A novel amphiphilic GdPCTA-[12] derivative has been prepared. The complex formed micelles in aqueous solution with a relatively low CMC, 0.15 mM (25 degrees C). The concentration dependent T1-relaxivity (r1) of the system has been described. The maximum T1-relaxivity, 29.2 s-1 mM-1 (20 MHz, 25 degrees C), was higher than for previously described micellar MRI contrast agents. This high T1-relaxivity is a consequence of the favourable water residence time (tau M) and the fact that the complex is heptadentate allowing two water molecules to coordinate to the gadolinium ion (q = 2).
A series of Gd-DO3A derivatives (8a-d) mimicking phospholipids have been prepared. Two of the complexes, Gd-HADB-DO3A (8a) and Gd-HADO-DO3A (8b), have been evaluated as pH responsive MRI contrast agents in vitro. The T 1 -relaxivity (r 1 ) of Gd-HADO-DO3A (8b) increased 142% on changing the pH from 6 to 8. The pH dependence is thought to arise from the formation of supramolecular structures caused by deprotonation of the amphiphilic complex at alkaline pH.Scheme 1 Reagents and conditions: i, 1) epichlorohydrin, H 2 O, rt or ∆, 2) NaOH, H 2 O, rt.
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