Phytochlorin‐Based Sonosensitizers Combined with Free‐Field Ultrasound for Immune‐Sonodynamic Cancer Therapy

Abstract Phytochlorins, a class of plant‐derived tetrapyrroles, show great potential as sonosensitizers in sonodynamic therapy (SDT). The development of new phytochlorin‐based sonosensitizers has significantly improved SDT, yet the absence of specialized sonodynamic systems limits their clinical translation. Herein, a dedicated ultrasound system along with a detailed step‐by‐step sonodynamic process from in vitro to in vivo is developed to activate phytochlorin‐based sonosensitizers. Compared to standing‐wave ultrasound, free‐field ultrasound maintains stable acoustic pressure amplitudes and minimizes mechanical damage to cell membranes. In vitro experiments demonstrate that free‐field ultrasound effectively activates naturally occurring phytochlorin, reducing the cavitation threshold for reactive oxygen species production and triggering immunogenic cell death. Furthermore, the intravenously injectable phytochlorin‐based sonosensitizer ( C34 ) enhances sonodynamic efficiency by reducing interfacial tension. Driven by in vivo free‐field ultrasound, C34 effectively inhibits tumor growth in an orthotopic murine breast cancer model and elicits an immune response, preventing tumor metastasis. The reliable protocol provided by the free‐field ultrasound system facilitates the activation of phytochlorin‐based sonosensitizers while simultaneously stimulating the immune system, highlighting the potential of immune‐sonodynamic therapy.