The cellular uptake and cytotoxicity of chlorambucil-monoclonal antibody conjugates

To investigate the mode of entry and action of the alkylating agent chlorambucil (CBL), conjugated to monoclonal antibodies (MoAbs), CBL was coupled with three different MoAbs--to the transferrin receptor, to L3T4 and to Ly-2 molecules--and the activity of these conjugates was compared with free CBL. It was clear that CBL and CBL-MoAb conjugates enter cells and are transported differently within the cell prior to their cytotoxic action. Evidence favouring a separate entry point of CBL and CBL-MoAb conjugates is the differential effect of temperature and metabolic inhibitors (2-deoxyglucose and sodium azide) on the processing of both moieties. In addition, the likely sites of cleavage of CBL-MoAb complexes, the lysosomes, were effected by NH4Cl and chloroquine, which inhibited the activity of CBL-MoAb but not free CBL. Thus, it is likely that CBL-MoAb conjugates enter via the antibody binding sites and the CBL is internalised and transported as a passenger.