The long-term goal of this proposed research is to lower recurrence and improve survival in breast cancer treatment. In early breast cancer, surgery can remove primary tumors, but undetected, residual cancer cells develop into life-threatening recurrence. The proposed research will develop and test novel photodynamic therapy (PDT) for killing cancer cells in the tumor margin and regional lymph nodes with minimal damage to normal tissues and the lymphatic system (high therapeutic index). Conventional PDT uses photosensitizers as drug that, when activated by light, produce free radicals, causing damages to cells. With this unique mechanism, PDT can be an effective modality to kill chemo-resistant cells and TNBC subtypes. However, current PDT is not a viable option for breast cancer treatment because the limited penetration of light in tissue prevents it from being effective at depths beyond several millimeters. This project explores an innovative solution to this problem in which bioluminescence (BL) of molecules locally injected into the tissue enables PDT at deep regions that are not accessible by external light illumination. This technique termed BL-PDT will be developed and valuated in state-of-the-art murine models, including patient-derived xenograft murine models and genetically induced spontaneous models of breast cancer, as well as various heterogeneous breast cancer cell lines in vitro. There are three specific aims: (1) Develop and test luciferase-photosensitizer conjugates for treating metastatic breast cancer. Non-toxic organic agents will be developed and tested. (2) Establish optimal therapeutic index for targeted BL-PDT for breast cancer. (3) Develop and validate adjuvant BL-PDT to treat residual disease in the breast and LNs to lower recurrence and increase survival. This project is expected to demonstrate the preclinical feasibility of BL-PDT as a novel, effective, minimally invasive, adjuvant therapy and will add to the existing arsenal of tools to combat breast cancer. The long-term impact of this project is high as it can reduce recurrence and improve prognosis. Besides invasive breast cancer, BL-PDT is expected to be applicable to ductal carcinoma in situ and other types of recurrent cancers. BL-PDT removes the limitations of near infrared agents necessary in PDT currently. Furthermore, biochemical activation by BL energy may have far-reaching impacts in light- based photomedicine beyond PDT.