# Enhancement of EPR Effect for Passive Tumor Targeting: Current Status and Future Perspectives
## 4 The Use of EPR in Passive Tumor Targeting with Nanomedicines
### Hardness Dependence
This surface modification increases plasma half-life and consequently enhances drug retention in circulation and tumor accumulation [110; 144]. The absorption of nanoparticles by proteins of the reticuloendothelial system depends on particle size, surface charge, and surface hydrophobicity. Particles around 100 nm in size with hydrophilic surfaces undergo relatively less opsonization and clearance, extending their circulation time in the blood [21]. Nanoparticles' Surface Charge Requirement  
The surface charge of nanoparticles plays a crucial role in their clearance and, consequently, their retention time within the body [72]. Specifically, because the luminal side of blood vessels, i.e., the surface of the vascular endothelium, carries carboxyl, phosphate, and sulfate groups that create a negative charge, positively charged drug molecules or cationic nanocarriers easily bind to the surface of endothelial cells. They are then removed from systemic circulation, reducing their plasma concentration (and the area under the curve, AUC) and thus their tumor accumulation [110; 143]. However, positively charged nanodrugs exhibit favorable binding to cellular membranes and can enter cells [143]. In contrast, negatively charged nanodrugs do not show this behavior, as they have an increased plasma half-life, but are quickly removed by the RES [110; 143]. Additionally, both positively and negatively charged nanoparticles have an affinity for binding to the extracellular matrix, thereby reducing their interstitial diffusion [72]. Therefore, considering all of the above, the ideal surface charge for nanomedicines is neutral or slightly negative [143]. Conjugation with Polymers--Example of Polyethylene Glycol (PEG)  
The surface characteristics of nanoparticles and their pharmaceutical complexes are a primary factor in determining their biocompatibility within the organism [148]. These characteristics can be modified by the surface functionalization of nanocarriers with polymers to avoid detection by the patient's immune system and prevent inactivation by the reticuloendothelial system (RES) [127; 143; 148]. This system consists of a group of 35 proteins that are inactive and found in the blood either dissolved or attached to the surface of blood cells. The activation of this system involves the recognition, opsonization, and removal of pathogens or foreign materials, significantly influencing the immune response to a material [148]. The importance of polymeric drug delivery systems for parenteral administration, which bypasses the digestive system, has grown exponentially. For polymers to be suitable carriers for drugs, they must meet several requirements. Specifically, they should have a neutral or slightly negative surface charge, possess appropriate functional groups for covalent bonding with the drug, and be compatible with it. They should also be inert, non-toxic, biocompatible, biodegradable, and have a molecular weight below the renal excretion threshold. Additionally, they should have a long circulation half-life to maintain therapeutic levels in the blood, appropriate biodegradation kinetics, mechanical properties, and ease of processing, as well as be abundantly available and cost-effective. Finally, they should be designed to encapsulate either hydrophilic or hydrophobic drug molecules and provide increased solubility for the latter [21; 143; 150; 124]. Proteins or nanoparticles conjugated with biocompatible polymers are referred to as \"stealth\" particles. Their main feature is that they are not recognized by the immune system and interact with the environment similarly to endothelial cells and plasma proteins, affecting the stability, distribution, efficacy, and toxicity of nanoparticles [143]. Synthetic biodegradable polymers are increasingly used for drug delivery because they are free from most of the problems associated with natural polymers (e.g., albumin, chitosan, heparin, etc.) [21].