Anti-Cancer Agents in Medicinal Chemistry (Formerly Current Medicinal Chemistry - Anti-Cancer Agents) - Volume 11, Issue 4, 2011

As a continuation of Part I, herein Garry Buettner [1] addressed the critical role of manganese superoxide dismutase, MnSOD - the enzyme that we cannot live without. MnSOD is a central player in the redox biology of cells and tissues [1]. It is critical for establishing the appropriate balance in redox circuitry of the mitochondria. Thus, there has been an increasing demand to develop powerful SOD mimics. In the studies where MnSOD was overexpressed, the increased levels of H2O2 were found, which suggests that H2O2 has a major role in metastases. The loss of MnSOD is likely an early event in tumor progression allowing for further propagation of the tumorigenic phenotype resulting from the steady state increases in free radical production [2]. Garry Buettner contribution, and manuscripts by Irwin Fridovich [3] and Lee Ann MacMillan Crow and John Crow [4], as well as from Melendez [2] and St. Clair [5] groups addressed the possible, and still controversial origin of the increased peroxide as a direct or indirect consequence of MnSOD overexpression. More work is needed to gain a profound insight into the dichotomous role of MnSOD as a tumor suppressor or oncogene [2]. Similarly, much is to be learned about redoxbased compounds developed originally as SOD mimics. Major classes of such compounds are addressed in this Issue: Mn porphyrins, metallotexaphyrins, Mn salens, metallocorroles, nitroxides, nitrones, and quinones. Many were shown to exert anticancer effects, acting as tumor suppressors as exemplified with Mn porphyrins in contribution from Keir et al., [6], corroles by Zeev Gross group [7], texaphyrins by Jonathan Sessler group [8], nitrones by Robert Floyd et al., [9], and quinones by Pedro Buc Calderon group [10]. However, the mechanism of action of those compounds is not yet fully understood. Cationic Mn(III) N-substituted pyridylporphyrins are potent SOD mimics and protect SOD-deficient E. coli when it grows aerobically. Ines Batinic-Haberle and Ludmil Benov groups showed that in the presence of cellular reductant ascorbate, which is abundant in vivo, Mn porphyrins suppressed E. coli growth via cytotoxic H2O2 production. Under milder conditions and in a rich growing medium, over time the adaptive response of E. coli was observed, whereby oxyR regulon was induced and endogenous antioxidants - peroxide-removing enzymes, peroxidases and catalases upregulated [11]. The data exemplify how a pro-oxidative event could exert antioxidative effects [11]. Such data caution us to differentiate between the nature of the actions of synthetic antioxidants and the type of the effects observed. A manuscript by Robert Floyd et al., [9] discusses the potent anticancer effects of nitrones observed in three experimental cancer models: (1) the rat choline-deficiency liver cancer model; (2) the rat C6 glioma model; and (3) the mouse APCMin/+ colon cancer model. Originally, nitrones were developed as spin traps for free radicals. The two mostly studied nitrones are α-phenyl-tert-butylnitrone (PBN) and its derivative, 2,4-disulfophenyl-tert-butylnitrone (OKN-007, formerly known as NXY-059 and developed earlier for stroke therapy [9]). The ionic PBN derivative was shown to cause shrinkage of fully formed tumors in the rat C6 glioma model. The extensive human safety studies, showing that it is a safe compound to use, combined with its demonstrated potency to decrease the size of tumors, makes it an ideal candidate for clinical trials, which Robert Floyd is currently pursuing. Again, the mechanism of action has not yet been fully understood. The decreased .NO production due to the PBN-mediated suppression of iNOS expression, S-nitrosylation of critical proteins such as caspases and Bcl-2, as well as free radical scavenging, may play a role. Nitrones were previously shown to exert general anti-inflammatory effects. The exacerbated inflammatory cellular signaling processes are suppressed by their administration [9]. The inhibition of NF-κB master transcription factor was reported with PBN, and needs to be tested with OKN-007 [9]. Mn porphyrin, MnTnHex-2-PyP5+ has been shown in this Issue [6] to exert anticancer effect in mouse glioma study. Both Mn porphyrin and nitrones have been shown to affect cellular transcriptional activity as they inhibit NF-κB activation [6,9]. Both also scavenge reactive species [6,9]. It may be challenging, from therapeutic and mechanistic perspective, to compare Mn porphyrin action to the anticancer effect of nitrones in the same glioma cell line. The other papers in Part II of this Issue relate to three different classes of redox-active compounds developed originally as SOD mimics. The metal-containing compounds are Mn(III) corroles developed by Zeev Gross group [7], and Mn(III) salen derivatives developed by Susan Doctrow group [12]. The third group comprises the nitroxides, described by Ryan Davies et al., [13]......

Cationic Mn(III) N-alkylpyridyl (MnTalkyl-2(or 3)-PyP5+) and N, N'-dialkylimidazolylporphyrins (MnTDalkyl-2-ImP5+) have been regarded as the most powerful SOD mimics/peroxynitrite scavengers - i. e. antioxidants. The ethyl-, MnTE-2-PyP5+ (AEOL10113), and hexylpyridyl-, MnTnHex-2-PyP5+ and diethylimidazolylporphyrin, MnTDE-2-ImP5+ (AEOL10150) have been mostly studied in vitro and in vivo. Given the in vivo abundance of cellular reductants, MnPs can couple with them in removing superoxide. Thus, they could be readily reduced from MnIIIP to MnIIP with ascorbate and glutathione, and in a subsequent step reduce either O2 .- (while acting as superoxide reductase) or oxygen (while exerting pro-oxidative action). Moreover, MnPs can catalyze ascorbate oxidation and in turn hydrogen peroxide production. The in vivo type of MnP action (anti- or pro-oxidative) will depend upon the cellular levels of reactive species, endogenous antioxidants, availability of oxygen, ratio of O2 .-- to peroxide-removing systems, redox ability of MnPs and their cellular localization/bioavailibility. To exemplify the switch from an anti- to pro-oxidative action we have explored a very simple and straightforward system - the superoxide-specific aerobic growth of SOD-deficient E. coli. In such a system, cationic MnPs, ortho and meta MnTE-2-(or 3)-PyP5+ act as powerful SOD mimics. Yet, in the presence of exogenous ascorbate, the SOD mimics catalyze the H2O2 production, causing oxidative damage to both wild and SOD-deficient strains and inhibiting their growth. Catalase added to the medium reversed the effect indicating that H2O2 is a major damaging/signaling species involved in cell growth suppression. The experiments with oxyR- and soxRS-deficient E. coli were conducted to show that E. coli responds to increased oxidative stress exerted by MnP/ascorbate system by induction of oxyR regulon and thus upregulation of antioxidative defenses such as catalases and peroxidases. As anticipated, when catalase was added into medium to remove H2O2, E. coli did not respond with upregulation of its own antioxidant systems.

Superoxide dismutases (SOD) are considered to be antioxidant enzymes. This view came about because its substrate, superoxide, is a free radical; in the era of their discovery, 1960's - 1970's, the general mindset was that free radicals in biology must be damaging. Indeed SOD blunts the cascade of oxidations initiated by superoxide. However in the late 1970's it was observed that cancer cells that have low activity of the mitochondrial form of SOD, MnSOD, grow faster than those with higher activities of MnSOD. These observations indicated that SOD, superoxide, and hydrogen peroxide affected the basic biology of cells and tissues, not just via damaging oxidation reactions. It is now realized that superoxide and hydrogen peroxide are essential for normal cellular and organism function. MnSOD appears to be a central player in the redox biology of cells and tissues.

Nitroxides are low molecular weight (150-400 Da) superoxide dismutase mimics that exhibit antioxidant, radical scavenging, and radioprotective activity. Additionally, the paramagnetic nature of nitroxides makes them viable as both spin probes for electron paramagnetic resonance imaging as well as contrast agents for magnetic resonance imaging. These imaging techniques enable in vivo monitoring of nitroxide metabolism. In biological systems, nitroxide metabolism occurs predominantly via reduction of the nitroxide to a hydroxylamine. The rate of nitroxide reduction can increase or decrease due to oxidative stress, suggesting that nitroxides can provide an imaging-based assay of tissue redox status. The current review briefly summarizes the potential clinical applications of nitroxides, and focuses on the biochemical and tumor microenvironmental factors that affect the rate of nitroxide reduction. The potential therapeutic applications and bio-reduction mechanisms are discussed in the context of their relevance to oncology.

Salen Mn complexes, including EUK-134, EUK-189 and a newer cyclized analog EUK-207, are synthetic SOD/catalase mimetics that have beneficial effects in many models of oxidative stress. As oxidative stress is implicated in some forms of delayed radiation injury, we are investigating whether these compounds can mitigate injury to normal tissues caused by ionizing radiation. This review describes some of this research, focusing on several tissues of therapeutic interest, namely kidney, lung, skin, and oral mucosa. These studies have demonstrated suppression of delayed radiation injury in animals treated with EUK-189 and/or EUK-207. While an antioxidant mechanism of action is postulated, it is likely that the mechanisms of radiation mitigation by these compounds in vivo are complex and may differ in the various target tissues. Indicators of oxidative stress are increased in lung and skin radiation injury models, and suppressed by salen Mn complexes. The role of oxidative stress in the renal injury model is unclear, though EUK-207 does mitigate. In certain experimental models, salen Mn complexes have shown “mito-protective” properties, that is, attenuating mitochondrial injury. Consistent with this, EUK-134 suppresses effects of ionizing radiation on mitochondrial function in rat astrocyte cultures. In summary, salen Mn complexes could be useful to mitigate delayed radiation injury to normal tissues following radiation therapy, accidental exposure, or radiological terrorism. Optimization of their mode of delivery and other key pharmaceutical properties, and increasing understanding of their mechanism(s) of action as radiation mitigators, are key issues for future study.

The nitrone compound PBN, α-phenyl-tert-butylnitrone, and closely related nitrones have anti-cancer activity in several experimental cancer models. The three experimental models most extensively studied include A) the rat choline deficiency liver cancer model, B) the rat C6 glioma model and C) the mouse APCMin/+ colon cancer model. The two PBN-nitrones mostly studied are PBN and a PBN derivative 2,4-disulfophenyl-tert-butylnitrone, referred as OKN-007. OKN-007 is a proprietary compound that has had extensive commercial development (designated as NXY-059) for another indication, acute ischemic stroke, and after extensive clinical studies was shown to lack efficacy for this indication but was shown to be very safe for human use. This compound administered orally in the rat glioma model has potent activity in treating fully formed gliomas. In this report observations made on the PBN-nitrones in experimental cancer models will be summarized. In addition the experimental results will be discussed in the general framework of the properties of the compounds with a view to try to understand the mechanistic basis of how the PBN-nitrones act as anti-cancer agents. Possible mechanisms related to the suppression of NO production, S-nitrosylation of critical proteins and inhibition of NF-κB activation are discussed.

We report here an investigation that focuses on the organ distribution of metal complexes that are chelated by the amphipolar corrole whose macrocycle is decorated by two sulphonic acid head groups, which are emerging potential therapeutics against cancer (the cytotoxic Ga chelate) and diseases that are characterized by excessive production of ROS and RNS (the cytoprotective Mn and Fe derivatives). We show that the intraperitoneally injected fluorescent gallium(III) derivative accumulates in tissues sections of the kidney, liver, lung, heart, and pancreas. It also reaches the brain blood vessels, but does not cross the blood brain barrier. These findings are of prime importance for future in vivo studies on disease models, as they point toward a large utility of this kind of corrole chelates for treating cancer, neurodegenerative diseases characterized by “leaking BBB”, cardiovascular diseases and diabetes.

Sphingolipids are major constituents of the cells with emerging roles in the regulation of cellular processes. Deregulation of sphingolipid metabolism is reflected as various pathophysiological conditions including metabolic disorders and several forms of cancer. Ceramides, ceramide-1-phosphate (C1P), glucosyl ceramide (GluCer), sphingosine and sphingosine-1-phosphate (S1P) are among the bioactive sphingolipid species that have important roles in the regulation of cell death, survival and chemotherapeutic resistance. Some of those species are known to accumulate in the cells upon chemotherapy while some others are known to exhibit an opposite pattern. Even though the length of fatty acid chain has a deterministic effect, in general, upregulation of ceramides and sphingosine is known to induce apoptosis. However, S1P, C1P and GluCer are proliferative for cells and they are involved in the development of chemoresistance. Therefore, sphingolipid metabolism appears as a good target for the development of novel therapeutics or supportive interventions to increase the effectiveness of the chemotherapeutic drugs currently in hand. Some approaches involve manipulation of the synthesis pathways yielding the increased production of apoptotic sphingolipids while the proliferative ones are suppressed. Some others are trying to take advantage of cytotoxic sphingolipids like short chain ceramide analogs by directly delivering them to the malignant cells as a distinct chemotherapeutic intervention. Numerous studies in the literature show the feasibility of those approaches especially in acute and chronic leukemias. This review compiles the current knowledge about sphingolipids and their roles in chemotherapeutic response with the particular attention to leukemias.

The medicinal properties of garlic (Allium sativum) have been well known and widely used since historical times. Garlic compounds have received increasing attention during the last few years due to their cancer chemopreventive properties. The anti-cancer activity of garlic-derived organosulfur compounds (OSCs) are extensively reported in many cancers but only a few in the pediatric tumor neuroblastoma, which warrants exploration of new therapy for its management. There are some recent reports suggesting that garlicderived OSCs cause cell cycle arrest, generate reactive oxygen species (ROS), activate stress kinases, and also stimulate the mitochondrial pathway for apoptosis in malignant neuroblastoma. The comprehensive mechanisms of anti-cancer action of OSCs still remain unclear and require more studies in neuroblastoma. This review is designed to highlight the known molecular mechanisms of anti-cancer actions of garlic-derived OSCs in neuroblastoma and as well as in several other cancers. Further studies should be conducted to establish the clinical expediency of garlic-derived OSCs for treatment of malignant neuroblastoma in humans.