# Enhancement of EPR Effect for Passive Tumor Targeting: Current Status and Future Perspectives
## 4 The Use of EPR in Passive Tumor Targeting with Nanomedicines
### Biological Barriers Encountered by Nanoparticles
#### 4.2.1 Nanocarrier Specifications
However, the toxicity of metallic nanoparticles remains a serious concern due to the generated reactive oxygen species and their effects on cellular structure [132; 134; 139; 140; 141]. Finally, hybrid nanoparticles combine the properties of both organic and inorganic nanoparticles, offering improved biocompatibility and stability [142; 134]. Almost all nanoparticle delivery systems approved by the FDA or currently in clinical trials are based on liposomes or polymers [21]. The nanoparticles used also present defined physicochemical properties, including size, shape, and surface chemistry, which are further analyzed below [21; 123]. Dependence of the Phenomenon on Nanoparticles' Size and Shape  
Nanoparticles intended for drug delivery are manufactured by various methods, which determine their size and shape, endowing them with unique properties [21]. The size or molecular weight of the macromolecule of interest is a key parameter that determines the effectiveness of the EPR effect for the targeted delivery of anticancer drugs to solid tumors [8; 126; 127]. The EPR effect depends on the size of the molecules, as it influences various biological phenomena such as circulation half-life, the ability to pass through the endothelial gaps of blood vessels, diffusion, retention within tumor tissues, and uptake by macrophages [5; 20]. Molecules with sizes ranging from 10 to 200 nm or 40 to 800 kDa in mass, such as nanoparticles and plasma proteins, exhibit a strong EPR effect, as they can permeate the tumor's blood vessels [101; 126; 127]. Specifically, nanoparticles smaller than 70 nm tend to accumulate in the tumor if it is highly permeable due to the endothelial pores, which are about 0.1-3 \(\upmu\)m in size [72; 101]. For effective drug accumulation using the EPR effect, long-circulating nanoparticles must not only circulate but also be capable of extravasation from the blood vessels to be transported into the tumor's interstitial tissue [110; 125]. Therefore, they should exhibit a hydrodynamic diameter that exceeds the renal clearance threshold, considering the size of the renal glomerular pores, approximately 6 nm [110; 125]. Nanoparticles, liposomes, and macromolecular drugs larger than 6-8 nm or 40 kDa, which is also the lower limit of renal clearance, penetrate the vascular system and accumulate in tumor tissues [110]. They tend to diffuse in and out of the blood vessels, exhibiting prolonged circulation time, i.e., half-life (t\({}_{1/2}\)), as well as very slow clearance from the body, resulting in selective penetration and retention in tumor tissues and high AUC (a pharmacokinetic indicator that demonstrates drug absorption and retention in the body) [101; 126; 143; 104; 101]. However, low-molecular-weight molecules do not exhibit significant accumulation due to their removal through renal excretion [126; 143; 110]. Moreover, these molecules easily pass through the normal vascular system into healthy tissues and organs, causing unwanted effects in these areas [110]. Furthermore, smaller drugs, up to 1-2 \(\upmu\)m, exhibit easy extrusion from the IFP, while very large molecules have low tissue penetration, making the size of drugs critical [101; 110; 144]. Typically, nanodrugs used have a diameter ranging from 10 to 100 nm, providing satisfactory accumulation through the EPR effect [143]. Another study observed that the optimal delivery efficiency using the EPR effect is with nanoparticles sized 100-200 nm [110]. The impact of nanoparticle shape on their effective delivery and accumulation within solid tumors is not fully defined, as it is influenced by the material properties of the nanoparticles and the type of cancer [5]. However, shape plays a very important role in circulation time in the blood, the ability to remain in blood vessels, and absorption by cancer cells [127]. Dendrimers exhibit a three-dimensional structure with a highly branched architecture, offering high drug loading capacity, and can be used for the delivery of genes, drugs, and anticancer agents.