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Poly (ADP-ribose) polymerase inhibitors (PARPi) have revolutionized the treatment landscape for patients suffering from BRCA1-mutated breast and ovarian cancers. However, responses are not durable. We demonstrate that treatment with PARPi, niraparib, increases programmed death-ligand 1 (PD-L1) expression in BRCA1-deficient cancer cells, contributing to immune evasion. To circumvent this, we developed P-selectin-targeted poly (lactic-co-glycolic) acid (PLGA)-poly (ethylene glycol) (PEG)-based nanoparticles (NPs) encapsulating PARP and PD-L1 inhibitors at a synergistic ratio. To further enhance tumor targeting, we leveraged radiation-induced P-selectin upregulation in BRCA1-deficient cancer cells and their associated angiogenic endothelial cells, improving NP accumulation in the primary tumors and hard-to-target metastatic sites, including brain metastasis. Using a combination of traditional 2-dimensional (2D) cell cultures, advanced 3-dimensional (3D) spheroids, tumor-on-a-chip platforms, and in vivo models, we demonstrate the enhanced accumulation and efficacy of the radiation-guided P-selectin-targeted NPs in primary and brain-metastatic BRCA1-deficient tumors.