Development of phosphonate derivatives of gadolinium chelates for NMR imaging of calcified soft tissues

We have synthesized several classes of gadolinium (Gd) complexes for use as NMR contrast agents in the detection of soft-tissue calcification. Class I was made up of strongly chelated GdDTPA complexes with one carboxylate arm coupled to a phosphonate-containing molecule through an amide link. Class II complexes were formed by Gd with several aminophosphonates and phosphono carboxylic acids. Class III were Gd complexes of weak chelates containing no phosphonate. The calcium-seeking ability of each complex was assessed by in vivo bone uptake. Tissue distribution in normal rats showed that only the complexes of GdDTPA modified with a diphosphonate group and GdEDTMP (EDTMP is ethylenediaminetetrakis(methylenephosponate] showed adequate bone localization at the concentrations required for NMR contrast enhancement (approximately 20% of a 100 mumol/kg dose).