Relaxation-Based In Vivo Discrimination of Oxidized and Reduced States of a Redox-Switchable <sup>19</sup>F MRI Probe
Zoltán Garda, Frédéric Szeremeta, Csilla Noémi Tóth, Szilvia Bunda, Carlo Pifferi, Rudy Clémençon, Sandra Même, Gyula Tircsó, Éva Tóth
Abstract
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 Fe 2+ and Fe 3+ ions, and the redox potential of the Fe 3+ L1/Fe 2+ L1 couple (+240 mV vs NHE) is adapted to biological redox sensing. Fe 2+ L1 undergoes instantaneous oxidation in the presence of H 2 O 2, and Fe 3+ L1 is reduced by cysteine, glutathione, and ascorbate. Fe 2+ L1 and Fe 3+ 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 19 F relaxation times ( T 1 = 71–130 ms; T 2 = 60–117 ms and T 1 = 2.43 ms; T 2 = 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 Fe 2+ L1 and UTE for Fe 3+ L1, combined with appropriate acquisition parameters), both redox forms are detected in 19 F MR phantom images with good sensitivity and signal-to-noise ratios linearly dependent on probe concentration. Fe 2+ L1 and Fe 3+ L1 can be readily visualized and unambiguously discriminated based on their 19 F relaxation times in living mice, following intramuscular injection. The possibility of monitoring the redox switch in 1 H MRI as well is an additional advantage of this bioresponsive probe.