Carbohydrate-appended 2,2′-dipicolylamine metal complexes as potential imaging agents

Three discrete carbohydrate-appended 2,2'-dipicolylamine ligands were complexed to the {M(CO)3}(+) (M = Tc-99m/ Re) core: 2-(bis(2-pyridinylmethyl)amino)ethyl-beta-D-glucopyranoside (L-1), 2-(bis(2-pyridinylmethyl)amino)ethyl-beta-Dxylopyranoside (L-2), and 2-(bis(2-pyridinylmethyl)amino)ethyl-alpha-D-mannopyranoside (L-3). An ethylene spacer is used to separate the carbohydrate moiety and the dipicolylamine (DPA) function in all three ligands. The Re complexes [Re(L1-3)(CO)(3)]Br were characterized by H-1 and C-13 1D/2D NMR spectroscopies, which confirmed the pendant nature of the carbohydrate moieties in solution. NMR measurements also established the long-range asymmetric effect of the carbohydrate functions on the chelating portion of the ligand. One analogue, [Re(L-1)(CO)(3)]Cl, was characterized in the solid state by X-ray crystallography. Further characterization was provided by IR spectroscopy, elemental analysis, conductivity, and mass spectrometry. Radiolabeling of L-1-L-3 with [Tc-99m(H2O)(3)(CO)(3)](+) afforded high yield compounds of identical character to the Re analogues. The radiolabeled compounds were found to be stable toward ligand exchange in the presence of a large excess of either cysteine or histidine over a 24-h period.