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Esophageal squamous cell carcinoma (ESCC) is a disease characterized by a high mutation rate of the TP53 gene, which plays pivotal roles in the DNA damage response (DDR) and is regulated by checkpoint kinase (CHK) 2. CHK1 is another key DDR-related protein, and its selective inhibition is suggested to be particularly sensitive to TP53-mutated cancers, because a loss of both pathways (CHK1 and/or CHK2-p53) is lethal due to the serious impairment of DDR. Such a therapeutic strategy is termed synthetic lethality. Here, we propose a novel therapeutic strategy based on synthetic lethality combining trifluridine/tipiracil and prexasertib (CHK1 inhibitor) as a treatment for ESCC. Trifluridine is a key component of the antitumor drug combination with trifluridine/tipiracil (an inhibitor of trifluridine degradation), also known as TAS-102. In this study, we demonstrate that trifluridine increases CHK1 phosphorylation in ESCC cells combined with a reduction of the S-phase ratio as well as the induction of ssDNA damage. Because CHK1 phosphorylation is considered to be induced as DDR for trifluridine-mediated DNA damage, we examined the effects of CHK1 inhibition on trifluridine treatment. Consequently, CHK1 inhibition by short hairpin RNA or treatment with the CHK1 inhibitor, prexasertib, markedly enhanced trifluridine-mediated DNA damage, represented by an increase of gH2AX expression. Moreover, the combination of trifluridine/tipiracil and CHK1 inhibition significantly suppressed tumor growth of ESCC-derived xenograft tumors. Furthermore, the combination of trifluridine and prexasertib enhanced radiosensitivity both in vitro and in vivo. Thus, the combination of trifluridine/tipiracil and a CHK1 inhibitor exhibits effective antitumor effects, suggesting a novel therapeutic strategy for ESCC.

Cancer cell lines are widely used in basic research to study cancer development, growth, invasion, or metastasis. They are also used for the development and screening of anticancer drugs. However, there are no clear criteria for choosing the most suitable cell lines among the wide variety of cancer cell lines commercially available for research, and the choice is often based on previously published reports. Here, we investigated the characteristics of liver cancer cell lines by analyzing the gene expression data available in the Cancer Cell Line Encyclopedia. Unsupervised clustering analysis of 28 liver cancer cell lines yielded two main clusters. One cluster showed a gene expression pattern similar to that of hepatocytes, and the other showed a pattern similar to that of fibroblasts. Analysis of hepatocellular carcinoma gene expression profiles available in The Cancer Genome Atlas showed that the gene expression patterns in most hepatoma tissues were similar to those in the hepatocyte-like cluster. With respect to liver cancer research, our findings may be useful for selecting an appropriate cell line for a specific study objective. Furthermore, our approach of utilizing a public database for comparing the properties of cell lines could be an attractive cell line selection strategy that can be applied to other fields of research.

Background Tumor mutational burden (TMB) measured via next‐generation sequencing (NGS)‐based gene panel is a promising biomarker for response to immune checkpoint inhibitors (ICIs) in solid tumors. However, little is known about the preanalytical factors that can affect the TMB score. Materials and Methods Data of 199 patients with solid tumors who underwent multiplex NGS gene panel (OncoPrime), which was commercially provided by a Clinical Laboratory Improvement Amendments‐licensed laboratory and covered 0.78 megabase (Mb) of capture size relevant to the TMB calculation, were reviewed. Associations between the TMB score and preanalytical factors, including sample DNA quality, sample type, sampling site, and storage period, were analyzed. Clinical outcomes of patients with a high TMB score (≥10 mutations per megabase) who received anti‐programmed cell death protein 1 antibodies (n = 22) were also analyzed. Results Low DNA library concentration (<5 nM), formalin‐fixed paraffin‐embedded tissue (FFPE), and the prolonged sample storage period (range, 0.9–58.1 months) correlated with a higher TMB score. After excluding low DNA library samples from the analysis, FFPE samples, but not the sample storage period, exhibited a marked correlation with a high TMB score. Of 22 patients with a high TMB score, we observed the partial response in 2 patients (9.1%). Conclusion Our results indicate that the TMB score estimated via NGS‐based gene panel could be affected by the DNA library concentration and sample type. These factors could potentially increase the false‐positive and/or artifactual variant calls. As each gene panel has its own pipeline for variant calling, it is unknown whether these factors have a significant effect in other platforms. Implications for Practice A high tumor mutational burden score, as estimated via next‐generation sequencing‐based gene panel testing, should be carefully interpreted as it could be affected by the DNA library concentration and sample type.