Though chemotherapy plays an important role in cancer treatment, there are still many problems during clinical application. resistance in tumor involves multiple mechanisms, which mainly includes lowered extracellular pH, hypoxic region, and irregular vasculature in physiological level, and the alteration of apoptotic machineries, over-expression of efflux transporters, and enhanced repair mechanism of drug induced DNA damage in cellular level. With the increasing Betonicine role of tumor microenvironment in multidrug resistance, cell proliferation and metastasis, this review will focus on the characteristics of tumor microenvironment and their targeting mechanisms with PEG-based amphiphilic nanoparticles to overcome chemoresistance. == 1 . Introduction == Cancer is among the most serious diseases faced by the humanity. Following cardiovascular diseases, malignant tumor occupies the second place for the cause of Betonicine death. According to the statistical data from the American Cancer Society, 1658370 cancer cases have been registered and 589430 cancer deaths are projected to occur in 2015 only in the United States1. In the past few decades, with the progress of medical technologies various treatments for cancer have been developed, which include surgery, chemotherapy, radiation therapy, immunotherapy, hormone therapy, targeted therapy et al (Table 1). Depending on different conditions of cancers, each type of treatment can be given alone or associated with other forms of cancer therapy. == Table 1 . == Advantages and disadvantages of different types of cancer treatment. Among the various treatments, chemotherapy is one of the most common types of cancer treatment. Compared to other therapies, chemotherapy with the advantages of easy operation, good patients compliance Rabbit Polyclonal to ATG16L2 and better treatment effects, remains the first-line treatment of choice for clinic2, 3. Though chemotherapy plays an important role in cancer treatment, there are still many problems during clinical application. The major drawback is side effect as chemotherapeutic agents would kill cells in both healthy and tumor tissue46. Whats more, the surrounding environment of tumors is another obstacle for chemotherapeutic efficiency. Some special pathophysiology changes such as growth-induced solid stress, abnormal blood vessel networks, elevated interstitial fluid pressure and dense interstitial structure can form the transport barriers that limit the rate and extent of the chemotherapeutic drugs delivery to Betonicine both primary and metastatic tumors713. In addition , the other limitation of chemotherapy is the multi-drug resistance. Tumor cells can develop drug resistance which leads Betonicine to reduced or complete absence of antitumor effect14. Drug resistance mechanisms could appear at the tumor level such as low pH or high interstitial pressure, and at the cellular level including certain overexpressed enzyme systems, increased drug efflux and reduced uptake1517. It is worth mentioning that the tumor microenvironment also play an important role on the multi-drug resistance. All of those have limited the effect of chemotherapy treatment. In order to solve these issues in chemotherapy, various approaches had been applied. One of the most promising methods is applying nanomedicines for delivering anticancer drugs to their action site. Nanomedicine is nanoscale complex system which is fabricated from different materials and applicable to various drugs, proteins, nucleic acids. They comply with the requirements of stability, safety, biocompatibility and biodegradability18. Application of nanomedicine will be able to overcome many problems such as poor water solubility of therapeutic agents, short blood circulation time, non-specific distribution, toxicity, tumor resistance (both at the tissue and cellular levels), low therapeutic index, which not only help to decrease systemic toxicity of chemotherapy but also enhance the efficacy of anticancer drugs. During the last few decades many types of nanomedicine for cancer treatment have been fabricated19. Among them amphiphilic polymers is one of the most promising nanomaterials with desirable properties, which can self-assemble above critical micellar concentration (CMC) and form micelles with an average size 20100nm18. Structural units of the micelles are hydrophilic and hydrophobic moieties. The hydrophilic part is usually presented by polyethylene glycol (PEG) which forms the hydrophilic outer layer (corona) that can enhance the circulation time by evading plasma protein adsorption20. The hydrophobic part forms the core of the system capability to incorporate the anticancer substances that solve water-insolubility issue21. Micelles are dynamic systems and undergo rapid modification when compared to solid stable structures. In addition , amphiphilic polymers represent better kinetic and thermodynamic properties with high biocompatibility and degradation rate18, 21. In this review, we will focus on the PEG-based amphiphilic nano structures which were developed to target tumor microenvironment and overcome multi drug resistance. == 2 . Use PEG-based amphiphilic nanostructure.