Comparison of three remote radiolabelling methods for long-circulating liposomes

Long-circulating liposomes (LCL) are often used as a drug carrier system to improve the therapeutic index of water-soluble drugs. To track these LCL in vivo, they can be radiolabelled with 111In-oxine. For this labelling method, generally DTPA is encapsulated in the aqueous phase of LCL (DTPA-LCL). Alternatively, LCL can be labelled with 111InCl3 after incorporation of DTPA-conjugated DSPE in the lipid bilayer (DTPA-DSPE LCL). Here, we compared the in vitro properties of DTPA-DSPE LCL with those of DTPA LCL and empty LCL. Additionally, we compared the in vivo performance of DTPA-DSPE LCL with those of DTPA LCL in mice.

DTPA LCL (88 nm) and empty LCL (84 nm) were labelled with 111In-oxine, and DTPA-DSPE LCL (83 nm) were labelled with 111InCl3. Labelling efficiency at increasing specific activity was determined. In vitro stability of 111In-labelled LCL was determined in human serum at 37 °C. The in vivo properties of 111In-labelled LCL were determined in mice with a Staphylococcus aureus infection in the thigh muscle. Image acquisition, blood sampling and biodistribution studies were performed 1, 4 (blood sampling only), 24, 48 and 72 h p.i. of 111In-labelled LCL.

DTPA-DSPE LCL could be labelled efficiently at a much higher specific activity compared to DTPA LCL and empty LCL: > 90% at 15 GBq/mmol, > 90% at 150 MBq/mmol and 60–65% at 150 MBq/mmol, respectively. 111In-labelled DTPA-DSPE LCL and DTPA LCL were stable in human serum, regarding label retention, for at least 48 h at 37 °C (> 98% retention of the radiolabel). In contrast, only 68% radiolabel was retained in empty LCL after 48 h. In vivo targeting of 111In-DTPA-DSPE LCL to the abscess was comparable to targeting of 111In-DTPA LCL (3.5 ± 0.9%ID/g and 3.4 ± 0.9%ID/g abscess uptake respectively, 48 h p.i.).

In conclusion, labelling of DTPA-DSPE LCL with 111InCl3 represents a robust, easy and fast procedure which is preferred over the more laborious conventional labelling of DTPA-LCL with 111In-oxine.