Complete on/off responsive ParaCEST MRI contrast agents for copper and zinc

Two thulium-based ParaCEST responsive contrast agents, Tm-DOTAm-py and Tm-DOTAm-βAla-py, have been synthesized and evaluated for imaging copper and zinc. Unusual for responsive MRI contrast agents, both agents display a complete on/off response in the presence of transition metals. Both complexes function as paraCEST agents in the absence of copper and zinc, with the positively charged Tm-DOTAm-py being more sensitive than the neutrally charged Tm-DOTAm-βAla-py. In each case, the CEST signal arises from amide protons rather than from a water molecule coordinated to Tm³⁺ ions. Upon binding to Cu⁺, Cu²⁺, or Zn²⁺, the exchange rate of the amide protons increases substantially, resulting in a complete loss of the CEST signal. This efficient mode of action along with the lack of inner-sphere water molecules both in the presence and absence of transition metals was confirmed by 1/T₁ NMRD profiles, ¹⁷O NMR measurements, and molecular modelling simulations. Neither complex is selective for copper over zinc. Both form either a 1:1 TmL:Cu⁺ or a 2:1 TmL:Cu²⁺ and TmL:Zn²⁺ complexes with binding affinities comparable to that of other responsive MRI contrast agents and sensitivity comparable to that of other CEST contrast agents.