Biomedical applications of electric pulses with special emphasis on antitumor electrochemotherapy

Electrochemotherapy in the Treatment of Head and Neck Cancer: Current Conditions and Future Directions

Simple Summary

Electrochemotherapy (ECT) was first introduced in the late 1980s and was initially used mainly on cutaneous tumors. It has now evolved into a clinically verified treatment approach. Thanks to its high feasibility, it has been extended to treating mucosal and deep-seated tumors, including head and neck cancer (HNC) and in heavily pretreated settings. This review describes current knowledge and data on the use of ECT in various forms of HNCs across different clinical settings, with attention to future clinical and research perspectives.

Abstract

Despite recent advances in the development of chemotherapeutic drug, treatment for advanced cancer of the head and neck cancer (HNC) is still challenging. Options are limited by multiple factors, such as a prior history of irradiation to the tumor site as well as functional limitations. Against this background, electrochemotherapy (ECT) is a new modality which combines administration of an antineoplastic agent with locally applied electric pulses. These pulses allow the chemotherapeutic drug to penetrate the intracellular space of the tumor cells and thereby increase its cytotoxicity. ECT has shown encouraging efficacy and a tolerable safety profile in many clinical studies, including in heavily pre-treated HNC patients, and is considered a promising strategy. Efforts to improve its efficacy and broaden its application are now ongoing. Moreover, the combination of ECT with recently developed novel therapies, including immunotherapy, represented by immune checkpoint inhibitor (ICI)s, has attracted attention for its potent theoretical rationale. More extensive, well-organized clinical studies and timely updating of consensus guidelines will bring this hopeful treatment to HNC patients under challenging situations.

Electropermeabilization of metastatic chondrosarcoma cells from primary cell culture

Primary cell cultures are challenging, but reliable model reflecting tumor response in vitro. The study was designed to examine if the increased electropermeabilization can overcame initial drug insensitivity in chondrosarcoma cells from lung metastasis. We established a primary cell culture and evaluated the cytotoxic impact of four drugs-cisplatin (CDDP), camptothecin, 2-methoxyestradiol, and leucovorin calcium (LeuCa). After determination of parameters allowing for electropermeabilization, we performed electrochemotherapy in vitro with the least toxic drugs-CDDP and LeuCa. Although combining CDDP and leucovorin together increased their toxicity and supported apoptosis, application of pulsed electric fields (PEFs) brought no advantage for their efficacy. The study emphasizes the need for introduction of primary cell cultures into studies on pulse electric fields as model frequently less sensitive to PEF-based treatments than continuous cell lines.

Electrochemotherapy in Veterinary Oncology: State-of-the-Art and Perspectives

Tumor microenvironment represents a key obstacle for the effectiveness of anticancer drugs. Electrochemotherapy involves the systemic or local delivery of lipophobic drugs such as bleomycin and cisplatin, with the application of permeabilizing electric pulses having appropriate amplitude and waveforms. This greatly enhances the uptake of these drugs by an estimated factor of 700-fold for bleomycin and 4 to 8 times for cisplatin. Because of its efficacy and limited morbidity, this therapeutic option is becoming more and more available in veterinary oncology either as an adjuvant to surgery or as first line of treatment with palliative or curative purposes.

Characterization of Conductivity Changes During High-Frequency Irreversible Electroporation for Treatment Planning

For irreversible-electroporation (IRE)-based therapies, the underlying electric field distribution in the target tissue is influenced by the electroporation-induced conductivity changes and is important for predicting the treatment zone.

OBJECTIVE

In this study, we characterized the liver tissue conductivity changes during high-frequency irreversible electroporation (H-FIRE) treatments of widths 5 and 10 μs and proposed a method for predicting the ablation zones.

METHODS

To achieve this, we created a finite-element model of the tissue treated with H-FIRE and IRE pulses based on experiments conducted in an in-vivo rabbit liver study. We performed a parametric sweep on a Heaviside function that captured the tissue conductivity versus electric field behavior to yield a model current close to the experimental current during the first burst/pulse. A temperature module was added to account for the current increase in subsequent bursts/pulses. The evolution of the electric field at the end of the treatment was overlaid on the experimental ablation zones determined from hematoxylin and eosin staining to find the field thresholds of ablation.

RESULTS

Dynamic conductivity curves that provided a statistically significant relation between the model and experimental results were determined for H-FIRE. In addition, the field thresholds of ablation were obtained for the tested H-FIRE parameters.

CONCLUSION

The proposed numerical model can simulate the electroporation process during H-FIRE.

SIGNIFICANCE

The treatment planning method developed in this study can be translated to H-FIRE treatments of different widths and for different tissue types.

The controversial role of electrochemotherapy in head and neck cancer: a systematic review of the literature

Electroporation, also known as electrochemotherapy, combines an antineoplastic agent with electroporation, causing localized progressive necrosis in the treated area. Today it is primarily used in the palliative treatment of cutaneous and subcutaneous metastases and has been found to be safe and efficacious in head and neck cancer recurrences. Despite the steady increase in the number of published studies this treatment is not universally available and used systematically in head and neck carcinomas. To shed light on its limitations and analyze treatment outcome we have, therefore, reviewed all available literature regarding this topic. This systematic review includes 16 studies on head and neck squamous cell carcinoma and reports the data of 200 treated patients. The combined results show a very heterogeneous overall response rate, ranging from 0 to 100%, while the complete response rate ranges between 0 and 83.3%. No major side effects have been described in those who used electrochemotherapy as a mono modality palliative treatment. This systematic review shows how standardization of treatment is still pivotal to achieve a more homogeneous response rate in the approach to head and neck tumors. In conclusion, due to the scarcity of alternatives of treatment in advanced stage cancer in this anatomical region and the good tolerability and mostly high success rates of electrochemotherapy, this palliative approach should be taken into consideration in these patients.

Novel Instruments for the Implementation of Electrochemotherapy Protocols: From Bench Side to Veterinary Clinic

Electrochemotherapy (ECT) is a medical strategy that allows an increased efficacy of chemotherapy agents after the application of permeabilizing electric pulses having appropriate characteristics (form, voltage, frequency). In the past 10 years, the clinical efficacy of this therapeutic approach in several spontaneous models of tumors in animals has been shown. Moreover, some of the molecular and cellular mechanisms responsible for this phenomenon have been elucidated. Our group has been deeply involved in the development of new ECT protocols for companion animals, implementing the use of the technique as first line treatment, and evaluating different chemotherapy agents in laboratory animals as well as pets. This article summarizes the most important advances in veterinary ECT, including the development of novel equipment, therapeutic protocols, and their translation to humans. J. Cell. Physiol. 232: 490-495, 2017. © 2016 Wiley Periodicals, Inc.

Theoretical analysis of transmembrane potential of cells exposed to nanosecond pulsed electric field

PURPOSE

Intracellular electroporation occurs when the cells are exposed to nanosecond pulsed electric field (nsPEF). It is believed the electroporation (formation and extension of pores on the membrane induced by external electric field) is affected significantly by the transmembrane potential. This paper analyzed transmembrane potential induced by nsPEF in the term of pulse frequency spectrum, aiming to provide a theoretical explanation to intracellular bio-effects.

METHODS

Based on the double-shelled spherical cell model, the frequency dependence of transmembrane potential was obtained by solving Laplace's equation, while the time course of transmembrane potential was obtained by a method combined with discrete Fourier transform and Laplace transform. First-order Debye equation was used to describe the dielectric relaxation of the cell medium.

RESULTS

Frequency-domain analysis showed that when the electric field frequency was higher than 10⁵ Hz, the transmembrane potential on the organelle membrane (ΔΦ_o) was increasing to exceed the transmembrane potential on the cellular membrane (ΔΦ_c). In the time-domain analysis, transmembrane potentials induced by four nsPEF (short trapezoid, long trapezoid, bipolar and sine shapes) with the same field strength were compared with each other. It showed that ΔΦ_o is obviously larger than ΔΦ_c if the curve of the normalized frequency spectrum of the pulse is more similar with the curve of normalized ΔΦ_o in frequency domain. Pulses with major frequency components higher than 10⁸ Hz lead to both small ΔΦ_o and ΔΦ_c. This may explain why high power pulsed microwave lead to unobvious bio-effects of cells than nsPEF with trapezoid form.

CONCLUSION

Through the pulse frequency spectrum it is clearer to understand the relationship between nsPEF and the transmembrane potential.

Theoretical analyses of cellular transmembrane voltage in suspensions induced by high-frequency fields

A change of the transmembrane voltage is considered to cause biophysical and biochemical responses in cells. The present study focuses on the cellular transmembrane voltage (Δφ) induced by external fields. We detail analytical equations for the transmembrane voltage induced by external high-frequency (above the relaxation frequency of the cell membrane) fields on cells of a spherical shape in suspensions and layers. At direct current (DC) and low frequencies, the cell membrane was assumed to be non-conductive under physiologic conditions. However, with increasing frequency, the permittivity of the cytoplasm/extracellular medium and conductivity of the membrane must be accounted for. Our main work is to extend application of the analytical solution of Δφ to the high-frequency range. We first introduce the transmembrane voltage generated by DC and low-frequency exposures on a single cell. Then, we focus on cell suspensions exposed to high-frequency fields. Using the effective medium theory and the reasonable assumption, the approximate analytical solution of Δφ on cells in suspensions and layers can be derived. Phenomenological effective medium theory equations cannot be used to calculate the local electric field of cell suspensions, so we raised a possible solution based on the Bergman theory.

Transient oscillation of shape and membrane conductivity changes by field pulse-induced electroporation in nano-sized phospholipid vesicles

The results of electrooptical and conductometrical measurements on unilamellar lipid vesicles (of mean radius a = 90 nm), filled with 0.2 M NaCl solution, suspended in 0.33 M sucrose solution of 0.2 mM NaCl, and exposed to a stepwise decaying electric field (time constant τE = 154 μs) in the range 10 ≤ E0 (kV cm(-1)) ≤ 90, are analyzed in terms of cyclic changes in vesicle shape and vesicle membrane conductivity. The two peaks in the dichroitic turbidity relaxations reflect two cycles of rapid membrane electroporation and slower resealing of long-lived electropores. The field-induced changes reflect structural transitions between closed (C) and porated (P) membrane states, qualified by pores of type P1 and of type P2, respectively. The transient change in the membrane conductivity and the transient shape oscillation are based on changes in the pore density of the (larger) P2-pores along a hysteresis cycle. The P2-pore formation leads to transient net ion flows across the P2-pores and to transient changes in the membrane field. The kinetic data are numerically processed in terms of coupled structural relaxation modes. Using the torus-hole pore model, the mean inner pore radii are estimated to be r1 = 0.38 (±0.05) nm and r2 = 1.7 (±0.1) nm, respectively. The observation of a transient oscillation of membrane electroporation and of shape changes in a longer lasting external field pulse is suggestive of potential resonance enhancement, for instance, of electro-uptake by, and of electro-release of biogenic molecules from, biological cells in trains of long-lasting low-intensity voltage pulses.

The effects of metallic implants on electroporation therapies: feasibility of irreversible electroporation for brachytherapy salvage

PURPOSE

Electroporation-based therapies deliver brief electric pulses into a targeted volume to destabilize cellular membranes. Nonthermal irreversible electroporation (IRE) provides focal ablation with effects dependent on the electric field distribution, which changes in heterogeneous environments. It should be determined if highly conductive metallic implants in targeted regions, such as radiotherapy brachytherapy seeds in prostate tissue, will alter treatment outcomes. Theoretical and experimental models determine the impact of prostate brachytherapy seeds on IRE treatments.

MATERIALS AND METHODS

This study delivered IRE pulses in nonanimal, as well as in ex vivo and in vivo tissue, with and in the absence of expired radiotherapy seeds. Electrical current was measured and lesion dimensions were examined macroscopically and with magnetic resonance imaging. Finite-element treatment simulations predicted the effects of brachytherapy seeds in the targeted region on electrical current, electric field, and temperature distributions.

RESULTS

There was no significant difference in electrical behavior in tissue containing a grid of expired radiotherapy seeds relative to those without seeds for nonanimal, ex vivo, and in vivo experiments (all p > 0.1). Numerical simulations predict no significant alteration of electric field or thermal effects (all p > 0.1). Histology showed cellular necrosis in the region near the electrodes and seeds within the ablation region; however, there were no seeds beyond the ablation margins.

CONCLUSION

This study suggests that electroporation therapies can be implemented in regions containing small metallic implants without significant changes to electrical and thermal effects relative to use in tissue without the implants. This supports the ability to use IRE as a salvage therapy option for brachytherapy.

Molecular dynamic simulation of transmembrane pore growth

A molecular dynamic approach was applied for simulation of dynamics of pore formation and growth in a phospholipid bilayer in the presence of an external electric field. Processing the simulation results permitted recovery of the kinetic coefficients used in the Einstein-Smoluchowski equation describing the dynamics of pore evolution. Two different models of the bilayer membrane were considered: membrane consisting of POPC and POPE lipids. The simulations permitted us to find nonempirical values of the pore energy parameters, which are compared with empirical values. It was found that the parameters are sensitive to membrane type.

Electrochemotherapy: technological advancements for efficient electroporation-based treatment of internal tumors

Electrochemotherapy, a combination of high voltage electric pulses and of an anticancer drug, has been demonstrated to be highly effective in treatment of cutaneous and subcutaneous tumors. Unique properties of electrochemotherapy (e.g., high specificity for targeting cancer cells, high degree of localization of treatment effect, capacity for preserving the innate immune response and the structure of the extracellular matrix) are facilitating its wide spread in the clinics. Due to high effectiveness of electrochemotherapy in treatment of cutaneous and subcutaneous tumors regardless of histological origin, there are now attempts to extend its use to treatment of internal tumors. To advance the applicability of electrochemotherapy to treatment of internal solid tumors, new technological developments are needed that will enable treatment of these tumors in daily clinical practice. New electrodes through which electric pulses are delivered to target tissue need to be designed with the aim to access target tissue anywhere in the body. To increase the probability of complete tumor eradication, the electrodes have to be accurately positioned, first to provide an adequate extent of electroporation of all tumor cells and second not to damage critical healthy tissue or organs in its vicinity. This can be achieved by image guided insertion of electrodes that will enable accurate positioning of the electrodes in combination with patient-specific numerical treatment planning or using a predefined geometry of electrodes. In order to be able to use electrochemotherapy safely for treatment of internal tumors located in relative proximity of the heart (e.g., in case of liver metastases), the treatment must be performed without interfering with the heart’s electrical activity. We describe recent technological advances, which allow treatment of liver and bone metastases, soft tissue sarcomas, brain tumors, and colorectal and esophageal tumors. The first clinical experiences in these novel application areas of electrochemotherapy are also described.

Molecular-level characterization of lipid membrane electroporation using linearly rising current

We present experimental and theoretical results of electroporation of small patches of planar lipid bilayers by means of linearly rising current. The experiments were conducted on ~120-μm-diameter patches of planar phospholipid bilayers. The steadily increasing voltage across the bilayer imposed by linearly increasing current led to electroporation of the membrane for voltages above a few hundred millivolts. This method shows new molecular mechanisms of electroporation. We recorded small voltage drops preceding the breakdown of the bilayer due to irreversible electroporation. These voltage drops were often followed by a voltage re-rise within a fraction of a second. Modeling the observed phenomenon by equivalent electric circuits showed that these events relate to opening and closing of conducting pores through the bilayer. Molecular dynamics simulations performed under similar conditions indicate that each event is likely to correspond to the opening and closing of a single pore of about 5 nm in diameter, the conductance of which ranges in the 100-nS scale. This combined experimental and theoretical investigation provides a better quantitative characterization of the size, conductance and lifetime of pores created during lipid bilayer electroporation. Such a molecular insight should enable better control and tuning of electroporation parameters for a wide range of biomedical and biotechnological applications.

In vivo muscle electroporation threshold determination: realistic numerical models and in vivo experiments

In vivo electroporation is used as an effective technique for delivery of therapeutic agents such as chemotherapeutic drugs or DNA into target tissue cells for different biomedical purposes. In order to successfully electroporate a target tissue, it is essential to know the local electric field distribution produced by an application of electroporation voltage pulses. In this study three-dimensional finite element models were built in order to analyze local electric field distribution and corresponding tissue conductivity changes in rat muscle electroporated either transcutaneously or directly (i.e., two-plate electrodes were placed either on the skin or directly on the skeletal muscle after removing the skin). Numerical calculations of electroporation thresholds and conductivity changes in skin and muscle were validated with in vivo measurements. Our model of muscle with skin also confirms the in vivo findings of previous studies that electroporation "breaks" the skin barrier when the applied voltage is above 50 V.

A brief overview of electroporation pulse strength-duration space: a region where additional intracellular effects are expected

Electroporation (EP) of outer cell membranes is widely used in research, biotechnology and medicine. Now intracellular effects by organelle EP are of growing interest, mainly due to nanosecond pulsed electric fields (nsPEF). For perspective, here we provide an approximate overview of EP pulse strength-duration space. This overview locates approximately some known effects and applications in strength-duration space, and includes a region where additional intracellular EP effects are expected. A feature of intracellular EP is direct, electrical redistribution of endogenous biochemicals among cellular compartments. For example, intracellular EP may initiate a multistep process for apoptosis. In this hypothesis, initial EP pulses release calcium from the endoplasmic reticulum, followed by calcium redistribution within the cytoplasm. With further EP pulses calcium penetrates mitochondrial membranes and causes changes that trigger release of cytochrome c and other death molecules. Apoptosis may therefore occur even in the presence of apoptotic inhibitors, using pulses that are smaller, but longer, than nsPEF.

Case for applying subnanosecond high-intensity, electrical pulses to biological cells

In this paper, model analysis into the time-dependent transmembrane potential at the outer cell membrane is presented, for applied high-intensity electric pulses having durations in the nanosecond range or smaller. It is argued that the frequency-dependent dielectric response of cell membranes could be used to advantage for stronger bioeffects by employing shorter pulses. Our model calculations predict faster transmembrane voltages and larger electroporation densities for a given external energy with pulse durations in the subnanosecond regime. This temporal regime would be used, for example, in the electrotherapy of mixed cell ensembles having different dielectric response properties.

Synergistic effects of local temperature enhancements on cellular responses in the context of high-intensity, ultrashort electric pulses

Results of self-consistent analyses of cells show the possibility of temperature increases at membranes in response to a single nanosecond, high-voltage pulse, at least over small sections of the membrane. Molecular Dynamics simulations indicate that such a temperature increase could facilitate poration, which is one example of a bio-process at the plasma membrane. Our study thus suggests that the use of repetitive high-intensity voltage pulses could open up possibilities for a host of synergistic bio-responses involving both thermal and electrically driven phenomena.

Analysis of cell membrane permeabilization mechanics and pore shape due to ultrashort electrical pulsing

Cell membrane permeabilization mechanics and the resulting shape of nanopores in response to electrical pulsing are probed based on a continuum approach. This has implications for electropermeabilization and cell membrane transport. It is argued that small pores resulting from high-intensity (approximately 100 kV/cm), nanosecond pulsing would have an initial asymmetric shape. This would lead to asymmetric membrane current-voltage characteristics, at least at early times. The role of the cytoskeleton is ignored here, but can be expected to additionally contribute to such asymmetries. Furthermore, we show that the pore shape and membrane conduction would be dynamic, and evolve toward a symmetric characteristic over time. This duration has been shown to be in the micro-second range.

An electrically active microneedle array for electroporation

We have designed and fabricated a microneedle array with electrical functionality with the final goal of electroporating skin's epidermal cells to increase their transfection by DNA vaccines. The microneedle array was made of polymethylmethacrylate (PMMA) by micromolding technology from a polydimethylsiloxane (PDMS) mold, followed by metal deposition, patterning using laser ablation, and electrodeposition. This microneedle array possessed sufficient mechanical strength to penetrate human skin in vivo and was also able to electroporate both red blood cells and human prostate cancer cells as an in vitro model to demonstrate cell membrane permeabilization. A computational model to predict the effective volume for electroporation with respect to applied voltages was constructed from finite element simulation. This study demonstrates the mechanical and electrical functionalities of the first MEMS-fabricated microneedle array for electroporation, designed for DNA vaccine delivery.

Cellular apoptosis by nanosecond, high-intensity electric pulses: model evaluation of the pulsing threshold and extrinsic pathway

A simple, bistable rate-equation based model is used to predict trends of cellular apoptosis following electric pulsing. The caspase-8 extrinsic pathway with inherent delays in its activation, cytochrome c release, and an internal feedback mechanism between caspase-3 and cleavage of Bid are incorporated. Results obtained were roughly in keeping with the experimental cell-survival data and include an electrical pulse-number threshold followed by a near-exponential fall-off. The extrinsic caspase-8 mechanism is predicted to be more sensitive than the mitochondrial intrinsic pathway for electric pulse induced cell apoptosis. Also, delays of about an hour are predicted for detectable molecular concentration increases following electrical pulsing. Finally, our results suggest that multi-needle electrode systems with adjustable field orientations would likely enhance apoptosis in the context of pulsed voltage-induced inactivation of tumor cells.

Electroporation and alternating current cause membrane permeation of photodynamic cytotoxins yielding necrosis and apoptosis of cancer cells

In order to increase the permeability of cell membranes for low doses of cytostatic drugs, two bioelectrochemical methods have been compared: (a) electric pore formation in the plasma membranes by single electric impulses (electroporation), and (b) reordering of membrane structure by alternating currents (capacitively coupled). These treatments were applied to human leukemic K-562 cells and human lymphoma U-937 cells, yielding apoptotic and necrotic effects, determined by flow cytometry. Additional cell death occurs after exposure to light irradiation at wavelengths lambda > 600 nm, of cells which were electroporated and had incorporated actinomycin-C or daunomycin (daunorubicin). It is observed that drug uptake after an exponentially decaying electroporation pulse of the initial field strength Eo=1.4 kV/cm and pulse time constants in the time range 0.5-3 ms is faster than during PEMF-treatment, i.e., application of an alternating current of 16 kHz, voltage U<100 V, I=55 mA, and exposure time 20 min. However, at the low a.c. voltage of this treatment, more apoptotic and necrotic cells are produced as compared to the electroporation treatment with one exponentially decaying voltage pulse. Thus, additional photodynamic action appears to be more effective than solely drugs and electroporation as applied in clinical electrochemotherapy, and more effective than the noninvasive pulsed electromagnetic fields (PEMFs), for cancer cells in general and animals bearing tumors in particular.

Synthesis of new heterometallic macromolecules: their DNA binding, cleavage activity and in vitro model electrochemotherapy study

The homodinuclear C(16)H(30)N(8)O(5)Sn(2)Cl(4) (1), heterotetranuclear C(16)H(38)N(8)O(9)Sn(2)Cu(2)Cl(8) (2) and C(16)H(38)N(8)O(9)Sn(2)Mn(2)Cl(8) (3) macrocyclic complexes were synthesized and characterized by elemental analysis, spectroscopic techniques and molar conductance measurements. The interaction studies of 1-3 with calf thymus DNA (CT-DNA) were carried out by UV-vis titration, fluorescence, cyclic voltammetry and viscosity measurements. These results were further authenticated by carrying out interaction studies of 1-3 with plasmid pBR322 DNA employing gel electrophoresis. To overcome the dose resistance, auto toxicity of the drugs, a model study based on electrochemotherapy (ECT) was carried out and the results were compared in the presence and in the absence of the applied electrical potential.

The feasibility of irreversible electroporation for the treatment of breast cancer and other heterogeneous systems

Developments in breast cancer therapies show potential for replacing simple and radical mastectomies with less invasive techniques. Localized thermal techniques encounter difficulties, preventing their widespread acceptance as replacements for surgical resection. Irreversible electroporation (IRE) is a non-thermal, minimally invasive focal ablation technique capable of killing tissue using electric pulses to create irrecoverable nano-scale pores in the cell membrane. Its unique mechanism of cell death exhibits benefits over thermal techniques including rapid lesion creation and resolution, preservation of the extracellular matrix and major vasculature, and reduced scarring. This study investigates applying IRE to treat primary breast tumors located within a fatty extracellular matrix despite IREs dependence on the heterogeneous properties of tissue. In vitro experiments were performed on MDA-MB-231 human mammary carcinoma cells to determine a baseline electric field threshold (1000 V/cm) to cause IRE for a given set of pulse parameters. The threshold was incorporated into a three-dimensional numerical model of a heterogeneous system to simulate IRE treatments. Treatment-relevant protocols were found to be capable of treating targeted tissue over a large range of heterogeneous properties without inducing significant thermal damage, making IRE a potential modality for successfully treating breast cancer. Information from this study may be used for the investigation of other heterogeneous tissue applications for IRE.

Electric pulses used in electrochemotherapy and electrogene therapy do not significantly change the expression profile of genes involved in the development of cancer in malignant melanoma cells

Background

Electroporation is a versatile method for in vitro or in vivo delivery of different molecules into cells. However, no study so far has analysed the effects of electric pulses used in electrochemotherapy (ECT pulses) or electric pulses used in electrogene therapy (EGT pulses) on malignant cells. We studied the effect of ECT and EGT pulses on human malignant melanoma cells in vitro in order to understand and predict the possible effect of electric pulses on gene expression and their possible effect on cell behaviour.

Methods

We used microarrays with 2698 different oligonucleotides to obtain the expression profile of genes involved in apoptosis and cancer development in a malignant melanoma cell line (SK-MEL28) exposed to ECT pulses and EGT pulses.

Results

Cells exposed to ECT pulses showed a 68.8% average survival rate, while cells exposed to EGT pulses showed a 31.4% average survival rate. Only seven common genes were found differentially expressed in cells 16 h after exposure to ECT and EGT pulses. We found that ECT and EGT pulses induce an HSP70 stress response mechanism, repress histone protein H4, a major protein involved in chromatin assembly, and down-regulate components involved in protein synthesis.

Conclusion

Our results show that electroporation does not significantly change the expression profile of major tumour suppressor genes or oncogenes of the cell cycle. Moreover, electroporation also does not changes the expression of genes involved in the stability of DNA, supporting current evidence that electroporation is a safe method that does not promote tumorigenesis. However, in spite of being considered an isothermal method, it does to some extent induce stress, which resulted in the expression of the environmental stress response mechanism, HSP70.

Kinetics, statistics, and energetics of lipid membrane electroporation studied by molecular dynamics simulations

Membrane electroporation is the method to directly transfer bioactive substances such as drugs and genes into living cells, as well as preceding electrofusion. Although much information on the microscopic mechanism has been obtained both from experiment and simulation, the existence and nature of possible intermediates is still unclear. To elucidate intermediates of electropore formation by direct comparison with measured prepore formation kinetics, we have carried out 49 atomistic electroporation simulations on a palmitoyl-oleoyl-phosphatidylcholine bilayer for electric field strengths between 0.04 and 0.7 V/nm. A statistical theory is developed to facilitate direct comparison of experimental (macroscopic) prepore formation kinetics with the (single event) preporation times derived from the simulations, which also allows us to extract an effective number of lipids involved in each pore formation event. A linear dependency of the activation energy for prepore formation on the applied field is seen, with quantitative agreement between experiment and simulation. The distribution of preporation times suggests a four-state pore formation model. The model involves a first intermediate characterized by a differential tilt of the polar lipid headgroups on both leaflets, and a second intermediate (prepore), where a polar chain across the bilayer is formed by 3-4 lipid headgroups and several water molecules, thereby providing a microscopic explanation for the polarizable volume derived previously from the measured kinetics. An average pore radius of 0.47 +/- 0.15 nm is seen, in favorable agreement with conductance measurements and electrooptical experiments of lipid vesicles.

Electrochemotherapy: aspects of preclinical development and early clinical experience

OBJECTIVE

To develop an optimized, reproducible system of electrochemotherapy, and to investigate its clinical application in patients with cutaneous or subcutaneous recurrences of inoperable or progressive disease recalcitrant to current anticancer treatments.

BACKGROUND

Electrochemotherapy is the application of electric pulses to tumor tissue, rendering the cell membranes permeable to otherwise impermeant or poorly permeant anticancer drugs. This facilitates a potent local cytotoxic effect.

STUDY DESIGN

The optimal parameters for electrical pulses and bleomycin concentration were obtained in vitro and then applied to tumors derived from 4 histologically distinct human cancer cell lines (7860, PC3, OE19, MCF-7) established in athymic nude mice. Comparison was made with tumors that received bleomycin alone, electric pulses alone, and untreated controls. The optimized electrochemotherapy was then applied to patients with cutaneous or subcutaneous tumors, of any histologic type, recurrent or metastatic and unresponsive to standard chemotherapy and/or radiotherapy regimens. Tumors were assessed at monthly intervals to determine response to the treatment.

RESULTS

In vivo: Using the optimal parameters ascertained in vitro, all tumors treated by electrochemotherapy with bleomycin (n = 24) had significantly regressed (P < 0.001, all 4 lines) compared with control tumors (n = 72). Twelve tumors completely regressed (50%) following a single application, with 12 partial regressions (50%). Clinical: In 30 patients (111 tumors), none of the treated tumors progressed. Sixty percent of tumors (66 of 111) showed complete regression, 22% (24 of 111) partial response, and 18% (21 of 111) no change. Electrochemotherapy was more effective in smaller tumors (<3 cm), 71% (64 of 90) showing complete regression, 20% (18 of 90) partial response, and 9% (8 of 90) no change.

CONCLUSIONS

Electrochemotherapy parameters optimized in vitro are applicable in vivo. This treatment is effective in athymic nude mice for all histologic types indicating a nonimmunologic mode of action. In clinical application, electrochemotherapy is an effective, safe, and reproducible therapy. Patients with cutaneous or subcutaneous tumors previously refractory to surgical intervention, systemic chemotherapy, and/or radiotherapy responded successfully irrespective of histologic type.

Electric field effects on membranes: Gramicidin A as a test ground

Electric fields due to transmembrane potential differences or ionic gradients across the membrane are presumably crucial for many reactions across membranes or close to membranes like signal transduction, control of ion channels or the generation of neural impulses. Molecular dynamics simulations have been used to study the influence of external electric fields on a mixed gramicidin/phospholipid bilayer system. At high field strengths, formation of membrane electropores occurred both close and distal to the gramicidin. Gramicidin was found to stabilize the membrane adjacent to the protein but also at larger distances of up to 2-3 nm. As a result, membrane pore formation was found to be significantly suppressed for the mixed gramicidin/DMPC system. Moderate field strengths only weakly affected the structure and dynamics of the gramicidin. Spontaneous potassium passage events in external electric fields were observed for both the head-to-head helical conformation as well as for the double helical conformation of gramicidin A. The double-helical conformation was found to facilitate ion passage compared to the head-to-head helical dimer.

Sequential finite element model of tissue electropermeabilisation

Sequential model of liver tissue electropermeabilisation around two needle electrodes was designed by computing electric field (E) distribution by means of the finite element (FE) method. Sequential model consists of a sequence of static FE models which represent E distribution during tissue permeabilisation. In the model an S-shaped dependency between specific conductivity and E was assumed. Parameter estimation of S-shaped dependency was performed on a set of current measurements obtained by in vivo experiments. Another set of in vivo measurements was used for model validation. Model validation was carried out in three different ways by comparing experimental measurements and modelled results. The model validation showed good agreement between modelled and measured results. The model also provided means for better understanding processes that occur during permeabilisation. Based on the model, the permeabilised volume of tissue exposed to electrical treatment can be predicted. Therefore, the most important contribution of the model is its potential to be used as a tool for determining the electrode position and pulse amplitude needed for effective tissue permeabilisation.

Adsorption of DNA and electric fields decrease the rigidity of lipid vesicle membranes

The adsorption of calf-thymus DNA-fragments of 300 +/- 50 base pairs (bp) to the outer membrane monolayer of unilamellar lipid vesicles in the presence of Ca2+ ions has been quantified by the standard method of chemical relaxation spectrometry using polarized light. The vesicles of radius a = 150 +/- 45 nm are prepared from bovine brain extract type III containing 80-85% phosphatidylserine (PS) and palmitoyl-oleoyl-phosphatidylcholine (POPC) in the molar ratio PS : 2POPC; total lipid concentration [L(t)] = 1 mM in 1 mM HEPES buffer, pH 7.4 at T = 293 K (20 degrees C). The turbidity relaxations of vesicle suspensions, at the wavelength lambda = 365 nm at two characteristic electric field strengths are identified as electroelongation of the whole vesicle coupled to smoothing of thermal membrane undulations and membrane stretching, and at higher fields, to membrane electroporation (MEP). The elongation kinetics indicates that the DNA adsorption renders the membrane more flexible and prone to membrane electroporation (MEP). Remarkably, it is found that the Ca-mediated adsorption of DNA (D) decreases both, bending rigidity kappa and stretching modulus K, along an unique Langmuir adsorption isotherm for the fraction of bound DNA at the given Ca concentration [Ca(t)] = 0.25 mM. The characteristic chemo-mechanical parameter of the isotherm is the apparent dissociation equilibrium constant K(D,Ca) = 100 +/- 10 microM (bp) of the ternary complex DCaB of DNA base pairs (bp) and Ca binding to sites B on the outer vesicle surface. Whereas both kappa and K decrease in the presence of high electric fields (E), the key parameter K(D,Ca) is independent of E in the range 0 < or = E/(kV cm(-1)) < or = 40.

Interfacial ternary complex DNA/Ca/lipids at anionic vesicle surfaces

The electroporative transfer of gene DNA and other bioactive substances into tissue cells by electric pulses gains increasing importance in the new disciplines of electrochemotherapy and electrogenetherapy. The efficiency of the electrotransfer depends crucially on the adsorption of the gene DNA and oligonucleotides to the plasma cell membranes. Here it is shown that the adsorption of larger oligonucleotides such as fragments (ca. 300 bp) of sonicated calf-thymus DNA, to anionic lipids of unilamellar vesicles (diameter Phi=300+/-90 nm) is greatly enhanced by divalent cations such as Ca(2+)-ions. Applying centrifugation, bound and free DNA are monitored optically at the wavelength lambda=260 nm. Using arsenazo III as a Ca(2+)-indicator and atomic absorption spectroscopy (AAS), Ca(2+)-titrations of DNA and vesicles yield the individual equilibrium constants of Ca(2+)- and DNA-binding not only for the binary complexes: Ca/lipids, Ca/DNA and DNA/lipids, respectively, but also for the various processes to form the ternary complex DNA/Ca/lipids. The data provide the basis for goal-directed optimization protocols for the adsorption and thus efficient electrotransfer of oligonucleotides and polynucleotides into cells.

Sequential finite element model of tissue electropermeabilization

Permeabilization, when observed on a tissue level, is a dynamic process resulting from changes in membrane permeability when exposing biological cells to external electric field (E). In this paper we present a sequential finite element model of E distribution in tissue which considers local changes in tissue conductivity due to permeabilization. These changes affect the pattern of the field distribution during the high voltage pulse application. The presented model consists of a sequence of static models (steps), which describe E distribution at discrete time intervals during tissue permeabilization and in this way present the dynamics of electropermeabilization. The tissue conductivity for each static model in a sequence is determined based on E distribution from the previous step by considering a sigmoid dependency between specific conductivity and E intensity. Such a dependency was determined by parameter estimation on a set of current measurements, obtained by in vivo experiments. Another set of measurements was used for model validation. All experiments were performed on rabbit liver tissue with inserted needle electrodes. Model validation was carried out in four different ways: 1) by comparing reversibly permeabilized tissue computed by the model and the reversibly permeabilized area of tissue as obtained in the experiments; 2) by comparing the area of irreversibly permeabilized tissue computed by the model and the area where tissue necrosis was observed in experiments; 3) through the comparison of total current at the end of pulse and computed current in the last step of sequential electropermeabilization model; 4) by comparing total current during the first pulse and current computed in consecutive steps of a modeling sequence. The presented permeabilization model presents the first approach of describing the course of permeabilization on tissue level. Despite some approximations (ohmic tissue behavior) the model can predict the permeabilized volume of tissue, when exposed to electrical treatment. Therefore, the most important contribution and novelty of the model is its potentiality to be used as a tool for determining parameters for effective tissue permeabilization.

Combined electric field and ultrasound therapy as a novel anti-tumour treatment

The permeabilising effects of electric pulses on cell membranes and the use of ultrasound energy of various intensities, for both thermal effects and enhancement of drug and gene delivery, have led to extensive research into the potential applications of these systems in the development of novel anti-cancer treatments. In the present study we have demonstrated for the first time that the application of brief electric pulses 'sensitises' tumour cells to the effects of low intensity ultrasound. The studies were conducted in human tumours established in athymic nude mice and in many instances resulted in the reduction of tumour mass. The combined electric field and ultrasound approach (CEFUS) was applied in vivo to a murine colon adenocarcinoma (C26) and a human oesophageal adenocarcinoma (OE19). The experiments performed demonstrated the anti-tumour effects of the combined therapy. Varying the electrosensitisation parameters used (voltage, waveform, electrode type) contributed to optimise the procedure. Exponential electric pulses with a peak of 1000 V/cm were initially used, but square wave pulses (1000 V/cm, 1 ms, x2, 1 Hz) were found to be just as effective. All ultrasound application parameters were kept constant during the study. The growth rate of C26 tumours treated with CEFUS was significantly reduced with respect to untreated controls at day 7 (96% of average initial tumour volume in CEFUS group versus 615% for controls, P < 0.05). Similar reduction was observed in OE19 tumours treated with CEFUS by day 4 (82% versus 232%, P < 0.032). Our preliminary data suggest that this novel technology could potentially be of wide application in clinical practice for the treatment of solid tumours and is worth further investigation.

Membrane electroporation: a molecular dynamics simulation

We present results of molecular dynamics simulations of lipid bilayers under a high transverse electrical field aimed at investigating their electroporation. Several systems are studied, namely 1), a bare bilayer, 2), a bilayer containing a peptide nanotube channel, and 3), a system with a peripheral DNA double strand. In all systems, the applied transmembrane electric fields (0.5 V.nm(-1) and 1.0 V.nm(-1)) induce an electroporation of the lipid bilayer manifested by the formation of water wires and water channels across the membrane. The internal structures of the peptide nanotube assembly and that of the DNA strand are hardly modified under field. For system 2, no perturbation of the membrane is witnessed at the vicinity of the channel, which indicates that the interactions of the peptide with the nearby lipids stabilize the bilayer. For system 3, the DNA strand migrates to the interior of the membrane only after electroporation. Interestingly enough, switching of the external transmembrane potential in cases 1 and 2 for few nanoseconds is enough to allow for complete resealing and reconstitution of the bilayer. We provide evidence that the electric field induces a significant lateral stress on the bilayer, manifested by surface tensions of magnitudes in the order of 1 mN.m(-1). This study is believed to capture the essence of several dynamical phenomena observed experimentally and provides a framework for further developments and for new applications.

Finite-element modeling of needle electrodes in tissue from the perspective of frequent model computation

Information about electric field distribution in tissue is very important for effective electropermeabilization. In heterogeneous tissues with complex geometry, finite-element (FE) models provide one of alternative sources of such information. In the present study, modeling of needle electrode geometry in the FE model was investigated in order to determine the most appropriate geometry by considering the need for frequent FE model computation present in electroporation models. The 8-faceted needle electrode geometry proposed--determined on a model with a single needle electrode pair by means of criteria function--consisted of the weighted sum of relative difference between measured and computed total current, the relative difference in CPU time spent on solving model, and the relative difference in cross section surface of electrodes. Such electrode geometry was further evaluated on physical models with needle arrays by comparison of computed total current and measured current. The agreement between modeled and measured current was good (within 9% of measurement), except in cases with very thin gel. For voltage above 50 V, a linear relationship between current and voltage was observed in measurements. But at lower voltages, a nonlinear behavior was detected resulting from side (electrochemical) effects at electrode-gel interface. This effect was incorporated in the model by introducing a 50-V shift which reduced the difference between the model and the measurement to less than 3%. As long as material properties and geometry are well described by FE model, current-based validation can be used for a rough model validation. That is a routine assay compared with imaging of electric field, which is otherwise employed for model validation. Additionally, current estimated by model, can be preset as maximum in electroporator in order to protect tissue against damage.

Joule heating during solid tissue electroporation

The application of high-voltage pulses to biological tissue causes not only electroporation, a non-thermal phenomenon of pore creation within a lipid membrane due to an elevated electric field, but also significant heating. Once a biological membrane is porated, the current density increases several times, causing Joule heating. A combined experimental and theoretical study is reported. The theoretical temperature rise for a 1.25 kV cm(-1), 6 ms pulse is about 11.2 K for a tissue conductivity of 0.5 S m(-1) (i.e. myocardial tissue) during high-voltage application. Owing to the inhomogeneous electric field obtained with the use of needle electrodes, the temperature rises first at the electrodes, where the field strength reaches a maximum. Only for highly conductive tissue such as muscle was a temperature effect primarily observed in the bulk. Even if the temperature effect is biologically insignificant, it can affect the creation of stabile aqueous pathways by electroporation. The calculation of temperature distribution during high-voltage application, taking the electric field strength and the heat transfer into account, can be a useful tool for electrode optimisation.