Targeting of liposomes containing methotrexate towards Tetrahymena pyriformis cells

The uptake of liposomes containing methotrexate by Tetrahymena pyriformis cells was investigated with the aim of producing liposome-cell association enabling methotrexate to be introduced into the cytoplasm of intact cells. Incubation of liposomes containing methotrexate with tetrahymena pyriformis cells resulted in a time and concentration-dependent uptake of entrapped methotrexate by the cells. The uptake by Tetrahymena pyriformis cells (at 1 hr) of liposomes prepared by phospholipids and gangliosides extracted from Tetrahymena pyriformis cells was approximately three fold higher than that of liposomes prepared by commercial phospholipids. Approximately 90% of liposome uptake could be inhibited by cytochalasin B and also by NaN3 and 2-deoxyglucose. This was consistent with the uptake being the result of endocytosis. The remaining uptake was probably the result of adhesion of liposomes to the cell membrane. The rate of efflux vs time of methotrexate entrapped in liposomes was much slower than that of free methotrexate which reinforces the concept that endocytosis is the main mode of liposomes uptake by the cells. Liposomes containing methotrexate at concentrations as low as 4.5 microM effectively inhibited the activity of dihydrofolate reductase which was used as a function parameter in this study. Similar inhibition of the enzyme activity by free methotrexate was achieved only at concentrations as high as 880 microM. The influence of liposomes lipid composition on the targeting of liposomes to Tetrahymena pyriformis cells was discussed.