Electrotherapies for Glioblastoma

Abstract

Non-thermal, intermediate frequency (100-500 kHz) electrotherapies present a unique therapeutic strategy to treat malignant neoplasms. Here, pulsed electric fields (PEFs) which induce reversible or irreversible electroporation (IRE) and tumour-treating fields (TTFs) are reviewed highlighting the foundations, advances, and considerations of each method when applied to glioblastoma (GBM). Several biological aspects of GBM that contribute to treatment complexity (heterogeneity, recurrence, resistance, and blood-brain barrier(BBB)) and electrophysiological traits which are suggested to promote glioma progression are described. Particularly, the biological responses at the cellular and molecular level to specific parameters of the electrical stimuli are discussed offering ways to compare these parameters despite the lack of a universally adopted physical description. Reviewing the literature, a disconnect is found between electrotherapy techniques and how they target the biological complexities of GBM that make treatment difficult in the first place. An attempt is made to bridge the interdisciplinary gap by mapping biological characteristics to different methods of electrotherapy, suggesting important future research topics and directions in both understanding and treating GBM. To the authors' knowledge, this is the first paper that attempts an in-tandem assessment of the biological effects of different aspects of intermediate frequency electrotherapy methods, thus offering possible strategies toward GBM treatment.

Keywords: bioelectronics; electroporation; electrotherapy; glioblastoma; tumour-treating fields.

Figure 1

Stages and potential biological modulation of electrotherapies. A) Pulsed electric fields (including nanosecond‐pulsed electric fields (nsPEFs), reversible electroporation and IRE techniques) targeting GBM are in pre‐clinical validation with most studies in vitro and others in canine and rat models. These techniques typically involve mono‐ or bi‐phasic waveforms. B) TTFs is FDA approved to treat new and recurrent GBM in combination with current standard of care. It typically involves sinusoidal AC fields at 200 kHz. C) Potential mechanisms (from left to right): Neuron signaling contributes to GBM growth which feeds back to increase neuron excitation. PEF techniques can target ion channels, lipid bilayers, blood‐brain barrier opening, and other intracellular mechanisms (not shown). TTFs target mitotic cells through dipoles and DEP forces. Created with BioRender.com

Figure 2

Stages of electroporation. Short but intense electric fields (induced V _m≈0.5 V) force water molecules to penetrate and disrupt the lipid bilayer in the cell membrane leading to unstable hydrophobic pore formation (stage 1). The hydrophilic heads of the lipids begin to reorientate to form a metastable pore of ≈2 nm wide allowing small molecules to enter the cell (stage 2). Depending on the strength and duration of the applied field, modulation of ion channels/transporters have been reported. When the field strength is very intense (V _m ≥ 1 V), the cell membrane breaks down due to lack of homeostasis and several cell death pathways have been suggested including pyroptosis, necrosis and apoptosis. Created with BioRender.com

Figure 3

Various electrical regimes used in electroporation techniques. a) ECT are typically monophasic, long in duration (up to 20 ms) with tolerable, reversible field strengths. b) nsPEFs are biphasic and use the shortest duration of all electroporation techniques thus require substantial field strengths to achieve electropermeablization or irreversible effects. However, in this regime, most effects are observed intracellularly. c) IRE traditionally involves a short duration (≈100 µs), monophasic regime at lethal field strengths. d) High‐frequency IRE (H‐FIRE) are second generation IRE waveforms involving biphasic short duration bursts at lethal thresholds to induce cell death. e) H‐FIRE regimes with asymmetric inter‐pulse and inter‐phase delay.

Figure 4

Single cell level mechanism of action of TTFs. TTFs targets mitotic cells inducing, DNA damage, replication stress, and mitotic arrest leading to cell death. Small arrows indicate increase (upward) or decrease (downward). Created with BioRender.com.

Figure 5

How can electrotherapy be improved in order to better treat GBM? Here we depict both the complex nature of GBM and what strategies of electrotherapy have attempted to overcome these challenges. The prospect of electrotherapies ought to address numerous challenges which are currently poorly understood including various aspects of heterogeneity, BBB, treatment resistance, and recurrence and how innate immunity can be harnessed. Created with BioRender.com

Figure 6

A cartoon representation of the current pathway newly diagnosed GBM patients follow and the most common progression of events (Created with BioRender.com). A) Patient presents to clinic with a brain tumour and receives gross total resection (GTR) surgery. B) Despite GTR and margins, some tumour cells remain close to the resection cavity, and some may have begun spreading further through the parenchyma. C) A representation of the complex heterogeneity of cell types surrounding GBM cells in the brain microenvironment. D) Following GTR, the patient receives TMZ, and radiotherapy which aims to kill remaining sensitive tumour cells; however some resistant cells persist following the cessation of therapies (shown in purple). E) These persisting cells begin the process of re‐populating the tumour and re‐establishing the original heterogeneity. F) Despite standard of care, the tumour recurs either at sites close to or further afield from the original tumour. Recurrences may present on the ipsilateral or contralateral side.

Figure 7

The “vascular lock effect” in reversible conditions. Electroporation induces vasoconstriction, reduce blood flow, and structural changes to tight endothelial cell junctions between 0 and ≈2 h after application. The gaps allow paracellular permeability of ions (i.e., cytotoxic agents) to reach the tumour and extracellular space. ≈8–12 h post application, the vessel is able to gradually recover its shape and function and full recovery is typically evident within 24 h. Created with BioRender.com.