Treatment of solid tumors with immunotherapy has met with limited success. One approach to tumor therapy has been to target tumors with anti-cancer monoclonal antibodies (mAbs). One of the first mAbs approved by the FDA for treatment of solid tumors is Trastuzumab which targets the HER2 receptor and is used to treat 25 to 30% of metastatic breast cancers that overexpress the HER2 receptor. Mechanisms thought to contribute to the efficacy of Trastuzumab include an inhibitory effect on tumor cell growth, downregulation of angiogenic factors, and most importantly to the Aims of this application, recruitment and activation of cancer-killing cells of the innate immune system. Cancer-killing cells such as Natural Killer (NK) cells express activatory Fcg receptors (FcgRs) on the cell surface. Tumor cells coated with Trastuzumab are attacked by FcgR+ NK cells. Recruitment and activation of FcgR+ cancer-killing cells by Trastuzumab is an important contributor to its efficacy. Trastuzumab treatment fails in approximately 75% of breast cancer patients. This failure may link to sub-optimal engagement and activation of cancer-killing cells through insufficient engagement of FcgRs. We have developed recombinant Fcg receptor-targeted ligands (FcRTL) that are potently activate NK cells. FcRTLs bind FcgRs with higher avidity than monomeric or aggregated IgG. We propose to use our FcRTL ligand system to develop novel therapeutics comprised of recombinant fusion proteins that contain heavy and light chain variable region sequences from anti-HER2 mAbs fused to the FcRTLs, R2 and R4. Use of FcRTLs to develop anti-HER2 mAb analogs represents a novel approach for the treatment of cancer. The goal of the application is to develop novel mAb analogs built upon an FcRTL structure. PUBLIC HEALTH RELEVANCE: New Technology for making a better immunotherapeutic for treating breast cancer: Anti-HER2 antibody, Trastuzumab, is used for the treatment of HER2-overexpressing breast cancer. The efficacy of Trastuzumab is limited by insufficient binding and crosslinking of Fcg receptors (FcgRs) on tumor-attacking innate immune system cells. We are pursuing a novel strategy to develop anti-HER2 monoclonal antibody (mAb) analogs. We have developed recombinant Fcg receptor-targeted ligands (FcRTL) that bind low affinity FcgRs and activate NK cells. We propose to apply the FcRTL ligand system to the development of novel therapeutics comprised of recombinant fusion proteins that contain heavy and light chain variable region sequences from anti-HER2 mAbs fused to the FcRTLs, R2 and R4. Use of FcRTLs in the development of anti-HER2 mAb analogs represents a novel approach to optimize mAbs for the treatment of cancer.