Electrochemical Treatment of Tumours: Brief Considerations

Introduction

In electrochemical treatment (ECHT) or electrolytic ablation, a direct electric current streams through the tumour cellular and interstitial compartments, the latter including mostly a complex conglomerate of collagen, glycoproteins, proteoglycans and hyaluronic acid. Tissue destruction has been described via this procedure in an extensive range of solid tumours, with greater effectiveness detected in skin cancer, oral cavity and thyroid malignancies [1]. In tissue destruction, some contributory components look to be involved. But these respective characteristics in creating the anti-tumours effects have not been fully comprehended. The electricity power means fascination and its usage in oncology spans from the late 18th century [2,3]. A summary of the landmarks in the utilisation of different electrical usages has been illustrated in (Table 1). The ECHT usage Information could be discovered as far back in history as the mid- 19th century [4-12]. While it is tough to critical evaluation these reports a few attempts have been made to express the ECHT usage [1,13-15].

The electrochemical treatment in contemporary history began in 1959 when Humphrey and Seal described encouraging consequences with sarcoma tumours in mice [16]. After this several experiments were led in which animal tumour models and in vitro tests would serve as the base for the ECHT introduction in the clinical setting. It is suitable at this point to attend that, although recognized since the 19th century middle, Bjorn Nordenstrom, from Sweden, is considered to be a forerunner in tumour therapy with electric current and mixture treatments in patients [9,10]. In the late seventies, primary lung cancers were started to be treated via applying current between two platinum wire electrodes by Nordenstrom and, in his book of 1983 [9], he reported results from the treatment of 26 lung tumours in 20 patients. Relapse was achieved in 12 out of 26 tumours and regrowth symptoms were not determined after a 2–5-year follow-up period.

Following Nordenstrom’s works, Xin Yu-Ling and his group in China extended ECHT to the whole country (more than 1500 patients have been treated in the period 1990-2005) [17-19]. Initially, due to the low quality, the studies were not taken seriously in the Western world. A few years later, the procedure has been changed and many articles have been established by groups in Europe, the USA and Australia that utilize a mixed variety of the methods reported by the Chinese [20-23]. Moreover, in 1991, the effects of ECHT on tumours in mice were investigated by Miklavčič et al in Slovenia in a series of articles [24-27]. Especially, in 1992, Serša et al investigated the antitumour effectiveness of electrochemical treatment in combination with anticancer drugs like bleomycin [28]. Fifteen years later, Von Euler et al reported ECHT treatment effectiveness on cell proliferation and apoptosis in rat mammary cancer [29]. Nowadays, several groups are working in Australia, Cuba, Japan, Slovenia, Sweden and USA [30-40].

Table 1. Electrotherapeutic methods to tumour destruction.

Some of the ECHT benefits are its simplicity, effectiveness, inexpensive and few side effects. This treatment is especially shown for superficial, non-operable, or chemotherapy-resistant tumours. It has also been recommended that the ECHT would potentiate the antineoplastic effects of radio and chemotherapy and diminish their side effect [10,41]. The induced transmembrane potential in a cell subjected to an electric field leads to an increment in membrane permeability, so permitting specific molecules to be transferred into the cell [42]. This procedure is regularly named electroporation or electropermeabilization or irreversible electroporation (IRE) and has been broadly utilized in molecular biology and associated fields [37,38]. In comparison to thermodependent modalities for tissue ablation (e.g. radiofrequency ablation, microwave ablation, laser interstitial therapy, highintensity focused ultrasound and cryoablation), IRE which is a commonly utilized thermal-independent modality, destroys cancer cells by interrupting membrane integrity [43]. IRE exerts high electric potential microsecond pulses (up to 3000V) between two or more electrodes [38].

Whereas the tendency for heat to be produced scales with the voltage applied amplitude, IRE does not mechanistically depend on hyperthermia to lead to cell death. It is a widely held view that this cell dead instead moves from the induced transmembrane potential which irretrievably disrupts the lipid bilayer integrity; particularly, in order for cell death to happen, a potential of 1-2V across a cell membrane is needed [44,45]. This method’s specific advantage is that the extracellular matrix keeps mostly intact. The IRE primary defects are secondary side effects related to the high magnitude of the applied voltages. The transported pulses voltages have the potential to induce cardiac arrhythmias and muscle contractions, which make necessary the usage of general anaesthesia [38]. Moreover, precise electrode alignment is needed to ensure sufficient charge deposition and to alleviate thermal injury to nontarget tissues [46].

Despite relating electrochemotherapy (ECT) and gene electrotransfer procedures to IRE, they are distinguished via their usage of either fewer electrical pulses or lower voltage magnitudes, respectively. These modalities induce impermanent and cell membranes sublethal permeabilization that simplifies the cargo delivery to cells. Electrochemotherapy is utilized in cancer treatment to increase the uptake of chemotherapeutic agents. Gene electrotransfer is similar to electrochemotherapy, however, instead simplifies the transfer of a gene or genes to cells to enable the downstream production of a therapeutic protein [47]. Whereas these methods illustrate similar restrictions to those observed with IRE, their usefulness matters from their selectivity and wellcharacterized mechanisms of action. ECHT, also recognized as electrochemical treatment causes necrosis and apoptosis via the application of a direct current between multiple electrodes at relatively low electric potentials in comparison with IRE, sometimes lower than 50 V. Therefore, this method suggests unique benefits in comparison with other ablation procedures [31].

Particularly, electrolytic ablation may permit the development of exactly defined, shapeable ablation zones, that are uninfluenced via heat sink effects and could be checked and regulated in realtime by magnetic resonance imaging (MRI) [1,29,48]. Electrolytic ablation has been used for the therapy of various kinds of human malignancies; although, its progress has been restricted by uncertainty considering the underlying mechanism of induced cell death [31,49,50]. Whereas it has been well proved that electrolytic ablation induces electrolysis, as argued in the next section, a diversity of potential mechanisms have been recognized that may underlie the detected cell death. These comprise electric charge deposition, the making of a cellular transmembrane potential, toxic materials production, water extraction by electroosmosis, and alteration of microenvironmental pH [51-54]. Cell death mechanistic comprehending induced by electrolytic ablation has been needed to enable its additional progress for extensive clinical application.

Moreover, comprehending cellular alteration induced via electrolytic ablation will increase parameter choice for associated electrochemical treatments, comprising IRE, electrochemotherapy, and gene electrotransfer, that might include synergistic effects when compounded with electrolytic ablation [40,55]. Following this introductory review we briefly describe the mechanism by which the ECHT causes cell death, then we report the main destructive mechanisms, and finally, for a better understanding the fundamental electrolytic mechanisms involved, we describe the combined methodology proposed for studying ECHT in tumours, which are a powerful tool for viewing this complex problem while assuring that experimental and /or numerical artefacts are more easily detected.

ECHT Principle

When two or more electrodes have been injected inside of the tissue a series of electrochemical reactions have been occurring. When a practically insolvable electrode substance like platinum has been utilized, the main electrode reactions are the water splitting with materials oxidation and reduction solved in the tissue. At the anode the oxygen evolution, along with chlorine acidification and formation, happen:

Moreover, chlorine may react with water causing further acidification:

Regarding experimental results along with theoretical assessments, the hydrogen ions spreading in the tissue surrounding the anode are significantly larger than the chlorine spreading [56]. At the cathode, water is broken down into hydrogen and hydroxyl ions:

Considering the acidic and basic heamatin formation, the tissue around the anode and cathode converts to dark brown [57]. The acidic heamatin comprises methaemoglobin whereas the dark area around the cathodic lesion includes haemochromogens [58, 59]. Due to the spreading of The chlorine, a grey-coloured zone near the anode is created, which is noticeably smaller than the heamatin area [58].

The delivered charge (dose) has been usually described in Coulomb (C). Coulomb is the electrical charge unit equal to the charge amount delivered by a current of 1 ampere in 1 second (As). Diverse plans have been applied for therapies; some investigators have utilized constant voltage and a variable current, whereas others have remained the current constant to let the voltage vary instead. For both methods, the Coulomb dose could be detected. EChT treatment and electrolysis schematic have been depicted in (Figure 1).

Figure 1: A schematic on the principle for electrochemical treatment of tumours.

The Destructive Mechanisms

Many suggestions have been made on which should be the main cause of tissue destruction after electrochemical treatment. According to some scientists’ reports, the electric field has a significant impact on cell death or tumour tissue remodelling [60,61]. The electric field leads to interstitial water flux, and electro-osmosis, from the anode to the cathode, since the water molecules operate as a dipole. Therefore, the tissue around the anode dehydrates whereas oedema has been obtained around the cathode [9,61]. Charged materials, solved or suspended in tissue, transferred in the electric field and ions accumulation and charged tissue constituents have been acquired at specified and diverse zones in the electric field. The electric field affects the ion exchange across the cell membranes. Thus, the membrane potential has been changed and therewith the circumstances e.g. for many vital enzyme-regulated reactions [9,60].

Chlorine (CI) has been considered as being the most toxic agent by other scientists [59,62]. With little hesitation though, the extreme pH gradient that happens at ECHT has been explained by most articles. At the anode, as low pH as 1 has been evaluated [9,17]. Whereas at the cathode the prominent alkalisation produces pH levels as high as 13 [9,63]. Due to extreme pH amounts, the tissue proteins convert denaturated and the cell structure collapses and the cell finally dies [9,57,63]. The pH changes during ECHT have also been predicted by many theoretical computations. The hydrogen ions and molecular chlorine spreading around spherical and planar platinum anodes that have been investigated by Berendson et al. [56,64]. The formation and spreading of potential toxic species around both the anode and cathode, rather with a spherical electrode configuration have been studied by Nilsson et al. [54,65,66].

The above are true in an environment where the therapy electrodes are inert. If the electrode has been made from a soluble substance like copper (Cu), rhodium (Rh) or brass (Zn-Cu alloy) the electrolysis leads to metal ions solving in the tissue [67]. Therefore, the metal ions could include toxic capacity by themselves. Miklavčič, Serša et al, have illustrated both the pH change importance in tissue during ECHT along with the metal ions formation during electrolysis when utilizing other materials than platinum (Pt) [26]. In another research, they also revealed that the intralesional temperature change is marginal and hence, most likely, does not influence cell survival [27].

To sum up, the results described above demonstrate that ECHT of cancer results from the introduction of toxic acid and base that modulates the tumour microenvironment primarily involving a pH-dependent mechanism that mitigates the limitations of leading ablation modalities. Studies in progress demonstrate that cancer cells placed in an environment at a pH below 4.8 or above 10.6 undergo cell death. These unique insights will be essential for leveraging cancer cell susceptibility to altered microenvironments as well as furthering the development of electrolytic ablation for clinical application.

In Vivo, In Vitro and In Silico Modeling

Animal models have an important role in the investigation of human cancer. One of the major causes is that both have almost the same genes complement and signal pathways (tumours in mice and humans mutate in the same genes class which is an index of similar mechanisms leading tumour growth). Actually, the article reveals that for better preclinical modelling, mice are appropriate [68]. In the human tumour invitro EChT modeling has been utilized a gel of collagen I expose to an electric field where it has been supposed that the gel includes physicochemical and hydrodynamic properties near to those found in the solid tumour interstitium. Collagen usage is according to the fact that it comprises more than 70% of the tumour interstitium. Furthermore, the previous document indicates that collagen type I gels might be a good initial simplified model of the transport of species in the tumour extracellular matrix [69,70]. a gel of agar has been also used because it is more appropriate for optical studies and more chemically resistant.

In silico cancer modeling is an effective tool that could supply more vision into the mechanisms that control tumour evolution and growth. It points both mathematical modeling and numerical simulation. Generally, it comprises in a system of reaction-transport differential equations in a fixed or moving domain (Stefan-like problem) explaining physicochemical conservation laws whose solution has been achieved by numerical techniques. Presently, the Preziosi book could be accessed as an effective area of research and a review [71]. Jain et al did revolutionary work in the subject [72-75]. Nilsson et al in a series of articles developed EChT in silico modeling [53,54,65,66]. They described ion transport in an area close to one of the electrodes (cathode or anode) via a quasi-onedimensional model utilizing the Nernst-Planck equations for ion transport under the electroneutrality hypothesis. Considering their initial model, the tissue matrix was approximated to a saline (NaCl) solution with a specified buffer volume and organic content.

Regarding consecutive refinements, the model was extended to comprise the bicarbonate buffer effects on anodic hydrogen ions and the transfer and reaction of chlorine and chlorinated species. The consequences were compared with experimental information from in vivo rat normal tissue giving a good explanation of the n’ profile near the anode after ECHT. In their final model [54] ion transfer close to the cathode in which they reveal their simulated pH profiles to be strongly associated with the size of experimentally calculated lesions has been investigated by them, so approving that the spreading of hydroxyl ions detected the lesion size around the cathode. They also recommended that the model could be utilized for anticipating the size of the lesion produced by ECHT..

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