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 (Arg ₈CG2), a new copper-responsive magnetic resonance imaging (MRI) contrast agent that combines a Gd ³⁺-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. Arg ₈CG2 exhibits a 220% increase in relaxivity (r 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, Arg ₈CG2 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. In cellulo relaxivity measurements and T ₁-weighted phantom images using a Menkes disease model cell line demonstrate the utility of Arg ₈CG2 to report on biological perturbations of exchangeable copper pools.