An exploratory, single-center, single-arm trial of R-ISV-FOLactis in situ vaccination for advanced soft tissue sarcoma.

2592 Background: Advanced soft tissue sarcomas (STSs) have limited treatment options and exhibit low responsiveness to immunotherapy, largely due to an immunosuppressive tumor microenvironment (TME). In situ vaccination (ISV) represents a promising strategy to convert immunologically “cold” tumors into “hot” ones. FOLactis, a food-grade probiotic Lactococcus lactis engineered to express a fusion protein of Fms-like tyrosine kinase 3 ligand (Flt3L) and OX40 ligand (OX40L), has shown antitumor immune activity in preclinical models. This study investigates the synergistic antitumor effect and safety of intratumoral FOLactis in combination with hypofractionated radiotherapy and anti-PD-1 therapy in clinical settings. Methods: In this investigator-initiated, single-arm trial, patients with advanced STS who had failed standard therapies received intratumoral injections of FOLactis combined with hypofractionated radiotherapy and PD-1 blockade. Primary endpoints included safety and efficacy, secondary endpoints encompassed abscopal responses and mechanistic exploration of treatment efficacy. Results: Between July 2022 to July 2025, 16 advanced STS patients were enrolled. The R-ISV-FOLactis regimen was well-tolerated, with grade 3 adverse events occurring in 37.5% of patients. Injected lesions showed a best objective response rate (ORR) of 56.25% and a disease control rate (DCR) of 100%. Median progression-free survival (mPFS) for all lesions was 6.3 months, while mPFS for injected lesions was not reached. Abscopal responses were observed in 56.25% of patients. Immunological analyses revealed increased frequencies of peripheral memory CD8⁺ T cells, PD-1⁺ CD8⁺ T cells, and dendritic cells (DCs), alongside decreased regulatory T cells (Tregs) and tissue-resident CD103⁺CD4⁺ T cells in responders, indicating systemic immune activation. Furthermore, reshaped TME with reduced fibroblasts and increased immune cell infiltration were associated with enhanced antitumor immunity. Moreover, significant increase of CD8⁺ T cell density in regions distal to neutrophil clusters, along with elevated 4−Cresol Sulfate levels suggesting neutrophil modulation. Conclusions: R-ISV-FOLactis exhibited a favorable safety profile and encouraging efficacy against both local and distant tumors. The treatment induced systemic immune activation and metabolic remodeling, supporting its further development as a novel ISV strategy for advanced STSs. Clinical trial information: ChiCTR2200060660.