Rational Design of Superoxide Dismutase (SOD) Mimics: The Evaluation of the Therapeutic Potential of New Cationic Mn Porphyrins with Linear and Cyclic Substituents

Abstract

Our goal herein has been to gain further insight into the parameters which control porphyrin therapeutic potential. Mn porphyrins (MnTnOct-2-PyP5+, MnTnHexOE-2-PyP5+, MnTE-2-PyPhP5+, and MnTPhE-2-PyP5+) that bear the same positive charge and same number of carbon atoms at meso positions of porphyrin core were explored. The carbon atoms of their meso substituents are organized to form either linear or cyclic structures of vastly different redox properties, bulkiness, and lipophilicities. These Mn porphyrins were compared to frequently studied compounds, MnTE-2-PyP5+, MnTE-3-PyP5+, and MnTBAP3–. All Mn(III) porphyrins (MnPs) have metal-centered reduction potential, E1/2 for MnIIIP/MnIIP redox couple, ranging from −194 to +340 mV versus NHE, log kcat(O2•–) from 3.16 to 7.92, and log kred(ONOO–) from 5.02 to 7.53. The lipophilicity, expressed as partition between n-octanol and water, log POW, was in the range −1.67 to −7.67. The therapeutic potential of MnPs was assessed via: (i) in vitro ability to prevent spontaneous lipid peroxidation in rat brain homogenate as assessed by malondialdehyde levels; (ii) in vivo O2•– specific assay to measure the efficacy in protecting the aerobic growth of SOD-deficient Saccharomyces cerevisiae; and (iii) aqueous solution chemistry to measure the reactivity toward major in vivo endogenous antioxidant, ascorbate. Under the conditions of lipid peroxidation assay, the transport across the cellular membranes, and in turn shape and size of molecule, played no significant role. Those MnPs of E1/2 ∼ +300 mV were the most efficacious, significantly inhibiting lipid peroxidation in 0.5–10 μM range. At up to 200 μM, MnTBAP3– (E1/2 = −194 mV vs NHE) failed to inhibit lipid peroxidation, while MnTE-2-PyPhP5+ with 129 mV more positive E1/2 (−65 mV vs NHE) was fully efficacious at 50 μM. The E1/2 of MnIIIP/MnIIP redox couple is proportional to the log kcat(O2•–), i.e., the SOD-like activity of MnPs. It is further proportional to kred(ONOO–) and the ability of MnPs to prevent lipid peroxidation. In turn, the inhibition of lipid peroxidation by MnPs is also proportional to their SOD-like activity. In an in vivo S. cerevisiae assay, however, while E1/2 predominates, lipophilicity significantly affects the efficacy of MnPs. MnPs of similar log POW and E1/2, that have linear alkyl or alkoxyalkyl pyridyl substituents, distribute more easily within a cell and in turn provide higher protection to S. cerevisiae in comparison to MnP with bulky cyclic substituents. The bell-shape curve, with MnTE-2-PyP5+ exhibiting the highest ability to catalyze ascorbate oxidation, has been established and discussed. Our data support the notion that the SOD-like activity of MnPs parallels their therapeutic potential, though species other than O2•–, such as peroxynitrite, H2O2, lipid reactive species, and cellular reductants, may be involved in their mode(s) of action(s).

A comprehensive evaluation of catalase-like activity of different classes of redox-active therapeutics

“…The E 1/2 for any couple that involves species in high-oxidation states (eqs [3] and [4]) are similar for all Mn porphyrins (see under Catalysis of H 2 O 2 dismutation by Mn porphyrins); thus the 2 nd step, electron transfer, has no impact on the magnitude of k cat (H 2 O 2 ). In turn, the p K a(ax) was reported and confirmed here with new MnPs [49] to parallel the E 1/2 of Mn III P/Mn II P redox couple (C) [50, 68]. Consequently, the log k cat (H 2 O 2 ) paralells the E 1/2 of Mn III P/Mn II P redox couple (A) though this couple is not involved in H 2 O 2 dismutation.…”

“…Same dramatic impact of hydrogen bonding was demonstrated on the metalation rate of H 2 TnBuOE-2-PyP 4+ [47]. With MnTnHexOE-2-PyP 5+ , however, oxygen atoms are buried deeper into the hexoxyethyl chains resulting in lesser impact; in turn the log k cat (H 2 O 2 ) is closer to the value for alkyl analogs, MnTnHex(or Oct)-2-PyP 5+ [49]. The hindrance of the oxygen atoms from the solvent is also reflected in the lipophilicity of MnTnHexOE-2-PyP 5+ which is closer to the one of the alkyl analog of same number of carbon atoms in pyridyl substituents, MnTnNon-2-PyP 5+ .…”

Anticancer therapeutic potential of Mn porphyrin/ascorbate system

“…The ascorbate oxidation was catalyzed by endogenous metalloproteins, presumably metal-containing porphyrins such as cyt P450 family of enzymes [39-41]. We have already indicated that cationic MnPs may be better catalysts for ascorbate oxidation than endogenous metalloproteins [33, 42-45]. Others subsequently confirmed that MnP-driven catalysis of ascorbate oxidation bears anticancer therapeutic potential [39, 46-49].…”

Redox-Active Mn Porphyrin-based Potent SOD Mimic, MnTnBuOE-2-PyP5+, Enhances Carbenoxolone-Mediated TRAIL-Induced Apoptosis in Glioblastoma Multiforme

“…Its oxidation, catalyzed by endogenous metalloproteins resulting in cytotoxic H 2 O 2 production, was proposed as its mode of action. However, we have shown that cationic Mn(III) ortho N -substituted pyridylporphyrins, one of which will be used herein, are superior to endogenous metalloproteins as they are optimized as catalysts for ascorbate oxidation [30, 40]. Thus, such MnPs are prospective for clinical development as enhancers of ascorbate/H 2 O 2 driven cancer cell killing.…”