Anti-Cancer Agents in Medicinal Chemistry (v.11, #4)

As a continuation of Part I, herein Garry Buettner [1] addressed the critical role of manganese superoxide dismutase, MnSOD - theenzyme that we cannot live without. MnSOD is a central player in the redox biology of cells and tissues [1]. It is critical for establishing theappropriate balance in redox circuitry of the mitochondria. Thus, there has been an increasing demand to develop powerful SOD mimics. Inthe studies where MnSOD was overexpressed, the increased levels of H2O2 were found, which suggests that H2O2 has a major role inmetastases. The loss of MnSOD is likely an early event in tumor progression allowing for further propagation of the tumorigenic phenotyperesulting 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 stillcontroversial origin of the increased peroxide as a direct or indirect consequence of MnSOD overexpression. More work is needed to gain aprofound insight into the dichotomous role of MnSOD as a tumor suppressor or oncogene [2]. Similarly, much is to be learned about redoxbasedcompounds 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 astumor suppressors as exemplified with Mn porphyrins in contribution from Keir et al., [6], corroles by Zeev Gross group [7], texaphyrins byJonathan Sessler group [8], nitrones by Robert Floyd et al., [9], and quinones by Pedro Buc Calderon group [10]. However, the mechanism ofaction 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, Mnporphyrins suppressed E. coli growth via cytotoxic H2O2 production. Under milder conditions and in a rich growing medium, over time theadaptive 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 datacaution 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 itsderivative, 2,4-disulfophenyl-tert-butylnitrone (OKN-007, formerly known as NXY-059 and developed earlier for stroke therapy [9]). Theionic 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 candidatefor 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, aswell as free radical scavenging, may play a role. Nitrones were previously shown to exert general anti-inflammatory effects. The exacerbatedinflammatory cellular signaling processes are suppressed by their administration [9]. The inhibition of NF-κB master transcription factor wasreported with PBN, and needs to be tested with OKN-007 [9]. Mn porphyrin, MnTnHex-2-PyP5+ has been shown in this Issue [6] to exertanticancer effect in mouse glioma study. Both Mn porphyrin and nitrones have been shown to affect cellular transcriptional activity as theyinhibit NF-kB activation [6,9]. Both also scavenge reactive species [6,9]. It may be challenging, from therapeutic and mechanisticperspective, 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 SusanDoctrow 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+) havebeen 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 vitroand in vivo. Given the in vivo abundance of cellular reductants, MnPs can couple with them in removing superoxide. Thus, they could bereadily 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 hydrogenperoxide 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 cellularlocalization/bioavailibility. To exemplify the switch from an anti- to pro-oxidative action we have explored a very simple and straightforwardsystem - the superoxide-specific aerobic growth of SOD-deficient E. coli. In such a system, cationic MnPs, ortho and metaMnTE-2-(or 3)-PyP5+ act as powerful SOD mimics. Yet, in the presence of exogenous ascorbate, the SOD mimics catalyze the H2O2production, causing oxidative damage to both wild and SOD-deficient strains and inhibiting their growth. Catalase added to the mediumreversed the effect indicating that H2O2 is a major damaging/signaling species involved in cell growth suppression. The experiments withoxyR- and soxRS-deficient E. coli were conducted to show that E. coli responds to increased oxidative stress exerted by MnP/ascorbatesystem 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 bedamaging. Indeed SOD blunts the cascade of oxidations initiated by superoxide. However in the late 1970’s it was observed that cancercells that have low activity of the mitochondrial form of SOD, MnSOD, grow faster than those with higher activities of MnSOD. Theseobservations indicated that SOD, superoxide, and hydrogen peroxide affected the basic biology of cells and tissues, not just via damagingoxidation 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 as Cancer Imaging Agents by Ryan M. Davis, James B. Mitchell, Murali C. Krishna (347-358).
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 electronparamagnetic resonance imaging as well as contrast agents for magnetic resonance imaging. These imaging techniques enable in vivomonitoring of nitroxide metabolism. In biological systems, nitroxide metabolism occurs predominantly via reduction of the nitroxide toa hydroxylamine. The rate of nitroxide reduction can increase or decrease due to oxidative stress, suggesting that nitroxides can providean 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 therapeuticapplications and bio-reduction mechanisms are discussed in the context of their relevance to oncology.

Salen Mn Complexes Mitigate Radiation Injury in Normal Tissues by Rosalind A. Rosenthal, Brian Fish, Richard P. Hill, Karl D. Huffman, Zelmira Lazarova, Javed Mahmood, Meetha Medhora, Robert Molthen, John E. Moulder, Stephen T. Sonis, Philip J. Tofilon, Susan R. Doctrow (359-372).
Salen Mn complexes, including EUK-134, EUK-189 and a newer cyclized analog EUK-207, are synthetic SOD/catalasemimetics that have beneficial effects in many models of oxidative stress. As oxidative stress is implicated in some forms of delayedradiation injury, we are investigating whether these compounds can mitigate injury to normal tissues caused by ionizing radiation. Thisreview 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 anantioxidant mechanism of action is postulated, it is likely that the mechanisms of radiation mitigation by these compounds in vivo arecomplex 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, accidentalexposure, or radiological terrorism. Optimization of their mode of delivery and other key pharmaceutical properties, and increasingunderstanding of their mechanism(s) of action as radiation mitigators, are key issues for future study.

Anti-Cancer Activity of Nitrones and Observations on Mechanism of Action by Robert A. Floyd, Hema K. Chandru, Ting He, Rheal Towner (373-379).
The nitrone compound PBN, α-phenyl-tert-butylnitrone, and closely related nitrones have anti-cancer activity in severalexperimental cancer models. The three experimental models most extensively studied include A) the rat choline deficiency liver cancermodel, B) the rat C6 glioma model and C) the mouse APCMin/+ colon cancer model. The two PBN-nitrones mostly studied are PBN and aPBN derivative 2,4-disulfophenyl-tert-butylnitrone, referred as OKN-007. OKN-007 is a proprietary compound that has had extensivecommercial development (designated as NXY-059) for another indication, acute ischemic stroke, and after extensive clinical studies wasshown to lack efficacy for this indication but was shown to be very safe for human use. This compound administered orally in the ratglioma model has potent activity in treating fully formed gliomas. In this report observations made on the PBN-nitrones in experimentalcancer models will be summarized. In addition the experimental results will be discussed in the general framework of the properties ofthe compounds with a view to try to understand the mechanistic basis of how the PBN-nitrones act as anti-cancer agents. Possiblemechanisms related to the suppression of NO production, S-nitrosylation of critical proteins and inhibition of NF-κB activation arediscussed.

Cellular Uptake and Organ Accumulation of Amphipolar Metallocorroles with Cytoprotective and Cytotoxic Properties by Zoya Okun, Lana Kupershmidt, Moussa B. H. Youdim, Zeev Gross (380-384).
We report here an investigation that focuses on the organ distribution of metal complexes that are chelated by the amphipolarcorrole whose macrocycle is decorated by two sulphonic acid head groups, which are emerging potential therapeutics against cancer (thecytotoxic 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 primeimportance for future in vivo studies on disease models, as they point toward a large utility of this kind of corrole chelates for treatingcancer, neurodegenerative diseases characterized by “leaking BBB”, cardiovascular diseases and diabetes.

Bioactive Sphingolipids in Response to Chemotherapy: A Scope on Leukemias by Huseyin Atakan Ekiz, Yusuf Baran (385-397).
Sphingolipids are major constituents of the cells with emerging roles in the regulation of cellular processes. Deregulation ofsphingolipid 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 thebioactive sphingolipid species that have important roles in the regulation of cell death, survival and chemotherapeutic resistance. Some ofthose species are known to accumulate in the cells upon chemotherapy while some others are known to exhibit an opposite pattern. Eventhough the length of fatty acid chain has a deterministic effect, in general, upregulation of ceramides and sphingosine is known to induceapoptosis. 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 toincrease the effectiveness of the chemotherapeutic drugs currently in hand. Some approaches involve manipulation of the synthesispathways yielding the increased production of apoptotic sphingolipids while the proliferative ones are suppressed. Some others are tryingto take advantage of cytotoxic sphingolipids like short chain ceramide analogs by directly delivering them to the malignant cells as adistinct chemotherapeutic intervention. Numerous studies in the literature show the feasibility of those approaches especially in acute andchronic leukemias. This review compiles the current knowledge about sphingolipids and their roles in chemotherapeutic response withthe particular attention to leukemias.

Molecular Mechanisms of Anti-cancer Action of Garlic Compounds in Neuroblastoma by Surajit Karmakar, Subhasree Roy Choudhury, Naren L. Banik, Swapan K. Ray (398-407).
The medicinal properties of garlic (Allium sativum) have been well known and widely used since historical times. Garliccompounds have received increasing attention during the last few years due to their cancer chemopreventive properties. The anti-canceractivity of garlic-derived organosulfur compounds (OSCs) are extensively reported in many cancers but only a few in the pediatric tumorneuroblastoma, which warrants exploration of new therapy for its management. There are some recent reports suggesting that garlicderivedOSCs cause cell cycle arrest, generate reactive oxygen species (ROS), activate stress kinases, and also stimulate the mitochondrialpathway for apoptosis in malignant neuroblastoma. The comprehensive mechanisms of anti-cancer action of OSCs still remainunclear and require more studies in neuroblastoma. This review is designed to highlight the known molecular mechanisms of anti-canceractions of garlic-derived OSCs in neuroblastoma and as well as in several other cancers. Further studies should be conducted to establishthe clinical expediency of garlic-derived OSCs for treatment of malignant neuroblastoma in humans.