Background: The four ErbB/HER receptor tyrosine kinases and their multiple EGF-Iike growth factors constitute a layered signaling network, whose action is unleashed in human cancer. ErbB interceptors (e.g., Erbitux, and Iressa) have already established the therapeutic significance of the network, but lack of patient selection criteria, low drug effectiveness, and development of resistance limit clinical efficacy. Our observations implicate a sub-set of low-affinity ErbB ligands in signaling and highlight the potential of harnessing protein endocytosis/degradation pathways. Objectives: The following lines of research are designed to resolve the molecular bases of drug action and pave the way for next generation ErbB therapeutics: (i) Ligands and soluble receptors- We hypothesize that autocrine loops involving low-affinity and other ErbB ligands drive the network in cancer and may limit drug efficacy. These possibilities will be examined and the therapeutic potential of blocking such loops by using monoclonal antibodies to specific ligands, as well as engineered bi-specific soluble ErbB receptors, will be tested in model systems. (ii) Monoclonal antibodies and their mechanisms of action- We hypothesize that endocytic removal of antibody-receptor complexes from the cell surface enhances efficacy of immunotherapy. Because certain combinations of anti-ErbB-2 antibodies better down regulate ErbB-2, we will design antibodies that simultaneously bind two epitopes of ErbB-2. The efficacy of the bi-epitopic antibodies will be compared to the parental antibodies in tumor-bearing animals. (iii) Chaperone and kinase inhibitors promoting ErbB degradation- Our results implicate Hsp90 as a suppressor of ligand-induced heterodimerization of ErbB-2. This model will be tested by using chaperone antagonists, as well as kinase inhibitors that occlude binding of Hsp90 to ErbB-2. Using chemical libraries and novel ErbB-2 degradation assays as read-out, we will select lead compounds and examine their tumor-inhibitory action. Significance: By highlighting the role of ErbB ligands, especially a previously ignored class of low-affinity growth factors, we hope to gain knowledge on patient stratification, response and resistance to therapy. Through blocking of autocrine loops, anti-ligand antibodies and bi-specific soluble ErbB proteins, we will construct may prevent resistance to immunotherapy and arrest tumors. Likewise, recombinant bi-epitopic monoclonal antibodies to ErbB-2 described herein may emerge as next generation immunotherapeutics. Finally, deeper understanding of the regulation of ErbB signaling by molecular chaperones, as well as selection of compounds that block chaperone-ErbB interactions, will enable clinical harnessing of the 26s proteasome to selective degradation of ErbB-2.