1 research outputs found
Enhancing <i>r</i><sub>1</sub> 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 r1 relaxivity
from 3.0 to 6.3 mMā1 sā1 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 r1 with changes in pH, the
unique r1 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 r1 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
acidābase prototropic mechanisms