Nanocarriers to Solid Tumors: Considerations on Tumor Penetration and Exposure of Tumor Cells to Therapeutic Agents

Solid tumors constitute the majority of diagnosed cancers. For effective killing, therapeutic agents should ideally be delivered uniformly and at lethal doses to all cancer cells comprising the tumors, while keeping normal organ toxicities to a minimum. This requirement sets two of the major challenges in drug delivery to solid cancers: uniformity in delivery, and delivery of at least a minimum amount of therapeutics per cancer cell. Herein we review various approaches that aim to improve the penetration and content release of delivered therapeutic agents from nanocarriers of selfassembling nature. Biophysical characteristics of solid tumors are briefly discussed to motivate and rationalize the design of reported nanoparticle structures. This review does not aim to be exhaustive of the various designs and strategies, but to mostly give a flavor of the general current directions aiming to address these challenges.