Gd 3+ Complexes for MRI Detection of Zn 2+ in the Presence of Human Serum Albumin: Structure-Activity Relationships.

Zn 2+ -responsive magnetic resonance imaging (MRI) contrast agents are typically composed of a Gd chelate conjugated to a Zn 2+ -binding moiety via a linker. They allow for Zn 2+ detection in the presence of human serum albumin (HSA). In order to decipher the key parameters that drive their Zn 2+ -dependent MRI response, we designed a pyridine-based ligand, PyAmC2mDPA , and compared the properties of GdPyAmC2mDPA to those of analogue complexes with varying Gd core, Zn-binding moiety, or linker sizes. The stability constants determined by pH potentiometry showed the good selectivity of PyAmC2mDPA for Gd 3+ (log K Gd = 16.27) versus Zn 2+ (log K Zn = 13.58), proving that our modified Zn 2+ -binding DPA moiety prevents the formation of previously observed dimeric species. Paramagnetic relaxation enhancement measurements indicated at least three sites that are available for GdPyAmC2mDPA binding on HSA, as well as a 2-fold affinity increase when Zn 2+ is present ( K D = 170 μM versus K DZn = 60 μM). Fluorescence competition experiments provided evidence of the higher affinity for site II vs site I, as well as the importance of both the Zn-binding part and the Gd core in generating enhanced HSA affinity in the presence of Zn 2+ . Finally, an analysis of nuclear magnetic relaxation dispersion (NMRD) data suggested a significantly increased rigidity for the Zn 2+ -bound system, which is responsible for the Zn 2+ -dependent relaxivity response.