A biocompatible redox MRI probe based on a Mn(ii)/Mn(iii) porphyrin

For the development of redox responsive MRI probes based on the MnIII/MnII couple, stable complexation of both reduced and oxidized forms of the metal ion and appropriate tuning of the redox potential in the biologically relevant range are key elements. The water soluble fluorinated Mn-porphyrin derivative Mn-3 satisfies both requirements. In aqueous solutions, it can reversibly switch between MnIII/MnII oxidation states. In the presence of ascorbic acid or β-mercaptoethanol, the MnIII form undergoes reduction, which is slowly but fully reversed in the presence of air oxygen. A UV-Vis kinetic study of MnIII/MnII reduction under oxygen-free conditions yielded second-order rate constants, k2, of 46.1 M-1 s-1 and 13.8 M-1 s-1 for the reaction with ascorbic acid and β-mercaptoethanol, respectively. This could correspond, in the absence of oxygen, to a half-life of a few minutes in blood plasma and a few seconds in circulating immune cells where ascorbic acid reaches 20-40 μM and a few mM concentrations, respectively. In contrast to expectations based on the redox potential, reduction with glutathione or cysteine does not occur. It is prevented by the coordination of the glutathione carboxylate group(s) to MnIII in the axial position, as was evidenced by NMR data. Therefore, MnIII-3 acts as an ascorbate specific turn-on MRI probe, which in turn can be re-oxidized by oxygen. The relaxivity increase from the oxidized to the reduced form is considerably improved at medium frequencies (up to 80 MHz) with respect to the previously studied Mn-TPPS4 analogues; at 20 MHz, it amounts to 150%. No in vitro cytotoxicity is detectable for Mn-3 in the typical MRI concentration range. Finally, 19F NMR resonances of MnIII-3 are relatively sharp which could open further opportunities to exploit such complexes as paramagnetic 19F NMR probes.