Purpose

The purpose of this study is to test the hypothesis that redox active iron complexes provide an straightforward and effective means to biochemically responsive MRI contrast.

Methods

Fe(3+)-PyC3A were characterized by high-pressure liquid chromatography (HPLC), mass spectrometry (MS), spectrophotometry, relaxometry, and cyclic voltammetry. The redox potential was measured by cyclic voltammetry. T1-relaxivity in water was determined from plots of 1/T1 vs. Fe concentration for at least 4 Mn concentrations, with T1 determined using an inversion recovery sequence Reductions and oxidation kinetics were measured spectrophotometrically at 310 nm in the presence cysteine and enzymatically generated hydrogen peroxide (glucose/ glucose oxidase reaction), respectively. T1-weighted images of phantoms were acquired at 25 °C, 4.7T using a 2D FLASH sequence.

Results

The relaxivity of Fe(3+)-PyC3A is at least an order of magnitude greater than Fe(2+)-PyC3A between 1.4T and 11.7T, Fig 1A. Fig 1B compares T1-weighted images of phantoms containing water, 0.5 mM Fe(2+)-PyC3A and 0.5 mM Fe(3+)-PyC3A. Contrast generated with the Fe(2+) chelate is barely perceptible, but the Fe(3+) containing solution is strongly contrast enhanced. The Fe(3+/2+) redox potential is 230 mV vs NHE and is poised within range for reaction with reactive species generated during oxidative stress (ie. H2O2, myeloperoxidase) as well as reduction by thiols responsible for governing tissue redox status, Fig 2. Fe(3+)-PyC3A is reduced by cysteine with an rate law of k= 1.5±0.28 M-1s-1. Oxidation by hydrogen peroxide is so fast that an empirical rate law could not be measured, but the rate of oxidation to Fe(3+)-PyC3A correlates tightly with the rate of enzymatically generated hydrogen peroxide production, Fig 3.

Discussion

Switching between iron oxidation states provides a field independent, order of magnitude relaxivity change. The signal modulating capabilities of Fe-PyC3A far exceeds what has been demonstrated with Gd based contrast agents.1 Oxidation state interchange is mediated by biochemical reduction and oxidation using cysteine and hydrogen peroxide, respectively. Redox active iron complexes offer a new paradigm for the design of biochemically responsive MRI contrast agents.

Conclusion

Redox active iron complexes offer a highly effective mechanism for biochemically responsive MRI contrast agents.

Acknowledgements

This work was supported by grants from the National Institutes of Health: HL128899, HL119145, EB022804, EB009062, RR014075, RR023385, and OD010650.

References

1. Angelovski G. What We Can Really Do with Bioresponsive MRI Contrast Agents. Angew. Chem. Int. Ed. 2016;55(25):7038-7046.