The endogenous mediator of vasodilation, nitric oxide (NO), has been shown to be a potent radiosensitizer. However, the underlying mode of action for its role as a radiosensitizer - while not entirely understood - is believed to arise from increased tumor blood flow, effects on cellular respiration, on cell signaling, and on the production of reactive oxygen and nitrogen species (RONS), that can act as radiosensitizers in their own right. NO activity is surprisingly long-lived and more potent in comparison to oxygen. Reports of the effects of NO with radiation have often been contradictory leading to confusion about the true radiosensitizing nature of NO. Whether increasing or decreasing tumor blood flow, acting as radiosensitizer or radioprotector, the effects of NO have been controversial. Key to understanding the role of NO as a radiosensitizer is to recognize the importance of biological context. With a very short half-life and potent activity, the local effects of NO need to be carefully considered and understood when using NO as a radiosensitizer. The systemic effects of NO donors can cause extensive side effects, and also affect the local tumor microenvironment, both directly and indirectly. To minimize systemic effects and maximize effects on tumors, agents that deliver NO on demand selectively to tumors using hypoxia as a trigger may be of greater interest as radiosensitizers. Herein we discuss the multiple effects of NO and focus on the clinical molecule RRx-001, a hypoxia-activated NO donor currently being investigated as a radiosensitizer in the clinic.

In an effort to develop cancer therapies that maximize cytotoxicity, while minimizing unwanted side effects, we studied a series of novel compounds based on the highly energetic heterocyclic scaffold, dinitroazetidine. In this study, we report the preclinical validation of 1-bromoacetyl-3,3-dinitroazetidine (ABDNAZ), a representative lead compound currently in a phase I clinical trial in patients with cancer. In tumor cell culture, ABDNAZ generated reactive free radicals in a concentration- and time-dependent manner, modulating intracellular redox status and triggering apoptosis. When administered to mice as a single agent, ABDNAZ exhibited greater cytotoxicity than cisplatin or tirapazamine under hypoxic conditions. However, compared with cisplatin, ABDNAZ was better tolerated at submaximal doses, yielding significant tumor growth inhibition in the absence of systemic toxicity. Similarly, when combined with radiation, ABDNAZ accentuated antitumor efficacy along with the therapeutic index. Toxicity studies indicated that ABDNAZ was not myelosuppressive and no dose-limiting toxicity was apparent following daily administration for 14 days. Taken together, our findings offer preclinical proof-of-concept for ABDNAZ as a promising new anticancer agent with a favorable toxicity profile, either as a chemotherapeutic agent or a radiosensitizer. Cancer Res; 72(10); 2600–8. ©2012 AACR.

The popular theory six degrees of separation is used in this review as an analogy to relate all radiosensitization to oxygen. As the prime mover of all radiosensitizers, the pervasive influence of oxygen has consciously or unconsciously influenced the direction of research and development and provided the benchmark against which all other compounds and approaches are measured. It is the aim of this review to develop the six degrees of separation from oxygen analogy as a unifying framework for conceptually organizing the field and for giving context to its varied subspecializations and theories. Under such a framework, it would become possible for one area to consider questions and problems found in other areas of radiosensitization, using a common analogy, that would allow for further development and unification of this multifaceted discipline. In this review, approaches to the development of radiosensitizers and the current state of research in this field are discussed, including promising new agents in various stages of clinical development.

As early as the 1920s, the eminent physician and chemist, Otto Warburg, nominated for a second Nobel Prize for his work on fermentation, observed that the core metabolic signature of cancer cells is a high glycolytic flux. Warburg averred that the prime mover of cancer is defective mitochondrial respiration, which drives a switch to an alternative energy source, aerobic glycolysis in lieu of Oxidative Phosphorylation (OXPHOS), in an attempt to maintain cellular viability and support critical macromolecular needs. The cell, deprived of mitochondrial ATP production, must reprogram its metabolism as a secondary survival mechanism to maintain sufficient ATP and NADH levels for macromolecule production, membrane integrity and DNA synthesis as well as maintenance of membrane ionic gradients. A time-tested method to identify and disrupt criminal activity is to “follow the money” since the illicit proceeds from crime are required to underwrite it. By analogy, strategies to target cancer involve following and disrupting the flow of ATP and NADH, the energetic and redox “currencies” of the cell, respectively, since the tumor requires high levels of ATP and NADH, not only for metastasis and proliferation, but also, on a more basic level, for survival. Accordingly, four broad ATP reduction strategies to impact and potentially derail cancer energy production are highlighted herein: 1) small molecule energy-restriction mimetic agents (ERMAs) that target various aspects of energy metabolism, 2) reduction of energy ‘subsidization’ with autophagy inhibitors, 3) acceleration of ATP turnover to increase energy inefficiency, and 4) dietary energy restriction to limit the energy supply.