2. of advantages over conventional low molecular weight agents including a large loading capacity, the ability to protect the payload from degradation, specific targeting and controlled or DGKD sustained release.1-3These features can be enhanced by changing characteristics such as size, the nature of the payload and surface features.4,5A variety of nano-sized, diagnostic and therapeutic agents have recently been synthesized for possible clinical application.6-9Nano-drugs are particularly relevant to cancer because tumors often possess a leaky vasculature compared with healthy vessels in normal organs.10When administered intravenously, nano-sized brokers tend to circulate for longer occasions, if they are not small enough to be excreted by the kidney or large enough to be rapidly acknowledged and trapped by the reticuloendothelial system (RES).11Therefore, nano-sized agents with long circulation times leak preferentially into tumor tissue through a leaky tumor vasculature and are then retained in the tumor bed due to reduced lymphatic drainage. This process is known as the enhanced permeability and retention (EPR) effect.12Most nano-sized brokers accumulate within tumors due to the EPR effect and then release their therapeutic payloads. However, EPR effects provide relatively modest specificity offering 20-30% increases in delivery compared with critical normal organs. Nano-sized cancer drugs have shown CHMFL-ABL/KIT-155 efficacy in animal models of malignancy and several brokers are in testing in clinical trials.13,14However, response rates vary, likely related to the broad heterogeneity of EPR effects observed among tumor types and within individual tumors. The aggregate EPR effect is dependent on factors,15,16in which tumor-specific biological features are considered to affect the heterogeneity, including 1. The degree of angiogenesis and lymphangiogenesis. 2. The degree of perivascular tumor growth adjacent to the vasculature and the density of the stromal response. 3. Intratumoral pressure. By manipulating these conditions, EPR effects can be enhanced leading to superior nano-sized drug delivery, thereby enhancing their anti-cancer effects. In this review, we first overview the basis of nano-sized drug delivery into cancer tissue, and then, discuss non-selective and selective molecular targeting methods for further improving the permeability and retention of nano-sized brokers in cancer tissue compared with intrinsic EPR effects. == 2. Physiology in tumor tissue == In normal tissues, low molecular weight agents enter a mature, organized hierarchical vascular network, beginning with arteries, continuing to arterioles and ending in capillaries, whereupon the brokers leak from the vasculature and disperse homogenously within the tissue according to a concentration gradient. In solid tumors several factors inhibit the homogenous distribution of low molecular weight agents, particularly within deep and central parts of the tumor. In contrast to low molecular weight agents, CHMFL-ABL/KIT-155 nano-sized brokers have a number of advantages in this setting. In this section, we will describe the physiological characteristics of tumor tissues which present barriers to drug delivery, especially for non-targeted low molecular weight molecules (Physique1). == Fig 1. == Physiological characteristics of tumor tissue and vasculatures that can facilitate or prevent cancer drug delivery. == 2.1. Vascular structures == In order to grow, tumor cells recruit a CHMFL-ABL/KIT-155 neovasculature to ensure an adequate supply of nutrients and oxygen. As tumors grow they recruit new vessels or engulf existing blood vessels. The imbalance of pro- and anti-angiogenic signaling within different parts of tumors creates an abnormal vascular network that is characterized by dilated, tortuous, and saccular channels with haphazard patterns of interconnection and branching.17-19Unlike the microvasculature of normal tissue, which has an organized and regular branching order, tumor microvasculature shows disorganization and lack of the conventional hierarchy of blood vessels.20Arterioles, capillaries, and venules are not identifiable as such and instead, vessels are enlarged and are often interconnected by bidirectional shunts.21One physiological consequence of these vascular abnormalities is heterogeneity of tumor blood flow,22which interferes with the homogeneous distribution of a drug within the tumor. In addition to vascular.