The 100 Most Influential Studies in Chimeric Antigen Receptor T-Cell: A Bibliometric Analysis

Background: Bibliometric analyses are used to provide information on trends within a specific research field, along with indicators of the impact of a publication. With such an analysis, we map the scientific landscape of chimeric antigen receptor T-cell (CAR-T) research to see the emerging topics and infer directions the field might take. Methods: We extracted the 100 most-cited articles, published all periods (from 2008 to 2019) by the Web of Science Core Collection. Using their bibliographic details, including year of publication, country of author, research organization, author information, and keywords, we graph the networks created between the articles. Results: Of the 100 papers identified, the majority (93%) were written in the USA. Notable was that 34 papers were published from the University of Pennsylvania. Regarding authors, Carl H. June participated in 29 researches, followed by Bruce L. Levine who participated in 12. As for journals, Blood (n = 19) published the most papers, followed by Science Translational Medicine (n = 9) and Cancer Research (n = 9). Lastly, the most frequently used keywords were “adoptive immunotherapy” (n = 47), “lymphocytes” (n = 27), and “antitumor activity” (n = 22). Conclusion: By evaluating the top 100 most-cited papers in the CAR-T field, this study provides insight into the direction of the scientific growth and its trends, as well as information on the field's network structure.


INTRODUCTION
Since chimeric antigen receptors (CARs) were genetically engineered to express on T-cells three decades ago (1), they have become one of the most promising targeted immunotherapy research interests. These CAR T-cells (CAR-Ts) are modified to express cancer antigen-recognizing CARs and to stimulate the immune system (2)(3)(4). To produce CAR-Ts, leukapheresis is performed in the patient's blood: first, the T-cell is extracted (selection and activation phase) with the virus vector (retrovirus/lentivirus vector), and then the host T-cell is injected with the unique cancerspecific CAR DNA (CAR transduction) for cell proliferation (expansion). It is critical that these methods, such as the handling of the virus and the quality of the CAR-T, are properly verified as these manufactured CAR-Ts are reinfused into the patient, and any errors in selection may compromise their health. The success of these early researches and trials sets the basis for a larger clinical trial in a CD19-targeted CAR-T therapy called tisagenlecleucel (Kymriah TM ) for children and adolescents with acute lymphoblastic leukemia. Based on the clinical trial results, the Food and Drug Administration (FDA) approved tisagenlecleucel as the first treatment in August 2017. Currently, only two multinational pharmaceutical companies (Novartis and Gilead) have FDA approval for the treatment of some hematologic disorder ( Table 1).
The CAR-T studies registered at ClinicalTrials.gov as ongoing (i.e., not yet recruiting, recruiting, enrolling by invitation, and active but not recruiting) number over 300. Among them are studies that look into the development of new combination therapeutic options for malignant blood cancer, but also solid tumors, and other immunotherapies (5)(6)(7). In many therapeutic areas, immunotherapy using CD19-targeted CAR-T therapy is being introduced as a new and promising treatment for systemic lupus erythematosus (SLE), regulatory rheumatoid factor (regRF) bring into being lymphocytes also use therapeutic targets in rheumatoid arthritis (8)(9)(10).
Not only has the number of CAR-T experiments been growing, but over 3,000 articles regarding CAR-Ts have been published according to Clarivate Analytics' Web of Science Core Collection (WoSCC; www.webofknowledge.com). The WoSCC online database provides systematic literature information, including data under the scope of the Science Citation Index (SCI) and the Science Citation Index Expanded (SCIE), Social Science Citation Index (SSCI), Arts and Humanity Citation Index (A&HCI), and the Emerging Sources Citation Index (ESCI) (11).
Typically, citations express an author's consent to a study's presented insights, findings, and interpretations presented. Thus, citation analysis, as a quantitative bibliometric method, can be used to provide information on a study's trends, as well as an objective index of the scientific effect of publications through their citation frequency within a specific field (9)(10)(11). Consequently, the most-cited articles are analyzed through their bibliometrics to understand the direction of scientific growth and flow of the study area (12)(13)(14).
In this study, a bibliometric network analysis will be conducted on the 100 most-cited publications in the CAR-T area. Using their bibliographic information (i.e., year of publication, country, funding, institution, author information, and keywords), we can provide insights on the target cells and genes typically studied in the field, indications for major studies, and any hot topics regarding CAR-Ts after analyzing the simultaneous exposure and frequency of the core keywords.   Identification of predictive biomarkers for cytokine release syndrome after chimeric antigen receptor T-cell therapy for acute lymphoblastic leukemia  Monocyte-derived IL-1 and IL-6 are differentially required for cytokine-release syndrome and neurotoxicity due to CAR T cells  Phase I hepatic immunotherapy for metastases study of intra-arterial chimeric antigen receptor-modified T-cell therapy for CEA(+) liver metastases   (11). b ACPY, average citations per year (average number of times a record has been cited) (11).
Also, the information yielded may allow us to determine potential new study fields, the opportunities available to researchers, and leading funding organizations.

METHODS
Using "chimeric antigen receptor T cell, " "CAR-T, " or "CAR T" as keywords, we found a total of 3,871 articles published from January 1, 2008, to December 31, 2019. All of them were located using the WoSCC as of May 8, 2020. Based on the target period, the top 100 cited articles were reviewed, and there were excellent techniques and quantitative growth such as immunooncology and gene therapy area. The most highly cited papers are listed in Table 2. All publication information (i.e., journal name, country, enhanced-organization information, author, title, publisher, keyword, PubMed ID and citation frequency, and references) was downloaded as Bib-Text files and then converted into the XML format. The network graph was constructed using VOSviewer 1.6.13, a software tool for visualizing and exploring network data. The VOSviewer manual by Van Eck and Waltman states that VOSviewer is a program for building and visualizing networks of scientific publications, journals, researchers, research institutions, countries, keywords, or terms. It can be used to analyze bibliographic and other types of networks (15). In this study, we used the function of the VOSviewer 1.6.13 version to design the network graph. For clearer network visualization,  (11). c Impact factor is based on Journal Citation Reports in June 2019. keywords can be marked more than five times and author networks more than two times. For brevity of presentation, only information on the top 10 is included in the tables. This noninterference study conducted a bibliometric analysis not involving any human subjects; thus, no approval was required from any institutional review board or ethics committee.

RESULT
The publication rate and the citation frequency of the 100 mostcited publications are shown in Figure 1.  (Figure 1).
We observed that majority of the publications were original articles (80%), while the remaining were reviews (20%); all publications were released in English across 28 different journals. The top 100 most-cited articles were published by co-authors from 19 countries-most of who were from the USA (n = 93), Germany (n = 11), Italy (n = 6), the UK (n = 5), and Canada (n = 5) ( Table 3).
The results are able to note that the USA and Germany showed the most pronounced activity (Figure 2). The thickness of the link is expressed according to the number of connected nodes.
Hartmann et al. (16) corroborates this lead the USA possesses as he cites their overwhelming percentage of next-generation development and research in the rapidly developing field of  (11). CAR-T, similar to their development of new technologies and cost-intensive immunotherapies. The articles were published across 28 journals, focusing on subjects such as blood cancer, chemotherapy research, and immune-chemotherapies; all of them were located in the USA ( Table 3). Notably, it was the journals Blood (n = 19), Cancer Research (n = 9), and Science Translational Medicine (n = 9) that accounted for 37% of the publications, 14 of which had only published one paper ( Table 4).
The authorial organizations and affiliations of the top 10 are summarized in Table 5. We found that the most-cited publications were authored in the University of Pennsylvania (n = 34), followed by the University of Texas (n = 15). However, the difference in citation frequency between the two was more than double. These are then proceeded by the Memorial Sloan Kettering Cancer Center (n = 15), University of Washington (n = 12), and the Fred Hutchinson Cancer Center (n = 11; Table 5   Riddell have no clear connection bridge. It seems that each study has been conducted in small groups separately. Finally, keywords from the most-cited 100 publications were extracted using "Keywords Plus" function from WoSCC. "KeyWords Plus are words or phrases that frequently appear in the titles of an article's references, but do not appear in the title of the article itself. Based upon a special algorithm that is unique to Clarivate Analytics databases, KeyWords Plus enhances the power of cited-reference searching by searching across disciplines for all the articles that have cited references in common" (11). We analyzed natural languages such as "treatment, " "Cell, " and "CAR" through independent reviewers by classifying them into the meaningless words. The most frequently used keywords were "adoptive immunotherapy" (n = 47), "lymphocytes" (n = 27), and "antitumor activity" (n = 22; Table 6).
The density of the keywords is also accounted for and determined by their frequency of appearance ( Figure 5). Higher-density keywords are represented in yellow, and lower densities are represented in blue; shorter distances between keyword nodes indicate frequent expression as co-occurring keywords.

DISCUSSION
We extracted the top 100 cited articles in the CAR-T field from the WoSCC database to analyze the field's network characteristics. In this study, we tried to visually express the research trends and mainstream structure through the network mapping of the simultaneous exposure of countries, funding bodies, researchers' affiliations and organizations, and keywords. Our network analysis found the USA and Germany possessed the most nodes, followed by Italy, Canada, and China. While an overwhelming amount of studies have been conducted in the USA, there appears to be an exchange with the vast majority of other countries (Figure 2).
The study of CAR-Ts is led mainly by three institutions: first is the University of Pennsylvania, then there is University of Texas MD Anderson (UTMD) Cancer Center, and Memorial Sloan Kettering Cancer Center (MSKCC) (Figure 3). When looking at the authorial connection and citation relationships, there seems to be a network centered around the works of C. H. June and B. L. Levine, with M. Sadelain acting as a bridging node. Another cluster of nodes centers around S. R. Riddell, and one around G. Dotti, but neither has a bridging node between them (Figure 4). This may imply that each group is being led by their respective institution directing the research and that there is minimal to no research network cooperation in the researches listed in ClinicalTrials.gov (7). Finally, the most used keyword is "adoptive immunotherapy, " which lies at the center, while other keywords like "lymphocytes, " "antitumor activity, " "persistence, " and "B-cell" were connected through multi-frequency simultaneous exposures (Figure 5).
In our determination of effect and value, we only consider the citation frequency. As such, we also only consider the number of co-authorships, regardless of an author's actual achievements and contributions. Future studies may consider the use of high quality databases that considers both qualitative and quantitative evidence for their objective analysis. Should consideration be placed on a study's phase and design, and the author's achievement and contribution for their analytical database, more robust results may be gained. Even now, there may be studies in press or in the process of completion that can contribute to the development of the field in spite of their low citation frequency.
CAR-T is a highly technological and high-cost method of cancer therapy; and the top funding bodies, journals, organizations, and authors were based or from the USA. The USA is objectively leading the development and research of the CAR-T field that is growing at an incredibly rapid rate (17)(18)(19). However, these advancements are not limited only to those dealing with CAR-T research and general cost-intensive treatments but also with overall immuno-chemotherapy research (20). In addition, CAR-T is a highly advanced next-generation treatment that has been studied in various fields such as lymphoma, solid cancer, rheumatoid arthritis, and autoimmune treatment. However, we have to consider the side effects of all treatments. Multiple cardiovascular adverse events are frequently stated after CAR-T therapy associated with mortality (21).

CONCLUSION
Using the 100 most-cited papers in the CAR-T field, we attempted to provide insight into the direction of the scientific growth and core study areas and trends, and opportunity to understand information on the main network structure of those studies.
What we observed was that CAR-T engineering is a developing technology-and cost-intensive form of immunotherapy, with most of its studies funded and led by US-based institutions and researchers.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.