Abstract Triple-negative breast cancer (TNBC), which accounts for 15–20% of breast cancers, is characterized by rapid proliferation and oncogenic pathway activation, creating a permissive environment for oncolytic virus replication. JX-594 (pexastimogene devacirepvec) is a genetically engineered vaccinia virus in which the thymidine kinase gene is replaced with human GM-CSF, enabling selective tumor replication and immune activation. Our in vitro studies demonstrated high sensitivity of TNBC cell lines to JX-594. Based on these findings, we established TNBC syngeneic and patient-derived xenograft (PDX) models to investigate JX-594–induced molecular and immunologic changes within the tumor microenvironment. Syngeneic models were generated by implanting 4T1 and EMT6 cells into the mammary fat pads of BALB/c mice. When tumors reached approximately 30–60 mm3, mice received intratumoral mJX-594 injections (5 × 107 PFU, three doses every three days). On day 10, tumors were harvested for histopathologic and immunohistochemical (IHC) analyses. For translational validation, TNBC PDX models were established from human breast cancer tissues. When tumors reached approximately 30 mm3, mice received intratumoral injections of JX-594 (1 × 107 PFU) once weekly for four weeks. After 28 days, tumor samples were examined by IHC for Ki-67, CD31, and vaccinia antigen. Tumor growth and body weight were monitored throughout the treatment period in both models.In syngeneic TNBC models, mJX-594 treatment markedly suppressed tumor growth compared with controls and was associated with increased infiltration of CD4+ and CD8+ T lymphocytes, as well as reduced micro-vessel density as indicated by CD31 staining. In contrast, PD-1/PD-L1 and CD20 expression levels were not significantly altered. In TNBC PDX models, an initial transient increase in tumor size was observed following the first JX-594 administration, followed by pronounced tumor regression and sustained growth suppression after the second treatment. IHC analyses revealed extensive tumor necrosis, decreased Ki-67 and CD31 expression, and positive vaccinia antigen staining, indicating active viral replication and ongoing oncolysis within the tumor microenvironment.This study demonstrates that JX-594 exerts potent anti-tumor activity in TNBC through immune activation, anti-angiogenic effects, and direct oncolytic mechanisms. The consistent therapeutic efficacy observed in both syngeneic and PDX models highlights the translational significance of JX-594. Although several studies have explored oncolytic virus–based therapies for TNBC, preclinical validation using PDX models remains extremely limited. Our findings provide robust in vivo evidence supporting the therapeutic potential of JX-594 and establish a translational foundation for advancing oncolytic virotherapy in TNBC. Citation Format: Yelin KIM, Jihye B Lee, Yoonjin Cha, Keunhee Oh, Namhee Lee, Seoyoung Lee, Hei Cheul Jeung, Soong June Bae, Sung Gwe Ahn, Joon JEONG, Jee Hung Kim. Oncolytic virus exhibits potent antitumor and immunomodulatory effects in triple-negative breast cancer using syngeneic and PDX models abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 4321.
Kim et al. (Fri,) studied this question.