**2. Molecular mechanism of nanoparticle-mediated tumor ablation**

CRYO proved to be significantly superior to RFA in patients with larger tumors (i.e., those that were 3.1 to 4 cm in diameter). The two methods were not significantly different in terms of complication rates, which were less than 4% in both groups, or survival (overall and tumorfree) at 1, 3, and 5 years [18]. The superiority of CRYO over RFA in the larger tumors suggests that CRYO has the ability to necrotize larger volumes of tissue, hence increasing the chances

Irreversible electroporation (IRE) is a new treatment method with certain advantages over the existing ablative techniques that have gained widespread attention. With IRE, cell death is induced with electric energy. Under image guidance electrodes are placed around the tumor and through multiple and short high-voltage electric pulses, the existing cell membrane potential is disturbed. As a consequence, nanoscale defects appear in the lipid bilayer of the cell membrane. Although IRE is believed to destroy all cells within the ablation zone effec‐ tively, the non-thermal nature of IRE results in relative preservation of the extracellular matrix. Hence, the structural integrity of vessels and bile ducts remain intact. Moreover, IRE is not affected by the heat-sink effect [19]. All these advantages suggest that IRE may be more suitable for the treatment of HCCs ineligible for surgical resections or thermal ablation because of

Currently, there are no published clinical trials for the treatment of hepatic tumors using IRE. In a recent review, Scheffer J. et al. included 221 patients with 325 lesions in different organs: 227 hepatic tumors, 70 unresectable pancreatic adenocarcinoma, 17 renal tumors, 8 pulmonary tumors, 1 presacral tumor, and 2 lymph nodes. Most of the patients were treated by IRE owing to tumor proximity to bile ducts, bronchi, renal pelvis, presacral neural plexus or large vessels, making the tumor unsuitable for surgery or thermal ablation. They concluded that IRE is a safe procedure with a promising early efficacy on smaller hepatic tumors near vascular structures and portal triads, with reported ablation success reaching 90%, but rapidly decreas‐

Tremendous efforts have been made in the last decades to improve the currently available techniques. However, given that there is not a single method available that meets all the requirements of an ideal ablation system, based on what has been discussed above and on data

Firstly, all differences between the techniques in terms of results are modest. Secondly, one technique may be more difficult than another and more rapid than another. Thirdly, each technique has its own major advantages and disadvantages. Finally, the rate of recurrence is still high after tumor ablation despite the major advances in tumor ablation devices, optic fibers, and improved imaging guidance. A major limitation in its overall effectiveness is due to the difficulties of heating large tumors. Small regions of viable tumor may still remain even after apparently good tumor ablation. Moreover, simple heating techniques have trouble discriminating between tumors and surrounding healthy tissues leading to many side effects. In order to overcome these major limitations, numerous groups are investigating the use of different types of nanoparticles, including carbon nanotubes, gold nanoparticles, and magnetic

nanoparticles, placed/ introduced within tumor tissues to facilitate localized heating.

from the vast literature available, we can reasonably draw some conclusions.

of ablating microsatellite lesions that are always possible with lesions of this size.

unfavorable location.

226 Recent Advances in Liver Diseases and Surgery

ing with increasing tumor size [20].

A better understanding of the molecular mechanism of nanoparticle mediated tumor ablation is of great importance in order to improve the current available ablation techniques and also to increase the synergies between specific drugs and tumor ablation. There are several ways in which nanoparticles (NPs) alone can affect biological processes.

Several studies have shown that NPs can increase the production of reactive oxygen species (ROS). Cancer cells are generally deficient in antioxidative enzymes present in normal cells, making them more vulnerable to an oxidative assault. Iron oxide nanoparticles via direct uptake in cancer cells result in acutely elevated intracellular iron concentrations and subse‐ quent ROS generation by Fenton reaction [21]. Moreover, silver nanoparticles have also been linked to ROS generation via a mechanism affecting calcium homeostasis. Silver ions can act on the same sites as calcium ions that could perturbate calcium influx in and out of the mitochondria. As a consequence, mitochondrial membrane damage results in ROS production, inhibition of ATP synthesis, and initiation of apoptotic signaling pathways [22].

From a biological and molecular point of view, NPs can affect different structures of the cancer cells. For instance, cellular uptake of NPs results in changes to the cytoskeleton and further affects many biological processes including cell spreading and adhesion, cell growth, viability, and ECM production [23]. Moreover, the accumulation of NPs in the cytoplasm may lead to physical interactions with the cytoskeleton, an increase in size and/or number of endosomes leading to the rearrangement of the cytoskeleton components in order to form new trafficking routes [24]. We consider that by altering the intracellular trafficking routes many other fundamental processes, including intracellular signaling pathways, different types of crosstalks with other cells and proliferation may also be affected. Furthermore, NPs can be engi‐ neered to accumulate preferentially in the nucleus of cancer cells. One study used gold nanoparticles (AuNPs) coated with polyethylene glycocol, bioconjugated with an argininegyicine-aspartic acid peptide and a nuclear localization signal peptide in order to transport the nanoparticles into the cancer cell nucleus. The results showed that nuclear targeting of AuNPs in cancer cells cause cytokinesis arrest, leading to the failure of complete cell division and thereby resulting in apoptosis [25].

In the past, cancer was considered an isolated self-sufficient ball of aberrant cells. However nowadays, tumors are viewed as "organs" composed of multiple and highly interactive cell types. Thus, the tumor is made up of primary cancer cells and of a court of stromal cells including mesenchymal derived cells, inflammatory cells, and vascular cells. Each of these cell types can be found in normal stroma, but in a tumorigenic setting, the cancer has appropriated, modified, and corrupted these cells to do its bidding [26]. NPs can also be used to target the tumor stroma changing the tumor microenvironment from its pro-tumorigenic state to an antitumorigenic state. One study demonstrated the ability of nanoparticles to target the tumor endothelium and improve the anti-tumoral efficacy of paclitaxel, both *in vivo* and *in vitro* [27]. Another approach would be to target the macrophage because they are inherently phagocytic and may uptake nanoparticles either within the tumor or in the circulation and subsequently migrate towards to the tumor. Another ability of macrophage is to store iron; hence, iron oxide NPs have been shown to induce cytotoxic effects on themselves and surrounding cells via ROSmediated activation of the c-jun N-terminal kinase pathway [28].

Understanding how nanomaterials affect live cell function, controlling such effects, and using them in therapy (for example In tumor ablation), is now the most challenging aspects of nanobiotechnology. An ideal NP would be a multifunctional one, targeting both the tumor cells and tumor microenvironment with low toxicity, which is easy to engineer, and has low costs. However, there is still a long way and a great deal of research has to be performed in order to develop what we consider the ideal nanoparticle.
