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Biomedical subjects

D Papahadjopoulos

Publications and source records attributed to D Papahadjopoulos.

At least 73 records · Page 4Linked to original sources
PubMed1988

Liposome-based therapy of human ovarian cancer: parameters determining potency of negatively charged and antibody-targeted liposomes.

Liposomes containing cytotoxic agents may be highly efficacious for intracavitary therapy of malignancies such as ovarian carcinoma, which resides principally in the peritoneal cavity. We have examined in vitro the cytotoxicity of a variety of liposome-drug formulations against OVCAR-3, a human ovarian cancer cell line. Two drugs tested, methotrexate-gamma-aspartate and 5-fluoroorotate, show increased cytotoxicity on various cultured cell lines following encapsulation in liposomes and can be considered liposome-dependent agents. With the optimal lipid composition used in this study, the maximal increase in potency on OVCAR-3 is 2.6-fold for methotrexate-gamma-aspartate and 5.2-fold for 5-fluoroorotate. Studies on liposome-cell association suggest a low capacity of OVCAR-3 to bind and internalize phospholipid vesicles, which limits the in vitro potency of liposomes for these cells. OC-125, a monoclonal antibody recognizing an antigen common to a number of human ovarian cancers (CA-125), has been coupled covalently to the liposome surface. Liposomes bearing OC-125 and containing methotrexate-gamma-aspartate show an 8-fold increase in potency against OVCAR-3 cells in a 96-h growth inhibition assay. Briefer exposure of tumor cells to treatment accentuates the advantage of targeted liposomes. The cytostatic effect of 1 h exposure to OC-125 liposomes is 100-fold greater than the equivalent exposure to free drug and equal to the maximal cytostatic effect achieved with free drug for 96 h. Attachment of OC-125 antibody also confers upon liposomes the capacity to recognize OVCAR-3 cells growing as an ascites tumor in nude mice. After i.p. injection, control liposomes bind tumor cells in relatively low numbers, while fluorescent OC-125 liposomes can be observed bound specifically to tumor cell masses for periods of days.

Antibodies, Monoclonal↗
PubMed1988

Free fatty acid enhancement of cation-induced fusion of liposomes: synergism with synexin and other promoters of vesicle aggregation.

The effect of free fatty acids on the cation-induced fusion of large unilamellar vesicles (liposomes) was investigated by using fluorescent assays which monitor the mixing of aqueous contents of liposomes. Overall fusion was modeled as a two-step process involving aggregation of vesicles followed by actual fusion. Different experimental conditions were used which favored either aggregation or fusion as the rate-limiting step in the overall process. When phosphatidylserine liposomes were induced to fuse by 4 mM Ca2+ plus 5 mM Mg2+, preincubation with arachidonic acid showed a dramatically increased overall rate of fusion compared to the same liposomes not treated with fatty acid. When fusion was induced by 3 mM Ca2+, arachidonic acid had little effect. These results were interpreted in terms of the action of arachidonic acid only at the fusion step per se and not the aggregation step. Therefore, the enhancement of the overall fusion rate would be observed solely under conditions where the actual fusion of liposomes was rate limiting (Ca/Mg) rather than the aggregation of liposomes (Ca alone). When other liposome systems were tested, the effect of arachidonic acid was observed only under fusion rate-limiting conditions. Arachidonic acid was found to act synergistically with promoters of liposomal aggregation, such as Mg2+, spermine, and synexin, to enhance the overall rate of liposome fusion, as would be expected from action at separate kinetic steps. The dependence of the fusion rates on arachidonic acid concentration demonstrated an apparently cooperative effect. The structure of the fatty acid is of critical importance in determining its effects, as shown by the fact that 16-doxylstearic acid always increased the rate of fusion while 5-doxylstearic acid always decreased the rate of fusion under all conditions tested. A number of different fatty acids, including oleic acid, elaidic acid, 16-doxylstearic acid, myristic acid, and stearic acid, were effective at increasing the fusion rate to varying extents. In general, unsaturated fatty acids were more effective than saturated ones, either due to partitioning into the membrane or because of structural requirements for promotion of fusion.

Annexin A7↗
PubMed1988

Synexin enhances the aggregation rate but not the fusion rate of liposomes.

The effect of synexin on the calcium-induced fusion of large unilamellar liposomes was studied by using two assays for the mixing of aqueous contents. The results were analyzed in terms of the mass action kinetic model, which describes the overall fusion reaction as a two-step sequence consisting of a second-order process of liposome aggregation followed by a first-order fusion reaction. By using several different lipid compositions and varying the electrolyte composition, it was possible to select the rate-limiting step of the overall fusion process. When aggregation was the rate-limiting step, as in the case of Ca2+-induced fusion of phosphatidylserine (PS), phosphatidate (PA)/phosphatidylethanolamine (PE) (1:3), and PS/PE (1:3) liposomes, synexin increased the overall fusion kinetics by increasing the aggregation rate constant (up to 100-fold). When aggregation was rapid compared to destabilization of apposed membranes, i.e., fusion was rate limiting, synexin either had no effect or reduced the overall fusion kinetics. In one such case involving liposomes composed of PA/PS/PE/phosphatidylcholine (PC) (10:15:65:10), synexin reduced the fusion rate constant by 50%. The effect of calcium-induced synexin polymerization was investigated by preincubation of synexin with calcium prior to addition of liposomes. Prepolymerization by Ca2+ always decreased the activity of synexin such that it was less than the activity of an equal amount of untreated monomers. However, it was found that the activity of synexin monomers polymerized to an average hexameric size was greater than that of one-sixth as many untreated monomers, with respect to the liposome aggregation rate constant. Neither polymers nor monomers increased the fusion rate constant.

Animals↗
PubMed1988

Fluorometric detection of the bilayer-to-hexagonal phase transition in liposomes.

We have used the fluorescent probe N-(7-nitro-2,1,3-benzoxadiazol-4-yl)phosphatidylethanolamine (NBD-PE) to detect the bilayer-to-hexagonal phase transition. The fluorescence intensity of the probe was found to increase during the bilayer-to-hexagonal transition. The bilayer-to-hexagonal transitions of various types of phosphatidylethanolamine or cardiolipin measured by this method are consistent with results obtained by differential scanning calorimetry. To establish this method for wider use, agents known to alter the bilayer-to-hexagonal transition were examined, and the results are comparable with the published data. The added advantage of this fluorometric method over other currently available techniques is that it is applicable not only for aggregated lipid samples but also for dilute liposome suspensions. This is especially important when one of the components of the system under study can partition between lipid and aqueous phase. Since NBD is located near the headgroup region of the bilayer, it most likely detects the change of the environment surrounding that region. On the basis of our present study, it appears that NBD-PE is sufficiently sensitive to detect bilayer-to-hexagonal phase transition.

Fluorescent Dyes↗
PubMed1988

Liposome formulations with prolonged circulation time in blood and enhanced uptake by tumors.

The rapid clearance of circulating liposomes from the bloodstream, coupled with their high uptake by liver and spleen, has thus far been an obstacle to any attempts at targeting to tumors. We have assessed the impact of liposome composition on their clearance from the circulation in normal and tumor-bearing mice and on their uptake by tumors and various normal tissues. By selective changes in lipid composition, while maintaining a mean particle diameter of approximately equal to 100 nm, we have achieved up to a 60-fold increase in the fraction of recovered dose present in blood 24 hr after i.v. injection. Concomitantly, there was a decrease by a factor of 4 of the recovered dose localizing in the liver and spleen, the major organs of the reticuloendothelial system. Parallel experiments in tumor-bearing mice demonstrated a 25-fold increase of the liposome concentration in the tumor when formulations with long and short blood residence time were compared. The most favorable results were obtained with liposomes containing a small molar fraction of a negatively charged glycolipid, such as monosialoganglioside or phosphatidylinositol, and a solid-phase neutral phospholipid as the bulk component. The bio-distribution of such formulations is of considerable therapeutic potential in cancer for increasing the concentration of cytotoxic agents in tumors while minimizing the likelihood of toxicity to the reticuloendothelial system.

Animals↗
PubMed1987

Lipid mixing during membrane aggregation and fusion: why fusion assays disagree.

The kinetics of lipid mixing during membrane aggregation and fusion was monitored by two assays employing resonance energy transfer between N-(7-nitro-2,1,3-benzoxadiazol-4-yl)phosphatidylethanolamine (NBD-PE) and N-(lissamine Rhodamine B sulfonyl)phosphatidylethanolamine (Rh-PE). For the "probe mixing" assay, NBD-PE and Rh-PE were incorporated into separate populations of phospholipid vesicles. For the "probe dilution" assay, both probes were incorporated into one population of vesicles, and the assay monitored the dilution of the molecules into the membrane of unlabeled vesicles. The former assay was found to be very sensitive to aggregation, even when the internal aqueous contents of the vesicles did not intermix. Examples of this case were large unilamellar vesicles (LUV) composed of phosphatidylserine (PS) in the presence of Mg2+ and small unilamellar vesicles (SUV) composed of phosphatidylserine in the presence of high concentrations of Na+. No lipid mixing was detected in these cases by the probe dilution assay. Under conditions where membrane fusion (defined as the intermixing of aqueous contents with concomitant membrane mixing) was observed, such as LUV (PS) in the presence of Ca2+, the rate of probe mixing was faster than that of probe dilution, which in turn was faster than the rate of contents mixing. Two assays monitoring the intermixing of aqueous contents were also compared. The Tb/dipicolinic acid assay reported slower fusion rates than the 1-aminonaphthalene-3,6,8-trisulfonic acid/N,N'-p-xylylene-bis(pyridinium bromide) assay for PS LUV undergoing fusion in the presence of Ca2+. These observations point to the importance of utilizing contents mixing assays in conjunction with lipid mixing assays to obtain the rates of membrane destabilization and fusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Chemical Phenomena↗
PubMed1987

Antibody-directed targeting of liposomes to human cell lines: role of binding and internalization on growth inhibition.

Small unilamellar liposomes containing methotrexate or methotrexate-gamma-aspartate were conjugated to Staphylococcus aureus protein A and were thus able to bind cell-specific immunoglobulins for targeting to malignant human B- and T-cell lines. We were able to demonstrate enhanced protein A liposome uptake and growth inhibition by targeting with an anti-major histocompatibility complex class II antibody recognizing two different B-cell lines. The enhanced growth inhibition was specific for the targeting antibody and amounted to a 2- to 3-fold lowering of the concentration of drug required to inhibit cell growth by 50% as compared to nontargeted liposomes or liposomes targeted with an antibody not recognizing a cell surface antigen. A strong association between enhanced growth inhibition and liposome internalization as assessed by fluorescent-activated cell sorter analysis of carboxyfluorescein containing protein A liposomes was seen. By contrast, specific enhancement of growth inhibition was not seen with several anti-idiotype antibodies or antibodies to T-cell differentiation antigens. Liposome internalization did not occur with these antibodies. Failure of growth inhibition and PA liposome internalization could not be explained by differences in cell binding of the antibody PA liposomes or the degree of protein A binding of the targeting antibody. Although the ability of the targeting antibody to bind to the cell and to protein A are important, these factors alone are not sufficient to guarantee internalization and growth inhibition. Variations in rates of internalization of various cell surface antigen-antibody complexes may account for different protein A liposome mediated cytotoxicities.

Animals↗
PubMed1987

Targeting of anti-Thy 1.1 monoclonal antibody conjugated liposomes in Thy 1.1 mice after intravenous administration.

125I-labeled liposomes, conjugated to an anti-Thy 1.1 monoclonal antibody (MRCOX7), demonstrated up to 7.4-fold greater lymph node uptake than liposomes conjugated to non-specific monoclonal antibody (R-10) after intravenous injection into Thy 1.1 (AKR-J) mice. Uptake of anti-Thy 1.1-conjugated liposomes by the lymph nodes of AKR-J mice was 3-times greater than their uptake by lymph nodes of Thy 1.2 (AKR-Cu) mice. Lymph node localization of anti-Thy 1.1-liposomes was equal to that of control monoclonal antibody-liposomes in Thy 1.2 mice. Conjugation to either monoclonal antibody substantially increased liposome clearance by the liver, while decreasing liposome uptake in a number of organs outside the reticuloendothelial system. Changes in liposome size and phospholipid composition did not significantly alter these results. Administration of a large predose of unconjugated liposomes prior to injection of MRCOX7-conjugated liposomes increased blood levels and reduced liver uptake of the monoclonal antibody-liposome conjugates, but did not further enhance lymph node uptake. This study demonstrates that targeting of liposomes by conjugation to the appropriate monoclonal antibody, can significantly increase their uptake in lymph nodes which contain high levels of cells expressing the target antigen. However, conjugation to monoclonal antibody also increases clearance of liposomes by the liver. To increase the uptake of monoclonal antibody-conjugated liposomes in target tissue, substantial reduction of their clearance by the reticuloendothelial system will be required.

Animals↗
PubMed1987

Synexin-like proteins from human polymorphonuclear leukocytes. Identification and characterization of granule-aggregating and membrane-fusing activities.

We have used Ca2+-dependent binding to a phospholipid vesicle affinity column to isolate a mixture of three synexin-like proteins from the cytosol of human polymorphonuclear leukocytes (PMN), with relative molecular weights of approximately 67,000, 47,000, and 28,000. Rabbit antibodies raised against bovine liver synexin recognized the 47,000 molecular weight PMN protein. These PMN proteins, like bovine liver synexin, promoted aggregation of isolated PMN specific granules in the presence of Ca2+ and increased the overall rate of Ca2+-induced fusion of liposomes composed of phosphatidate (PA)/phosphatidylethanolamine (PE) (1:3) and phosphatidylserine/PE (1:3), but decreased the rate of spermine-induced fusion of PA/PE (1:3) liposomes. Using fluorescent lipid probes, rapid fusion of PA/PE liposomes with PMN specific granules (50% maximum signal within a few minutes) was observed when 1 mM Ca2+ was added in the presence of both synexin and free arachidonic acid. Dilution of the aqueous contents of liposomes was also observed under the same conditions. The rate of fusion increased monotonically with Ca2+ and arachidonic acid concentrations, but synexin exhibited an optimum concentration. Lack of any one of the components precluded rapid fusion. These results suggest that PMN contain a protein similar to, or identical with, synexin that may be involved in calcium-dependent fusion of intracellular membranes.

Animals↗
PubMed1987

Successful treatment with aerosolized pentamidine of Pneumocystis carinii pneumonia in rats.

We examined both the therapeutic efficacy and tissue distribution of aerosolized pentamidine in immunosuppressed rats with Pneumocystis carinii pneumonia. In rats immunosuppressed by 5 weeks of pretreatment with dexamethasone, a 2-week course of 5 mg of aerosolized pentamidine per kg per day, administered free or encapsulated in the drug carrier system (liposomes), eradicated P. carinii pneumonia in 75% of treated animals. At this dose, extrapulmonary drug uptake as measured by a sensitive high-pressure liquid chromatography assay was negligible. No significant differences in tissue distribution were noted between aerosolized free and liposome-encapsulated pentamidine. In rats receiving dexamethasone for 6 weeks prior to treatment with pentamidine, both lung uptake and therapeutic efficacy of aerosolized pentamidine (5 mg/kg per day) were substantially reduced. Aerosolized pentamidine appears to be an effective therapy for P. carinii pneumonia in rats and produces significantly lower extrapulmonary drug deposition than parenteral administration. The severity of P. carinii involvement at the time of treatment influences both the level of drug delivery to the lung and the response to aerosolized pentamidine therapy.

Aerosols↗
PubMed1986

Liposome-mediated delivery of pteridine antifolates to cells in vitro: potency of methotrexate, and its alpha and gamma substituents.

We have examined the growth-inhibitory potency of several pteridines encapsulated in negatively charged liposomes, including methotrexate, methotrexate-gamma-methylamide, methotrexate-gamma-dimethylamide, methotrexate-alpha-aspartate, and a lipophilic methotrexate-phosphatidylethanolamine conjugate. The potency of encapsulated methotrexate is greater than the potency of the free drug for CV1-P cells, but not for other cell lines. The potency of methotrexate-gamma-methylamide and methotrexate-gamma-dimethylamide is only minimally improved by encapsulation. The potency of methotrexate-alpha-aspartate is increased by encapsulation. In addition, the lipophilic methotrexate derivative has demonstrable potency when incorporated in liposomes. We have also examined the potency of several pteridines under conditions where the cells are exposed to the drug for periods shorter than the entire growth assay. Reduction of the exposure time decreases the potency of both encapsulated and free drugs. However, the difference in potency between the encapsulated and free drug is increased, because the potency of the encapsulated drug is affected less. Consequently, encapsulated methotrexate-gamma-aspartate is 300-fold more potent than free drug, if CV1-P cells are exposed to drug for 4 h. Moreover, encapsulated methotrexate is more potent than free methotrexate for growth inhibition of L929 fibroblasts, if the term of exposure is less than 8 h. Potency is least affected by reduction of exposure length for the lipophilic methotrexate derivative.

Cell Line↗
PubMed1986

Spermine as a modulator of membrane fusion: interactions with acidic phospholipids.

The interaction of spermine with acidic phospholipids was investigated for its possible relevance to membrane fusion. Equilibrium dialysis was used to measure the binding of spermine and calcium to large unilamellar vesicles (liposomes) of phosphatidate (PA) or phosphatidylserine (PS). Spermine bound to isolated PA and PS liposomes with intrinsic association constants of approximately 2 and 0.2 M-1, respectively. Above the aggregation threshold of the liposomes, the binding of spermine increased dramatically, especially for PA. The increased binding upon aggregation of PA liposomes was interpreted as evidence for the formation of a new binding complex after aggregation. Spermine enhanced calcium binding to PA, while it inhibited calcium binding to PS, under the same conditions. This difference explained the small effect of spermine on the overall rate of calcium-induced fusion of PS liposomes as opposed to the large effect on PA liposomes. The rate increase could be modeled by a spermine-induced increase in the liposome aggregation rate. The preference for binding of spermine to PA over PS suggested a preference for accessible monoesterified phosphate groups by spermine. This preference was confirmed by the large effects of spermine on aggregation and overall fusion rates of liposomes containing phosphatidylinositol 4,5-diphosphate. The large spermine effects on these liposomes compared with phosphatidate- or phosphatidylinositol-containing liposomes suggested that spermine has a strong specific interaction with phosphatidylinositol 4,5-diphosphate. Clearly, phosphorylation of phosphatidylinositol can lead to a large change in the spermine sensitivity of membrane fusion.

Calcium↗
PubMed1986

Antibody-directed liposomes: comparison of various ligands for association, endocytosis, and drug delivery.

We have developed and compared the cytotoxicity of methotrexate-gamma-aspartate encapsulated in several liposome formulations which bind mouse monoclonal antibody in order to define conditions for screening cell lines and antibodies for liposomal efficacy. Liposomes conjugated to Staphylococcus aureus Protein A were more potent than liposomes conjugated to either rabbit or affinity-purified goat anti-mouse immunoglobulin (Ig) when incubated with AKR/J SL2 cells sensitized with specific antibody. The antibody-directed Protein A liposomes were also 10-fold more potent than liposomes conjugated directly to specific antibody against the AKR/J SL2. We examined the effect of antibody specificity, concentration, and isotype on liposome-mediated drug delivery to AKR/J SL2 cells. The growth-inhibitory effect of the drug in the antibody-directed Protein A liposomes varied with the target antigen on the cell. The potency of the liposomes with a given antibody was proportional to their relative binding and endocytosis by the cells, and to the reactivity of the particular antibody with the cell as demonstrated by indirect immunofluorescence. The Protein A liposomes maintained maximal potency down to antibody concentrations as low as 1 microgram/ml with the anti-Thy 1.1-sensitized AKR/J SL2 cells, thus demonstrating the possible use of these liposomes for hybridoma screening. Use of isotype-switched variants of the anti-Thy 1.1 antibody with the AKR/J SL2 cells showed the superior efficacy of the IgG2a, IgG2b, and IgG3 isotypes to the IgG1 with the Protein A liposomes. The large differential potency of the free drug and the drug encapsulated in antibody-directed Protein A liposomes was maintained even at short incubation times, thus providing a system which may be useful for eradication of tumor cells from bone marrow in vitro.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1985

The effects of liposome size and surface charge on liposome-mediated delivery of methotrexate-gamma-aspartate to cells in vitro.

We have studied the liposome-mediated delivery of methotrexate-gamma-aspartate to five cell lines. The sensitivity of the cells to encapsulated drug varies widely in accordance with their ability to take up the liposomes. CV1-P cells can be 150-times more sensitive to encapsulated methotrexate-gamma-aspartate than to free drug, while AKR/J SL2 cells are only twice as sensitive to the encapsulated drug. Negatively-charged liposomes are much more efficient for delivery than are neutral liposomes, and cholesterol is an essential component of the liposome membrane for optimal drug delivery. The optimal liposome size for drug delivery is 0.1 micron, although the amount of cell-associated lipid is the same for all liposome sizes. The effect of the encapsulated drug is inhibited by NH4Cl, suggesting an endocytic mechanism for delivery. The potency of the encapsulated drug is not affected by wide variations in the drug: lipid ratio.

Ammonium Chloride↗
PubMed1985

Interaction of clathrin with liposomes: pH-dependent fusion of phospholipid membranes induced by clathrin.

Clathrin, dissociated from coated vesicles of bovine brain and purified by gel chromatography, was found to interact with the lipid bilayer as shown by the spontaneous release of encapsulated fluorescent dye in liposome. Clathrin-induced dye release was enhanced at acidic pH in phosphatidylserine-containing vesicles. A strong correlation between dye release and fusion of liposomes was observed. In general, when there was a fast release of encapsulated dye induced by clathrin, a pH-dependent, clathrin-induced fusion was observed. Clathrin did not induce either dye release or fusion of egg phosphatidylcholine liposomes. The self-association of clathrin at low pH diminished the fusogenic activity. Fusion induced by clathrin at low pH could be stopped at pH above 5.0 and resumed by lowering the pH below 5.0. This suggests that the interaction of clathrin with phospholipid membranes can be regulated by pH.

Animals↗