Merkel cell carcinoma (MCC) is an aggressive neuroendocrine cancer of the skin with a dismal five-year survival rate of less than 18% in advanced disease and a mortality rate 3-times higher than melanoma. MCC disproportionately and predominantly affects Caucasian males older than 65, who are well represented among Veterans, and more than 5000 new MCC cases have been diagnosed among Veterans. Hence, MCC has a growing impact on Veterans’ health and VA healthcare system. Notwithstanding the approval of immunotherapy, treating metastatic MCC remains a challenge. Thus, we are compelled to seek novel therapies to overcome resistance, or to serve as definitive treatments for MCC patients who are ineligible for immunotherapy due to comorbidities. In this proposal, we will collaborate with VA Medical Centers encountering most Veteran MCC patients to define the role of B7-H3 (encoded by CD276) in MCC patient survival in a large cohort of Veteran patients. Moreover, we will determine the efficacy of CD276-targeted therapy, as its overexpression has been associated with poor outcome in a myriad of advanced human cancer including MCC. Our collaborator, Dr. St. Croix at the National Cancer Institute, has developed a novel antibody-drug conjugate (ADC, m276-SL-PBD), which couples CD276 antibody with a cytotoxic DNA-binding agent PBD. Notably, m276-SL-PBD confers robust anti-tumor activities and long-term durability in a range of preclinical models, including our MCC cell line-derived xenograft (CDX) models (preliminary data leading to this proposal). Remarkably, m276-SL-PBD effectively prevents resistance by directly eradicating heterogenous cancer cells including cancer stem cells, tumor vasculatures and tumor stromal cells, where CD276 upregulation is endowed. Therefore, we postulate that m276-SL-PBD is a superior anti-MCC drug with sustainable response, underscoring a novel therapeutic advance in MCC. To overcome barriers, we have successfully established unique and robust models for MCC preclinical studies including multiple primary MCC cell lines, CDX, and patient tumor-derived xenograft (PDX) models with reconstituted human immune system. To simulate clinical scenario, we have first established human metastatic MCC cell lines with propensity for spontaneous visceral dissemination after subcutaneous inoculation in mice. Due to its finite resource, we will utilize single mouse testing (SMT)-base PDX trial strategies that are increasingly used to better capture inter-tumor heterogeneity and generate highly clinically relevant preclinical evidence. The cost-effectiveness of SMT-based studies allow to encompass a large cohort of patient tumors and evaluate heterogeneous drug effects across patients, which is of particular importance in rare cancer preclinical studies including MCC. Hence leveraging our singularly powerful patient MCC-derived models, we are ideally positioned to pursue proposed studies: 1) to determine safety, therapeutic efficacy, and direc...