Abstract: Objective To investigate the effect and safety of Escherichia coli (E.coli) expressing gas vesicle (GVs) for enhancing the efficacy of tumor ablation by high intensity focused ultrasound (HIFU) in tumor-bearing mice. Methods Thirty-two female BALB/c mice were used to establish mouse models bearing 4T1 tumor, which were randomized into GVs group [E.coli BL21 (AI)-PET28a-Arg1] and control group (PBS), and the efficacy of HIFU ablation was evaluated by examining coagulative necrotic volume and pathology of the tumors. Another 104 BALB/c mice were also randomly divided into GVs group and control group, and body weight changes of the mice were recorded on days 1, 4 and 15 after intravenous injection of E.coli containing GVs or PBS. White blood cells, red blood cells, hemoglobin and platelet counts and liver and renal function parameters of the mice were detected, and serum levels of TNF-α and IL-1β were examined using ELISA. The pathological changes in the liver and spleen were evaluated using HE staining to assess the safety of the treatments. Results HIFU ablation resulted in a significantly greater volume of coagulative necrosis and severer tissue damage in GVs group than in the control group (P<0.001). In the 104 BALB/c mice without tumor cell inoculation, intravenous injection of E.coli expressing GVs, as compared with PBS, did not significantly affect body weight or cause changes in white blood cell, red blood cell and platelet counts or hemoglobin level (P1=0.59, P2=0.27, P3=0.76, P4=0.81). The liver and kidney function parameters (P1=0.12, P2=0.46, P3=0.62, P4=0.86) and serum levels of TNF-α and IL-1β (P1=0.48, P2=0.56) were all comparable between GVs group and control group. No obvious pathological changes were detected in the liver and spleen tissues in either GVs group or the control group. Conclusion E.coli expressing GVs is safe for enhancing the ablation effect of HIFU in tumor-bearing mice.

Key words: gas vesicles; Escherichia coli; neoplasms; high-intensity focused ultrasound