Microbubble ultrasound improves the efficiency of gene transduction in skeletal muscle in vivo with reduced tissue damage

Optical microscopic findings of the behavior of perflubutane microbubbles outside and inside Kupffer cells during diagnostic ultrasound examination

PURPOSE

To investigate the behavior of perflubutane microbubbles outside and inside Kupffer cells during diagnostic ultrasound (US) examination, and to determine the thresholds of the acoustic pressure of different kinds of behavior.

METHODS

Acoustic behavior of perflubutane microbubbles inside and outside Kupffer cells in an acoustic field induced by a clinical US transducer and equipment was optically observed in vitro. The acoustic pressure was measured simultaneously by a calibrated hydrophone and an oscilloscope.

RESULTS

The acoustic behavior of microbubbles was optically categorized as stabilization, oscillation, transposition, shrinkage, and destruction. The mechanical index (MI) displayed on the US equipment correlated well with the acoustic pressure at the level of microbubbles measured hydrophonically. At a frame rate of 15 Hz with a frequency of 3.5 MHz and pulse repetition frequency of 3 KHz, the thresholds in term of MI for free microbubbles to begin oscillation, reach best oscillation, transposition, shrinkage, and destruction were 0.21, 0.44, 0.53, 0.75, and 1.03, respectively. Although adherent and phagocytosed microbubbles showed more stability enduring insonation compared with free microbubbles, the thresholds of shrinkage and destruction were MI 1.03 and 1.18 for adherent microbubbles, and 1.18 and 1.37 for phagocytosed microbubbles, respectively. Neither oscillation nor transposition of microbubbles inside Kupffer cells was observed microscopically. No cell damage because of microbubbles destruction was found in the present study.

CONCLUSION

Perflubutane microbubbles outside and inside Kupffer cells respond to external US insonation with same parameters of a clinical contrast-enhanced US study according to the acoustic pressure. Free microbubbles behave as stabilization, oscillation, transposition, shrinkage, and destruction under insonation. The adherent and phagocytosed microbubbles are more stable under insonation than free microbubbles, but still respond showing shrinkage and destruction when MI is over 1.03.

Application of ultrasonic gas-filled liposomes in enhancing transfer for breast cancer-related antisense oligonucleotides: an experimental study

The aim of this study was to investigate the application of ultrasonic gas-filled liposomes in enhancing transfer for breast cancer-related antisense oligonucleotides in vitro. An antisense oligodeoxynucleotide (AS-ODN) sequence, HA2741, modified with luciferase reporter plasmid, was used in evaluating the enhancing effect of gas-filled liposomes for gene transfer in breast cancer cells. Some important factors on HA 2741 transfection efficiency, such as wave intensity, ultrasound duration, gas-filled liposome concentration, and HA2741 concentration, were tested, respectively. Transfection efficiency was detected by fluorescence microscopy. Cell viability was verified by propidium iodide assay. Reverse-transcriptase polymerase chain reaction and immunocytochemistry were used to detect the inhibitory effect of HA2741 on HER-2 expression. All the four factors (wave intensity, ultrasound duration, gas-filled liposome concentration, and HA2741 concentration) showed a positive effect on AS-ODN transfection efficiency. However, these factors had a negative effect on cell viability. Considering all the factors investigated, the maximum transfection efficiency with minimum cell viability achieved under 2% gas-filled liposome mixed with 80 nmol/L HA2741 for 30-second ultrasound exposure at -3.0 dB wave intensity, which gave an overall transfection efficiency exceeding 90% and a cell viability near 90%. Under controlled conditions, ultrasound-mediated AS-ODN transfer, enhanced by gas-filled liposomes, may represent an effective, safe avenue for cancer-related gene delivery.

Physical methods of nucleic acid transfer: general concepts and applications

Physical methods of gene (and/or drug) transfer need to combine two effects to deliver the therapeutic material into cells. The physical methods must induce reversible alterations in the plasma membrane to allow the direct passage of the molecules of interest into the cell cytosol. They must also bring the nucleic acids in contact with the permeabilized plasma membrane or facilitate access to the inside of the cell. These two effects can be achieved in one or more steps, depending upon the methods employed. In this review, we describe and compare several physical methods: biolistics, jet injection, hydrodynamic injection, ultrasound, magnetic field and electric pulse mediated gene transfer. We describe the physical mechanisms underlying these approaches and discuss the advantages and limitations of each approach as well as its potential application in research or in preclinical and clinical trials. We also provide conclusions, comparisons, and projections for future developments. While some of these methods are already in use in man, some are still under development or are used only within clinical trials for gene transfer. The possibilities offered by these methods are, however, not restricted to the transfer of genes and the complementary uses of these technologies are also discussed. As these methods of gene transfer may bypass some of the side effects linked to viral or biochemical approaches, they may find their place in specific clinical applications in the future.

Ultrasound targeted microbubble destruction for drug and gene delivery

BACKGROUND

Gas-filled microbubbles have been used as ultrasound contrast agents for some decades. More recently, such microbubbles have evolved as experimental tools for organ- and tissue-specific drug and gene delivery. When sonified with ultrasound near their resonance frequency, microbubbles oscillate. With higher ultrasound energies, oscillation amplitudes increase, leading to microbubble destruction. This phenomenon can be used to deliver a substance into a target organ, if microbubbles are co-administered loaded with drugs or gene therapy vectors before i.v. injection.

OBJECTIVE

This review focuses on different experimental applications of microbubbles as tools for drug and gene delivery. Different organ systems and different classes of bioactive substances that have been used in previous studies will be discussed.

METHODS

All the available literature was reviewed to highlight the potential of this non-invasive, organ-specific delivery system.

CONCLUSION

Ultrasound targeted microbubble destruction has been used in various organ systems and in tumours to successfully deliver drugs, proteins, gene therapy vectors and gene silencing constructs. Many proof of principle studies have demonstrated its potential as a non-invasive delivery tool. However, too few large animal studies and studies with therapeutic aims have been performed to see a clinical application of this technique in the near future. Nevertheless, there is great hope that preclinical large animal studies will confirm the successful results already obtained in small animals.

Physical approaches for nucleic acid delivery to liver

The liver is a key organ for numerous metabolic pathways and involves many inherited diseases that, although being different in their pathology, are often caused by lack or overproduction of a critical gene product in the diseased cells. In principle, a straightforward method to fix such problem is to introduce into these cells with a gene-coding sequence to provide the missing gene product or with the nucleic acid sequence to inhibit production of the excessive gene product. Practically, however, success of nucleic acid-based pharmaceutics is dependent on the availability of a method capable of delivering nucleic acid sequence in the form of DNA or RNA to liver cells. In this review, we will summarize the progress toward the development of physical methods for nucleic acid delivery to the liver. Emphasis is placed on the mechanism of action, pros, and cons of each method developed so far. We hope the information provided will encourage new endeavor to improve the current methodologies or develop new strategies that will lead to safe and effective delivery of nucleic acids to the liver.

Sonoporation of the minicircle-VEGF(165) for wound healing of diabetic mice

PURPOSE

The purpose of this study is to examine the efficiency of sonoporation with minicircle DNA for the skin wound healing in diabetic mice.

METHODS

Minicircle DNA containing the human VEGF(165) was constructed and tested in vitro. Diabetes was induced in 2-week old male C57BL/6J mice via streptozotocin (STZ) injection. 6 mm circular skin wounds were made on the mice back. After the subcutaneous injection of the minicircle DNA at the edge of the wound, the mice were exposed to the ultrasound irradiation for the sonoporation. Wound areas were analyzed until the day 12. Blood perfusion and angiogenesis were evaluated using a laser Doppler imaging and CD31 immunostaining, respectively. Re-epithelialization was assessed by histochemical analysis using hematoxylin and eosin staining.

RESULTS

Accelerated wound closure was observed in the mice receiving sonoporation of minicircle-VEGF(165), which corresponds to the markedly increased skin blood perfusion and CD31 expression. Histological analysis revealed that the minicircle treated wound tissues showed fully restored normal architectures as compared with the non-treated diabetic controls with the markedly edematous and chaotic morphologies.

CONCLUSIONS

Ultrasound mediated gene therapy with the minicircle-VEGF(165) is effective for the healing of the skin wound of the diabetic mice.

Acoustic attenuation by contrast agent microbubbles in superficial tissue markedly diminishes petechiae bioeffects in deep tissue

OBJECTIVE

To measure how ultrasound attenuation by contrast agent microbubbles (MBs) in superficial tissue affects petechiae creation in underlying deep tissue.

MATERIALS AND METHODS

Studies using Sprague-Dawley rats were approved by the Animal Care and Use Committee. MBs were injected intravenously, and 12 ultrasound pulses (100 sinusoids of 1 MHz ultrasound per pulse) were applied through the skin overlying the hindlimb adductors at intervals of 10 or 60 seconds. In some groups, the skin was resected and immediately returned without re-establishing vascular connections. Muscle petechiae were counted.

RESULTS

Applying ultrasound through unperfused skin after bolus and continuous intravenous MB injection yielded, respectively, 30-fold and 3.5-fold more petechiae than for perfused skin. Surprisingly, petechiae/mm2 decreased with a higher MB dosage [0.12 +/- 0.05 (1 x 10 MBs/g) vs. 0.04 +/- 0.02 (3 x 10 MBs/g)] when ultrasound was applied through perfused skin. In contrast, petechiae/mm2 was approximately proportional to MB dosage for unperfused skin [0.17 +/- 0.10(5) (1 x 10 MBs/g) vs. 0.42 + 0.14 (3 x 10(5) MBs/g)]. In comparison to MB-free controls, MB solutions in this concentration range reduced the peak-negative pressure of ultrasound by 65% to 85%.

CONCLUSIONS

Acoustic attenuation by MBs in skin markedly reduces petechiae creation in deep muscle. Petechiae inhibition is dependent on [MB]2.1 and, therefore, dominates the otherwise proportional relationship between petechiae and [MB] in muscle. The drop of peak-negative pressure below a critical microvessel rupturing threshold is the probable mechanism for petechiae inhibition. These results indicate that high MB doses could, paradoxically, reduce the potential for petechiae creation and may have important bearing on the design of contrast ultrasound-based therapeutics.

Microwave irradiation enhances gene and oligonucleotide delivery and induces effective exon skipping in myoblasts

Microwave (MW) energy consists of electric and magnetic fields and is able to penetrate deep into biological materials. We investigated the effect of MW (2450 MHz) irradiation on gene delivery in cultured mouse myoblasts and observed enhanced transgene expression. This effect is, however, highly variable and critically dependent on the power levels, duration and cycle conditions of MW exposure. MW irradiation greatly enhances delivery of 2'O methyl-phosphorothioate antisense oligonucleotide (AON) targeting mouse dystrophin exon 23 and induces specific exon skipping in cultured myoblasts. Effective delivery of AON by MW irradiation is able to correct the dystrophin reading frame disrupted by a nonsense point mutation in the H2K mdx myoblasts, resulting in the restoration of dystrophin expression. MW-mediated nucleic acid delivery does not directly link to the increase in system temperature. The high variability in gene and oligonucleotide delivery is most likely the result of considerable irregularity in the distribution of the energy and magnetic field produced by MW with the current device. Therefore, achieving effective delivery of the therapeutic molecules would require new designs of MW devices capable of providing controllable and evenly distributed energy for homogenous exposure of the target cells.

Molecular-targeted therapy for Duchenne muscular dystrophy: progress and potential

Duchenne muscular dystrophy (DMD) is a lethal heritable childhood myodegenerative condition caused by a mutation within the gene encoding the dystrophin protein within the X chromosome. While, historically, patients with this condition rarely lived into their thirties, they are now living substantially longer as a result of new treatments based on multi-disciplinary care. Despite these advances, the prognosis for DMD patients is limited, and a progressive reduction in quality of life and early death in adulthood cannot be prevented using currently available treatment regimens. The best hopes for a cure lies with cellular and gene therapy approaches that target the underlying genetic defect. In the past several years, viral and nonviral gene therapy methodologies based on adeno-associated viruses, naked plasmid delivery, antisense oligonucleotides, and oligonucleotide-mediated gene editing have advanced to a high degree of sophistication, to the extent that research has moved from the laboratory setting to the clinic. Notwithstanding these accomplishments, shortcomings with each therapy remain, so more work is required to devise an appropriate therapeutic strategy for the management and eventual cure of this debilitating disease.

Liposomes in ultrasonic drug and gene delivery

Liposome-based drug and gene delivery systems have potential for significant roles in a variety of therapeutic applications. Recently, liposomes have been used to entrap gas and drugs for ultrasound-controlled drug release and ultrasound-enhanced drug delivery. Echogenic liposomes have been produced by different preparation methods, including lyophilization, pressurization, and biotin-avidin binding. Presently, significant in vivo applications of liposomal ultrasound-based drug and gene delivery are being made in cardiac disease, stroke and tumor therapy. Translation of these vehicles into the clinic will require a better understanding of improved physical properties to avoid rapid clearance, as well as of possible side effects, including those of the ultrasound. The aim of this review is to provide orientation for new researchers in the area of ultrasound-enhanced liposome drug and gene delivery.

Ultrasonic gene and drug delivery to the cardiovascular system

Ultrasound targeted microbubble destruction has evolved as a promising tool for organ specific gene and drug delivery. This technique has initially been developed as a method in myocardial contrast echocardiography, destroying intramyocardial microbubbles to characterize refill kinetics. When loading similar microbubbles with a bioactive substance, ultrasonic destruction of microbubbles may release the transported substance in the targeted organ. Furthermore, high amplitude oscillations of microbubbles lead to increased capillary and cell membrane permeability, thus facilitating tissue and cell penetration of the released substance. While this technique has been successfully used in many organs, its application in the cardiovascular system has dominated so far. Drug delivery using microbubbles has played a minor role in the cardiovascular system. In contrast, gene transfer has been successfully achieved in many studies. Both viral and non-viral vectors were used for loading on microbubbles. This review article will give an overview on studies that have applied ultrasound targeted microbubble destruction to deliver substances in the heart and blood vessels. It will show potential therapeutic targets, especially for gene therapy, describe feasible substances that can be loaded on microbubbles, and critically discuss prospects and limitations of this technique.

Ultrasound, cavitation bubbles and their interaction with cells

This article reviews the basic physics of ultrasound generation, acoustic field, and both inertial and non-inertial acoustic cavitation in the context of localized gene and drug delivery as well as non-linear oscillation of an encapsulated microbubble and its associated microstreaming and radiation force generated by ultrasound. The ultrasound thermal and mechanical bioeffects and relevant safety issues for in vivo applications are also discussed.

Nonviral approaches for targeted delivery of plasmid DNA and oligonucleotide

Successful gene therapy depends on the development of efficient delivery systems. Although pDNA and ODN are novel candidates for nonviral gene therapy, their clinical applications are generally limited owing to their rapid degradation by nucleases in serum and rapid clearance. A great deal of effort had been devoted to developing gene delivery systems, including physical methods and carrier-mediated methods. Both methods could improve transfection efficacy and achieve high gene expression in vitro and in vivo. As for carrier-mediated delivery in vivo, since gene expression depends on the particle size, charge ratio, and interaction with blood components, these factors must be optimized. Furthermore, a lack of cell-selectivity limits the wide application to gene therapy; therefore, the use of ligand-modified carriers is a promising strategy to achieve well-controlled gene expression in target cells. In this review, we will focus on the in vivo targeted delivery of pDNA and ODN using nonviral carriers.

Ultrasound with microbubbles enhances gene expression of plasmid DNA in the liver via intraportal delivery

Current ultrasound (US)-mediated gene delivery methods are inefficient due, in part, to a lack of US optimization. We systematically explored the use of microbubbles (MBs), US parameters and plasmid delivery routes to improve gene transfer into the mouse liver. Co-presentation of plasmid DNA (pDNA), 10% Optison MBs and pulsed 1-MHz US at a peak negative pressure of 4.3 MPa significantly increased luciferase gene expression with pDNA delivered by intrahepatic injection to the left liver lobe. Intraportal injection delivered pDNA and MBs to the whole liver; with insonation, all lobes expressed the transgene, thus increasing total gene expression. Gene expression was also dependent on acoustic pressure over the range of 0-4.3 MPa, with a peak effect at 3 MPa. An average of 85-fold enhancement in gene delivery was achieved. No enhancement was observed below 0.25 MPa. Increasing pulse length while decreasing pulse repetition frequency and exposure time to maintain a constant total energy during exposure did not further improve transfection efficiency, nor did extend the US exposure pre- or postinjection of pDNA. The results indicate that coupled with MBs, US can more efficiently and dose-dependently enhance gene expression from pDNA delivered via portal vein injection by an acoustic mechanism of inertial cavitation.

Nanopolymers improve delivery of exon skipping oligonucleotides and concomitant dystrophin expression in skeletal muscle of mdx mice

Background

Exon skipping oligonucleotides (ESOs) of 2'O-Methyl (2'OMe) and morpholino chemistry have been shown to restore dystrophin expression in muscle fibers from the mdx mouse, and are currently being tested in phase I clinical trials for Duchenne Muscular Dystrophy (DMD). However, ESOs remain limited in their effectiveness because of an inadequate delivery profile. Synthetic cationic copolymers of poly(ethylene imine) (PEI) and poly(ethylene glycol) (PEG) are regarded as effective agents for enhanced delivery of nucleic acids in various applications.

Results

We examined whether PEG-PEI copolymers can facilitate ESO-mediated dystrophin expression after intramuscular injections into tibialis anterior (TA) muscles of mdx mice. We utilized a set of PEG-PEI copolymers containing 2 kDa PEI and either 550 Da or 5 kDa PEG, both of which bind 2'OMe ESOs with high affinity and form stable nanoparticulates with a relatively low surface charge. Three weekly intramuscular injections of 5 μg of ESO complexed with PEI2K-PEG550 copolymers resulted in about 500 dystrophin-positive fibers and about 12% of normal levels of dystrophin expression at 3 weeks after the initial injection, which is significantly greater than for injections of ESO alone, which are known to be almost completely ineffective. In an effort to enhance biocompatibility and cellular uptake, the PEI2K-PEG550 and PEI2K-PEG5K copolymers were functionalized by covalent conjugation with nanogold (NG) or adsorbtion of colloidal gold (CG), respectively. Surprisingly, using the same injection and dosing regimen, we found no significant difference in dystrophin expression by Western blot between the NG-PEI2K-PEG550, CG-PEI2K-PEG5K, and non-functionalized PEI2K-PEG550 copolymers. Dose-response experiments using the CG-PEI2K-PEG5K copolymer with total ESO ranging from 3–60 μg yielded a maximum of about 15% dystrophin expression. Further improvements in dystrophin expression up to 20% of normal levels were found at 6 weeks after 10 twice-weekly injections of the NG-PEI2K-PEG550 copolymer complexed with 5 μg of ESO per injection. This injection and dosing regimen showed over 1000 dystrophin-positive fibers. H&E staining of all treated muscle groups revealed no overt signs of cytotoxicity.

Conclusion

We conclude that PEGylated PEI2K copolymers are efficient carriers for local delivery of 2'OMe ESOs and warrant further development as potential therapeutics for treatment of DMD.

Non-viral gene transfection technologies for genetic engineering of stem cells

The recent rapid progress of molecular biology together with the steady progress of genome projects has given us some essential and revolutionary information about DNA and RNA to elucidate various biological phenomena at a genetic level. Under these circumstances, the technology and methodology of gene transfection have become more and more important to enhance the efficacy of gene therapy for several diseases. In addition, gene transfection is a fundamental technology indispensable to the further research development of basic biology and medicine regarding stem cells. Stem cells genetically manipulated will enhance the therapeutic efficacy of cell transplantation. In this paper, the carrier and technology of gene delivery are briefly overviewed while the applications to the basic researches of biology and medicine as well as regenerative medical therapy are introduced. A new non-viral carrier and the cell culture system are described to efficiently manipulate stem cells.

The use of ultrasound in transfection and transgene expression

The interaction of ultrasound with tissue leads to radiation pressure, heat generation, and cavitation. These phenomena have been utilised for local gene delivery, transfection and control of expression. Specially designed nanocarriers or adapted ultrasound contrast agents can further enhance local delivery by: (1) increased permeability of cell membranes; (2) local release of genes. Biological carriers may also be used for local gene delivery. Stem cells and immune cells appear especially promising because of their homing capabilities to lesion sites. Imaging methods can be employed for pharmacodistribution and pharmacokinetics. MRI contrast agents can serve as non-invasive reporters on gene distribution when co-delivered with the gene. They can be used to label nanocarriers and cellular transport systems in gene therapy strategies such as those based on stem cells. Finally, ultrasound heating together with the use of a temperature sensitive promoter allows a local, physical, spatio-temporal control of transgene expression, in particular when combined with MRI temperature mapping for monitoring and even controlling ultrasound heating.

Effects of extracellular calcium on cell membrane resealing in sonoporation

Sonoporation has been exploited as a promising strategy for intracellular drug and gene delivery. The technique uses ultrasound to generate pores on the cell membrane to allow entry of extracellular agents into the cell. Resealing of these non-specific pores is a key factor determining both the uptake and post-ultrasound cell survival. This study examined the effects of extracellular Ca(2+) on membrane resealing in sonoporation, using Xenopus oocytes as a model system. The cells were exposed to tone burst ultrasound (1.06 MHz, duration 0.2 s, acoustic pressure 0.3 MPa) in the presence of 0.1% Definity at various extracellular [Ca(2+)] (0-3 mM). Sonoporation inception and resealing in a single cell were monitored in real time via the transmembrane current of the cell under voltage clamp. The time-resolved measurements of transmembrane current revealed the involvement of two or more Ca(2+) related processes with different rate constants and characteristics. Rapid resealing occurred immediately after ultrasound application followed by a much slower resealing process. Complete resealing required [Ca(2+)] above 0.54 mM. The cells resealed in 6-26 s at 1.8 mM Ca(2+), but took longer at lower concentrations, up to 58-170 s at 0.54 mM Ca(2+).

Ultrasound-based nonviral gene delivery induces bone formation in vivo

Nonviral gene delivery is a promising, safe, therapeutic tool in regenerative medicine. This study is the first to achieve nonviral, ultrasound-based, osteogenic gene delivery that leads to bone tissue formation, in vivo. We hypothesized that direct in vivo sonoporation of naked DNA encoding for the osteogenic gene, recombinant human bone morphogenetic protein-9 (rhBMP-9) would induce bone formation. A luciferase plasmid (Luc), encoding rhBMP-9 or empty pcDNA3 vector mixed with microbubbles, was injected into the thigh muscles of mice. After injection, noninvasive sonoporation was applied. Luc activity was monitored noninvasively, and quantitatively using bioluminescence imaging in vivo, and found for 14 days with a peak expression on day 7. To examine osteogenesis in vivo, rhBMP-9 plasmid was sonoporated into the thigh muscles of transgenic mice that express the Luc gene under the control of a human osteocalcin promoter. Following rhBMP-9 sonoporation, osteocalcin-dependent Luc expression lasted for 24 days and peaked on day 10. Bone tissue was formed in the site of rhBMP-9 delivery, as was shown by micro-computerized tomography and histology. The sonoporation method was also compared with previously developed electrotransfer-based gene delivery and was found significantly inferior in its efficiency of gene delivery. We conclude that ultrasound-mediated osteogenic gene delivery could serve as a therapeutic solution in conditions requiring bone tissue regeneration after further development that will increase the transfection efficiency.

The effects of radial shock waves on gene transfer in rabbit chondrocytes in vitro

OBJECTIVE

The purpose of this study was to develop a new technique of gene transfer utilizing radial shock waves. The effects of radial shock waves on gene transfer in rabbit chondrocytes were examined by varying the parameters of exposure conditions in vitro.

METHODS

Chondrocytes were obtained from New Zealand white rabbits and cultured in a monolayer. A luciferase-encoding gene expression vector, or vector alone, was added to chondrocyte cell suspensions, and the cells were then exposed to radial shock waves. Parameters such as pressure amplitude, number of pulses, frequency, and DNA concentration were varied, and luciferase activity was measured 48h after transfection. Transfection efficiency of radial shock waves was compared with the FuGENE6 transfection method using a green fluorescence protein (GFP)-encoding gene vector by fluorescent-activated cell sorter (FACS) analysis.

RESULTS

Radial shock wave exposure significantly increased luciferase activity over 140-fold as compared to the control under the optimal exposure conditions. Both pressure amplitude and number of pulses were relevant to transfection efficiency and cell viability, but frequency was not. Transfection efficiency increased in a dose-dependent manner with DNA concentration. FACS analysis showed 4.74% of GFP-encoding gene using radial shock waves. FuGENE6 transfection was almost similar in transfection efficiency to radial shock wave.

CONCLUSION

In spite of certain degree of cell disruption, radial shock waves significantly augmented reporter gene transfection in rabbit chondrocytes in vitro. Radial shock waves may potentially contribute to the treatment of the cartilage morbidities by enhancing the potency of tissue healing and gene transfection of growth factors.

Ultrasound microbubble contrast agents: fundamentals and application to gene and drug delivery

This review offers a critical analysis of the state of the art of medical microbubbles and their application in therapeutic delivery and monitoring. When driven by an ultrasonic pulse, these small gas bubbles oscillate with a wall velocity on the order of tens to hundreds of meters per second and can be deflected to a vessel wall or fragmented into particles on the order of nanometers. While single-session molecular imaging of multiple targets is difficult with affinity-based strategies employed in some other imaging modalities, microbubble fragmentation facilitates such studies. Similarly, a focused ultrasound beam can be used to disrupt delivery vehicles and blood vessel walls, offering the opportunity to locally deliver a drug or gene. Clinical translation of these vehicles will require that current challenges be overcome, where these challenges include rapid clearance and low payload. The technology, early successes with drug and gene delivery, and potential clinical applications are reviewed.

Sonoporation mediated transduction of pDNA/siRNA into joint synovium in vivo

The purpose of this study was to investigate the usefulness of sonoporation method on in vivo transduction of plasmid DNA (pDNA) and small interfering RNA (siRNA) into joint tissue. pGEG.GL3 plasmid was mixed with microbubble and injected into knee joints of rats. Ultrasound sonication was performed percutaneously. Three days after injection, GL3 expression of synovial tissue was determined by luciferase assay and RT-PCR. siRNA specific for GL3 (siGL3) or nonspecific siRNA were mixed with pGEG.GL3 plasmid and transduced by sonoporation. siRNA specific for EGFP (siEGFP) was transduced into the knee joints of EGFP transgenic rats, and gene silencing effects for endogenous gene were examined. To determine the localization of transduced siRNA, fluorescently labeled siRNA was transduced into joints. The expression of GL3 in the synovium was significantly enhanced by sonoporation. The gene expression was only seen in the synovium of the knee joint. The expression of GL3 was remarkably suppressed by co-transduction of siGL3, but not suppressed by nonspecific siRNA. siEGFP transduced by sonoporation attenuated green fluorescence on the surface layer of synovium of EGFP transgenic rats. The fluorescently labeled siRNA was seen in the synovium around the patella, femur, and tibia. Sonoporation is examined as a recent, novel, gene transduction method, and the advantage of this technique is minimal invasiveness. In this study, we showed that pDNA/siRNA can be transduced specifically into the joint synovium using sonoporation. The present method may be useful in nucleic acid therapy for joint disorders.

Cavitation threshold of microbubbles in gel tunnels by focused ultrasound

The investigation of inertial cavitation in micro-tunnels has significant implications for the development of therapeutic applications of ultrasound such as ultrasound-mediated drug and gene delivery. The threshold for inertial cavitation was investigated using a passive cavitation detector with a center frequency of 1 MHz. Micro-tunnels of various diameters (90 to 800 microm) embedded in gel were fabricated and injected with a solution of Optison(trade mark) contrast agent of concentrations 1.2% and 0.2% diluted in water. An ultrasound pulse of duration 500 ms and center frequency 1.736 MHz was used to insonate the microbubbles. The acoustic pressure was increased at 1-s intervals until broadband noise emission was detected. The pressure threshold at which broadband noise emission was observed was found to be dependent on the diameter of the micro-tunnels, with an average increase of 1.2 to 1.5 between the smallest and the largest tunnels, depending on the microbubble concentration. The evaluation of inertial cavitation in gel tunnels rather than tubes provides a novel opportunity to investigate microbubble collapse in a situation that simulates in vivo blood vessels better than tubes with solid walls do.

Therapeutic ultrasound facilitates antiangiogenic gene delivery and inhibits prostate tumor growth

Gene therapy clinical trials are limited due to several hurdles concerning the type of vector used, particularly, the viral vectors, and transfection efficacy when non-viral vectors are used. Therapeutic ultrasound is a promising non-viral technology that can be used in the clinical setting. Here, for the first time, we show the efficacy of therapeutic ultrasound to deliver genes encoding for hemopexin-like domain fragment (PEX), an inhibitor of angiogenesis, to prostate tumors in vivo. Moreover, the addition of an ultrasound contrast agent (Optison) to the transfection process was evaluated. Prostate cancer cells and endothelial cells (EC) were transfected in vitro with cDNA-PEX using therapeutic ultrasound alone (TUS + pPEX) or with Optison (TUS + pPEX + Optison). The biological activity of the expressed PEX was assessed using proliferation, migration, and apoptosis assays done on EC and prostate cancer cells. TUS + pPEX + Optison led to the inhibition of EC and prostate cancer cell proliferation (<65%), migration (<50%), and an increase in apoptosis. In vivo, C57/black mice were inoculated s.c. with prostate cancer cells. The tumors were treated with TUS + pPEX and TUS + pPEX + Optison either once or repeatedly. Tumor growth was evaluated, after which histology and immunohistochemistry analyses were done. A single treatment of TUS + pPEX led to a 35% inhibition in tumor growth. Using TUS + PEX + Optison led to an inhibition of 50%. Repeated treatments of TUS + pPEX + Optison were found to significantly (P < 0.001) inhibit prostate tumor growth by 80%, along with the angiogenic indices, with no toxicity to the surrounding tissues. These results depict the efficacy of therapeutic ultrasound as a non-viral technology to efficiently deliver genes to tumors in general, and to deliver angiogenic inhibitors to prostate cancer in particular.

Targeted retroviral gene delivery using ultrasound

BACKGROUND

Achieving specificity of delivery represents a major problem limiting the clinical application of retroviral vectors for gene therapy, whilst lack of efficiency and longevity of gene expression limit non-viral techniques. Ultrasound and microbubble contrast agents can be used to effect plasmid DNA delivery. We therefore sought to evaluate the potential for ultrasound/microbubble-mediated retroviral gene delivery.

METHODS

An envelope-deficient retroviral vector, inherently incapable of target cell entry, was combined with cationic microbubbles and added to target cells. The cells were exposed to pulsed 1 MHz ultrasound for 5 s and subsequently analysed for marker gene expression. The acoustic pressure profile of the ultrasound field, to which transduction efficiency was related, was determined using a needle hydrophone.

RESULTS

Ultrasound-targeted gene delivery to a restricted area of cells was achieved using virus-loaded microbubbles. Gene delivery efficiency was up to 2% near the beam focus. Significant transduction was restricted to areas exposed to > or = 0.4 MPa peak-negative acoustic pressure, despite uniform application of the vector. An acoustic pressure-dependence was demonstrated that can be exploited for targeted retroviral transduction. The mechanism of entry likely involves membrane perturbation in the vicinity of oscillating microbubbles, facilitating fusion of the viral and cell membranes.

CONCLUSIONS

We have established the basis of a novel retroviral vector technology incorporating favourable aspects of existing viral and non-viral gene delivery vectors. In particular, transduction can be controlled by means of ultrasound exposure. The technology is ideally suited to targeted delivery following systemic vector administration.

Nonviral transfection of suspension cells in ultrasound standing wave fields

Ultrasound-induced cavitation has been widely used for delivering DNA vectors into cells. However, this approach may seriously disrupt cell membranes and cause lethal damage when cells are exposed to the inertial cavitation field. In this study, instead of using sonoporation, ultrasound standing wave fields (USWF) were explored for nonviral transfection of suspension cells. Acoustic resonance in a tubular chamber was generated from the interference of waves emitted from a piezoelectric transducer and consequently reflected from a borosilicate glass coverslip. The suspended K562 erythroleukemia cells were transfected by polyethyleneimine (PEI)/DNA complexes with and without exposure to 1-MHz USWF for 5 min. During USWF exposure, K562 cells moved to the pressure nodal planes first and formed cell bands by the primary radiation force. Nanometer-sized PEI/DNA complexes, circulated between nodal planes by acoustic microstreaming, then used the cell agglomerates as the nucleating sites on which to attach. After incubation at 37 degrees C for 48 h, the efficiency of nonviral transfection based on EGFP transgene expression was determined by fluorescent microscopy and fluorometry. Both studies showed that USWF brought suspended K562 cells and PEI/DNA complexes into close contact at the pressure nodal planes, yielding an approximately 10-fold increment of EGFP transgene expression compared with the group without ultrasonic treatment.

Transcription factor decoy oligonucleotide-based therapeutic strategy for renal disease

Renal disease, including slight renal injuries, has come to be seen as one of the risk factors for cardiovascular events. At present, most conventional therapy is inefficient, and tends to treat the symptoms rather than the underlying causes of the disorder. Gene therapy based on oligonucleotides (ODN) offers a novel approach for the prevention and treatment of renal diseases. Gene transfer into somatic cells to interfere with the pathogenesis contributing to renal disease may provide such an approach, leading to the better prevention and treatment of renal disease. The major development of gene transfer methods has made an important contribution to an intense investigation of the potential of gene therapy in renal diseases. Amazing advances in molecular biology have provided the dramatic improvement in the technology that is necessary to transfer target genes into somatic cells. Gene transfer methods, especially when mediated by several viral vectors, have improved to a surprising extent. In fact, some (retroviral vectors, adenoviral vectors, or liposome-based vectors, etc.) have already been used in clinical trials. On the other hand, recent progress in molecular biology has provided new techniques to inhibit target gene expression. The transfer of cis-element double-stranded ODN (= decoy) has been reported to be a powerful novel tool in a new class of antigene strategies for gene therapy. The transfer of decoy ODN corresponding to the cis sequence will result in attenuation of the authentic cis-trans interaction, leading to the removal of trans-factors from the endogenous cis-elements with a subsequent modulation of gene expression.

Gene therapy progress and prospects: Ultrasound for gene transfer

Ultrasound exposure (USE) in the presence of microbubbles (MCB) (e.g. contrast agents used to enhance ultrasound imaging) increases plasmid transfection efficiency in vitro by several orders of magnitude. Formation of short-lived pores in the plasma membrane ('sonoporation'), up to 100 nm in effective diameter lasting a few seconds, is implicated as the dominant mechanism, associated with acoustic cavitation. Ultrasound enhanced gene transfer (UEGT) has also been successfully achieved in vivo, with reports of spatially restricted and therapeutically relevant levels of transgene expression. Loading MCB with nucleic acids and/or disease-targeting ligands may further improve the efficiency and specificity of UEGT such that clinical testing becomes a realistic prospect.

Use of ultrasound to enhance nonviral lung gene transfer in vivo

We have assessed if high-frequency ultrasound (US) can enhance nonviral gene transfer to the mouse lung. Cationic lipid GL67/pDNA, polyethylenimine (PEI)/pDNA and naked plasmid DNA (pDNA) were delivered via intranasal instillation, mixed with Optison microbubbles, and the animals were then exposed to 1 MHz US. Addition of Optison alone significantly reduced the transfection efficiency of all three gene transfer agents. US exposure did not increase GL67/pDNA or PEI/pDNA gene transfer compared to Optison-treated animals. However, it increased naked pDNA transfection efficiency by approximately 15-fold compared to Optison-treated animals, suggesting that despite ultrasound being attenuated by air in the lung, sufficient energy penetrates the tissue to increase gene transfer. US-induced lung haemorrhage, assessed histologically, increased with prolonged US exposure. The left lung was more affected than the right and this was mirrored by a lesser increase in naked pDNA gene transfer, in the left lung. The positive effect of US was dependent on Optison, as in its absence US did not increase naked pDNA transfection efficiency. We have thus established proof of principle that US can increase nonviral gene transfer, in the air-filled murine lung.

Efficient transfection of tumors facilitated by long-term therapeutic ultrasound in combination with contrast agent: from in vitro to in vivo setting

Therapeutic ultrasound (TUS) is a promising non-viral clinical approach for the delivery of genes. This study demonstrates the efficient delivery and localization of DNA in subcutaneous tumors facilitated by TUS application and examines the contribution of ultrasound contrast-agent (USCA) addition on transfection. The study addresses the importance of in vivo optimization when using long-term TUS and USCA based on data achieved in vitro. In vitro results showed that transfection of TrampC2 prostate cancer (Pca) cells using genes encoding for luciferase and green fluorescent protein was enhanced when DNA and Optison were added together and TUS was applied for 20 or 30 min. In vivo results showed that the highest transfection was achieved when Optison and DNA were co-injected intratumorally, and TUS was applied for 20 min. Using Optison significantly increased protein distribution in the tumor. However, in vivo expression level was decreased by two and four fold at 7 and 14 days, respectively, post-TUS. The study establishes the potential of intratumoral delivery of DNA-Optison, followed by TUS as an effective, non-toxic, gene delivery method that could provide a safe, clinical alternative to current viral gene delivery approaches where short-term gene expression is needed.

Ultrasound-induced cavitation: applications in drug and gene delivery

Ultrasound, which has been conventionally used for diagnostics until recently, is now being extensively used for drug and gene delivery. This transformation has come about primarily due to ultrasound-mediated acoustic cavitation - particularly transient cavitation. Acoustic cavitation has been used to facilitate the delivery of small molecules, as well as macromolecules, including proteins and DNA. Controlled generation of cavitation has also been used for targeting drugs to diseased tissues, including skin, brain, eyes and endothelium. Ultrasound has also been employed for the treatment of several diseases, including thromboembolism, arteriosclerosis and cancer. This review provides a detailed account of mechanisms, current status and future prospects of ultrasonic cavitation in drug and gene delivery applications.

Gene therapy progress and prospects: Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a severe muscle wasting disorder affecting 1/3500 male births. There is currently no effective treatment, but gene therapy approaches are offering viable avenues for treatment development. The last 10 years have seen the development of a number of strategies and tools for muscle gene therapy. However, the major hurdle has been the inability to deliver vectors at high enough efficiency via a systemic route. The last 2-3 years (reviewed here) have seen unrivalled progress in efficient systemic delivery of viral and non-viral gene transfer agents and antisense oligonucleotides. This progress, coupled with the successful completion of the first gene therapy clinical trial for DMD, has led to three more clinical trials planned for the immediate future.

Emerging vectors and targeting methods for nonviral gene therapy

Until recently, nonviral vectors were outside the mainstream of gene transfer technology. Recent problems in clinical trials using viral vectors renewed interest in these methods. The clinical usefulness of nonviral methods is still hindered by their relatively low gene delivery/transgene expression efficiencies. Vectors must navigate a series of obstacles before the therapeutic gene can be expressed. This review considers these barriers and the properties of components of nonviral vectors that are essential for nucleic acid transfer. Although developments of new physical methods (hydrodynamic delivery, ultrasound, electroporation) have made a significant impact on gene transfer efficiency, various chemical carriers (lipids and polymers) have been shown to achieve high-level gene delivery and functional expression. Success of nonviral gene targeting will depend not only on the efficacy, but also safety of this methodology, and this aspect is also discussed. Understanding problems associated with nonviral targeting can also help in designing better viral vectors. In fact, interplay between viral and nonviral technologies should lead to a continued refinement of both methodologies.

Physical parameters affecting ultrasound/microbubble-mediated gene delivery efficiency in vitro

Ultrasound (US)/microbubble-mediated gene delivery is a technology with many potential advantages suited to clinical application. Previous studies have demonstrated transfection but many are unsatisfactory in respect to the exposure apparatus, lack of definition of the US field or the limitations on parameters that can be explored using clinical diagnostic US machines. We investigated individual exposure parameters using a system minimising experimental artefacts and allowing control of many parameters of the US field. Using a 1-MHz transducer we systematically varied US parameters, the duration of exposure and the microbubble and DNA concentrations to optimise gene delivery. Delivery was achieved, using lipid microbubbles (SonoVue) and clinically acceptable US exposures, to adherent cells at efficiencies of approximately 4%. The acoustic pressure amplitude (0.25 MPa peak-negative), pulse repetition frequency (1-kHz) and duration of exposure (10 s) were important in optimising gene delivery with minimal impact on cell viability. These findings support the hypothesis that varying the physical parameters of US-mediated gene delivery has an affect on both efficiency and cell viability. These data are the first in terms of their thorough exploration of the US parameter space and will be the basis for more-informed approaches to developing clinical applications of this technology.

Sustained transgene expression in intervertebral disc cells in vivo mediated by microbubble-enhanced ultrasound gene therapy

STUDY DESIGN

In vivo studies using a rat model were performed to determine the feasibility of microbubble-enhanced ultrasound gene transfer technique to the intervertebral disc.

OBJECTIVES

1) To establish this microbubble-enhanced ultrasound gene therapy technique for intervertebral disc cells in vivo without using viral vectors and 2) to estimate the duration of transgene expression in vivo.

SUMMARY OF BACKGROUND DATA

Intervertebral disc degeneration and associated spinal disorders remain a formidable problem. Although gene therapy approaches have been reported as having promising therapeutic potential to regenerate the disc, concerns over safety issues using recombinant viral vectors limits its application. Successful gene transfer using ultrasound has been reported in muscle and cardiovascular systems in vivo.

MATERIALS AND METHODS

Two different reporter plasmid DNA encoding green fluorescent protein (GFP) and firefly luciferase were used. Plasmid DNA was mixed with ultrasonography contrast agent (microbubbles) and injected into coccygeal intervertebral discs of Sprague-Dawley rats. The therapeutic ultrasound was irradiated on the surface of injected discs. Rats were killed 1, 3, 6, 12, and 24 weeks after gene transduction. Harvested nucleus pulposus tissues were used for evaluation of transgene expression. The intact discs were used as a control.

RESULTS

Seven days after gene transfection, considerable numbers of GFP-positive cells were observed in nucleus pulposus from the GFP-transfected group. Luciferase assay revealed that the ultrasound group demonstrated approximately an 11-fold increase in luciferase activity over the plasmid DNA-only group. Furthermore, transgene expression mediated by this method was observed, at least up to 24 weeks.

CONCLUSIONS

Our study indicated that ultrasound transfection method with microbubbles significantly enhanced transfection efficiency of plasmid DNA into the nucleus pulposus cells in vivo. Furthermore, the sustained transgene expression in vivo was possible up to 24 weeks. The long-term gene expression mediated by simple and safe procedure has important clinical applications, including the treatment of chronic types of disease such as degenerative disc diseases.

Transfection effect of microbubbles on cells in superposed ultrasound waves and behavior of cavitation bubble

The combination of ultrasound and ultrasound contrast agents (UCAs) is able to induce transient membrane permeability leading to direct delivery of exogenous molecules into cells. Cavitation bubbles are believed to be involved in the membrane permeability; however, the detailed mechanism is still unknown. In the present study, the effects of ultrasound and the UCAs, Optison on transfection in vitro for different medium heights and the related dynamic behaviors of cavitation bubbles were investigated. Cultured CHO-E cells mixed with reporter genes (luciferase or beta-gal plasmid DNA) and UCAs were exposed to 1 MHz ultrasound in 24-well plates. Ultrasound was applied from the bottom of the well and reflected at the free surface of the medium, resulting in the superposition of ultrasound waves within the well. Cells cultured on the bottom of 24-well plates were located near the first node (displacement node) of the incident ultrasound downstream. Transfection activity was a function determined with the height of the medium (wave traveling distance), as well as the concentration of UCAs and the exposure time was also determined with the concentration of UCAs and the exposure duration. Survival fraction was determined by MTT assay, also changes with these values in the reverse pattern compared with luciferase activity. With shallow medium height, high transfection efficacy and high survival fraction were obtained at a low concentration of UCAs. In addition, capillary waves and subsequent atomized particles became significant as the medium height decreased. These phenomena suggested cavitation bubbles were being generated in the medium. To determine the effect of UCAs on bubble generation, we repeated the experiments using crushed heat-treated Optison solution instead of the standard microbubble preparation. The transfection ratio and survival fraction showed no additional benefit when ultrasound was used. These results suggested that cavitation bubbles created by the collapse of UCAs were a key factor for transfection, and their intensities were enhanced by the interaction of the superpose ultrasound with the decreasing the height of the medium. Hypothesizing that free cavitation bubbles were generated from cavitation nuclei created by fragmented UCA shells, we carried out numerical analysis of a free spherical bubble motion in the field of ultrasound. Analyzing the interaction of the shock wave generated by a cavitation bubble and a cell membrane, we estimated the shock wave propagation distance that would induce cell membrane damage from the center of the cavitation bubble.

The effects of albumin-coated microbubbles in DNA delivery mediated by therapeutic ultrasound

The application of therapeutic ultrasound (TUS) in combination with contrast agents (USCA) to mediate gene delivery relies on the understanding of the bioeffects involved. The objective of this study was to evaluate the various bioeffects generated by albumin-coated microbubbles: Optison, an USCA, when applied with TUS operated for 10-30 min, on cells and on DNA transfection. This study reveals that Optison microbubbles were still acoustically active after long-term TUS application of 30 min. Optison enhances TUS-gene transfection by increasing the number of plasmids in the cells and also by distributing the plasmids to more cells, without significant decrease in cell viability. Optison also interacts with the DNA to further enhance transfection in a mechanism not necessarily involving cavitation. However, Optison affects mainly the cell cytoplasmatic membrane, without interfering with DNA intracellular trafficking. Using high-resolution scanning electron microscopy (HRSEM), the bioeffects on cell membrane induced by TUS-Optison were observed, demonstrating that Optison lead to a rougher surface, characterized by depressions that are reversible within 24-h post TUS. These effects are different from those observed when only TUS was applied. The findings from this study suggest that albumin-coated microbubbles enhances transfection when using TUS for 10-30 min, and that microbubbles play a major role in elevating cell transfection level and efficiency.

Application of liposomes to sonoporation

A method to prepare liposomes is presented. Liposomes made in our laboratory were characterized acoustically and optically. The phase velocity and attenuation of liposomes in suspension (concentration = 10(9)/mL) were measured, ranging from 2 to 14 MHz, using ultrasound spectroscopy. Anti-rabbit IgG conjugated with Alexafluor 647 was delivered into Jurkat cells in suspension, using the liposomes, by 10 % duty cycle ultrasound tonebursts of 2.2 MHz (the in situ spatial peak-pressure amplitude = 80 W/cm2) with an efficiency of 13 %. It has been experimentally shown that liposomes may be an alternative stable agent to Optison for delivering macromolecules into cells.

Induction of dystrophin expression by exon skipping in mdx mice following intramuscular injection of antisense oligonucleotides complexed with PEG-PEI copolymers

Antisense oligonucleotides (AOs) with 2-O-methyl modifications can circumvent dystrophin mutations via exon skipping and, it is hoped, can become drugs for treatment of Duchenne muscular dystrophy (DMD). However, AO-based approaches are hindered by a lack of effective carriers to facilitate delivery of AOs to myonuclei. We examined whether copolymers composed of cationic poly(ethylene imine) (PEI) and polyethylene glycol (PEG) can enhance AO transfection in skeletal muscle of mdx mice. Single intramuscular injections of AO complexed with low Mw PEI2000(PEG550) copolymers into TA muscles of mdx mice resulted in widespread distribution of dystrophin-positive fibers at 3 weeks after injection, with no apparent cytotoxicity. Overall, injections of these low Mw polyplexes, which formed 250-nm aggregate particles, resulted in about sixfold more dystrophin-positive fibers than AO alone. Western analysis confirmed the dystrophin expression in these muscles. Surprisingly, injections of AO complexed with high Mw PEI25000(PEG5000) copolymers, which formed smaller nonaggregated particles, produced about threefold fewer dystrophin-positive fibers than injections of the low Mw polyplexes. We conclude that low Mw PEI2000(PEG550) copolymers function as high-capacity, nontoxic AO carriers suitable for in vivo transfection of skeletal muscle and are promising compounds for potential use in molecular therapy of DMD.

Fluorescein isothiocynate-dextran uptake by chinese hamster ovary cells in a 1.5 MHz ultrasonic standing wave in the presence of contrast agent

Uptake of fluorescein isothiocynate-dextran (FITC-dextran) by Chinese hamster ovary cells was studied after exposure to ultrasonic standing wave (USW) in presence of Optison, an ultrasound contrast agent. Confluent Chinese hamster ovary cells were harvested and suspended in phosphate-buffered saline + 0.1% bovine serum albumin containing FITC-dextran (10, 40, and 500 kDa) at 10 microM final concentration. The suspension was seeded with contrast agent (75 microL/mL) and exposed to a 1.5 MHz USW system at acoustic pressures ranging from 0.98 to 4.2 MPa. Macromolecular uptake was assessed by fluorescent microscopy and quantified by flow cytometry 10 min after exposure. FITC-dextran positive cells, as assessed by flow cytometry, were 1 +/- 0.05% and 2.58 +/- 0.27% for acoustic pressures of 1.96 and 4.2 MPa, respectively (p = 0.006). Fluorescent microscopy indicated a degree of macromolecular loading at 0.98 MPa with 46% of peripherally FITC-dextran- and/or propidium iodide-stained cells coincident with the appearance of significant frequency (f0/2 and 2 f0) emission signals. At higher pressures, high macromolecular loading with 6% peripherally stained cells at 1.96 MPa was associated with lower order emission signals and white noise. The study conclusively demonstrates macromolecular loading in an USW, a significantly higher macromolecular loading at higher pressures and indicates potential of emission signals for a feedback loop to control the acoustic power outputs and fine-tune the biologic effects associated with sonoporation.

Targeted Therapeutic Applications of Acoustically Active Microspheres in the Microcirculation

The targeted delivery of intravascular drugs and genes across the endothelial barrier with only minimal side effects remains a significant obstacle in establishing effective therapies for many pathological conditions. Recent investigations have shown that contrast agent microbubbles, which are typically used for image enhancement in diagnostic ultrasound, may also be promising tools in emergent, ultrasound-based therapies. Explorations of the bioeffects generated by ultrasound-microbubble interactions indicate that these phenomena may be exploited for clinical utility such as in the targeted revascularization of flow-deficient tissues. Moreover, development of this treatment modality may also include using ultrasound-microbubble interactions to deliver therapeutic material to tissues, and reporter genes and therapeutic agents have been successfully transferred from the microcirculation to tissue in various animal models of normal and pathological function. This article reviews the recent studies aimed at using interactions between ultrasound and contrast agent microbubbles in the microcirculation for therapeutic purposes. Furthermore, the authors present investigations involving microspheres that are of a different design compared to current microbubble contrast agents, yet are acoustically active and demonstrate potential as tools for targeted delivery. Future directions necessary to address current challenges and advance these techniques to clinical practicality are also discussed.

Pluronic block copolymers: novel functions in ultrasound-mediated gene transfer and against cell damage

Pluronics have been investigated as vectors for drug and gene delivery in vitro and in vivo and were demonstrated to have high efficiency for gene transfer in vivo. However, they alone do not enhance gene transfer in vitro. We examined three pluronics, F127, L61 and P85, for their effects on ultrasound (US)-mediated gene transfer in three cell lines, 3T3-MDEI, C2C12 and CHO. The polymers showed differential effects on cell viability and transfection efficiency in a dose-dependent manner. All the polymers were unable to facilitate gene transfer when used alone, but enhanced US-mediated gene transfer significantly at concentrations around the critical micelle concentration in the three cell lines. F127 showed no significant toxicity at any concentration and protected the cells against US-mediated damage at a high concentration. L61 decreased cell viability significantly in a dose-dependent manner, whereas P85 showed mild toxicity when its concentration was at or above 0.05%.