Studies of methotrexate-monoclonal antibody conjugates for immunotherapy.

Methotrexate (MTX) was covalently bound to two different murine monoclonal antibodies, one reactive with human colon carcinoma and the other reactive with the transferrin receptor. The drug-antibody complexes were examined for their in vitro and in vivo potency against tumors. The conditions of coupling were closely monitored, with particular attention being paid to the preservation of both drug and antibody activity. After activation of MTX with N-hydroxysuccinimide, MTX was bound to the antibodies under conditions leading to maximum protein and antibody recovery. Although the coupling conditions for both antibodies were different, up to 13 molecules of MTX-antibody molecule could be attached with retention of antibody activity. Such conjugates were active in vitro and could inhibit the growth [( 3H]deoxyuridine uptake) of cells in culture. The conjugates were highly specific, having no effect on tumors lacking the antigen; however, MTX complexed to antibody was less potent than the free drug. In vivo, the MTX-antibody impaired the growth of established tumors. Thus MTX-antibody complexes can be successfully produced and can be used for the immunotherapy of tumors.