Lipid vesicles loaded with thymopentin: characterization and in vitro activity on tumoral cells

The cytotoxic activity on tumoral cells of thymopentin (TP5), a synthetic pentapeptide corresponding to residues 32–36 of thymopoietin, was evaluated after inclusion in multilamellar liposomes. These were made by all neutral (zwitterionic) phospholipids as well as those containing positively (stearylamine, SA) or negatively charged (DPPS) lipids. Vesicles were also prepared containing cholesterol, and were characterized by light scattering dimensional analysis and differential scanning calorimetry. The effect of charged species and of the presence of cholesterol on TP5 incorporation was studied. For the different kinds of liposomes, loaded with sodium fluoresceinate, the rate of interaction after incubation with lymphocytes in a normal or tumoral (IM9 lymphoblastoid cells) state was also evaluated. TP5-loaded vesicles were assayed on two lines of tumoral cells (IM9 and K562) to assess the ability of liposomal carriers to potentiate the cell growth inhibitory activity of the drug. SA-containing positive liposomes appeared as the most efficacious, resulting in 87% inhibition on IM9 cells, with an amount of liposomes containing 100 ng of TP5.