Derivatives of GdAAZTA Conjugated to Amino Acids: A Multinuclear and Multifrequency NMR Study

Abstract

The GdAAZTA (AAZTA = 6-amino-6-methylperhydro-1,4-diazepinetetraacetic acid) complex represents a platform of great interest for the design of innovative MRI probes due to its remarkable magnetic properties, thermodynamic stability, kinetic inertness, and high chemical versatility. Here, we detail the synthesis and characterization of new derivatives functionalized with four amino acids with different molecular weights and charges: l -serine, l -cysteine, l -lysine, and l -glutamic acid. The main reason for conjugating these moieties to the ligand AAZTA is the in-depth study of the chemical properties in aqueous solution of model compounds that mimic complex structures based on polypeptide fragments used in molecular imaging applications. The analysis of the 1 H NMR spectra of the corresponding Eu(III)-complexes indicates the presence of a single isomeric species in solution, and measurements of the luminescence lifetimes show that functionalization with amino acid residues maintains the hydration state of the parent complex unaltered ( q = 2). The relaxometric properties of the Gd(III) chelates were analyzed by multinuclear and multifrequency NMR techniques to evaluate the molecular parameters that determine their performance as MRI probes. The relaxivity values of all of the novel chelates are higher than that of GdAAZTA over the entire range of applied magnetic fields because of the slower rotational dynamics. Data obtained in reconstituted human serum indicate the occurrence of weak interactions with the proteins, which result in larger relaxivity values at the typical imaging fields. Finally, all of the new complexes are characterized by excellent chemical stability in biological matrices over time, by the absence of transmetallation processes, or the formation of ternary complexes with oxyanions of biological relevance. In particular, the kinetic stability of the new complexes, measured by monitoring the release of Gd 3+ in the presence of a large excess of Zn 2+ , is ca. two orders of magnitude higher than that of the clinical MRI contrast agent GdDTPA.

Kilogram scale facile synthesis and systematic characterization of a Gd-macrochelate as T1-weighted magnetic resonance imaging contrast agent

“…Considering non-nitrogen atmosphere and room temperature conditions are conducive to subsequent large-scale synthesis, the Gd-HPMA12 macrochelate ( r 1 = 50.46 ± 1.01 mM −1 s −1 , r 2 / r 1 = 1.21 ± 0.06, 3.0 T; r 1 = 16.36 ± 0.87 mM −1 s −1 , r 2 / r 1 = 2.17 ± 0.12, 7.0 T) was considered as the optimal sample for the following characterizations. Significantly, the ascertained r 1 value of the Gd-HPMA12 macrochelate stands notably superior, eclipsing commercial GBCAs by an order of magnitude [ 29 , 30 ]. That’s because the high molecular weight of Gd-HPMA12 can limit the Gd tumbling, increase rotational correlation times and induce strong T 1 water proton spin relaxation, thus exhibiting an excellent r 1 value compared with commercial T 1 -weighted MRI CAs [ 30 , 31 ].…”