In vivo stability of ester- and ether-linked phospholipid-containing liposomes as measured by perturbed angular correlation spectroscopy

To evaluate liposome formulations for use as intracellular sustained-release drug depots, we have compared the uptake and degradation in rat liver and spleen of liposomes of various compositions, containing as their bulk phospholipid an ether-linked phospholipid or one of several ester-linked phospholipids, by perturbed angular correlation spectroscopy. Multilamellar and small unilamellar vesicles (MLVs and SUVs), composed of egg phosphatidylcholine, sphingomyelin, distearoyl phosphatidylcholine (DSPC), dipalmitoyl phosphatidylcholine (DPPC) or its analog dihexadecylglycerophosphorylcholine (DHPC), and cholesterol plus phosphatidylserine, and containing (111)In complexed to nitrilotriacetic acid, were injected intravenously in rats. Recovery of (111)In-labeled liposomes in blood, liver, and spleen was assessed at specific time points after injection and the percentage of liposomes still intact in liver and spleen was determined by measurement of the time-integrated angular perturbation factor ([G22(∞)] of the (111)In label. We found that MLVs but not SUVs, having DHPC as their bulk phospholipid, showed an increased resistance against lysosomal degradation as compared to other phospholipid-containing liposomes. The use of diacyl phospholipids with a high gel/liquid-crystalline phase-transition temperature, such as DPPC and DSPC, also retarded degradation of MLV, but not of SUV in the dose range tested, while the rate of uptake of these liposomes by the liver was lower

Structure of the cell envelope of Halobacterium halobium

The structure of the isolated cell envelope of Halobacterium halobium is studied by X-ray diffraction, electron microscopy, and biochemical analysis. The envelope consists of the cell membrane and two layers of protein outside. The outer layer of protein shows a regular arrangement of the protein or glycoprotein particles and is therefore identified as the cell wall. Just outside the cell membrane is a 20 A-thick layer of protein. It is a third structure in the envelope, the function of which may be distinct from that of the cell membrane and the cell wall. This inner layer of protein is separated from the outer protein layer by a 65 Å-wide space which has an electron density very close to that of the suspending medium, and which can be etched after freeze-fracture. The space is tentatively identified as the periplasmic space. At NaCl concentrations below 2.0 M, both protein layers of the envelope disintegrate. Gel filtration and analytical ultracentrifugation of the soluble components from the two protein layers reveal two major bands of protein with apparent mol wt of ~16,000 and 21,000. At the same time, the cell membrane stays essentially intact as long as the Mg++ concentration is kept at ≥ 20 mM. The cell membrane breaks into small fragments when treated with 0.1 M NaCl and EDTA, or with distilled water, and some soluble proteins, including flavins and cytochromes, are released. The cell membrane apparently has an asymmetric core of the lipid bilayer

Liposomal phosphatidylserine inhibits tumor cytotoxicity of liver macrophages induced by muramyl dipeptide and lipopolysaccharide

Liposomes can very efficiently deliver immunomodulators to macrophages so as to induce tumor cytotoxicity. Liposomes most widely used for that purpose contain negatively charged lipids, in particular phosphatidylserine (PS), to enhance liposome uptake by the macrophages. We investigated the effect of three negatively charged liposomal lipids on the in vitro activation of liver macrophages to tumor cytotoxicity by muramyl dipeptide (MDP) and lipopolysaccharide (LPS). Both MDP- and LPS-induced tumor cytotoxicity towards murine colon adenocarcinoma cells were strongly inhibited by PS-containing liposomes. Under comparable conditions phosphatidylglycerol (DPPG)-containing or dicetyl phosphate (DCP)-containing liposomes did not inhibit or only marginally inhibited the induction of tumor cytotoxicity. We did not observe PS-mediated inhibition of tumor cell toxicity when the exposure of the macrophages to PS-liposomes was limited to the 4-h activation period prior to addition of the tumor target cells, suggesting that the inhibitory effect is accomplished at the level of the later stages of the activation process. Previously, we showed that macrophages which are activated to tumor cytotoxicity during a 24-h incubation with MDP become refractory to a second activation with MDP. Now we observed that simultaneous incubation with PS-containing liposomes partially prevents this refractoriness, which is also compatible with an interfering action of PS at a relatively late stage in the activation process. We conclude that PS, despite its reported stimulatory effect on liposome uptake by macrophages, can seriously antagonize the effectiveness of immunomodulating, agents acting on macrophages. This bears relevance to the use of PS-containing liposomes as a vehicle for such agents. The results are discussed in perspective of earlier reported pharmacological effects of PS and its metabolites.</p

Antiproliferative effect of immunoliposomes containing 5-fluorodeoxyuridine-dipalmitate on colon cancer cells

We have investigated the antiproliferative action towards CC531 colon adenocarcinoma cells of target cell-specific immunoliposomes containing the amphiphilic dipalmitoyl derivative of 5-fluorodeoxyuridine (FUdR-dP). FUdR-dP incorporated in immunoliposomes caused a 13-fold stronger inhibition of CC531 cell growth in vitro, during a 72-h treatment, than FUdR-dP in liposomes without antibody, demonstrating that the prodrug is efficiently hydrolysed to yield the active drug, FUdR, intracellularly. The intracellular release of active FUdR was confirmed by determining the fate of H-3-labelled immunoliposomal FUdR-dP. Treatments shorter than 72 h with FUdR-dP in immunoliposomes resulted in anti-tumour activities comparable to, or even higher than, that of free FUdR. The shorter treatments reflect more closely the in vivo situation and illustrate the potential advantage of the use of immunoliposomes over non-targeted liposomal FUdR-dP or free FUdR. Association of tumour cell-specific immunoliposomes with CC531 cells was up to tenfold higher than that of liposomes without antibody or with irrelevant IgG coupled, demonstrating a specific interaction between liposomes and target cells which causes an efficient intracellular delivery of the drug. Since biochemical evidence indicates a lack of internalization or degradation of the liposomes as such; we postulate that entry of the drug most likely involves the direct transfer of the prodrug from the immunoliposome to the cell membrane during its antigen-specific interaction with the cells. followed by hydrolysis of FUdR-dP leading to relatively high intracellular FUdR-levels. In conclusion, we describe a targeted liposomal formulation for the anticancer drug FUdR, which is able to deliver the active drug to colon carcinoma cells with high efficiency, without the need for the cells to internalize the liposomes as such