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In this dissertation three novel targeted liposomal formulations for the delivery of cytotoxic drugs in tumor therapy were generated and characterized. In order to achieve active targeting the surfaces of the liposomes were modified with tumor-specific ligands, which were generated by phage display. For targeting liposomes to the tumor vasculature a human single-chain Fv fragment (immunoliposomes), directed against Endoglin, and a RGD-peptide, directed against alpha-v-Integrins, were utilized. Tumor-cell specific liposomes were generated using a modified human Epidermal Growth Factor (EGF) protein. The single-chain Fv fragment was coupled to maleimide containing liposomes and the EGF-protein was linked to liposomes, containing the coupling lipid NHS-PEG-DSPE, whereas the RGD-peptide was incorporated as a lipopeptide into liposomes. First all liposomal formulations were characterized by physico-chemical parameters, such as size and zeta-potential. In vitro all targeted liposomes showed selective binding and internalization to receptor-positive cells. In addition, competition experiments demonstrated that binding and internalization was mediated by the ligands. Immunoliposomes, loaded with doxorubicin, showed in vitro an increased cytotoxicity towards endoglin-expressing cells compared to untargeted liposomes and free doxorubicin. In vivo experiments applying doxorubicin-loaded RGD-liposomes in a C26 colon carcinoma mouse model demonstrated improved efficiency compared with free doxorubicin and untargeted liposomes. Furthermore the cytotoxic drug cytarabine was encapsulated into a novel liposomal formulation. In vivo, these liposomes showed an improved anti-tumor effect compared with free cytarabine.