Flow-injection EPR investigation on OH radical scavenging activity of Gd(III) containing MRI contrast media

The OH radical scavenging activity of a series of Gd(III) MRI contrast media, such as Gd(III)DTPA, Gd(III)BMA and Gd(III)DO3A, were evaluated by means of EPR spin trapping measurements. The second order reaction rate constant (k2) occurring between Gd(III)DTPA and OH radical was estimated to be 3.26 x 10(10) (M(-1)s(-1)), which was ten times larger than that of the free ligand DTPA (3.86 x 10(9) M(-1)s(-1). The k2 values of Gd(III)BMA and Gd(III)DO3A were also determined to be 1.31 x 10(10) and 1.77 x 10(10) (M(-1)s(1)), respectively. The present results suggest that widely used Gd(III) containing MRI reagent exhibit OH radical scavenging activity, and these values of k2 are same order as that of ascorbic acid (1.16 x 10(10) M(-1)s(-1)) which has been well established to be the most powerful OH radical scavenger. Based on the EPR measurements performed for these Gd(III) complexes, a possible reaction mechanism of the OH radical scavenging action of these MRI contrast media will be discussed.