Relaxation-Based In Vivo Discrimination of Oxidized and Reduced States of a Redox-Switchable 19F MRI Probe

MRI assessment of the tissue redox state is important for revealing and understanding various pathologies, and redox-responsive imaging probes capable of generating discrete and quantifiable signals in both their reduced and oxidized forms can provide enhanced detection reliability. The small fluorinated, redox-active FeL1 chelate is a prototype of such agents. L1 forms stable and inert complexes with both Fe²⁺ and Fe³⁺ ions, and the redox potential of the Fe³⁺L1/Fe²⁺L1 couple (+240 mV vs NHE) is adapted to biological redox sensing. Fe²⁺L1 undergoes instantaneous oxidation in the presence of H₂O₂, and Fe³⁺L1 is reduced by cysteine, glutathione, and ascorbate. Fe²⁺L1 and Fe³⁺L1 have very different proton relaxivities (0.1 mM^-1 s^-1 and 2.83 mM^-1 s^-1, respectively, 60 MHz, 298 K), as well as ¹⁹F relaxation times (T₁ = 71-130 ms; T₂ = 60-117 ms and T₁ = 2.43 ms; T₂ = 1.81 ms, respectively, 400 MHz, 298 K), in accordance with the different paramagnetic relaxation enhancement capacity of the two iron redox states. Upon application of specific MRI pulse sequences adapted to the relaxation rate (RARE for Fe²⁺L1 and UTE for Fe³⁺L1, combined with appropriate acquisition parameters), both redox forms are detected in ¹⁹F MR phantom images with good sensitivity and signal-to-noise ratios linearly dependent on probe concentration. Fe²⁺L1 and Fe³⁺L1 can be readily visualized and unambiguously discriminated based on their ¹⁹F relaxation times in living mice, following intramuscular injection. The possibility of monitoring the redox switch in ¹H MRI as well is an additional advantage of this bioresponsive probe.