Calcium-responsive paramagnetic CEST agents

Abstract

The assessment of changes in the extracellular calcium concentration by magnetic resonance imaging would be a valuable biomedical research tool to monitor brain neuronal activity. In this perspective, we report here the synthesis of novel ligands consisting of tetraamide and bisamide derivatives of cyclen, L1 and L2, respectively, each bearing imino(diacetate) moieties for Ca2+ binding. Yb3+ and Eu3+ complexes are investigated as chemical exchange saturation transfer (CEST) agents that respond to the presence of Ca2+. A CEST effect is observed for both YbL1 and EuL1 complexes (B = 11.7 T), originating from the slow exchange of the amide protons and those of the coordinated water, respectively, whilst no CEST is detected for complexes of L2. Upon calcium binding, the CEST effect decreases considerably (from 60% to 20% for YbL1 and from 35% to 10% for EuL1). A similar variation is observed in the presence of Mg2+. The affinity constants between the lanthanide complexes and the alkaline earth metal ions have been estimated from the variation of the CEST effect to be KaffYbL1-Ca=8±2M-1, KaffYbL1-Mg=23±3M-1 and KaffEuL1-Ca= 10±3M-1. These low values imply the coordination of the alkaline earth ions to a single iminodiacetate arm. Ca2+/Mg2+ binding to the lanthanide complexes slows down the exchange of the amide protons on YbL1 which is responsible for the diminished CEST effect. This has been evidenced by assessing the proton exchange rates from the dependency of the CEST effect on the saturation time and the saturation power, in the absence and in the presence of Ca2+ and Mg2+. The applicability of the PARACEST MRI agents for Ca2+ detection has been evaluated on a 16 T MRI scanner. © 2010 Elsevier Ltd. All rights reserved.