Comparison of a tartaric acid derived polymeric MRI contrast agent to a small molecule model chelate

Contrast agents with high relaxivity are needed to increase the sensitivity of magnetic resonance imaging (MRI) for novel clinical and research applications. For this reason, polymeric structures containing multiple Gd(III) chelates are of current interest. Described in this communication are the syntheses and characterization of a glycopolymer derived from L-tartaric acid, Gd4(H₂O), as well as a low molecular weight compound, Gd10(H ₂O), that models the Gd(III) chelate structure in the repeat unit of polymer Gd4(H₂O). Luminescence lifetime measurements in H ₂O and D₂O for Eu(III) analogues of Gd4(H₂O) and Gd10(H₂O) [named Eu4(H₂O) and Eu10(H₂O)] reveal that the lanthanide in both structures likely has one water ligand in the primary coordination sphere. The relaxivity of the model chelate Gd10(H ₂O) at 400 MHz and 310 K was determined to be 4.7 mmol ^-1·s^-1, representing a nearly 50% increase over Magnevist (3.2 mmol^-1·s^-1). Relaxivity values on a per Gd basis for the polymeric structure Gd4(H₂O) prepared at two degrees of polymerization, n = 12 and 19, are similar, but slightly lower than Gd10(H₂O) (4.4 mmol^-1·s^-1 and 4.5 mmol^-1·s^-1, respectively). However, their molecular relaxivities of 51 mmol^-1·s^-1 and 80 mmol ^-1·s^-1, respectively, provide a substantial increase over that of Magnevist.