Research frontiers of electroporation-based applications in cancer treatment: a bibliometric analysis

Background

Electroporation, also called electropermeabilization, is a biophysical phenomenon referring to high electric fields induced formation of a swarm of nanoscale aqueous pores in the lipid bilayer [1], [2], [3]. Several treatment protocols were proposed based on electroporation for its possible feasibility in cancer treatment in the past decades, and part of them have been proven to be promising for clinical trials and treatments.

On the one hand, the formation of aqueous pores breaks the barrier of the cellular membrane that separates cytoplasm and subcellular organelles from extracellular space, facilitating the transmembrane delivery of exogenous macromolecules, such as drugs [4, 5], genes [6, 7], etc. The protocol of the electroporation-based intracellular delivery of chemotherapy drugs is known as electrochemotherapy, which could alleviate the damage to normal body tissues and consequent adverse effects, and increase the absorption of cytotoxic agents into the cancer cells [3, 8], [9], [10]. Numerous pharmaceutical agents have been preclinically and clinically tested for their antitumor effect in electrochemotherapy, and bleomycin and cisplatin are the most efficient and most commonly used chemotherapeutic drugs at present. The cytotoxicity of bleomycin and cisplatin in electrochemotherapy is increased 100–5000-fold and 1.8–12.2-fold, respectively [10, 11]. Electrochemotherapy has been clinically proven to be available, safe, and efficient for different cancer histotypes, including melanoma, breast cancer, pancreatic cancer, head and neck squamous cell carcinoma, etc. [12, 13]. A great milestone in electrochemotherapy was achieved in 2006 when the standard operating procedures were enacted during the ESOPE (European Standard Operating Procedures of Electrochemotherapy) project [8]. Besides, combined with gene therapy, genetic information can be introduced into cells via aqueous pores, known as gene electrotransfer, which can be used in correcting a defective gene, introducing a gene encoded with therapeutic proteins or cytokines provoking tumor cell death [3, 14].

On the other hand, high-frequency high-intensity pulsed electric fields can trigger multiple cell death pathways, which is termed irreversible electroporation [15, 16]. It requires a higher frequency of the pulsed electric fields to penetrate into the interior of cells and higher intensity to electroporate the lipid bilayer of subcellular organelles than the electric field used in electrochemotherapy [17]. Irreversible electroporation, as a non-thermal, non-pharmacological, minimally invasive, ultra-short modality, can ablate the target lesion by piercing electrode needles without damaging the surrounding normal tissues, vessels, and nerves [18]. Distinguished from traditional tumor ablation therapies, advanced studies showed that irreversible electroporation ablation could change the local tumor immune microenvironment inhibiting tumor growth and preventing tumor recurrence, and it could activate systemic immune response suppressing tumor metastasis [15]. Clinical trials provided scientific proof and justifications for irreversible electroporation as a promising therapy for hepatocellular carcinoma, pancreatic adenocarcinoma, prostate cancer, kidney tumor, and so on [15, 19].

Bibliometric analysis can be employed in a wide range of disciplines and literature, which enables researchers to measure scientific progress and uncover emerging trends. Herein, we collected the published articles and reviews about electroporation-based techniques in cancer treatment during 2001–2021 from the Web of Science Core Collection. By using the bibliometrics analysis software, CiteSpace 6.1.R2 [20], this study quantified the thematic patterns and topics for researchers to outline the research status and predict emerging trends of electroporation-based cancer therapies.

Bibliometric analysis

Data collection

To analyze the research network mapping, the bibliometric articles during 2001–2021 were collected from the Web of Science Core Collection through the following strategies:

1. Topic #1 = (“electroporation” OR “electropermeabilization” OR “electrochemotherapy”)

2. Topic #2=(“tumor*” OR “tumour*” OR “cancer*” OR “carcinoma*” OR “nodule*” OR “melanoma” OR “ablat*” OR “neoplasm*”)

3. Language=(English)

4. Document Type=(Article OR Review)

5. Publication Date=(2001-01-01 to 2021-12-31)

Topic #1 included all electroporation-related studies, and topic #2 included various cancer types. The * mark in the search formula presented the neglection on the subsequent letters, for example, “ablat*” can stand for “ablate”, “ablates”, “ablation”, etc. Based on topic #1 and topic #2, a total of 4,740 bibliometric records were obtained from the database of the Web of Science Core Collection, and the bibliographic information includes author(s), affiliation(s), title, keywords, abstract, citations, categories, publication date, and journal information. Excluded according to the document type (n=701) and language (n=72), 3,967 bibliometric records were then input into the bibliometrics analysis tool, CiteSpace 6.1.R2, for records pre-processing. Then, after removing duplicate records (n=1), 3,966 research and review articles in total were selected for bibliometric analysis. The process of data collection and the schematic flowchart of this study were detailed in Figure 1.

Figure 1:

The schematic flowchart illustrating the search strategy and data preprocessing. Topic #1=(“electroporation” OR “electropermeabilization” OR “electrochemotherapy”); Topic #2=(“tumor*” OR “tumour*” OR “cancer*” OR “carcinoma*” OR “nodule*” OR “melanoma” OR “ablat*” OR “neoplasm*”).

Panorama of electroporation-based cancer therapies

The number of annual publications and citations gradually grows in the past 21 years, as shown in Figure S1A. The annual publications surged 515.15 % from 2001 to 2021, indicating that electroporation-based cancer therapies have received considerably growing scholarly attention in recent years. Additionally, the number of papers published in 2022 related to this subject is 383, implying that electroporation-based cancer therapies are still a hot area of research. The increased interest in this novel method of cancer treatment also indicates that traditional cancer therapies (chemotherapy or radiotherapy, for instance) can no longer cope with the ever-growing new cancer cases and cancer deaths due to their unsatisfactory curative effect and undesirable side effect [23].

Figure S1B shows the disciplinary category distribution of the search results. Oncology (26 %) and Medicine Research Experimental (12 %), of course, are the top two most outstanding disciplines. Experimental research on the underlying mechanism of electroporation and its impact on cancer cell metabolism and tumor microenvironment paved the way to combine the established therapies and obtain a better curative effect. According to the category distribution, researchers have investigated the possibility of combining the electroporation effect with drug therapy (Biochemistry Molecular Biology, 11 %; Pharmacology Pharmacy, 6 %), radiotherapy (Radiology Nuclear Medicine Medical Imaging, 9 %), immunotherapy (Immunology, 8 %), and gene therapy (Genetics Heredity, 6 %). Therefore, to improve the clinical applications of electroporation-based technologies, medical engineering cooperation and multidisciplinary integration are indispensable.

Analysis of authors, countries, and institutions

The bibliometric maps of the top productive and influential authors are shown in Figure 2A and B. Sersa G. published 177 articles or reviews during 2001–2021, ranking as the most productive researcher in the field of electroporation-based cancer therapies, with Cemazar M. (n_publication=163) and Miklavčič D. (n_publication=135) not far behind. According to the order of citation times, Mir L. M. was the most influential researcher in this field, whose papers were co-cited 991 times, followed by Sersa G. (n_citation=641) and Davalos R. V. (n_citation=638). The top 10 authors and co-cited authors are listed in Table 1.

Figure 2:

The bibliometric network maps of authors (A), co-cited authors (B), countries (C), institutions (D), and co-cited journals (E), contributing to electroporation-based cancer therapies.

Sersa G., professor of molecular biology at the University of Ljubljana and head of the Department of Experimental Oncology at the Ljubljana Institute of Oncology, is devoted to utilizing the electroporation effect to facilitate the transmembrane delivery of chemotherapeutic agents and explores the clinical potential of electroporation for gene therapy [8, 9, 11, 13, 24, 25]. As the deputy head of the Department of Experimental Oncology at the Ljubljana Institute of Oncology, Cemazar M. is an important partner of Sersa G., who focuses on the efficient drug delivery method based on electroporation [9, 13, 25], [26], [27]. The Department of Experimental Oncology at the Ljubljana Institute of Oncology frequently cooperated with Miklavčič D., the professor at the faculty of electrical engineering of the University of Ljubljana, who is actively exploring the hypostasis and mechanism of electroporation and investigating electroporation-based treatments and therapies, including electrochemotherapy, cardiac tissue ablation by irreversible electroporation, and gene transfer for gene therapy and DNA vaccination [2, 3, 11, 13, 16, 24, 25, 28, 29]. All of them participated in the drawing-up of the standard operating procedures of electrochemotherapy during the ESOPE project, led by Mir L. M., who is the directeur de recherche, exceptional class (DRCE) in the centre national de la recherche scientifique (the French National Centre for Scientific Research, CNRS). Mir L. M. is a pioneer of electrochemotherapy, and his research on electrochemotherapy, genetic electro-transfection, and tissue ablation by irreversible electroporation promotes the clinical practices of electroporation-based therapies [8, 14, 30], [31], [32], [33]. As one of the most influential researchers on electroporation-based technologies in the United States, Davalos R. V., the L. Preston Wade Professor in the Virginia Tech - Wake Forest University School of Biomedical Engineering and Sciences and the department of biomedical engineering and mechanics, is a pathfinder of issue ablation with high-frequency irreversible electroporation and its impact on immune microenvironment [3, 15, 17, 19, 31, 34]. Because bibliometric analysis was conducted based on the retrieved bibliographic records from 2001 to 2021, the outstanding contributors in this field who were active and incorporated with each other before 2001 were not mentioned in detail. for example, Mir L. M. used to incorporate with Miklavčič D., Gehl J. and Sersa G. in the 20th century.

The network maps of the top countries and institutions are shown in Figure 2C and D, and the detailed rankings are listed in Table 1. The top five countries with the most publications are USA (n_publication=1,306), China (n_publication=499), Italy (n_publication=429), Germany (n_publication=334), and Slovenia (n_publication=287). The number of papers published in the United States accounted for 33 % of the total number of retrieved literatures. The top five institutions with the most publications are University of Ljubljana (n_publication=210), Institute of Oncology Ljubljana (n_publication=150), Old Dominion University (n_publication=94), University of Primorska (n_publication=90), and University of California Berkeley (n_publication=70). The link line between the two nodes indicates there was a collaborative relationship between the two authors, countries, or institutions.

Analysis of journals and cited journals

Table 2 lists the top 10 productive and influential journals in the field of electroporation-based cancer therapies. Bioelectrochemistry was the journal that published the largest number of articles and reviews related to the subject (n_publication=105), followed by PLOS ONE (n_publication=81) and Scientific Reports (n_publication=77). Regarding the citation numbers as the index of importance, Cancer Research was the core journal in the field of electroporation-based cancer therapies (n_citation=1,559).

Several scientific studies published in Cancer Research are seen as the cornerstones of electroporation-based cancer therapies and the forerunners of advanced methods of treating cancer. Guo et al. [35] delivered intense electrical pulses to ablate the hepatocellular carcinoma tumor transplanted on Sprague-Dawley rats, whose result suggested irreversible electroporation can be an effective strategy for targeted ablation of liver tumors. Frandsen et al. [36] proposed an approach that utilized electroporation to introduce high calcium concentrations into cells, leading to acute and severe ATP depletion, and proved the therapeutic effect in vitro and in vivo. Poirot et al. [37] merged the electroporation-based technology with chimeric antigen receptor T-cell therapy (CAR-T Cell Therapy), allowing highly efficient multiplex gene editing in primary human T cells. In addition, PLOS ONE was also an important journal in the field of electroporation-based cancer therapies (n_citation=1,400), in which Mir LM (from CNRS), Davalos RV (from Virginia Tech), and Rubinsky B (from Univ Calif Berkeley) cooperatively published the guiding article about tumor ablation with irreversible electroporation [38]. Proceedings of the National Academy of Sciences was another important journal that published several groundbreaking scientific studies on novel cancer treatment methods based on electroporation. For instance, Schumann et al. [39] introduced single-guide RNA ribonucleoproteins (Cas9 RNPs) to modify the target genome of human CD4+ T cells, which can suppress the cell-surface expression of PD-1, an “immune checkpoint” cell-surface receptor that can inhibit effective T-cell-mediated clearance of cancers.

Analysis of references

The co-citation network map of co-cited references is shown in Figure 3A, and the top 10 co-cited references are listed in Table 3. They can be simply divided into two main cancer treatment protocols, tumor ablation with irreversible electroporation and electrochemotherapy.

Figure 3:

The bibliometric analysis of co-cited references (A) and co-occurring keywords (B). The cluster analysis according to the basic concepts of collected bibliographic records (C), and its timeline view (D).

Most of the references listed in Table 3 are related to irreversible electroporation. Reference [40] is a multicenter clinical trial on the efficacy of irreversible electroporation on locally advanced pancreatic cancer (LAPC), one of the deadliest cancers on the earth. Median overall survival was extended from 12 months to 24.9 months [48]. Reference [41] is a single-center clinical trial performed to investigate the safety of irreversible electroporation for advanced malignancy of the liver, kidney, or lung. 46 of the 69 separate tumors were completely targetedly ablated, while 8 of the 38 patients suffered minor adverse events. Reference [43, 47] both investigate the safety and efficacy of irreversible electroporation on hepatic tumors in clinical trials. Reference [43] reports that the local recurrence free survival (LRFS) for ablated lesions at 6 months was 94.6 %, and Reference [47] reports that there were only 4 of 65 tumors with persistent disease or recurrence, proving irreversible electroporation is a safe and efficacious treatment for hepatic tumors in proximity to vital structures. Reference [46] is one of the pioneering scientific studies on irreversible electroporation, which preclinically demonstrated the feasibility of irreversible electroporation tissue ablation methodology, and propose an ablation protocol through a mathematical prediction with the Laplace equation. Reference [18] gives a systematic review on the safety or efficacy of irreversible electroporation tumor ablation with 16 clinical studies, 221 patients, and 325 tumors. In their reviewed trials, no major adverse events were reported, the tumor complete ablation rate was significantly increased, and the overall survival of patients was prolonged. Reference [15], on the other hand, reviews the basic mechanism of irreversible electroporation and the experimental studies in vitro and in vivo (preclinical animal experiments and clinical trials), providing a guiding reference for future practice with irreversible electroporation on treatment protocol planning, electrode delivery design, and postoperative injury evaluation.

References related to electrochemotherapy included [44, 45]. Reference [44] is an update of the previous standard operating procedures for electrochemotherapy of cutaneous tumors and skin metastases proposed during the ESOPE project. Reference [45] utilizes the statistical data analysis using two-sided common statistical methods and meta-analysis to perform a systematic bibliometric analysis, consolidating the knowledge about the clinical effectiveness of electrochemotherapy.

Reference [42] comprehensively reviews the fundamental principles of electroporation, medical applications of electroporation, including electrochemotherapy, tissue ablation, gene therapy, transdermal drug delivery, and treatment planning protocols.

Burst detection function in Citespace identifies the citation frequency surge during a short time interval [49]. In this study, citation bursts were also extracted to tell the research trends of electroporation-based cancer therapies to some extent, which can indicate the research interests and future trends. The top 10 references with the strongest citation bursts are listed in Table 4, in which the burst strength is positively related to the citation surge per unit time. In Table 4, each block means a year within the duration, the beginning of a yellow block depicts when an article is published, the beginning of a red block marks the beginning of a period of the burst, and if there are no yellow ones between the blue and red blocks, the citation burst of this article starts in the same year it is published. It can be seen that there was a large overlap between the top 10 most co-cited references and the top 10 references with the strongest citation burst. Similarly, they can be divided into two main categories, irreversible electroporation, and electrochemotherapy.

Table 4:

The top 10 references with the strongest citation burst.

Reference	Citation bursts
	Strength	Begin	End	Duration (2001–2021)
Martin et al. [40]	60.2	2016	2021
Scheffer et al. [18]	48.57	2015	2019
Thomson et al. [41]	48.26	2012	2016
Yarmush et al. [42]	44.77	2015	2019
Rubinsky et al. [46]	44.27	2007	2012
Cannon et al. [43]	42.53	2014	2018
Sersa et al. [24]	35.69	2008	2013
Kingham et al. [47]	35.63	2013	2017
Martin et al. [50]	33.39	2014	2018
Mali et al. [45]	33.36	2014	2018

The most compelling trend that probably leads to future research is the application of irreversible electroporation to pancreatic cancer, which is found to be difficult to be controlled or treated with traditional physical resection, chemotherapy, or radiotherapy [40, 47, 50]. The citation burst of “Martin et al. 2015 [40]” ends in 2021, the end of the time span of our retrieved literature, indicating that it can be a research hotspot in near future. Another leading application of irreversible electroporation is the nonthermal ablation of hepatic tumors. The citation bursts of “Kingham et al. 2012 [47]” and “Cannon et al. 2013 [43]” end in 2017 and 2018, respectively, which may influence the future trend of liver cancer treatment. Comparatively, it seems that the research burst of electrochemotherapy has gradually faded out of the view of researchers. That is because its efficacy and safety had been fully proved in the 2000s, and the standard operating procedures have been implemented in many countries, including France, Germany, the USA, the UK, and so on [24, 45]. Gene therapy and transdermal drug delivery, two novel theologies based on electroporation, are only mentioned in “Yarmush et al. 2014 [42]”, meaning that there is still a long way to go for these two novel theologies to final clinical applications.

Analysis of keywords

Summary and outlook

In this article, 3,967 scientific articles and reviews were collected from the Web of Science Core Collection, and these bibliographic records were analyzed by CiteSpace to trace the development of electroporation-based cancer therapies and reveal its future trend. First, we pointed out the critical journals, authors, and institutions that have been actively promoting the scientific development of electroporation-based technologies over the past decades. Then, the key articles providing the scientific foundation of electroporation-based cancer therapies were recognized, which has great reference significance. At last, we concluded the possible research directions in the near future by analyzing references and keywords. However, the topic of this study was relatively broad, so the detailed development process of a specific kind of electroporation-based cancer therapy cannot be expatiated.

Although electroporation-based cancer therapies seem promising in clinical applications, following efforts should be made to bring them from laboratories to our life:

1. No direct evidence of electroporation was witnessed. The phenomenon can only be studied with indirect indicators using fluorescent dyes as an example.

2. The radical mechanism of electroporation is still opaque. The current dominant electromechnical theory cannot universally applied to the full range of experimental observations.

3. The deep integration of education, research, production and medicine should be promoted to accelerate commercialization of electroporation-based technologies.