Toward stable electron paramagnetic resonance oximetry probes: synthesis, characterization, and metabolic evaluation of new ester derivatives of a tris-(para-carboxyltetrathiaaryl)methyl (TAM) radical.

Tris(p-carboxyltetrathiaaryl)methyl (TAM) radicals, such as 1a ("Finland" radical), are useful EPR probes for oximetry. However, they are rapidly metabolized by liver microsomes in the presence of NADPH, with the formation of diamagnetic quinone-methide metabolites resulting from an oxidative decarboxylation of one of their carboxylate substituents. In an effort to obtain TAM derivatives potentially more metabolically stable in vivo, we have synthesized four new TAM radicals in which the carboxylate substituents of 1a have been replaced with esters groups bearing various alkyl chains designed to render them water-soluble. The new compounds were completely characterized by UV-vis and EPR spectroscopies, high resolution mass spectrometry (HRMS), and electrochemistry. Two of them were water-soluble enough to undergo detailed microsomal metabolic studies in comparison with 1a. They were found to be stable in the presence of the esterases present in rat liver microsomes and cytosol, and, contrary to 1a, stable to oxidation in the presence of NADPH-supplemented microsomes. A careful study of their possible microsomal reduction under anaerobic or aerobic conditions showed that they were more easily reduced than 1a, in agreement with their higher reduction potentials. They were reduced into the corresponding anions not only under anaerobic conditions but also in the presence of dioxygen. These anions were much more stable than that of 1a and could be characterized by UV-vis spectroscopy, MS, and at the level of their protonated product. However, they were oxidized by O₂, giving back to the starting ester radicals and catalyzing a futile cycle of O₂ reduction. Such reactions should be considered in the design of future stable EPR probes for oximetry in vivo.