catalase-like activity and radical-radical reactions. However, more recent studies have focused on whether nitroxide treatment impacts gene expression. Cells treated with non-toxic concentrations of several different nitroxide analogues; including a nitroxide incapable of cellular entry (and protection) all exhibit similar patterns of gene expression. Prominent genes up-regulated by nitroxide treatment include the heat shock protein (HSP) family, reductive enzyme genes, and genes associated with the Wnt/beta-catenin pathway. We are currently comparing nitroxide-mediated gene expression profiles with other forms of oxidative stress including hydrogen peroxide, superoxide, and ionizing radiation. Additionally we are conducting gene expression studies of selected tissues taken from animals maintained on Tempol in the food. We have shown that long-term administration of Tempol (in the food or drinking water) results in dramatic weight reduction and a decrease in spontaneous tumor incidence in mice. Recently we have shown that Tempol administration delays the onset of tumors in Atm and p53 deficient mice. These studies will hopefully enable us to better understand the complex cellular/molecular mechanisms of nitroxides that trigger responses important in the antioxidant properties of nitroxides as well as those related to weight and the chemopreventive findings. Lastly, we are continuing our studies on the differential radioprotection of Tempol toward normal tissues as opposed to tumor. Recent studies indicate that Tempol administered 10 min prior to fractionated radiation treatment does not protect SCC and HT-29 tumor growth. We have recently shown that nitroxides administered prior to radiation protects against salivary gland damage and preliminary studies show protection against radiation-induced mucositis and damage to the rectum. We have also demonstrated that nitroxides can be followed non-invasively using MRI. We have shown that the disappearance of the nitroxide MRI signal represents reduction of the nitroxide to the hydroxylamine. A recent phase I clinical trial has shown that topically applied nitroxides protect against alopecia for patients receiving whole brain radiotherapy. Our pre-clinical studies support the concept that nitroxides such as Tempol will provide selective radioprotection of normal tissues. 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, prevention of cataracts, inflammatory processes, and aging.
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