-Tc/Re-tricarbonyl complexes with tridentate aminocarboxyphosphonate ligands: suitability of the phosphonate group in chelate ligand design of new imaging agents.

Ligands that coordinate via dianionic phosphonate groups have not been widely utilized in radiopharmaceuticals. -(phosphonomethyl)iminodiacetic acid (, PMIDA) and (phosphonomethyl)glycine (, PMG) were investigated as new chelators for the Tc/Re-tricarbonyl "core" (-M(CO), M = Tc, Re) present in a major class of radiopharmaceuticals. fac-M(CO)(PMIDA) and fac-M(CO)(PMG) complexes were studied by HPLC and H/C/P NMR methods for M = Re ( and ) and by HPLC for M = Tc ( and ). and complexes exhibit a similar pH-dependent equilibrium between geometric linkage isomers ( and ). However, only one isomer exists for under all conditions. Structural characterization by X-ray crystallography reveals the presence of a bond between a phosphonate oxygen and the Re(I) center in Re(CO)(PMG) (). Detailed comparisons of NMR data for conclusively demonstrate that the phosphonate group is coordinated in (isomer favored at high pH) but not in , which has a dangling -(phosphonomethyl) group. To our knowledge, and and their Tc analogs are the first well-documented examples of complexes with these metal-tricarbonyl cores having a dianionic phosphonate group directly coordinated in a -M(CO)-O-P grouping. Pharmacokinetic studies using Sprague-Dawley rats reveal that is a robust tracer. Hence, phosphonate groups should be considered in designing Tc and Re radiopharmaceuticals, including agents with bioactive moieties attached to dangling carboxylate or phosphonate groups.