Anti-Cancer Agents in Medicinal Chemistry (Formerly Current Medicinal Chemistry - Anti-Cancer Agents) - Volume 11, Issue 2, 2011

The Special Issue titled “SOD enzymes and their mimics in cancer: pro- vs anti-oxidative mode of action” aimed at challenging our thoughts on what is the true mechanism behind the beneficial effects we observe with redox-active compounds, both endogenous and exogenous, in different in vitro and animal models of diseases, which could help us in treating human diseases more successfully. SOD enzymes were proved essential for the functioning of our body in numerous publications since their mechanism of action had been established nearly half a century ago by McCord and Fridovich in a seminal J Biol Chem paper on Cu,ZnSOD [1]. Moreover, the mitochondrial Mn form of superoxide dismutase, MnSOD, is absolutely essential to our lives; the MnSOD-knockout mice survive only days after birth. It is therefore no wonder that therapeutic strategies targeting mitochondria have been sought. The review on MnSOD in this Issue was prepared by Daret St. Clair who has dedicated herself to understanding the critical role of this enzyme under physiological and pathological conditions and most specifically to the role of mitochondria in cancer [2]. Through the years a substantial body of evidence has emerged from a number of groups for an anticancer role of MnSOD. While this is true for many cancers Melendez and coworkers, in their chapter, discuss the strong connection between high level MnSOD expression and redox-signaling in metastatic disease. The effects were ascribed to the increased levels of H2O2, that was thought to be a direct consequence of overexpression of MnSOD. However the possibility is allowed, and elaborated by Melendez group in this Issue, that MnSOD overexpression could modify the cellular redox environment to indirectly enhance the production of O2˙- and H2O2 [3]. Several possible mechanisms of peroxide increase as a consequence of MnSOD overexpression, other than that resulting directly from MnSOD action, were proposed by Irwin Fridovich [4]. Further, Lee Ann MacMillan-Crow and John Crow reported that in their hands overexpression of MnSOD did not lead to increased levels of peroxide [5]. MacMillan-Crow and John Crow also noted that both peroxynitrite and peroxide behave similarly with respect to the effects that they produce in assays which we routinely use for measurements of reactive species. Hardly an assay exists that is specific to a particular reactive species. Thus, what we attribute to the increased levels of peroxide might indeed relate to the increased peroxynitrite levels. Based on their experiments, the authors illustrate how MnSOD overexpression could lead to peroxynitrite formation. Recently it became obvious that both the activity of redox-active proteins and their expression must be studied in order to fully comprehend their in vivo actions and their role in cellular signaling processes. For example as shown by Terry Oberley, the thioredoxin levels may be upregulated, while its activity suppressed due to the excessive oxidation of thioredoxin SH groups [6]. The references listed in the contribution from the Melendez group in this Issue have shown that such scenario may be operative for MnSOD also [3]. Understanding the impact of SOD enzymes on our health has resulted in a desire to mimic their activity. A number of compounds have been developed and used in different models of diseases with varying success over the last three decades: metalloporphyrins, Mn(III) salens, Mn(II) cyclic polyamines, nitroxides, different metal oxides [reviewed in 7], quinones and others. A number of naturally occurring antioxidants have been tested also. For a long time, all of those compounds were viewed as antioxidants. To be as successful SOD mimic as the enzyme itself, the compound must reduce and oxidize O2˙- with equal rate constants. For that to happen, the metal-centered reduction potential must be positioned at the midway between the potential for the oxidation and reduction of O2˙- (∼+300 mV vs NHE); that fact immediately tells us that powerful SOD mimics are equally good pro-oxidants as they are antioxidants. Years ago, Barry Halliwell wrote an article questioning the actions of polyphenols [8]. The role of ascorbate was questioned too [9], particularly in the presence of metals and metal complexes [10]. Are such compounds pro- or antioxidants? The same question holds true for the metalloporphyrins also. The Mn(III) cationic N-substituted pyridylporphyrins are very active in the dismution of O2˙- [11]. Further, it has been shown that under certain circumstances in the presence of ascorbate, the pro-oxidative action of Mn porphyrins is enhanced. In such a scenario metal complexes (or other redox-active compounds) catalyze the ascorbate-driven peroxide production, as evidenced by us and others [12, 13], and proposed by Chen et al., to result in anticancer effects [14, 15]. An ascorbate/menadione system has been approved by FDA for the treatment of metastatic, locally advanced, inoperable transitional cell carcinoma of the urothelium (stage III and IV bladder cancer). The ascorbate/ menadione system is discussed by Pedro Buc Calderon group in this Issue [16]. Ines Batinic-Haberle and Ludmil Benov groups (Part II) have recently shown that Mn porphyrins clearly act as SOD mimics in allowing SOD-deficient E. coli to grow aerobically, but become cytotoxic in the presence of ascorbate. Depending upon the circumstances, such action provoked an adaptive response and we observed the upregulation of endogenous antioxidative defenses [12]. Under stringent conditions, at high concentrations and longer exposure to proxidants, the cells were not able to adapt and died [12]. With redundant peroxide-removing systems (catalases, glutathione peroxidases, glutathione reductases, glutathione transferases, thioredoxins, peroxyredoxins) such pro-oxidative action of Mn porphyrins would not create a problem to a normal cell. Tumors are frequently under oxidative stress and also have compromised reactive species-removing systems, particularly an inadequate ratio of superoxide- to peroxide-removing systems. Thus they may not be able to tolerate the additional oxidative burden and would perish. Therefore, under particular circumstances and in the presence of cellular reductants such as ascorbate and glutathione, and at inadequate ratio of superoxide- to peroxide removing systems, metalloporphyrins could act as powerful cytotoxic agents and kill tumor via excessive production of reactive species.....

The family of superoxide dismutases (SODs) are well known for their antioxidant actions exerted by catalyzing the conversion of O2˙- into H2O2 plus oxygen. The importance of this action is revealed by the multiple phenotypic deficits exhibited by a variety of organisms that have been made to lack one or more of the SODs. Never the less there have been reports of deleterious consequences caused by overproduction of SOD. Several explanations have been proposed for these counter intuitive effects; one of which is that elevated SOD causes increased formation of H2O2. The reasons for dismissing this explanation are explored.

Superoxide dismutases (SODs) are a family of important antioxidant enzymes that catalyze the conversion of superoxide to hydrogen peroxide and oxygen. Hydrogen peroxide is then detoxified by a host of antioxidant enzymes. A common misconception is that the increased MnSOD levels will result in increased hydrogen peroxide levels. Herein we offer some potential reasons for this confusion, as well as some potential resolutions. Data are offered that demonstrate the ability of MnSOD, in the presence of nitric oxide, to utilize hydrogen peroxide to produce superoxide and the more toxic oxidant, peroxynitrite.

Mitochondrial superoxide dismutase (MnSOD) neutralizes the highly reactive superoxide radical (O2˙-), the first member in a plethora of mitochondrial reactive oxygen species (ROS). Over the past decades, research has extended the prevailing view of mitochondrion well beyond the generation of cellular energy to include its importance in cell survival and cell death. In the normal state of a cell, endogenous antioxidant enzyme systems maintain the level of reactive oxygen species generated by the mitochondrial respiratory chain. Mammalian mitochondria are important to the production of reactive oxygen species, which underlie oxidative damage in many pathological conditions and contribute to retrograde redox signaling from the organelle to the cytosol and nucleus. Mitochondria are further implicated in various metabolic and aging-related diseases that are now postulated to be caused by misregulation of physiological systems rather than pure accumulation of oxidative damage. Thus, the signaling mechanisms within mitochondria, and between the organelle and its environment, have gained interest as potential drug targets. Here, we discuss redox events in mitochondria that lead to retrograde signaling, the role of redox events in disease, and their potential to serve as therapeutic targets.

Manganese superoxide dismutase (Sod2) has emerged as a key enzyme with a dual role in tumorigenic progression. Early studies were primarily directed at defining the tumor suppressive function of Sod2 based on its low level expression in many tumor types. It is now commonly held that loss of Sod2 expression is likely an early event in tumor progression allowing for further propagation of the tumorigenic phenotype resulting from steady state increases in free radical production. Increases in free radical load have also been linked to defects in mitochondrial function and metastatic disease progression. It was initially believed that Sod2 loss may propagate metastatic disease progression, in reality both epidemiologic and experimental evidence indicate that Sod2 levels increase in many tumor types as they progress from early stage non-invasive disease to late stage metastatic disease. Sod2 overexpression in many instances enhances the metastatic phenotype that is reversed by efficient H2O2 scavenging. This review evaluates the many sequelae associated with increases in Sod2 that impinge on the metastatic phenotype. The ability to use Sod2 to modulate the cellular redox-environment has allowed for the identification of redox-responsive signaling events that drive malignancy, such as invasion, migration and prolonged tumor cell survival. Further studies of these redox-driven events will help in the development of targeted therapeutic strategies to efficiently restrict redox-signaling essential for malignant progression.

Despite intensive efforts to improve multimodal treatment of brain tumors, survival remains limited. Current therapy consists of a combination of surgery, irradiation and chemotherapy with predisposition to long-term complications. Identifying novel targeted therapies is therefore at the forefront of brain tumor research. This study explores the utility of a manganese porphyrin in a brain tumor model. The compound used is ortho isomer, manganese(III) meso-tetrakis(N-n-hexylpyridinium-2-yl)porphyrin, MnTnHex-2-PyP5+. It is a powerful SOD mimic and peroxynitrite scavenger and a potent modulator of redox-based cellular transcriptional activity, able to suppress excessive immune and inflammatory responses and in turn proliferative pathways. It is further one of the most lipophilic compounds among cationic Mn(III) N-alkylpyridylporphyrins, and thus accumulates predominantly in mitochondria relative to cytosol. In mitochondria, MnTnHex-2-PyP5+ could mimic our key antioxidant system, mitochondrial superoxide dismutase, MnSOD, whose overexpression has been widely shown to suppress tumor growth. Importantly, MnTnHex-2-PyP5+ crosses blood brain barrier in sufficient amounts to demonstrate efficacy in treating CNS injuries. For those reasons we elected to test its effects in inhibiting brain tumor growth. This study is the first report of the antitumor properties of MnTnHex-2-PyP5+ as a single agent in adult and pediatric glioblastoma multiforme (D-54 MG, D-245 MG, D-256 MG, D-456 MG) and pediatric medulloblastoma (D-341 MED), and is the first case where a redox-able metal complex has been used in glioma therapy. When given subcutaneously to mice bearing subcutaneous and intracranial xenografts, MnTnHex-2-PyP5+ caused a significant (P ≤ 0.001) growth delay in D-245 MG, D-256 MG, D-341 MED, and D-456 MG tumors. Growth delay for mice bearing subcutaneous xenografts ranged from 3 days in D-54 MG to 34 days in D-341 MED. With mice bearing intracranial xenografts, MnTnHex-2-PyP5+ increases median survival by 33% in adult glioblastoma multiforme (D-256 MG; P ≤ 0.001) and 173% in pediatric medulloblastoma (D-341 MED, ≤ 0.001). The beneficial effects of MnTnHex-2-PyP5+ are presumably achieved either (1) indirectly via elimination of signaling reactive oxygen and nitrogen species (in particular superoxide and peroxynitrite) which in turn would prevent activation of transcription factors; or (2) directly by coupling with cellular reductants and redox-sensitive signaling proteins. The former action is antioxidative while the latter action is presumably pro-oxidative in nature. Our findings suggest that the use of Mn porphyrin-based SOD mimics, and in particular lipophilic analogues such as MnTnHex-2-PyP5+, is a promising approach for brain tumor therapy.

Cancer cells are particularly vulnerable to treatments impairing redox homeostasis. Reactive oxygen species (ROS) can indeed play an important role in the initiation and progression of cancer, and advanced stage tumors frequently exhibit high basal levels of ROS that stimulate cell proliferation and promote genetic instability. In addition, an inverse correlation between histological grade and antioxidant enzyme activities is frequently observed in human tumors, further supporting the existence of a redox dysregulation in cancer cells. This biochemical property can be exploited by using redox-modulating compounds, which represent an interesting approach to induce cancer cell death. Thus, we have developed a new strategy based on the use of pharmacologic concentrations of ascorbate and redox-active quinones. Ascorbate-driven quinone redox cycling leads to ROS formation and provokes an oxidative stress that preferentially kills cancer cells and spares healthy tissues. Cancer cell death occurs through necrosis and the underlying mechanism implies an energetic impairment (ATP depletion) that is likely due to glycolysis inhibition. Additional mechanisms that participate to cell death include calcium equilibrium impairment and oxidative cleavage of protein chaperone Hsp90. Given the low systemic toxicity of ascorbate and the impairment of crucial survival pathways when associated with redox-active quinones, these combinations could represent an original approach that could be combined to standard cancer therapy.

Texaphyrins, a class of tumor selective expanded porphyrins capable of coordinating large metals, have been found to act as redox mediators within biological systems. This review summarizes studies involving their experimentaluse in cancer chemotherapy. Mechanistic insights involving their presumed mode of action are also described, as well as certain structure activity relationships. Finally, newer texaphyrin-based applications associated with targeted drug delivery are presented.

Reactive oxygen species (ROS) are considered to be a main cause for cancer development, but they can also be used for cancer eradication. Because of this dual nature of ROS action, both antioxidant and prooxidant therapeutic agents have been developed and some have shown clinical promise. Selective uptake of porphyrins by malignant cells has for a long time been used for tumor imaging and for targeted delivery of ROS. Redox-active Mn porphyrins can act both as antioxidants and as prooxidants, and may thus be used in anticancer therapy. Porphyrins, which chelate redox inactive metals, for example Zn, demonstrate photo-sensitizing activity and thus can produce singlet oxygen and other reactive oxygen species in cancer cells on irradiation with visible light. Here we review the properties of Zn(II) N-alkylpyridylporphyrin-based photosensitizers, and their ability to damage selected cellular targets.

A concept that currently steers the development of cancer therapies has been that agents directed against specific proteins that facilitate tumorigenesis or maintain a malignant phenotype will have greater efficacy, less toxicity and a more sustained response relative to traditional cytotoxic chemotherapeutic agents. The clinical success of the targeted agent Imatinib mesylate as an inhibitor of the tyrosine kinase associated with the breakpoint cluster region-Abelson oncogene locus (BCR-ABL) in the treatment of Philadelphiapositive chronic myelogenous leukemia (CML) has served as a paradigm. While intellectually gratifying, the selective targeting of a single driver event by a small molecule, e.g., kinase inhibitor, to dampen a tumor-promoting pathway in the treatment of solid tumors is limited by many factors. Focus can alternatively be placed on targeting fundamental cellular processes that regulate multiple events, e.g., protein degradation, through the Ubiquitin (Ub)+Proteasome System (UPS). The UPS plays a critical role in modulating numerous cellular proteins to regulate cellular processes such as signal transduction, growth, proliferation, differentiation and apoptosis. Clinical success with the proteasome inhibitor bortezomib revolutionized treatment of B-cell lineage malignancies such as Multiple Myeloma (MM). However, many patients harbor primary resistance and do not respond to bortezomib and those that do respond inevitably develop resistance (secondary resistance). The lack of clinical efficacy of proteasome inhibitors in the treatment of solid tumors may be linked mechanistically to the resistance detected during treatment of hematologic malignancies. Potential mechanisms of resistance and means to improve the response to proteasome inhibitors in solid tumors are discussed.