Ultrasound-Activated Prodrugs for Tumor-Specific Immunotherapy

Precise spatiotemporal control over drug activation is essential to mitigate the systemic toxicity of potent immunotherapeutics. While ultrasound offers a safe modality with intrinsic deep tissue penetration, the scarcity of high-sensitivity chemical triggers has severely hindered the development of direct ultrasound-activated prodrugs. Herein, we report the rational design of a library of ultrasound-activated prodrugs based on benzyloxycarbonyl scaffolds. By systematically modulating the electronic properties of the aromatic ring, we established a structure-activity relationship governing the responsiveness to sonolytically generated hydroxyl radicals (·OH). Among the screened candidates, the 3,5-bis(methylamino)-substituted linker (BMBC) was identified as an optimal trigger, enabling efficient and instantaneous drug activation. Density functional theory (DFT) calculations reveal that BMBC's superior performance stems from a unique synergy: a high-lying HOMO energy level indicative of enhanced electron-donating character and high susceptibility toward electrophilic radical attack, and a minimized activation barrier for the rate-determining self-immolation step. This platform demonstrates broad universality, efficiently caging amines, hydroxyls, and carboxyls. As a proof of concept, a BMBC-caged TLR7 agonist elicited robust antitumor immunity and durable immunological memory in a murine model. Notably, the prodrug exhibited a significantly widened therapeutic window, remaining safe even at a 10-fold therapeutic dose. This work provides a versatile chemical toolkit for safe and spatiotemporally controlled drug activation.