In recent years, an increasing number of studies have been devoted to the development of more and more sophisticated targeted delivery systems. First attempts in this direction were accomplished using monoclonal antibodies or antibody fragments coupled with active compounds, such as antitumor drugs (1). The antibodies were used to deliver the drug to the target cells expressing specific antigens on their surface. One of the major problems in the development and utilization of these drug-antibody conjugates was the preservation of both pharmacological activity and binding ability after production of the drug-antibody conjugates. In fact, to maintain the binding activity of the antibody, only a relatively low amount of a drug can be coupled to it, which might be insufficient to obtain the desired therapeutic effect. If a higher amount of a drug is coupled to the antibody, the risk of inactivating the recognition sites of the antibody is generally rather high. This fact promoted the investigation of other strategies and one of the most interesting and challenging strategies consists of the entrapment of drugs into liposomes or nanoparticles coated with specific ligands, such as antibodies.