Separation and characterization of the two diastereomers for [Gd(DTPA-bz-NH2)(H2O)]2-, a common synthon in macromolecular MRI contrast agents:: Their water exchange and isomerization kinetics

Chiral, bifunctional poly(amino carboxylate) ligands are commonly used for the synthesis of macromolecular, Gd-III-based MRI contrast agents, prepared in the objective of increasing relaxivity or delivering the paramagnetic Gd-III to a specific site (targeting). Complex formation with such ligands results in two diastereomeric forms for the complex which can be separated by HPLC. We demonstrated that the diastereomer ratio for Ln(III) DTPA derivatives (similar to 60:40) remains constant throughout the lanthanide series, in contrast to Ln(III) EPTPA derivatives, where it varies as a function of the cation size with a maximum for the middle lanthanides (DTPA(5-) = diethylenetriaminepentaacetate; EPTPA(5-) = ethylenepropylenetriaminepentaacetate). The interconversion of the two diastereomers, studied by HPLC, is a proton-catalyzed process (k(obs) = k(1)[H+]). It is relatively fast for [Gd(EPTPA-bz-NH2)(H2O)](2-) but slow enough for [Gd(DTPA-bz-NH2)(H2O)](2-) to allow investigation of pure individual isomers (isomerization rate constants are k(1) = (3.03 +/- 0.07) x 10(4) and 11.6 +/- 0.5 s(-1) M-1 for [Gd(EPTPA-bz-NH2)(H2O)](2-) and [Gd(DTPA-bz-NH2)(H2O)](2-), respectively). Individual water exchange rates have been determined for both diastereomers of [Gd(DTPA-bz-NH2)(H2O)](2-) by a variable-temperature O-17 NMR study. Similarly to Ln(III) EPTPA derivatives, k(ex) values differ by a factor of 2 (k(ex)(298) = (5.7 +/- 0.2) x 10(6) and (3.1 +/- 0.1) x 10(6) s(-1)). This variance in the exchange rate has no consequence on the proton relaxivity of the two diastereomers, since it is solely limited by fast rotation. However, such difference in kex will affect proton relaxivity when these diastereomers are linked to a slowly rotating macromolecule. Once the rotation is optimized, slow water exchange will limit relaxivity; thus, a factor of 2 in the exchange rate can lead to a remarkably different relaxivity for the diastereomer complexes. These results have implications for future development of Gd-III-based, macromolecular MRI contrast agents, since the use of chiral bifunctional ligands in their synthesis inevitably generates diastereomeric complexes.