Catalytic antioxidant mimetics
Endogenous antioxidant enzymes are examples of catalytic antioxidants and have been used as models for the development of catalytic antioxidant mimetics. The three most prominent classes are the SOD, catalase, and GPx mimics. Most of these compounds contain a ligated transitional metal or selenium. They are generally broad-spectrum antioxidants that can scavenge O2−, H2O2, ONOO−, and a variety of lipid peroxides. The SOD and catalase mimic class include macrocyclics, metallo-porphyrins, salens, and nitroxides. The GPx class includes selenium- and tellurium-based compounds.
The potencies of these catalytic antioxidants are often compared by using their rate constants obtained under tightly defined simple chemical systems, which may or may not be relevant in more complex biologic systems. Another important note is that many of these agents can obtain electrons from cellular sources (54
). These properties have two important consequences that affect the in vivo
rate constant for the reaction with ROS and can also result in the inhibition of ROS production. The finding that many of these diverse compounds are effective in similar oxidative stress models confirms the basic concept that small, efficient, catalytic antioxidants show promise in the treatment of ROS-mediated conditions associated with injury and tissue dysfunction.
The pentaazamacrocyclic ligand-based mimetics [M40403 is currently being developed by ActivBiotics (http://www.activbiotics.com
)] are unique in that they are relatively specific O2−
scavengers, because the manganese atom (Mn) is held by five coordination points in the macrocyclic structure and is available only for one-electron transfers (). Mn(II) macrocylics function in the dismutation reaction with O2−
by alternate oxidation and reduction, changing its valence between Mn(II) and Mn(III) (11
). This unique aspect of these compounds gives them selectivity toward O2−
under highly defined conditions. However, in biologic systems, it is unclear whether it is only with O2−
that these compounds interact. A number of endogenous compounds also can partake in one-electron reactions beside O2−
; these include flavins and ubiquinones. The macrocyclics are effective in many of the same oxidative paradigms in which nonselective catalytic antioxidant mimetics have been used. The different classes of catalytic antioxidant mimetics have not been directly compared in experimental models; therefore, the conditions under which one class holds an advantage over the others are currently not known.
Chemical structures of macrocyclic catalytic antioxidant mimetics with SOD activity.
Metalloporphyrins [AEOL series is currently being developed by Aeolus Pharmaceuticals (http://www.aeoluspharma.com
)] are structurally different from endogenous protoporphyrins and are classified as synthetic meso
-substituted porphyrins (). Metalloporphyrins have been shown to possess at least four distinct antioxidant properties, which include scavenging O2−
), and lipid peroxides (51
). Most metalloporphyrins contain either an Fe or Mn that is coordinated by four nitrogen axial ligands. The catalase-like activity of metalloporphyrins is thought to be due to their extensive conjugated ring system that can undergo reversible one-electron transfer in addition to the one-electron transfer on the metal center (70
). This mechanism is similar to that proposed for the heme prosthetic groups of endogenous catalase and peroxidases. The two classes of metalloporphyrins include one group in which the SOD activities track with their catalase activities, and another group that has very little SOD activity and high catalase activity. An example of a manganese porphyrin with both high SOD and catalase-like activities is AEOL 10150 (98
), whereas an example of a compound with low SOD activity and high catalase activity is AEOL 11207 (122
). The compounds with high catalase-like activity still only possess a fraction of the native catalase enzyme activity under chemically defined conditions, yet they can protect cells from H2
-mediated toxicity (53
). This may not be a fair comparison because catalase is hard to saturate with H2
and has a relative high km
. Under biologically relevant steady-state levels of H2
, the metalloporphyrins are more comparable to catalase (32
). Metalloporphyrins have been shown to be effective in ameliorating oxidative stress, inflammation, and injury in a large number of animal models of human disease (50
). Metalloporphyrins have plasma half-lives that range from 4 to 48 hours. Most metalloporphyrins are not extensively metabolized by the body and are largely excreted unchanged in the urine. A previous limitation of the metalloporphyrin class of compounds has been poor oral bioavailability, but several compounds in the AEOL 112 series have good oral bioavailability and longer plasma half-lives that should make them better candidates for treating chronic diseases (122
Chemical structures of metalloporphyrin catalytic antioxidant mimetics with SOD and catalase activities.
The salen class of catalytic antioxidant mimetics (EUK series) is currently being developed by Proteome Systems (http://www.proteomesystems.com
) (). Generically, salens are aromatic, substituted ethylenediamine metal complexes. The Mn(III)-containing salen complexes have both O2−
dismutation activities (63
). However, like the metalloporphyrins, these compounds are not selective and can react with O2−
and other peroxides, including ONOO−
). The Mn moiety of the salen is coordinated by four axial ligands. One of the unique features of these compounds is that the metal center is coordinated to oxygen and nitrogen atoms, which is in contrast to the porphyrins, in which the metal is coordinated to nitrogen atoms. The coordination of Mn by four axial ligands results in the formation of several possible valance states that give these compounds their broad ROS-scavenging capabilities. The rates at which reported salens scavenge H2
are similar to those reported for metalloporphyrins, but are many orders less than those documented for catalase under similarly defined conditions (63
). Salens have also been shown to protect cells against oxidative stress and are protective in a large number of animal models of human diseases (50
). One of the current limitations of the salens is the stability of the parent compounds in biologic matrix, which makes it difficult to determine tissue levels and half-lives.
Chemical structures of salen catalytic antioxidant mimetics with SOD and catalase activities.
A number of compounds initially developed as free radical spin traps have been shown to have antioxidant properties in cell and animal systems (136
). These compounds react with free radicals and form more-stable free radical products. The most frequently used compounds are the nitroxides and include α
-butylnitrone (PBN) and 2,2,6,6-tetramethylpiperidine N
-oxyl (TEMPO) (). These compounds have also been described as non–metal-containing SOD mimics (5
). The rate of reaction with O2−
is relatively low and thus requires large amounts (often millimolar levels) of these compounds to be present in the system to be effective (190
). Fortunately, these compounds are well tolerated in animals and can achieve high tissue levels (136
). These agents have a number of properties other than the reaction with ROS that could also explain some of their protective properties in models of oxidative stress. Many of these compounds can be metabolized to release NO and can inhibit enzymes that are endogenous sources of ROS (34
Chemical structures of nitroxide catalytic antioxidant mimetics with SOD activity.
GPx enzymes are found in every compartment within the cell and tissues and are effective scavengers of cellular peroxides. The GPx mimetic class includes mono- and diselenium–containing compounds (). One of the best-studied GPx-like mimics is 2-phenyl-1,2-benzisoselenazol-3(2H)-one, also known as ebselen or PZ51. Ebselen was one of the first selenium-based GPx mimics developed and catalytically scavenges peroxides in the presence of reducing equivalents such as GSH, N
-acetylcysteine (NAC), and dihydrolipoate (DHLA) (179
). The mechanism by which this occurs is still debated and may differ under different conditions. Ebselen has also been shown to stimulate the decomposition of a number of ROS, including hypochlorous acid (HOCl) (17
), singlet oxygen (174
), and ONOO−
). Ebselen can readily bind cellular thiol groups on proteins, which may complicate the interpretation of biologic effects, because many cellular proteins have reactive thiols in their catalytic domains. It has been documented that ebselen can inhibit lipoxygenases (168
), NADPH oxidases (46
), and nitric oxide synthases (226
). All of these enzymes are also potential sources of endogenous ROS. Ebselen has been shown to be protective in a number of cell-culture systems (159
) and animal models of human disease (179
). Ebselen is orally active and appears to be well tolerated in animals and humans. Newer analogues of ebselen have been developed, including BXT-51072, which has increased activity and potency in cell systems. These analogues [BXT series are being developed by Oxis International (http://www.oxis.com
)] have been shown to be protective in a limited number of cell-culture systems and animal models of human disease (203
Chemical structures of selenium-containing catalytic antioxidant mimetics with glutathione peroxidase activity.
A number of diselenide- and ditelluride-containing compounds have been reported to catalytically scavenge peroxides with higher GPx-like activity than ebselen (75
). Sulfur, selenium, and tellurium belong to group IV of the periodic table and have similar chemical properties. A major difference with these types of compounds is that they usually contain a diselenide bond. Earlier compounds, such as the diphenyl diselenide (DPDS), were electrophilic agents that had cytotoxic, genotoxic, and mutagenic issues (4
). Many previously reported diselenide compounds release free selenium during the catalytic cycle, and this may be problematic in their development as therapeutic agents. A unique aspect of a newer series of these compounds is the cyclodextrin group, which may help in directing hydrophobic peroxides toward the selenium or tellurium active site. The diselenide, 2,2′-deseleno-bis-β
-cyclodextrin (2-SeCD), can scavenge a variety of peroxides including H2
-butyl hydroperoxide, and cumenyl hydroperoxide by using GSH as a cofactor (124
). Only a limited number of cell-culture studies have been reported for these compounds (140
), and it is still unclear whether these compounds can be successfully used in animal models of lung fibrosis.
The largest categories of antioxidants are those that are reactive toward ROS, and the product of the reaction results in a less-toxic species. The naturally occurring vitamins E (α-tocopherol) and C (ascorbate) are such examples. Both the ascorbate and α-tocopherol radicals are less reactive and can be recycled by cellular reductases. Glutathione is a thiol-containing tripeptide that readily reacts with peroxides and forms a less-toxic disulfide product that is recycled by glutathione reductase. A number of synthetic compounds have been models after these endogenous antioxidants and have been shown to be protective in models of oxidative stress ().
Chemical structures of antioxidant scavengers.
A number of polyphenolic-based antioxidants are known, such as the water-soluble analogue of α
-tocopherol, known as trolox, hindered phenols that include butylated hydroxytoluene (BHT), and various plant phenolics such as curcumin and flavonoids. These compounds are often chain-breaking antioxidants, and some have been used in the food industry as preservatives (42
). In general, they require larger doses or concentrations to produce antioxidant effects in model systems because of their lower rates of reaction with ROS and their limited ability to be recycled endogenously.
Another group of compounds use a steroid nucleus substituted with antioxidant side groups and are known as lazaroids. Lazaroids are very effective at inhibiting iron-dependent lipid peroxidation (39
). Lazaroids have been extensively tested as neuroprotective agents, but it is still not clear whether their neuroprotective effects are directly related to their antioxidant properties.
A large class of antioxidants is the thiol-containing compounds. The most extensively studied thiol compound is N
-acetyl cysteine (NAC). NAC is a direct-acting antioxidant and can scavenge several ROS such as hypochlorous acid, peroxides, ·
OH, and ONOO−
). NAC can also serve as a cellular source of cysteine for the endogenous synthesis of GSH. NAC can suppress the activation of transcription factors such as NF-κ
B as a way to modulate cell-signaling pathways. A homocysteine derivative, erdosteine, is a prodrug that, when metabolized, produces an active thiol antioxidant metabolite (56
). Erdosteine has beneficial effects in COPD patients (156
). Amifostine is another prodrug that, when metabolized, produces an active thiol antioxidant used clinically as a radioprotective agent (37
). Thiol-containing agents can also act as metal chelators and decrease oxidative stress by limiting the ability of transitional metal to participate in ROS formation. Paradoxically, some thiol-containing agents have the potential to create a more-reactive species when they react with ROS, which is often dependent on availability of oxygen and transitional metals.