The kinetic stability of the complex [Gd(DTPA)]2- (H5DTPA = diethylenetriamine-N,N,N′,N″,N″-penta-acetic 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±0.17M-1S-1 and (5.6±0.4) × 10-2M-1 S-1, respectively). The proton-assisted dissociation of [Gd(DTPA)]2- is relatively slow (k1 = 0.58±0.22M-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.
|Number of pages||6|
|Journal||Chemistry - A European Journal|
|Publication status||Published - febr. 18 2000|
ASJC Scopus subject areas
- Organic Chemistry