Using MRI as a kinetic tool Viliyana G. Lewis, Gemma-Louise Davies University College, UK
Magnetic Resonance imaging (MRI) is a non-invasive technique widely used for soft tissue scans due to its high spatial resolution (<1 mm) and the use of non-damaging radiofrequency (RF) radiation.1,2 Contrast agents are frequently employed to improve tissue contrast and Gd3+-based contrast agents improve the T1 relaxation times of water protons, resulting in brighter images. Interactions of contrast agents and guest molecules can be quantified through the relaxivity enhancement parameter (r1=∆(1/T1)/[Gd3+], mM-1 s-1) or by calculating association constants from titrations using fluorescent analogues to the MRI contrast agents.1,3-6These methods work well for stronger interactions; however, very weak binding cannot be detected using these standard techniques. In this work kinetic profiles of the weak interactions of Dotarem® (MRI contrast agent) and guest compounds were generated by recording the relaxivity enhancement (r1, mM-1 s-1) for different concentrations of the guest molecule. Association constants of these weak interactions were calculated by fitting the MRI kinetic curves in Dynafit software. The results were compared with existing techniques. Temperature-dependent MRI of Dotarem® was performed to investigate whether the water exchange mode changed upon adding lysine to theDotarem® solution. Furthermore, nuclear magnetic resonance dispersion (NMRD) profiles were generated and the contribution of the inner and second/outer solvation spheres of Dotarem® to the relaxivity was calculated using Fitteia software. In all cases the association constants based on chemical interactions in MRI were higher than the binding association constants calculated using fluorescent analogues. In most cases fluorescent titrations showed no change in signal at all, whereas equivalent MRI titrations showed titration curves which could be fitted effortlessly. This was attributed to the richness of the MRI signal derived from all three solvation spheres in contrast to the fluorescent signal, which can only detect inner-sphere contributions. Other standard techniques used to detect binding, such as diffusion NMR (DOSY) were tested and failed to detect these weak interactions. Temperature- dependent MRI profiles showed no change in the water exchange regime upon addition of lysine. Both Dotarem® and Dotarem® -lysine were in fast-exchange regime (τM < T1M). Fitting of the NMRD profiles revealed dominant second/outer sphere effects upon addition of most guest molecules explaining why these interactions could not be detected using standard fluorescent titrations. This new way of using MRI can be utilised to detect and quantify much weaker interactions than current methods. In addition, association constants calculated using MRI are more representative of the signal compared with current fluorescent titrations used in the literature. References 1. G. L. Davies, et al., Chem. Commun., 2013, 49, 9704–9721. 2. Y. Shen,et al., Invest. Radiol., 2015, 50, 330–338
3. J. L. Major, et al., Proc. Natl. Acad. Sci., 2007, 104, 13881–13886. 4. P. Caravan,et al., J. Am. Chem. Soc., 2002, 124, 3152–3162. 5. G. Angelovski,et al., ChemBioChem, 2008, 9, 1729–1734. 6. E. L. Que et al., J. Am. Chem. Soc., 2006, 128, 15942–15943.
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