Vitamin B2 Derivative-Enabled Sonochemical Thioketal Cleavage for Ultrasound-Activated In Situ Tumor Vaccination

Toll-like receptor (TLR) agonists are promising for in situ tumor vaccination but suffer from poor tumor selectivity and systemic toxicity. Here, we establish a sonochemical activation strategy using a vitamin B2-derived sonosensitizer for spatiotemporally controlled immunotherapy. Riboflavin tetrabutyrate (TBR) was identified as an efficient sonosensitizer for thioketal (TK) cleavage under low-intensity ultrasound (US) and coformulated with a TK-masked TLR7/8 agonist (R848) to construct sonosensitive nanoparticles (R848/TBR NPs). Upon US irradiation, TBR generated singlet oxygen to induce immunogenic cell death and trigger R848 release through an oxidation-hydrolysis pathway, resulting in a 6.0-fold increase in intratumoral active R848. The activated in situ vaccine elicited robust antitumor immunity with >150-fold enhancement of tumor-specific T-cell responses, achieving tumor inhibition rates of 98.1% in CT26 and 95.7% in highly metastatic 4T1 models while markedly suppressing metastasis and recurrence. This work establishes a mechanistically defined and biocompatible sonochemical framework for controllable drug activation and cancer immunotherapy.