The Rates of the Exchange Reactions between Gd(DTPA)2− and the Endogenous Ions Cu2+ and Zn2+: A Kinetic Model for the Prediction of the In Vivo Stability of Gd(DTPA)2−, Used as a Contrast Agent in Magnetic Resonance Imaging

The kinetic stability of the complex Gd(DTPA)2- (H5DTPA = diethylenetriamine-N,N,N',N",N"-pentaacetic acid), used as a contrast-enhancing agent in magnetic resonance imaging (MRI), is characterised by the rates of the exchange reactions that take place with the endogenous ions Cu2+ and Zn2+. The reactions predominantly occur through the direct attack of Cu2+ and Zn2+ on the complex (rate constants are 0.93+/-0.17 M(-1) s(-1) and (5.6+/-0.4) x 10(-2)M(-1) S(-1), respectively). The proton-assisted dissociation of Gd(DTPA)2- is relatively slow (k1 = 0.58+/-0.22 M(-1) s(-1)), and under physiological conditions the release of Gd3+ predominantly occurs through the reactions of the complex with the Cu2+ and Zn2+ ions. To interpret the rate data, the rate-controlling role of a dinuclear intermediate was assumed in which a glycinate fragment of DTPA is coordinated to Cu2+ or Zn2+. In the exchange reactions between Gd-(DTPA)2- and Eu3+, smaller amounts of Cu2+ and Zn2+ and their complexes with the amino acids glycine and cysteine have a catalytic effect. In a model of the fate of the complex in the body fluids, the excretion and the "dissociation" of Gd(DTPA)2- are regarded as parallel first-order processes, and by 10 h after the intravenous administration the ratio of the amounts of "dissociated" and excreted Gd(DTPA)2- is constant. From about this time, 1.71% of the injected dose of Gd(DTPA)2- is "dissociated". The results of equilibrium calculations indicate that the Gd3+ released from the complex is in the form of Gd3+-citrate.