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Abstract: Liposomes radiolabelling with diagnostic radionuclides is an excellent tool for studying pharmacokinetics with the view of developing liposome-based drug delivery agents. The aim of the present study is to evaluate the in vitro and in vivo behavior of a 99mTc-labeled liposome by applying either a direct labeling strategy via a carboxyl group, LP–COOH, or a surface chelating method via pyridyl ethyl cysteine compound (with the intermediate [99mTc(I)(CO)3(H2O)3]+), LP-PEC. 99mTc-LP-COOH was obtained in high radiolabelling yield and radiochemical purity, while 99mTc(I)(CO)3-LP-PEC was initially purified before being in vitro and in vivo evaluated. 99mTc-LP-COOH was less stable in the presence of competitive for 99mTc ligands than 99mTc(I)(CO)3-LP-PEC. According to DLS measurements, the presence of serum as well as the applied radiolabelling conditions did not affect the liposomes’ size. The different radiolabelling methods seemed to exert an influence on the biodistribution pattern of the liposomes with the 99mTc(I)(CO)3-LP-PEC showing slow blood clearance, which was also confirmed by in vivo scintigraphic imaging. Nevertheless, passive tumor targeting was attained at a similar extent no matter which radiolabelling technique was followed. [Copyright &y& Elsevier]