Biomimetic exosome-delivered STAT3 inhibitor for radiosensitization in oral squamous cell carcinoma cells

Background: Signal transducer and activator of transcription 3 (STAT3) is a key driver of resistance to radiotherapy in advanced oral squamous cell carcinoma (OSCC), promoting an immunosuppressive microenvironment via upregulation of immune checkpoints including PD-L1. Although STAT3 inhibition represents a promising radiosensitizing strategy, its clinical application is limited by poor drug delivery. Exosomes, as natural nanocarriers, present an ideal solution with their excellent biocompatibility, low immunogenicity, and inherent targeting capabilities. This study aimed to develop an exosome-based targeted delivery system for a STAT3 inhibitor to improve the efficacy of radiotherapy in OSCC. Methods: We constructed a biomimetic delivery system (Exo@S3I-201) by loading the STAT3 inhibitor (S3I-201) into exosomes derived from mouse oral squamous cell carcinoma (MOC2) cells via sonication. The system was characterized by transmission electron microscopy (TEM), nanoparticle tracking analysis (NTA), and Western Blot. Drug loading efficiency and pH-dependent release profile were determined by ultraviolet spectroscopy. Cellular uptake and targeting were visualized by fluorescence microscopy. The radiosensitizing effect of Exo@S3I-201 on MOC2 cells were evaluated in vitro through CCK-8 assay, colony formation assay, flow cytometry, live-dead staining. Results: The successfully constructed Exo@S3I-201 exhibited similar size, dispersion, and stability to plain exosomes. UV absorption confirmed that Exo@S3I-201 retained the characteristic absorption peak of S3I-201, exhibited high drug loading efficiency, and displayed pH-responsive drug release behavior, with maximal release under acidic conditions (pH=5.5) mimicking the tumor microenvironment. Fluorescence microscopy revealed efficient cellular uptake of Exo@S3I-201 into MOC2 cells. Western Blot analysis revealed that Exo@S3I-201 combined with radiotherapy more effectively suppressed STAT3 phosphorylation and expression of its downstream target genes, compared with free S3I-201. Functionally, compared to radiotherapy alone or free S3I-201, the combined treatment markedly inhibited MOC2 cell proliferation and colony-forming ability and induced higher levels of cellular apoptosis. Conclusion: Our study presented an autologous exosome-based system for targeted STAT3 inhibitor delivery. Exo@S3I-201 significantly enhanced radiosensitivity in OSCC by inhibiting the STAT3 pathway, demonstrating its potential as a novel combination therapy to overcome radioresistance.