PROJECT ABSTRACT (Application submitted in response to NOSI NOT-CA-21-100) Colorectal cancer (CRC) with peritoneal metastases (PM) has evolved from a terminal diagnosis to a condition treatable through cytoreductive surgery. Observed in up to 10-15% of patients undergoing surgery for CRC and ultimately seen in up to 35% of patients with CRC, cytoreductive surgery has revolutionized the treatment of patients with CRC-PM. A critical factor predictive of oncologic outcomes after a cytoreductive surgery is the completeness of cytoreduction; failure to do so is associated with earlier recurrence and poor survival. Current methods of detection during surgery rely on visual and tactile inspection, posing challenges in achieving complete cytoreduction. There is a critical need for improved methods of cancer detection and treatment in the surgical setting. In response, new approaches such as in-vivo macroscopic immunohistochemical staining using tumor-targeting fluorescent dyes for fluorescence image-guided surgery (FIGS) are in development. FIGS helps identify tumor deposits, but its effectiveness is limited to surgically removable lesions. Another approach, targeted photoimmunotherapy (PIT), combines a phototoxic fluorophore with an anti-tumor targeting antibody to both identify and treat lesions. A lead target for PIT is CEA, as it is specifically overexpressed in many gastrointestinal cancers, especially CRC. An ideal candidate for this approach is carcinoembryonic antigen (CEA), a well- studied tumor-marker overexpressed in many gastrointestinal cancers, particularly CRC. Its specificity makes CEA an ideal target for our innovative approach, combining targeted photoimmunotherapy (PIT) with fluorescence image-guided surgery (FIGS). Leveraging my expertise in fluorescence and surgery to develop theranostic approaches along with a strong mentorship team, I propose to conduct a series of initial studies to determine efficacy of PIT for FIGS and immune activation using CEA as the target. I hypothesize that a humanized antibody targeting CEA (M5A) conjugated to the phototoxic fluorophore (IRDye700DX) can effectively label CEA-expressing CRC for FIGS and upon NIR light activation, induce phototoxic tumor-cell lysis, and elicit a cell-mediated immune response. This novel approach aims to enhance surgical precision, offer local therapy in surgically challenging areas, and potentially activate therapeutic immune responses. It could transform an immunosuppressive microenvironment into an immune-activated one, significantly improving patient outcomes in CRC-PM with minimal side effects and potential immune activation.