Engineering Nanoparticles to Navigate Biological Barriers for Deep Tumor Penetration.

Deep and spatially uniform drug delivery remains a major challenge in nanomedicine for solid tumor therapy. During delivery, nanoparticles must sequentially traverse multiple biological barriers, from systemic circulation and tumor accumulation to vascular extravasation, basement-membrane crossing, interstitial penetration, and cellular internalization. In this review, we distinguish the sheet-like perivascular basement membrane from the fibrillar, hydrated interstitial extracellular matrix and discuss how intertumoral, intratumoral, and stage-specific heterogeneity alters the dominant transport barriers across different tumor contexts. We then summarize three complementary strategies for engineering nanoparticles to improve deep tumor penetration: 1) Nanocarrier design, which tunes particle shape, size, surface charge, mechanical stiffness, and ligand presentation to balance cellular interactions and interstitial mobility; 2) Vascular and Stromal Modulation, which alleviates transport barriers through vascular normalization, transient basement-membrane modulation, controlled extracellular-matrix remodeling, and regulation of matrix production by cancer-associated fibroblasts; 3) Active and Assisted Transport, which promotes trans-barrier delivery through transcytosis, self-propelled nanomotors, and externally applied physical energy. We discuss the associated challenges and optimization strategies, particularly those related to spatial penetration, tumor accumulation, model-dependent efficacy, and safety. Finally, we highlight the translational limitations and future opportunities of tumor-penetrating nanomedicines from a clinical perspective.