A copper-activated magnetic resonance imaging contrast agent with improved turn-on relaxivity response and anion compatibility

We present the synthesis and characterization of Copper-Gad-7 (CG7), a new copper-activated magnetic resonance imaging (MRI) contrast agent that possesses a Gd³⁺-DO3A scaffold with an appended thioether-rich receptor for copper recognition. Installation of additional carboxylate groups on the periphery of the CG scaffold affords a practical strategy to increase the absolute relaxivity of these types of copper-responsive MRI sensors as well as reduce their sensitivity to biologically abundant anions. Due in large part to restricted access of inner-sphere water molecules to the paramagnetic Gd ³⁺ core, in the absence of copper ions, CG7 exhibits a relatively low relaxivity value of r₁ = 2.6 mM^-1 s^-1; addition of Cu⁺ triggers a 340% enhancement in relaxivity to r ₁ = 11.4 mM^-1 s^-1. For comparison, the relaxivity of the analogous CG2 sensor without peripheral carboxylates increases from r₁ = 1.5 to 6.9 mM^-1 s^-1 upon Cu ⁺ binding. CG7 features high selectivity for Cu⁺ over a range of biologically relevant metal ions, including the cellular abundant alkali and alkaline earth cations and d-block ions Zn²⁺ and Cu ²⁺. Moreover, the Cu⁺-response of the CG7 sensor is not significantly affected by bicarbonate, phosphate, citrate, and lactate anions at cellular levels. ¹⁷O NMR dysprosium-induced shift (DIS) and nuclear magnetic relaxation dispersion (NMRD) experiments suggest that the origin of the improved anion compatibility of CG7 is a reduced q modulation compared to previous members of the CG family, and T₁-weighted phantom images confirm that CG7 can monitor changes in copper levels by MRI at clinically relevant field strengths.