Species dependence on plasma protein binding and relaxivity of the gadolinium-based MRI contrast agent MS-325

Rationale and Objectives: We sought to determine whether there is a species dependence on plasma protein and serum album binding and/or relaxivity of the MR contrast agent MS-325. Methods: Equilibrium binding of MS-325 to plasma proteins or purified serum albumin was determined as a function of chelate concentration. T-1 and T-2 values were determined at 0.47 and 1.41 T, and NMRD profiles were measured to determine the changes in relaxivity over varying field strengths from 0.002 to 1.2 T. Results: The binding of MS-325 to either animal plasma or serum albumin plateaus at chelate concentrations less than 0.1 mM with human, pig, and rabbit plasmas showing maximum binding. Human and pig plasmas show the greatest observed relaxivity enhancement in the presence of MS-325. Conclusions: MS-325 exhibits increased relaxivity in blood plasma as the result of plasma protein binding. Binding ranged from 64% to 91% and was species dependent: human > pig similar to rabbit > dog similar to rat similar to mouse.