4028 Background: Immune checkpoint inhibitor (ICI) plus chemotherapy is an approved therapy for EBVaGC whose tumors express PD-L1 (CPS ≥1). However, some patients are resistant to ICI or exhibit myelosuppression, making them unable to tolerate chemotherapy. BCV, an antiviral drug with no myelotoxicity, is effective against EBV (EC50: 30 nM), and has demonstrated anti-tumor activity against head and neck cancer and EBV-associated lymphoma. Recently, we found that SNU-719, an EBVaGC cell line, is sensitive to BCV in vitro (IC50: 1.95 µM). To seek conventional chemo-free regimens for EBVaGC, we have studied the preclinical efficacy of BCV monotherapy and combination therapy with ICI against EBVaGC. Methods: BCV was tested for its ability to induce dsDNA breaks in SNU-719 cells by γ-H2A.X immunostaining. To assess synergy of BCV with ICI, we analyzed BCV-induced immunogenic cell death (ICD) in SNU-719 cells using calreticulin and HMGB-1 assays. Additionally, we assessed BCV-induced expression of PD-L1 using qPCR and FACS. Since BCV induced both ICD and PD-L1 expression in vitro , we then evaluated the in vivo anti-tumor activity of BCV with or without ICI in humanized NOG mice bearing SNU-719 xenografts. Specifically, mice received intraperitoneal administration of vehicle, BCV, pembrolizumab (Pembro), or Pembro+BCV. Subcutaneous tumor growth was monitored for each group. After euthanasia, the SNU-719 xenografts were also used to measure the mRNA expression levels of EBV genes including EBNA1 . Results: BCV induced both dsDNA breaks and ICD in SNU-719 cells, similar to the positive control bleomycin. It also induced PD-L1 mRNA and protein expression. Subsequent in vivo studies showed that, while BCV or Pembro monotherapy showed partial efficacy, the combination therapy of BCV and Pembro was effective in all mice. Notably, treatment with BCV significantly reduced the expression levels of EBNA1 mRNA ( p < 0.05) in the tumor tissue of SNU-719 xenografts. Conclusions: EBV is an oncovirus and its gene product EBNA1 is an oncoprotein for EBVaGC. This study showed a significant reduction of EBNA1 mRNA expression by BCV. Therefore, in addition to its own anti-tumor effects, BCV may contribute significantly to the suppression of EBVaGC tumorigenesis via its anti-EBV activity. From a clinical perspective, BCV was effective against EBVaGC xenografts when combined with ICI. This less toxic regimen may be beneficial for EBVaGC patients, especially those with myelosuppression.
Kamitani et al. (Wed,) studied this question.