Diagnostic ultrasound activation of contrast agent gas bodies induces capillary rupture in mice

Theranostics in the vasculature: bioeffects of ultrasound and microbubbles to induce vascular shutdown

Ultrasound-triggered microbubbles destruction leading to vascular shutdown have resulted in preclinical studies in tumor growth delay or inhibition, lesion formation, radio-sensitization and modulation of the immune micro-environment. Antivascular ultrasound aims to be developed as a focal, targeted, non-invasive, mechanical and non-thermal treatment, alone or in combination with other treatments, and this review positions these treatments among the wider therapeutic ultrasound domain. Antivascular effects have been reported for a wide range of ultrasound exposure conditions, and evidence points to a prominent role of cavitation as the main mechanism. At relatively low peak negative acoustic pressure, predominantly non-inertial cavitation is most likely induced, while higher peak negative pressures lead to inertial cavitation and bubbles collapse. Resulting bioeffects start with inflammation and/or loose opening of the endothelial lining of the vessel. The latter causes vascular access of tissue factor, leading to platelet aggregation, and consequent clotting. Alternatively, endothelium damage exposes subendothelial collagen layer, leading to rapid adhesion and aggregation of platelets and clotting. In a pilot clinical trial, a prevalence of tumor response was observed in patients receiving ultrasound-triggered microbubble destruction along with transarterial radioembolization. Two ongoing clinical trials are assessing the effectiveness of ultrasound-stimulated microbubble treatment to enhance radiation effects in cancer patients. Clinical translation of antivascular ultrasound/microbubble approach may thus be forthcoming.

Development of a Gene and Nucleic Acid Delivery System for Skeletal Muscle Administration via Limb Perfusion Using Nanobubbles and Ultrasound

Strategies for gene and nucleic acid delivery to skeletal muscles have been extensively explored to treat Duchenne muscular dystrophy (DMD) and other neuromuscular diseases. Of these, effective intravascular delivery of naked plasmid DNA (pDNA) and nucleic acids into muscles is an attractive approach, given the high capillary density in close contact with myofibers. We developed lipid-based nanobubbles (NBs) using polyethylene-glycol-modified liposomes and an echo-contrast gas and found that these NBs could improve tissue permeability by ultrasound (US)-induced cavitation. Herein, we delivered naked pDNA or antisense phosphorodiamidate morpholino oligomers (PMOs) into the regional hindlimb muscle via limb perfusion using NBs and US exposure. pDNA encoding the luciferase gene was injected with NBs via limb perfusion into normal mice with application of US. High luciferase activity was achieved in a wide area of the limb muscle. DMD model mice were administered PMOs, designed to skip the mutated exon 23 of the dystrophin gene, with NBs via intravenous limb perfusion, followed by US exposure. The number of dystrophin-positive fibers increased in the muscles of mdx mice. Combining NBs and US exposure, which can be widely delivered to the hind limb muscles via the limb vein, could be an effective therapeutic approach for DMD and other neuromuscular disorders.

A Review on Biological Effects of Ultrasounds: Key Messages for Clinicians

Ultrasound (US) is acoustic energy that interacts with human tissues, thus, producing bioeffects that may be hazardous, especially in sensitive organs (i.e., brain, eye, heart, lung, and digestive tract) and embryos/fetuses. Two basic mechanisms of US interaction with biological systems have been identified: thermal and non-thermal. As a result, thermal and mechanical indexes have been developed to provide a means of assessing the potential for biological effects from exposure to diagnostic US. The main aims of this paper were to describe the models and assumptions used to estimate the "safety" of acoustic outputs and indices and to summarize the current state of knowledge about US-induced effects on living systems deriving from in vitro models and in vivo experiments on animals. This review work has made it possible to highlight the limits associated with the use of the estimated safety values of thermal and mechanical indices relating above all to the use of new US technologies, such as contrast-enhanced ultrasound (CEUS) and acoustic radiation force impulse (ARFI) shear wave elastography (SWE). US for diagnostic and research purposes has been officially declared safe, and no harmful biological effects in humans have yet been demonstrated with new imaging modalities; however, physicians should be adequately informed on the potential risks of biological effects. US exposure, according to the ALARA (As Low As Reasonably Achievable) principle, should be as low as reasonably possible.

Stable Cavitation-Mediated Delivery of miR-126 to Endothelial Cells

In endothelial cells, microRNA-126 (miR-126) promotes angiogenesis, and modulating the intracellular levels of this gene could suggest a method to treat cardiovascular diseases such as ischemia. Novel ultrasound-stimulated microbubbles offer a means to deliver therapeutic payloads to target cells and sites of disease. The purpose of this study was to investigate the feasibility of gene delivery by stimulating miR-126-decorated microbubbles using gentle acoustic conditions (stable cavitation). A cationic DSTAP microbubble was formulated and characterized to carry 6 µg of a miR-126 payload per 109 microbubbles. Human umbilical vein endothelial cells (HUVECs) were treated at 20−40% duty cycle with miR-126-conjugated microbubbles in a custom ultrasound setup coupled with a passive cavitation detection system. Transfection efficiency was assessed by RT-qPCR, Western blotting, and endothelial tube formation assay, while HUVEC viability was monitored by MTT assay. With increasing duty cycle, the trend observed was an increase in intracellular miR-126 levels, up to a 2.3-fold increase, as well as a decrease in SPRED1 (by 33%) and PIK3R2 (by 46%) expression, two salient miR-126 targets. Under these ultrasound parameters, HUVECs maintained >95% viability after 96 h. The present work describes the delivery of a proangiogenic miR-126 using an ultrasound-responsive cationic microbubble with potential to stimulate therapeutic angiogenesis while minimizing endothelial damage.

Overcoming Hypoxia-Induced Drug Resistance via Promotion of Drug Uptake and Reoxygenation by Acousto–Mechanical Oxygen Delivery

Hypoxia-induced drug resistance (HDR) is a critical issue in cancer therapy. The presence of hypoxic tumor cells impedes drug uptake and reduces the cytotoxicity of chemotherapeutic drugs, leading to HDR and increasing the probability of tumor recurrence and metastasis. Microbubbles, which are used as an ultrasound contrast agent and drug/gas carrier, can locally deliver drugs/gas and produce an acousto–mechanical effect to enhance cell permeability under ultrasound sonication. The present study applied oxygen-loaded microbubbles (OMBs) to evaluate the mechanisms of overcoming HDR via promotion of drug uptake and reoxygenation. A hypoxic mouse prostate tumor cell model was established by hypoxic incubation for 4 h. After OMB treatment, the permeability of HDR cells was enhanced by 23 ± 5% and doxorubicin uptake was increased by 11 ± 7%. The 61 ± 14% reoxygenation of HDR cells increased the cytotoxicity of doxorubicin from 18 ± 4% to 58 ± 6%. In combination treatment with OMB and doxorubicin, the relative contributions of uptake promotion and reoxygenation towards overcoming HDR were 11 ± 7% and 28 ± 10%, respectively. Our study demonstrated that reoxygenation of hypoxic conditions is a critical mechanism in the inhibition of HDR and enhancing the outcome of OMB treatment.

Simulation, Implementation and Measurement of Defined Sound Fields for Blood-Brain Barrier Opening in Rats

The blood-brain barrier (BBB) is the most important obstacle to delivery of therapeutics to the central nervous system. Low-intensity pulsed focused ultrasound (FUS) in combination with microbubbles applied under magnetic resonance imaging (MRI) control provides a non-invasive and safe technique for BBB opening (BBBo). In rodent models, however, settings and application protocols differ significantly. Depending on the strain and size, important variables include ultrasound attenuation and sound field distortion caused by the skull. We examined the ultrasound attenuation of the skull of Wistar rats using a targeted FUS system. By modifying the transducer elements and by varying and simulating the acoustic field of the FUS system, we measured a skull attenuation of about 60%. To evaluate potential application of the targeted FUS system in genetically modified animals with increased sensitivity to brain hemorrhage caused by vascular dysfunction, we assessed safety in healthy animals. Histological and MRI analyses of the central nervous system revealed an increase in the number and severity of hyperacute bleeds with focal pressure. At a pressure of 0.4 MPa, no bleeds were induced, albeit at the cost of a weaker hyperintense MRI signal post BBBo. These results indicate a relationship between pressure and the dimension of permeabilization.

Ultrasonic particles: An approach for targeted gene delivery

Gene therapy has been widely investigated for the treatment of genetic, acquired, and infectious diseases. Pioneering work utilized viral vectors; however, these are suspected of causing serious adverse events, resulting in the termination of several clinical trials. Non-viral vectors, such as lipid nanoparticles, have attracted significant interest, mainly due to their successful use in vaccines in the current COVID-19 pandemic. Although they allow safe delivery, they come with the disadvantage of off-target delivery. The application of ultrasound to ultrasound-sensitive particles allows for a direct, site-specific transfer of genetic materials into the organ/site of interest. This process, termed ultrasound-targeted gene delivery (UTGD), also increases cell membrane permeability and enhances gene uptake. This review focuses on the advances in ultrasound and the development of ultrasonic particles for UTGD across a range of diseases. Furthermore, we discuss the limitations and future perspectives of UTGD.

Contrast-enhanced ultrasound: a comprehensive review of safety in children

Contrast-enhanced ultrasound (CEUS) has been increasingly used in pediatric radiology practice worldwide. For nearly two decades, CEUS applications have been performed with the off-label use of gas-containing second-generation ultrasound contrast agents (UCAs). Since 2016, the United States Food and Drug Administration (FDA) has approved the UCA Lumason for three pediatric indications: the evaluation of focal liver lesions and echocardiography via intravenous administration and the assessment of vesicoureteral reflux via intravesical application (contrast-enhanced voiding urosonography, ceVUS). Prior to the FDA approval of Lumason, numerous studies with the use of second-generation UCAs had been conducted in adults and children. Comprehensive protocols for clinical safety evaluations have demonstrated the highly favorable safety profile of UCA for intravenous, intravesical and other intracavitary uses. The safety data on CEUS continue to accumulate as this imaging modality is increasingly utilized in clinical settings worldwide. As of August 2021, 57 pediatric-only original research studies encompassing a total of 4,518 children with 4,906 intravenous CEUS examinations had been published. As in adults, there were a few adverse events; the majority of these were non-serious, although very rarely serious anaphylactic reactions were reported. In the published pediatric-only intravenous CEUS studies included in our analysis, the overall incidence rate of serious adverse events was 0.22% (10/4,518) of children and 0.20% (10/4,906) of all CEUS examinations. Non-serious adverse events from the intravenous CEUS were observed in 1.20% (54/4,518) of children and 1.10% (54/4,906) of CEUS examinations. During the same time period, 31 studies with the intravesical use of UCA were conducted in 12,362 children. A few non-serious adverse events were encountered (0.31%; 38/12,362), but these were most likely attributable to the bladder catheterization rather than the UCA. Other developing clinical applications of UCA in children, including intracavitary and intralymphatic, are ongoing. To date, no serious adverse events have been reported with these applications. This article reviews the existing pediatric CEUS literature and provides an overview of safety-related information reported from UCA uses in children.

Ultrasound-Enabled Therapeutic Delivery and Regenerative Medicine: Physical and Biological Perspectives

The role of ultrasound in medicine and biological sciences is expanding rapidly beyond its use in conventional diagnostic imaging. Numerous studies have reported the effects of ultrasound on cellular and tissue physiology. Advances in instrumentation and electronics have enabled successful in vivo applications of therapeutic ultrasound. Despite path breaking advances in understanding the biophysical and biological mechanisms at both microscopic and macroscopic scales, there remain substantial gaps. With the progression of research in this area, it is important to take stock of the current understanding of the field and to highlight important areas for future work. We present herein key developments in the biological applications of ultrasound especially in the context of nanoparticle delivery, drug delivery, and regenerative medicine. We conclude with a brief perspective on the current promise, limitations, and future directions for interfacing ultrasound technology with biological systems, which could provide guidance for future investigations in this interdisciplinary area.

Therapeutic Ultrasound Parameter Optimization for Drug Delivery Applied to a Murine Model of Hepatocellular Carcinoma

Ultrasound and microbubble (USMB)-mediated drug delivery is a valuable tool for increasing the efficiency of the delivery of therapeutic agents to cancer while maintaining low systemic toxicity. Typically, selection of USMB drug delivery parameters used in current research settings are either based on previous studies described in the literature or optimized using tissue-mimicking phantoms. However, phantoms rarely mimic in vivo tumor environments, and the selection of parameters should be based on the application or experiment. In the following study, we optimized the therapeutic parameters of the ultrasound drug delivery system to achieve the most efficient in vivo drug delivery using fluorescent semiconducting polymer nanoparticles as a model nanocarrier. We illustrate that voltage, pulse repetition frequency and treatment time (i.e., number of ultrasound pulses per therapy area) delivered to the tumor can successfully be optimized in vivo to ensure effective delivery of the semiconducting polymer nanoparticles to models of hepatocellular carcinoma. The optimal in vivo parameters for USMB drug delivery in this study were 70 V (peak negative pressure = 3.4 MPa, mechanical index = 1.22), 1-Hz pulse repetition frequency and 100-s therapy time. USMB-mediated drug delivery using in vivo optimized ultrasound parameters caused an up to 2.2-fold (p < 0.01) increase in drug delivery to solid tumors compared with that using phantom-optimized ultrasound parameters.

Therapeutic Ultrasound in Cardiovascular Medicine

An advantage of therapeutic ultrasound (US) is the ability to cause controlled biological effects noninvasively. Depending on the magnitude and frequency of exposure parameters, US can interact in different ways with a variety of biological tissues. The development and clinical utility of therapeutic US techniques are now rapidly growing, especially with regard to the application of US pulses for cardiac pacing and the potential treatment of cardiovascular diseases. This review outlines the basic principles of US-based therapy in cardiology, including the acoustic properties of the cardiovascular tissue, and the use of US in therapeutic cardiovascular medicine.

Research methodology for in vivo measurements of resting energy expenditure, daily body temperature, metabolic heat and non-viral tissue-specific gene therapy in baboons

A large number of studies have shown that the baboon is one of the most commonly used non-human primate (NHP) research model for the study of immunometabolic complex traits such as type 2 diabetes (T2D), insulin resistance (IR), adipose tissue dysfunction (ATD), dyslipidemia, obesity (OB) and cardiovascular disease (CVD). This paper reports on innovative technologies and advanced research strategies for energetics and translational medicine with this NHP model. This includes the following: measuring resting energy expenditure (REE) with the mobile indirect calorimeter Breezing®; monitoring daily body temperature using subcutaneously implanted data loggers; quantifying metabolic heat with veterinary infrared thermography (IRT) imaging, and non-viral non-invasive, tissue-specific ultrasound-targeted microbubble destruction (UTMD) gene-based therapy. These methods are of broad utility; for example, they may facilitate the engineering of ectopic overexpression of brown adipose tissue (BAT) mUCP-1 via UTMD-gene therapy into baboon SKM to achieve weight loss, hypophagia and immunometabolic improvement. These methods will be valuable to basic and translational research, and human clinical trials, in the areas of metabolism, cardiovascular health, and immunometabolic and infectious diseases.

Functional Activity and Endothelial-Lining Integrity of Ex Vivo Arteries Exposed to Ultrasound-Mediated Microbubble Destruction

Ultrasound-mediated microbubble destruction (UMMD) is a promising strategy to improve local drug delivery in specific tissues. However, acoustic cavitation can lead to harmful bioeffects in endothelial cells. We investigated the side effects of UMMD treatment on vascular function (contraction and relaxation) and endothelium integrity of ex vivo Wistar rat arteries. We used an isolated organ system to evaluate vascular responses and confocal microscopy to quantify the integrity and viability of endothelial cells. The arteries were exposed for 1-3 min to ultrasound at a 100 Hz pulse-repetition frequency, 0.5 MPa acoustic pressure, 50% duty cycle and 1%-5% v/v microbubbles. The vascular contractile response was not affected. The acetylcholine-dependent maximal relaxation response was reduced from 78% (control) to 60% after 3 min of ultrasound exposure. In arteries treated simultaneously with 1 min of ultrasound exposure and 1%, 2%, 3% or 5% microbubble concentration, vascular relaxation was reduced by 19%, 58%, 80% or 93%, respectively, compared with the control arteries. Fluorescent labeling revealed that apoptotic death, detachment of endothelial cells and reduced nitric oxide synthase phosphorylation are involved in relaxation impairment. We demonstrated that UMMD can be a safe technology if the correct ultrasound and microbubble parameters are applied. Furthermore, we found that tissue-function evaluation combined with cellular analysis can be useful to study ultrasound-microbubble-tissue interactions in the optimization of targeted endothelial drug delivery.

Delayed contrast enhancement of hepatic parenchyma after intravenous sonographic contrast agent: unusual phenomenon. Case report and review of literature

Aim

A case of heterogeneous late-phase hepatic enhancement (HLHE) using contrast‐enhanced ultrasound (CEUS) with SonoVue is presented, where HLHE lasted after 50 min of injection.

Methods

This study aims to review prior literature on this topic, to characterize the features of HLHE in the liver, and to find possible and reliable explanations for this phenomenon.

Results

From literature, thus far five publications discuss this phenomenon with a total of 21 patients.

Conclusion

We suggest that phagocytosis of contrast agent microbubbles by macrophages, and lymphocytosis of peripheral blood due to stress conditions of the patients might be in the background of HLHE.

Ultrasound Imaging

Ultrasound imaging plays an important role in oncological imaging for more than five decades now. It can be applied in all tissues that are not occluded by bone or gas-filled regions. The quality of ultrasound images benefitted strongly from improved electronics and increased computational power. To the morphological imaging, several functional imaging methods were added: Flow visualization became possible by Doppler techniques and as a recent addition the elastic properties of tissues can be imaged by elastographic methods with transient shear wave imaging. In the beginning of molecular imaging, ultrasound with its contrast based on mechanical tissue properties was an unlikely candidate to play a role. However, with contrast agents consisting of micrometer-sized gas bubbles, which can be imaged with high sensitivity, ligands addressing targets in the vascular wall could be used. Because even single bubbles can be detected, this led to various ultrasound molecular imaging techniques and the ongoing development of clinical molecular contrast media. In this chapter, the basic properties of ultrasonic imaging like its contrast mechanisms and spatiotemporal resolution are discussed. The image formation and its ongoing change from line-oriented scanning to full-volume reconstructions are explained. Then, the ultrasound contrast media and imaging techniques are introduced and emerging new methods like super-resolution vascular imaging demonstrate the ongoing development in this field.

Photoacoustic simulations of microvascular bleeding: spectral analysis and its application for monitoring vascular-targeted treatments

Solid tumors are typically supplied nutrients by a network of irregular blood vessels. By targeting these vascular networks, it might be possible to hinder cancer growth and metastasis. Vascular disrupting agents induce intertumoral hemorrhaging, making photoacoustic (PA) imaging well positioned to detect bleeding due to its sensitivity to hemoglobin and its various states. We introduce a fractal-based numerical model of intertumoral hemorrhaging to simulate the PA signals from disrupted tumor blood vessels. The fractal model uses bifurcated cylinders to represent vascular trees. To mimic bleeding from blood vessels, hemoglobin diffusion from microvessels was simulated. In the simulations, the PA signals were detected by a linear array transducer (30 MHz center frequency) of four different vascular trees. The power spectrum of each beamformed PA signal was computed and fitted to a straight line within the −6-dB bandwidth of the receiving transducer. The spectral slope and midband fit (MBF) based on the fit decreased by 0.11  dB  /  MHz and 2.12 dB, respectively, 1 h post bleeding, while the y-intercept increased by 1.21 dB. The results suggest that spectral PA analysis can be used to measure changes in the concentration and spatial distribution of hemoglobin in tissue without the need to resolve individual vessels. The simulations support the feasibility of using PA imaging and spectral analysis in cancer treatment monitoring by detecting microvessel disruption.

Capillary Hemorrhage Induced by Contrast-Enhanced Diagnostic Ultrasound in Rat Intestine

Contrast-enhanced diagnostic ultrasound (CEDUS) can lead to microvascular injury and petechial hemorrhage by the cavitational mechanism of ultrasonic bioeffects. Capillary hemorrhage has been noted in the heart and kidney, which are common targets of CEDUS examination. CEDUS has also become useful for monitoring intestinal inflammation. In the 1990s, the risk of intestinal microvascular hemorrhage was investigated both for incidental exposure by lithotripter shockwaves and for contrast agent microbubbles acting as cavitation nuclei with laboratory pulsed ultrasound systems. This study was initiated to update the risk assessment for intestine exposed to diagnostic imaging simulating CEDUS. The abdomens of anesthetized rats were scanned by a 1.6 MHz phased array probe during infusion of microbubble suspensions simulating Definity ultrasound contrast agent. Dual image frames were triggered intermittently, and the output power was varied to assess the exposure response. Petechiae counts in small intestine mucosa and muscle layers increased with increasing trigger interval from 2 s to 10 s, indicative of a slow refill after microbubble destruction. The counts increased with increasing output above a threshold of 1.4 MPa peak rarefactional pressure amplitude. Petechiae were also seen in Peyer's patches, and occult blood was detected in many affected segments of intestine. These results are consistent with early laboratory pulsed-ultrasound results.

Hepatocyte Injury Induced by Contrast-Enhanced Diagnostic Ultrasound

OBJECTIVES

Contrast-enhanced diagnostic ultrasound (US) has a potential to induce localized biological effects. The potential for contrast-enhanced diagnostic US bioeffects in liver were researched, with guidance from a report by Yang et al (Ultrasonics 2012; 52:1065-1071).

METHODS

Contact and standoff scanning was performed for 10 minutes with a diagnostic US phased array at 1.6 MHz during bolus injection or infusion of a contrast agent at a high dose. The impact of the imaging on rat liver was investigated by measuring enzyme release, microvascular leakage, and staining of injured hepatocytes.

RESULTS

The results showed liver enzyme release at 30 minutes, indicating liver injury, and elevated extraction of Evans blue dye, indicating microvascular leakage. In addition, Evans blue and trypan blue vital-staining methods revealed scattered stained cells within the US scan plane. For the Evans blue method, fluorescent cell counts in frozen sections were greatest for standoff exposure with contrast infusion. The count decreased strongly with depth for bolus injection, which was probably reflective of the high attenuation noted for this agent delivery method.

CONCLUSIONS

The results qualitatively confirmed the report by Yang et al and additionally showed hepatocyte vital staining. Research is needed to determine the threshold for the effects and the contrast agent dose response.

Advances in acoustic monitoring and control of focused ultrasound-mediated increases in blood-brain barrier permeability

Transcranial focused ultrasound (FUS) combined with intravenously circulating microbubbles can transiently and selectively increase blood-brain barrier permeability to enable targeted drug delivery to the central nervous system, and is a technique that has the potential to revolutionize the way neurological diseases are managed in medical practice. Clinical testing of this approach is currently underway in patients with brain tumors, early Alzheimer's disease, and amyotrophic lateral sclerosis. A major challenge that needs to be addressed in order for widespread clinical adoption of FUS-mediated blood-brain barrier permeabilization to occur is the development of systems and methods for real-time treatment monitoring and control, to ensure that safe and effective acoustic exposure levels are maintained throughout the procedures. This review gives a basic overview of the oscillation dynamics, acoustic emissions, and biological effects associated with ultrasound-stimulated microbubbles in vivo, and provides a summary of recent advances in acoustic-based strategies for detecting, controlling, and mapping microbubble activity in the brain. Further development of next-generation clinical FUS brain devices tailored towards microbubble-mediated applications is warranted and required for translation of this potentially disruptive technology into routine clinical practice.

Enhanced microbubble contrast agent oscillation following 250 kHz insonation

Microbubble contrast agents are widely used in ultrasound imaging and therapy, typically with transmission center frequencies in the MHz range. Currently, an ultrasound center frequency near 250 kHz is proposed for clinical trials in which ultrasound combined with microbubble contrast agents is applied to open the blood brain barrier, since at this low frequency focusing through the human skull to a predetermined location can be performed with reduced distortion and attenuation compared to higher frequencies. However, the microbubble vibrational response has not yet been carefully evaluated at this low frequency (an order of magnitude below the resonance frequency of these contrast agents). In the past, it was assumed that encapsulated microbubble expansion is maximized near the resonance frequency and monotonically decreases with decreasing frequency. Our results indicated that microbubble expansion was enhanced for 250 kHz transmission as compared with the 1 MHz center frequency. Following 250 kHz insonation, microbubble expansion increased nonlinearly with increasing ultrasonic pressure, and was accurately predicted by either the modified Rayleigh-Plesset equation for a clean bubble or the Marmottant model of a lipid-shelled microbubble. The expansion ratio reached 30-fold with 250 kHz at a peak negative pressure of 400 kPa, as compared to a measured expansion ratio of 1.6 fold for 1 MHz transmission at a similar peak negative pressure. Further, the range of peak negative pressure yielding stable cavitation in vitro was narrow (~100 kPa) for the 250 kHz transmission frequency. Blood brain barrier opening using in vivo transcranial ultrasound in mice followed the same trend as the in vitro experiments, and the pressure range for safe and effective treatment was 75-150 kPa. For pressures above 150 kPa, inertial cavitation and hemorrhage occurred. Therefore, we conclude that (1) at this low frequency, and for the large oscillations, lipid-shelled microbubbles can be approximately modeled as clean gas microbubbles and (2) the development of safe and successful protocols for therapeutic delivery to the brain utilizing 250 kHz or a similar center frequency requires consideration of the narrow pressure window between stable and inertial cavitation.

Ultrasound Triggered Conversion of Porphyrin/Camptothecin-Fluoroxyuridine Triad Microbubbles into Nanoparticles Overcomes Multidrug Resistance in Colorectal Cancer

Multidrug resistance remains one of the main obstacles to efficient chemotherapy of colorectal cancer. Herein, an efficient combination therapeutic strategy is proposed based on porphyrin/camptothecin-floxuridine triad microbubbles (PCF-MBs) with high drug loading contents, which own highly stable co-delivery drug combinations and no premature release. The triad PCF-MBs can act not only as a contrast agent for ultrasound (US)/fluorescence bimodal imaging but also a multimodal therapeutic agent for synergistic chemo-photodynamic combination therapy. Upon local ultrasound exposure under the guidance of ultrasound imaging, in situ conversion of PCF-MBs into porphyrin/camptothecin-floxuridine nanoparticles (PCF-NPs) leads to high accumulation of chemo-drugs and photosensitizer in tumors due to the induced high permeability of the capillary wall and cell membrane temporarily via sonoporation effect, greatly reducing the risk of systemic exposure. Most importantly, it was found that the PCF-MB-mediated photodynamic therapy could significantly reduce the expression of adenosine-triphosphate (ATP)-binding cassette subfamily G member 2 (ABCG₂), which is responsible for the drug resistance in chemotherapy, resulting in a prominent intracellular camptothecin increase. In vivo experiments revealed that the PCF-MBs in combination with ultrasound and laser irradiation could achieve a 90% tumor inhibition rate of HT-29 cancer with no recurrence. Therefore, such triad PCF-MB-based combination therapeutic strategy shows great promise for overcoming drug resistance of colorectal cancer and other cancers.

Three-dimensional transcranial microbubble imaging for guiding volumetric ultrasound-mediated blood-brain barrier opening

Focused ultrasound (FUS)-mediated blood-brain barrier (BBB) opening recently entered clinical testing for targeted drug delivery to the brain. Sources of variability exist in the current procedures, motivating the development of real-time monitoring and control techniques to improve treatment safety and efficacy. Here we used three-dimensional (3D) transcranial microbubble imaging to calibrate FUS exposure levels for volumetric BBB opening.

Methods: Using a sparse hemispherical transmit/receive ultrasound phased array, pulsed ultrasound was focused transcranially into the thalamus of rabbits during microbubble infusion and multi-channel 3D beamforming was performed online with receiver signals captured at the subharmonic frequency. Pressures were increased pulse-by-pulse until subharmonic activity was detected on acoustic imaging (p_sub), and tissue volumes surrounding the calibration point were exposed at 50-100%p_sub via rapid electronic beam steering.

Results: Spatially-coherent subharmonic microbubble activity was successfully reconstructed transcranially in vivo during calibration sonications. Multi-point exposures induced volumetric regions of elevated BBB permeability assessed via contrast-enhanced magnetic resonance imaging (MRI). At exposure levels ≥75%p_sub, MRI and histological examination occasionally revealed tissue damage, whereas sonications at 50%p_sub were performed safely. Substantial intra-grid variability of FUS-induced bioeffects was observed via MRI, prompting future development of multi-point calibration schemes for improved treatment consistency. Receiver array sparsity and sensor configuration had substantial impacts on subharmonic detection sensitivity, and are factors that should be considered when designing next-generation clinical FUS brain therapy systems.

Conclusion: Our findings suggest that 3D subharmonic imaging can be used to calibrate exposure levels for safe FUS-induced volumetric BBB opening, and should be explored further as a method for cavitation-mediated treatment guidance.

Imaging with ultrasound contrast agents: current status and future

Microbubble ultrasound contrast agents (UCAs) were recently approved by the Food and Drug administration for non-cardiac imaging. The physical principles of UCAs, methods of administration, dosage, adverse effects, and imaging techniques both current and future are described. UCAs consist of microbubbles in suspension which strongly interact with the ultrasound beam and are readily detectable by ultrasound imaging systems. They are confined to the blood pool when administered intravenously, unlike iodinated and gadolinium contrast agents. UCAs have a proven safety record based on over two decades of use, during which they have been used in echocardiography in the U.S. and for non-cardiac imaging in the rest of the world. Adverse effects are less common with UCAs than CT/MR contrast agents. Compared to CT and MR, contrast-enhanced ultrasound has the advantages of real-time imaging, portability, and reduced susceptibility to metal and motion artifact. UCAs are not nephrotoxic and can be used in renal failure. High acoustic amplitudes can cause microbubbles to fragment in a manner that can result in short-term increases in capillary permeability or capillary rupture. These bioeffects can be beneficial and have been used to enhance drug delivery under appropriate conditions. Imaging with a mechanical index of < 0.4 preserves the microbubbles and is not typically associated with substantial bioeffects. Molecularly targeted ultrasound contrast agents are created by conjugating the microbubble shell with a peptide, antibody, or other ligand designed to target an endothelial biomarker associated with tumor angiogenesis or inflammation. These microbubbles then accumulate in the microvasculature at target sites where they can be imaged. Ultrasound contrast agents are a valuable addition to the diagnostic imaging toolkit. They will facilitate cross-sectional abdominal imaging in situations where contrast-enhanced CT and MR are contraindicated or impractical.

Hemocoagulase Combined with Microbubble-Enhanced Ultrasound Cavitation for Augmented Ablation of Microvasculature in Rabbit VX2 Liver Tumors

We investigated a new method for combining microbubble-enhanced ultrasound cavitation (MEUC) with hemocoagulase (HC) atrox. Our goal was to induce embolic effects in the vasculature and combine these with an anti-angiogenic treatment strategy. Fourteen days after being implanted with a single slice of the liver VX2 tumor, rabbits were randomly divided into five groups: (i) a control group injected intra-venously with saline using a micropump; (ii) a group given only an injection of HC; (iii) a group treated only with ultrasound cavitation; (iv) a group treated with MEUC; (v) a group treated with MEUC + HC. Contrast-enhanced ultrasound was performed before treatment and 1 h and 7 d post-treatment to measure tumor size, enhancement and necrosis range. QontraXt software was used to determine the time-intensity curve of tumor blood perfusion and microvascular changes. At 1 h and 7 d after treatment with MEUC + HC, the parameters of the time-intensity curve, which included peak value, regional blood volume, regional blood flow and area under the curve value and which were measured using contrast-enhanced ultrasound, were significantly lower than those of the other treatment groups. The MEUC + HC treatment group exhibited significant growth inhibition relative to the ultrasound cavitation only, HC and MEUC treatment groups. No damage was observed in the surrounding normal tissues. These results support the feasibility of reducing the blood perfusion of rabbit VX2 liver tumors using a new method that combines MEUC and HC.

Mechanical and Biological Effects of Ultrasound: A Review of Present Knowledge

Ultrasound is widely used for medical diagnosis and increasingly for therapeutic purposes. An understanding of the bio-effects of sonography is important for clinicians and scientists working in the field because permanent damage to biological tissues can occur at high levels of exposure. Here the underlying principles of thermal mechanisms and the physical interactions of ultrasound with biological tissues are reviewed. Adverse health effects derived from cellular studies, animal studies and clinical reports are reviewed to provide insight into the in vitro and in vivo bio-effects of ultrasound.

Increasing of Blood-Brain Tumor Barrier Permeability through Transcellular and Paracellular Pathways by Microbubble-Enhanced Diagnostic Ultrasound in a C6 Glioma Model

Most of the anticancer agents cannot be efficiently delivered into the brain tumor because of the existence of blood-brain tumor barrier (BTB). The objective of this study was to explore the effect of microbubble-enhanced diagnostic ultrasound (MEUS) on the BTB permeability and the possible mechanism. Glioma-bearing rats were randomized into three groups as follows: the microbubble-enhanced continued diagnostic ultrasound (MECUS) group; the microbubble-enhanced intermittent diagnostic ultrasound (MEIUS) group and the control group. The gliomas were insonicated through the skull with a diagnostic ultrasound and injected with microbubbles through the tail veins. Evans Blue (EB) and dynamic contrast-enhanced-MRI were used to test changes in the BTB permeability. Confocal laser scanning microscopy was used to observe the deposition of the EB in the tumor tissues. The distribution and expression of junctional adhesion molecule-A (JAM-A) and calcium-activated potassium channels (K_Ca channels) were detected by a Western blot, qRT-PCR, and immunohistochemical staining. In the MEUS groups, the EB extravasation (11.0 ± 2.2 μg/g in MECUS group and 17.9 ± 2.3 μg/g in MEIUS group) exhibited a significant increase compared with the control group (5.3 ± 0.9 μg/g). The MEIUS group had more EB extravasation than the MECUS group. The K_trans value of the dynamic contrast-enhanced-MRI in the MEUS groups was higher than that of the control group and correlated strongly with the EB extravasation in the tumor (R² = 0.97). This showed that the K_trans value might be a non-invasive method to evaluate the BTB permeability in rat glioma after microbubble-enhanced ultrasound treatment.Western blot, qRT-PCR and immunohistochemical staining revealed that MEUS increased the K_Ca channels expression and reduced JAM-A expression in glioma. This change was more obvious in the MEIUS group than in the MECUS group. The results demonstrated that MEUS effectively increased the BTB permeability in glioma. The mechanisms might involve the up-regulation of K_Ca channels expression and affecting the formation of tight junctions in the BTB by a reduction of JAM-A expression. These findings might provide some new guidance for glioma drug therapy.

Disruption of Prostate Microvasculature by Combining Microbubble-Enhanced Ultrasound and Prothrombin

Previous studies have shown a unique method to disrupt tumor vasculature using pulsed, high-pressure amplitude therapeutic ultrasound combined with microbubbles. In this study, we attempted to destroy the prostate vasculature of canine prostates using microbubble-enhanced ultrasound (MEUS) and prothrombin. The prostates of 43 male mongrel canines were surgically exposed. Twenty-two prostates were treated using MEUS (n = 11) or MEUS and prothrombin (PMEUS, n = 11). The other 21 prostates, which were treated using microbubbles (n = 7), ultrasound (n = 7) or prothrombin (n = 7) only, served as the controls. Prothrombin was intravenously infused at 20 IU/kg. MEUS was induced using a therapeutic ultrasound device at a peak negative pressure of 4.47 MPa and a microbubble injection. Contrast-enhanced ultrasound was performed to assess the blood perfusion of the prostates. Then, the prostate tissue was harvested immediately after treatment and at 48 hours later for pathological examination. The contrast-enhanced ultrasound peak value of the prostate decreased significantly from 36.2 ± 5.6 to 27.1 ± 6.3 after treatment in the PMEUS group, but it remained unchanged in the other groups. Histological examination found severe microvascular rupture, hemorrhage and thrombosis in both MEUS- and PMEUS-treated prostates immediately after treatment, while disruption in the PMEUS group was more severe than in the MEUS group. Forty-eight hours after treatment, massive necrosis and infiltration of white blood cells occurred in the PMEUS group. This study demonstrated that PMEUS disrupted the normal microvasculature of canine prostates and induced massive necrosis. PMEUS could potentially become a new noninvasive method used for the treatment of benign prostatic hyperplasia.

Frequency Dependence of Petechial Hemorrhage and Cardiomyocyte Injury Induced during Myocardial Contrast Echocardiography

Myocardial contrast echocardiography (MCE) for perfusion imaging can induce microscale bio-effects during intermittent high-Mechanical Index scans. The dependence of MCE-induced bio-effects on the ultrasonic frequency was examined in rats at 1.6, 2.5 and 3.5 MHz. Premature complexes were counted in the electrocardiogram, petechial hemorrhages with microvascular leakage on the heart surface were observed at the time of exposure, plasma troponin elevation was measured after 4 h and cardiomyocyte injury was detected at 24 h. Increasing response to exposure above an apparent threshold was observed for all endpoints at each frequency. The effects decreased with increasing ultrasonic frequency, and the thresholds increased. Linear regressions for frequency-dependent thresholds indicated coefficients and exponents of 0.6 and 1.07 for petechial hemorrhages, respectively, and 1.02 and 0.8 for cardiomyocyte death, compared with 1.9 and 0.5 (square root) for the guideline limit of the mechanical index. The results clarify the dependence of cardiac bio-effects on frequency, and should allow development of theoretical descriptions of the phenomena and improved safety guidance for MCE.

Mechanical index

Mechanical index (MI) is a measure of acoustic power. It is generally omitted during routine echocardiographic imaging. By adjusting the MI, an echocardiographer can perform various contrast-specific imaging modalities during the same session.

Adenosine receptor agonist NECA increases cerebral extravasation of fluorescein and low molecular weight dextran independent of blood-brain barrier modulation

Conventional methods for therapeutic blood-brain barrier (BBB) disruption facilitate drug delivery but are cumbersome to perform. A previous study demonstrated that adenosine receptor (AR) stimulation by 5′-N-ethylcarboxamide adenosine (NECA) increased the extravasation of intravascular tracers into the brain and proposed that AR agonism may be an effective method for therapeutic BBB disruption. We attempted to confirm the extravasation of tracers into the brain and also investigated tracer extravasation into peripheral organs and tracer retention in the blood. We found that NECA not only increased the extravasation of intravascular fluorescein and low molecular weight dextran into the brain of mice but also increased the concentrations of these tracers in the blood. In fact, the brain:blood ratio-normalized BBB permeability for either tracer is actually decreased by NECA administration. Elevated blood urea nitrogen levels in mice following NECA treatment suggested that renal function impairment was a probable cause of tracer retention. Therefore, NECA has almost no effect on the extravasation of intravascular Evans blue dye (EBD), an albumin-binding tracer with little renal clearance. Rather than inducing BBB disruption, our study demonstrated that NECA increased tracer extravasation into the brain by increasing the concentration gradient of the tracer across the BBB.

Microbubble oscillating in a microvessel filled with viscous fluid: A finite element modeling study

Understanding the dynamics of coated-microbubble oscillating in an elastic microvessel is important for effective and safe applications of ultrasound contrast agents (UCAs) in imaging and therapy. Numerical simulations are performed based on a two-dimensional (2D) asymmetric finite element model to investigate the influences of both acoustic driving parameters (e.g., pressure and frequency) and material properties (vessel size, microbubble shell visco-elastic parameters and fluid viscosity) on the dynamic interactions in the bubble-blood-vessel system. The results show that, the constrained effect of the blood vessel along the radial direction will induce the asymmetric bubble oscillation and vessel deformation, as well as shifting the bubble resonance frequency toward the higher frequency range. For a bubble (1.5-μm radius) activated by 1-MHz ultrasound pulses in a microvessel with a radius varying between 2 and 6.5 μm, up to 26.95 kPa shear stress could be generated on the vessel wall at a driving pressure of 0.2 MPa, which should be high enough to damage the vascular endothelial cells. The asymmetrical oscillation ratio of the bubble can be aggravated from 0.12% to 79.94% with the increasing acoustic driving pressure and blood viscosity, or the decreasing vessel size and microbubble shell visco-elastic properties. The maximum compression velocity on the bubble shell will be enhanced from 0.19 to 22.79 m/s by the increasing vessel size and acoustic pressure, or the decreasing microbubble shell visco-elasticity and blood viscosity. As the results, the peak values of microstreaming-induced shear stress on the vessel wall increases from 0.003 to 26.95 kPa and the deformation degree of vessel is raised from 1.01 to 1.49, due to the enhanced acoustic amplitude, or the decreasing vessel size, blood viscosity and microbubble shell visco-elasticity. Moreover, it also suggests that, among above impact parameters, microbubble resonance frequency and UCA shell elasticity might play more dominant roles in dynamic interactions of the bubble-blood-vessel system.

Improving Nanoparticle Penetration in Tumors by Vascular Disruption with Acoustic Droplet Vaporization

Drug penetration influences the efficacy of tumor therapy. Although the leaky vessels of tumors can improve the penetration of nanodrugs via the enhanced permeability and retention (EPR) effect, various aspects of the tumor microenvironment still restrict this process. This study investigated whether vascular disruption using the acoustic vaporization of micro- or nanoscale droplets (MDs or NDs) induced by ultrasound sonication can overcome the limitations of the EPR effect to allow drug diffusion into extensive regions. The intravital penetration of DiI-labeled liposomes (as a drug model with red fluorescence) was observed using an acousto-optical integrated system comprising a 2-MHz focused ultrasound transducer (transmitting a three-cycle single pulse and a peak negative pressure of 10 MPa) in a window-chamber mouse model. Histology images of the solid tumor were also used to quantify and demonstrate the locations where DiI-labeled liposomes accumulated. In the intravital image analyses, the cumulative diffusion area and fluorescence intensity at 180 min were 0.08±0.01 mm(2) (mean±standard deviation) and 8.5±0.4%, respectively, in the EPR group, 0.33±0.01 mm(2) and 13.1±0.4% in the MD group (p<0.01), and 0.63±0.01 mm(2) and 18.9±1.1% in the ND group (p<0.01). The intratumoral accumulations of DiI-labeled liposomes were 1.7- and 2.3-fold higher in the MD and ND groups, respectively, than in the EPR group. These results demonstrate that vascular disruption induced by acoustic droplet vaporization can improve drug penetration more than utilizing the EPR effect. The NDs showed longer lifetime in vivo than MDs and provided potential abilities of long periods of treatment, intertissue ND vaporization, and intertissue NDs-converted bubble cavitation to improve the drug penetration and transport distance.

The partitioning of nanoparticles to endothelium or interstitium during ultrasound-microbubble-targeted delivery depends on peak-negative pressure

Patients diagnosed with advanced peripheral arterial disease often face poor prognoses and have limited treatment options. For some patient populations, the therapeutic growth of collateral arteries (i.e. arteriogenesis) that bypass regions affected by vascular disease may become a viable treatment option. Our group and others are developing therapeutic approaches centered on the ability of ultrasound-activated microbubbles to permeabilize skeletal muscle capillaries and facilitate the targeted delivery of pro-arteriogenic growth factor-bearing nanoparticles. The development of such approaches would benefit significantly from a better understanding of how nanoparticle diameter and ultrasound peak-negative pressure affect both total nanoparticle delivery and the partitioning of nanoparticles to endothelial or interstitial compartments. Toward this goal, using Balb/C mice that had undergone unilateral femoral artery ligation, we intra-arterially co-injected nanoparticles (50 and 100 nm) with microbubbles, applied 1 MHz ultrasound to the gracilis adductor muscle at peak-negative pressures of 0.7, 0.55, 0.4, and 0.2 MPa, and analyzed nanoparticle delivery and distribution. As expected, total nanoparticle (50 and 100 nm) delivery increased with increasing peak-negative pressure, with 50 nm nanoparticles exhibiting greater tissue coverage than 100 nm nanoparticles. Of particular interest, increasing peak-negative pressure resulted in increased delivery to the interstitium for both nanoparticle sizes, but had little influence on nanoparticle delivery to the endothelium. Thus, we conclude that alterations to peak-negative pressure may be used to adjust the fraction of nanoparticles delivered to the interstitial compartment. This information will be useful when designing ultrasound protocols for delivering pro-arteriogenic nanoparticles to skeletal muscle.

Contrast Ultrasound Imaging Does Not Affect Heat Shock Protein 70 Expression in Cholesterol-Fed Rabbit Aorta

OBJECTIVES

Diagnostic ultrasound imaging is enhanced by the use of circulating microbubble contrast agents (UCAs), but the interactions between ultrasound, UCAs, and vascular tissue are not fully understood. We hypothesized that ultrasound with a UCA would stress the vascular tissue and increase levels of heat shock protein 70 (Hsp70), a cellular stress protein.

METHODS

Male New Zealand White rabbits (n = 32) were fed a standard chow diet (n = 4) or a 1% cholesterol, 10% fat, and 0.11% magnesium diet (n = 28). At 21 days, 24 rabbits on the cholesterol diet were either exposed to ultrasound (3.2-MHz f/3 transducer; 2.1 MPa; mechanical index, 1.17; 10 Hz pulse repetition frequency; 1.6 microseconds pulse duration; 2 minutes exposure duration at 4 sites along the aorta) with the UCA Definity (1× concentration, 1 mL/min; Lantheus Medical Imaging, North Billerica, MA) or sham exposed with a saline vehicle injection (n = 12 per group). Four rabbits on the cholesterol diet and 4 on the chow diet served as cage controls and were not exposed to ultrasound or restrained for blood sample collection. Animals were euthanized 24 hours after exposure, and aortas were quickly isolated and frozen in liquid nitrogen. Aorta lysates from the area of ultrasound exposure were analyzed for Hsp70 level by Western blot. Blood plasma was analyzed for cholesterol, Hsp70, and von Willebrand factor, a marker of endothelial function.

RESULTS

Plasma total cholesterol levels increased to an average of 705 mg/dL. Ultrasound did not affect plasma von Willebrand factor, plasma Hsp70, or aorta Hsp70. Restraint increased Hsp70 (P < .001, analysis of variance).

CONCLUSIONS

Restraint, but not ultrasound with the UCA or cholesterol feeding, significantly increased Hsp70.

Contrast Ultrasound Imaging of the Aorta Does Not Affect Progression of Atherosclerosis or Cardiovascular Biomarkers in ApoE-/- Mice

OBJECTIVES

Ultrasound contrast agents (UCAs) enhance cardiovascular ultrasound imaging. Adverse biological effects have occurred after administration of UCAs, and more research is needed for a comprehensive understanding of the risks involved. We used the ApoE(-/-) mouse model of atherosclerosis to characterize the effects of ultrasound and UCAs on atherosclerosis and plasma biomarkers.

METHODS

Male ApoE(-/-) mice (8 weeks old; n = 24) were intravenously infused with a UCA (2 × 10(10) Definity microbubbles per hour; Lantheus Medical Imaging, North Billerica, MA) and exposed to 2.8-MHz center frequency ultrasound (10 Hz pulse repetition frequency, 1.4 microseconds pulse duration, 2 minutes exposure duration, and 2 sites) at 1 of 3 derated peak rarefactional pressure amplitudes (0, 1.9, or 3.8 MPa), and then consumed either a chow or Western diet for 4 weeks (n = 4 per group). Blood plasma samples were collected before ultrasound exposure and at 2 and 4 weeks after exposure and assayed for total cholesterol and von Willebrand Factor (vWF). A pathologist measured atheroma thickness in formalin-fixed, hematoxylin-eosin-stained transverse aorta sections and scored them for severity of atherosclerosis.

RESULTS

Plasma total cholesterol initially averaged 286 mg/dL in the Western diet group and increased to 861 mg/dL after 4 weeks on the diet (P < .0001). Total cholesterol did not increase significantly in the chow diet group. Plasma vWF increased after 2 weeks on the Western diet (P < .0001). Atheroma thickness was greater in animals consuming the Western diet than in chow-fed animals (P < .05). Ultrasound had no significant effect on plasma total cholesterol, plasma vWF, or atheroma thickness.

CONCLUSIONS

Contrast ultrasound did not increase the severity of atherosclerosis or alter cardiovascular biomarkers in the ApoE(-/-) mouse model.

An overview of the influence of therapeutic ultrasound exposures on the vasculature: high intensity ultrasound and microbubble-mediated bioeffects

It is well established that the interaction of ultrasound with soft tissues can induce a wide range of bioeffects. One of the most important and complex of these interactions in the context of therapeutic ultrasound is with the vasculature. Potential vascular effects range from enhancing microvascular permeability to inducing vascular damage and vessel occlusion. While aspects of these effects are broadly understood, the development of improved approaches to exploit these effects and gain a more detailed mechanistic understanding is ongoing and largely anchored in preclinical research. Here a general overview of this established yet rapidly evolving topic is provided, with a particular emphasis on effects arising from high-intensity focused ultrasound and microbubble-mediated exposures.

Ultrasound technologies for biomaterials fabrication and imaging

Ultrasound is emerging as a powerful tool for developing biomaterials for regenerative medicine. Ultrasound technologies are finding wide-ranging, innovative applications for controlling the fabrication of bioengineered scaffolds, as well as for imaging and quantitatively monitoring the properties of engineered constructs both during fabrication processes and post-implantation. This review provides an overview of the biomedical applications of ultrasound for imaging and therapy, a tutorial of the physical mechanisms through which ultrasound can interact with biomaterials, and examples of how ultrasound technologies are being developed and applied for biomaterials fabrication processes, non-invasive imaging, and quantitative characterization of bioengineered scaffolds in vitro and in vivo.

Influence of repetitive contrast agent injections on functional and molecular ultrasound measurements

Quantitative contrast-enhanced ultrasound plays an important role in tumor characterization and treatment assessment. Besides established functional ultrasound techniques, ultrasound molecular imaging using microbubbles targeted to disease-associated markers is increasingly being applied in pre-clinical studies. Often, repeated injections of non-targeted or targeted microbubbles during the same imaging session are administered. However, the influence of repeated injections on the accuracy of the quantitative data is unclear. Therefore, in tumor-bearing mice, we investigated the influence of multiple injections of non-targeted microbubbles (SonoVue) on time to peak and peak enhancement in liver and tumor tissue and of vascular endothelial growth factor receptor 2 (VEGFR2)-targeted contrast agents (MicroMarker) on specific tumor accumulation. We found significantly decreasing values for time to peak and a tendency for increased values for peak enhancement after multiple injections. Repeated injections of VEGFR2-targeted microbubbles led to significantly increased tumor accumulation, which may result from the exposure of additional binding sites at endothelial surfaces caused by mechanical forces from destroyed microbubbles.

Albumin acts like transforming growth factor β1 in microbubble-based drug delivery

Unlike lipid-shelled microbubbles (MBs), albumin-shelled microbubbles (MBs) have not been reported to be actively targeted to cells without the assistance of antibodies. Recent studies indicate that the albumin molecule is similar to transforming growth factor β (TGF-β) both structurally and functionally. The TGF-β superfamily is important during early tumor outgrowth, with an elevated TGF-β being tumor suppressive; at later stages, this switches to malignant conversion and progression, including breast cancer. TGF-β receptors I and II play crucial roles in both the binding and endocytosis of albumin. However, until now, no specific albumin receptor has been found. On the basis of the above-mentioned information, we hypothesized that non-antibody-conjugated albumin-shelled MBs can be used to deliver drugs to breast cancer cells. We also studied the possible roles of TGF-β1 and radiation force in the behavior of cells and albumin-shelled MBs. The results indicate that albumin-shelled MBs loaded with paclitaxel (PTX) induce breast cancer cell apoptosis without the specific targeting produced by an antibody. Applying either an acoustic radiation force or cavitation alone to cells with PTX-loaded albumin MBs increased the apoptosis rate to 23.2% and 26.3% (p < 0.05), respectively. We also found that albumin-shelled MBs can enter MDA-MB-231 breast cancer cells and remain there for at least 24 h, even in the presence of PTX loading. Confocal micrographs revealed that 70.5% of the breast cancer cells took up albumin-shelled MBs spontaneously after 1 d of incubation. Applying an acoustic radiation force further increased the percentage to 91.9% in our experiments. However, this process could be blocked by TGF-β1, even with subsequent exposure to the radiation force. From these results, we conclude that TGF-β1 receptors are involved in the endocytotic process by which albumin-shelled MBs enter breast cancer cells. The acoustic radiation force increases the contact rate between albumin-shelled MBs and tumor cells. Combining a radiation force and cavitation yields an apoptosis rate of 31.3%. This in vitro study found that non-antibody-conjugated albumin-shelled MBs provide a useful method of drug delivery. Further in vivo studies of the roles of albumin MBs and TGF-β in different stages of cancer are necessary.

Ultrasound-assisted siRNA delivery via arginine-grafted bioreducible polymer and microbubbles targeting VEGF for ovarian cancer treatment

The major drawback hampering siRNA therapies from being more widely accepted in clinical practice is its insufficient accumulation at the target site mainly due to poor cellular uptake and rapid degradation in serum. Therefore, we designed a novel polymeric siRNA carrier system, which would withstand serum-containing environments and tested its performance in vitro as well as in vivo. Delivering siRNA with a system combining an arginine-grafted bioreducible polymer (ABP), microbubbles (MBs), and ultrasound technology (US) we were able to synergize the advantages each delivery system owns individually, and created our innovative siRNA-ABP-MB (SAM) complexes. SAM complexes show significantly higher siRNA uptake and VEGF protein knockdown in vitro with serum-containing media when compared to naked siRNA, and 25k-branched-polyethylenimine (bPEI) representing the current standard in nonviral gene therapy. SAM complexes activated by US are also able to improve siRNA uptake in tumor tissue resulting in decelerating tumor growth in vivo.

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.