In vivo results of a new focal tissue ablation technique: irreversible electroporation

The effect of high frequency steep pulsed electric fields on in vitro and in vivo antitumor efficiency of ovarian cancer cell line skov3 and potential use in electrochemotherapy

BACKGROUND

Patients received electrochemotherapy often associated with unpleasant sensations mainly result from low-frequency electric pulse induced muscle contractions. Increasing the repetition frequency of electric pulse can reduce unpleasant sensations. However, due to the specificity of SPEF, frequency related antitumor efficiency need to be further clarified. The aim of this study was to compare in vitro cytotoxic and in vivo antitumor effect on ovarian cancer cell line SKOV3 by SPEF with different repetition frequencies. Explore potential benefits of using high frequency SPEF in order to be exploitable in electrochemotherapy.

METHODS

For in vitro experiment, SKOV3 cell suspensions were exposed to SPEF with gradient increased frequencies (1, 60, 1000, 5000 Hz) and electric field intensity (50, 100, 150, 200, 250, 300, 350, 400 V/cm) respectively. For in vivo test, SKOV3 subcutaneous implanted tumor in BALB/c nude mice (nu/nu) were exposure to SPEF with gradient increased frequencies (1, 60, 1000, 5000 Hz) and fixed electric field intensity (250 V/cm) (7 mice for each frequency and 7 for control). Antitumor efficiency was performed by in vitro cytotoxic assay and in vivo tumor growth inhibition rate, supplemented by histological and TEM observations. Data were analyzed using one-way ANOVA followed by the comparisons of multiple groups.

RESULTS

SPEF with a given frequency and appropriate electric field intensity could achieve similar cytotoxicity until reached a plateau of maximum cytotoxicity (approx. 100%). SPEF with different frequencies had significant antitumor efficiency in comparison to the control group (P < 0.05). However, there was no difference in tumor responses among test groups (P > 0.05). Histological and TEM observations demonstrated obvious cell damages in response to SPEF exposure. Furthermore, SPEF with 5 kHz could induce apoptosis under TEM observations both in vitro and in vivo.

CONCLUSION

SPEF with high frequency could also achieve similar antitumor efficiency which can be used to reduce unpleasant sensations in tumor electrical treatment. Our research proposed potential applications of using high frequency SPEF in clinical cancer treatment.

Non thermal irreversible electroporation: novel technology for vascular smooth muscle cells ablation

Background

Non thermal Irreversible electroporation (NTIRE) is a new tissue ablation method that induces selective damage only to the cell membrane while sparing all other tissue components. Our group has recently showed that NTIRE attenuated neointimal formation in rodent model. The goal of this study was to determine optimal values of NTIRE for vascular smooth muscle cell (VSMC) ablation.

Methods and Results

33 Sprague-Dawley rats were used to compare NTIRE protocols. Each animal had NTIRE applied to its left common carotid artery using a custom-made electrodes. The right carotid artery was used as control. Electric pulses of 100 microseconds were used. Eight IRE protocols were compared: 1–4) 10 pulses at a frequency of 10 Hz with electric fields of 3500, 1750, 875 and 437.5 V/cm and 5–8) 45 and 90 pulses at a frequency of 1 Hz with electric fields of 1750 and 875 V/cm. Animals were euthanized after one week. Histological analysis included VSMC counting and morphometry of 152 sections. Selective slides were stained with elastic Van Gieson and Masson trichrome to evaluate extra-cellular structures. The most efficient protocols were 10 pulses of 3500 V/cm at a frequency of 10 Hz and 90 pulses of 1750 V/cm at a frequency of 1 Hz, with ablation efficiency of 89±16% and 94±9% respectively. Extra-cellular structures were not damaged and the endothelial layer recovered completely.

Conclusions

NTIRE is a promising, efficient and simple novel technology for VMSC ablation. It enables ablation within seconds without causing damage to extra-cellular structures, thus preserving the arterial scaffold and enabling endothelial regeneration. This study provides scientific information for future anti-restenosis experiments utilizing NTIRE.

Activation of transgene expression in skeletal muscle by focused ultrasound

To correlate thermal dose from focused ultrasound (FUS) with gene expression and tissue injury, a temperature plateau strategy was employed. Plasmids encoding luciferase gene under the control of hsp70B promoter were transfected into the right gastrocnemius muscle in a rat via electroporation. One day after transfection, hind limbs were treated with 3.3-MHz focused ultrasound, using one of four different temperature plateaus with spatial-peak time-average focal temperatures (T(SPTA)) of 46 degrees C, 48 degrees C, 51 degrees C and 62 degrees C. The treatment duration at the plateau temperature was varied from 0 to 30s. Gene expression was analyzed in vivo one day following FUS treatment, and H&E staining was employed to assess tissue injury. Gene activation and tissue damage correlated closely with thermal dose. The highest level of gene activation was induced by FUS at T(SPTA)=51 degrees C for 20s, which was found to be statistically equivalent to that produced by water-bath hyperthermia.

Irreversible electroporation attenuates neointimal formation after angioplasty

Restenosis following coronary angioplasty represents a major clinical problem. Irreversible electroporation (IRE) is a nonthermal, nonpharmacological cell ablation method. IRE utilizes a sequence of electrical pulses that produce permanent damage to tissue within a few seconds.

METHODS AND RESULTS

The left carotid arteries of eight rats underwent in vivo intimal damage using two Fogarty angioplasty catheters. The procedure was immediately followed by IRE ablation in four rats, while the remaining four were used as the control group. The IRE ablation was performed using a sequence of ten dc pulses of 3800 V/cm, 100 micros each, at a frequency of ten pulses per second, applied across the blood vessel between two parallel electrodes. The electrical conductance of the treated tissue was measured during the electroporation to provide real-time feedback of the process. Left carotid arteries were excised and fixated after a 28-day follow-up period. Neointimal formation was evaluated histologically. The use of IRE was successful in three out of four animals in a way that is consistent with the measurements of blood vessel electrical properties. The integrity of the endothelial layer was recovered in the IRE-treated animals, compared with control. Successful IRE reduced neointima to media ratio (0.57 +/-0.4 versus 1.88 +/-1.0, P = 0.02).

CONCLUSIONS

We report for the first time the in vivo results of attenuation of neointimal formation using IRE. Our study shows that IRE might be able to attenuate neointimal formation after angioplasty damage in a rodent model of restenosis. This approach may open new venues in the treatment of coronary artery restenosis after balloon angioplasty.

Tumor ablation with irreversible electroporation

We report the first successful use of irreversible electroporation for the minimally invasive treatment of aggressive cutaneous tumors implanted in mice. Irreversible electroporation is a newly developed non-thermal tissue ablation technique in which certain short duration electrical fields are used to permanently permeabilize the cell membrane, presumably through the formation of nanoscale defects in the cell membrane. Mathematical models of the electrical and thermal fields that develop during the application of the pulses were used to design an efficient treatment protocol with minimal heating of the tissue. Tumor regression was confirmed by histological studies which also revealed that it occurred as a direct result of irreversible cell membrane permeabilization. Parametric studies show that the successful outcome of the procedure is related to the applied electric field strength, the total pulse duration as well as the temporal mode of delivery of the pulses. Our best results were obtained using plate electrodes to deliver across the tumor 80 pulses of 100 µs at 0.3 Hz with an electrical field magnitude of 2500 V/cm. These conditions induced complete regression in 12 out of 13 treated tumors, (92%), in the absence of tissue heating. Irreversible electroporation is thus a new effective modality for non-thermal tumor ablation.

The effect of irreversible electroporation on blood vessels

We present a pilot study on the long term effects of irreversible electroporation (IRE) on a large blood vessel. The study was motivated by the anticipated use of IRE for treatment of cancer tumors abutting large blood vessels. A sequence of 10 direct current IRE pulses of 3800 V/cm, 100 micros each, at a frequency of 10 pulses per second, were applied directly to the carotid artery in six rats. Measuring tissue conductivity during the procedure showed, as predicted, an increase in conductivity during the application of the pulse, which suggests that this measurement can be used to control the application of IRE. All the animals survived the procedure and showed no side effects. Histology performed 28 days after the procedure showed that the connective matrix of the blood vessels remained intact and the number of vascular smooth muscle cells (VSMC) in the arterial wall decreased with no evidence of aneurysm, thrombus formation or necrosis. Average VSMC density was significantly lower following IRE ablation compared with control (24 +/- 11 vs. 139 +/- 14, P<0.001), with no apparent damage to extra cellular matrix components and structure. In addition to the relevance of this study to treatment of cancer near large blood vessels these findings tentatively suggest that IRE has possible applications to treatment of pathological processes in which it is desired to reduce the proliferation of VSMC population, such as restenosis and for attenuating atherosclerotic processes in clinical important locations such as coronary, carotid and renal arteries.

Imaging guided percutaneous irreversible electroporation: ultrasound and immunohistological correlation

Preliminary results of percutaneous irreversible electroporation (PIE) on swine liver as a novel non-thermal ablation are presented. The goal of this study was to evaluate the feasibility of using irreversible electroporation in more clinically applicable manner, a percutaneous method, and to investigate a possible role of apoptosis in PIE-induced cell death. We performed PIE on four swine livers under real-time ultrasound guidance. The lesions created by PIE were imaged with ultrasound and were correlated with histology data, including pro-apoptotic marker. A total of 11 lesions were created with a mean size of 16.8 cm(3) in 8.4 +/- 1.8 minutes. Real-time monitoring was performed and a correlation of (+) 2 +/- 3.2 mm in measurement comparison between ultrasound and gross pathologic measurements was demonstrated. Complete hepatic cell death without structural destruction, unaffected by heat-sink effect, and with a sharp demarcation between the ablated zone and the non-ablated zone were observed. Immunohistological analysis confirmed complete apoptotic cell death by PIE on Von Kossa, BAX, and H&E staining. In summary, PIE can provide a novel and unique ablative method with real-time monitoring capability, ultra-short procedure time, non-thermal ablation, and well-controlled and focused apoptotic cell death.

Towards solid tumor treatment by irreversible electroporation: intrinsic redistribution of fields and currents in tissue

Local and drug-free tissue treatment by irreversible electroporation (IRE) involves the creation of aqueous pores in a cell's plasma membrane (PM) and leads to non-thermal cell death by necrosis. To investigate explicit pore-based effects we use two-dimensional system models with different spatial scales. The first is a multicellular system model (spatial scale 100 mum) that has irregularly shaped cells, and quantitatively describes dynamic (creation and destruction, evolution in pore size) pore behavior at the PM. The second is a tissue model (spatial scale 200 mm) that is constructed from a unit cell and uses the asymptotic (fixed pore size) electroporation model. Both system models show that significant redistribution of fields and currents occurs through transient PM pores. Pore histograms for the multicellular model demonstrate the simultaneous presence of small and large pores during IRE pulses. The associated significant increase of PM permeability may prove to be essential to understanding how cell death by necrosis occurs. The averaged tissue conductivity in both models increases during IRE pulses because of electroporation. This leads to greater electrical dissipation (heating) and, thus, to larger temperature increases than suggested by tissue models with passive and static electrical properties.

Irreversible electroporation in medicine

This is a brief introduction to the emerging field of irreversible electroporation in medicine. Certain electrical fields when applied across a cell can have as a sole effect the permeabilization of the cell membrane, presumable through the formation of nanoscale defects in the cell membrane. Sometimes this process leads to cell death, primarily when the electrical fields cause permanent permeabilization of the membrane and the consequent loss of cell homeostasis, in a process known as irreversible electroporation. This is an unusual mode of cell death that is not understood yet. While the phenomenon of irreversible electroporation may have been known for centuries it has become only recently rigorously considered in medicine for various applications of tissue ablation. A brief historical perspective of irreversible electroporation is presented and recent studies in the field are discussed.

A study of the immunological response to tumor ablation with irreversible electroporation

Immune cell recruitment during the treatment of sarcoma tumors in mice with irreversible electroporation was studied by immunohistochemistry. Irreversible electroporation is a non-thermal tissue ablation technique in which certain short duration electrical fields are used to permanently permeabilize the cell membrane, presumably through the formation of nanoscale defects in the membrane. Employing irreversible electroporation parameters known to completely ablate the tumors without thermal effects we did not find infiltration of immune cells probably because of the destruction of infiltration routes. We confirm here that immune response is not instrumental in irreversible electroporation efficacy, and we propose that irreversible electroporation may be, therefore, a treatment modality of interest to immunodepressed cancer patients.

Methods of optimization of electrical impedance tomography for imaging tissue electroporation

Tissue electroporation is a medical technique in which electrical pulses of microsecond to millisecond length are applied to a tissue in order to permeabilize the membrane of targeted cells, either temporarily or permanently, for the purpose of drug delivery and gene therapy or tissue ablation, respectively. Electrical impedance tomography (EIT) has been suggested as an effective means of imaging the treated area and thereby providing control of electroporation. In this simulation based study we introduce methods for optimizing the use of EIT under the special conditions of electroporation. First, we address the issue of the rapid changes in tissue conductivity, during and after the application of pulses. We propose a solution through a method of simultaneously collecting data from all the electrodes, essentially capturing the state of the tissue at a single instant. This method, which employs several distinct frequencies, one for each electrode, allows a speedy and continuous collection of data, a vital part of real-time electroporation monitoring. The second issue is taking advantage of the presence of electroporation electrodes for the EIT process. We show how the electroporation electrodes that are normally found inside the tissue may help improve the reconstruction compared to data collected only from the body's boundary. This mathematical study employs recently collected in vivo data of rat liver electroporation to obtain a model which represents, as closely as possible, the reality of electroporation procedures.

Electrochemotherapy in treatment of tumours

AIM

Electrochemotherapy is a local drug delivery approach aimed at treatment with palliative intent of cutaneous and subcutaneous tumour nodules of different histologies. Electrochemotherapy, via cell membrane permeabilising electric pulses, potentiates the cytotoxicity of non-permeant or poorly permeant anticancer drugs with high intrinsic cytotoxicity, such as bleomycin or cisplatin, at the site of electric pulse application.

METHODS

An overview of preclinical and clinical studies is presented, and the treatment procedure is further critically evaluated.

RESULTS

In clinical studies electrochemotherapy has proved to be a highly efficient and safe approach for treating cutaneous and subcutaneous tumour nodules. The treatment response for various tumours (predominantly melanoma) was approximately 75% complete and 10% partial response of the treated nodules.

CONCLUSIONS

Electrochemotherapy is a new, clinically acknowledged method for the treatment of cutaneous and subcutaneous tumours. Its advantages are high effectiveness on tumours with different histologies, simple application, minimal side effects and the possibility of effective repetitive treatment.

A novel nonthermal energy source for surgical epicardial atrial ablation: irreversible electroporation

BACKGROUND

All currently used energy sources in surgical ablation for atrial fibrillation create lesions via thermal injury. We report for the first time the in vivo results of a new nonthermal modality of epicardial atrial ablation called irreversible electroporation (IRE). IRE utilizes a sequence of electrical pulses that produce permanent nonthermal damage to tissue in a few seconds with sharp borders between affected and unaffected regions.

METHODS

Five pigs underwent beating heart surgical epicardial ablations of their right and/or left atrial appendages, utilizing a sequence of 8, 16, or 32 direct current pulses of 1500 to 2000 V, 100 micros each, at a frequency of 5 per second, applied between two 4-cm long parallel electrodes with an IRE pulse generator. Local temperature measurements were performed during ablations followed by electrical isolation testing by pacing. Animal hearts were excised 24 hours after surgery and processed histologically to evaluate the degree of myocardial tissue necrosis and transmurality.

RESULTS

A clear demarcation line between ablated and normal tissue, with no tissue disruption or charring, was observed on gross inspection of all lesions. Staining results showed complete transmural destruction of atrial tissue at the site of the electrode application in all 10 atrial lesions, measuring a mean of 0.9 cm in depth. Each 3- to 3.5-cm long lesion was created in 1 to 4 seconds with no local temperature change and with demonstration of electrical isolation.

CONCLUSIONS

We propose a new modality to perform atrial ablations, which holds the potential of providing very swift, precise, and complete transmurality with no local heating effects.

Irreversible electroporation: a new ablation modality--clinical implications

Irreversible electroporation (IRE) is a new tissue ablation technique in which micro to millisecond electrical pulses are delivered to undesirable tissue to produce cell necrosis through irreversible cell membrane permeabilization. IRE affects only the cell membrane and no other structure in the tissue. The goal of the study is to test our IRE tissue ablation methodology in the pig liver, provide first experience results on long term histopathology of IRE ablated tissue, and discuss the clinical implications of the findings. The study consists of: a) designing an IRE ablation protocol through a mathematical analysis of the electrical field during electroporation; b) using ultrasound to position the electroporation electrodes in the predetermined locations and subsequently to monitor the process; c) applying the predetermined electroporation pulses; d) performing histopathology on the treated samples for up to two weeks after the procedure; and e) correlating the mathematical analysis, ultrasound data, and histology. We observed that electroporation affects tissue in a way that can be imaged in real time with ultrasound, which should facilitate real time control of electroporation during clinical applications. We observed cell ablation to the margin of the treated lesion with several cells thickness resolution. There appears to be complete ablation to the margin of blood vessels without compromising the functionality of the blood vessels, which suggests that IRE is a promising method for treatment of tumors near blood vessels (a significant challenge with current ablation methods). Consistent with the mechanism of action of IRE on the cell membrane only, we show that the structure of bile ducts, blood vessels, and connective tissues remains intact with IRE. We report extremely rapid resolution of lesions, within two weeks, which is consistent with retention of vasculature. We also document tentative evidence for an immunological response to the ablated tissue. Last, we show that mathematical predictions with the Laplace equation can be used in treatment planning. The IRE tissue ablation technique, as characterized in this report, may become an important new tool in the surgeon armamentarium.