Mapping intratumoral coverage of neural stem cell–delivered anti-HER2 antibody in breast cancer brain metastasis

Cell-based therapies can formulate and deliver biologics within the brain, but their development has relied largely on survival outcomes that do not distinguish inadequate payload potency from inadequate tumor distribution. We evaluated intratumoral coverage as a quantitative endpoint for cell-mediated delivery using neural stem cells (NSCs) engineered to secrete an anti-HER2 antibody in breast cancer brain metastasis. Intracranial HER2-positive BT474Br tumors received a single dose of 5 × 10 5 anti-HER2 antibody-secreting NSCs. Secreted human IgG was reconstructed across consecutive 10-µm sections spanning the entire tumor depth, and therapeutic activity of the same cell dose administered once was assessed in an independent survival experiment. A single NSC dose produced a mean section-level antibody coverage of 60.6%; coverage was ≥ 25% in 96.8% of sections and ≥ 50% in 74.2%. Coverage was greatest in the tumor core and declined toward both poles, a pattern consistent with carrier-cell localization limiting therapeutic reach. The same dose extended median survival from 67 days with vector-control NSCs to 73.5 days ( P = 0.0009) and increased restricted mean survival by 10.8 days. These data provide an initial spatial benchmark for a therapeutically active dose and establish tumor coverage as a mechanistically informative endpoint for intracranial cell-based delivery.