**6.1 Renal elimination**

The kidneys have the potential for rapid removal of particles from the vascular system without the need for decomposition. Renal elimination involves the mechanisms of glomerular filtration and tubular secretion to end in urinary excretion [120]. The nanoparticles reach the nephrons through the afferent arteriole, where they meet three endothelial barriers: the fenestrated endothelium; the highly negatively charged glomerular basement membrane; and the podocyte extensions of glomerular epithelial cells. The fenestrated epithelium has pores with a functional physiological diameter of between 9 and 10 nm, and a few (ca. 1%) pores of 15 to 23 nm in diameter [121], which means that nanoparticles with diameters less

than 10 nm can spread freely regardless of the charge of the particle. The second barrier presented by the glomerular basement membrane filters particles between 6 and 8 nm depending on the electrostatic interactions between the nanoparticle and the membrane [122]. In this way, low-charged or positively charged nanoparticles can diffuse more freely. After glomerular filtration, the nanoparticles enter the lumen of Bowman's capsule, where they can be reabsorbed. Because the proximal tube epithelium is negatively charged, positively charged nanoparticles can be more easily reabsorbed.

Choi et al. [123] administered quantum-dots (inorganic nanoparticles) intravenously to rodents to study their renal elimination. The results indicated that particles with a hydrodynamic diameter less than 5.5 nm present rapid elimination and the increase in this diameter is inversely proportional to the retention time of the quantum-dots in the body.

#### **6.2 Hepatic clearance and the reticulum endothelial system**

Those nanoparticles that are too large to be excreted by the renal system must be eliminated by the hepatobiliary system. In 1924, Karl Albert Ludwig Aschoff coined the term reticuloendothelial system (RES) to describe a functional cellular system widely distributed in the body, composed of sessile and circulating macrophages of mesenchymal origin. These cells have a marked phagocytic capacity towards particulate matter. Macrophages stored in the RES can be found in the central nervous system (microglia), in the spleen, lymph nodes, tonsils, in the bone marrow (reticular cells) and, particularly, in the liver (90% of all macrophages) [124]. The exogenous structures are subjected to very intensive phagocytosis by the RES as well as the foreign proteins of higher molecular weight. Total blood flow must pass through the liver, making it a central organ to monitor the blood for endogenous, foreign substances and particles that must be removed for physiological reasons. In order to perform their functions, RES cells have special abilities such as: phagocytosis, pinocytosis, the release of signaling substances (cytokines, eicosanoids) and elimination of endotoxins, among others [124]. In addition, these are equipped with numerous pores of various diameters, depending on their different functions, which gives them the ability to filter larger molecules and particles, keeping them away from the liver parenchyma. The Kupffer cells and the endothelial sinus are in a privileged position to engulf any colloid foreign to the body. For this purpose, Kupffer cells are equipped with a branched and ciliated surface that act as capture mechanics. Besides, they possess specific receptors for carbohydrate components, as well as for the Fc region of IgG and for complement C3, allowing them to differentiate the opsonized matter. They also possess lysosomal enzymes, although in much lower amounts than sinus endothelial cells.

In a very complete study, Poon et al. [125] proposed an algorithm to infer how nanoparticles can be eliminated in vivo (**Figure 4**). Most of the nanoparticles with diameters smaller than the glomerular filtration size limit (∼5.5 nm) are eliminated by the kidneys and leave the body through the urine [123] although fecal elimination of small nanoparticles is also observed [125]. Biodegradable nanocarriers or nanoparticles larger than 5.5 nm can be decomposed [126, 127] or metabolized [128] and can be returned to the systemic circulation. Most non-biodegradable nanoparticles larger than 5.5 nm are retained long-term in Kupffer cells [129]. If the nanoparticles can evade Kupffer cells or if Kupffer cells are incapacitated, the nanoparticles can undergo hepatobiliary clearance. Similar to the glomerular filtration size limit, the authors proposed that there is a filtration size limit in hepatic sinusoidal endothelium. Nanoparticles larger than the fenestra of sinusoidal endothelium in the liver have restricted access to hepatocytes, whereas nanoparticles smaller than the

#### **Figure 4.**

*Flow diagram for removal of nanoparticles in vivo. Reprinted with permission from reference [125]. Copyright 2019 American Chemical Society.*

fenestrae have better access through the fenestra to enter the perisinusoidal space. In general, nanoparticles must escape these barriers established by non-parenchymal cells in the liver before they have the potential to enter the perisinusoidal space and interact with hepatocytes for elimination. Once the nanoparticles successfully interact with them, they can transit to enter the bile ducts. Finally, the nanoparticles enter the intestine and are eliminated from the body through the feces.

#### **7. Conclusions**

The academy and industry have made extraordinary advances in a wide variety of areas due to the development of nanotechnology and the control of structures at the nanoscopic levels. Particularly in the field of medicine, nanotechnology has the potential to generate a significant impact on human health, being able to improve the diagnosis, prevention and treatment of diseases. In this field, nanotechnology seeks to encapsulate drugs and/or tracer compounds in nanoparticles to increase their efficiency by allowing direct delivery to target tissues, while they reduce their toxicity avoiding accumulation and the consequent side effects in healthy tissues. The encapsulation of drugs also allows their controlled release, thus avoiding maximum levels of highly harmful or subtherapeutic concentrations. Moreover, nanoparticles are of great value in the transport of drugs with low solubility in water, which turns out to be the major problem when introducing new drugs to the market because it limits their bioavailability in the body. A wide variety of materials can be used for the preparation of nanoparticles depending on the intended function of the system.
