Novel gadolinium(III) polyaminocarboxylate macrocyclic complexes as potential magnetic resonance imaging contrast agents

Two novel Gd(III) complexes with functionalised polyaminocarboxylate macrocycles, 1,4,7-tris(carboxymethyl)-9,24-dioxo-14,19-dioxa-1,4,7,10,23- pentaazacyclododecane (L¹) and 1,4,7-tris(carboxymethyl)-9,25-dioxo- 14,17,20-trioxa-1,4,7,10,23- pentaazacyclotridecane (L²), were prepared in good yield. Their potential use as magnetic resonance imaging (MRI) contrast agents (CAs) was evaluated by investigating their relaxation behaviour as a function of pH, temperature and magnetic field strength. The 1/T ₁ proton relaxivities at 20 MHz and 25°C of GdL¹ (5.87 mM^-1 s^-1) and GdL² (6.14 mM^-1 s^-1) were found to be significantly higher than the clinically used Gd 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (Gd(DOTA) ^-) and Gd diethylenetriaminepentaethanoic acid (Gd(DTPA) ^2-). The complexes possess one water molecule in the inner coordination sphere whose mean residence lifetime was estimated to be 1.1 and 1.5 μs at 25°C by variable temperature (VT) ¹⁷O NMR spectroscopy.