Uptake of encapsulated 99mTc-MIBI into simple or pegylated liposomes in cultured cells and in tumour-bearing nude mice

Encapsulating 99mTc-MIBI into liposomes could prolong its circulation half-life in blood without alteration of tracer abilities. In addition, surface coating of liposomes with polyethylene-glycol (PEG) have been shown to be efficient vehicles for antibiotics or 99mTc-tracers. The uptake of encapsulated 99mTc-MIBI into liposomes, simple or pegylated, in cancerous cells and its biodistribution were compared to the free 99mTc-MIBI. The encapsulation of 99mTc-MIBI into liposomes was obtained using a K+ diffusion potential method. Untrapped 99mTc-MIBI into liposomes preparations 'Small Unilamellar Vesicles' (SUVs) was removed by passing the SUVs through a chromatography column. 99mTc-MIBI uptake in cells was qualified by measuring radioactivity retained in K562 and MCF7-ras cells incubated with encapsulated or free 99mTc-MIBI. The biodistribution was explored in tumour-bearing nude mice. The efficiency with which 99mTc-MIBI was encapsulated in liposomes was 45% - 50% for pegylated or not. In the two cell lines, the accumulation of 99mTc-MIBI was similar either the tracer was free or encapsulated into liposomes. One hour after injection, the biodistribution showed a higher clearance for free 99mTc-MIBI than for encapsulated tracer into liposomes. The tumour accumulated in a greater extent the encapsulated form than the free 99mTc-MIBI. Encapsulated 99mTc-MIBI into PEG-liposomes would be a promising radiopharmaceutical for tumour imaging in vivo. (author)