99mTc-HMPAO-labeled liposomes: an investigation into the effects of some formulation factors on labeling efficiency and in vitro stability

Technetium-99m complex of hexamethyl-propylene-amineoxime (HMPAO) is used as an efficient agent to label liposomes. For this, 99mTc-HMPAO is incubated with preformed liposomes that contain glutathione (GSH). Effect of GSH and lipid concentration on labeling efficiency, as well as the effect of lipid composition on in vitro stability of labeled liposomes, was investigated in the present study. d,l-HMPAO was synthesized and kits including d,l-HMPAO and SnCl2·2H2O were optimized at 0.5 mg HMPAO, 5.0 μg SnCl2·2H2O and pH 7, and lyophilized. DSPC/CHOL (molar ratio 2:1) liposomes encapsulating GSH were labeled with 99mTc-HMPAO prepared kits. Increase of GSH concentration in hydration buffer from 5 to 200 mM during liposome preparation resulted in a broad labeling efficiency of liposomes ranging from 4.16% to 69.81%. An initial approximate concentration of 100 mM GSH in the hydration buffer seems to be appropriate for a good labeling efficiency. At the optimum concentration of GSH, change of the total initial lipid concentration from 10 to 70 mM did not produce a remarkable difference in labeling efficiency. Study of the effect of lipid composition on the stability of liposomes showed that all three kinds of labeled liposomes composed of DSPC/CHOL, DPPC/CHOL and DMPC/CHOL (molar ratio 2:1) had good in vitro stability in human plasma at 37°C for 48 h; however, employing DSPC resulted in the most stable ones.