Beyond the EPR effect: Intravital microscopy analysis of nanoparticle drug delivery to tumors

Delivery of nanoparticles (NPs) to solid tumors has long relied on enhanced permeability and retention (EPR) effect, involving permeation of NPs through a leaky vasculature with prolonged retention by reduced lymphatic drainage in tumor. Recent research studies and clinical data challenge EPR concept, revealing alternative pathways and approaches of NP delivery. The area was significantly impacted by the implementation of intravital optical microscopy, unraveling delivery mechanisms at cellular level in vivo. This review presents analysis of the reasons for EPR heterogeneity in tumors and describes non-EPR based concepts for drug delivery, which can supplement the current paradigm. One of the approaches is targeting tumor endothelium by NPs with subsequent intravascular drug release and gradient-driven drug transport to tumor interstitium. Others exploit various immune cells for tumor infiltration and breaking endothelial barriers. Finally, we discuss the involvement of active transcytosis through endothelial cells in NP delivery. This review aims to inspire further understanding of the process of NP extravasation in tumors and provide insights for developing next-generation nanomedicines with improved delivery.