A cell-permeable gadolinium contrast agent for magnetic resonance imaging of copper in a Menkes disease model

We present the synthesis and characterization of octaarginine-conjugated Copper-Gad-2 (Arg₈CG2), a new copper-responsive magnetic resonance imaging (MRI) contrast agent that combines a Gd³⁺-DO3A scaffold with a thioether-rich receptor for copper recognition. The inclusion of a polyarginine appendage leads to a marked increase in cellular uptake compared to previously reported MRI-based copper sensors of the CG family. Arg₈CG2 exhibits a 220% increase in relaxivity (<i>r</i>₁ = 3.9 to 12.5 mM^-1 s^-1) upon 1 : 1 binding with Cu⁺, with a highly selective response to Cu⁺ over other biologically relevant metal ions. Moreover, Arg₈CG2 accumulates in cells at nine-fold greater concentrations than the parent CG2 lacking the polyarginine functionality and is retained well in the cell after washing. <i>In cellulo</i> relaxivity measurements and <i>T</i>₁-weighted phantom images using a Menkes disease model cell line demonstrate the utility of Arg₈CG2 to report on biological perturbations of exchangeable copper pools.