Importance of the purification grade of 5(6)-carboxyfluorescein on the stability and permeability properties of N-acylphosphatidylethanolamine liposomes

In vitro cytotoxic study of immunoliposomal doxorubicin targeted to human CD34+ leukemic cells

The expression of CD34 antigen in acute myelogenous leukemias is considered an unfavourable prognosis marker for response to anticancer drugs and duration of remission. This study investigated the applicability of long-circulating immunoliposomes loaded with doxorubicin targeted to CD34 antigen present on MDR(+) human myelogenous leukemia KG-1a cell line. Immunoliposomal doxorubicin showed a higher cytotoxicity against KG-1a cells than non-targeted liposomal doxorubicin, but it did not improve over that of free drug. Although no reversal of doxorubicin resistance was found to occur through its liposomal encapsulation, a therapeutic benefit can be obtained by the selective cytotoxicity observed. Endocytosis studies demonstrated that, after binding to CD34 antigen, the immunoliposomes are not internalized by the KG-1a cells and so the cytotoxic effect might be due to drug released into the space near the cell membrane. Thus, immunotargeting of liposomal doxorubicin to CD34(+) leukemic cells may only provide an ex vivo strategy for site-selective CD34(+) leukemia cell killing.

Design and characterization of liposomes containing long-chain N-acylPEs for brain delivery: penetration of liposomes incorporating GM1 into the rat brain

PURPOSE

To develop a suitable liposomal carrier to encapsulate neu roactive compounds that are stable enough to carry them to the brain across the blood-brain barrier with the appropriate surface characteri tics for an effective targeting and for an active membrane transport.

METHODS

Liposomes containing glycosides and a fusogenic lipid were prepared by extrusion. Photon correlation spectroscopy, fluorescent spectroscopy, and differential scanning calorimetry were used to characterize liposomal preparations. Tissue distribution was determined by using 3H-cholesterylhexadecylether as a marker.

RESULTS

The incorporation of glycoside determinants and N-palmitoylphosphatidylethanolamine gives liposomes with similar in tial size, trapped volume, negative surface charge, bilayer fluidity, and melting temperature, except for monosialoganglioside-containing liposomes, which showed less negative surface charge and the highe size, trapped volume and melting temperature. All glycosilated formulations gave liposomes able to retain up to the 95% of encapsulated carboxyfluorescein after 90 min at physiologic temperature even in the presence of serum. Monosialoganglioside liposomes were recovered in the cortex, basal ganglia, and mesencephalon of both brain hemispheres. The liver uptake was higher for sulfatide- and glucose-liposomes, whereas the higher blood levels were observed for glucose- and mannose-liposomes.

CONCLUSIONS

These results show the suitability of such liposomal formulations to hold encapsulated drugs. Moreover, the brain uptake of monosialoganglioside liposomes makes them good candidates as drug delivery systems to the brain.

Preparation of long-circulating immunoliposomes using PEG-cholesterol conjugates: effect of the spacer arm between PEG and cholesterol on liposomal characteristics

Poly(ethylene glycol)-coated liposomes were prepared with two new synthesised pegylated cholesterol (Chol) derivatives linked via carbamate bond. Poly(ethylene glycol) (PEG) was directly linked to Chol (PEG-Chol) or through a space arm of diaminebutane (PEG-L-Chol). In buffer, the physicochemical properties of PC/Chol liposomes (2/1, molar ratio) containing up to 10 mol% of pegylated Chol derivatives did not change significantly and the PEG layer at liposome surface inhibited the agglutination of biotin-liposomes induced by streptavidin. On the other hand, in serum, PEG-L-Chol seemed to reduce the interactions of liposomes with serum proteins, much more than PEG-Chol. The low steric hindrance of PEG-Chol derivative may be due to the slow conformational transition rate of the polymer, since PEG may be deeper located in the membrane. The coupling efficiency of the ligand to the functionalised amino group at the polymer end was also affected, but, its antigen-binding activity was preserved. The basic physical-chemical characteristics studied in this work are relevant to assess the application of pegylated Chol liposomes as drug delivery systems.

Effect of the exposure to gentamicin and diltiazem on the permeability of model membranes

Preliminary observations showed that the calcium-antagonist diltiazem enhances the 'in vitro' bactericidal action of the aminoglycoside gentamicin, especially against Gram-positive bacteria. To verify if a non-specific interaction of these two drugs with biomembranes may play a role in their synergic effect on bacterial cells, we have studied the effect of exposure to gentamicin, in the absence or presence of diltiazem, on the release of carboxyfluorescein (CF) trapped in phosphatidylcholine (PC) unilamellar vesicles (LUVs) used as model membranes. A significant leakage of trapped CF from PC LUVs was registered when liposomes were treated with gentamicin and diltiazem together, employed at doses (50 and 100 microg/ml, respectively) unable to affect CF release if applied alone; the combined effect of gentamicin and diltiazem was synergic and not cumulative. The present findings demonstrate that the simultaneous exposure to gentamicin and diltiazem may induce significant alterations in the permeability of phospholipid membranes and, so, very likely, in functional properties of bacterial membranes, targets of their action.

Preparation of immunoliposomes bearing poly(ethylene glycol)-coupled monoclonal antibody linked via a cleavable disulfide bond for ex vivo applications

Several methods for the preparation of sterically stabilized immunoliposomes (SIL) have recently been described. This report examines an established method for coupling anti-CD34 My10 mAb to poly(ethylene glycol)-liposomes (PEG-liposomes) containing the anchor pyridyldithiopropionylamino-PEG-phosphatidylethanolamine (PDP-PEG-PE) via a cleavable disulfide bond. Efficient attachment of pyridyldithio-derivatized mAb took place (equivalent to coupling ca. 70% of total input protein) at 2 mol percent of the functionalized PEG-lipid. The My10-SIL bound specifically to CD34+ cells (human leukemic KG-1a and hematopoietic progenitor cells) and the extent of binding was a function of liposomal lipid concentration, the mAb density in the liposome surface and the CD34 cell expression. In mixtures with CD34- cells (CHO or Jurkat), CD34+KG-1a cells were determined by flow cytometry at percentages (1-4%) similar to those reported in clinical samples (such as cord blood, mobilized peripheral blood and bone marrow) using a direct immunostaining with My10-SIL. The disulfide bond was stable in cell culture medium (10% of fetal calf serum) during 8 h and cell-bound SIL can be released from cells by treatment with dithiothreitol as reducing agent under mild conditions (1 h of incubation with 50 mM DTT at 20 degrees C). SIL binding and subsequent dithiothreitol treatment did not influence the cell viability. Our approach should contribute to the development of targetable liposomal vehicles to CD34+ cells for use in ex vivo conditions as sorting of hematopoietic stem cells.

Preparation of immunoliposomes directed against CD34 antigen as target

The My-10 monoclonal antibody has facilitated the search of haematopoietic stem cells by recognizing selectively the human CD34 antigen. In the present work, My-10 immunoliposomes directed specifically against CD34+ cells were prepared, characterized and tested in vitro. Binding to target cells at 4 degreesC of immunoliposomes containing carboxyfluorescein as aqueous marker was evaluated by flow cytometry and fluorescence microscopy. These immunoliposomes demonstrated their capacity to bind specifically to CD34+ cells. Studies have shown that 9 antibodies/vesicle were sufficient to obtain a good binding efficiency. The product was stable over one month at 4 degreesC in terms of leakage of encapsulated carboxyfluorescein, particle size and antigen binding capacity.

N-palmitoylphosphatidylethanolamine stabilizes liposomes in the presence of human serum: effect of lipidic composition and system characterization

Liposomes containing negatively-charged phospholipid, N-palmitoylphosphatidylethanolamine (NPPE) were examined for stability in the presence of human serum, using the release of the entrapped 5,6-carboxyfluorescein as an aqueous marker. Either small unilamellar vesicles (SUV) or large unilamellar vesicles (LUV) were used. Incorporation of NPPE into PC SUV decreases leakage in the presence of serum or phosphate-buffered saline, no strictly related to size increase observed and to the surface negative charge present. The stabilizing effect of NPPE and Chol were synergistic. Inhibition of destabilization induced by serum of PC/Chol liposomes was observed when NPPE concentrations were above 12 mol%. Change in the membrane fluidity or incorporation of a monosialoganglioside into liposomes do not significantly change the half-life of liposomes in the presence of a high NPPE concentration. Incorporation of NPPE into PC/Chol liposomes increases membrane rigidity which does not change after serum incubation. The presence of NPPE in liposomes decreases lipid transfer/exchange between liposomes and lipoproteins although the same amount of serum proteins were incorporated as in PC/Chol liposomes. As expected, these proteins are accessible to trypsin digestion. In accordance with these results, the liposome agglutination assay shows no steric barrier activity. As a whole, the results obtained in this paper suggest a complex mechanism for stabilization of NPPE containing liposomes in human serum.

Incorporation of N-acylethanolamine phospholipids into egg phosphatidylcholine vesicles: characterization and permeability properties of the binary systems

We have studied the effect of the N-acylphosphatidylethanolamine (N-acylPE) on the permeability properties of liposomes composed primarily of egg phosphatidylcholine using a fluorescent anionic dye, carboxyfluorescein, as model solute. Leakage from liposomes decreased and vesicle size increased with increasing N-acylPE content. In addition, measurement of the trapped aqueous space, using the same dye marker, showed a correlation between trapped volume and vesicle size determined by dynamic light scattering. Permeability parameters were calculated according to the pseudo-first-order analysis. It appears that N-acylPE stabilizes liposomes at least in part through its ability to impart surface negative charge, in accord with the results obtained with potassium chloride as encapsulated solute. These results agreed well with osmotic response of anionic lipid vesicles. Cholesterol stabilizes N-acylPE liposomes in a proportional manner to the molar fraction of the effector.