Drug-monoclonal antibody conjugates for cancer therapy: potentials and limitations.

The development of monoclonal antibodies against antigens associated preferentially with human tumors has reawakened interest in the use of antibodies as site-specific targeting agents for cancer therapy. There are now many reports of the construction of conjugates of drugs and toxins with antibodies which have in vitro toxic properties directed by the antibody moiety. Only recently, however, have limitations to the in vivo therapeutic application of these conjugates become appreciated. These include limited levels of antibodies accumulating in tumor deposits in patients, sometimes marked differences in the biodistribution of antibody-drug conjugates from those seen with free antibody, limited penetration of antibody and drug-antibody conjugates into tumor tissue together with heterogeneity of antigen expression within tumor, and limitation to the cytotoxicity of conjugates caused by the low drug to antibody molar ratios possible without denaturation. It is now becoming appreciated that effective development of this approach will require more than chemical conjugation of existing or even novel drugs or toxins to antibodies so that they are cytotoxic in vitro. Additional strategies will probably have to be employed, including the use of intermediate carriers to increase drug to antibody ratios in conjugates, the production of conjugates without extreme overall charge or conformation changes, techniques to increase antibody localization into tumors by, for example, regional perfusion, blood flow enhancement, or increase in antigenic expression, or the use of conjugates containing mixtures of antibodies or fragments directed against different tumor-associated antigens.