Solution Structure and Relaxivity of Ln-DOTXAZA Derivatives

Abstract

Herein we report the synthesis and structural analysis of tunable 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-derivatives bearing azidoethyl side chains. According to the number of azidoethyl side chains, the compounds are termed DOTXAZA (with X=1–4). All derivatives retain the well-approved DOTA scaffold for metal complexation with 8 coordinating functionalities and are applicable to Cu-catalyzed alkyne-azide cycloaddition (CuAAC). They may thus be used to assemble new Gd-based contrast agents (GBCAs) with tailored pharmacokinetic or MRI properties retaining the high stability of clinically used DOTA-derived GBCAs. The complex geometry of all DOTXAZA derivatives was investigated in solution with NMR-spectroscopy of the corresponding Eu-complexes. A correlation of complex geometry with the longitudinal relaxivity of corresponding Gd-complexes revealed an increase in relaxivity with increasing substitution number in the DOTXAZA series. Moreover, CuAAC-modified DOTAZA-derivatives with neutral, charged, and zwitterionic groups were prepared. Although of similar molecular mass, the corresponding Gd-complexes revealed pronounced differences in relaxivities from r1=3.9–7.9 mm−1 s−1 in water at 1.5 T and 37 °C. Relaxivities of these complexes were found to be strongly dependent on side chain charge and are most likely determined by second-sphere water effects.