Ultrasound and microbubble potentiated sensitization of cells to ionizing radiation Free

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Objective: The aim of this work is to develop a novel ultrasound and microbubble mediated vascular-based method to enhance the effects of radiation on tumours. Microbubble contrast agents are used in oncologic research to enhance visualization of tumour vasculature, and recently have been exploited to enhance drug delivery and facilitate sonoporation. Our ultimate goal is to use them to radiosensitize vascular endothelial cells in tumours. As a first step, in this study our objective was to test the hypothesis that ultrasound microbubble agents can be used as radiosensitizers in vitro with a standard leukemia cell line.

Method: Acute myeloid leukemia (AML) cells in suspension were exposed to ultrasound and ionizing radiation at various ultrasound exposure settings (0, 125, 240, 570 kPa acoustic pressure), microbubble concentrations (0-3%) and with different radiation doses (0, 2, 4 and 8 Gy). Cell death was measured using flow cytometry, microscopy and standard histologic techniques. Cell viability was estimated using propidium iodide for membrane integrity and confirmed with clonogenic assays.

Results: The results indicate that selective exposure to ultrasound in the presence of microbubbles can sensitize cells to ionizing radiation. This phenomenon depended on ultrasound exposure parameters and microbubble concentration. Higher acoustic pressures (Pneg=570kPa) and bubble concentrations (1.67-3.33% v/v) were more effective at radiosensitizing cells. Cell viability in samples exposed to both ultrasound and ionizing radiation was less (30% to 40%) compared to samples exposed to only ultrasound or ionizing radiation at 24, 48 and 72 hours post treatment. The effect of ionizing radiation on cell viability was evident after 24 hours, whereas the effect of ultrasound on cells was measurable immediately. By 48 hours, cell viability in samples exposed to ultrasound and ionizing radiation decreased to 42%±6% (Pneg=570kPa and X-ray dose=2Gy) compared to cells exposed to ultrasound alone (77%±5% at Pneg=570kPa) or to ionizing radiation alone (71%±7% at X-ray dose=2Gy). The ability of ultrasound to sensitize cells to ionizing radiation was apparent also at higher doses (4 to 8 Gy). The ionizing radiation dose affects cell viability as expected however at higher radiation doses the relative advantage of ultrasound and microbubbles on radiosensitizing cells was less evident.

Conclusions: In summary, ultrasound mediated microbubble disruption can enhance the response of cells to radiation in vitro. This study forms a basis for future work with endothelial cells in vitro and applications of these findings to in vivo models. (The first two authors have made equal contributions to this publication).