Irreversible Electroporation in a Swine Lung Model
This study was designed to evaluate the safety and tissue effects of IRE in a swine lung model.
This study was approved by the institutional animal care committee. Nine anesthetized domestic swine underwent 15 percutaneous irreversible electroporation (IRE) lesion creations (6 with bipolar and 3 with 3–4 monopolar electrodes) under fluoroscopic guidance and with pancuronium neuromuscular blockade and EKG gating. IRE electrodes were placed into the central and middle third of the right mid and lower lobes in all animals. Postprocedure PA and lateral chest radiographs were obtained to evaluate for pneumothorax. Three animals were sacrificed at 2 weeks and six at 4 weeks. Animals underwent high-resolution CT scanning and PA and lateral radiographs 1 h before sacrifice. The treated lungs were removed en bloc, perfused with formalin, and sectioned. Gross pathologic and microscopic changes after standard hematoxylin and eosin staining were analyzed within the areas of IRE lesion creation.
No significant adverse events were identified. CT showed focal areas of spiculated high density ranging in greatest diameter from 1.1–2.2 cm. On gross inspection of the sectioned lung, focal areas of tan discoloration and increased density were palpated in the areas of IRE. Histological analysis revealed focal areas of diffuse alveolar damage with fibrosis and inflammatory infiltration that respected the boundaries of the interlobular septae. No pathological difference could be discerned between the 2- and 4-week time points. The bronchioles and blood vessels within the areas of IRE were intact and did not show signs of tissue injury.
IRE creates focal areas of diffuse alveolar damage without creating damage to the bronchioles or blood vessels. Short-term safety in a swine model appears to be satisfactory.
Electroporation is a new nonthermal ablative technique that is being investigated for the treatment of solid malignancies. To date, there have been few preclinical studies published for electroporation [1–6]. The technology can be applied in a reversible (RE) or an irreversible manner (IRE). In either technique, high-voltage electrical impulses are delivered to tissue in rapid, short intervals (microseconds). The result is disruption of the lipid bilayer of the cell, which creates small pores that allow molecules to enter and leave the cell; if permanent, this leads to cell dysregulation and death . IRE like thermal ablative techniques is a minimally invasive technique proposed for controlled elimination of small volumes of tissue. However, IRE kills cells through a nonthermal mediated technique. Thermal ablative techniques are limited in certain anatomic regions due to thermal sinks from larger vessels and potential thermal injury to collateral tissues, such as hollow viscera, airways, nerves, and skin . Lung tumors have been treated with thermal ablation, but collateral injury to central structures as well as prominent central thermal sinks make it less safe and efficacious in the inner third of the lung and mediastinum. To date, IRE has not been evaluated in the lung. If tumors closer to the chest wall, hilum, and mediastinum could be ablated with IRE due to lack of blood flow interference and lack of thermal injury to adjacent healthy tissue, then many patients with thoracic malignancies who are not surgical candidates may benefit . In this study, we evaluated the safety and tissue effects of IRE in a swine lung model.
Materials and Methods
This study was approved by the institutional animal care and use committee. Nine domestic swine (sus scrofula) were premedicated with glycopyrrolate (parasympatholytic) (003 mg/kg IM), Telazol (sedative) (Fort Dodge Laboratories, Fort Dodge, IA) (5 mg/kg IM), and xylazine (alpha blockade anesthetic) (2 mg/kg IM), and surgical anesthesia further induced with sodium thiopental (20 mg/kg IV) via an indwelling catheter placed in an auricular vein, as needed to intubate. The animal was then placed on a mechanical ventilator and maintained at a surgical plane of anesthesia with isoflurane (2–4%) in oxygen. A surgical plane of anesthesia was determined by using jaw tone and pedal reflex; anesthesia was increased if response to stimuli was noted. A baseline evoked motor response was obtained with a nerve stimulator (ulnar nerve). The right lateral thoracic wall was sterilely prepared for percutaneous placement of the IRE device. Because the high direct current voltages of the IRE pulses cause muscle contraction, neuromuscular blockade is necessary. Before administering pancuronium (paralytic) (0.2 mg/kg) IV, a small thoracic incision was made at the insertion point of the IRE electrode. ECG, heart rate, respiratory rate, rectal temperature, pulse-oximetry, and end-tidal CO2 were monitored and documented at least every 15 min. Evoked motor response was monitored during the interval of muscle relaxation and was continued until full muscle function was restored. The muscle relaxation was reversed, if necessary, with a mixture of edrophonium (0.5 mg/kg) and atropine (0.005 mg/kg) IV.
Treatment parameters for IRE
Pulse length (μs)
Maximum ampere range
Irreversible electroporation results in tissue necrosis presumably due to apoptotic cell death. Previous reports suggest a narrow transition between the zone of tissue necrosis and healthy tissue. We have shown similar findings in swine lung where the IRE lesion is demarcated by the interlobular septae and the bronchioles and blood vessels within the affected region were not damaged. Importantly, tissue adjacent to these major vascular structures showed tissue injury, suggesting a potential major advantage over currently available thermal ablation technologies whereby thermal sinks can affect the ablation zone. This tissue healing effects seen in lung is different than what Lee and colleagues observed in liver whereby healing appears more complete . The differences may be related to the less sterile respiratory environment in lung tissue in the swine model compared with the liver. Lee and colleagues showed larger IRE ablation zones averaging 3.4 cm in greatest diameter in swine liver tissue with preservation of blood vessels and bile ducts . The larger ablation zone in their study can be explained by better electrical current conduction in solid tissue compared with aerated lung. In fact in our study the applied current was typically much lower, whereas in liver tissue IRE the current deposition is usually higher. Demarcation of the IRE lesion by the interlobular septae may be due to the underlying healing process or due to a relative insulating effect of the IRE energy by the interlobular septae. Compared with more acute (less than 24 h) IRE, lesions in lung tissue would help to determine which effect is involved. There are no human or animal data in lung tissue for comparison.
Miller et al. performed IRE in vitro on human hepatocarcinoma cells . They reported complete cancer cell ablation with the application of 1500 V/cm in three sets of ten pulses of 300 ms. Other investigators have reported success with IRE in cutaneous tumors implanted in mice, dog prostate, and pig liver [10–12]. At the margin of an IRE lesion, surrounding cells may undergo RE. In theory this effect may be used to augment chemotherapy for certain tumors. RE has been used to promote uptake of chemotherapy into tumor cells: electropermeabilization. Allegretti and Panje reported the use of electroporation with intralesional bleomycin for the treatment of 14 patients with head and neck cancers . In this series, six patients had a complete response, six patients had a partial response, and two patients did not respond, for an overall response rate of 86%. They reported no treatment-related deaths and a low procedure-related morbidity.
There are multiple limitations to this study. First, we only studied the 2-and 4-week time points, therefore, earlier and more delayed pathological changes are unknown. Second, we did not use CT guidance to place the electrodes, so our placement was not as central as we would have liked, and the range of distances from the individual monopolar electrode tips was greater than we would have liked. Monopolar tip distances as large as 2.0 cm may have accounted for the smaller lesions seen with the monopolar IRE. Third, lesion visualization can be difficult after fixation, and size measurement can be underestimated due to fixation and lung deflation.
Our study showed relatively small treatment effects in aerated lung, which may make obtaining a margin in human lung tumors difficult. Combining chemotherapy to IRE in human lung tumors in anatomic areas that are too difficult to treat with radiotherapy due to previous radiotherapy or standard thermal ablation techniques may overcome this effect. Future human lung tumor trials looking at the safety and effectiveness of IRE should be performed.
IRE creates focal areas of diffuse alveolar damage without creating damage to the bronchioles or blood vessels. Short-term safety in a swine model appears to be satisfactory. Continued study in human tumors is necessary.
Conflict of interest
Damian E. Dupuy, M.D. received grant support from Angiodynamics (Latham, NY) for this study and has received speaking honoraria from Angiodynamics.