Gene therapy progress and prospects: Ultrasound for gene transfer

Passive imaging with pulsed ultrasound insonations

Previously, passive cavitation imaging has been described in the context of continuous-wave high-intensity focused ultrasound thermal ablation. However, the technique has potential use as a feedback mechanism for pulsed-wave therapies, such as ultrasound-mediated drug delivery. In this paper, results of experiments and simulations are reported to demonstrate the feasibility of passive cavitation imaging using pulsed ultrasound insonations and how the images depend on pulsed ultrasound parameters. The passive cavitation images were formed from channel data that was beamformed in the frequency domain. Experiments were performed in an invitro flow phantom with an experimental echo contrast agent, echogenic liposomes, as cavitation nuclei. It was found that the pulse duration and envelope have minimal impact on the image resolution achieved. The passive cavitation image amplitude scales linearly with the cavitation emission energy. Cavitation images for both stable and inertial cavitation can be obtained from the same received data set.

Cationic versus neutral microbubbles for ultrasound-mediated gene delivery in cancer

PURPOSE

To test whether plasmid-binding cationic microbubbles (MBs) enhance ultrasound-mediated gene delivery efficiency relative to control neutral MBs in cell culture and in vivo tumors in mice.

MATERIALS AND METHODS

Animal studies were approved by the institutional animal care committee. Cationic and neutral MBs were characterized in terms of size, charge, circulation time, and DNA binding. Click beetle luciferase (CBLuc) reporter plasmids were mixed with cationic or neutral MBs. The ability of cationic MBs to protect bound plasmids from nuclease degradation was tested by means of a deoxyribonuclease (DNase) protection assay. Relative efficiencies of ultrasound-mediated transfection (ultrasound parameters: 1 MHz, 1 W/cm(2), 20% duty cycle, 1 minute) of CBLuc to endothelial cells by using cationic, neutral, or no MBs were compared in cell culture. Ultrasound-mediated gene delivery to mouse hind limb tumors was performed in vivo (n = 24) with insonation (1 MHz, 2 W/cm(2), 50% duty cycle, 5 minutes) after intravenous administration of CBLuc with cationic, neutral, or no MBs. Tumor luciferase activity was assessed by means of serial in vivo bioluminescence imaging and ex vivo analysis. Results were compared by using analysis of variance.

RESULTS

Cationic MBs (+15.8 mV; DNA binding capacity, 0.03 pg per MB) partially protected bound DNA from DNase degradation. Mean CBLuc expression of treated endothelial cells in culture was 20-fold higher with cationic than with neutral MBs (219.0 relative light units [RLUs]/µg protein ± 92.5 [standard deviation] vs 10.9 RLUs/µg protein ± 2.7, P = .001) and was significantly higher (P < .001) than that in the no MB and no ultrasound control groups. Serial in vivo bioluminescence of mouse tumors was significantly higher with cationic than with neutral MBs ([5.9 ± 2.2] to [9.3 ± 5.2] vs [2.4 ± 0.8] to [2.9 ± 1.1] × 10(4) photons/sec/cm(2)/steradian, P < .0001) and versus no MB and no ultrasound controls (P < .0001). Results of ex vivo analysis confirmed these results (ρ = 0.88, P < .0001).

CONCLUSION

Plasmid-binding cationic MBs enhance ultrasound-mediated gene delivery efficiency relative to neutral MBs in both cell culture and mouse hind limb tumors.

Enhancing effect of ultrasound-mediated microbubble destruction on gene delivery into rat kidney via different administration routes

OBJECTIVE

To investigate the efficiency of β-galactosidase gene transfer into rat kidney with ultrasound-mediated microbubble destruction via different injection routes.

METHODS

A total of 25 Wistar rats were randomly divided into 5 groups. Four groups received a mixture of optison microbubbles (0.2 mL) and lacz plasmids (25 μg) injection via renal artery, tail vein, anterior tibial muscle and renal parenchyma, respectively. The control group received a mixture of PBS (xx mL) and lacz plasmids (25 μg) via renal artery. Three days after the gene transfer, ultrasound with fixed frequency and power (1 MHz, xxW) was delivered to the kidneys for 3 min. The efficiency of the gene transfer and expression was evaluated on the basis of β-galactosidase expression. The side effects of this method were evaluated by immunohistological method.

RESULTS

β-galactosidase expression could be observed only in tubules but not in glomeruli and interstitial area. The efficiency of renal artery group was higher than that of tail vein, anterior tibial muscle and renal parenchyma group (P<0.05). Immunohistochemical analysis revealed co-expression of β-galactosidase with a roximal tubule marker, megalin, which suggested that ultrasound enhanced gene transfer into the proximal tubular epithelial cells. No β-galactosidase expression was observed in the extrarenal organs. There were no evident pathological and biochemical changes after gene transfer.

CONCLUSIONS

Ultrasound-mediated microbubble destruction can transfer gene into kidney via renal artery, tail vein, anterior tibial muscle and renal parenchyma. Compared with renal artery, administrating microbubbles via tail vein and anterior tibial muscle are more convenient and less vulnerarious.

Relationship between plasmid size and shock wave-mediated bacterial transformation

Bacterial transformation is a fundamental tool in molecular biology; nevertheless, there is still a lack of efficient methods for gene delivery. The use of shock waves has been proposed as an alternative. Recently, our group demonstrated that shock wave-induced transfer of deoxyribonucleic acid (DNA) into bacteria can be increased by enhancing acoustic cavitation; however, so far, little information exists about the effects of shock waves on DNA. The objective of this study was to identify the size regimes of plasmids (DNA molecules that are separate from the chromosomal DNA), which promote shock wave-induced transformation. The transformation efficiency of shock waves and the integrity of DNA were studied for six different plasmid sizes, using the parameters that led to the best results in our previous study.

Ultrasound-microbubble mediated cavitation of plant cells: effects on morphology and viability

The interaction between ultrasound pulses and microbubbles is known to generate acoustic cavitation that may puncture biological cells. This work presents new experimental findings on the bioeffects of ultrasound-microbubble mediated cavitation in plant cells with emphasis on direct observations of morphological impact and analysis of viability trends in tobacco BY-2 cells that are widely studied in higher plant physiology. The tobacco cell suspensions were exposed to 1 MHz ultrasound pulses in the presence of 1% v/v microbubbles (10% duty cycle; 1 kHz pulse repetition frequency; 70 mm between probe and cells; 1-min exposure time). Few bioeffects were observed at low peak negative pressures (<0.4 MPa) where stable cavitation presumably occurred. In contrast, at 0.9 MPa peak negative pressure (with more inertial cavitation activities according to our passive cavitation detection results), random pores were found on tobacco cell wall (observed via scanning electron microscopy) and enhanced exogenous uptake into the cytoplasm was evident (noted in our fluorescein isothiocyanate dextran uptake analysis). Also, instant lysis was observed in 23.4% of cells (found using trypan blue staining) and programmed cell death was seen in 23.3% of population after 12 h (determined by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling [TUNEL]). These bioeffects generally correspond in trend with those for mammalian cells. This raises the possibility of developing ultrasound-microbubble mediated cavitation into a targeted gene transfection paradigm for plant cells and, conversely, adopting plant cells as experimental test-beds for sonoporation-based gene therapy in mammalian cells.

Can ultrasound enable efficient intracellular uptake of molecules? A retrospective literature review and analysis

Most applications of therapeutic ultrasound (US) for intracellular delivery of drugs, proteins, DNA/RNA and other compounds would benefit from efficient uptake of these molecules into large numbers of cells without killing cells in the process. In this study we tested the hypothesis that efficient intracellular uptake of molecules can be achieved with high cell viability after US exposure in vitro. A search of the literature for studies with quantitative data on uptake and viability yielded 26 published papers containing 898 experimental data points. Analysis of these studies showed that just 7.7% of the data points corresponded to relatively efficient uptake (>50% of cells exhibiting uptake). Closer examination of the data showed that use of Definity US contrast agent (as opposed to Optison) and elevated sonication temperature at 37°C (as opposed to room temperature) were associated with high uptake, which we further validated through independent experiments carried out in this study. Although these factors contributed to high uptake, almost all data with efficient uptake were from studies that had not accounted for lysed cells when determining cell viability. Based on retrospective analysis of the data, we showed that not accounting for lysed cells can dramatically increase the calculated uptake efficiency. We further argue that if all the data considered in this study were re-analyzed to account for lysed cells, there would be essentially no data with efficient uptake. We therefore conclude that the literature does not support the hypothesis that efficient intracellular uptake of molecules can be achieved with high cell viability after US exposure in vitro, which poses a challenge to future applications of US that require efficient intracellular delivery.

Plasmid pUDK-HGF encoding human hepatocyte growth factor gene attenuates gentamicin-induced kidney injury in rats

The clinical application of gentamicin has been limited by its nephrotoxicity, which is characterized by kidney injury, interstitial fibrosis and progressive renal impairment. In this paper, we examine effects of plasmid pUDK-HGF which encodes the human hepatocyte growth factor (HGF) gene on gentamicin-induced renal injury in rats. The kidney injury was intentionally induced by injecting gentamicin intraperitoneally. On the third day after last gentamicin treatment, pUDK-HGF was injected into the left kidney tissue only once via a sterile back incision. At day 30 after gentamicin treatment, RI, Scr, BUN, 24 h-UTP and apoptotic cell death were determined. Tubulointerstitial injury and the renal interstitial vessel regeneration were evaluated by histological scoring. pUDK-HGF treatment significantly improved the renal function with decreasing RI, Scr and BUN. 24 h-UTP also presented ameliorating trend compared to the control group with kidney injury. pUDK-HGF treatment significantly decreased the score of tubulointerstitial injury and enhanced angiogenesis, also prevented kidney cells from apoptosis. The tubulointerstitial injury was significantly reduced in the pUDK-HGF injected left kidney and right kidney also showed some improvements. Our results showed that pUDK-HGF may become a novel therapeutic agent for kidney injury and renal fibrosis.

Microbubble-induced sonoporation involved in ultrasound-mediated DNA transfection in vitro at low acoustic pressures

In the present work, human breast cancer cells MCF-7 mixed with polyethylenimine: deoxyribonucleic acid complex and microbubbles were exposed to 1-MHz ultrasound at low acoustic driving pressures ranging from 0.05 to 0.3 MPa. The sonoporation pores generated on the cell membrane were examined with scanning electron microscopy. The transfection efficiency and cell viability were evaluated with flow cytometry. The results showed that ultrasound sonication under the current exposure condition could generate cell pores with mean size ranging from about 100 nm to 1.25 μm, and that larger sonoporation pores would be generated with the increasing acoustic pressure or longer treatment time, leading to the enhancement of transfection efficiency and the reduction of cell viability. The simulations based on the Marmottant model were performed to test the hypothesis that the microstreaming-induced shear stress might be involved in the mechanisms of the low-intensity ultrasound induced sonoporation. The calculated shear stress resulting from the micro-streaming ranged from 15 to 680 Pa corresponding to the applied acoustic pressures 0.05-0.3 MPa, which is sufficient to induce reversible sonoporation. This study indicates that the shear stress related bio-effects may provide a base for strategies aimed at targeted drug delivery.

Augmentation of transgenic expression by ultrasound‑mediated liposome microbubble destruction

Non-invasive, efficient and tissue-specific transgenic technologies could be valuable in gene therapy. Although non-viral carriers may be safer and cheaper, they have a much lower transfection efficiency than viral gene carriers. The present study was designed to test the transgenic expression and safety of red fluorescent protein (RFP) in HeLa cells in vitro and in transplanted tumors of nude mice in vivo under ultrasound-mediated liposome microbubble destruction (UMLMD) conditions. Plasmids containing RFP were gently mixed with liposome microbubbles (LMs). The mixture was added to HeLa cells or injected into BALB/c mice by the tail vein under various ultrasound exposure and LM parameters, and then the transfection efficiencies were examined. The results in vivo and in vitro demonstrated that, following a comparison of the plasmid group, the ultrasound + plasmid group and the LM + plasmid group, UMLMD significantly increased the transgenic expression (P<0.01) without causing any apparent detrimental effect. From the study, we concluded that UMLMD could be a non-invasive, effective and promising non-viral technique for gene therapy and transgenic research.

PLGA Nanoparticles for Ultrasound-Mediated Gene Delivery to Solid Tumors

This paper focuses on novel approaches in the field of nanotechnology-based carriers utilizing ultrasound stimuli as a means to spatially target gene delivery in vivo, using nanoparticles made with either poly(lactic-co-glycolic acid) (PLGA) or other polymers. We specifically discuss the potential for gene delivery by particles that are echogenic (amenable to destruction by ultrasound) composed either of polymers (PLGA, polystyrene) or other contrast agent materials (Optison, SonoVue microbubbles). The use of ultrasound is an efficient tool to further enhance gene delivery by PLGA or other echogenic particles in vivo. Echogenic PLGA nanoparticles are an attractive strategy for ultrasound-mediated gene delivery since this polymer is currently approved by the US Food and Drug Administration for drug delivery and diagnostics in cancer, cardiovascular disease, and also other applications such as vaccines and tissue engineering. This paper will review recent successes and the potential of applying PLGA nanoparticles for gene delivery, which include (a) echogenic PLGA used with ultrasound to enhance local gene delivery in tumors or muscle and (b) PLGA nanoparticles currently under development, which could benefit in the future from ultrasound-enhanced tumor targeted gene delivery.

Ultrasound microbubbles combined with liposome-mediated pNogo-R shRNA delivery into neural stem cells

In the present study, ultrasound-mediated microbubble destruction (UMMD) alone and combined with liposome technology was used as a novel nonviral technique to transfect a Nogo receptor (Nogo-R) shRNA plasmid (pNogo-R shRNA) into neural stem cells (NSCs). Using green fluorescent protein as a reporter gene, transfection efficiency of NSCs was significantly higher in the group transfected with UMMD combined with liposomes compared with that of the group transfected with UMMD or liposomes alone, and did not affect cell vitality. In addition, Nogo-R mRNA and protein expression was dramatically decreased in the UMMD combined with liposome-mediated group compared with that of other groups after 24 hours of transfection. The UMMD technique combined with liposomes is a noninvasive gene transfer method, which showed minimal effects on cell viability and effectively increased transfer of Nogo-R shRNA into NSCs.

Sonoporation induces apoptosis and cell cycle arrest in human promyelocytic leukemia cells

Despite being a transient biophysical phenomenon, sonoporation is known to disturb the homeostasis of living cells. This work presents new evidence on how sonoporation may lead to antiproliferation effects including cell-cycle arrest and apoptosis through disrupting various cell signaling pathways. Our findings were obtained from sonoporation experiments conducted on HL-60 human promyelocytic leukemia cells (with 1% v/v microbubbles; 1 MHz ultrasound; 0.3 or 0.5MPa peak negative pressure; 10% duty cycle; 1 kHz pulse repetition frequency; 1 min exposure period). Membrane resealing in these sonoporated cells was first verified using scanning electron microscopy. Time-lapse flow cytometry analysis of cellular deoxyribonucleic acid (DNA) contents was then performed at four post-sonoporation time points (4 h, 8 h, 12 h and 24 h). Results indicate that an increasing trend in the apoptotic cell population can be observed for at least 12 h after sonoporation, whilst viable sonoporated cells are found to temporarily accumulate in the G(2)/M (gap-2/mitosis) phase of the cell cycle. Further analysis using western blotting reveals that sonoporation-induced apoptosis involves cleavage of poly adenosine diphosphate ribose polymerase (PARP) proteins: a pro-apoptotic hallmark related to loss of DNA repair functionality. Also, mitochondrial signaling seems to have taken part in triggering this cellular event as the expression of two complementary regulators for mitochondrial release of pro-apoptotic molecules, Bcl-2 (B-cell lymphoma 2) and Bax (Bcl-2-associated X), are seen to be imbalanced in sonoporated cells. Furthermore, sonoporation is found to induce cell-cycle arrest through perturbing the expression of various cyclin and Cdk (cyclin-dependent kinase) checkpoint proteins that play an enabling role in cell-cycle progression. These bioeffects should be taken into account when using sonoporation for therapeutic purposes.

Sonoporation-mediated anti-angiogenic gene transfer into muscle effectively regresses distant orthotopic tumors

Ultrasound (US) is an effective tool for local delivery of genes into target tumors or organs. In combination with microbubbles, US can temporarily change the permeability of cell membranes by cavitation and facilitate entry of plasmid DNA into cells. Here, we demonstrate that repeated US-mediated delivery of anti-angiogenic genes, endostatin or calreticulin, into muscle significantly inhibits the growth of orthotopic tumors in the liver, brain or lung. US-mediated anti-angiogenic gene therapy also seems to function as an adjuvant therapy that significantly enhances the antitumor effects of the chemotherapeutic drug doxorubicin and adenovirus-mediated cytokine gene therapy. Significantly higher levels of tumor apoptosis or tumor-infiltrating lymphocytes were observed after combined therapy consisting of either anti-angiogenic therapy and chemotherapy, or anti-angiogenic therapy and immunotherapy. Taken together, our experiments demonstrate that intramuscular delivery of anti-angiogenic genes by US exposure can effectively treat distant orthotopic tumors, and thus has great therapeutic potential in terms of clinical treatment.

Elucidating the mechanisms behind sonoporation with adeno-associated virus-loaded microbubbles

Microbubbles are Food and Drug Administration (FDA) approved contrast agents for ultrasound imaging. It has been reported that applying ultrasound on drug-loaded microbubbles facilitates drug uptake by cells, due to so-named sonoporation. However, the biophysics behind sonoporation are not fully understood. It is believed that sonoporation results in a "direct" delivery of drugs in the cytoplasm of cells, though it has been suggested as well that sonoporation facilitates endocytosis which would improve the internalization of drugs by cells. To get a better understanding of sonoporation, this study reports on the ultrasound assisted delivery of adeno-associated virus (AAV) loaded on the surface of microbubbles. AAVs rely on endocytosis for efficient transduction of cells and are, consequently, an elegant tool to evaluate whether endocytosis is involved in ultrasound-induced sonoporation. Applying ultrasound on AAV-loaded microbubbles clearly improved the internalization of AAVs by cells, though transduction of the cells did not occur, indicating that by sonoporation substances become directly delivered in the cytosol of cells.

In vivo gene transfer into the ocular ciliary muscle mediated by ultrasound and microbubbles

This study aimed to assess application of ultrasound (US) combined with microbubbles (MB) to transfect the ciliary muscle of rat eyes. Reporter DNA plasmids encoding for Gaussia luciferase, β-galactosidase or the green fluorescent protein (GFP), alone or mixed with 50% Artison MB, were injected into the ciliary muscle, with or without US exposure (US set at 1 MHz, 2 W/cm(2), 50% duty cycle for 2 min). Luciferase activity was measured in ocular fluids at 7 and 30 days after sonoporation. At 1 week, the US+MB treatment showed a significant increase in luminescence compared with control eyes, injected with plasmid only, with or without MB (×2.6), and, reporter proteins were localized in the ciliary muscle by histochemical analysis. At 1 month, a significant decrease in luciferase activity was observed in all groups. A rise in lens and ciliary muscle temperature was measured during the procedure but did not result in any observable or microscopic damages at 1 and 8 days. The feasibility to transfer gene into the ciliary muscle by US and MB suggests that sonoporation may allow intraocular production of proteins for the treatment of inflammatory, angiogenic and/or degenerative retinal diseases.

The phagocytosis of gas-filled microbubbles by human and murine antigen-presenting cells

This study was designed to evaluate the potential of gas-filled microbubbles (MB) to be internalized by antigen-presenting cells (APC). Fluorescently labeled MB were prepared, thus permitting to track binding to, and internalization in, APC. Both human and mouse cells, including monocytes and dendritic cells (DC), prove capable to phagocyte MB in vitro. Observation by confocal laser scanning microscopy showed that interaction between MB and target cells resulted in a rapid internalization in cellular compartments and to a lesser extent in the cytoplasm. Capture of MB by APC resulted in phagolysosomal targeting as verified by double staining with anti-lysosome-associated membrane protein-1 monoclonal antibody and decrease of internalization by phagocytosis inhibitors. Fluorescent MB injected subcutaneously (s.c.) in mice were found to be associated with CD11c(+)DC in lymph nodes draining the injection sites 24 h after administration. Altogether, our study demonstrates that MB can successfully target APC both in vitro and in vivo, and thus may serve as a potent Ag delivery system without requirement for ultrasound-based sonoporation. This adds to the potential of applications of MB already extensively used for diagnostic imaging in humans.

Controlled permeation of cell membrane by single bubble acoustic cavitation

Sonoporation is the membrane disruption generated by ultrasound and has been exploited as a non-viral strategy for drug and gene delivery. Acoustic cavitation of microbubbles has been recognized to play an important role in sonoporation. However, due to the lack of adequate techniques for precise control of cavitation activities and real-time assessment of the resulting sub-micron process of sonoporation, limited knowledge has been available regarding the detail processes and correlation of cavitation with membrane disruption at the single cell level. In the current study, we developed a combined approach including optical, acoustical, and electrophysiological techniques to enable synchronized manipulation, imaging, and measurement of cavitation of single bubbles and the resulting cell membrane disruption in real-time. Using a self-focused femtosecond laser and high frequency ultrasound (7.44MHz) pulses, a single microbubble was generated and positioned at a desired distance from the membrane of a Xenopus oocyte. Cavitation of the bubble was achieved by applying a low frequency (1.5MHz) ultrasound pulse (duration 13.3 or 40μs) to induce bubble collapse. Disruption of the cell membrane was assessed by the increase in the transmembrane current (TMC) of the cell under voltage clamp. Simultaneous high-speed bright field imaging of cavitation and measurements of the TMC were obtained to correlate the ultrasound-generated bubble activities with the cell membrane poration. The change in membrane permeability was directly associated with the formation of a sub-micrometer pore from a local membrane rupture generated by bubble collapse or bubble compression depending on ultrasound amplitude and duration. The impact of the bubble collapse on membrane permeation decreased rapidly with increasing distance (D) between the bubble (diameter d) and the cell membrane. The effective range of cavitation impact on membrane poration was determined to be D/d=0.75. The maximum mean radius of the pores was estimated from the measured TMC to be 0.106±0.032μm (n=70) for acoustic pressure of 1.5MPa (duration 13.3μs), and increased to 0.171±0.030μm (n=125) for acoustic pressure of 1.7MPa and to 0.182±0.052μm (n=112) for a pulse duration of 40μs (1.5MPa). These results from controlled cell membrane permeation by cavitation of single bubbles revealed insights and key factors affecting sonoporation at the single cell level.

Sonoporation delivery of interleukin-27 gene therapy efficiently reduces prostate tumor cell growth in vivo

We have examined the potential of a novel cytokine, interleukin-27 (IL-27), for gene therapy of prostate cancer. IL-27 is the most recently characterized member of the family of heterodimeric IL-12-related cytokines and has shown promise in halting tumor growth and mediating tumor regression in several cancer models. In the present study, we examined the efficacy of a new mode of gene delivery to prostate tumors: low-frequency ultrasound irradiation or "sonoporation." We also examined the potential of IL-27 gene delivery by sonoporation to treat and reduce the growth of prostate cancer in vivo. We used three models of immune-competent prostate adenocarcinoma and characterized the tumor-growth reduction, gene-profile expression, and effector cellular profiles. Our results suggest that IL-27 can be effective in reducing tumor growth and can help enhance accumulation of effector cells in prostate tumors in vivo. These results are promising, because they are potentially relevant to developing novel therapies that can be translated by using the novel and effective sonoporation gene-therapy delivery strategy.

Introduction to the ultrasound targeted microbubble destruction technique

In UTMD, bioactive molecules, such as negatively charged plasmid DNA vectors encoding a gene of interest, are added to the cationic shells of lipid microbubble contrast agents. In mice these vector-carrying microbubbles can be administered intravenously or directly to the left ventricle of the heart. In larger animals they can also be infused through an intracoronary catheter. The subsequent delivery from the circulation to a target organ occurs by acoustic cavitation at a resonant frequency of the microbubbles. It seems likely that the mechanical energy generated by the microbubble destruction results in transient pore formation in or between the endothelial cells of the microvasculature of the targeted region. As a result of this sonoporation effect, the transfection efficiency into and across the endothelial cells is enhanced, and transgene-encoding vectors are deposited into the surrounding tissue. Plasmid DNA remaining in the circulation is rapidly degraded by nucleases in the blood, which further reduces the likelihood of delivery to non-sonicated tissues and leads to highly specific target-organ transfection.

Cellular Injury of Cardiomyocytes during Hepatocyte Growth Factor Gene Transfection with Ultrasound-Triggered Bubble Liposome Destruction

We transfected naked HGF plasmid DNA into cultured cardiomyocytes using a sonoporation method consisting of ultrasound-triggered bubble liposome destruction. We examined the effects on transfection efficiency of three concentrations of bubble liposome (1 × 10⁶, 1 × 10⁷, 1 × 10⁸/mL), three concentrations of HGF DNA (60, 120, 180 μg/mL), two insonification times (30, 60 sec), and three incubation times (15, 60, 120 min). We found that low concentrations of bubble liposome and low concentrations of DNA provided the largest amount of the HGF protein expression by the sonoporated cardiomyocytes. Variation of insonification and incubation times did not affect the amount of product. Following insonification, cardiomyocytes showed cellular injury, as determined by a dye exclusion test. The extent of injury was most severe with the highest concentration of bubble liposome. In conclusion, there are some trade-offs between gene transfection efficiency and cellular injury using ultrasound-triggered bubble liposome destruction as a method for gene transfection.

Enhanced shock wave-assisted transformation of Escherichia coli

The objective of the study was to demonstrate that shock wave-induced transfer of DNA into bacteria can be increased by enhancing cavitation using dual-pulse (tandem) shock waves. Escherichia coli and plasmid were transferred to test vials. Competent cells were prepared at different concentrations of CaCl(2). Single pulses and tandem shock waves were compared as were three treatment temperatures: 0, 10 and 25 °C. Three delays (250, 500, 750 μs) between double pulses were tested. Characterization was achieved by using a plasmid that provided green fluorescent protein expression. At 0 °C double pulses generated at a delay of 750 μs significantly increased the number of fluorescent colonies compared with single pulses. In general, the lowest temperature enhanced the mean number of transformants compared with the two higher temperatures. A strong influence of the CaCl(2) concentration on the transformation efficiency was also found. The main conclusion is that gene transfer to target cells may be increased up to 50 times at 0 °C by enhancing cavitation using pairs of shock waves.

P85, Optison microbubbles and ultrasound cooperate in mediating plasmid DNA transfection in mouse skeletal muscles in vivo

Pluronic block copolymers, a kind of non-ionic surfactant, also known as poloxamers, and ultrasound-targeted microbubble destruction have been respectively investigated as vectors for gene delivery in vitro and in vivo. However, they are limited for clinical application due to the relatively low transfer efficiency of each individual vector. In the present study, we explored if the combination of P85, a pluronic block copolymer, Optison, a microbubble contrast agent and ultrasound enhances the transfection of plasmid DNA in vivo using mouse skeletal muscle models. Plasmid encoding green fluorescent protein (GFP) was respectively conjugated with 0.05%P85, 10%Optison, or 0.05%P85 plus 10%Optison, and injected into mouse tibialis anterior (TA) muscles with or without ultrasound irradiation (1 MHz, 1 W/cm(2), 2 min and 20% duty cycle). Mice were sacrificed 1 week after injection. The TA muscles were collected and cryo-sectioned into a series of 7 μm slices. To assess the efficiency of plasmid DNA transfection, tissue sections were counterstained with DAPI and scored by counting the number of GFP-positive fibers. Meanwhile the area of damaged muscles was measured based on the tissues stained with hematoxylin and eosin. Both P85 and Optison significantly enhanced the delivery of plasmid DNA in mouse TA skeletal muscles (P<0.01 and P<0.05 respectively, compared to saline control). In combination with Ultrasound irradiation, P85 (P<0.01, compared to P85 alone) but not Optison (P>0.05, compared to Optison alone) exerted a more pronounced effect on the transfection efficiency. Furthermore P85-induced gene delivery was higher than that by Optison regardless of the presence of ultrasound (P<0.01). The highest transfection efficiency was observed when P85, Optison and ultrasound irradiation were administrated together (P<0.01, compared to any other treatment in this study). The area of damaged muscles was enlarged by ultrasound irradiation in the presence of Optison microbubbles (P<0.01, compared to those groups without ultrasound irradiation). These results suggest that P85, microbubbles and ultrasound irradiation synergistically enhance plasmid DNA delivery in mouse skeletal muscles in vivo.

Buckling resistance of solid shell bubbles under ultrasound

Thin solid shell contrast agents bubbles are expected to undergo different volume oscillating behaviors when the acoustic power is increased: small oscillations when the shell remains spherical, and large oscillations when the shell buckles. Contrary to bubbles covered with thin lipidic monolayers that buckle as soon as compressed: the solid shell bubbles resist compression, making the buckling transition abrupt. Numerical simulations that explicitly incorporate a shell bending modulus give the critical buckling pressure and post-buckling shape, and show the appearance of a finite number of wrinkles. These findings are incorporated in a model based on the concept of effective surface tension. This model compares favorably to experiments when adjusting two main parameters: the buckling tension and the rupture shell tension. The buckling tension provides a direct estimation of the acoustic pressure threshold at which buckling occurs.

Self-assembled liposome-loaded microbubbles: The missing link for safe and efficient ultrasound triggered drug-delivery

Liposome-loaded microbubbles have been recently introduced as a promising drug delivery platform for ultrasound guided drug delivery. In this paper we design liposome-loaded (lipid-shelled) microbubbles through the simple self-assembly of the involved compounds in a single step process. We thoroughly characterized the liposome-loading of the microbubbles and evaluated the cell killing efficiency of this material using doxorubicin (DOX) as a model drug. Importantly, we observed that the DOX liposome-loaded microbubbles allowed killing of melanoma cells even at very low doses of DOX. These findings clearly prove the potential of liposome-loaded microbubbles for ultrasound targeted drug delivery to cancer tissues.

The actin cytoskeleton has an active role in the electrotransfer of plasmid DNA in mammalian cells

Electrotransfer of molecules is a well established technique which finds extensive use for gene transfer and holds great promise for anticancer treatment. Despite its widespread application, the mechanisms governing the entry of DNA into the cell and its intracellular trafficking are not yet known. The aim of this study is to unravel the role of the actin cytoskeleton during gene electrotransfer in cells. We performed single-cell level approaches to observe the organization of the actin cytoskeleton in Chinese hamster ovary (CHO) cells. In addition, we performed experiments at the multiple-cell level to evaluate the efficiency of DNA transfer after alteration of the actin cytoskeleton using the drug latrunculin B. Actin patches colocalizing with the DNA at the plasma membrane were observed with additional characteristics similar to those of the DNA aggregates in terms of time, number, and size. The disruption of the microfilaments reduces the DNA accumulation at the plasma membrane and the gene expression. This is the first direct experimental evidence of the participation of the actin cytoskeleton in DNA electrotransfer.

Metodologie per il trasferimento genico

L’esito di qualsiasi approccio di terapia genica, sia che esso preveda l’inoculazione del materiale genetico direttamente in vivo sia che venga effettuato ex vivo nelle cellule prelevate dal paziente, dipende strettamente dall’efficienza con cui gli acidi nucleici con funzione terapeutica vengono internalizzati dalle cellule bersaglio. Di fatto, l’efficienza del trasferimento genico probabilmente rappresenta a tutt’oggi il parametro più importante che ancora limita le applicazioni di terapia genica, o comunque ne condiziona il successo.

A new method for measuring edge tensions and stability of lipid bilayers: effect of membrane composition

We report a novel and facile method for measuring edge tensions of lipid membranes. The approach is based on electroporation of giant unilamellar vesicles and analysis of the pore closure dynamics. We applied this method to evaluate the edge tension in membranes with four different compositions: egg phosphatidylcholine (eggPC), dioleoylphosphatidylcholine (DOPC), and mixtures of DOPC with cholesterol and dioleoylphosphatidylethanolamine. Our data confirm previous results for eggPC and DOPC. The addition of 17 mol % cholesterol to the DOPC membrane causes an increase in the membrane edge tension. On the contrary, when the same fraction of dioleoylphosphatidylethanolamine is added to the membrane, a decrease in the edge tension is observed, which is an unexpected result considering the inverted-cone shape geometry of the molecule. It is presumed that interlipid hydrogen bonding is the origin of this behavior. Furthermore, cholesterol was found to lower the lysis tension of DOPC bilayers. This behavior differs from that observed on bilayers made of stearoyloleoylphosphatidylcholine, suggesting that cholesterol influences the membrane mechanical stability in a lipid-specific manner.

Methods for Gene Delivery

The success of any gene transfer procedure, either through in vivo inoculation of the genetic material or after gene transfer into the patient’s cells ex vivo, strictly depends upon the efficiency of nucleic acid internalization by the target cells. As a matter of fact, making gene transfer more efficient continues to represent the most relevant challenge to the clinical success of gene therapy.

In vitro gene transfer by electrosonoporation

Among the nonviral methods for gene delivery in vitro, electroporation is simple, inexpensive and safe. To upregulate the expression level of transfected gene, we investigated the applicability of electrosonoporation. This approach consists of a combination of electric pulses and ultrasound assisted with gas microbubbles. Cells were first electroporated with plasmid DNA encoding-enhanced green fluorescent protein and then sonoporated in presence of contrast microbubbles. Twenty-four hours later, cells that received electrosonoporation demonstrated a four-fold increase in transfection level and a six-fold increase in transfection efficiency compared with cells having undergone electroporation alone. Although electroporation induced the formation of DNA aggregates into the cell membrane, sonoporation induced its direct propulsion into the cytoplasm. Sonoporation can improve the transfer of electro-induced DNA aggregates by allowing its free and rapid entrance into the cells. These results demonstrated that in vitro gene transfer by electrosonoporation could provide a new potent method for gene transfer.

Molecular imaging of inflammation and intraplaque vasa vasorum: a step forward to identification of vulnerable plaques?

Current developments in cardiovascular biology and imaging enable the noninvasive molecular evaluation of atherosclerotic vascular disease. Intraplaque neovascularization sprouting from the adventitial vasa vasorum has been identified as an independent predictor of intraplaque hemorrhage and plaque rupture. These intraplaque vasa vasorum result from angiogenesis, most likely under influence of hypoxic and inflammatory stimuli. Several molecular imaging techniques are currently available. Most experience has been obtained with molecular imaging using positron emission tomography and single photon emission computed tomography. Recently, the development of targeted contrast agents has allowed molecular imaging with magnetic resonance imaging, ultrasound and computed tomography. The present review discusses the use of these molecular imaging techniques to identify inflammation and intraplaque vasa vasorum to identify vulnerable atherosclerotic plaques at risk of rupture and thrombosis. The available literature on molecular imaging techniques and molecular targets associated with inflammation and angiogenesis is discussed, and the clinical applications of molecular cardiovascular imaging and the use of molecular techniques for local drug delivery are addressed.

Ultrasound-mediated DNA transformation in thermophilic gram-positive anaerobes

Background

Thermophilic, Gram-positive, anaerobic bacteria (TGPAs) are generally recalcitrant to chemical and electrotransformation due to their special cell-wall structure and the low intrinsic permeability of plasma membranes.

Methodology/Principal Findings

Here we established for any Gram-positive or thermophiles an ultrasound-based sonoporation as a simple, rapid, and minimally invasive method to genetically transform TGPAs. We showed that by applying a 40 kHz ultrasound frequency over a 20-second exposure, Texas red-conjugated dextran was delivered with 27% efficiency into Thermoanaerobacter sp. X514, a TGPA that can utilize both pentose and hexose for ethanol production. Experiments that delivered plasmids showed that host-cell viability and plasmid DNA integrity were not compromised. Via sonoporation, shuttle vectors pHL015 harboring a jellyfish gfp gene and pIKM2 encoding a Clostridium thermocellum β-1,4-glucanase gene were delivered into X514 with an efficiency of 6×10² transformants/µg of methylated DNA. Delivery into X514 cells was confirmed via detecting the kanamycin-resistance gene for pIKM2, while confirmation of pHL015 was detected by visualization of fluorescence signals of secondary host-cells following a plasmid-rescue experiment. Furthermore, the foreign β-1,4-glucanase gene was functionally expressed in X514, converting the host into a prototypic thermophilic consolidated bioprocessing organism that is not only ethanologenic but cellulolytic.

Conclusions/Significance

In this study, we developed an ultrasound-based sonoporation method in TGPAs. This new DNA-delivery method could significantly improve the throughput in developing genetic systems for TGPAs, many of which are of industrial interest yet remain difficult to manipulate genetically.

A model for the dynamics of ultrasound contrast agents in vivo

The Rayleigh-Plesset (RP) equation for a clean gas bubble in an incompressible and infinite liquid has previously been applied to approximately simulate the behavior of ultrasound contrast agents (UCA) in vivo, and extended RP equations have been proposed to account for the effects of the UCA shell or surrounding soft tissue. These models produce results that are consistent with experimental measurements for low acoustic pressure scenarios. For applications of UCAs in therapeutic medicine, the transmitted acoustic pulse can have a peak negative pressure (PNP) up to a few megapascals, resulting in discrepancies between measurements and predictions using these extended RP equations. Here, a model was developed to describe the dynamics of UCAs in vivo while taking account of the effects of liquid compressibility, the shell and the surrounding tissue. Liquid compressibility is approximated to first order and the shell is treated either as a Voigt viscoelastic solid or a Newtonian viscous liquid. Finite deformation of the shell and tissue is derived. Dynamics of UCAs with a shell of lipid, polymer, albumin and liquid are investigated for typical therapeutic ultrasound pulses. The effects of liquid compressibility and shell and tissue parameters are analyzed.

Ultrasound-enhanced drug transport and distribution in the brain

Drug delivery in the brain is limited by slow drug diffusion in the brain tissue. This study tested the hypothesis that ultrasound can safely enhance the permeation of drugs in the brain. In vitro exposure to ultrasound at various frequencies (85 kHz, 174 kHz, and 1 MHz) enhanced the permeation of tritium-labeled molecules with molecular weight up to 70 kDa across porcine brain tissue. A maximum enhancement of 24-fold was observed at 85 kHz and 1,200 J/cm². In vivo exposure to 1-MHz ultrasound further demonstrated the ability of ultrasound to facilitate molecule distribution in the brain of a non-human primate. Finally, ultrasound under conditions similar to those used in vivo was shown to cause no damage to plasmid DNA, siRNA, adeno-associated virus, and fetal rat cortical neurons over a range of conditions. Altogether, these studies demonstrate that ultrasound can increase drug permeation in the brain in vitro and in vivo under conditions that did not cause detectable damage.

Optimum conditions of ultrasound-mediated destruction of bubble liposome for siRNA transfer in bladder cancer

Background: We investigated the effectiveness of ultrasound-mediated destruction of bubble liposome (UBL) for siRNA transfer by observing reduction in the luciferase activity of human bladder tumor RT-112 cells transfected with the luciferase gene (RT-112Luc) following luciferase siRNA transfer into the cells. Results: siRNA was transferred to 26% of RT-112Luc cells by UBL and the luciferase activity of RT-112Luc cells was significantly suppressed by UBL using the luciferase siRNA, compared with that using nonspecific siRNA in vitro (p = 0.036). The luciferase activity of RT-112Luc tumor was suppressed by UBL using luciferase siRNA compared with that using nonspecific siRNA 2 days after the in vivo treatment. Conclusion: This study showed that UBL is suitable for siRNA transfer to mammalian cells.

Exploiting ultrasound-mediated effects in delivering targeted, site-specific cancer therapy

Although the concept of employing ultrasound for the treatment of cancer is not a new one, virtually all existing ultrasound-based clinical cancer treatments are based on hyperthermic ablation. This review seeks to highlight the potential offered by more subtle ultrasound-triggered phenomena such as sonoporation in delivering novel targeted cancer treatment modalities.

Focused ultrasound and microbubbles for enhanced extravasation

The permeability of blood vessels for albumin can be altered by using ultrasound and polymer or lipid-shelled microbubbles. The region in which the microbubbles were destroyed with focused ultrasound was quantified in gel phantoms as a function of pressure, number of cycles and type of microbubble. At 2MPa the destruction took place in a fairly wide area for a lipid-shelled agent, while for polymer-shelled agents at this setting, distinct destruction spots with a radius of only 1mm were obtained. When microbubbles with a thicker shell were used, the pressure above which the bubbles were destroyed shifts to higher values. In vivo both lipid and polymer microbubbles increased the extravasation of the albumin binding dye Evans Blue, especially in muscle leading to about 6-8% of the injected dose to extravasate per gram muscle tissue 30 min after start of the treatment, while no Evans Blue could be detected in muscle in the absence of microbubbles. Variation in the time between ultrasound treatment and Evans Blue injection, demonstrated that the time window for promoting extravasation is at least an hour at the settings used. In MC38 tumors, extravasation already occurred without ultrasound and only a trend towards enhancement with about a factor of 2 could be established with a maximum percentage injected dose per gram of 3%. Ultrasound mediated microbubble destruction especially enhances the extravasation in the highly vascularized outer part of the MC38 tumor and adjacent muscle and would, therefore, be most useful for release of, for instance, anti-angiogenic drugs.

Novel polymeric scaffolds using protein microbubbles as porogen and growth factor carriers

Polymeric tissue engineering scaffolds prepared by conventional techniques like salt leaching and phase separation are greatly limited by their poor biomolecule-delivery abilities. Conventional methods of incorporation of various growth factors, proteins, and/or peptides on or in scaffold materials via different crosslinking and conjugation techniques are often tedious and may affect scaffold's physical, chemical, and mechanical properties. To overcome such deficiencies, a novel two-step porous scaffold fabrication procedure has been created in which bovine serum albumin microbubbles (henceforth MB) were used as porogen and growth factor carriers. Polymer solution mixed with MB was phase separated and then lyophilized to create porous scaffold. MB scaffold triggered substantially lesser inflammatory responses than salt-leached and conventional phase-separated scaffolds in vivo. Most importantly, the same technique was used to produce insulin-like growth factor-1 (IGF-1)-eluting porous scaffolds, simply by incorporating IGF-1-loaded MB (MB-IGF-1) with polymer solution before phase separation. In vitro such MB-IGF-1 scaffolds were able to promote cell growth to a much greater extent than scaffold soaked in IGF-1, confirming the bioactivity of the released IGF-1. Further, such MB-IGF-1 scaffolds elicited IGF-1-specific collagen production in the surrounding tissue in vivo. This novel growth factor-eluting scaffold fabrication procedure can be used to deliver a range of single or combination of bioactive biomolecules to substantially promote cell growth and function in degradable scaffold.

Sonoporation-mediated gene transfer into adult rat dorsal root ganglion cells

Background

Gene transfer into many cell types has been successfully used to develop alternative and adjunct approaches to conventional medical treatment. However, effective transfection of postmitotic neurons remains a challenge. The aim of this study was to develop a method for gene transfer into rat primary dorsal root ganglion neurons using sonoporation.

Methods

Dissociated cells from adult rat dorsal root ganglion (DRG) cells were sonicated for 1-8 s at 2.5-10 W to determine the optimal ultrasound duration and power for gene transfection and cell survival. Transfection efficiency was compared between sonoporation, liposome and lentiviral vector gene transfer techniques.

Results

The optimum ultrasound intensity was 5 W for 2 s and yielded an efficiency of gene transfection of 31% and a survival rate of 35%.

Conclusions

Sonoporation can be optimized to minimize cell death and yield a high percentage of transfected neurons and that this technique can be easily applied to primary cultures of rat dorsal root ganglion neurons.

Ultrasound-mediated gene delivery

IMPORTANCE OF THE FIELD

The use of ultrasound with microbubbles raises the possibility of an efficient and safe gene delivery.

AREAS COVERED IN THIS REVIEW

This review summarizes the current state of the art of gene delivery by sonoporation under the following topics. First, the basic ultrasound parameters and the characteristics of microbubble in biological systems are discussed. Second, the extensions of sonoporation to other fields of gene delivery such as viral and non-viral vector are briefly reviewed. Finally, recent applications in an animal model for various diseases are introduced.

WHAT THE READER WILL GAIN

Information and comments on gene delivery by sonoporation or enhanced cell membrane permeability by means of ultrasound.

TAKE HOME MESSAGE

Ultrasound-mediated gene delivery combined with microbubble agents provides significant safety advantages over other methods of local gene delivery.

Sonoporation, drug delivery, and gene therapy

Ultrasound is a very effective modality for drug delivery and gene therapy because energy that is non-invasively transmitted through the skin can be focused deeply into the human body in a specific location and employed to release drugs at that site. Ultrasound cavitation, enhanced by injected microbubbles, perturbs cell membrane structures to cause sonoporation and increases the permeability to bioactive materials. Cavitation events also increase the rate of drug transport in general by augmenting the slow diffusion process with convective transport processes. Drugs and genes can be incorporated into microbubbles, which in turn can target a specific disease site using ligands such as the antibody. Drugs can be released ultrasonically from microbubbles that are sufficiently robust to circulate in the blood and retain their cargo of drugs until they enter an insonated volume of tissue. Local drug delivery ensures sufficient drug concentration at the diseased region while limiting toxicity for healthy tissues. Ultrasound-mediated gene delivery has been applied to heart, blood vessel, lung, kidney, muscle, brain, and tumour with enhanced gene transfection efficiency, which depends on the ultrasonic parameters such as acoustic pressure, pulse length, duty cycle, repetition rate, and exposure duration, as well as microbubble properties such as size, gas species, shell material, interfacial tension, and surface rigidity. Microbubble-augmented sonothrombolysis can be enhanced further by using targeting microbubbles.