Theranostics 2014; 4(1):81-89. doi:10.7150/thno.7193 This issue
Molecular Imaging Program, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892, United States.
Nano-sized therapeutic agents have several advantages over low molecular weight agents such as a larger loading capacity, the ability to protect the payload until delivery, more specific targeting due to multivalency and the opportunity for controlled/sustained release. However, the delivery of nano-sized agents into cancer tissue is problematic because it mostly relies on the enhanced permeability and retention (EPR) effect that depends on the leaky nature of the tumor vasculature and the prolonged circulation of nano-sized agents, allowing slow but uneven accumulation in the tumor bed. Delivery of nano-sized agents is dependent on several factors that influence the EPR effect; 1. Regional blood flow to the tumor, 2. Permeability of the tumor vasculature, 3. Structural barriers imposed by perivascular tumor cells and extracellular matrix, 4. Intratumoral pressure. In this review, these factors will be described and methods to enhance nano-agent delivery will be reviewed.
Keywords: Cancer, Nano-delivery, Tumor physiology, Enhanced permeability and retention effects.