In Vivo Magnetic Resonance Imaging Detection of Paramagnetic Liposomes Loaded with Amphiphilic Gadolinium(III) Complexes: Impact of Molecular Structure on Relaxivity and Excretion Efficiency

The magnetic resonance imaging (MRI) performance of two liposome formulations incorporating amphiphilic 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-like GdIII complexes has been investigated both in vitro and in vivo. The complexes differ in one donor group of the coordination cage (carboxylate versus carboxoamide), and in the length (C12 versus C18) and the point of attachment of the aliphatic chains to the chelators. The in vitro 1H relaxometric characterisation of the systems, performed with a newly developed relaxation model that takes into account the contributions of the GdIII chelates pointing in- and outwards of the liposome, indicates that their efficacy is optimal in the range 0.5–1.5 T. The tetracarboxylic C12-containing liposomes (LIPO-GdDOTA(GAC12)2; GA=glutaric acid) are four-fold more efficient than the monoamide C18-based analogue (LIPO-GdDOTAMA(C18)2). Such a difference is also found in vivo at 1 T in a melanoma tumour model on mice. A few hours after intravenous injection, the T1 contrast enhancement in the organs where the nanovesicles typically distribute (liver, spleen, kidneys and tumour) is much higher for LIPO-GdDOTA(GAC12)2. Interestingly, after about 7 h post-injection the contrast enhancement observed for the more efficient liposomes decreases rapidly and becomes lower than for LIPO-GdDOTAMA(C18)2. The relaxometric data and the quantification of the GdIII complexes in the organs, determined ex vivo by inductively coupled plasma mass spectrometry, indicate that: 1) the differences in the contrast enhancement can be attributed to the different rate of water exchange and rotational dynamics of the Gd complexes, and 2) the rapid contrast decrease is caused by a faster clearance of GdDOTA(GAC12)2 from the organs. This is also confirmed by using a newly synthesised amphiphilic cyanine-based fluorescent probe (Cy5-(C16)2). As one of the main limitations for the clinical translation of liposomes incorporating amphiphilic imaging agents is related to their very long persistence in the body, the results reported herein suggest that the clearance of the probes can be accelerated, without compromising their role, by a proper selection of the lipophilic portion of the incorporated compound as well as of the ligand site at which the aliphatic tails are linked. Then, GdDOTA(GAC12)2 complex may represent a good candidate for the development of improved MRI protocols based on paramagnetically labelled lipidic nanoparticles.