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Schwendener, R. A. and Trüb, T. and Schott, H. and Langhals, Heinz and Barth, R. F. and Groscurth, P. and Hengartner, H. (1990): Comparative studies of the preparation of immunoliposomes with the use of two bifunctional coupling agents and investigation of in vitro immunoliposome-target cell binding by cytofluorometry and electron microscopy. In: Biochimica et Biophysica Acta (BBA) - Biomembranes, Vol. 1026, No. 1: pp. 69-79
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Abstract

The two coupling agents SPDP (N-succinimidyl-3-(2-pyridyldithio)propionate) and SATA (N-succinimidyl-S-acetylthioacetate) were compared in their efficiency and feasibility to couple monoclonal antibodies (Abs) via thioether linkage to liposomes functionalized by various lipophilic maleimide compounds like Full-size image methyl ester (MP-PL), N-(3-maleimidopropionyl)phosphatidylethanolamide (MP-PE), Full-size image methyl ester (EMC-PL), and N-(6-maleimidocaproyl)phosphatidylethanolamine (EMC-PE). The composition of the liposomes was soy phosphatidylcholine (SPC), cholesterol, maleimide compounds and -tocopherol (1:0.2:0.02:0.01, mol parts), plus N4-oleylcytosine arabinoside (NOAC) as cytostatic prodrug (0.2 mol parts) and a new, lipophilic and highly fluorescent dye N,N′-bis(1-hexylheptyl)-3,4:9,10-perylenebis(dicarboximide) (BHPD, 0.006 mol parts). From the maleimide derivatives MP-PL was the most effective in terms of preservation of the coupling activity in dependence of liposome storage. The coupling of the monoclonal A B8-24.3 (mouse IgG2b, MHC class I, anti H-2kb) and IB16-6 (rat IgG2a, anti B16 mouse melanoma) to the drug carrying liposomes was more effective and easier to accomplish with SATA as compared to SPDP. Coupling rates of 60–65% were obtained with SATA at molar ratios of 12 SATA:1 Ab:40 maleimide spacer groups on the surface of one liposome. The highest coupling rates with SPDP were obtained at the ratio of 24 SPDP:1 Ab:40 liposomal maleimide groups, with an Ab binding efficiency of only 20–25%. The optimal in vitro binding conditions to specific target cells (EL4 for B8-24.3-liposomes and B16-F10 for IB16-6-liposomes) were determined by cytofluorometric measurement of the liposomal BHPD fluorescence with SATA linked Abs. Optimal immunoliposome binding to specific epitopes on the target cells was achieved with 1–2 Ab molecules coupled to one liposome, with immunoliposome concentrations of 20–130 nM and with a small incubation volume of 0.3–0.4 ml. The specificity of the binding of B8-24.3-liposomes to EL4 target cells was visualized by scanning electron microscopy. Antibody mediated endocytic uptake of immunoliposomes could be demonstrated by transmission electron microscopy.