Synthesis, complexation and water exchange properties of Gd(iii)–TTDA-mono and bis(amide) derivatives and their binding affinity to human serum albumin

Abstract

With the objective of tuning the lipophilicity of ligands and maintaining the neutrality and stability of Gd(III) chelate, we designed and synthesized two bis(amide) derivatives of TTDA, TTDA-BMA and TTDA-BBA, and a mono(amide) derivative, TTDA-N-MOBA. The ligand protonation constants and complex stability constants for various metal ions were determined in this study. The identification of the microscopic sites of protonation of the amide ligand by ¹H NMR titrations show that the first protonation site occurs on the central nitrogen atom. The values of the stability constant of TTDA-mono and bis(amide) complex are significantly lower than those of TTDA and DTPA, but the selectivity constants of these ligands for Gd(III) over Zn(II) and Cu(II) are slightly higher than those of TTDA and DTPA. On the basis of the water-exchange rate values available for [Gd(TTDA-BMA)(H₂O)], [Gd(TTDA-BBA)(H₂O)] and [Gd(TTDA-N-MOBA)(H₂O)]⁻, we can state that, in general, the replacement of one carboxylate group by an amide group decreases the water-exchange rate of the gadolinium(III) complexes by a factor of about three to five. The decrease in the exchange rate is explained in terms of a decreased steric crowding and charge effect around the metal ion when carboxylates are replaced by an amide group. In addition, to support the HSA protein binding studies of lipophilic [Gd(TTDA-N-MOBA)(H₂O)]⁻ and [Gd(TTDA-BBA)(H₂O)] complexes, further protein–complex binding was studied by ultrafiltration and relaxivity studies. The binding constants (K_A) of [Gd(TTDA-N-MOBA)(H₂O)]⁻ and [Gd(TTDA-BBA)(H₂O)] are 8.6 × 10² and 1.0 × 10⁴ dm³ mol⁻¹, respectively. The bound relaxivities (r₁^b) are 51.8 and 52 dm³ mmol⁻¹ s⁻¹, respectively. The K_A value of [Gd(TTDA-BBA)(H₂O)] is similar to that of MS-325 and indicates a stronger interaction of [Gd(TTDA-BBA)(H₂O)] with HSA.