Copper-responsive magnetic resonance imaging contrast agents

The design, synthesis, and evaluation of the Copper-Gad (CG) family, a new class of copper-activated magnetic resonance imaging (MRI) contrast agents, are presented. These indicators comprise a Gd³⁺-DO3A core coupled to various thioether-rich receptors for copper-induced relaxivity switching. In the absence of copper ions, inner-sphere water binding to the Gd³⁺ chelate is restricted, resulting in low longitudinal relaxivity values (r ₁ = 1.2-2.2 mM^-1 s^-1 measured at 60 MHz). Addition of Cu⁺ to CG2, CG3, CG4, and CG5 and either Cu⁺ or Cu²⁺ to CG6 triggers marked enhancements in relaxivity (r ₁ = 2.3-6.9 mM^-1 s^-1). CG2 and CG3 exhibit the greatest turn-on responses, going from r₁ = 1.5 mM^-1 s^-1 in the absence of Cu⁺ to r₁ = 6.9 mM ^-1 s^-1 upon Cu⁺ binding (a 360% increase). The CG sensors are highly selective for Cu⁺ and/or Cu²⁺ over competing metal ions at cellular concentrations, including Zn²⁺ at 10-fold higher concentrations. ¹⁷O NMR dysprosium-induced shift and nuclear magnetic relaxation dispersion measurements support a mechanism in which copper-induced changes in the coordination environment of the Gd³⁺ core result in increases in q and r₁. T₁-weighted phantom images establish that the CG sensors are capable of visualizing changes in copper levels by MRI at clinical field strengths.