Serum extracellular vesicles containing adenoviral E1A-DNA as a predictive biomarker for liquid biopsy in oncolytic adenovirus therapy

Oncolytic adenoviruses replicate selectively in tumor cells and induce immunogenic cell death, but predictive biomarkers for early therapeutic response are lacking. This study evaluated extracellular vesicle-encapsulated adenoviral E1A-DNA (EV-E1A-DNA) as a minimally invasive biomarker for monitoring responses to telomerase-specific oncolytic adenoviruses OBP-301 and OBP-502. EVs were isolated from human and murine cancer cell lines and from the serum of treated mice using ultracentrifugation. EV-associated E1A-DNA levels were measured by quantitative polymerase chain reaction and found to correlate with cytotoxicity in vitro and tumor regression in vivo. In xenograft models, serum EV-E1A-DNA levels at 2 days post-treatment showed strong correlations with final tumor volume and survival, supporting their utility as an early predictive biomarker. In immunocompetent mice pre-immunized with wild-type adenovirus, free viral DNA was undetectable in serum due to neutralizing antibodies, whereas EV-E1A-DNA remained detectable. This "stealth effect" indicates that EVs protect viral components from immune clearance. These results demonstrate that EV-E1A-DNA is a sensitive and virus-specific biomarker that enables early assessment of therapeutic efficacy, even in the presence of antiviral immunity. This strategy offers a promising liquid biopsy approach for personalized monitoring of oncolytic virotherapy and may be applicable to other virus-based therapies.