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The purpose of this study was to further understand the mechanisms involved in ultrasound-mediated delivery of DNA (sonoporation); in particular, to understand how a plasmid should be formulated with an ultrasound contrast agent (UCA). Different polymer adjuvant-UCA combinations were formulated, and their impact on in vitro DNA transfection, was determined, under various experimental conditions. When present in the medium surrounding a cell suspension, and in the presence of a plasmid encoding for the green fluorescent protein (GFP), expression following sonoporation was increased by more than 1.5-fold compared to that achieved in control experiments (without the adjuvants). The effects of the adjuvants were not influenced by the nature of the UCA, nor by that of the transfected cells; in contrast, the adjuvant concentrations, their physico-chemical properties, and the manner in which they were used, did have an impact on transfection. Close association of the adjuvants to the UCA inhibited their action, suggesting that these substances must have access to the cell membrane to be effective. Indeed, Pluronic F127 appeared to improve the efficacy of transfection (percentage of GFP-positive cells and cell viability), via fluidization of the cell membrane, perhaps facilitating thereby the formation of transient pores and their re-sealing. The mechanism of action of polyethylene glycols, on the other hand, remains unclear.

MAT B III cells were insonified at ultrasound frequencies of 1.15 and 2.25 MHz in the presence of different ultrasound contrast agents (UCA) and a plasmid encoding for the green fluorescent protein. The transfection efficiency was assessed by flow cytometry, while contrast agent destruction by ultrasound was evaluated using optical and scanning electronic microscopies. It was found that the gas and shell properties of the UCA have an important influence on cell transfection. A good correlation was observed between bubble destruction and transfection rate, and it was demonstrated, for the first time, that hard-shelled contrast agents (gas microcapsules) are promising candidates for ultrasound-mediated gene delivery.

Sonoporation, in the presence of ultrasound contrast agents (UCA), is a technique that permits the transfer of drugs, including genes, into cells. In this study, the size of the pores created by ultrasound application, and the duration of pore opening, have been characterized via indirect molecular probing and microscopic observation. Internalization of molecules with diameters up to 37 nm was efficient and generally well-tolerated; on the other hand, confocal microscopy revealed that 75 nm particles entered only a few cells when sonoporation was applied. In general, the larger the species to internalize, the poorer the transfer. Direct visualization of pores following insonification, using scanning electron microscopy, was hampered by the presence of numerous villi on the surface of the cells employed (MAT B III), and by the short duration of pore opening. Clearer observations of porated regions were possible using red blood cells. This research (i) confirms that sonoporation is a means with which to achieve macromolecule delivery into cells, and (ii) characterizes in some detail the phenomenon of ultrasound induction of transient pores in the cell membrane.

Sonoporation is an approach that can be used to transfer DNA or drugs into cells. However, very little is known about the mechanism of ultrasound-mediated membrane permeabilization. In this investigation, DNA transport post-sonoporation and the subsequent plasmid internalization and protein expression kinetics have been studied. Using a plasmid encoding for the green fluorescent protein (GFP), labelled or not with an intercalating agent (YOYO-1), it was found that, as compared to lipofection that requires endocytosis, sonoporation allowed a rapid and direct transfer of naked DNA into the cell cytoplasm probably via ultrasound-induced pores in the membrane. The kinetics of protein expression were significantly faster for sonoporation than for lipofection, the mechanism of which requires endocytosis. However, unprotected DNA in the cytoplasm could be degraded by resident cytosolic DNases, thereby decreasing ultrasound-mediated gene delivery efficiency.

One factor critical to successful gene therapy is the development of efficient delivery systems. Although advances in gene transfer technology, including viral and non-viral vectors, have been made, an ideal vector system has not yet been constructed. This review describes the basic principles behind various physical methods for gene transfer and assesses the advantages and performance of such approaches, compared to other transfection systems. In particular, the kinetics and efficiency of gene delivery, the toxicity, in vivo feasibility, and targeting ability of different physical methodologies are discussed and evaluated.