Towards Metabolic Organic Radical Contrast Agents (mORCAs) for Magnetic Resonance Imaging

We report two conjugates of <i>gem</i>-diethyl pyrroline nitroxide radicals with D-mannosamine as potential metabolic organic radical contrast agents, mORCAs, circumventing the need for biorthogonal reactions. In-cell EPR spectroscopy, using Jurkat cells and analogous conjugate, based on a pyrrolidine nitroxide radical, shows an efficient incorporation of highly immobilized nitroxides, with a correlation time of <i>τ</i>_cor = 20 ns. In vivo MRI experiments in mice show that the paramagnetic nitroxide radical shortens the <i>T</i>₁ and <i>T</i>₂ relaxation times of protons in water located in the kidney and brain by only up to ~10% after 3 d. Ex vivo EPR spectroscopic analyses indicate that the contrast agents in mouse tissues are primarily localized in the kidney, lung, liver, heart, and blood, which primarily contain immobilized nitroxide radicals with <i>τ</i>_cor = 4-9 ns. The spin concentrations in tissues remain low (1-3 nmol g⁻¹) at 24 h after the third mORCA injection, approximately one to two orders of magnitude lower than those of ORCAFluor and BASP-ORCA (measured at ~24 h post-injection). These low spin concentrations explain the small proton <i>T</i>₁ and <i>T</i>₂ relaxation changes observed in in vivo MRI.