The synthesis and in vitro testing of a zinc-activated MRI contrast agent

doi:10.1073/pnas.0706247104

The synthesis and in vitro testing of a zinc-activated MRI contrast agent

Departments of *Chemistry,
^‡Biochemistry and Molecular and Cell Biology,
^§Neurobiology and Physiology, and
^¶Radiology, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208; and
^†Magnetic Resonance Center (CERM), Fiorgen Pharmacogenomic Foundation, and Department of Agricultural Biotechnology, University of Florence, Via Luigi Sacconi, 6, 50019 Florence, Italy

Communicated by Brian M. Hoffman, Northwestern University, Evanston, IL, July 3, 2007 (received for review May 20, 2007)

Abstract

Zinc(II) plays a vital role in normal cellular function as an essential component of numerous enzymes, transcription factors, and synaptic vesicles. While zinc can be linked to a variety of physiological processes, the mechanisms of its cellular actions are less discernible. Here, we have synthesized and tested a Zn(II)-activated magnetic resonance imaging (MRI) contrast agent in which the coordination geometry of the complex rearranges upon binding of Zn(II). In the absence of Zn(II) water is restricted from binding to a chelated Gd(III) ion by coordinating acetate arms resulting in a low relaxivity of 2.33 mM⁻¹·s⁻¹ at 60 MHz. Upon addition of Zn(II) the relaxivity of the Gd(III)–Zn(II) complex increases to 5.07 mM⁻¹·s⁻¹ and is consistent with one water molecule bound to Gd(III). These results were confirmed by nuclear magnetic relaxation dispersion analysis. There was no observed change in relaxivity of the Gd(III) complex when physiologically competing cations Ca(II) and Mg(II) were added. A competitive binding assay gave a dissociation constant of 2.38 × 10⁻⁴ M for the Gd(III)–Zn(II) complex. In vitro magnetic resonance images confirm that Zn(II) concentrations as low as 100 μM can be detected by using this contrast agent.

Footnotes

^‖To whom correspondence should be addressed. E-mail: tmeade{at}northwestern.edu
Author contributions: J.L.M. and T.J.M. designed research; J.L.M. and G.P. performed research; J.L.M. contributed new reagents/analytical tools; J.L.M., G.P., and C.L. analyzed data; and J.L.M., G.P., C.L., and T.J.M. wrote the paper.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/cgi/content/full/0706247104/DC1.
↵ ** Gd diethylenetriaminepentaacetate (DTPA) has a relaxivity of 3.8 mM⁻¹·s⁻¹ at 60 MHz and 310 K. In the absence of Zn(II), Gd-daa3 has a 40% lower relaxivity (2.3 mM⁻¹·s⁻¹) than Gd-DTPA, and in the presence of Zn(II), it has 33% higher relaxivity (5.1 mM⁻¹·s⁻¹) than Gd-DTPA.
Abbreviations:

daa3,

diaminoacetate with three methylenes;

DO3A,

1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid;

DTPA,

diethylenetriaminepentaacetate;

MR,

magnetic resonance;

NMRD,

nuclear magnetic relaxation dispersion;

SBM,

Solomon–Bloembergen–Morgan;

ZFS,

zero field splitting.