Manganese(III) complexes with porphyrins and related compounds as catalytic scavengers of superoxide

DFT Method Used for Prediction of Molecular and Electronic Structures of Mn(VI) Macrocyclic Complexes with Porhyrazine/Phthalocyanine and Two Oxo Ligands

By using the data of the DFT quantum chemical calculation in the OPBE/TZVP and B3PW91/TZVP levels, the possibility of the existence of a manganese(VI) heteroligand complex containing porphyrazine or its tetra[benzo] derivative (phthalocyanine) and two oxygen (O^2-) ligands, which is still unknown for this element, is shown. The parameters of the molecular structure, multiplicity of the ground state, NBO analysis data and standard thermodynamic parameters (enthalpy ΔH⁰_f, entropy S⁰_f and Gibbs's energy ΔG⁰_f of formation) of each of these metal macrocyclic compounds are presented. Additionally, it is noted that, based on the totality of structural data obtained by the above versions of the DFT method, the existence of a similar complex of manganese with di[benzo] derivative of porhyrazine and two oxygen (O^2-) ligands seems doubtful.

Antioxidant cytochrome c-like activity of para-Mn(III)TMPyP

Manganese porphyrins are well-known protectors against the deleterious effects of pro-oxidant species such as superoxide ions and hydrogen peroxide. The present study investigated the antioxidant cytochrome c-like activities of Mn(III)TMPyP [meso-tetrakis (4-N-methyl pyridinium) porphyrin] against superoxide ion and hydrogen peroxide that remained unexplored for this porphyrin. The association of TMPyP with a model of the inner mitochondrial membrane, cardiolipin (CL)-containing liposomes, shifted +30 mV vs. NHE (normal hydrogen electrode) redox potential of the Mn^(II)/Mn^(III) redox couple. In CL-containing liposomes, Mn^(III)TMPyP was reduced by superoxide ions and recycled by Fe^(III)cytochrome c to the oxidized form. Similarly, isolated rat liver mitoplasts added to a sample of Mn^(II)TMPyP promoted immediate porphyrin reoxidation by electron transfer to the respiratory chain. These results show that Mn^(III)TMPyP can act as an additional pool of Fe^(III)cytochrome c capable of transferring electrons that escape from the IV complex back into the respiratory chain. Unlike Fe^(II)cytochrome c, Mn^(II)TMPyP was not efficient for hydrogen peroxide clearance. Therefore, by reducing cytochrome c, Mn^(II)TMPyP can indirectly contribute to hydrogen peroxide elimination.

Photodynamic properties of aza-analogues of phthalocyanines

Spectral and photophysical properties and in vitro photodynamic activity of aza-analogues of phthalocyanines are summarized.

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.

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.

Thermal stability of the prototypical Mn porphyrin-based superoxide dismutase mimic and potent oxidative-stress redox modulator Mn(III) meso-tetrakis(N-ethylpyridinium-2-yl)porphyrin chloride, MnTE-2-PyP(5+)

Cationic Mn porphyrins are among the most potent catalytic antioxidants and/or cellular redox modulators. Mn(III) meso-tetrakis(N-ethylpyridinium-2-yl)porphyrin chloride (MnTE-2-PyPCl(5)) is the Mn porphyrin most studied in vivo and has successfully rescued animal models of a variety of oxidative stress-related diseases. The stability of an authentic MnTE-2-PyPCl(5) sample was investigated hereon by thermogravimetric, derivative thermogravimetric, and differential thermal analyses (TG/DTG/DTA), under dynamic air, followed by studies at selected temperatures to evaluate the decomposition path and appropriate conditions for storage and handling of these materials. All residues were analyzed by thin-layer chromatography (TLC) and UV-vis spectroscopy. Three thermal processes were observed by TG/DTG. The first event (endothermic) corresponded to dehydration, and did not alter the MnTE-2-PyPCl(5) moiety. The second event (endothermic) corresponded to the loss of EtCl (dealkylation), which was characterized by gas chromatography-mass spectrometry. The residue at 279°C had UV-vis and TLC data consistent with those of the authentic, completely dealkylated analog, MnT-2-PyPCl. The final, multi-step event corresponded to the loss of the remaining organic matter to yield Mn(3)O(4) which was characterized by IR spectroscopy. Isothermal treatment at 188°C under static air for 3h yielded a mixture of partially dealkylated MnPs and traces of the free-base, dealkylated ligand, H(2)T-2-PyP, which reveals that dealkylation is accompanied by thermal demetallation under static air conditions. Dealkylation was not observed if the sample was heated as a solid or in aqueous solution up to ∼100°C. Whereas moderate heating changes sample composition by loss of H(2)O, the dehydrated sample is indistinguishable from the original sample upon dissolution in water, which indicates that catalytic activity (on Mn basis) remains unaltered. Evidently, dealkylation at high temperature compromises sample activity.

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.

β-Pyrrole brominated meso-tetraphenylporphyrins: synthesis, spectral and electrochemical properties

The synthesis, catalytic properties, UV-visible spectra and electrochemistry of β-pyrrole brominated porphyrins are summarized in this brief review. The effect of the Br substituents of the porphyrin ring on the redox behavior, and on axial CO or pyridine binding to the porphyrins is also discussed.

Oleic acid peroxidation in the presence of metallo-porphyrins

The influence of free bases of meso-tetrakis(3,5-di-tert-butyl-4-hydroxy-phenyl)porphyrins and meso-tetraphenylporphyrins and their complexes of Co , Ni , Cu , Pt upon the radical chain oxidation of oleic acid as model substrates for lipid peroxidation has been studied. It was shown that the introduction of the antioxidative phenolic fragment into the porphyrin ring leads to the dual activity of phenolic porphyrins when compared with their tetraphenyl substituted analogues in substrate oxidation. Free base phenolic porphyrins and their nickel, copper, platinum complexes demonstrate acute inhibitory effect upon oleic acid peroxidation due to the key role of 2,6-di-tert-butylphenol moities, whereas cobalt porphyrin exhibits dual activity associated with the presence of both the redox active metal center and phenolic group.

Metal-containing polymers via electropolymerization

Electropolymerization represents a suitable and well-established approach for the assembly of polymer structures, in particular with regard to the formation of thin, insoluble films. Utilization of monomers that are functionalized with metal complex units allows the combination of structural and functional benefits of polymers and metal moieties. Since a broad range of both electropolymerizable monomers and metal complexes are available, various structures and, thus, applications are possible. Recent developments in the field of synthesis and potential applications of metal-functionalized polymers obtained via electropolymerization are presented, highlighting the significant advances in this field of research.

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.

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.

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.

Water exchange rates of water-soluble manganese(III) porphyrins of therapeutical potential

The activation parameters and the rate constants of the water-exchange reactions of Mn(III)TE-2-PyP(5+) (meso-tetrakis(N-ethylpyridinium-2-yl)porphyrin) as cationic, Mn(III)TnHex-2-PyP(5+) (meso-tetrakis(N-n-hexylpyridinium-2-yl)porphyrin) as sterically shielded cationic, and Mn(III)TSPP(3-) (meso-tetrakis(4-sulfonatophenyl)porphyrin) as anionic manganese(iii) porphyrins were determined from the temperature dependence of (17)O NMR relaxation rates. The rate constants at 298 K were obtained as 4.12 x 10(6) s(-1), 5.73 x 10(6) s(-1), and 2.74 x 10(7) s(-1), respectively. On the basis of the determined entropies of activation, an interchange-dissociative mechanism (I(d)) was proposed for the cationic complexes (DeltaS(double dagger) = approximately 0 J mol(-1) K(-1)) whereas a limiting dissociative mechanism (D) was proposed for Mn(III)TSPP(3-) complex (DeltaS(double dagger) = +79 J mol(-1) K(-1)). The obtained water exchange rate of Mn(III)TSPP(3-) corresponded well to the previously assumed value used by Koenig et al. (S. H. Koenig, R. D. Brown and M. Spiller, Magn. Reson. Med., 1987, 4, 52-260) to simulate the (1)H NMRD curves, therefore the measured value supports the theory developed for explaining the anomalous relaxivity of Mn(III)TSPP(3-) complex. A magnitude of the obtained water-exchange rate constants further confirms the suggested inner sphere electron transfer mechanism for the reactions of the two positively charged Mn(iii) porphyrins with the various biologically important oxygen and nitrogen reactive species. Due to the high biological and clinical relevance of the reactions that occur at the metal site of the studied Mn(iii) porphyrins, the determination of water exchange rates advanced our insight into their efficacy and mechanism of action, and in turn should impact their further development for both diagnostic (imaging) and therapeutic purposes.

Impact of electrostatics in redox modulation of oxidative stress by Mn porphyrins: protection of SOD-deficient Escherichia coli via alternative mechanism where Mn porphyrin acts as a Mn carrier

Understanding the factors that determine the ability of Mn porphyrins to scavenge reactive species is essential for tuning their in vivo efficacy. We present herein the revised structure-activity relationships accounting for the critical importance of electrostatics in the Mn porphyrin-based redox modulation systems and show that the design of effective SOD mimics (per se) based on anionic porphyrins is greatly hindered by inappropriate electrostatics. A new strategy for the beta-octabromination of the prototypical anionic Mn porphyrins Mn(III) meso-tetrakis(p-carboxylatophenyl)porphyrin ([Mn(III)TCPP](3-) or MnTBAP(3-)) and Mn(III) meso-tetrakis(p-sulfonatophenyl)porphyrin ([Mn(III)TSPP](3-)), to yield the corresponding anionic analogues [Mn(III)Br(8)TCPP](3-) and [Mn(III)Br(8)TSPP](3-), respectively, is described along with characterization data, stability studies, and their ability to substitute for SOD in SOD-deficient Escherichia coli. Despite the Mn(III)/Mn(II) reduction potential of [Mn(III)Br(8)TCPP](3-) and [Mn(III)Br(8)TSPP](3-) being close to the SOD-enzyme optimum and nearly identical to that of the cationic Mn(III) meso-tetrakis(N-methylpyridinium-2-yl)porphyrin (Mn(III)TM-2-PyP(5+)), the SOD activity of both anionic brominated porphyrins ([Mn(III)Br(8)TCPP](3-), E(1/2)=+213 mV vs NHE, log k(cat)=5.07; [Mn(III)Br(8)TSPP](3-), E(1/2)=+209 mV, log k(cat)=5.56) is considerably lower than that of Mn(III)TM-2-PyP(5+) (E(1/2)=+220 mV, log k(cat)=7.79). This illustrates the impact of electrostatic guidance of O(2)(-) toward the metal center of the mimic. With low k(cat), the [Mn(III)TCPP](3-), [Mn(III)TSPP](3-), and [Mn(III)Br(8)TCPP](3-) did not rescue SOD-deficient E. coli. The striking ability of [Mn(III)Br(8)TSPP](3-) to substitute for the SOD enzymes in the E. coli model does not correlate with its log k(cat). In fact, the protectiveness of [Mn(III)Br(8)TSPP](3-) is comparable to or better than that of the potent SOD mimic Mn(III)TM-2-PyP(5+), even though the dismutation rate constant of the anionic complex is 170-fold smaller. Analyses of the medium and E. coli cell extract revealed that the major species in the [Mn(III)Br(8)TSPP](3-) system is not the Mn complex, but the free-base porphyrin [H(2)Br(8)TSPP](4-) instead. Control experiments with extracellular MnCl(2) showed the lack of E. coli protection, indicating that "free" Mn(2+) cannot enter the cell to a significant extent. We proposed herein the alternative mechanism where a labile Mn porphyrin [Mn(III)Br(8)TSPP](3-) is not an SOD mimic per se but carries Mn into the E. coli cell.

Redox modulation of oxidative stress by Mn porphyrin-based therapeutics: the effect of charge distribution

We evaluate herein the impact of positive charge distribution on the in vitro and in vivo properties of Mn porphyrins as redox modulators possessing the same overall 5+ charge and of minimal stericity demand: Mn(III) meso-tetrakis(trimethylanilinium-4-yl)porphyrin (MnTTriMAP(5+)), Mn(III) meso-tetrakis(N,N'-dimethylpyrazolium-4-yl)porphyrin (MnTDM-4-PzP(5+)), Mn(III) meso-tetrakis(N,N'-dimethylimidazolium-2-yl)porphyrin (MnTDM-2-ImP(5+)), and the ortho and para methylpyridinium complexes Mn(III) meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (MnTM-4-PyP(5+)) and Mn(III) meso-tetrakis(N-methylpyridinium-2-yl)porphyrin (MnTM-2-PyP(5+)). Both Mn(III)/Mn(II) reduction potential and SOD activity within the series follow the order: MnTTriMAP(5+)<MnTDM-4-PzP(5+)<MnTM-4-PyP(5+)<MnTM-2-PyP(5+)<MnTDM-2-ImP(5+). The kinetic salt effect (KSE) on the catalytic rate constant for superoxide dismutation (k(cat)) indicates that the electrostatic contribution to the O(2)*(-) dismutation is the greatest with MnTM-2-PyP(5+) and follows the order: MnTM-4-PyP(5+)<MnTDM-4-PzP(5+) approximately MnTDM-2-ImP(5+)<MnTM-2-PyP(5+). The KSE observed on k(cat) suggests that the charges are relatively confined within specific regions of the aryl rings. Whereas the charges in imidazolium, pyrazolium, and MnTM-4-PyP(5+) compounds are distributed in-plane with the porphyrin ring, the charges of MnTM-2-PyP(5+) are either above or below the plane, which channels the negatively-charged superoxide toward the axial positions of the Mn porphyrin more efficiently, and leads to the highest KSE. This mimics the tunneling effect observed in the SOD enzymes themselves. The modulation of the reactivity of the Mn center by the electronic perturbations caused by the meso-aryl substituent could be explained by DFT calculation, whereby a correlation between the Mn(III)/Mn(II) reduction potential (and/or SOD activity) and meso-aryl fragment softness descriptors for nucleophilic (s(f)(+)) and radical (s(f)(o)) attacks was observed. MnTDM-4-PzP(5+) and MnTM-4-PyP(5+) did not protect SOD-deficient E. coli grown aerobically, which is in agreement with their low k(cat). MnTM-2-PyP(5+) and MnTDM-2-ImP(5+) have similar high k(cat), but MnTDM-2-ImP(5+) was significantly less protective to E. coli, probably due to its bulkier size, decreased cellular uptake, and/or observed toxicity. The placement of charges closer to the metal center and spatial charge localization increases both the in vitro and the in vivo SOD activity of the compound.

Porphyrazines: Designer Macrocycles by Peripheral Substituent Change

It is rare that such a diverse array of applications can be realized from a single basic molecular unit, however, such is the power of the tetrapyrrolic macrocycle motif. Indeed, their potential in areas such as chemical dyes, optical sensors, optoelectronics, and biomedical agents is a function of their rich electronic and optical properties. While the naturally occurring porphyrins and the synthetic phthalocyanines have been extensively studied, the related tetraazaporphyrins or porphyrazines remain comparatively underdeveloped. Since porphyrazines maintain a unique position in this family: analogous derivatives are virtually inaccessible for the porphyrins, and direct fusion of heteroatomic substituents onto the porphyrazine β-positions results in a more pronounced effect compared with the substitution of an equivalent group onto the benzenoid rings of the phthalocyanine; a driving force exists to further explore the synthesis and applications of these novel macrocycles. This review will provide a historical overview of the synthetic strategies towards functionalized porphyrazines and describe new strategies towards the preparation and applications of heteroatom-appended porphyrazines, particularly in the context of their multimetallic complexes, catalysis, surface chemistry, and as biomedical agents.

Design and synthesis of manganese porphyrins with tailored lipophilicity: investigation of redox properties and superoxide dismutase activity

Thirteen new manganese porphyrins and two porphodimethenes bearing one to three different substituents at the meso positions in a variety of architectures have been synthesized. The substituents employed generally are (i) electron-withdrawing to tune the reduction potential to the desirable range (near +0.3V vs NHE), and/or (ii) lipophilic to target the interior of lipid bilayer membranes and/or the blood-brain barrier. The influence of the substituents on the Mn(III)/Mn(II) reduction potentials has been characterized, and the superoxide dismutase activity of the compounds has been examined.

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.

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.

Partially and fully β-brominated Mn-porphyrins in P450 biomimetic systems: Effects of the degree of bromination on electrochemical and catalytic properties

Beta-hexabromo-5,10,15,20-tetrakis(4-carbomethoxyphenyl)porphyrinatomanganese(III) chloride (Mn(III)(Br6TCMPP)Cl) was prepared by selective Br2-hexabromation of its parent non-brominated manganese complex (Mn(III)(TCMPP)Cl), whereas the octabrominated analogue beta-octabromo-5,10,15,20-tetrakis(4-carbomethoxyphenyl)porphyrinatomanganese(III) chloride (Mn(III)(Br8TCMPP)Cl) was synthesized via metallation of the corresponding free-base. Beta-octabromo-5,10,15,20-tetrakis(4-carbomethoxyphenyl)porphyrin was obtained by demetallation of its brominated Cu(II) derivative, which, in its turn, was prepared by either a Br2 or an N-bromosuccinimide protocol. Relative to Mn(III)(TCMPP)Cl (E(1/2) = -0.16 V vs. normal hydrogen electrode, CH2Cl2), the Mn(III)/Mn(II) reduction potential of Mn(III)(Br8TCMPP)Cl and Mn(III)(Br6TCMPP)Cl showed anodic shifts of 0.43 and 0.33 V, respectively, which corresponded to a linear shift of 0.05 V per bromine added. These manganese complexes were evaluated as cytochrome P450 mimics in catalytic iodosylbenzene (PhIO)-oxidations of cyclohexane and cyclohexene. In aerobic PhIO-oxidation of cyclohexene, epoxidation and allylic autoxidation reactions were inversely related, competitive processes; the most efficient P450-mimics were the least effective autoxidation catalysts. Mn(III)(Br6TCMPP)Cl was more efficient as epoxidation or hydroxylation catalyst than both its fully and non-beta-brominated counterparts were. There was no linear relationship between the catalytic efficiency and both the number of bromine substituents and the Mn(III)/Mn(II) potential; these observations were compared to Lyons system literature data and discussed. Analogously to enzymatic optimum pH effects, an optimum redox potential effect is suggested as relevant in designing and understanding cytochrome P450 biomimetic catalysts.

Complementation of SOD-deficient Escherichia coli by manganese porphyrin mimics of superoxide dismutase activity

Cationic Mn(III) porphyrins substituted on the methine bridge carbons (meso positions) with N-alkylpyridinium or N,N'-diethylimidazolium groups have been prepared and characterized, both chemically and as SOD mimics. The ortho tetrakis N-methylpyridinium compound was substantially more active than the corresponding para isomer. This ortho compound also exhibited a more positive redox potential and greater ability to facilitate the aerobic growth of a SOD-deficient Escherichia coli. Analogs with longer alkyl side chains and with methoxyethyl side chains, as well as with N,N'-diethylimidazolium and N,N'-dimethoxyethylimidazolium groups on the meso positions, have been prepared in anticipation of greater penetration of the cells due to greater lipophilicity. We now report that the more lipophilic compounds were effective at complementing the SOD-deficient E. coli at lower concentrations than were needed with the less lipophilic compounds. The greater efficacy of the more lipophilic compounds was achieved at the cost of greater toxicity that became apparent when these compounds were applied at higher concentrations.

New class of potent catalysts of O2.-dismutation. Mn(III) ortho-methoxyethylpyridyl- and di-ortho-methoxyethylimidazolylporphyrins

Three new Mn(III) porphyrin catalysts of O2.-dismutation (superoxide dismutase mimics), bearing ether oxygen atoms within their side chains, were synthesized and characterized: Mn(III) 5,10,15,20-tetrakis[N-(2-methoxyethyl)pyridinium-2-yl]porphyrin (MnTMOE-2-PyP(5+)), Mn(III)5,10,15,20-tetrakis[N-methyl-N'-(2-methoxyethyl)imidazolium-2-yl]porphyrin (MnTM,MOE-2-ImP(5+)) and Mn(III) 5,10,15,20-tetrakis[N,N'-di(2-methoxyethyl)imidazolium-2-yl]porphyrin (MnTDMOE-2-ImP(5+)). Their catalytic rate constants for O2.-dismutation (disproportionation) and the related metal-centered redox potentials vs. NHE are: log k(cat)= 8.04 (E(1/2)=+251 mV) for MnTMOE-2-PyP(5+), log k(cat)= 7.98 (E(1/2)=+356 mV) for MnTM,MOE-2-ImP(5+) and log k(cat)= 7.59 (E(1/2)=+365 mV) for MnTDMOE-2-ImP(5+). The new porphyrins were compared to the previously described SOD mimics Mn(III) 5,10,15,20-tetrakis(N-ethylpyridinium-2-yl)porphyrin (MnTE-2-PyP(5+)), Mn(III) 5,10,15,20-tetrakis(N-n-butylpyridinium-2-yl)porphyrin (MnTnBu-2-PyP(5+)) and Mn(III) 5,10,15,20-tetrakis(N,N'-diethylimidazolium-2-yl)porphyrin (MnTDE-2-ImP(5+)). MnTMOE-2-PyP(5+) has side chains of the same length and the same E(1/2), as MnTnBu-2-PyP(5+)(k(cat)= 7.25, E(1/2)=+ 254 mV), yet it is 6-fold more potent a catalyst of O2.-dismutation , presumably due to the presence of the ether oxygen. The log k(cat)vs. E(1/2) relationship for all Mn porphyrin-based SOD mimics thus far studied is discussed. None of the new compounds were toxic to Escherichia coli in the concentration range studied (up to 30 microM), and protected SOD-deficient E. coli in a concentration-dependent manner. At 3 microM levels, the MnTDMOE-2-ImP(5+), bearing an oxygen atom within each of the eight side chains, was the most effective and offered much higher protection than MnTE-2-PyP(5+), while MnTDE-2-ImP(5+) was of very low efficacy.

Synthesis and in vitro antioxidant properties of manganese(III) beta-octabromo-meso-tetrakis(4-carboxyphenyl)porphyrin

Manganese(III) meso-tetrakis(4-carboxypheny)porphyrin (MnTBAP) is a readily available and widely used agent to scavenge reactive oxygen species. A major limitation of MnTBAP is its relatively weak potency due to its low metal centered redox potential. The goal of these studies was to prepare a more potent analog of MnTBAP by increasing its redox potential through beta-substitution on the porphyrin ring by bromination. Manganese(III) beta-octabromo-meso-tetrakis(4-carboxyphenyl)porphyrin (MnBr(8)TBAP) was prepared in three steps starting from the methyl ester of the free ligand meso-tetrakis(4-carboxyphenyl)porphyrin, with an overall yield of 50%. The superoxide dismutase (SOD)-like activity of MnBr(8)TBAP (IC(50)=0.7 microM) was the same as manganese(III) meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (MnTM-4-PyP(5+)), while the metal-centered redox potential of the first was considerably higher than the second (E(1/2)=+128 and 0 mV vs. normal hydrogen electrode, respectively). However, a number of these cationic Mn-porphyrins (such as MnTM-4-PyP(5+)) redox-cycle with cytochrome P450 reductase in the presence of oxygen and NADPH whereas MnTBAP and its halogenated analog, MnBr(8)TBAP do not. The enhanced ability of MnBr(8)TBAP to inhibit paraquat- and hypoxia-induced injuries in vitro is also reported. In these in vitro models, in which cationic Mn-porphyrins exhibit very low activity, MnBr(8)TBAP appears to be at least eightfold more active than the non-brominated analog MnTBAP.

Electrostatic contribution in the catalysis of O2*- dismutation by superoxide dismutase mimics. MnIIITE-2-PyP5+ versus MnIIIBr8T-2-PyP+

The Mn(III) meso-tetrakis(N-ethylpyridinium-2-yl)porphyrin (Mn(III)TE-2-PyP(5+)) is a potent superoxide dismutase (SOD) mimic in vitro and was beneficial in rodent models of oxidative stress pathologies. Its high activity has been ascribed to both the favorable redox potential of its metal center and to the electrostatic facilitation assured by the four positive charges encircling the metal center. Its comparison with the non-alkylated, singly charged analogue Mn(III) beta-octabromo meso-tetrakis(2-pyridyl)porphyrin (Mn(III)Br(8)T-2-PyP(+)) enabled us to evaluate the electrostatic contribution to the catalysis of O(2)() dismutation. Both compounds exhibit nearly identical metal-centered redox potential for Mn(III)/Mn(II) redox couple: +228 mV for Mn(III)TE-2-PyP(5+) and +219 mV versus NHE for Mn(III)Br(8)T-2-PyP(+). The eight electron-withdrawing beta pyrrolic bromines contribute equally to the redox properties of the parent Mn(III)T-2-PyP(+) as do four quaternized cationic meso ortho pyridyl nitrogens. However, the SOD-like activity of the highly charged Mn(III)TE-2-PyP(5+) is >100-fold higher (log k(cat) = 7.76) than that of the singly charged Mn(III)Br(8)T-2-PyP(+) (log k(cat) = 5.63). The kinetic salt effect showed that the catalytic rate constants of the Mn(III)TE-2-PyP(5+) and of its methyl analogue, Mn(III)TM-2-PyP(5+), are exactly 5-fold more sensitive to ionic strength than is the k(cat) of Mn(III)Br(8)T-2-PyP(+), which parallels the charge ratio of these compounds. Interestingly, only a small effect of ionic strength on the rate constant was found in the case of penta-charged para (Mn(III)TM-4-PyP(5+)) and meta isomers (Mn(III)TM-3-PyP(5+)), indicating that the placement of the positive charges in the close proximity of the metal center (ortho position) is essential for the electrostatic facilitation of O(2)() dismutation.

Rotational isomers of N-alkylpyridylporphyrins and their metal complexes. HPLC separation, (1)H NMR and X-ray structural characterization, electrochemistry, and catalysis of O(2)(.-) disproportionation

Rotational (atropo-) isomers of Mn(III) meso-tetrakis(N-alkylpyridinium-2-yl)porphyrins and corresponding metal-free porphyrin ligands (where alkyl is methyl, ethyl, n-butyl, n-hexyl) and Zn(II) meso-tetrakis(N-methyl(ethyl,n-hexyl)pyridinium-2-yl)porphyrins were separated and isolated by reverse-phase HPLC. The identity of the rotational isomers of metal-free meso-tetrakis(N-methylpyridinium-2-yl)porphyrin was established by (1)H NMR spectra and by the crystal structure of the fastest eluting fraction (R(f) = 7.7%, R(w) = 9.2%, P2(1)/c, Z = 8, a = 14.2846(15) A, b = 22.2158(24) A, c = 29.369(3) A, beta = 95.374(2) degrees ) which, in accordance with (1)H NMR interpretation, proved to be the alphabetaalphabeta isomer. This result, together with elution intensity patterns, was used to identify the fractions of other Mn(III)-porphyrins, Zn(II)-porphyrins, and corresponding metal-free ligands in the series. All of the atropoisomers were inert toward isomerization which was not observable for 30 days at room temperature and reached only 50% in 16 days at 90 degrees C in the case of the Mn(III)-ethyl analogue. However, a complete freeze-dry removal of the mobile phase from the HPLC fractions caused an almost 100% isomerization. The Mn(III) meso-tetrakis(N-ethylpyridinium-2-yl)porphyrin, as a mixture of atropoisomers (AEOL-10113), has been shown to offer protection in oxidative stress injury ascribed to its high reactivity toward superoxide (k(cat) = 5.8 x 10(7) M(-1) s(-1)) as a consequence of its favorable redox potential (E(1/2) = +228 mV vs NHE). In this work, the atropoisomers were found to have similar redox potentials ranging from +240 to +220 mV, to be similarly potent catalysts of O(2)(.-) disproportionation (dismutation), with k(cat) ranging from 5.5 x 10(7) to 6.8 x 10(7) M(-1) s(-1), and not to preferentially bind to biological tissue.

A small molecular weight catalytic metalloporphyrin antioxidant with superoxide dismutase (SOD) mimetic properties protects lungs from radiation-induced injury

Radiation therapy (RT) is an important therapeutic modality in the treatment of thoracic tumors. The maximum doses to these tumors are often limited by the radiation tolerance of lung tissues. Lung injury from ionizing radiation is believed to be a consequence of oxidative stress and a cascade of cytokine activity. Superoxide dismutase (SOD) is a key enzyme in cellular defenses against oxidative damage. The objective of this study was to determine whether the SOD mimetic AEOL 10113 [manganese (III) mesotetrakis (N-ethylpyridinium-2-yl) porphyrin (MnTE-2-PyP(5+))] increases the tolerance of lung to ionizing radiation. AEOL 10113 was able to significantly reduce the severity of RT-induced lung injury. This was strongly supported with histopathology results and measurements of collagen deposition (hydroxyproline content). There was a significant reduction in the plasma level of the profibrogenic cytokine transforming growth factor-beta (TGF-beta) in the group of rats receiving RT + AEOL 10113. In conclusion, the novel SOD mimetic, AEOL 10113, demonstrates a significant protective effect from radiation-induced lung injury.

Isomeric N-alkylpyridylporphyrins and their Zn(II) complexes: inactive as SOD mimics but powerful photosensitizers

The ortho, meta, and para isomers of cationic N-alkylpyridylporphyrins and their Zn(II) complexes were compared in terms of their photodynamic properties. The ortho Zn(II) complex was found to be the most efficient in causing photooxidation of NADH in vitro. In Escherichia coli, however, the para and meta isomers were better photosensitizers than their ortho analogs. The lower potency of the ortho compound in vivo seems to be due to its lower intracellular concentration. All porphyrins tested were more efficient in killing E. coli and in photooxidizing NADH than the hematoporphyrin derivative. Antibiotic resistance did not affect the photokill, which implies that the cationic N-alkylpyridylporphyrins, as their Zn(II) complexes, can be used as bactericidal agents against antibiotic-resistant strains of gram-negative bacteria.