REACTION OF GADOLINIUM CHELATES WITH ENDOGENOUSLY AVAILABLE IONS

The extent of reaction of Gd-153-radiolabeled Gd(L) chelates with 25 mM CO3(2-)(25 mF), PO4(3-), Zn2+ and Cu2+ at pH 7 was determined for L = EDTA, DTPA, DOTA, HP-DO3A, and DO3A. Gd(EDTA)- and Gd(DTPA)2- reacted (> 20% in 10 min) with Cu2+ and Zn2+ in the presence of PO4(3-). These double replacement reactions yielded precipitated GdPO4 and chelated Cu(L). Gd(HP-DO3A), Gd(DO3A) and Gd(DOTA)- were inert to reaction with all four ions at room temperature (less-than-or-equal-to 1% reaction detected). The thermodynamic binding constants of the ligands for Gd3+ and Cu2+ were found to be equal (10(20) M-1) for DO3A, while DOTA and HP-DO3A favored Gd3+ over Cu2+ by greater-than-or-equal-to 10(2) M-1. The low order of reactivity of Gd(DOTA)- and Gd(HP-DO3A) was anticipated by the binding constants, but the lack of reactivity of Gd(DO3A) is attributed to kinetic inertia. This latter property, desirable in MRI contrast agents, is promoted by the conformational stability of the tetraazacyclododecane macrocycle, which forms the backbone of the ligand. It is concluded that this class of chelates is exceptionally inert in solutions of endogenously available ions, and that thermodynamics alone is an insufficient predictor of the reactivity of the highly inert Gd complexes based on the tetraazamacrocycle.