Diagnostic magnetic resonance (MR) imaging increasingly utilizes molecular imaging probes to correlate fundamental biochemical processes with soft tissue anatomy. Imaging probes interact with molecular markers through a chemical reaction that alters MR signal. In MR shift probes, a paramagnetic metal influences the frequency of reporter nuclei. This multichannel imaging approach enables specific detection of molecular markers and ratiometric imaging to quantify activation. The frequency of reporter nuclei is dependent on the pseudocontact shift (PCS) field, which is sensitive to the ligand field. Therefore, interactions of probes with molecular markers can be visualized as distinct signal frequencies. Herein, we report on the synthesis and characterization of Tb(III) and Dy(III) MR shift probes and a Y(III) diamagnetic analogue that respond to the reporter enzyme nitroreductase (NTR). The activation of probes L1 and L2 to L1* and L2*, respectively, by NTR unmasks a binding ligand and changes the PCS field. The probes performance is evaluated by <sup>1</sup>H NMR for three essential characteristics of the reporter signal: 1) initial shift beyond diamagnetic NMR region, 2) detectable shift in signal frequency in response to activation, and 3) high sensitivity.

Magnetic resonance shift probes for multichannel MR imaging

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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