Irreversible Electroporation

The objective of this chapter is to present a historical review of the field of irreversible electroporation (IRE) in the context of its medical applications. Although relevant scientific observations were made since the 18th century, the electroporation phenomenon was not identified as an increase of membrane permeability until mid 20th century. After that, multiple applications of reversible electroporation emerged in vitro (DNA electrotransfer) and in vivo (electrogenetherapy and electrochemotherapy). Irreversible electroporation was tested commercially in the 60s as a bactericidal method for liquids and foods but its use in the context of medical applications was not studied until the early 2000s as an ablative method. The cell destruction mechanism of IRE is not based on thermal damage and this fact provides to IRE an important advantage over other physical ablation methods: the extracellular scaffolding, including the vessels, is preserved. Several surgical applications are now under study or even under clinical trial: ablation of hepatocarcinomas, ablation of prostate tumors, treatment of atrial fibrillation and treatment of vascular occurrences such as restenosis and atherosclerotic processes.

Electroporation is the phenomenon in which cell membrane permeability to ions and macromolecules is increased by exposing the cell to short (microsecond to millisecond) high electric field pulses. In living tissues, such permeabilization boost can be used in order to enhance the penetration of drugs (electrochemotherapy) or DNA plasmids (electrogenetherapy) or to destroy undesirable cells (irreversible electroporation). The main purpose of the present chapter is to provide an overview of the electrical concepts related to electroporation for those not familiar with electromagnetism. It is explained that electroporation is a dynamic phenomenon that depends on the local transmembrane voltage and it is shown how a voltage difference applied though a pair of electrodes generates an electric field which in turn induces the required transmembrane voltage for electroporation to occur. Quite exhaustive details are given on how electroporation changes the passive electrical properties of living tissues. Furthermore, some remarks are given about the effects of electroporation on other bioelectric phenomena such as cardiac arrhythmias.

In the following chapter we wish to explore the fundamental physical properties of electroporation that have emerged from experiments conducted with cells. Since, up to date, a uniform comprehensive theory that explains the mechanism that underlies the electroporation phenomena has not yet been proposed, it is interesting to look into the experimental facts that such a theory needs to encompass.

In the last 20 years, electroporation studies have focused primarily on reversible electroporation because of its importance for drug and gene delivery. Irreversible electroporation (IRE) was mostly studied in the context of the delayed tissue damage in high-voltage accidents [70, 69], the postshock arrhythmias during defibrillation [51, 50], and biofouling control [46, 95]. Very recently, IRE has shown great promise as a nonthermal technique for the ablation of tumors and arrhythmogenic regions in the heart [20, 27, 91, 85, 4, 68]. IRE’s ability to create a complete and predictable cell ablation with a sharp transition between normal and necrotic tissue, while sparing neighboring blood vessels, connective tissue, and nerves, has great advantages in a variety of medical applications.

Irreversible electroporation (IRE) is a promising new technique for the ablation of tumors and arrhythmogenic regions in the heart (Davalos, Mir et al. 2005; Edd, Horowitz et al. 2006; Al-Sakere, Andre et al. 2007; Edd and Davalos 2007; Onik, Mikus et al. 2007; Rubinsky 2007). One of its primary advantages over other ablation techniques lies in its mechanism to kill undesirable cells by affecting the cell membrane without thermally damaging major blood vessels, connective tissue, nerves and the surrounding tissue. IRE’s ability to create complete and predictable tissue ablation with sharp transition between normal and necrotic tissue will have great advantages in a variety of medical applications (Rubinsky 2007).

In the past, irreversible electroporation of tissue was studied for two applications: a) in the food industry for food processing and b) as a means to determine the upper limit of electrical parameters for reversible electroporation of tissue. Non-thermal irreversible electroporation (NTIRE) is a new modality for tissue ablation that applies the irreversible electroporation pulses in such a way as to avoid thermal damage to tissue components while irreversible affecting the cells. This particular aspect of irreversible electroporation was not studied before. This chapter reviews experimental studies on non-thermal irreversible electroporation in tissue done by our group. The studies will be discussed in the chronological order in which they were done. In all the studies discussed in this chapter, treatment planning was done prior to performing the experiments. Treatment planning is done to identify the appropriate sequence of electrical pulses, which produce the desired cell ablation without causing damage to the remaining tissue structure, [1]. It is not trivial as the range of parameters used can vary from tissue type to tissue type and from circumstance to circumstance. In the absence of other information, in these earlier studies modeling the electrical field and the temperature distribution caused by Joule heating during the application of the electrical pulses identified the treatment planning. It is possible that in the future it will be also valuable to model that change in pH in the tissue, as they could also affect the tissue components, e.g. [2]. It should be emphasized that when thermal or chemical damage do occur during irreversible electroporation, the outcome of the procedure will most likely be different from those described in this chapter.

As a simple definition, a mathematical model is a representation of chosen essential aspects of a real system (may it be a living, engineering or social system), described by a set of variables and a set of equations that establish relationships between the variables. The aim of such a model is to gain knowledge about the represented system, explaining some of its phenomena, or help designing it. Mathematical modeling is a vast scientific and engineering field and is therefore not possible to explain in detail in one single book, let alone in one paper. It is used in countless scientific, engineering and even social studies and represents an important field in the study of the effects of the electromagnetic fields and accompanying coupled phenomena on cells, tissues and organs (Fear and Stuchly, 1998; Debruin and Krassowska, 1999; Debruin and Krassowska, 1999a).

When you overhear someone mention treatment planning, you can bet the conversation is about one of the forms of radiation therapy (RT). In the 1960s, when high powered x-ray delivery systems were readily available to radiation oncologists, the biggest challenge remained to improve the accuracy in locating the tumor and in directing the beams of charged particles. This changed when the first computed tomography (CT) scanners were invented (Lampert et al, 1974). Availability of 3D anatomical data and ever increasing computer processing power gave rise to numerical treatment planning. Together with improved beam generation and delivery technology, treatment planning has enabled RT to better target the tumor and to reduce adverse effects on vital organs (Jaffray et al, 2007). This is exactly what researchers would also like to achieve with irreversible electroporation (IRE): ablate the target tissue and spare as much healthy tissue as possible. Just as treatment planning in RT provides radiation oncologist with the radiation beam intensities and directions that cover the tumor without causing extensive damage to healthy tissue, so can treatment planning in IRE provide physicians with electrode configurations and amplitudes of electric pulses that result in adequate electric field distribution in and around the target tissue.

Irreversible electroporation (IRE) and electrochemotherapy are two electroporation-based antitumor treatments, both relying on the delivery of short electric pulses of a typical duration of 100 microseconds. Constant current, radiofrequency electromagnetic fields and other types of pulses are also used in the clinics or are tested in preclinical trials. In electrochemotherapy, cell reversible electropermeabilization allows the uptake of non-permeant or low-permeant anticancer drugs and the tumor cells killer is the drug. In IRE, the killer is the electric field which irreversibly perturbs membranes structure and cells homeostasis. The characteristics of IRE and electrochemotherapy are compared, showing the differences and the complementarities of these two antitumor approaches.

Irreversible electroporation (IRE) is a new non-thermal ablation modality that uses short pulses of DC electric current to create irreversible pores in the cell membrane thus causing cell death. This method has been shown by Rubinsky et al. to have significant advantages in ablating prostatic tissue, such as rapid lesion creation, rapid lesion resolution, sparing of structures such as vessels, nerves and urethra, and uniform destruction throughout the IRE lesion (1). The underlying principles are well covered in other chapters in this book. This discussion will deal with the first human applications of IRE and whether its theoretical promises of improved clinical outcomes, have been delivered. For a number of reasons, including the ability to carry out extensive post operative biopsies to confirm the adequate ablation of cancer, the first human experience was carried out in the prostate.

Considering the remarkable opportunities and safety profile demonstrated in the animal studies using irreversible electroporation, it would seem intuitive that this method would provide a safer, more effective and more widely applicable treatment for solid tumours in humans.

Electroporation is a technique that uses micro to milliseconds electric pulses to create pores in the cell membrane, thus allowing molecules that, due to their physical and/or chemical properties, would normally not be able to cross the cell membrane, to enter the cell (1-5). Electroporation finds applications in many fields in particular for gene insertion in cells (electrogenetherapy) (6,7) and for the treatment of cancer (electrochemotherapy). In electrochemotherapy, the combination of chemotherapy and electroporation of tumour cells, the effects of drugs that usually show little cytotoxicity are greatly increased (8). The opening of pores in the cell membrane allows the chemotherapeutic agent to enter the cell at greater, more effective concentration and exert its cytotoxic action by killing the target cell (9-11).

The need for food preservation has faced mankind since ancient times. Food preservation technologies have evolved as a result of the progression of human knowledge. The more knowledge we have about the environment we live in, the more sophisticated tools we develop to control it. Any significant breakthrough in physics and engineering has resulted in the improvement of food preservation methods. For example,the invention of electricity in 1600 undoubtedly led to major breakthroughs in fresh food storage. Primarily,various thermal methods were invented; thermal sterilization, refrigeration, and cooled storage are among them. However, as years passed, the additional method of pulsed electric field ( PEF) treatment was developed based on the observation of the effect of certain electric fields on cell membranes when delivered as high amplitude short length pulses. Stimulated by customer demand for high quality products displaying the same properties as untreated food, this method is currently in the latest stages of research progress before being implemented industrially. The exact molecular mechanism by which PEF inactivates cells is not yet known. One proposition is that PEF causes the formation of nanoscale pores in the cell membrane, a phenomenon termed electroporation. In the particular case when the process causes cell death, it is called irreversible electroporation (IRE). In the last four decades, IRE food disinfection has been successfully performed on numerous products and bacteria types. Particular contamination problems concerned with specific products have been evaluated, resulting in the investigation of various process parameters and protocol development. This will allow the implementation of IRE technology in the food industry.