Xin-Hua Chen, Stephen J Beebe and Shu-Sen Zheng

Hangzhou, China

Editorial

Tumor ablation with nanosecond pulsed electric fields

Xin-Hua Chen, Stephen J Beebe and Shu-Sen Zheng

Hangzhou, China

Liver cancer is one of the most malignant cancers. It is reported that 600 000 patients died from liver cancer every year.[1,2]Hepatocellular carcinoma (HCC) is a particular problem because symptoms are not evident until the disease has progressed and hepatitis B, which is prominent specific regions of Asia, is a common precursor of the disease. There are many liver cancer treatment methods, but each has its own limitations and it is difficult to eradicate HCC by the single application of any method. Treatments for HCC include surgical procedures such as resection and transplantation as well as image-guided tumor ablation using percutaneous therapeutic procedures. Image-guided ablations include chemical ablation with injection of ethanol or acetic acid, thermal therapies such as ablation with radiofrequencies, microwaves, lasers, or cryoablation with liquid nitrogen. Because oral chemotherapeutic agents are not very effective, image-guided transcatheter arterial chemoembolization using chemotherapeutic agents such as cisplatin or doxorubicin has been used. However, sorafenib, a tyrosine kinase inhibitor, and other targeted therapies for HCC have shown some therapeutic efficacy in liver cancer.[2]Nevertheless, tumors <5 cm are most commonly treated with ablation with radiofrequencies. Newer approaches include the use of electric fields. Electrochemotherapy uses conventional electroporation with electric field durations in the micro- to millisecond range. This causes large "pores" or defects in plasma membranes, allowing the entry of poorly permeable drugs such as cisplatin to ablate tumors. Conventional electroporation has been extended to irreversible electroporation (IRE), which uses similar pulse durations, but electric fields are high enough to destroy plasma membranes causing tumor necrosis without using drugs. More recently, nanosecond pulsed electric fields (nsPEFs) extend conventional electroporation by using electric pulses with shorter durations, in the nanosecond range, and even higher electric fields than IRE, in the kilovolt per centimeter (kV/cm) range, e.g. 40 kV/cm electrical pulses to treat melanomas[3-5]or 35, 50 and 68 kV/cm to treat HCC.[6,7]

Nanosecond pulses are a high technology used by the military for the immediate release of high power. In the last 10 years, researchers have applied this pulse power technology to biology and medicine, including the elimination of tumors. It was first used to reduce mouse fibrosarcoma tumor size and was shown to induce several markers for apoptotic cell death.[8]A number of studies have confirmed these observations, showing that nsPEFs do induce apoptosis in a number of cancer cell typesin vitro[9-13]and tumorsin vivo.[3]The potential for nsPEFs to ablate tumors was confirmed by showing that they can eliminate melanoma,[3,4]HCC and basal cell carcinoma.[14]

A nanosecond is one billionth of a second. When stored electrical charges are released in ultra-short times, the power is very high, but the energy density is low and thereby non-thermal. Unlike electroporation, which primarily targets plasma membranes, nsPEFs target sub-cellular membranes as well as plasma membranes. Modeling[15,16]and experimental[17]evidence indicates that nsPEFs induce "nanopores" in cell membranes, limiting transport to ions and molecules smaller than a nanometer. This is referred to as supra-electroporation with large numbers of small pores in all cell membranes.[15,16]The acting time of nsPEFs is so short that they are faster than membrane charging times. This allows electric fields to affect intracellular membranes. Effects have been reported on plasma membranes[17]and intracellular membranes in the cytoskeleton,[11]endoplasmic reticulum,[10,18,19]mitochondria,[12]and nuclear DNA.[3,4,8]Plasma mem-brane resistance decreases and cells are depolarized. Intracellular calcium is mobilized, not only through plasma membranes, but also from the endoplasmic reticulum. Likewise, the mitochondrial membrane is depolarized and nuclear effects include DNA damage as well as effects on small nuclear ribonucleoproteins (nuclear speckles), suggesting changes in RNA-protein complexes.[20]In addition to direct electric field effects, nsPEFs stimulate secondary, downstream effects. Given this wide range of nsPEF-induced effects on cell structures, it has been of interest to determine which primary and/or secondary determinants change cell functions, such as apoptotic cell death.

Nanosecond pulses also damage tumor and host blood vessels that are within sufficiently high electric fields.[4]As a result, tumor-derived blood vessel growthstimulating factors such as VEGF and PD-ECGF decrease as do microvascular density markers.[5-7]The balance between pro-angiogenic and anti-angiogenic cytokines is affected and new vessel formation is inhibited, all of which contribute to tumor self-destruction.[4]

Tumor ablation with nsPEFs has specific advantages for tumor ablation. (1) Because pulse durations are so short, there is no heat accumulation, only electric field effects, thereby avoiding thermal injury; (2) nsPEFs have well-defined treatment zones with localized effects determined by electrode placement, thereby minimizing adjacent and systemic side effects; (3) unlike IRE, the use of nsPEFs does not require paralytic agents, because muscle contractions are mostly eliminated; (4) nsPEFs can be used alone, which also avoids the systemic side effects of chemotherapy drugs; (5) unlike electroporation, nsPEF effects are independent of tumor size or shape, equally effecting all heterogeneous tumor cells; (6) nsPEFs target multiple, well-characterized cancer therapeutic processes including apoptotic cell death and anti-angiogenesis/anti-vessel effects; and (7) since nsPEFs do not eliminate cells based on proliferation rates like chemotherapeutic agents, they not only eliminate cancer cells, but also eradicate cancer stem cells and host cells that collaborate with tumors.

Because of the remarkable advantages of nsPEFs, an image-guided interventional nsPEF device has been designed to treat human malignant tumors. AngioDynamics has announced the first patient treatment and growing enrollment in a pilot study of the NanoKnife in the treatment of early stages of HCC. As of 2011, physicians have treated 538 patients in 11 medical centers around the world. Procedures have been performed on many organs, including the prostate, liver, lung and pancreas. But the new cancer-treatment device that uses nsPEFs to zap tumors needs even larger randomized clinical trials to build up evidence that demonstrates its efficacy and safety for the treatment of specific cancers.

nsPEFs are a novel and promising physical ablation treatment. However, many questions remain to be answered. nsPEFs do not exist under natural conditions; therefore cells must have default responses to them. One response, albeit not the only one, is cell death, in part by apoptosis. Events that initiate cell death and their mechanisms remain to be determined. How to achieve the optimal treatment parameters also remains to be determined. A significant effort in basic and pre-clinical research needs to be pursued.

Current medicine has entered the translational era. Medical research should address clinical applications and nsPEF treatment for malignant tumors should be tested to accumulate medical evidence based on preclinical and clinical trials. nsPEFs add a new, effective, and safe therapeutic strategy for tumor therapy.

Acknowledgements:We thank Mr. Frank Reidy for proof-reading.Contributors:ZSS proposed the study. CXH wrote the first draft. BSJ polished the manuscript. All authors contributed to the reviews to further drafts. ZSS is the guarantor.

Funding:This study was supported by grants from the National Natural Science Foundation of China (3070078), a National S&T Major Project (2012ZX10002017), the National Basic Research Program of China (973 Program) (2009CB522403) and Zhejiang Medical Research Funding (2008B079).

Ethical approval:Not needed.

Competing interest:No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article.

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November 25, 2011

Accepted after revision January 3, 2012

Author Affiliations: Division of Hepatobiliary Pancreatic Surgery, First Affiliated Hospital, Zhejiang University School of Medicine, Hangzhou 310003, China (Chen XH and Zheng SS); Frank Reidy Research Center for Bioelectrics, Old Dominion University, Norfolk, VA 23508, USA (Beebe SJ)

Shu-Sen Zheng, MD, PhD, FACS, Division of Hepatobiliary Pancreatic Surgery, First Affiliated Hospital, Zhejiang University School of Medicine, Hangzhou 310003, China (Tel: 86-571-87236601; Fax: 86-571-87236601; Email: shusenzheng@zju.edu.cn)

? 2012, Hepatobiliary Pancreat Dis Int. All rights reserved.

10.1016/S1499-3872(12)60135-0