Gene transfer into adult rat spinal cord using naked plasmid DNA and ultrasound microbubbles

The road ahead to successful BBB opening and drug-delivery with focused ultrasound

This review delves into the innovative technology of Blood-Brain Barrier (BBB) opening with low-intensity focused ultrasound in combination with microbubbles (LIFU-MB), a promising therapeutic modality aimed at enhancing drug delivery to the central nervous system (CNS). The BBB's selective permeability, while crucial for neuroprotection, significantly hampers the efficacy of pharmacological treatments for CNS disorders. LIFU-MB emerges as a non-invasive and localized method to transiently increase BBB permeability, facilitating the delivery of therapeutic molecules. Here, we review the procedural stages of LIFU-MB interventions, including planning and preparation, sonication, evaluation, and delivery, highlighting the technological diversity and methodological challenges encountered in current clinical applications. With an emphasis on safety and efficacy, we discuss the crucial aspects of ultrasound delivery, microbubble administration, acoustic feedback monitoring and assessment of BBB permeability. Finally, we explore the critical choices for effective BBB opening with LIFU-MB, focusing on selecting therapeutic agents, optimizing delivery methods, and timing for delivery. Overcoming existing barriers to integrate this technology into clinical practice could potentially revolutionize CNS drug delivery and treatment paradigms in the near future.

Determining Ultrasound Parameters for Bursting Polymer Microbubbles for Future Use in Spinal Cord Injury

OBJECTIVE

We believe our poly(lactic acid) (PLA) microbubbles are well suited for therapeutic delivery to spinal cord injury (SCI) using ultrasound-triggered bursting. We investigated the feasibility of clinical ultrasound bursting in situ, the optimal bursting parameters in vitro and the loading and release of a model bio-active DNA.

METHODS

Microbubbles were tested using clinical ultrasound in a rat cadaver SCI model. Burst pressure thresholds were determined using the change in enhancement after ultrasound exposure. Resonance frequency, acoustic enhancement, sizing and morphology were evaluated by comparing two microbubble porogens, ammonium carbonate and ammonium carbamate. Oligonucleotides were loaded into the shell and released using the found optimized ultrasound bursting parameters.

RESULTS

In situ imaging and bursting were successful. In vitro bursting thresholds using frequencies 1, 2.25 and 5 MHz were identified between peak negative pressures 0.2 and 0.5 MPa, believed to be safe for spinal cord. The pressure threshold decreased with decreasing frequencies. PLA bursting was optimized near the resonance frequency of 2.5 to 3.0 MHz using 2.25 MHz and not at lower frequencies. PLA microbubbles, initially with a mean size of approximately 2 µm, remained in one piece, collapsed to between 0.5 and 1 µm and did not fragment. Significantly more oligonucleotide was released after ultrasound bursting of loaded microbubbles. Microbubble-sized debris was detected when using ammonium carbamate, leading to inaccurate microbubble concentration measurements.

CONCLUSION

PLA microbubbles made with ammonium carbonate and burst at appropriate parameters have the potential to safely improve intrathecal therapeutic delivery to SCI using targeted ultrasound.

Safety Review of Therapeutic Ultrasound for Spinal Cord Neuromodulation and Blood-Spinal Cord Barrier Opening

New focused ultrasound spinal cord applications have emerged, particularly those improving therapeutic agent delivery to the spinal cord via blood-spinal cord barrier opening and the neuromodulation of spinal cord tracts. One hurdle in the development of these applications is safety. It may be possible to use safety trends from seminal and subsequent works in focused ultrasound to guide the development of safety guidelines for spinal cord applications. We collated data from decades of pre-clinical studies and illustrate a clear relationship between damage, time-averaged spatial peak intensity and exposure duration. This relationship suggests a thermal mechanism underlies ultrasound-induced spinal cord damage. We developed minimum and mean thresholds for damage from these pre-clinical studies. When these thresholds were plotted against the parameters used in recent pre-clinical ultrasonic spinal cord neuromodulation studies, the majority of the neuromodulation studies were near or above the minimum threshold. This suggests that a thermal neuromodulatory effect may exist for ultrasonic spinal cord neuromodulation, and that the thermal dose must be carefully controlled to avoid damage to the spinal cord. By contrast, the intensity-exposure duration threshold had no predictive value when applied to blood-spinal cord barrier opening studies that employed injected contrast agents. Most blood-spinal cord barrier opening studies observed slight to severe damage, except for small animal studies that employed an active feedback control method to limit pressures based on measured bubble oscillation behavior. The development of new focused ultrasound spinal cord applications perhaps reflects the recent success in the development of focused ultrasound brain applications, and recent work has begun on the translation of these technologies from brain to spinal cord. However, a great deal of work remains to be done, particularly with respect to developing and accepting safety standards for these applications.

Focused ultrasound gene delivery for the treatment of neurological disorders

The transformative potential of gene therapy has been demonstrated in humans. However, there is an unmet need for non-invasive targeted gene delivery and regulation in the treatment of brain disorders. Transcranial focused ultrasound (FUS) has gained tremendous momentum to address these challenges. FUS non-invasively modulates brain cells and their environment, and is a powerful tool to facilitate gene delivery across the blood-brain barrier (BBB) with millimeter precision and promptly regulate transgene expression. This review highlights technical aspects of FUS-mediated gene therapies for the central nervous system (CNS) and lessons learned from discoveries in other organs. Understanding the possibilities and remaining obstacles of FUS-mediated gene therapy will be necessary to harness remarkable technologies and create life-changing treatments for neurological disorders.

Gene-Modified Stem Cells for Spinal Cord Injury: a Promising Better Alternative Therapy

Stem cell therapy holds great promise for the treatment of spinal cord injury (SCI), which can reverse neurodegeneration and promote tissue regeneration via its pluripotency and ability to secrete neurotrophic factors. Although various stem cell-based approaches have shown certain therapeutic effects when applied to the treatment of SCI, their clinical efficacies have been disappointing. Thus, it is an urgent need to further enhance the neurological benefits of stem cells through bioengineering strategies including genetic engineering. In this review, we summarize the progress of stem cell therapy for SCI and the prospect of genetically modified stem cells, focusing on the genome editing tools and functional molecules involved in SCI repair, trying to provide a deeper understanding of genetically modified stem cell therapy and more applicable clinical strategies for SCI repair.

Ultrasound in Traumatic Spinal Cord Injury: A Wide-Open Field

Traumatic spinal cord injury (SCI) is a common and devastating condition. In the absence of effective validated therapies, there is an urgent need for novel methods to achieve injury stabilization, regeneration, and functional restoration in SCI patients. Ultrasound is a versatile platform technology that can provide a foundation for viable diagnostic and therapeutic interventions in SCI. In particular, real-time perfusion and inflammatory biomarker monitoring, focal pharmaceutical delivery, and neuromodulation are capabilities that can be harnessed to advance our knowledge of SCI pathophysiology and to develop novel management and treatment options. Our review suggests that studies that evaluate the benefits and risks of ultrasound in SCI are severely lacking and our understanding of the technology's potential impact remains poorly understood. Although the complex anatomy and physiology of the spine and the spinal cord remain significant challenges, continued technological advances will help the field overcome the current barriers and bring ultrasound to the forefront of SCI research and development.

Nerve growth factor delivery by ultrasound-mediated nanobubble destruction as a treatment for acute spinal cord injury in rats

Background

Spinal cord injuries (SCIs) can cause severe disability or death. Treatment options include surgical intervention, drug therapy, and stem cell transplantation. However, the efficacy of these methods for functional recovery remains unsatisfactory.

Purpose

This study was conducted to explore the effect of ultrasound (US)-mediated destruction of poly(lactic-co-glycolic acid) (PLGA) nanobubbles (NBs) expressing nerve growth factor (NGF) (NGF/PLGA NBs) on nerve regeneration in rats following SCI.

Materials and methods

Adult male Sprague Dawley rats were randomly divided into four treatment groups after Allen hit models of SCI were established. The groups were normal saline (NS) group, NGF and NBs group, NGF and US group, and NGF/PLGA NBs and US group. Histological changes after SCI were observed by hematoxylin and eosin staining. Neuron viability was determined by Nissl staining. Terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling staining was used to examine cell apoptosis. NGF gene and protein expressions were detected by quantitative reverse transcription polymerase chain reaction and Western blotting. Green fluorescent protein expression in the spinal cord was examined using an inverted fluorescence microscope. The recovery of neural function was determined using the Basso, Beattie, and Bresnahan test.

Results

NGF therapy using US-mediated NGF/PLGA NBs destruction significantly increased NGF expression, attenuated histological injury, decreased neuron loss, inhibited neuronal apoptosis in injured spinal cords, and increased BBB scores in rats with SCI.

Conclusion

US-mediated NGF/PLGA NBs destruction effectively transfects the NGF gene into target tissues and has a significant effect on the injured spinal cord. The combination of US irradiation and gene therapy through NGF/PLGA NBs holds great promise for the future of nanomedicine and the development of noninvasive treatment options for SCI and other diseases.

Multimodal micro, nano, and size conversion ultrasound agents for imaging and therapy

Ultrasound (US) is one of the most commonly used clinical imaging techniques. However, the use of US and US-based intravenous agents extends far beyond imaging. In particular, there has been a surge in the fabrication of multimodality US contrast agents and theranostic US agents for cancer imaging and therapy. The unique interaction of US waves with microscale and nanoscale agents has attracted much attention in the development of contrast agents and drug-delivery vehicles. The dimensions of the agent not only dictate how it behaves in vivo, but also how it interacts with US for imaging and drug delivery. Furthermore, these agents are also unique due to their ability to convert from the nanoscale to the microscale and vice versa, having imaging and therapeutic utility in both dimensions. Here, we review multimodality and multifunctional US-based agents, according to their size, and also highlight recent developments in size conversion US agents. WIREs Nanomed Nanobiotechnol 2016, 8:796-813. doi: 10.1002/wnan.1398 For further resources related to this article, please visit the WIREs website.

Gene delivery strategies to promote spinal cord repair

Gene therapies hold great promise for the treatment of many neurodegenerative disorders and traumatic injuries in the central nervous system. However, development of effective methods to deliver such therapies in a controlled manner to the spinal cord is a necessity for their translation to the clinic. Although essential progress has been made to improve efficiency of transgene delivery and reduce the immunogenicity of genetic vectors, there is still much work to be done to achieve clinical strategies capable of reversing neurodegeneration and mediating tissue regeneration. In particular, strategies to achieve localized, robust expression of therapeutic transgenes by target cell types, at controlled levels over defined time periods, will be necessary to fully regenerate functional spinal cord tissues. This review summarizes the progress over the last decade toward the development of effective gene therapies in the spinal cord, including identification of appropriate target genes, improvements to design of genetic vectors, advances in delivery methods, and strategies for delivery of multiple transgenes with synergistic actions. The potential of biomaterials to mediate gene delivery while simultaneously providing inductive scaffolding to facilitate tissue regeneration is also discussed.

The dual effect of ultrasound-targeted microbubble destruction in mediating recombinant adeno-associated virus delivery in renal cell carcinoma: transfection enhancement and tumor inhibition

BACKGROUND

Recombinant adeno-associated virus (rAAV) is recognized as a promising vector for cancer gene therapy, although its low transfer efficiency in less permissive cells limits extensive application. Our previous studies reported that ultrasound-targeted microbubble (MB) destruction (UTMD) enhanced rAAV transfer in its permissive retinal cells. In the present study, we investigated whether UTMD increased rAAV transfer in less permissive human renal cell carcinoma (hRCC) cells and tumors.

METHODS

hRCC cells were treated with rAAV2 under different conditions of UTMD, and the viral transfer efficiency and cell viability were analyzed. Fifty-two male nude mice (BALB/c) implanted with hRCC cells were randomly assigned to four groups consisting of rAAV, rAAV + ultrasound and rAAV + UTMD (20 µl and 40 µl of MBs). UTMD was initiated immediately after intratumoral viral injection, and viral transfer efficiency and tumor volumes were analyzed at 12 weeks after infection.

RESULTS

The efficiency of non-augmented transfer of rAAV2 into hRCC cells was low (17.28 ± 2.44%). The use of UTMD enhanced viral transfer efficiency by two- to three-fold, and enhanced viral genomic DNA by more than nine-fold, without decreasing cell viability. In vivo studies also showed that UTMD increased rAAV2 transfer in tumor. The enhancements were maintained for a period of 12 weeks. Tumor growth in mice was inhibited by UTMD treatment, and UTMD treatment augmented by MBs (40 µl) produced an even stronger effect.

CONCLUSIONS

UTMD enhanced rAAV2 transfer into less permissive RCC cells and tumors, resulting in inhibition of tumor growth, which suggests that UTMD may be a useful delivery tool for cancer gene therapy.

Gene therapy for peripheral arterial disease

INTRODUCTION

Gene therapy has emerged as a novel therapy to promote angiogenesis in patients with critical limb ischemia (CLI) caused by peripheral artery disease. Researchers working in this area have focused on pro-angiogenic factors, such as VEGF, fibroblast growth factor (FGF) and hepatocyte growth factor (HGF). Based on the elaborate studies and favorable results of basic research using naked plasmid DNA (pDNA) encoding these growth factors, some clinical Phase I and Phase II trials have been performed. The results of these studies demonstrate the safety of these approaches and their potential for symptomatic improvement in CLI patients. However, the Phase III clinical trials have so far been limited to HGF gene therapy. Because one pitfall of the Phase III trials has been the limited transgene expression achieved using naked pDNA alone, the development of more efficient gene transfer systems, such as ultrasound microbubbles and the needleless injector, as well as the addition of other genes will make these novel therapies more effective and ease the symptoms of CLI.

AREAS COVERED

This study reviews the previously published basic research and clinical trials that have studied VEGF, FGF and HGF gene therapies for the treatment of CLI. Adjunctive therapies, such as the addition of prostacyclin synthase genes and the development of more efficient gene transfer techniques for pDNA, are also reviewed.

EXPERT OPINION

To date, clinical studies have demonstrated the safety of gene therapy in limb ischemia but the effectiveness of this treatment has not been determined. Larger clinical studies, as well as the development of more effective gene therapy, are needed to achieve and confirm beneficial effects.

Enhancement of adenovirus delivery after ultrasound-stimulated therapy in a cancer model

Improving the efficiency of adenovirus (Ad) delivery to target tissues has the potential to advance the translation of cancer gene therapy. Ultrasound (US)-stimulated therapy uses microbubbles (MBs) exposed to low-intensity US energy to improve localized delivery. We hypothesize that US-stimulated gene therapy can improve Ad infection in a primary prostate tumor through enhanced tumor uptake and retention of the Ad vector. In vitro studies were performed to analyze the degree of Ad infectivity after application of US-stimulated gene therapy. A luciferase-based Ad on a ubiquitous cytomegalovirus (CMV) promoter (Ad5/3-CMV-Luc) was used in an animal model of prostate cancer (bilateral tumor growth) to evaluate Ad transduction efficiency after US-stimulated therapy. Bioluminescence imaging was employed for in vivo analysis to quantify Ad infection within the tumor. In vitro studies revealed no difference in Ad transduction between groups receiving US-stimulated therapy using high, low or sham US intensity exposures at various multiplicities of infection (MOIs) (p = 0.80). In vivo results indicated that tumors receiving US-stimulated therapy after intra-tumoral injection of Ad5/3-CMV-Luc (1 × 10(6) plaque-forming units) exhibited a 95.1% enhancement in tumor delivery compared with control tumors receiving sham US (p = 0.03). US-stimulated therapy has significant potential to immediately affect Ad-based cancer gene therapy by improving virus bioavailability in target tissues.

Micro- and nanobubbles: a versatile non-viral platform for gene delivery

Micro- and nanobubbles provide a promising non-viral strategy for ultrasound mediated gene delivery. Microbubbles are spherical gas-filled structures with a mean diameter of 1-8 μm, characterised by their core-shell composition and their ability to circulate in the bloodstream following intravenous injection. They undergo volumetric oscillations or acoustic cavitation when insonified by ultrasound and, most importantly, they are able to resonate at diagnostic frequencies. It is due to this behaviour that microbubbles are currently being used as ultrasound contrast agents, but their use in therapeutics is still under investigation. For example, microbubbles could play a role in enhancing gene delivery to cells: when combined with clinical ultrasound exposure, microbubbles are able to favour gene entry into cells by cavitation. Two different delivery strategies have been used to date: DNA can be co-administered with the microbubbles (i.e. the contrast agent) or 'loaded' in purposed-built bubble systems - indeed a number of different technological approaches have been proposed to associate genes within microbubble structures. Nanobubbles, bubbles with sizes in the nanometre order of magnitude, have also been developed with the aim of obtaining more efficient gene delivery systems. Their small sizes allow the possibility of extravasation from blood vessels into the surrounding tissues and ultrasound-targeted site-specific release with minimal invasiveness. In contrast, microbubbles, due to their larger sizes, are unable to extravasate, thus and their targeting capacity is limited to specific antigens present within the vascular lumen. This review provides an overview of the use of microbubbles as gene delivery systems, with a specific focus on recent research into the development of nanosystems. In particular, ultrasound delivery mechanisms, formulation parameters, gene-loading approaches and the advantages of nanometric systems will be described.

Effect of low-intensity focused ultrasound on the middle ear in a mouse model of acute otitis media

We hypothesized that low-intensity focused ultrasound (LIFU) increases vessel permeability and antibacterial drug activity in the mouse middle ear. We determined appropriate settings by applying LIFU to mouse ears with the external auditory canal filled with normal saline and performed histologic and immunohistologic examination. Acute otitis media was induced in mice with nontypable Haemophilus influenzae, and they were given ampicillin (50, 10, or 2 mg/kg) intraperitoneally once daily for 3 days with or without LIFU (1.0 W/cm(2), 20% duty cycle, 30 s). In the LIFU(+) groups receiving the 2- and 10-mg/kg doses, viable bacteria counts, number of inflammatory cells and IL-1β and TNF-α levels in middle ear effusion were significantly lower than in the LIFU(-) groups on the same doses. Severity of AOM also tended to be reduced more in the LIFU(+) groups than in the LIFU(-) groups. LIFU application with antibiotics may be effective for middle ear infection.

Photomechanical wave-driven delivery of siRNAs targeting intermediate filament proteins promotes functional recovery after spinal cord injury in rats

The formation of glial scars after spinal cord injury (SCI) is one of the factors inhibiting axonal regeneration. Glial scars are mainly composed of reactive astrocytes overexpressing intermediate filament (IF) proteins such as glial fibrillary acidic protein (GFAP) and vimentin. In the current study, we delivered small interfering RNAs (siRNAs) targeting these IF proteins to SCI model rats using photomechanical waves (PMWs), and examined the restoration of motor function in the rats. PMWs are generated by irradiating a light-absorbing material with 532-nm nanosecond laser pulses from a Q-switched Nd:YAG laser. PMWs can site-selectively increase the permeability of the cell membrane for molecular delivery. Rat spinal cord was injured using a weight-drop device and the siRNA(s) solutions were intrathecally injected into the vicinity of the exposed SCI, to which PMWs were applied. We first confirmed the substantial uptake of fluorescence-labeled siRNA by deep glial cells; then we delivered siRNAs targeting GFAP and vimentin into the lesion. The treatment led to a significant improvement in locomotive function from five days post-injury in rats that underwent PMW-mediated siRNA delivery. This was attributable to the moderate silencing of the IF proteins and the subsequent decrease in the cavity area in the injured spinal tissue.

Effective gene transfer into central nervous system following ultrasound-microbubbles-induced opening of the blood-brain barrier

To investigate whether ultrasound-targeted microbubble destruction (UTMD) could transfer gene into central nervous system (CNS) following blood-brain barrier disruption (BBBD), DNA-loaded microbubbles were infused into the mice intravenously following ultrasonic exposure. Opening of the BBB, changes of mRNA and expression of enhanced green fluorescent protein (EGFP), and safety evaluation were measured. By UTMD, EGFP were substantially expressed in the cytoplasm of the neurons at the sonicated area with minor erythrocytes extravasation and the mRNA and expression of EGFP were markedly enhanced by about 15-fold and 10-fold, respectively, than that with US alone (p < 0.01). No EGFP was detected in the mice treated with DNA-loaded microbubbles or plasmid alone. The gene expression reached a climax at 48 h, gradually reduced to a much lower level thereafter. These results demonstrated UTMD could effectively enhance exogenous gene delivery and expression in CNS following BBBD, and this technique may provide a new method for CNS gene therapy.

Non-viral gene therapy for spinal cord regeneration

Spinal cord injury (SCI) normally results in life-long disabilities and a broad range of secondary complications. Advances in therapeutic delivery during the past few decades offer hope for such victims. However, the limited functional improvement shown in in vivo studies hinders effective therapeutic application in clinical practice. Recent studies showed that gene vectors can transfect cells present in the lesion of an injured spinal cord (endogenous cells) and thereby produce therapeutic molecules with long-lasting biological effects that promote neural tissue regeneration. In this article we review recent advances in non-viral gene delivery into neural cells and their use for gene therapy in SCI.

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.

Site-specific gene transfer into the rat spinal cord by photomechanical waves

Nonviral, site-specific gene delivery to deep tissue is required for gene therapy of a spinal cord injury. However, an efficient method satisfying these requirements has not been established. This study demonstrates efficient and targeted gene transfer into the spinal cord by using photomechanical waves (PMWs), which were generated by irradiating a black laser absorbing rubber with 532-nm nanosecond Nd:YAG laser pulses. After a solution of plasmid DNA coding for enhanced green fluorescent protein (EGFP) or luciferase was intraparenchymally injected into the spinal cord, PMWs were applied to the target site. In the PMW application group, we observed significant EGFP gene expression in the white matter and remarkably high luciferase activity only in the spinal cord segment exposed to the PMWs. We also assessed hind limb movements 24 h after the application of PMWs based on the Basso-Beattie-Bresnahan (BBB) score to evaluate the noninvasiveness of this method. Locomotor evaluation showed no significant decrease in BBB score under optimum laser irradiation conditions. These findings demonstrated that exogenous genes can be efficiently and site-selectively delivered into the spinal cord by applying PMWs without significant locomotive damage.

Explorations of high-intensity therapeutic ultrasound and microbubble-mediated gene delivery in mouse liver

Ultrasound (US) combined with microbubbles (MBs) is a promising technology for non-viral gene delivery. Significant enhancements of gene expression have been obtained in our previous studies. To optimize and prepare for application to larger animal models, the luciferase reporter gene transfer efficacy of lipid-based Definity MBs of various concentrations, pressure amplitudes and a novel unfocused high-intensity therapeutic US (HITU) system were explored. Luciferase expression exhibited a dependence on MB dose over the range of 0-25 vol%, and a strong dependence on acoustic peak negative pressure at over the range of 0-3.2 MPa. Gene expression reached an apparent plateau at MB concentration ≥2.5 vol% or at negative pressures >1.8 MPa. Maximum gene expression in treated animals was 700-fold greater than in negative controls. Pulse train US exposure protocols produced an upward trend of gene expression with increasing quiescent time. The hyperbolic correlation of gene expression and transaminase levels suggested that an optimum gene delivery effect can be achieved by maximizing acoustic cavitation-induced enhancement of DNA uptake and minimizing unproductive tissue damage. This study validated the new HITU system equipped with an unfocused transducer with a larger footprint capable of scanning large tissue areas to effectively enhance gene transfer efficiencies.

HeLa cell transfection using a novel sonoporation system

Sonoporation has been shown to have an important role in biotechnology for gene therapy and drug delivery. This paper presents a novel microfluidic sonoporation system that achieves high rates of cell transfection and cell viability by operating the sonoporation chamber at resonance. The paper presents a theoretical analysis of the resonant sonoporation chamber design, which achieves sonoporation by forming an ultrasonic standing wave across the chamber. A piezoelectric transducer (PZT 26) is used to generate the ultrasound and the different material thicknesses have been identified to give a chamber resonance at 980 kHz. The efficiency of the sonoporation system was determined experimentally under a range of sonoporation conditions and different exposures time (5, 10, 15, and 20 s, respectively) using HeLa cells and plasmid (peGFP-N1). The experimental results achieve a cell transfection efficiency of 68.9% (analysis of variance, ANOVA, p < 0.05) at the resonant frequency of 980 kHz at 100 V(p-p) (19.5 MPa) with a cell viability of 77% after 10 s of insonication.

Calibration of the 1-MHz Sonitron ultrasound system

Successful drug and gene delivery across cellular membranes can lead to improved therapeutic outcomes. Recent studies have suggested that sonoporation may enhance drug and gene delivery across cellular membranes. The enhancement may be a result of transient permeation of the membrane from cavitation or microstreaming effects of microbubbles exposed to ultrasound. Given limited acoustic pressure calibration and beam profile characterization of the Sonitron ultrasound systems in cellular bioeffects studies previously published, the objective of this work was to calibrate the acoustic output and explore the potential for standing waves in a cell-well plate. In this study, three 1-MHz transducers driven by Sonitron ultrasound systems, which have been used in a number of sonoporation studies, were calibrated. Transducers with 10-mm, 6-mm and 20-mm-diameter apertures (Sonitron 1000 and 2000, Rich-Mar, Inola, OK, USA) were calibrated using polyvinylidene fluoride (PVDF) needle hydrophones. Axial and transverse beam profiles were obtained, and the pressures were measured as a function of Sonitron intensity dial setting and duty cycle. The acoustic intensity was calculated and compared with the Sonitron intensity dial setting for duty cycles from 10-100%. Standing waves caused by reflections from the hydrophone holder were detected for each transducer. This observation may also have implications for in vitro sonoporation studies. Acoustic field characterization is an important first step in understanding the mechanisms of sonoporation and drug delivery across biomembranes.

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.

Plasmid releasing multiple channel bridges for transgene expression after spinal cord injury

The regeneration of tissues with complex architectures requires strategies that promote the appropriate cellular processes, and can direct their organization. Plasmid-loaded multiple channel bridges were engineered for spinal cord regeneration with the ability to support and direct cellular processes and promote gene transfer at the injury site. The bridges were manufactured with a gas foaming technique, and had multiple channels with controllable diameter and encapsulated plasmid. Initial studies investigating bridge implantation subcutaneously (SC) indicated transgene expression in vivo for 44 days, with gene expression dependent upon the pore size of the bridge. In the rat spinal cord, bridges implanted into a lateral hemisection supported substantial cell infiltration, aligned cells within the channels, axon growth across the channels, and high levels of transgene expression at the implant site with decreasing levels rostral and caudal. Immunohistochemistry revealed that the transfected cells at the implant site were present in both the pores and channels of the bridge and were mainly identified as Schwann cells, fibroblasts, and macrophages, in descending order of transfection. This synergy between gene delivery and the scaffold architecture may enable the engineering of tissues with complex architectures.

Low-intensity ultrasound and microbubbles enhance the antitumor effect of cisplatin

Cell permeabilization using microbubbles (MB) and low-intensity ultrasound (US) have the potential for delivering molecules into the cytoplasm. The collapsing MB and cavitation bubbles created by this collapse generate impulsive pressures that cause transient membrane permeability, allowing exogenous molecules to enter the cells. To evaluate this methodology in vitro and in vivo, we investigated the effects of low-intensity 1-MHz pulsed US and MB combined with cis-diamminedichloroplatinum (II) (CDDP) on two cell lines (Colon 26 murine colon carcinoma and EMT6 murine mammary carcinoma) in vitro and in vivo on severe combined immunodeficient mice inoculated with HT29-luc human colon carcinoma. To investigate in vitro the efficiency of molecular delivery by the US and MB method, calcein molecules with a molecular weight in the same range as that of CDDP were used as fluorescent markers. Fluorescence measurement revealed that approximately 10(6)-10(7) calcein molecules per cell were internalized. US-MB-mediated delivery of CDDP in Colon 26 and EMT6 cells increased cytotoxicity in a dose-dependent manner and induced apoptosis (nuclear condensation and fragmentation, and increase in caspase-3 activity). In vivo experiments with xenografts (HT29-luc) revealed a very significant reduction in tumor volume in mice treated with CDDP + US + MB compared with those in the US + CDDP groups for two different concentrations of CDDP. This finding suggests that the US-MB method combined with chemotherapy has clinical potential in cancer therapy.

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.

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.

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.

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.

Expression of hepatocyte growth factor and c-Met after spinal cord injury in rats

Since hepatocyte growth factor (HGF) plays a pivotal role in the development of the central nervous system and pathological conditions, we examined the long-term changes in the mRNA and protein expression of HGF and its receptor c-Met after spinal cord injury (SCI) in rats. HGF mRNA was significantly increased from 7 days after SCI in the injured segment, and the peak was at 7 days after SCI as assessed by real-time RT-PCR. Importantly, c-met mRNA expression was up-regulated from 1 day after SCI, and reached a peak at 14 days after SCI. Although up-regulation of HGF and c-met mRNA expression in the injured segment gradually decreased, the increased expression level persisted until 56 days after SCI. Consistent with HGF mRNA expression, HGF protein level was significantly increased mainly in the injured region, which persisted until 56 days after SCI. Immunohistochemistry showed that most of GFAP-positive reactive astrocytes expressed HGF and c-Met both on 14 days and 56 days after SCI. Staining with the mitotic indicator, bromodeoxyuridine (BrdU), revealed that a small number of BrdU-incorporated cells were co-localized with HGF/GFAP-positive or c-Met/GFAP-positive cells both on 14 and 56 days. These data suggest that HGF and c-Met were up-regulated mainly in the reactive astrocytes around the injured region in the subacute to chronic stage of spinal cord injury. Since HGF plays a critical role in neurotrophic activity, activation of the HGF/c-Met signaling system might be involved in the process of post-traumatic regeneration.

Key factors that affect sonoporation efficiency in in vitro settings: the importance of standing wave in sonoporation

Ultrasound-induced intracellular drug delivery, sonoporation, is an appealing and promising technique for next generation drug delivery system. Many types of molecules, such as plasmid DNAs, siRNAs and peptides, have been demonstrated to be delivered into the cell by ultrasound with the aid of microbubbles both in vitro and in vivo. Although there are many reports on in vitro sonoporation, the efficiency of successful sonoporation and the viabilities of cells after the procedure documented in each report vary in a wide range, and the reasons for these differences are not fully understood. In this study, we have investigated how different experimental settings would affect sonoporation efficiency and cell viabilities after the procedure. Our results show that the fashion of cell culture (e.g. in suspension or in monolayer culture) and the presence of standing wave have a great impact on the overall results. These results indicate that in vitro sonoporation settings should be carefully evaluated in each experiment. The fact that standing wave is necessary to achieve high sonoporation efficiency may be a problematic issue for clinical application of sonoporation, as it may be difficult (although not impossible) to create standing wave in a human body.

[Development of plasmid DNA-based gene transfer]

Gene therapy based on ultrasound with microbubbles offers a novel approach for the prevention and treatment of variety of diseases. The major development of gene transfer has importantly contributed to intense investigation of the potential of gene therapy in cancer or cardiovascular medicine. The amazing advances in molecular biology have provided a dramatic improvement of the technology that is necessary to transfer target genes into somatic cells. Gene transfer methods have been surprisingly improved. In fact, some of them (retroviral vectors, adenoviral vectors or liposome based vectors, etc.) have been used in the clinical trials already. But some severe side effects were reported in clinical gene therapy using such viral, so people desire safe and efficient clinical gene therapy. Recently, ultrasound-mediated gene transfer has been reported to augment the transfection efficiency and facilitate local gene expression. Interestingly, gene transfer into the fetal central nervous system was successfully achieved by intrauterine injection with microbubble-enhanced ultrasound. Compared to other viral vectors, there are some theoretical advantages including safety, simplicity of preparation, and local gene transfer. Thus, we focused on the development of gene transfer using naked plasmid DNA with an ultrasound or microbubble-enhanced ultrasound method.

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.

Spinal gene transfer using ultrasound and microbubbles

Spinal gene therapy is a promising option for treating various spinal-related disorders. Several previous studies using viral vectors reported successful transfer of therapeutic genes into the spinal nerve system. However, because of the considerable immunogenicity related to the use of viruses, non-viral gene transfer still needs to be developed. One possible approach is the combined use of ultrasound and echo-contrast microbubbles. The present study shows that this method can be applied for targeted intrathecal gene delivery. We intrathecally injected a mixture of plasmid-DNA encoded with luciferase and commercially available albumin microbubbles by needle puncture at the lower lumbar intervertebral space in mice. Subsequent percutaneous ultrasonication on the lumbar vertebrae significantly enhanced the luciferase expression, analyzed by imaging luciferin bioluminescence, in the dorsal meningeal cells at the insonated region. No apparent neurological damages were induced by the present spinal interventions. In addition to the general benefits of the combined use of ultrasound and microbubbles, our approach can offer some advantages specific to spinal gene transfection including minimal invasiveness of simple percutaneous dural puncture, targetability due to the limited access of ultrasound waves through anatomical apertures of the vertebrae, and possible paracrine delivery of therapeutic molecules to the spinal nerve system.

Spatial and acoustic pressure dependence of microbubble-mediated gene delivery targeted using focused ultrasound

BACKGROUND

Ultrasound/microbubble-mediated gene delivery has the potential to be targeted to tissue deep in the body by directing the ultrasound beam following vector administration. Application of this technology would be minimally invasive and benefit from the widespread clinical experience of using ultrasound and microbubble contrast agents. In this study we evaluate the targeting ability and spatial distribution of gene delivery using focused ultrasound.

METHODS

Using a custom-built exposure tank, Chinese hamster ovary cells in the presence of SonoVue microbubbles and plasmid encoding beta-galactosidase were exposed to ultrasound in the focal plane of a 1 MHz transducer. Gene delivery and cell viability were subsequently assessed. Characterisation of the acoustic field and high-resolution spatial analysis of transfection were used to examine the relationship between gene delivery efficiency and acoustic pressure.

RESULTS

In contrast to that seen in the homogeneous field close to the transducer face, gene delivery in the focal plane was concentrated on the ultrasound beam axis. Above a minimum peak-to-peak value of 0.1 MPa, transfection efficiency increased as acoustic pressure increased towards the focus, reaching a maximum above 1 MPa. Delivery was microbubble-dependent and cell viability was maintained.

CONCLUSIONS

Gene delivery can be targeted using focused ultrasound and microbubbles. Since delivery is dependent on acoustic pressure, the degree of targeting can be determined by appropriate transducer design to modify the ultrasound field. In contrast to other physical gene delivery approaches, the non-invasive targeting ability of ultrasound makes this technology an attractive option for clinical gene therapy.