Liposome-cell interactions A study of the interactions of liposomes containing entrapped anti-cancer drugs with the EMT6, S49 and AE1 (transport-deficient) cell lines

A study has been made to determine if the cytotoxicity observed when cells in culture were exposed to liposome-entrapped cytotoxic drugs was liposome mediated or resulted from leakage of drug from the liposomes with subsequent uptake of free drug by the cells. In preliminary experiments with the EMT6 cell line in monolayer culture, the cytotoxicity observed when the cells were exposed to a range of concentrations of liposome-entrapped methotrexate, actinomycin D and cytosine arabinoside for a variety of liposome compositions was somewhat less than that observed when the cells were exposed to similar concentrations of free drug. We suspected that the cytotoxicity was mediated via uptake of free drug leaked from liposomes. This was confirmed in experiments involving the EMT6 and S49 cell lines in monolayer or suspension culture, respectively, in the absence and presence of the nucleoside transport inhibitor, 6-((4-nitrobenzyl)thio)-9-β-d-ribofuranosylpurine. Additional experiments were performed on a transport-deficient mutant of the S49 cell line, the AE₁ cell line. No evidence for liposome-mediated cell death could be found in these cell lines when tubercidin 5′-monophosphate was entrapped in either large or small unilamellar liposomes composed of egg phosphatidylcholine/ cholesterol (2:1), bovine brain phosphatidylserine/egg phosphatidylcholine/ cholesterol (8:2:5) or egg phosphatidylcholine/stearylamine/cholesterol (10:1:5). Considerable toxicity due to empty liposomes of a variety of compositions was observed in the S49 cell line at high lipid concentrations.