Nitrosylation of manganese(II) tetrakis(N-ethylpyridinium-2-yl)porphyrin: a simple and sensitive spectrophotometric assay for nitric oxide

Azanone (HNO): generation, stabilization and detection

HNO (nitroxyl, azanone), joined the 'biologically relevant reactive nitrogen species' family in the 2000s. Azanone is impossible to store due to its high reactivity and inherent low stability. Consequently, its chemistry and effects are studied using donor compounds, which release this molecule in solution and in the gas phase upon stimulation. Researchers have also tried to stabilize this elusive species and its conjugate base by coordination to metal centers using several ligands, like metalloporphyrins and pincer ligands. Given HNO's high reactivity and short lifetime, several different strategies have been proposed for its detection in chemical and biological systems, such as colorimetric methods, EPR, HPLC, mass spectrometry, fluorescent probes, and electrochemical analysis. These approaches are described and critically compared. Finally, in the last ten years, several advances regarding the possibility of endogenous HNO generation were made; some of them are also revised in the present work.

Ortho Isomeric Mn(III) N-Alkyl- and Alkoxyalkylpyridylporphyrins-Enhancers of Hyaluronan Degradation Induced by Ascorbate and Cupric Ions

High levels of hyaluronic acid (HA) in tumors correlate with poor outcomes with several types of cancers due to HA-driven support of adhesion, migration and proliferation of cells. In this study we explored how to enhance the degradation of HA into low-molecular fragments, which cannot prevent the immune system to fight tumor proliferation and metastases. The physiological solution of HA was exposed to oxidative degradation by ascorbate and cupric ions in the presence of either one of three ortho isomeric Mn(III) substituted N-alkyl- and alkoxyalkylpyridylporphyrins or para isomeric Mn(III) N-methylpyridyl analog, commonly known as mimics of superoxide dismutase. The changes in hyaluronan degradation kinetics by four Mn(III) porphyrins were monitored by measuring the alteration in the dynamic viscosity of the HA solution. The ortho compounds MnTE-2-PyP⁵⁺ (BMX-010, AEOL10113), MnTnBuOE-2-PyP⁵⁺ (BMX-001) and MnTnHex-2-PyP⁵⁺ are able to redox cycle with ascorbate whereby producing H₂O₂ which is subsequently coupled with Cu(I) to produce the ^•OH radical essential for HA degradation. Conversely, with the para analog, MnTM-4-PyP⁵⁺, no catalysis of HA degradation was demonstrated, due to its inertness towards redox cycling with ascorbate. The impact of different Mn(III)-porphyrins on the HA decay was further clarified by electron paramagnetic resonance spectrometry. The ability to catalyze the degradation of HA in a biological milieu, in the presence of cupric ions and ascorbate under the conditions of high tumor oxidative stress provides further insight into the anticancer potential of redox-active ortho isomeric Mn(III) porphyrins.

Molecular Magnetic Resonance Imaging of Nitric Oxide in Biological Systems

Detection of nitric oxide (NO) in biological systems is challenging due to both physicochemical properties of NO and limitations of current imaging modalities and probes. Magnetic resonance imaging (MRI) could be applied for studying NO in living tissue with high spatiotemporal resolution, but there is still a need for chemical agents that effectively sensitize MRI to biological NO production. To develop a suitable probe, we studied the interactions between NO and a library of manganese complexes with various oxidation states and molecular structures. Among this set, the manganese(III) complex with N,N'-(1,2-phenylene)bis(5-fluoro-2-hydroxybenzamide) showed favorable changes in longitudinal relaxivity upon addition of NO-releasing chemicals in vitro while also maintaining selectivity against other biologically relevant reactive nitrogen and oxygen species, making it a suitable NO-responsive contrast agent for T₁-weighted MRI. When loaded with this compound, cells ectopically expressing nitric oxide synthase (NOS) isoforms showed MRI signal decreases of over 20% compared to control cells and were also responsive to NOS inhibition or calcium-dependent activation. The sensor could also detect endogenous NOS activity in antigen-stimulated macrophages and in a rat model of neuroinflammation in vivo. Given the key role of NO and associated reactive nitrogen species in numerous physiological and pathological processes, MRI approaches based on the new probe could be broadly beneficial for studies of NO-related signaling in living subjects.

Manganese Porphyrin-Based SOD Mimetics Produce Polysulfides from Hydrogen Sulfide

Manganese-centered porphyrins (MnPs), MnTE-2-PyP⁵⁺ (MnTE), MnTnHex-2-PyP⁵⁺ (MnTnHex), and MnTnBuOE-2-PyP⁵⁺ (MnTnBuOE) have received considerable attention because of their ability to serve as superoxide dismutase (SOD) mimetics thereby producing hydrogen peroxide (H₂O₂), and oxidants of ascorbate and simple aminothiols or protein thiols. MnTE-2-PyP⁵⁺ and MnTnBuOE-2-PyP⁵⁺ are now in five Phase II clinical trials warranting further exploration of their rich redox-based biology. Previously, we reported that SOD is also a sulfide oxidase catalyzing the oxidation of hydrogen sulfide (H₂S) to hydrogen persulfide (H₂S₂) and longer-chain polysulfides (H₂S_n, n = 3–7). We hypothesized that MnPs may have similar actions on sulfide metabolism. H₂S and polysulfides were monitored in fluorimetric assays with 7-azido-4-methylcoumarin (AzMC) and 3′,6′-di(O-thiosalicyl)fluorescein (SSP4), respectively, and specific polysulfides were further identified by mass spectrometry. MnPs concentration-dependently consumed H₂S and produced H₂S₂ and subsequently longer-chain polysulfides. This reaction appeared to be O₂-dependent. MnP absorbance spectra exhibited wavelength shifts in the Soret and Q bands characteristic of sulfide-mediated reduction of Mn. Taken together, our results suggest that MnPs can become efficacious activators of a variety of cytoprotective processes by acting as sulfide oxidation catalysts generating per/polysulfides.

Thiol regulation by Mn porphyrins, commonly known as SOD mimics

Superoxide dismutases play an important role in human health and disease. Three decades of effort have gone into synthesizing SOD mimics for clinical use. The result is the Mn porphyrins which have SOD-like activity. Several clinical trials are underway to test the efficacy of these compounds in patients, particularly as radioprotectors of normal tissue during cancer treatment. However, aqueous chemistry data indicate that the Mn porphyrins react equally well with multiple redox active species in cells including H2O2, O2•-, ONOO-, thiols, and ascorbate among others. The redox potential of the Mn porphyrins is midway between the potentials for the oxidation and reduction of O2•-. This positions them to react equally well as oxidants and reductants in cells. The result of this unique chemistry is that: 1) the species the Mn porphyrins react with in vivo will depend on the relative concentrations of the reactive species and Mn porphyrins in the cell of interest, and 2) the Mn porphyrins will act as catalytic (redox cycling) agents in vivo. The ability of the Mn porphyrins to catalyze protein S-glutathionylation means that Mn porphyrins have the potential to globally modulate cellular redox regulatory signaling networks. The purpose of this review is to summarize the data that indicate the Mn porphyrins have diverse reactions in vivo that are the basis of the observed biological effects. The ability to catalyze multiple reactions in vivo expands the potential therapeutic use of the Mn porphyrins to disease models that are not SOD based.

Mn Porphyrin-Based Redox-Active Drugs: Differential Effects as Cancer Therapeutics and Protectors of Normal Tissue Against Oxidative Injury

SIGNIFICANCE

After approximatelty three decades of research, two Mn(III) porphyrins (MnPs), MnTE-2-PyP⁵⁺ (BMX-010, AEOL10113) and MnTnBuOE-2-PyP⁵⁺ (BMX-001), have progressed to five clinical trials. In parallel, another similarly potent metal-based superoxide dismutase (SOD) mimic-Mn(II)pentaaza macrocycle, GC4419-has been tested in clinical trial on application, identical to that of MnTnBuOE-2-PyP⁵⁺-radioprotection of normal tissue in head and neck cancer patients. This clearly indicates that Mn complexes that target cellular redox environment have reached sufficient maturity for clinical applications. Recent Advances: While originally developed as SOD mimics, MnPs undergo intricate interactions with numerous redox-sensitive pathways, such as those involving nuclear factor κB (NF-κB) and nuclear factor E2-related factor 2 (Nrf2), thereby impacting cellular transcriptional activity. An increasing amount of data support the notion that MnP/H₂O₂/glutathione (GSH)-driven catalysis of S-glutathionylation of protein cysteine, associated with modification of protein function, is a major action of MnPs on molecular level.

CRITICAL ISSUES

Differential effects of MnPs on normal versus tumor cells/tissues, which support their translation into clinic, arise from differences in their accumulation and redox environment of such tissues. This in turn results in different yields of MnP-driven modifications of proteins. Thus far, direct evidence for such modification of NF-κB, mitogen-activated protein kinases (MAPK), phosphatases, Nrf2, and endogenous antioxidative defenses was provided in tumor, while indirect evidence shows the modification of NF-κB and Nrf2 translational activities by MnPs in normal tissue.

FUTURE DIRECTIONS

Studies that simultaneously explore differential effects in same animal are lacking, while they are essential for understanding of extremely intricate interactions of metal-based drugs with complex cellular networks of normal and cancer cells/tissues.

Manganese porphyrin redox state in endothelial cells: Resonance Raman studies and implications for antioxidant protection towards peroxynitrite

Cationic manganese(III) ortho N-substituted pyridylporphyrins (MnP) act as efficient antioxidants catalyzing superoxide dismutation and accelerating peroxynitrite reduction. Importantly, MnP can reach mitochondria offering protection against reactive species in different animal models of disease. Although an LC-MS/MS-based method for MnP quantitation and subcellular distribution has been reported, a direct method capable of evaluating both the uptake and the redox state of MnP in living cells has not yet been developed. In the present work we applied resonance Raman (RR) spectroscopy to analyze the intracellular accumulation of two potent MnP-based lipophilic SOD mimics, MnTnBuOE-2-PyP⁵⁺ and MnTnHex-2-PyP⁵⁺ within endothelial cells. RR experiments with isolated mitochondria revealed that the reduction of Mn(III)P was affected by inhibitors of the electron transport chain, supporting the action of MnP as efficient redox active compounds in mitochondria. Indeed, RR spectra confirmed that MnP added in the Mn(III) state can be incorporated into the cells, readily reduced by intracellular components to the Mn(II) state and oxidized by peroxynitrite. To assess the combined impact of reactivity and bioavailability, we studied the kinetics of Mn(III)TnBuOE-2-PyP⁵⁺ with peroxynitrite and evaluated the cytoprotective capacity of MnP by exposing the endothelial cells to nitro-oxidative stress induced by peroxynitrite. We observed a preservation of normal mitochondrial function, attenuation of cell damage and prevention of apoptotic cell death. These data introduce a novel application of RR spectroscopy for the direct detection of MnP and their redox states inside living cells, and helps to rationalize their antioxidant capacity in biological systems.

CNS bioavailability and radiation protection of normal hippocampal neurogenesis by a lipophilic Mn porphyrin-based superoxide dismutase mimic, MnTnBuOE-2-PyP5

Although radiation therapy can be effective against cancer, potential damage to normal tissues limits the amount that can be safely administered. In central nervous system (CNS), radiation damage to normal tissues is presented, in part, as suppressed hippocampal neurogenesis and impaired cognitive functions. Mn porphyrin (MnP)-based redox active drugs have demonstrated differential effects on cancer and normal tissues in experimental animals that lead to protection of normal tissues and radio- and chemo-sensitization of cancers. To test the efficacy of MnPs in CNS radioprotection, we first examined the tissue levels of three different MnPs – MnTE-2-PyP⁵⁺(MnE), MnTnHex-2-PyP⁵⁺(MnHex), and MnTnBuOE-2-PyP⁵⁺(MnBuOE). Nanomolar concentrations of MnHex and MnBuOE were detected in various brain regions after daily subcutaneous administration, and MnBuOE was well tolerated at a daily dose of 3 mg/kg. Administration of MnBuOE for one week before cranial irradiation and continued for one week afterwards supported production and long-term survival of newborn neurons in the hippocampal dentate gyrus. MnP-driven S-glutathionylation in cortex and hippocampus showed differential responses to MnP administration and radiation in these two brain regions. A better understanding of how preserved hippocampal neurogenesis correlates with cognitive functions following cranial irradiation will be helpful in designing better MnP-based radioprotection strategies.

•

Bioavailability of MnPs in individual brain regions were determined by LC-MS/MS.

•

CNS MnBuOE and MnHex levels were between 15 and 160 nM after daily administration.

•

MnBuOE administration ameliorated radiation effects on hippocampal neurogenesis.

•

MnBuOE preserved categories E&F Dcx+ neurons after cranial irradiation.

•

MnBuOE and irradiation lead to changes in protein S-glutathionylation in the CNS.

Mn porphyrin-based SOD mimic, MnTnHex-2-PyP(5+), and non-SOD mimic, MnTBAP(3-), suppressed rat spinal cord ischemia/reperfusion injury via NF-κB pathways

Herein we have demonstrated that both superoxide dismutase (SOD) mimic, cationic Mn(III) meso-tetrakis(N-n-hexylpyridinium-2-yl)porphyrin (MnTnHex-2-PyP(5+)), and non-SOD mimic, anionic Mn(III) meso-tetrakis(4-carboxylatophenyl)porphyrin (MnTBAP(3-)), protect against oxidative stress caused by spinal cord ischemia/reperfusion via suppression of nuclear factor kappa B (NF-κB) pro-inflammatory pathways. Earlier reports showed that Mn(III) N-alkylpyridylporphyrins were able to prevent the DNA binding of NF-κB in an aqueous system, whereas MnTBAP(3-) was not. Here, for the first time, in a complex in vivo system-animal model of spinal cord injury-a similar impact of MnTBAP(3-), at a dose identical to that of MnTnHex-2-PyP(5+), was demonstrated in NF-κB downregulation. Rats were treated subcutaneously at 1.5 mg/kg starting at 30 min before ischemia/reperfusion, and then every 12 h afterward for either 48 h or 7 days. The anti-inflammatory effects of both Mn porphyrins (MnPs) were demonstrated in the spinal cord tissue at both 48 h and 7 days. The downregulation of NF-κB, a major pro-inflammatory signaling protein regulating astrocyte activation, was detected and found to correlate well with the suppression of astrogliosis (as glial fibrillary acidic protein) by both MnPs. The markers of oxidative stress, lipid peroxidation and protein carbonyl formation, were significantly reduced by MnPs. The favorable impact of both MnPs on motor neurons (Tarlov score and inclined plane test) was assessed. No major changes in glutathione peroxidase- and SOD-like activities were demonstrated, which implies that none of the MnPs acted as SOD mimic. Increasing amount of data on the reactivity of MnTBAP(3-) with reactive nitrogen species (RNS) (.NO/HNO/ONOO(-)) suggests that RNS/MnTBAP(3-)-driven modification of NF-κB protein cysteines may be involved in its therapeutic effects. This differs from the therapeutic efficacy of MnTnHex-2-PyP(5+) which presumably occurs via reactive oxygen species and relates to NF-κB thiol oxidation; the role of RNS cannot be excluded.

Redox potential determines the reaction mechanism of HNO donors with Mn and Fe porphyrins: defining the better traps

Azanone ((1)HNO, nitroxyl) is a highly reactive molecule with interesting chemical and biological properties. Like nitric oxide (NO), its main biologically related targets are oxygen, thiols, and metalloproteins, particularly heme proteins. As HNO dimerizes with a rate constant between 10(6) and 10(7) M(-1) s(-1), reactive studies are performed using donors, which are compounds that spontaneously release HNO in solution. In the present work, we studied the reaction mechanism and kinetics of two azanone donors Angelís Salt and toluene sulfohydroxamic acid (TSHA) with eight different Mn porphyrins as trapping agents. These porphyrins differ in their total peripheral charge (positively or negatively charged) and in their Mn(III)/Mn(II) reduction potential, showing for each case positive (oxidizing) and negative (reducing) values. Our results show that the reduction potential determines the azanone donor reaction mechanism. While oxidizing porphyrins accelerate decomposition of the donor, reducing porphyrins react with free HNO. Our results also shed light into the donor decomposition mechanism using ab initio methods and provide a thorough analysis of which MnP are the best candidates for azanone trapping and quantification experiments.

Metalloporphyrins as therapeutic catalytic oxidoreductants in central nervous system disorders

SIGNIFICANCE

Metalloporphyrins, characterized by a redox-active transitional metal (Mn or Fe) coordinated to a cyclic porphyrin core ligand, mitigate oxidative/nitrosative stress in biological systems. Side-chain substitutions tune redox properties of metalloporphyrins to act as potent superoxide dismutase mimics, peroxynitrite decomposition catalysts, and redox regulators of transcription factor function. With oxidative/nitrosative stress central to pathogenesis of CNS injury, metalloporphyrins offer unique pharmacologic activity to improve the course of disease.

RECENT ADVANCES

Metalloporphyrins are efficacious in models of amyotrophic lateral sclerosis, Alzheimer's disease, epilepsy, neuropathic pain, opioid tolerance, Parkinson's disease, spinal cord injury, and stroke and have proved to be useful tools in defining roles of superoxide, nitric oxide, and peroxynitrite in disease progression. The most substantive recent advance has been the synthesis of lipophilic metalloporphyrins offering improved blood-brain barrier penetration to allow intravenous, subcutaneous, or oral treatment.

CRITICAL ISSUES

Insufficient preclinical data have accumulated to enable clinical development of metalloporphyrins for any single indication. An improved definition of mechanisms of action will facilitate preclinical modeling to define and validate optimal dosing strategies to enable appropriate clinical trial design. Due to previous failures of "antioxidants" in clinical trials, with most having markedly less biologic activity and bioavailability than current-generation metalloporphyrins, a stigma against antioxidants has discouraged the development of metalloporphyrins as CNS therapeutics, despite the consistent definition of efficacy in a wide array of CNS disorders.

FUTURE DIRECTIONS

Further definition of the metalloporphyrin mechanism of action, side-by-side comparison with "failed" antioxidants, and intense effort to optimize therapeutic dosing strategies are required to inform and encourage clinical trial design.

SOD therapeutics: latest insights into their structure-activity relationships and impact on the cellular redox-based signaling pathways

SIGNIFICANCE

Superoxide dismutase (SOD) enzymes are indispensable and ubiquitous antioxidant defenses maintaining the steady-state levels of O2·(-); no wonder, thus, that their mimics are remarkably efficacious in essentially any animal model of oxidative stress injuries thus far explored.

RECENT ADVANCES

Structure-activity relationship (half-wave reduction potential [E1/2] versus log kcat), originally reported for Mn porphyrins (MnPs), is valid for any other class of SOD mimics, as it is dominated by the superoxide reduction and oxidation potential. The biocompatible E1/2 of ∼+300 mV versus normal hydrogen electrode (NHE) allows powerful SOD mimics as mild oxidants and antioxidants (alike O2·(-)) to readily traffic electrons among reactive species and signaling proteins, serving as fine mediators of redox-based signaling pathways. Based on similar thermodynamics, both SOD enzymes and their mimics undergo similar reactions, however, due to vastly different sterics, with different rate constants.

CRITICAL ISSUES

Although log kcat(O2·(-)) is a good measure of therapeutic potential of SOD mimics, discussions of their in vivo mechanisms of actions remain mostly of speculative character. Most recently, the therapeutic and mechanistic relevance of oxidation of ascorbate and glutathionylation and oxidation of protein thiols by MnP-based SOD mimics and subsequent inactivation of nuclear factor κB has been substantiated in rescuing normal and killing cancer cells. Interaction of MnPs with thiols seems to be, at least in part, involved in up-regulation of endogenous antioxidative defenses, leading to the healing of diseased cells.

FUTURE DIRECTIONS

Mechanistic explorations of single and combined therapeutic strategies, along with studies of bioavailability and translational aspects, will comprise future work in optimizing redox-active drugs.

Mn porphyrin in combination with ascorbate acts as a pro-oxidant and mediates caspase-independent cancer cell death

Resistance to therapy-mediated apoptosis in inflammatory breast cancer, an aggressive and distinct subtype of breast cancer, was recently attributed to increased superoxide dismutase (SOD) expression, glutathione (GSH) content, and decreased accumulation of reactive species. In this study, we demonstrate the unique ability of two Mn(III) N-substituted pyridylporphyrin (MnP)-based SOD mimics (MnTE-2-PyP(5+) and MnTnBuOE-2-PyP(5+)) to catalyze oxidation of ascorbate, leading to the production of excessive levels of peroxide, and in turn cell death. The accumulation of peroxide, as a consequence of MnP+ascorbate treatment, was fully reversed by the administration of exogenous catalase, showing that hydrogen peroxide is essential for cell death. Cell death as a consequence of the action of MnP+ascorbate corresponded to decreases in GSH levels, prosurvival signaling (p-NF-κB, p-ERK1/2), and in expression of X-linked inhibitor of apoptosis protein, the most potent caspase inhibitor. Although markers of classical apoptosis were observed, including PARP cleavage and annexin V staining, administration of a pan-caspase inhibitor, Q-VD-OPh, did not reverse the observed cytotoxicity. MnP+ascorbate-treated cells showed nuclear translocation of apoptosis-inducing factor, suggesting the possibility of a mechanism of caspase-independent cell death. Pharmacological ascorbate has already shown promise in recently completed phase I clinical trials, in which its oxidation and subsequent peroxide formation was catalyzed by endogenous metalloproteins. The catalysis of ascorbate oxidation by an optimized metal-based catalyst (such as MnP) carries a large therapeutic potential as an anticancer agent by itself or in combination with other modalities such as radio- and chemotherapy.

Acid-base and electrochemical properties of manganese meso(ortho- and meta-N-ethylpyridyl)porphyrins: voltammetric and chronocoulometric study of protolytic and redox equilibria

Growing interest in redox-active compounds as therapeutics for oxidative stress-related diseases led to the design of metalloporphyrins as some of the most potent functional SOD-mimics. Herein we report a detailed electrochemical study of the protolytic and redox equilibria of manganese ortho and meta substituted N-ethylpyridyl porphyrins (MnPs), MnTE-2-PyP(5+) and MnTE-3-PyP(5+), in aqueous solutions. The electrochemical parameters of redox processes for all experimentally available species have been determined, as well as their diffusion coefficients and estimated sizes of aqueous cavities. The results indicate that possible changes of the intracellular acidity cannot affect the antioxidant activity of MnPs in vivo, since no change in the E(Mn(III)P/Mn(II)P) values was observed below pH 10. Furthermore, the results confirm that both of these MnPs can be efficient redox scavengers of peroxynitrite (ONOO(-)), another major damaging species in vivo. This can occur by either single-electron reduction or two-electron reduction of ONOO(-), involving either the Mn(IV)P/Mn(III)P redox couple or Mn(IV)P/Mn(II)P redox couple. In addition to kred(ONOO(-)) reported previously, the thermodynamic parameters calculated herein imply a strong and identical driving force for the reaction of both ortho and meta isomeric MnPs with ONOO(-). An enlargement of both Mn(III)P complexes upon an increase of the solution pH was also observed and attributed to the reduction of positive charge on the central ion caused by deprotonation of the axial water molecules. This expansion of aqueous cavities suggests the formation of a solvent cage and the increased lipophilicity of Mn(III)P complexes caused by increased electron density on the Mn ion.

Design, mechanism of action, bioavailability and therapeutic effects of mn porphyrin-based redox modulators

Based on aqueous redox chemistry and simple in vivo models of oxidative stress, Escherichia coli and Saccharomyces cerevisiae, the cationic Mn(III) N-substituted pyridylporphyrins (MnPs) have been identified as the most potent cellular redox modulators within the porphyrin class of drugs; their efficacy in animal models of diseases that have oxidative stress in common is based on their high ability to catalytically remove superoxide, peroxynitrite, carbonate anion radical, hypochlorite, nitric oxide, lipid peroxyl and alkoxyl radicals, thus suppressing the primary oxidative event. While doing so MnPs could couple with cellular reductants and redox-active proteins. Reactive species are widely accepted as regulators of cellular transcriptional activity: minute, nanomolar levels are essential for normal cell function, while submicromolar or micromolar levels impose oxidative stress, which is evidenced in increased inflammatory and immune responses. By removing reactive species, MnPs affect redox-based cellular transcriptional activity and consequently secondary oxidative stress, and in turn inflammatory processes. The equal ability to reduce and oxidize superoxide during the dismutation process and recently accumulated results suggest that pro-oxidative actions of MnPs may also contribute to their therapeutic effects. All our data identify the superoxide dismutase-like activity, estimated by log k(cat)O2-*), as a good measure for the therapeutic efficacy of MnPs. Their accumulation in mitochondria and their ability to cross the blood-brain barrier contribute to their remarkable efficacy. We summarize herein the therapeutic effects of MnPs in cancer, central nervous system injuries, diabetes, their radioprotective action and potential for imaging. Few of the most potent modulators of cellular redox-based pathways, MnTE2-PyP5+, MnTDE-2-ImP5+, MnTnHex-2-PyP5+ and MnTnBuOE-2-PyP5+, are under preclinical and clinical development.

Detailed mechanism of the autoxidation of N-hydroxyurea catalyzed by a superoxide dismutase mimic Mn(III) porphyrin: formation of the nitrosylated Mn(II) porphyrin as an intermediate

The in vitro autoxidation of N-hydroxyurea (HU) is catalyzed by Mn(III)TTEG-2-PyP(5+), a synthetic water soluble Mn(III) porphyrin which is also a potent mimic of the enzyme superoxide dismutase. The detailed mechanism of the reaction is deduced from kinetic studies under basic conditions mostly based on data measured at pH = 11.7 but also including some pH-dependent observations in the pH range 9-13. The major intermediates were identified by UV-vis spectroscopy and electrospray ionization mass spectrometry. The reaction starts with a fast axial coordination of HU to the metal center of Mn(III)TTEG-2-PyP(5+), which is followed by a ligand-to-metal electron transfer to get Mn(II)TTEG-2-PyP(4+) and the free radical derived from HU (HU˙). Nitric oxide (NO) and nitroxyl (HNO) are minor intermediates. The major pathway for the formation of the most significant intermediate, the {MnNO} complex of Mn(II)TTEG-2-PyP(4+), is the reaction of Mn(II)TTEG-2-PyP(4+) with NO. We have confirmed that the autoxidation of the intermediates opens alternative reaction channels, and the process finally yields NO(2)(-) and the initial Mn(III)TTEG-2-PyP(5+). The photochemical release of NO from the {MnNO} intermediate was also studied. Kinetic simulations were performed to validate the deduced rate constants. The investigated reaction has medical implications: the accelerated production of NO and HNO from HU may be utilized for therapeutic purposes.

Clair D, Batinic-Haberle I. Manganese superoxide dismutase, MnSOD and its mimics

Increased understanding of the role of mitochondria under physiological and pathological conditions parallels increased exploration of synthetic and natural compounds able to mimic MnSOD - endogenous mitochondrial antioxidant defense essential for the existence of virtually all aerobic organisms from bacteria to humans. This review describes most successful mitochondrially-targeted redox-active compounds, Mn porphyrins and MitoQ(10) in detail, and briefly addresses several other compounds that are either catalysts of O(2)(-) dismutation, or its non-catalytic scavengers, and that reportedly attenuate mitochondrial dysfunction. While not a true catalyst (SOD mimic) of O(2)(-) dismutation, MitoQ(10) oxidizes O(2)(-) to O(2) with a high rate constant. In vivo it is readily reduced to quinol, MitoQH(2), which in turn reduces ONOO(-) to NO(2), producing semiquinone radical that subsequently dismutes to MitoQ(10) and MitoQH(2), completing the "catalytic" cycle. In MitoQ(10), the redox-active unit was coupled via 10-carbon atom alkyl chain to monocationic triphenylphosphonium ion in order to reach the mitochondria. Mn porphyrin-based SOD mimics, however, were designed so that their multiple cationic charge and alkyl chains determine both their remarkable SOD potency and carry them into the mitochondria. Several animal efficacy studies such as skin carcinogenesis and UVB-mediated mtDNA damage, and subcellular distribution studies of Saccharomyces cerevisiae and mouse heart provided unambiguous evidence that Mn porphyrins mimic the site and action of MnSOD, which in turn contributes to their efficacy in numerous in vitro and in vivo models of oxidative stress. Within a class of Mn porphyrins, lipophilic analogs are particularly effective for treating central nervous system injuries where mitochondria play key role. This article is part of a Special Issue entitled: Antioxidants and Antioxidant Treatment in Disease.

Meso tetrakis ortho-, meta-, and para-N-alkylpyridiniopor-phyrins: kinetics of copper(II) and zinc(II) incorporation and zinc porphyrin demetalation

The relative reactivities of the tetrakis( N -alkylpyridinium- X - yl )-porphyrins where X = 4 (alkyl = methyl, ethyl, n -propyl) , X = 3 (methyl) , and X = 2 (methyl, ethyl, n -propyl, n -butyl, n -hexyl, n -octyl) were studied in aqueous solution. From the ionic strength dependence of the metalation rate constants, the effective charge of a particular cationic porphyrin was usually larger when copper(II) rather than zinc(II) was the reactant. The kinetics of ZnOH + incorporation and the acid catalyzed removal of zinc from the porphyrins in 1.0 M HCl were also studied. In general, the more basic 4- (para-) and 3- (meta-) isomers were the most reactive, followed by the less basic 2- (ortho-) methyl to n -butyl derivatives, with the lipophilic ortho n -hexyl and n -octyl porphyrins the least reactive.

Design of Mn porphyrins for treating oxidative stress injuries and their redox-based regulation of cellular transcriptional activities

The most efficacious Mn(III) porphyrinic (MnPs) scavengers of reactive species have positive charges close to the Mn site, whereby they afford thermodynamic and electrostatic facilitation for the reaction with negatively charged species such as O (2) (•-) and ONOO(-). Those are Mn(III) meso tetrakis(N-alkylpyridinium-2-yl)porphyrins, more specifically MnTE-2-PyP(5+) (AEOL10113) and MnTnHex-2-PyP(5+) (where alkyls are ethyl and n-hexyl, respectively), and their imidazolium analog, MnTDE-2-ImP(5+) (AEOL10150, Mn(III) meso tetrakis(N,N'-diethylimidazolium-2-yl) porphyrin). The efficacy of MnPs in vivo is determined not only by the compound antioxidant potency, but also by its bioavailability. The former is greatly affected by the lipophilicity, size, structure, and overall shape of the compound. These porphyrins have the ability to both eliminate reactive oxygen species and impact the progression of oxidative stress-dependent signaling events. This will effectively lead to the regulation of redox-dependent transcription factors and the suppression of secondary inflammatory- and oxidative stress-mediated immune responses. We have reported on the inhibition of major transcription factors HIF-1α, AP-1, SP-1, and NF-κB by Mn porphyrins. While the prevailing mechanistic view of the suppression of transcription factors activation is via antioxidative action (presumably in cytosol), the pro-oxidative action of MnPs in suppressing NF-κB activation in nucleus has been substantiated. The magnitude of the effect is dependent upon the electrostatic (porphyrin charges) and thermodynamic factors (porphyrin redox ability). The pro-oxidative action of MnPs has been suggested to contribute at least in part to the in vitro anticancer action of MnTE-2-PyP(5+) in the presence of ascorbate, and in vivo when combined with chemotherapy of lymphoma. Given the remarkable therapeutic potential of metalloporphyrins, future studies are warranted to further our understanding of in vivo action/s of Mn porphyrins, particularly with respect to their subcellular distribution.

Neuroprotective efficacy from a lipophilic redox-modulating Mn(III) N-Hexylpyridylporphyrin, MnTnHex-2-PyP: rodent models of ischemic stroke and subarachnoid hemorrhage

Intracerebroventricular treatment with redox-regulating Mn(III) N-hexylpyridylporphyrin (MnPorphyrin) is remarkably efficacious in experimental central nervous system (CNS) injury. Clinical development has been arrested because of poor blood-brain barrier penetration. Mn(III) meso-tetrakis (N-hexylpyridinium-2-yl) porphyrin (MnTnHex-2-PyP) was synthesized to include four six-carbon (hexyl) side chains on the core MnPorphyrin structure. This has been shown to increase in vitro lipophilicity 13,500-fold relative to the hydrophilic ethyl analog Mn(III) meso-tetrakis(N-ethylpyridinium-2-yl)porphyrin (MnTE-2-PyP). In normal mice, we found brain MnTnHex-2-PyP accumulation to be ∼9-fold greater than MnTE-2-PyP 24 h after a single intraperitoneal dose. We then evaluated MnTnHex-2-PyP efficacy in outcome-oriented models of focal cerebral ischemia and subarachnoid hemorrhage. For focal ischemia, rats underwent 90-min middle cerebral artery occlusion. Parenteral MnTnHex-2-PyP treatment began 5 min or 6 h after reperfusion onset and continued for 7 days. Neurologic function was improved with both early (P = 0.002) and delayed (P = 0.002) treatment onset. Total infarct size was decreased with both early (P = 0.03) and delayed (P = 0.01) treatment. MnTnHex-2-PyP attenuated nuclear factor κB nuclear DNA binding activity and suppressed tumor necrosis factor-α and interleukin-6 expression. For subarachnoid hemorrhage, mice underwent perforation of the anterior cerebral artery and were treated with intraperitoneal MnTnHex-2-PyP or vehicle for 3 days. Neurologic function was improved (P = 0.02), and vasoconstriction of the anterior cerebral (P = 0.0005), middle cerebral (P = 0.003), and internal carotid (P = 0.015) arteries was decreased by MnTnHex-2-PyP. Side-chain elongation preserved MnPorphyrin redox activity, but improved CNS bioavailability sufficient to cause improved outcome from acute CNS injury, despite delay in parenteral treatment onset of up to 6 h. This advance now allows consideration of MnPorphyrins for treatment of cerebrovascular disease.

Methoxy-derivatization of alkyl chains increases the in vivo efficacy of cationic Mn porphyrins. Synthesis, characterization, SOD-like activity, and SOD-deficient E. coli study of meta Mn(III) N-methoxyalkylpyridylporphyrins

Cationic Mn(III) N-alkylpyridylporphyrins (MnPs) are potent SOD mimics and peroxynitrite scavengers and diminish oxidative stress in a variety of animal models of central nervous system (CNS) injuries, cancer, radiation, diabetes, etc. Recently, properties other than antioxidant potency, such as lipophilicity, size, shape, and bulkiness, which influence the bioavailability and the toxicity of MnPs, have been addressed as they affect their in vivo efficacy and therapeutic utility. Porphyrin bearing longer alkyl substituents at pyridyl ring, MnTnHex-2-PyP(5+), is more lipophilic, thus more efficacious in vivo, particularly in CNS injuries, than the shorter alkyl-chained analog, MnTE-2-PyP(5+). Its enhanced lipophilicity allows it to accumulate in mitochondria (relative to cytosol) and to cross the blood-brain barrier to a much higher extent than MnTE-2-PyP(5+). Mn(III) N-alkylpyridylporphyrins of longer alkyl chains, however, bear micellar character, and when used at higher levels, become toxic. Recently we showed that meta isomers are ∼10-fold more lipophilic than ortho species, which enhances their cellular accumulation, and thus reportedly compensates for their somewhat inferior SOD-like activity. Herein, we modified the alkyl chains of the lipophilic meta compound, MnTnHex-3-PyP(5+) via introduction of a methoxy group, to diminish its toxicity (and/or enhance its efficacy), while maintaining high SOD-like activity and lipophilicity. We compared the lipophilic Mn(III) meso-tetrakis(N-(6'-methoxyhexyl)pyridinium-3-yl)porphyrin, MnTMOHex-3-PyP(5+), to a hydrophilic Mn(III) meso-tetrakis(N-(2'-methoxyethyl)pyridinium-3-yl)porphyrin, MnTMOE-3-PyP(5+). The compounds were characterized by uv-vis spectroscopy, mass spectrometry, elemental analysis, electrochemistry, and ability to dismute O(2)˙(-). Also, the lipophilicity was characterized by thin-layer chromatographic retention factor, R(f). The SOD-like activities and metal-centered reduction potentials for the Mn(III)P/Mn(II)P redox couple were similar-to-identical to those of N-alkylpyridyl analogs: log k(cat) = 6.78, and E(1/2) = +68 mV vs. NHE (MnTMOHex-3-PyP(5+)), and log k(cat) = 6.72, and E(1/2) = +64 mV vs. NHE (MnTMOE-3-PyP(5+)). The compounds were tested in a superoxide-specific in vivo model: aerobic growth of SOD-deficient E. coli, JI132. Both MnTMOHex-3-PyP(5+) and MnTMOE-3-PyP(5+) were more efficacious than their alkyl analogs. MnTMOE-3-PyP(5+) is further significantly more efficacious than the most explored compound in vivo, MnTE-2-PyP(5+). Such a beneficial effect of MnTMOE-3-PyP(5+) on diminished toxicity, improved efficacy and transport across the cell wall may originate from the favorable interplay of the size, length of pyridyl substituents, rotational flexibility (the ortho isomer, MnTE-2-PyP(5+), is more rigid, while MnTMOE-3-PyP(5+) is a more flexible meta isomer), bulkiness and presence of oxygen.

Bioavailability of metalloporphyrin-based SOD mimics is greatly influenced by a single charge residing on a Mn site

In the cell Mn porphyrins (MnPs) likely couple with cellular reductants which results in a drop of total charge from 5+ to 4+ and dramatically increases their lipophilicity by up to three orders of magnitude depending upon the length of alkylpyridyl chains and type of isomer. The effects result from the interplay of solvation, lipophilicit and stericity. Impact of ascorbate on accumulation of MnPs was measured in E. coli and in Balb/C mouse tumours and muscle; for the latter measurements, the LC/ESI-MS/MS method was developed. Accumulation was significantly enhanced when MnPs were co-administered with ascorbate in both prokaryotic and eukaryotic systems. Further, MnTnHex-2-PyP(5+) accumulates 5-fold more in the tumour than in a muscle. Such data increase our understanding of MnPs cellular and sub-cellular accumulation and remarkable in vivo effects. The work is in progress to understand how coupling of MnPs with ascorbate affects their mechanism of action, in particular with respect to cancer therapy.

Metalloporphyrin synergizes with ascorbic acid to inhibit cancer cell growth through fenton chemistry

Ascorbic acid (AA) has been reported to inhibit tumor cell growth through the generation of extracellular hydrogen peroxide (H(2)O(2)). However, the clinical utility of AA has been limited by relatively low potency and in vivo efficacy. This study reports that the metalloporphyrin, Mn(III) tetrakis(N-methylpyridinium-2-yl)porphyrin(5+) (MnTMPyP), has a potent synergistic cytotoxic effect when combined with AA in a variety of cancer cell lines. In the presence of MnTMPyP, the concentration of AA required to inhibit cancer cell growth was markedly reduced. In vitro (cell-free) experiments demonstrated that AA alone enhanced the Fenton reaction that produces cytotoxic hydroxyl radical (HO(*)); however, this reaction was limited by the low rate by which AA generates H(2)O(2) (Fenton reaction substrate) from O(2). MnTMPyP catalyzed H(2)O(2) generation through the AA-facilitated Mn(II <--> III)TMPyP redox cycle and thereby markedly potentiated the Fenton reaction. Accordingly, MnTMPyP and AA resulted in increased cellular levels of H(2)O(2) and HO(*) in cancer cells, which mediate the synergistic cytotoxicity of this combined treatment. This effect was inhibited by cellular enzymes that metabolize H(2)O(2), such as catalase and glutathione peroxidase, suggesting that selective killing of cancer cells deficient in such enzymes can be achieved in vivo.

Superoxide dismutase mimics: chemistry, pharmacology, and therapeutic potential

Oxidative stress has become widely viewed as an underlying condition in a number of diseases, such as ischemia-reperfusion disorders, central nervous system disorders, cardiovascular conditions, cancer, and diabetes. Thus, natural and synthetic antioxidants have been actively sought. Superoxide dismutase is a first line of defense against oxidative stress under physiological and pathological conditions. Therefore, the development of therapeutics aimed at mimicking superoxide dismutase was a natural maneuver. Metalloporphyrins, as well as Mn cyclic polyamines, Mn salen derivatives and nitroxides were all originally developed as SOD mimics. The same thermodynamic and electrostatic properties that make them potent SOD mimics may allow them to reduce other reactive species such as peroxynitrite, peroxynitrite-derived CO(3)(*-), peroxyl radical, and less efficiently H(2)O(2). By doing so SOD mimics can decrease both primary and secondary oxidative events, the latter arising from the inhibition of cellular transcriptional activity. To better judge the therapeutic potential and the advantage of one over the other type of compound, comparative studies of different classes of drugs in the same cellular and/or animal models are needed. We here provide a comprehensive overview of the chemical properties and some in vivo effects observed with various classes of compounds with a special emphasis on porphyrin-based compounds.

Peroxynitrite detoxification and its biologic implications

Peroxynitrite is a cytotoxic oxidant formed in vivo from the diffusional-controlled reaction between nitric oxide and superoxide radicals. Increased peroxynitrite formation has been related to the pathogenesis of multiple diseases, thus underlining the importance of understanding the mechanisms of its detoxification. In nature, different enzymatic routes for peroxynitrite decomposition have evolved. Among them, peroxiredoxins catalytically reduce peroxynitrite in vitro; modulation of their expression affects peroxynitrite-mediated cytotoxicity, and their content changes in pathologic conditions associated with increased peroxynitrite formation in vivo, thus indicating a physiologic role of these enzymes in peroxynitrite reduction. Selenium-containing glutathione peroxidase also catalyzes peroxynitrite reduction, but its role in vivo is still a matter of debate. In selected cellular systems, heme proteins also play a role in peroxynitrite detoxification, such as its isomerization by oxyhemoglobin in red blood cells. Moreover, different pharmacologic approaches have been used to decrease the toxicity related to peroxynitrite formation. Manganese or iron porphyrins catalyze peroxynitrite decomposition, and their protective role in vivo has been confirmed in biologic systems. Glutathione peroxidase mimetics also rapidly reduce peroxynitrite, but their biologic role is less well established. Flavonoids, nitroxides, and tyrosine-containing peptides decreased peroxynitrite-mediated toxicity under different conditions, but their mechanism of action is indirect.

Mn porphyrin-based superoxide dismutase (SOD) mimic, MnIIITE-2-PyP5+, targets mouse heart mitochondria

The Mn(III) meso-tetrakis(N-ethylpyridinium-2-yl)porphyrin, MnIIITE-2-PyP5+ (AEOL-10113) has proven effective in treating oxidative stress-induced conditions including cancer, radiation damage, diabetes, and central nervous system trauma. The ortho cationic pyridyl nitrogens of MnTE-2-PyP5+ are essential for its high antioxidant potency. The exceptional ability of MnIIITE-2-PyP5+ to dismute O2.- parallels its ability to reduce ONOO- and CO3-. Decreasing levels of these species are considered its predominant mode of action, which may also involve redox regulation of signaling pathways. Recently, Ferrer-Sueta at al. (Free Radic. Biol. Med. 41:503-512; 2006) showed, with submitochondrial particles, that>or=3 microM MnIIITE-2-PyP5+ was able to protect components of the mitochondrial electron transport chain from peroxynitrite-mediated damage. Our study complements their data in showing, for the first time that micromolar mitochondrial concentrations of MnIIITE-2-PyP5+ are obtainable in vivo. For this study we have developed a new and sensitive method for MnIIITE-2-PyP5+ determination in tissues. The method is based on the exchange of porphyrin Mn2+ with Zn2+, followed by the HPLC/fluorescence detection of ZnIITE-2-PyP4+. At 4 and 7 h after a single 10 mg/kg intraperitoneal administration of MnIIITE-2-PyP5+, the mice (8 in total) were anesthetized and perfused with saline. Mitochondria were then isolated by the method of Mela and Seitz (Methods Enzymol.55:39-46; 1979). We found MnIIITE-2-PyP5+ localized in heart mitochondria to 2.95 ng/mg protein. Given the average value of mitochondrial volume of 0.6 microL/mg protein, the calculated MnIIITE-2-PyP5+ concentration is 5.1 microM, which is sufficient to protect mitochondria from oxidative damage. This study establishes, for the first time, that MnIIITE-2-PyP5+, a highly charged metalloporphyrin, is capable of entering mitochondria in vivo at levels sufficient to exert there its antioxidant action; such a result encourages its development as a prospective therapeutic agent.

MnTMPyP, a metalloporphyrin-based superoxide dismutase/catalase mimetic, protects INS-1 cells and human pancreatic islets from an in vitro oxidative challenge

AIMS

Pancreatic islets can be lost early following allotransplantation from oxidative stress. Antioxidant enzyme overexpression could confer a beneficial effect on islets exposed to reactive oxygen species (ROS) and nitrogen species. Here, we tested the effect of MnTMPyP, a superoxide dismutase/catalase mimetic.

METHODS

INS-1 insulin-secreting cells or human islets were cultured with MnTMPyP and exposed to a superoxide donor (the hypoxanthine/xanthine oxidase (HX/XO) system), a nitric oxide donor [3-morpholinosydnonimine (SIN-1)] or menadione. Viability of INS-1 cells was assessed by WST-1 colorimetric assay and FACS analysis (Live/Dead test). ROS production was determined using fluorescent probes. Islet viability was estimated by WST-1 assay and endocrine function by static incubation.

RESULTS

Following MnTMPyP treatment, ROS production in INS-1 cells was reduced by 4- to 20-fold upon HX/XO challenge and up to 2-fold upon SIN-1 stress. This phenomenon correlated with higher viability measured by WST-1 or Live/Dead test. MnTMPyP preserved islet viability upon exposure to SIN-1 or menadione but not upon an HX/XO challenge. Similarly, decrease in insulin secretion tended to be less pronounced in MnTMPyP-treated islets than in control islet when exposed to SIN-1, but no changes were noticed during an HX/XO stress.

CONCLUSIONS

MnTMPyP was able to improve the viability of INS-1 cells and human islets exposed to oxidative challenges in vitro. Protection of INS-1 cells could be as high as 90%. This agent is therefore potentially attractive in situations involving the overproduction of ROS, such as islet transplantation.

Ab initio molecular dynamics study of manganese porphine hydration and interaction with nitric oxide

The authors use ab initio molecular dynamics and the density functional theory+U (DFT+U) method to compute the hydration environment of the manganese ion in manganese (II) and manganese (III) porphines (MnP) dispersed in liquid water. These are intended as simple models for more complex water soluble porphyrins, which have important physiological and electrochemical applications. The manganese ion in Mn(II)P exhibits significant out-of-porphine plane displacement and binds strongly to a single H2O molecule in liquid water. The Mn in Mn(III)P is on average coplanar with the porphine plane and forms a stable complex with two H2O molecules. The residence times of these water molecules exceed 15 ps. The DFT+U method correctly predicts that water displaces NO from Mn(III)P-NO, but yields an ambiguous spin state for the MnP(II)-NO complex.

New approach to the activation of anti-cancer pro-drugs by metalloporphyrin-based cytochrome P450 mimics in all-aqueous biologically relevant system

The low-molecular weight water-soluble Fe(III) and Mn(III) porphyrins--in biologically relevant phosphate-buffered saline medium with ascorbic acid as a source of electrons, under aerobic conditions but without co-oxidant - catalyze the hydroxylation of anti-cancer drug cyclophosphamide to active metabolite 4-hydroxycyclophosphamide in yields similar or higher than those typically obtained by the action of liver enzymes in vivo. The Fe(III) meso tetrakis(2,6-difluoro-3-sulfonatophenyl)porphyrin, highly electron-deficient at the metal site, was the most effective catalyst. If proven viable in vivo, this methodology could be expanded to localized or systemic activation of the entire family of oxazaphosphorine-based (and many other) anti-cancer drugs and become a powerful tool for an aggressive treatment of tumors with less toxic side effects to the patient.

New PEG-ylated Mn(iii) porphyrins approaching catalytic activity of SOD enzyme

Two new tri(ethyleneglycol)-derivatized Mn(III) porphyrins were synthesized with the aim of increasing their bioavailability, and blood-circulating half-life. These are Mn(III) tetrakis(N-(1-(2-(2-(2-methoxyethoxy)ethoxy)ethyl)pyridinium-2-yl)porphyrin, MnTTEG-2-PyP5+ and Mn(III) tetrakis(N,N'-di(1-(2-(2-(2-methoxyethoxy)ethoxy)ethyl)imidazolium-2-yl)porphyrin, MnTDTEG-2-ImP5+. Both porphyrins have ortho pyridyl or di-ortho imidazolyl electron-withdrawing substituents at meso positions of the porphyrin ring that assure highly positive metal centered redox potentials, E1/2 = +250 mV vs. NHE for MnTTEG-2-PyP5+ and E1/2 = + 412 mV vs. NHE for MnTDTEG-2-ImP5+. As expected, from established E1/2 vs. log kcat(O2 *-) structure-activity relationships for metalloporphyrins (Batinic-Haberle et al., Inorg. Chem., 1999, 38, 4011), both compounds exhibit higher SOD-like activity than any meso-substituted Mn(III) porphyrins-based SOD mimic thus far, log kcat = 8.11 (MnTTEG-2-PyP5+) and log kcat = 8.55 (MnTDTEG-2-ImP5+), the former being only a few-fold less potent in disproportionating O2*- than the SOD enzyme itself. The new porphyrins are stable to both acid and EDTA, and non toxic to E. coli. Despite elongated substituents, which could potentially lower their ability to cross the cell wall, MnTTEG-2-PyP5+ and MnTDTEG-2-ImP5+ exhibit similar protection of SOD-deficient E. coli as their much smaller ethyl analogues MnTE-2-PyP5+ and MnTDE-2-ImP5+, respectively. Consequently, with anticipated increased blood-circulating half-life, these new Mn(III) porphyrins may be more effective in ameliorating oxidative stress injuries than ethyl analogues that have been already successfully explored in vivo.

A manganese porphyrin superoxide dismutase mimetic enhances tumor radioresponsiveness

PURPOSE

To determine the effect of the superoxide dismutase mimetic Mn(III) tetrakis(N-ethylpyridinium-2-yl)porphyrin (MnTE-2-PyP(5+)) on tumor radioresponsiveness.

METHODS AND MATERIALS

Various rodent tumor (4T1, R3230, B16) and endothelial (SVEC) cell lines were exposed to MnTE-2-PyP(5+) and assayed for viability and radiosensitivity in vitro. Next, tumors were treated with radiation and MnTE-2-PyP(5+)in vivo, and the effects on tumor growth and vascularity were monitored.

RESULTS

In vitro, MnTE-2-PyP(5+) was not significantly cytotoxic. However, at concentrations as low as 2 mumol/L it caused 100% inhibition of secretion by tumor cells of cytokines protective of irradiated endothelial cells. In vivo, combined treatment with radiation and MnTE-2-PyP(5+) achieved synergistic tumor devascularization, reducing vascular density by 78.7% within 72 h of radiotherapy (p < 0.05 vs. radiation or drug alone). Co-treatment of tumors also resulted in synergistic antitumor effects, extending tumor growth delay by 9 days (p < 0.01).

CONCLUSIONS

These studies support the conclusion that MnTE-2-PyP(5+), which has been shown to protect normal tissues from radiation injury, can also improve tumor control through augmenting radiation-induced damage to the tumor vasculature.

A manganese porphyrin suppresses oxidative stress and extends the life span of streptozotocin-diabetic rats

Enhanced oxidative stress due to hyperglycemia has been implicated in diabetic complications and is considered a major cause of cell and tissue damage. The aim of the present study was to investigate whether synthetic manganese porphyrin, Mn(III) 5,10,15,20-tetrakis(N-methylpyridinium-2-yl)porphyrin (MnTM-2-PyP5+) can ameliorate diabetes-induced oxidative stress and affect life span of diabetic rats. Diabetes was induced by a single (60 mg/kg) intraperitoneal injection of streptozotocin in male Wistar rats. Oxidative stress was monitored by measuring malondialdehyde levels (MDA) in blood plasma and erythrocytes using HPLC. The antioxidant status was assessed by measuring the total radical-trapping potential (TRAP) of blood plasma. Life span of the animals was used as an indication of the overall effect of MnTM-2-PyP5+. MnTM-2-PyP5+ was administered subcutaneously at 1 mg/kg for the duration of the experiment, five times/week followed by one week of rest. Diabetes increased plasma and erythrocyte levels of MDA and decreased TRAP. MnTM-2-PyP5+ had no effect on blood glucose and glycosylated hemoglobin, but significantly increased TRAP and lowered MDA. This Mn porphyrin decreased mortality and markedly extended the life span of the diabetic animals. MnTM-2-PyP5+ suppressed diabetes-induced oxidative stress, which presumably accounts for its beneficial effect on the life span of the diabetic rats. The results indicate that Mn(III) N-alkylpyridylporphyrins can be used as potent therapeutic agents in diabetes.

Discrimination of nitroxyl and nitric oxide by water-soluble Mn(III) porphyrins

The water-soluble manganese(III) meso-tetrakis (N-ethylpyridinium-2-yl) porphyrin (Mn(III)TEPyP) and manganese(III) meso-(tetrakis(4-sulfonato-phenyl)) porphyrinate (Mn(III)TPPS) are able to chemically distinguish between HNO and NO donors, reacting with the former in a fast, efficient, and selective manner with concomitant formation of the {MnNO}(7) complex (k(on(HNO)) approximately equal to 10(5) M(-1) s(-1)), while they are inert or react very slowly with NO donors. DFT calculations and kinetic data suggest that HNO trapping is operative at least in the case of Mn(III)TPPS, while catalytic decomposition of the HNO donors (sodium trioxodinitrate and toluene sulfohydroxamic acid) seems to be the main pathway for Mn(III)TEPyP. In the presence of oxygen, the product Mn(II)TEPyP(NO) oxidizes back to Mn(III)TEPyP, making it possible to process large ratios of nitroxyl donor with small amounts of porphyrin.

Cryptococcus neoformans mitochondrial superoxide dismutase: an essential link between antioxidant function and high-temperature growth

Manganese superoxide dismutase is an essential component of the mitochondrial antioxidant defense system of most eukaryotes. In the present study, we used a reverse-genetics approach to assess the contribution of the Cryptococcus neoformans manganese superoxide dismutase (Sod2) for antioxidant defense. Strains with mutations in the SOD2 gene exhibited increased susceptibility to oxidative stress as well as poor growth at elevated temperatures compared to isogenic wild-type strains. The sod2Delta mutants were also avirulent in a murine model of inhaled cryptococcosis. Reconstitution of a sod2Delta mutant restored Sod2 activity, eliminated the oxidative stress and temperature-sensitive (ts) phenotypes, and complemented the virulence phenotype. Characterization of the ts phenotype revealed a dependency between Sod2 antioxidant activity and the ability of C. neoformans cells to adapt to growth at elevated temperatures. The ts phenotype could be suppressed by the addition of either ascorbic acid (10 mM) or Mn salen (200 muM) at 30 degrees C, but not at 37 degrees C. Furthermore, sod2Delta mutant cells that were incubated for 24 h at 37 degrees C under anaerobic, but not aerobic, conditions were viable when shifted to the permissive conditions of 25 degrees C in the presence of air. These data suggest that the C. neoformans Sod2 is a major component of the antioxidant defense system in this human fungal pathogen and that adaptation to growth at elevated temperatures is also dependent on Sod2 activity.

Oxidants, antioxidants and the ischemic brain

Despite numerous defenses, the brain is vulnerable to oxidative stress resulting from ischemia/reperfusion. Excitotoxic stimulation of superoxide and nitric oxide production leads to formation of highly reactive products, including peroxynitrite and hydroxyl radical, which are capable of damaging lipids, proteins and DNA. Use of transgenic mutants and selective pharmacological antioxidants has greatly increased understanding of the complex interplay between substrate deprivation and ischemic outcome. Recent evidence that reactive oxygen/nitrogen species play a critical role in initiation of apoptosis, mitochondrial permeability transition and poly(ADP-ribose) polymerase activation provides additional mechanisms for oxidative damage and new targets for post-ischemic therapeutic intervention. Because oxidative stress involves multiple post-ischemic cascades leading to cell death, effective prevention/treatment of ischemic brain injury is likely to require intervention at multiple effect sites.

Tetrahydrobiopterin rapidly reduces the SOD mimic Mn(III) ortho-tetrakis(N-ethylpyridinium-2-yl)porphyrin

Mn(III) ortho-tetrakis(N-ethylpyridinium-2-yl)porphyrin (Mn(III)TE-2-PyP(5+)) effectively scavenges reactive oxygen and nitrogen species in vitro, and protects in vivo, in different rodent models of oxidative stress injuries. Further, Mn(III)TE-2-PyP(5+) was shown to be readily reduced by cellular reductants such as ascorbic acid and glutathione. We now show that tetrahydrobiopterin (BH(4)) is also able to reduce the metal center. Under anaerobic conditions, in phosphate-buffered saline (pH 7.4) at 25 +/- 0.1 degrees C, reduction of Mn(III)TE-2-PyP(5+) occurs through two reaction steps with rate constants k(1) = 1.0 x 10(4) M(-1) s(-1) and k(2) = 1.5 x 10(3) M(-1) s(-1). We ascribe these steps to the formation of tetrahydrobiopterin radical (BH(4)(.+)) (k(1)) that then undergoes oxidation to 6,7-dihydro-8H-biopterin (k(2)), which upon rearrangement gives rise to 7,8-dihydrobiopterin (7,8-BH(2)). Under aerobic conditions, Mn(III)TE-2-PyP(5+) catalytically oxidizes BH(4). This is also true for its longer chain alkyl analog, Mn(III) ortho-tetrakis(N-n-octylpyridinium-2-yl)porphyrin. The reduced Mn(II) porphyrin cannot be oxidized by 7,8-BH(2) or by l-sepiapterin. The data are discussed with regard to the possible impact of the interaction of Mn(III)TE-2-PyP(5+) with BH(4) on endothelial cell proliferation and hence on tumor antiangiogenesis via inhibition of nitric oxide synthase.

Mouse spinal cord compression injury is ameliorated by intrathecal cationic manganese(III) porphyrin catalytic antioxidant therapy

This study evaluated the effects of the cationic manganese(III) tetrakis(N,N'-diethylimidazolium-2-yl)porphyrin catalytic antioxidant Mn(III)TDE-2-ImP5+ (AEOL 10150) on outcome from spinal cord compression (SCC) in the mouse. C57BL/6J mice were subjected to 60 min thoracic SCC after discontinuation of halothane anesthesia. In Experiment 1, mice were given intravenous Mn(III)TDE-2-ImP5+ (0.5 mg/kg bolus followed by 1 mg kg(-1) h(-1) for 24 h), methylprednisolone (30 mg/kg bolus followed by 5.4 mg kg(-1) h(-1) for 24 h), or vehicle (n = 25 per group). In Experiment 2, mice were given intrathecal Mn(III)TDE-2-ImP5+ (2.5 or 5.0 microg/kg) or vehicle (n = 18 per group). In both experiments, treatment began 5 min post-SCC onset. Rotarod performance was measured on post-SCC days 3, 7, 14, and 21. On post-SCC day 21, the spinal cord was histologically examined and a total damage score was calculated. Neither intravenous Mn(III)TDE-2-ImP5+ nor methylprednisolone altered rotarod performance (accelerated rate P = 0.11, fixed rate P = 0.11) or mean +/- S.D. total damage score (Mn(III)TDE-2-ImP5+ = 21 +/- 9, methylprednisolone = 24 +/- 8, vehicle = 22 +/- 10; P = 0.47; shams = 0). Intrathecal Mn(III)TDE-2-ImP5+ (both 2.5 and 5.0 microg) given at SCC-onset improved rotarod performance (P = 0.05) and total damage score (2.5 microg = 19 +/- 10, P = 0.04; 5.0 microg =19 +/- 8, P = 0.03) versus vehicle (26 +/- 10). These studies demonstrate sustained benefit from manganese(III) porphyrin catalytic antioxidant therapy after SCC. However, efficacy was dependent upon route of administration suggesting that bioavailability is critical in defining efficacy.