Fate of cholesterol-rich liposomes after subcutaneous injection into rats

The fate of small (30-60 nm) and large (about 400 nm diameter) liposomes composed of phosphatidylcholine or sphingomyelin and equimolar cholesterol and containing quenched carboxyfluorescein and 111In-labelled bleomycin after footpad injection of rats was investigated. Judging from the values of latent carboxyfluorescein in the plasma, phosphatidylcholine and sphingomyelin small liposomes entered the blood circulation intact, presumably via the lymphatics to reach 3 h after injection a peak of 16.9 and 23.1% of the dose per total blood, respectively. Of the 111In radioactivity given in phosphatidylcholine liposomes, about 32% was recovered in the liver. Hepatic uptake of 111In for sphingomyelin liposomes was lower (about 9%) reflecting their slower rate of clearance. No latent carboxyfluorescein could be detected in the blood after injection with phosphatidylcholine or sphingomyelin large liposomes and levels of 111In in the liver (and spleen) were very low. A proportionally much greater amount of liposomal 111In was intercepted by the popliteal and to a lesser extent, the lumbar lymph nodes. Such uptake was significantly higher (e.g. 463%) for small than for large (e.g. 195% per g popliteal nodes) liposomes. After foot pad injection of large liposomes into Walker 256 tumour-bearing rats, localization of 111In in the popliteal nodes was similar to that seen with control animals. However, 111In localization in the lumbar nodes was augmented more than 5-fold. These results suggest that large liposomes as used in this study, characterized by efficient drug entrapment, inability to reach the liver and spleen and improved localization in the lymph nodes of tumour-bearing animals may be preferable to vesicles of small size for the local treatment of lymphatic metastases.