Tumor-associated ganglio-N-triosylceramide. Target for antibody-dependent, avidin-mediated drug killing of tumor cells

Biotin conjugates in targeted drug delivery: is it mediated by a biotin transporter, a yet to be identified receptor, or (an)other unknown mechanism(s)?

Conjugation of drugs with biotin is a widely studied strategy for targeted drug delivery. The structure-activity relationship (SAR) studies through H3-biotin competition experiments conclude with the presence of a free carboxylic acid being essential for its uptake via the sodium-dependent multivitamin transporter (SMVT, the major biotin transporter). However, biotin conjugation with a payload requires modification of the carboxylic acid to an amide or ester group. Then, there is the question as to how/whether the uptake of biotin conjugates goes through the SMVT. If not, then what is the mechanism? Herein, we present known uptake mechanisms of biotin and its applications reported in the literature. We also critically analyse possible uptake mechanism(s) of biotin conjugates to address the disconnect between the results from SMVT-based SAR and "biotin-facilitated" targeted drug delivery. We believe understanding the uptake mechanism of biotin conjugates is critical for their future applications and further development.

Conjugation of von Willebrand factor-binding domain of platelet glycoprotein Ib alpha to size-controlled albumin microspheres

Albumin microspheres (AMS), of which the average diameter is 240 +/- 10 nm, were prepared by pH control and heat treatment. Cytochrome c and rGPIb alpha; a water-soluble fragment of the alpha chain of a recombinant platelet glycoprotein (GP)Ib containing a von Willebrand factor (vWf)-binding site were selected as a receptor protein. Cytochrome c was used as a probe protein for monitoring. Onto the surface of the AMS and those proteins, N-succinimidyl 3-(2-pyridyldithio)propionate (SPDP) was reacted through the amide linkage to obtain PD-AMS, PD-cytochrome c, and PD-rGPIb alpha, respectively. The latter two were further reduced to SH-cytochrome c and SH-rGPIb alpha by dithiothreitol and conjugated with PD-AMS by a thiol-disulfide exchange reaction. The resulting AMS contain cytochrome c or rGPIb alpha of about 25,000 or 2500 molecules, respectively. The addition of ristocetin to the rGPIb alpha-AMS in the presence of vWf caused specific aggregation. Furthermore, the rGPIb alpha-AMS enhanced the ristocetin induced platelet aggregation in a low platelet concentration (4.0 x 10(7)/mL).

Specific surface association of avidin with N-biotinylphosphatidylethanolamine membrane assemblies: effect on lipid phase behavior and acyl-chain dynamics

The interaction of avidin with aqueous dispersions of N-biotinylphosphatidylethanolamines, of acyl chain lengths C(14:0), C(16:0), and C(18:0), was studied by using spin-label electron spin resonance (ESR) spectroscopy, (31)P nuclear magnetic resonance ((31)P NMR) spectroscopy, differential scanning calorimetry, and chemical binding assays. In neutral buffer containing 1 M NaCl, binding of avidin is due to specific interaction with the biotinyl lipid headgroup because avidin presaturated with biotin does not bind. Saturation binding of the protein corresponds to a ratio of 50 lipid molecules per tetrameric avidin. Phospholipid probes spin-labeled at various positions between C-4 and C-14 in the sn-2 chain were used to characterize the effects of avidin binding on the lipid chain dynamics. In the fluid phase, protein binding results in a decrease of chain mobility at all positions of labeling while the flexibility gradient characteristic of a liquid-crystalline lipid phase is maintained. There is no evidence from the spin-label ESR spectra for penetration of the protein into the hydrophobic interior of the membrane. At temperatures corresponding to the gel phase, the lipid chain mobility increases on binding protein. The near constancy in mobility found with chain position, however, suggests that in the gel phase the lipid chains remain interdigitated upon binding avidin. Binding of increasing amounts of avidin results in a gradual decrease of the lipid chain-melting transition enthalpy with only small change in the transition temperature. At saturation binding, the calorimetric enthalpy is reduced to zero. (31)P NMR spectroscopy indicates that protein binding increases the surface curvature of dispersions of all three biotin lipids. The C(14:0) biotin lipid yields isotropic (31)P NMR spectra in the presence of avidin at all temperatures between 10 and 70 degrees C, in contrast to dispersions of the lipid alone, which give lamellar spectra at low temperature that become isotropic at the chain-melting temperature. In the presence of avidin, the C(16:0) and C(18:0) biotin lipids yield primarily lamellar (31)P NMR spectra at low temperature with a small isotropic component; the intensity of the isotropic component increases with temperature, and the spectra narrow and become totally isotropic at high temperature, in contrast to dispersions of the lipids alone, which give lamellar spectra in the fluid phase. The binding of avidin therefore reduces the cooperativity of the biotin lipid packing, regulates the mobility of the lipid chains, and enhances the surface curvature of the lipid aggregates. These effects may be important for both lateral and transbilayer communication in the membrane.

Spin-label electron paramagnetic resonance studies on the interaction of avidin with dimyristoyl-phosphatidylglycerol membranes

The interaction of avidin--a basic protein from hen egg-white--with dimyristoyl-phosphatidylglycerol membranes was investigated by spin-label electron paramagnetic resonance spectroscopy. Phosphatidylcholines, bearing the nitroxide spin label at different positions along the sn-2 acyl chain of the lipid were used to investigate the effect of protein binding on the lipid chain-melting phase transition and acyl chain dynamics. Binding of the protein at saturating levels results in abolition of the chain-melting phase transition of the lipid and accompanying perturbation of the lipid acyl chain mobility. In the fluid phase region, the outer hyperfine splitting increases for all phosphatidylcholine spin-label positional isomers, indicating that the chain mobility is decreased by binding avidin. However, there was no evidence for direct interaction of the protein with the lipid acyl chains, clearly indicating that the protein does not penetrate the hydrophobic interior of the membrane. Selectivity experiments with different spin-labelled lipid probes indicate that avidin exhibits a preference for negatively charged lipid species, although all spin-labelled lipid species indirectly sense the protein binding. The interaction with negatively charged lipids is relevant to the use of avidin in applications such as the ultrastructural localization of biotinylated lipids in histochemical studies.

Derivatised lipids in membranes. Physico-chemical aspects of N-biotinyl phosphatidylethanolamines, N-acyl phosphatidylethanolamines and N-acyl ethanolamines

The physical properties of N-biotinyl phosphatidylethanolamines, N-acyl phosphatidylethanolamines and of N-acyl ethanolamines, in aqueous dispersions, are reviewed. Emphasis is placed on the calorimetric (i.e. chain melting) properties, the thermotropic phase behaviour, certain aspects of the structure and dynamics, and the miscibility with other membrane lipids. In the case of N-biotinyl phosphatidylethanolamines, the specific binding of avidin, and in the case of N-acyl ethanolamines, the function of the third chain, is also considered. All of these properties are relevant to the role of these rather unusual lipids in membranes.

Mimicking rubella virus particles by using recombinant envelope glycoproteins and liposomes

The envelope glycoproteins E1 and E2 of rubella virus (RV) were engineered to display the FLAG epitope tag and a polyhistidine tag, at their amino and carboxy termini, respectively. These modified envelope proteins were produced in Sf9 insect cells utilizing baculovirus expression vectors, the E1 and E2 vectors giving rise to protein products of about 58 and 42 kDa, respectively. The recombinant proteins were purified by immobilized metal-ion affinity chromatography and reconstituted into liposomes via their hydrophobic transmembrane anchors. The liposomes were prepared by detergent dialysis in the presence of europium-DTPA chelate, enabling the subsequent measurement of the binding of the resultant proteoliposomes to the antibodies by time resolved fluorescence. RV mimicking proteoliposomes were recognized by antibodies specific for the E1 and E2 proteins, as well as the FLAG epitope tag. This type of virosome may prove useful for studies on the basic biological events of an RV infection or as diagnostic reagents.

Europium chelate-loaded liposomes: a tool for the study of binding and integrity of liposomes

Using the biotin-streptavidin interaction as a model, we investigated the suitability of lanthanide chelates as encapsulated liposomal labels in liposome-based binding assays. Large unilamellar phospholipid:cholesterol liposomes containing europium-DTPA chelate and biotinylated phosphatidylethanolamine were prepared by detergent dialysis. The resulting Eu-liposomes ([symbol: see text] 120 nm) bound specifically to streptavidin in microtiter wells as measured by time-resolved fluorometric assay (TRF). The intensity of fluorescence released from the bound liposomes was dependent on the concentration of biotin in the liposome membrane, the concentration of europium entrapped in the liposomes, the incubation time and the amount of liposomes used in the assay. The sensitivity of the TRF assay allowed the detection of binding of attomole quantities of liposomes. The streptavidin-immobilised liposomes subjected to porcine pancreatic phospholipase A2 (EC 3.1.1.4) and detergents displayed a dose-dependent release of the encapsulated europium. Lanthanide-chelate-liposomes should prove useful for studies addressing binding and stability of liposomes.

Structure of vitaminylated lipids in aqueous dispersion: X-ray diffraction and 31P NMR studies of N-biotinylphosphatidylethanolamines

The structures of the phases formed in excess buffer (at pH 7.4) by a homologous series of saturated diacylphosphatidylethanolamines in which the headgroup is N-derivatized with biotin have been investigated for chain lengths of C(12:0) to C(20:0), using both 31P nuclear magnetic resonance (NMR) spectroscopy and small-angle X-ray diffraction. In 1 M NaCl, all lipids display 31P NMR spectra characteristic of a lamellar gel phase at low temperature. In the fluid phase, the lipids of C(12:0) and C(14:0) chain lengths display isotropic 31P NMR spectra, corresponding to aggregated phases with high surface curvature, whereas those with C(18:0) and C(20:0) chain lengths display sharp axial powder patterns characteristic of a lamellar (L alpha) phase. The lipid of intermediate C(16:0) chain length displays a more complex temperature dependence of the 31P NMR spectra in the fluid phase. The spectra convert from an axial powder pattern of unusually low chemical shift anisotropy to one characteristic of a fluid lamellar (L alpha) phase with increasing temperature. The small-angle X-ray diffraction patterns of the lipids in 1 M NaCl have lamellar repeat spacings in the gel phase which increase linearly with chain length and are consistently lower than those in the fluid phase [for chain lengths of C(16:0) to C(20:0)]. In addition, the gradient in long spacing with chain length in the gel phase is approximately half that expected for a gel phase with untilted, all-trans chains, indicating that the lipid chains are interdigitated in the gel phase (L beta i).(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on carbohydrate-binding proteins using liposome-based systems--I. Preparation of neoglycoprotein-conjugated liposomes and the feasibility of their use as drug-targeting devices

1. Five types of neoglycoprotein-coupled liposomes were prepared in order to investigate their potential utility as new types of drug-targeting devices which exploit cellular functions of carbohydrate-binding proteins. 2. These preparations were shown to be stable at 37 degrees C for 24 hr and at 7 degrees C over 4 months. 3. An inhibition assay in an in vitro system using human adenocarcinoma cells indicated the high affinity binding of neoglycoprotein-conjugated liposomes. The inhibitory potency correlated with both the type and the amount of immobilized neoglycoproteins on liposomes. 4. A tissue distribution assay in an in vivo system using Ehrlich solid tumor-bearing mice showed the feasibility of the application of [125I]neoglycoprotein-conjugated liposomes as drug-targeting devices, based on carbohydrate-protein interactions.

Liposomes in haematology

Liposomes were first described nearly a quarter of a century ago and have been useful models for studying the physical chemistry of lipid bilayers and the biology of the cell membrane. It was also realised that they might be used as vehicles for the delivery of drugs but clinical applications have been slow to emerge. Proposed clinical uses have included vaccine adjuvancy, gene transfer and diagnostic imaging but the major effort has been in the development of liposomes as targetable drug carriers in the treatment of malignancy. Although based on good in vitro data and animal studies, the strategies have been mostly impractical due to the predominant but unwanted uptake by the reticuloendothelial system and the limited extent of extravasation. The same features have nonetheless been turned to advantage in the case of amphotericin B which has recently become the first liposomally formulated agent to be licensed for parenteral use. Liposomal doxorubicin is currently also being evaluated in clinical trials. The early evidence suggests that while liposomal encapsulation may not greatly enhance their efficacy the toxicity of these agents is greatly attenuated.

New indium-111 labeled biotin derivatives for improved immunotargeting

Investigations into the use of streptavidin-conjugated antibodies and labeled biotin to improve radioimmunotargeting have shown background levels drastically reduced over the conventional approach. Nevertheless, accumulation of 111In-biotin in normal tissue as well as streptavidin-independent accumulation in tumor, was observed. In this work, the effect of altering the biotin molecule to reduce this nonspecific uptake without decreasing specific localization has been investigated. Three EDTA and DTPA derivatives of biotin have been synthesized and investigated along with a commercial biotin derivative (DTPA-B2). The labeled biotin chelates were administered i.p. to normal mice implanted with avidin beads in one thigh. A wide variation in biodistribution was seen among the biotin derivatives. The most favorable results were obtained with biotinyl-hydrazino-EDTA (EDTA-B1), which showed the lowest accumulation in normal tissues but equivalent uptake in the target with respect to the other compounds. Averaged over 8 tissues sampled, the target-to-nontarget ratio was 140 vs 9 for EDTA-B1 vs DTPA-B2 (N = 6) at 24 h post administration. Similar observations have been made in culture with two tumor cell lines: positive accumulation of both DTPA-B2 and EDTA-B1 was measured in tumor cells independent of streptavidin-antibody conjugate, however in the case of the latter derivative, this accumulation was 3-5 fold lower. These studies show that modification of the biotin species can alter accumulation in normal tissues as well as the antibody-streptavidin independent accumulation in tumor tissue.

Characterization of in vivo immunoliposome targeting to pulmonary endothelium

Two rat monoclonal antibodies, 34A and 201B, which specifically bind to a surface glycoprotein (gp112) of the pulmonary endothelial cell surface, have been coupled to unilamellar liposomes of approximately 0.25 microns in diameter. The 34A- and 201B-liposomes (monoclonal antibodies 273-34A and 411-201B, respectively), but not antibody-free liposomes and liposomes coupled to 14, a nonspecific monoclonal antibody, accumulate efficiently (approximately 30% injected dose) in the lung of mice which have been injected via the tail vein. Immunoliposome targeting to lung is demonstrated both by using a 125I-labeled lipid marker and an entrapped water-soluble marker. Lung accumulation of 34A-liposomes is completely blocked by a preincubation of free antibody 34A, but not antibody 14, indicating that the immunoliposome accumulation at the target site is immunospecific. Time course studies have revealed that 34A-liposomes bind to lung antigens within 1 min after injection, indicating that the target binding takes place during the first few passages of immunoliposomes through the lung capillary bed. Unbound immunoliposomes are taken up by liver and spleen within 3-5 min after injection. The level of lung accumulation increases significantly as the protein:lipid ratio of the immunoliposome increases. Approximately 50% of injected dose is accumulated in lung for 34A-liposomes, with an average of 935 antibody molecules per liposome. Immunoliposomes of larger size accumulate in lung more significantly than those of smaller size. Injection with higher doses also enhances the level of lung accumulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Generic liposome reagent for immunoassays

We have derivatized liposomes with antibodies by using avidin to crosslink biotinylated phospholipid molecules in the liposome membranes with biotinylated antibody molecules. A comparison of the biotin binding activity of avidin in solution and avidin associated with liposomes shows that avidin bound to biotinylated phospholipid in liposome membranes retains full binding activity for additional biotin molecules. Changes in the fluorescence spectrum of avidin have been used to characterize the binding capacity of avidin for biotin in solution, and change in intensity of light scattered due to aggregation of liposomes was used to measure the biotin binding activity of avidin associated with liposomes. Relative amounts of the biotinylated phospholipid, avidin, and biotinylated antibody have been optimized to produce stable liposomes which are derivatized with up to 1.7 nmol of antibody/mumol of lipid. These derivatized liposomes are highly reactive to immunospecific aggregation in the presence of multivalent antigen. A linear increase in light scattering was recorded between 1 and 10 pmol of antigen. This work shows that liposomes containing biotinylated phospholipid can be a successful generic reagent for immunoassays.

Antibody-mediated targeting of differentiation inducers to tumor cells: inhibition of colonic cancer cell growth in vitro and in vivo. A preliminary note

A differentiation inducer (sodium butyrate) encapsulated in liposomes that are in turn covalently linked to anti-Lex monoclonal antibody, SH1 (IgG3 isotype), was successfully targeted to human colonic adenocarcinoma HRT-18 and HT29 cells expressing Lex antigen in vitro as well as in vivo in athymic nu/nu mice. Tumor cell growth was significantly inhibited and was associated with changes in cell morphology and increases in membrane-bound alkaline phosphatase and gamma-glutamyltranspeptidase, indicating the occurrence of butyrate-induced differentiation.

Monoclonal antibody to human endothelial cell surface internalization and liposome delivery in cell culture

A monoclonal antibody (mAb), E25, is described that binds to the surface of cultured human endothelial cells. Upon binding E25 is rapidly internalized and digested intracellularly. Selective liposome targeting to the surface of the cells is performed using a biotinylated E25 antibody and an avidin-biotin system. Up to 30% of the cell-adherent liposomal lipid is internalized.

Characterization of anti-cerebroside sulfate antisera using a theoretical model to analyse liposome immune lysis data

Antisera to the acidic glycolipid cerebroside sulfate (sulfogalactosyl ceramide) were raised in rabbits by several different methods. Reactivity with cerebroside sulfate was detected from complement-mediated lysis of liposomes composed of phosphatidylcholine/cholesterol/cerebroside sulfate and containing the spin label tempocholine chloride as a marker substance. Both cholesterol rich particles and lipid bilayer liposomes containing phosphatidylcholine and cholesterol were effective carriers for cerebroside sulfate, in combination with methylated bovine serum albumin for intravenous immunization, and with Freunds complete adjuvant for subcutaneous immunization. The antisera raised by the different methods were characterized with respect to their cross reactivity with other lipids and the relative concn of specific antibodies and their affinities for cerebroside sulfate using a theoretical model developed earlier [Vistnes A. I. (1984) J. Immun. Meth. 68, 251] for analysis of data from immune lysis of liposomes. Differences in these properties, both of which can affect antibody titer, could be detected for antisera raised by different methods and obtained at different times after immunization. Some of the antisera also reacted non-specifically to varying degrees with other anionic lipids indicating that the anti-cerebroside sulfate antibodies could bind non-specifically to anionic lipids by electrostatic interactions. This suggested that basic amino acid residues may be an important part of the antibody receptor binding site for the glycolipid head group. An important implication of this result is that antibodies raised against anionic glycolipids should be tested for non-specific binding to anionic phospholipids.

A non-covalent method of attaching antibodies to liposomes

A novel non-covalent method of attaching antibodies to liposomes which exploits the high affinity of streptavidin for biotin, is described. The two-step coupling protocol involves the initial attachment of streptavidin to liposomes containing biotin PE, followed by the coupling of biotinated antibodies to streptavidin-liposomes. The association of streptavidin with liposomes containing biotinated PE is rapid (less than 5 min), resulting in a maximum association of 40 molecules of streptavidin per 100 nm vesicle. In the presence of equimolar cholesterol, the amount of streptavidin bound is twice that observed when biotin PE/egg PC liposomes are used. Irrespective of the mole ratio of biotin to antibody (e.g. for 1-6 biotins per antibody), or the molar ratio of antibody to streptavidin in the second incubation step, equimolar amounts of antibody bind to streptavidin. It is shown that anti-rat-erythrocyte IgG or F(ab')2 complexed to liposomes via the streptavidin linker bind specifically to rat erythrocytes but not to human erythrocytes. This coupling protocol can be readily extended to other biotinated antibodies.

Comparison of inhibitory effect of galactose analogs on the binding and cytotoxicity of an anti-globotriaosylceramide monoclonal antibody coupled or not coupled to pokeweed antiviral protein

The results described here provided an example of a human IgM monoclonal antibody against a tumor-associated glycolipid and of the unusual properties of its corresponding immunotoxin (IT). The monoclonal antibody referred to as 38-13 has been previously described and reacted with the globotriaosylceramide [Gb3:Gal(alpha 1----4)-Gal(beta 1----4)-Glc(beta 1----1)ceramide] specifically expressed on surface membrane of Burkitt's lymphoma (BL) cells. An immunotoxin (38-13 IT) combined with the pokeweed antiviral protein (PAP) toxin via S-S bridges showed paradoxically a lower cytotoxic effect in BL Ramos cells than in non-BL cells such as leukemic mouse L1210 cells, while these cells appeared not to be involved by flow cytometric analysis and complement-dependent cytotoxicity. Consequently, the inhibitory effect of selective galactose analogs on the binding and the cytotoxicity of 38-13 antibody conjugated or not to PAP toxin was compared on BL and non-BL cells. Only the galactose blocked in alpha configuration provided a fine inhibition of 38-13 binding on BL Ramos cells and both alpha and beta-galactose allowed us to establish a clear distinction between the pathway entry of 38-13 IT in BL and non-BL cells; in close correlation with the 38-13 binding specificity the 38-13 IT cytotoxic effect in Ramos BL cells could also be prevented by alpha-Gal only, suggesting that this toxic action is probably mediated through the IT binding to Gb3 antigenic sites. In contrast, on apparently irrelevant L1210 cells, 38-13 IT showed a cytotoxic effect which was inhibited preferentially by lactose (Gal in beta configuration). It was discussed that IT binding alone to either antigenic sites which are inhibited by the hapten alpha-Gal, or nonspecific sites which can compete with the hapten beta-Gal is unable to induce efficient killing of cells. But cooperation of both bindings might give an attractive explanation of IT cytotoxic effect. It was concluded that the unexpected activity of 38-13 IT in non-BL cells probably could be mediated through an active macromolecular transport process which could implicate a beta-galactoside-binding protein (lectin).

On the possibility of the unification of drug targeting systems. Studies with liposome transport to the mixtures of target antigens

In order to make the drug targeting system more effective, simple and technological, we suggest creation of drug-bearing conjugates capable of simultaneous binding with different antigenic components of the target via specific antibodies. It is supposed that the targeted therapy should include sequential administration of the mixture of modified antibodies (or other specific vectors) against different components of affected tissue and, upon antibody accumulation in the desired region, administration of modified drugs or drug carrying systems which can recognize and bind with the target via accumulated antibodies due to the interaction between vector modifier and carrier modifier. Using as a model system monolayers consisting of the mixture of extracellular antigens and appropriated antibodies, it was shown that the treatment of the target with the mixture of biotinylated antibodies against all target components and subsequent binding with the target of biotinylated liposomes via avidin permits high liposome accumulation on the monolayer. The binding achieved is always higher than in the case of the utilization of single antibody-bearing liposomes. Besides, the system suggested is very simple and its components can be easily obtained on technological scale in standardized conditions.

Specific isolation of surface glycoproteins from intact cells by biotinylated concanavalin A and immobilized streptavidin

An indirect affinity chromatography procedure utilizing biotinylated lectins and designed for the specific isolation of surface glycoproteins is described. The method is illustrated with intact acute leukemic lymphoblastic cells (ALL cells) with biotin-epsilon-aminocaproyl-concanavalin A (biocap-Con A) and streptavidin-Sepharose 4B. Biocap-Con A, containing on average 27 biotin residues per tetrameric lectin molecule, is used to isolate Con A-binding glycoproteins from the surface of [35S]methionine-radiolabeled intact cells. The biocap-Con A/glycoprotein complexes, after solubilization in detergent, are retrieved on immobilized streptavidin. The surface glycoproteins isolated from intact ALL cells by this method are subjected to two-dimensional gel electrophoresis and detected by autoradiography. More than fifty Con A-binding glycoproteins can be separated from the ALL cells. These glycoproteins retrievable from the cell surface were compared to those retrieved by the indirect affinity chromatography procedure from isolated plasma membrane fractions. Certain groups of glycoproteins present in the fraction isolated from intact cells were not detected in that from the plasma membrane preparations. The advantage of using the biocap-con A/streptavidin system with intact cells rather than isolated plasma membranes for the detection of surface glycoproteins is discussed.

The development and application of protein-liposome conjugation techniques

Numerous techniques have been developed over the past 10 years for the conjugation of proteins to liposomes. Early procedures involved coupling with reagents such as glutaraldehyde or EDCI. Subsequently, more sophisticated approaches involving selective bifunctional coupling agents have been developed. These later procedures are also much more efficient for coupling in aqueous media. The techniques of coupling have become more rigorous because investigators have recognized the inherent problems of producing, purifying and characterizing protein conjugated liposomes. Protein-liposome coupling techniques were developed mainly for targeted drug delivery. The attachment of specific antibodies to the surface of the liposomes makes them able to bind to cells and to subsequently be internalised by the cells. Protein conjugated liposomes have also been used for various immunochemical and diagnostic purposes. These include the binding of labelled liposomes to cells and the agglutination of cells or latex particles by protein conjugated liposomes.

Gangliosides and related compounds as biological response modifiers

Several possibilities are discussed with regard to the biological potency of gangliosides and related compounds as biological response modifier. Evidence is presented that a ganglioside, GQ1b, but not other gangliosides examined exhibits a strong biological activity at a few nanomolar concentration for the promotion of neurite outgrowth as well as cell division in two cell lines of neuroblastoma.

Immunoglobulins as targeting agents for liposome encapsulated drugs

Recent literature dealing with the immunoglobulin-liposome combination as a tool for drug targeting is reviewed. Mechanisms for binding of immunoglobulins to liposomes are discussed. Results obtained so far by using this concept both to induce specific liposome-cell interactions in in vitro cell cultures and to target drug containing liposomes to selected tissues in animal studies are evaluated.

Inhibition of carbohydrate incorporation in transformed cells by a cancer-associated galactosyltransferase acceptor (CAGA)

The effect of cancer-associated galactosyltransferase acceptor (CAGA) on incorporation of a variety of macromolecular precursors has been studied in transformed and nontransformed cells. Incorporation of [3H]-mannose, [3H]-galactose, and [3H]-glucosamine into acid precipitable material after one-hour pulse was inhibited more than 70% within four hours after exposure to CAGA in polyoma-transformed BHK cells and within eight hours after exposure in chick embryo fibroblasts infected with a temperature-sensitive RSV mutant (Ts68) grown at the permissive temperature (CEF-RSV 37 degrees C). Initial short-term rate of uptake (less than one minute) and total long-term uptake (one hour) of the labelled carbohydrates (acid-soluble and acid-insoluble material) was inhibited less than 15% over this period. Incorporation of 14C-leucine, 3H-serine, 3H-uridine, and 3H-thymidine into acid-precipitable material was also inhibited greater than 85% in transformed cells, but more than 12-hour exposure to CAGA was required before maximal inhibition was detected. Uptake of these labelled precursors was inhibited less than 20% up to eight hours after exposure to CAGA. In nontransformed cells (BHK and CEF) incorporation of labelled monosaccharides as well as protein and nucleic acid precursors into acid-precipitable material was reduced less than 25% up to 12 hours following exposure to CAGA. Infected CEF grown at the nonpermissive temperature (CEF-RSV 41 degrees C) were affected to an extent similar to other nontransformed cells. These data suggest that the specific action of CAGA on transformed cells may be due to inhibition of glycoconjugate synthesis.

An improved method for covalent attachment of antibody to liposomes

A rapid and simple method is described for the incorporation of monoclonal antibody coupled with palmitic acid into liposomes prepared by the reverse-phase evaporation method (Szoka, F. and Papahadjopoulos, D. (1978) Proc. Natl. Acad. Sci. U.S.A. 75, 4194-4198). Palmitoyl antibody in 0.15% deoxycholate is added to a liposome suspension after the majority of the organic solvent has been removed by evaporation. Efficient incorporation (over 80%) of palmitoyl antibody occurred without leakage of the encapsulated drug. Native, unmodified antibody did not incorporate under identical conditions. About 50% of the incorporated antibodies could be readily digested by protease, while most of an internal protein marker was not, suggesting that about half of the antibodies were exposed on the outer surfaces of liposomes. Target-specific binding of antibody-liposomes has also been demonstrated in vitro with the RDM-4 lymphoma cells. This method offers a rapid and highly efficient attachment of functional antibody molecules to liposomes with high capture efficiency of drugs, and therefore should be useful in target-specific delivery of drugs mediated by liposomes.

Chemical synthesis of alpha-L-fucopyranosylceramide and its analogues and preparation of antibodies directed to this glycolipid

A novel glycolipid, alpha-L-fucopyranosylceramide, was previously isolated and characterized from metastatic human adenocarcinoma [Watanabe, K., Matsubara, T., & Hakomori, S. (1976) J. Biol. Chem. 251, 2385-2387]. For further investigation of the pathobiological significance of this glycolipid with its specific antibodies, the antigen glycolipid alpha-L-fucopyranosylceramide was chemically synthesized by Königs-Knorr-type condensation of 2,3,4-tri-O-benzyl-alpha-L-fucopyranosyl bromide and a ceramide, followed by removal of benzyl groups by catalytic hydrogenation. alpha-L-Fucopyranosyl-N-palmitoylethanolamine and 2,3,4-tri-O-benzyl-alpha-L-fucopyranosylsphingosine were also synthesized. The antibodies prepared against a synthetic alpha-L-fucopyranosyl-ceramide included in liposome reacted very well with the synthetic as well as the natural alpha-L-fucopyranosylceramide isolated from human adenocarcinoma, but the kinetics of the complement-dependent liposome lysis and the complement fixation pattern with the synthetic fucosylceramide were significantly different from those with the natural fucosylceramide. The natural fucosylceramide showed a much weaker immunogenicity than the synthetic fucosylceramide. The remarkable difference in liposome lysis, complement fixation, and immunogenicity between the synthetic and the natural fucosylceramide must be due to the difference in ceramide structures. The results indicate that the strength of antigenicity and immunogenicity of glycolipids may greatly depend on ceramide structure, whereas the specificity is solely determined by the sugar moiety. The anti-alpha-L-fucosylceramide antibodies display cross-reactivity with galactosylceramide, ceramide, and liposome alone that was eliminated by absorption with galactosylceramide liposomes.

NK cell-target interactions: approaches towards definition of recognition structures

NK cells lyse an uncommonly wide range of cell types, implying either that they (the NK cells) have clonally distributed receptors each of which is capable of interacting with a very limited number of cell types or, alternatively, that susceptible target cells share a common characteristic. A number of experimental approaches have suggested that the cytotoxic 'specificity' of NK cells is not clonally distributed. Thus, clones of NK cells, established from mouse spleen cell suspensions, showed no greater restriction in the spectrum of target cells which they could lyse, than did the parent spleen cell populations from which they were derived. It seems likely therefore that the wide range of target cell types that can be lysed by NK cells share common cell surface characteristics which render them susceptible. As lysis results from membrane-membrane interactions, it seemed logical that a search for 'hallmarks' of NK susceptibility should begin with a detailed examination of the plasma membrane of susceptible cells. Analysis of one pair of lymphoma cell variants, selected on account of their markedly different susceptibility to NK cells, suggests that cell surface glycoconjugates may be of significance in determining those effector cell-target cell interactions that lead to lysis. This review outlines attempts to characterize such glycoconjugates.