Preparation and Characterization of Paramagnetic Polychelates and Their Protein Conjugates

The gadolinium complexes of poly-L-lysine-poly(diethylenetriamine-N,N,N',N '',N ''-pentaacetic acid) (Gd-PL-DTPA) and poly-L-lysine-poly(1,4,7,10-tetraazacyclododecane-N,N',N '',N"'-tetraacetic acid) (Gd-PL-DOTA) and their conjugates with human serum albumin (HSA) have been prepared and characterized. Poly-L-lysine (PL, degree of polymerization approximate to 100) was N-acylated with a mixed anhydride of the chelating ligand (DTPA or DOTA). Sixty to ninety chelating groups per molecule of PL could be attached in this way. Following purification of the polychelate by size-exclusion chromatography, the gadolinium complexes were prepared by standard methods and conjugated to HSA with heterobifunctional cross-linking reagents. The molar relaxivities of these macromolecular species were 2-3-fold higher than those of the corresponding monomeric metal complexes ([Gd(DTPA)l and [Gd(DOTA)]). The conjugation conditions were optimized to produce conjugates containing 60-90 metal centers per molecule of HSA (ca. one polychelate per protein).