LAT1 as a Novel Therapeutic Target in Colorectal Cancer - ABSTRACT Colorectal cancer (CRC) is the second leading cause of cancer-related death in the United States, with limited treatment options, especially for patients with microsatellite stable (MSS) tumors, which are largely resistant to immune checkpoint inhibitors (ICIs). Despite advances in chemotherapy and targeted therapies, CRC treatment remains suboptimal, particularly in overcoming immunotherapy resistance. Therefore, there is an urgent need to identify novel therapeutic strategies that can modulate the immune microenvironment and enhance the efficacy of ICIs to increase survival outcomes in advanced-stage CRC. Our high-throughput metabolomic and transcriptomic data analysis from CRC patients identified significant upregulation of the essential amino acid (eAA) transporter LAT1 in CRC tissues, particularly in MSS tumors. Elevated LAT1 expression correlates with poor prognosis and immunosuppressive M2-like macrophages, suggesting that LAT1 contributes to tumor immune evasion. Our preliminary studies showed that LAT1 inhibition decreases tumor growth and improves T cell infiltration in the tumor. Based on these observations, we hypothesize that LAT1 inhibition will reduce tumor growth and enhance antitumor immune responses by increasing M1-like macrophages and CD8+ T cells in the tumor environment. We propose targeting LAT1 with the JPH203, a LAT1-specific inhibitor, to synergize with ICI therapy to improve therapeutic outcomes in advanced-stage CRC models. Aim 1: Evaluate the impact of LAT1 inhibition on tumor growth and immune cell infiltration using an orthotopic CRC model. Aim 2: Evaluate the effect of the LAT1-specific inhibitor JPH203, alone and in combination with ICIs, in reducing tumor burden and enhancing immune responses in advanced-stage CRC models. Impact: This project will elucidate the role of LAT1 in CRC and explore the therapeutic potential of LAT1 inhibition in combination with ICIs. By targeting LAT1, we aim to overcome immunotherapy resistance and improve outcomes for CRC patients, particularly those with MSS tumors.