An organolutetium nanosensitizer synergizes with PARP inhibition to unleash STING-mediated immunity for low-dose radioimmunotherapy.

The efficacy of radiotherapy in triple-negative breast cancer (TNBC) is often limited by an immunosuppressive tumor microenvironment (TME), requiring high radiation doses that cause systemic toxicity. There is a critical need for theranostic strategies capable of guiding therapy and amplifying the efficacy of low-dose radiation. We developed a multifunctional organolutetium nanosensitizer (LSPA) for image-guided, low-dose radioimmunotherapy. Lutetium (Lu) serves as both a contrast agent for CT imaging and a radiosensitizer through the generation of reactive oxygen species (ROS). The LSPA nanoparticles were engineered to selectively accumulate in tumors and release their therapeutic payload in response to the acidic TME. At a low 6 Gy X-ray dose, LSPA synergized with the PARP inhibitor Olaparib to induce extensive DNA damage. This activated the cGAS-STING pathway and remodeled the TME. The treatment promoted immunogenic cell death, dendritic cell maturation, and M1 macrophage repolarization. It also decreased regulatory T cells, leading to increased CD4 and CD8 T cell infiltration in both primary and metastatic tumors. This theranostic strategy suppressed primary and distant (abscopal) tumors, prevented recurrence, and established durable immune memory with low-dose irradiation. Our findings present a clinically translatable approach that combines a nanosensitizer with PARP inhibition to turn immunologically "cold" tumors into "hot" ones, thereby enhancing the efficacy of low-dose radioimmunotherapy while limiting systemic toxicity.