Tumor Targeting with Biotinylated Tumor Necrosis Factor α: Structure-Activity Relationships and Mechanism of Action on Avidin Pretargeted Tumor Cells¹

We have recently described a new strategy for targeting biotinylated tumor necrosis factor-α (TNF-α) to tumors, based on pretargeting with biotinylated antibodies and avidin. Here, we have analyzed the structure-activity relationships of several biotin-TNF-α conjugates and studied the mechanism of their interaction with avidin and TNF-α receptors on tumor cells.

The study has been carried out using an in vitro model based on human melanoma Colo 38 cells and monoclonal antibody 225, an antibody against the high molecular weight melanoma-associated antigen. Immunochemical and cytotoxicity studies showed that biotin-TNF-α but not TNF-α persists for several hours on the surface of cells pretargeted with biotin-monoclonal antibody 225 and avidin and triggers cytolytic effects. Studies on the mechanism of action showed that biotin-TNF-α trimers can slowly dissociate from targeted cells in a bioactive form, through trimer-monomer-trimer transitions. Structure-activity relationship studies showed that nonbiotinylated subunits must be present in the biotin-TNF-α trimers for efficient release of bioactive TNF-α. Colo 38 cells targeted with biotin-TNF-α were able to kill mouse L-M cells in coculture experiments, indicating that the released TNF-α can interact also with TNF-α receptors expressed by bystander cells. In conclusion, the targeting complex works as a system that slowly releases bioactive TNF-α in the microenvironment of the targeted cell. This opens up the possibility that cells other than those reached by the targeting antibody (e.g., endothelial cells and local cells of the immune system) can be affected in vivo.