Uptake of enzyme-laden liposomes by animal cells in vitro and in vivo

Liposome/white-cell interactions in vivo

The purpose of this study was to establish whether liposomes administered via the intravenous route promote granulocyte aggregation and pulmonary leukostasis. White cells (labelled with [111In]tropolanate) and positively and negatively charged liposomes (containing entrapped [99mTc]DTPA) were administered i.v. to rats. The blood clearance and tissue distribution of 111In label was not altered by the administration of liposomes (and vice versa) and it is inferred that on intravenous liposome administration liposome/white-cell interactions are unlikely to compromise lung function.

The effect of reticuloendothelial blockade on the blood clearance and tissue distribution of liposomes

The blood clearance and tissue distribution of liposomes have been studied in mice subjected to reticuloendothelial blockade with dextran sulphate or carbon. The liposomes have been labelled in the lipid membranes with [3H]-cholesterol, [14C]phosphatidylcholine and/or 99mTc and the content with [14C]inulin. Reticuloendothelial blockade has been shown to slow the rate of clearance of neutral, positively and negatively charged liposomes and of both small unilamellar vesicles and large multilamellar vesicles. In normal animals, the liver uptake accounted for only 20-55% of the total injected radioactivity, the amount varying with the charge and size of the liposomes. Following blockade, the liver uptake of charged and neutral multilamellar liposomes was depressed. This was also true for negatively charged small unilamellar vesicles. The degree of depression of hepatic uptake was between 25-50%, which contrasts with the 80-90% reduction in uptake of a wholly phagocytosed particle (sheep red cells). This difference suggests that mechanisms other than Kupffer cell phagocytosis are also responsible for the normal uptake of liposomes into the liver. In the case of neutral and positively charged small unilamellar vesicles, delayed clearance due to blockade was not associated with 'depressed' hepatic uptake. The site of action of blockading agents for these preparations is not clear. With all preparations of liposomes, blockade produced a slight and variable increase in uptake in the lung and spleen. The alteration of distribution of liposomes by reticuloendothelial blockade is therefore not great and the value of the technique in modifying the tissue distribution of substances within liposomes may be limited.

Entrapment of human leukocyte interferon in the aqueous interstices of liposomes

Human leukocyte interferon has been trapped in the aqueous interstices of multilamellar liposomes (phosphatidylcholine, dicetyl phosphate, and cholesterol [7:2:1]). Such liposomes trapped [3H]inulin (aqueous space marker) and interferon to the extent of 0.22 +/- 0.01 mg (n = 8) and 350 +/- 54 U (n = 4) per mumol of liquid, respectively, as judged by molecular sieve chromatography. Interferon trapped within liposomes was resistant to tryptic digestion under conditions which completely inactivated free interferon. Studies in which interferon was added to preformed liposomes excluded the possibility that interferon bound nonspecifically to the outer layer of the multilamellar liposomes. When interferon was added to the aqueous medium in which liposomes of various net surface charges were permitted to form, trapping of interferon varied directly with the interlamellar aqueous compartments of the liposomes. The demonstration that stable liposomes can entrap interferon suggests that these may constitute suitable vectors for the delivery of interferon to cells.

Liposomes. Effect of temperature on their mode of action on single frog skeletal muscle fibers

Liposomes containing the fluorescent dye 6-carboxyfluorescein were made from dipalmitoyl phosphatidylcholine and stearylamine. At 4 degrees C the liposomes are adsorbed on the fiber surface and when the temperature is raised to 21 degrees C, their contents are transferred directly into the fibers at a linear rate. Liposomes had little effect on the time course of the maximal twitch tension.