Radiolabeling and Quantitative In Vivo SPECT/CT Imaging Study of Liposomes Using the Novel Iminothiolane-99mTc-Tricarbonyl Complex

The in vivo biodistribution of liposomal formulations greatly influences the pharmacokinetics of these novel drugs; therefore the radioisotope labeling of liposomes and the use of nuclear imaging methods for in vivo studies are of great interest. In the present work, a new procedure for the surface labeling of liposomes is presented using the novel 99m Tc-tricarbonyl complex. Liposomes mimicking the composition of two FDA approved liposomal drugs were used. In the first step of the labeling, thiol-groups were formed on the surface of the liposomes using Traut’s reagent, which were subsequently used to bind 99m Tc-tricarbonyl complex to the liposomal surface. The labeling efficiency determined by size exclusion chromatography was 95%, and the stability of the labeled liposomes in bovine serum was found to be 94% over 2 hours. The obtained specific activity was 50 MBq per 1 μmol lipid which falls among the highest values reported for 99m Tc labeling of liposomes. Quantitative in vivo SPECT/CT biodistribution studies revealed distinct differences between the labeled liposomes and the free 99m Tc-tricarbonyl, which indicates the in vivo stability of the labeling. As the studied liposomes were non-PEGylated, fast clearance from the blood vessels and high uptake in the liver and spleen were observed.