Monensin is obligatory for the cytotoxic action of a disulfide linked methotrexate-anti-transferrin receptor conjugate.

Methotrexate (MTX) in the form of a gamma-cysteinylglycine derivative was disulfide linked to a monoclonal antibody reactive with the human transferrin receptor to give an antibody-MTX conjugate (anti-TfR-MTX). Antibody directed delivery of MTX to cell surface receptors was readily detected by flow cytometry using an anti-MTX antibody plus a secondary fluorescent antibody probe. Despite the presence of ample drug on the cell membrane, the conjugate alone was not cytotoxic over the course of several days. Expression of specific toxic activity, however, was obtained in conjunction with the carboxylic ionophore monensin, in whose presence anti-TfR-MTX displayed an IC50 of 8 X 10(-8) M. These results suggest that the ionophore causes antibody-drug conjugate to bypass the normal transferrin receptor cyclic pathway, allowing sufficient drug to reach, bind to, and inactivate intracellular dihydrofolate reductase.