Rational Design of Superoxide Dismutase (SOD) Mimics:
The Evaluation of the Therapeutic Potential of New Cationic Mn Porphyrins
with Linear and Cyclic Substituents
Our
goal herein has been to gain further insight into the parameters which
control porphyrin therapeutic potential. Mn porphyrins (MnTnOct-2-PyP<sup>5+</sup>, MnTnHexOE-2-PyP<sup>5+</sup>, MnTE-2-PyPhP<sup>5+</sup>, and MnTPhE-2-PyP<sup>5+</sup>) that bear the same positive charge
and same number of carbon atoms at <i>meso</i> positions
of porphyrin core were explored. The carbon atoms of their <i>meso</i> 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-PyP<sup>5+</sup>, MnTE-3-PyP<sup>5+</sup>, and MnTBAP<sup>3–</sup>. All Mn(III) porphyrins (MnPs) have
metal-centered reduction potential, <i>E</i><sub>1/2</sub> for Mn<sup>III</sup>P/Mn<sup>II</sup>P redox couple, ranging from
−194 to +340 mV versus NHE, log <i>k</i><sub>cat</sub>(O<sub>2</sub><sup>•–</sup>) from 3.16 to 7.92, and
log <i>k</i><sub>red</sub>(ONOO<sup>–</sup>) from
5.02 to 7.53. The lipophilicity, expressed as partition between n-octanol
and water, log <i>P</i><sub>OW</sub>, was in the range −1.67
to −7.67. The therapeutic potential of MnPs was assessed via:
(i) <i>in vitro</i> ability to prevent spontaneous lipid
peroxidation in rat brain homogenate as assessed by malondialdehyde
levels; (ii) <i>in vivo</i> O<sub>2</sub><sup>•–</sup> specific assay to measure the efficacy in protecting the aerobic
growth of SOD-deficient <i>Saccharomyces cerevisiae</i>;
and (iii) aqueous solution chemistry to measure the reactivity toward
major <i>in vivo</i> 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 <i>E</i><sub>1/2</sub> ∼ +300 mV were the most efficacious, significantly inhibiting
lipid peroxidation in 0.5–10 μM range. At up to 200 μM,
MnTBAP<sup>3–</sup> (<i>E</i><sub>1/2</sub> = −194
mV vs NHE) failed to inhibit lipid peroxidation, while MnTE-2-PyPhP<sup>5+</sup> with 129 mV more positive <i>E</i><sub>1/2</sub> (−65 mV vs NHE) was fully efficacious at 50 μM. The <i>E</i><sub>1/2</sub> of Mn<sup>III</sup>P/Mn<sup>II</sup>P redox
couple is proportional to the log <i>k</i><sub>cat</sub>(O<sub>2</sub><sup>•–</sup>), <i>i.e</i>.,
the SOD-like activity of MnPs. It is further proportional to <i>k</i><sub><i>r</i>ed</sub>(ONOO<sup>–</sup>) 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 <i>in vivo S. cerevisiae</i> assay, however, while <i>E</i><sub>1/2</sub> predominates,
lipophilicity significantly affects the efficacy of MnPs. MnPs of
similar log <i>P</i><sub>OW</sub> and <i>E</i><sub>1/2</sub>, that have linear alkyl or alkoxyalkyl pyridyl substituents,
distribute more easily within a cell and in turn provide higher protection
to <i>S. cerevisiae</i> in comparison to MnP with bulky
cyclic substituents. The bell-shape curve, with MnTE-2-PyP<sup>5+</sup> 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 O<sub>2</sub><sup>•–</sup>, such as peroxynitrite, H<sub>2</sub>O<sub>2</sub>, lipid reactive
species, and cellular reductants, may be involved in their mode(s)
of action(s)