Interaction of Gd-DTPA with phosphate and phosphite: toward the reaction intermediate in nephrogenic systemic fibrosis

Direct reactions of the MRI contrast agent K2[Gd(DTPA)(H2O)]·5H2O (1) (H5DTPA = diethylenetriaminepentaacetic acid) with dipotassium hydrogen phosphate (K2HPO4) or phosphite (K2HPO3) result in the isolation of well-defined Gd-DTPA phosphite K6[Gd2(DTPA)2(HPO3)]·7H2O (2) or phosphate K6[Gd2(DTPA)2(HPO4)]·10H2O (3), respectively. Their lanthanum analogs K4[La2(DTPA)2(H2O)]·8H2O (4), K6[La2(DTPA)2(HPO3)]·7H2O (5) and K6[La2(DTPA)2(HPO4)]·10H2O (6) are used for comparison. The phosphate and phosphite groups are able to substitute the coordinated water molecules in 1 and 4 in a close physiological aqueous solution, and act as bridging ligands to link adjacent Ln(DTPA)2− (Ln = Gd and La) into dimeric structures. Solid state and solution 13C NMR spectra of dimer 4 show complete dissociation into its monomeric species in solution, while no dissociation is observed for lanthanum phosphite 5 and phosphate 6 in solution, which show only one set of 13C spectra with the largest downfield shifts at 182.0 and 182.3 ppm respectively. Comparisons of the bond distances and spectral data indicate that the interaction between DTPA and central Ln3+ cations are weakened after the substitutions, which support phosphate substituted Gd-DTPA as an initial intermediate in nephrogenic systemic fibrosis.