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 micros 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.

Percutaneous prostate cryo-ablation has become an accepted treatment for primary prostate cancer. Thermal tissue ablation based on cold, however, does have some distinct limitations. These include, variable damage at the cryo lesions margin, injury to adjacent structures such as rectum, urethra and NVB (neurovascular bundle), and long procedure time due to the need for multiple freeze thaw cycles, that have limited the acceptance of this modality. Irreversible electroporation IRE, is a new non-thermal ablation modality that uses short pulses of DC electric current to create irreversible pore in the cell membrane, thus, causing cell death. This method theoretically should have significant advantages in ablating prostate tissue. Six males dogs had their prostates treated using IRE. Pulses were applied using a DC generator that delivered pulses in the microsecond range of duration, with a variable pulse interval and voltage range. IRE probes were placed percutaneously or trans-rectally using trans-rectal ultrasound guidance. In one of the dogs, the lesions were made purposely to include the rectum, urethra, and neurovascular bundle (NVB). Subjects were followed for 1 to 14 days before sacrifice. IRE lesions in the prostate had unique characteristics compared to thermal lesions. The margins of the IRE lesions was very distinct with a narrow zone of transition from normal to complete necrosis, there was complete destruction within the IRE lesion, and rapid resolution of the lesions with marked shrinkage within two weeks. Structures such as urethra, vessels, nerves, and rectum were unaffected by the IRE application. IRE lesions have characteristics that are distinctly different than thermal lesions. The differences could be very advantageous in a clinical setting, improving the results and acceptance of prostate ablation.

BACKGROUND

Bioelectrics, an interesting new area of medicine, combines pulsed high-voltage engineering with cell biology and has many potential applications. Pulsed electric current can be used to produce irreversible electroporation (IRE) of cell membranes with resulting cell death. This process has been shown to ablate tumors in animal studies.

METHODS

A clinical trial of IRE as a tumor ablation therapy was performed at our institution. A pulsating direct current of 20 to 50 A and 500 to 3000 V was delivered into metastatic or primary tumors in the liver, kidney, or lung via needle electrodes inserted under computed tomography (CT) or ultrasound guidance. Patients required a relaxant general anesthetic. We describe some challenges presented to anesthesiologists. Guidelines for anesthesia were produced and modified as issues became apparent. The patients' charts were audited throughout.

RESULTS

We noted a number of issues. The electrical discharge produced generalized upper body muscular contractions requiring neuromuscular blockade. Two patients developed positional neuropraxia because of the extended arm position requested for CT scanning. After experimentation, we have developed a modified arm position. Some patients developed self-limiting ventricular tachycardias that are now minimized by using an electrocardiogram synchronizer. Three patients developed pneumothoraces as a result of the needle electrode insertion.

CONCLUSIONS

Relaxant general anesthesia is required for IRE of the liver, lung, and kidney. An electrocardiogram synchronizer should be used to minimize the risk of arrhythmias. Attention to the position of the arms is required to maximize CT scan quality but minimize brachial plexus strain. Simple postoperative analgesia is all that is required in most patients.

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.