Modulation of the Prototropic Exchange Rate in pH-Responsive Yb-HPDO3A Derivatives as ParaCest Agents.

The aim of this work was to design new pH-sensitive paramagnetic chemical exchange saturation transfer (paraCEST) agents with a more acidic pH readout window with respect to the Yb complex of the 10-(2-hydroxypropyl)-1,4,7,10- tetraazacyclododecane-1,4,7-triacetic acid (HPDO3A). In particular, three HPDO3A derivatives have been synthesized by substituting the methyl group on the 2-hydroxypropyl arm with the electron withdrawing substituents -CF3, -CH2NH2 and -CH2OH. The Yb complexes have been characterized in terms of i) number and relative abundance of isomers present and, ii) pH sensitivity range in CEST applications. The influence of the introduced substituents on the -OH proton acidity and k(ex) has been investigated and the new agents have been compared with the parent Yb-HPDO3A. All complexes showed an increased acidity and an enhanced exchange rate of the -OH proton. As a consequence, the pH-CEST sensitivity has been shifted to more acidic values. Eventually, the -CH2OH bearing derivative has been tested to assess the intra-endosomal pH which is too acidic to be determined with Yb-HPDO3A.