A R₂/R₁ Ratiometric Procedure for a Concentration-Independent, pH-Responsive, Gd(III)-Based MRI Agent

Abstract

A novel procedure for making the pH responsiveness of a Gd-based probe independent of its concentration has been developed. The method is based on a ratiometric approach and requires the measurement, at magnetic fields higher than 1 T, of the paramagnetic contribution to the longitudinal and the transverse NMR relaxation rates of water protons in the presence of a Gd-based probe with a rotational mobility in the nanosecond time scale. The method has been tested and validated in vitro by using a macromolecular adduct based on the polyornithine backbone, whose rotational mobility is pH dependent. The results reported in this communication improve substantially the diagnostic potential of MRI responsive agents.