Formulation effects on delivery of propylthiouracil liposomes to U-937 macrophage cells

Abstract

Macrophages are prominent targets for several intracellular diseases such as leishmaniasis and tuberculosis. Moderation of macrophage functions can also be of therapeutic value in certain inflammatory conditions. Intracellular delivery of drugs to macrophages is thus of therapeutic value. The aim of this study was to explore the effects of formulation factors on role of liposomes as a drug carrier system for macrophages. Propylthiouracil (PTU) which possesses an antiproliferative activity was used as model drug. Liposomes were prepared by film-hydration method followed by extrusion through 100 nm polycarbonate membranes. The effects of three different negatively charged lipids (phosphatidylserine, PS; phosphatidylglycerol, PG; dicetylphosphate, DCP) and cholesterol (CH) were investigated. The negatively-charged lipids and CH were included at 10 and 30 mol%, respectively. Blank and PTU liposomes (0.075 and 0.15 mg/mL total lipid) were incubated with the human monocyte/macrophage U-937 cells (cells density 1 x 105 cells/mL) in the absence and presence of additional PTU. The antiproliferative activity, defined as percentage of total cells compared to the control, was used as evidence for liposomes uptake. The results suggest that surface charge was not the sole determinant of liposomes uptake. Inclusion of PS and PG, but not DCP, increased the antiproliferation of PC liposomes. Inclusion of CH significantly reduced the antiproliferation of PC/PS liposomes, but not other liposomes. Entrapment of PTU in liposomes did not enhance the antiproliferation seen with blank liposomes. However, synergism was observed when blank liposomes were co-administered with PTU solution. Increase in liposomes concentrations did not increase the antiproliferation nor show any sign of cytotoxicity for most formulations tested, except for PC/PG where increased antiproliferation was seen. Thus, formulation factors were significant in modifying biological effects introduced by liposomes. Preliminary study by fluorescence dequenching technique showed no evidence that PC liposomes underwent fusion with the U-937 cells. Further study would be necessary to elucidate the mechanisms by which these liposomes interacted with the U-937 cells. Such mechanisms would give insight into how liposomes enhanced drug effects and be helpful in development of liposomes drug carrier systems.