Enhancing r 1 Relaxivity in GdDOTA-Monoamide Complexes through Polar Group-Mediated Ordering of Second-Sphere Water Molecules.

This study was designed to test whether the single appended phosphonate group in GdDOTA-1AmP is sufficient for catalyzing the exchange of proton from the single inner-sphere water-exchanging molecule. Unlike the other phosphonate derivatives in this series, GdDOTA-1AmP showed a surprisingly smooth increase in r 1 relaxivity from 3.0 to 6.3 mM -1 s -1 at 20 MHz as the pH was lowered from 9 to 2.5. In comparison to the bis-, tris-, and tetrakis-phosphonate analogues, which all show a biphasic dependence of r 1 with changes in pH, the unique r 1 versus pH characteristics of GdDOTA-1AmP are shown to closely parallel deprotonation of the single appended phosphonate group. Although the tissue biodistribution and clearance rates of GdDOTA-1AmP are more favorable than the other more highly charged phosphonate derivatives, the pH dependency of r 1 is substantially reduced at magnetic fields typically used for small animal imaging (7 and 9.4T), so the attractiveness of this new molecule for quantitative imaging of tissue pH is diminished. However, this study provides some new insights into the feasibility of designing pH-responsive MRI contrast agents based upon fundamental acid-base prototropic mechanisms.