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

Direct reactions of the MRI contrast agent K-2[Gd(DTPA)(H2O)]center dot 5H(2)O (1) (H(5)DTPA = diethylene-triaminepentaacetic acid) with dipotassium hydrogen phosphate (K2HPO4) or phosphite (K2HPO3) result in the isolation of well-defined Gd-DTPA phosphite K-6[Gd-2(DTPA)(2)(HPO3)]center dot 7H(2)O (2) or phosphate K-6[Gd-2(DTPA)(2)(HPO4)]center dot 10H(2)O (3), respectively. Their lanthanum analogs K-4[La-2(DTPA)(2)(H2O)]center dot 8H(2)O (4), K-6[La-2(DTPA)(2)(HPO3)]center dot 7H(2)O (5) and K-6[La-2(DTPA)(2)(HPO4)]center dot 10H(2)O (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 C-13 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 C-13 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 Ln(3+) cations are weakened after the substitutions, which support phosphate substituted Gd-DTPA as an initial intermediate in nephrogenic systemic fibrosis.