Background: Pulsed high-intensity focused ultrasound(HIFU) has been demonstrated to non-invasively increase delivery of therapeutic agents (small molecules, liposomes, and plasmid DNA) to solid malignancies and improve therapeutic effects in murine models of cancer. These results are hypothesized to occur as a result of reversible structural changes in the targeted tissue which increase permeability. Prior to clinical translation, it is necessary to evaluate the safety of pulsed HIFU therapy. The objective of this study is to determine the effects of pulsed HIFU treatment on the metastatic potential of solid tumors.

Methods: We selected a human melanoma murine xenograft model (LOX) that consistently demonstrates spontaneous pulmonary metastasis. Retroviral infection of the LOX cells with a HSV1-tk/GFP/luciferase fusion reporter gene allows for in vivo and ex vivo fluorescent and bioluminescent imaging of the primary tumor and subsequent metastases. Two experimental groups, one receiving HIFU exposures and one (control) receiving sham treatment, were monitored with in vivo bioluminescent imaging. We utilized histologic examination, pulmonary metastatic index, western blot for GFP, and ex vivo luciferase assays for post-mortem quantification of pulmonary metastases.

Results: Initial experiments show that the triple reporter gene allows for successful in vivo characterization of the spontaneous metastatic behavior of the tumor cells and ex vivo fluorescent microscopy for evaluating pulmonary metastasis. We are using these imaging modalities in ongoing experiments to quantify and compare the extent of pulmonary metastases in the group of mice treated with pulsed HIFU against the control group.

Conclusions: Establishing the safety of this preclinical paradigm is a prerequisite for clinical translation. This study serves to clarify the effects of the pulsed HIFU on metastatic potential. With the successful addition of the triple reporter gene to the orthotopic murine xenograft model, multiparametric validation is possible. Corroboration of both in vivo and ex vivo imaging analysis as well as ex vivo assays should validate and improve the integrity of the model, which could be used to clarify mechanistic risks factors for metastases from thermal HIFU ablation and other ablative techniques as well.