Nanotherapy Targeting the Tumor Microenvironment.

Cancer is characterized by high mortality and low curability. Recent studies have shown that the mechanism of tumor resistance involves not only endogenous changes to tumor cells, but also to the tumor microenvironment (TME), which provides the necessary conditions for the growth, invasion, and metastasis of cancer cells, akin to Stephen Paget's hypothesis of "seed and soil." Hence, the TME is a significant target for cancer therapy via nanoparticles, which can carry different kinds of drugs targeting different types or stages of tumors. The key step of nanotherapy is the achievement of accurate active or passive targeting to trigger drugs precisely at tumor cells, with less toxicity and fewer side effects. With deepened understanding of the tumor microenvironment and rapid development of the nanomaterial industry, the mechanisms of nanotherapy could be individualized according to the specific TME characteristics, including low pH, cancer-associated fibroblasts (CAFs), and increased expression of metalloproteinase. However, some abnormal features of the TME limit drugs from reaching all tumor cells in lethal concentrations, and the characteristics of tumors vary in numerous ways, resulting in great challenges for the clinical application of nanotherapy. In this review, we discuss the essential role of the tumor microenvironment in the genesis and development of tumors, as well as the measures required to improve the therapeutic effects of tumor microenvironment-targeting nanoparticles and ways to reduce damage to normal tissue.