Targeted microbubbles in the experimental and clinical setting

Ultrasound-targeted microbubble technology facilitates SAHH gene delivery to treat diabetic cardiomyopathy by activating AMPK pathway

Diabetic cardiomyopathy (DCM) is a cardiovascular complication with no known cure. In this study, we evaluated the combination of ultrasound-targeted microbubble destruction (UTMD) and cationic microbubbles (CMBs) for cardiac S-adenosyl homocysteine hydrolase (SAHH) gene transfection as potential DCM therapy. Models of high glucose/fat (HG/HF)-induced H9C2 cells and streptozotocin-induced DCM rats were established. Ultrasound-mediated SAHH delivery using CMBs was a safe and noninvasive approach for spatially localized drug administration both in vitro and in vivo. Notably, SAHH overexpression increased cell viability and antioxidative stress and inhibited apoptosis of HG/HF-induced H9C2 cells. Likewise, UTMD-mediated SAHH delivery attenuated apoptosis, oxidative stress, cardiac fibrosis, and myocardial dysfunction in DCM rats. Activation of the AMPK/FOXO3/SIRT3 signaling pathway may be a key mechanism mediating the role of SAHH in regulating myocardial injury. Thus, UTMD-mediated SAHH transfection may be an important advancement in cardiac gene therapy for restoring ventricular function after DCM.

Dispersing and Sonoporating Biofilm-Associated Bacteria with Sonobactericide

Bacteria encased in a biofilm poses significant challenges to successful treatment, since both the immune system and antibiotics are ineffective. Sonobactericide, which uses ultrasound and microbubbles, is a potential new strategy for increasing antimicrobial effectiveness or directly killing bacteria. Several studies suggest that sonobactericide can lead to bacterial dispersion or sonoporation (i.e., cell membrane permeabilization); however, real-time observations distinguishing individual bacteria during and directly after insonification are missing. Therefore, in this study, we investigated, in real-time and at high-resolution, the effects of ultrasound-induced microbubble oscillation on Staphylococcus aureus biofilms, without or with an antibiotic (oxacillin, 1 μg/mL). Biofilms were exposed to ultrasound (2 MHz, 100-400 kPa, 100-1000 cycles, every second for 30 s) during time-lapse confocal microscopy recordings of 10 min. Bacterial responses were quantified using post hoc image analysis with particle counting. Bacterial dispersion was observed as the dominant effect over sonoporation, resulting from oscillating microbubbles. Increasing pressure and cycles both led to significantly more dispersion, with the highest pressure leading to the most biofilm removal (up to 83.7%). Antibiotic presence led to more variable treatment responses, yet did not significantly impact the therapeutic efficacy of sonobactericide, suggesting synergism is not an immediate effect. These findings elucidate the direct effects induced by sonobactericide to best utilize its potential as a biofilm treatment strategy.

Magnetic Nanostructures: Rational Design and Fabrication Strategies toward Diverse Applications

In recent years, the continuous development of magnetic nanostructures (MNSs) has tremendously promoted both fundamental scientific research and technological applications. Different from the bulk magnet, the systematic engineering on MNSs has brought a great breakthrough in some emerging fields such as the construction of MNSs, the magnetism exploration of multidimensional MNSs, and their potential translational applications. In this review, we give a detailed description of the synthetic strategies of MNSs based on the fundamental features and application potential of MNSs and discuss the recent progress of MNSs in the fields of nanomedicines, advanced nanobiotechnology, catalysis, and electromagnetic wave adsorption (EMWA), aiming to provide guidance for fabrication strategies of MNSs toward diverse applications.

Nano-Biomaterials for Retinal Regeneration

Nanoscience and nanotechnology have revolutionized key areas of environmental sciences, including biological and physical sciences. Nanoscience is useful in interconnecting these sciences to find new hybrid avenues targeted at improving daily life. Pharmaceuticals, regenerative medicine, and stem cell research are among the prominent segments of biological sciences that will be improved by nanostructure innovations. The present review was written to present a comprehensive insight into various emerging nanomaterials, such as nanoparticles, nanowires, hybrid nanostructures, and nanoscaffolds, that have been useful in mice for ocular tissue engineering and regeneration. Furthermore, the current status, future perspectives, and challenges of nanotechnology in tracking cells or nanostructures in the eye and their use in modified regenerative ophthalmology mechanisms have also been proposed and discussed in detail. In the present review, various research findings on the use of nano-biomaterials in retinal regeneration and retinal remediation are presented, and these findings might be useful for future clinical applications.

The Advent of Biomolecular Ultrasound Imaging

Ultrasound imaging is one of the most widely used modalities in clinical practice, revealing human prenatal development but also arterial function in the adult brain. Ultrasound waves travel deep within soft biological tissues and provide information about the motion and mechanical properties of internal organs. A drawback of ultrasound imaging is its limited ability to detect molecular targets due to a lack of cell-type specific acoustic contrast. To date, this limitation has been addressed by targeting synthetic ultrasound contrast agents to molecular targets. This molecular ultrasound imaging approach has proved to be successful but is restricted to the vascular space. Here, we introduce the nascent field of biomolecular ultrasound imaging, a molecular imaging approach that relies on genetically encoded acoustic biomolecules to interface ultrasound waves with cellular processes. We review ultrasound imaging applications bridging wave physics and chemical engineering with potential for deep brain imaging.

Targeting and deep-penetrating delivery strategy for stented coronary artery by magnetic guidance and ultrasound stimulation

Stent placement is an effective treatment for atherosclerosis, but is suffered from in-stent restenosis (ISR) caused by stent mechanical damage. Conventional ISR treatment such as drug-eluting stents (DES) is challenged by the low therapeutic efficacy and severe complications, unchangeable drug dosage for individuals, and limited drug penetration in the vascular tissue. We hypothesize that magnetic targeting and deep-penetrating delivery strategy by magnetic guidance and ultrasound stimulation might be an effective approach for ISR treatment. In the present study, antiproliferative drug (paclitaxel, PTX) loaded poly (lactide-co-glycolide) (PLGA) nanoparticles (PLGA-PTX) were embedded within the shells of the magnetic nanoparticle coated microbubbles (MMB-PLGA-PTX). Once being targeted to the stent under a magnetic field, a low intensity focused ultrasound (LIFU) is applied to activate stable microbubble oscillations, thereby triggering the release of PLGA-PTX. The generated mechanical force and microstreaming facilitate the penetration of released PLGA-PTX into the thickened vascular tissue and enhance their internalization by smooth muscle cells (SMCs), thereby reducing the clearance by blood flow. In an ex vivo experiment, magnetic targeting improved the accumulation amount of MMB-PLGA-PTX by 10 folds, while the LIFU facilitated the penetration of released PLGA-PTX into the tunica media region of the porcine coronary artery, resulting in prolonged retention time at the stented vascular tissue. With the combination effects, this strategy holds great promise in the precision delivery of antiproliferative drugs to the stented vascular tissue for ISR treatment.

Accelerating thrombolysis using a precision and clot-penetrating drug delivery strategy by nanoparticle-shelled microbubbles

Conventional thrombolytic drugs for vascular blockage such as tissue plasminogen activator (tPA) are challenged by the low bioavailability, off-target side effects and limited penetration in thrombi, leading to delayed recanalization. We hypothesize that these challenges can be addressed with the targeted and controlled delivery of thrombolytic drugs or precision drug delivery. A porous and magnetic microbubble platform is developed to formulate tPA. This system can maintain the tPA activity during circulation, be magnetically guided to the thrombi, and then remotely activated for drug release. The ultrasound stimulation also improves the drug penetration into thrombi. In a mouse model of venous thrombosis, the residual thrombus decreased by 67.5% when compared to conventional injection of tPA. The penetration of tPA by ultrasound was up to several hundred micrometers in thrombi. This strategy not only improves the therapeutic efficacy but also accelerates the lytic rate, enabling it to be promising in time-critical thrombolytic therapy.

Scoping Review of Targeted Ultrasound Contrast Agents in the Detection of Angiogenesis

A systematic search was conducted to categorize targeted ultrasound contrast agents (UCAs) used in cancer-related angiogenesis detection. We identified 15 unique contrast agents from 2008 to March 2018. Most primary research articles studied UCAs targeted to vascular endothelial growth factor receptor or α_v β₃ -integrin. Breast cancer and colon cancer are the most common neoplastic processes in which these agents were studied. BR55 (Bracco Research SA, Geneva, Switzerland), a vascular endothelial growth factor receptor-targeting UCA, is the first targeted UCA that has completed phase 0 trials. Our review identifies a gap in the literature regarding the application of targeted UCAs in cancer models beyond breast and colon cancers and identifies other promising UCAs.

Development of novel ST68/PLA-PEG stabilized ultrasound nanobubbles for potential tumor imaging and theranostic

Nanobubbles (NBs) have received wide attention as theranostic agents and been extensively explored in various applications, especially in cancer. The aim of this study was to develop a novel kind of NBs which possess high echogenicity and good stability. This novel ultrasonic nanobubbles (ST68/PLA-PEG NBs) consist of perfluoropropane gas stabilized by Span 60 and Tween 80 (ST68) surfactant and synthesized PLA-PEG-NH₂ block copolymers, and were prepared through the methods of mechanical shaking and low-speed centrifugation. A series of experiments were carried out to evaluate the physicochemical properties, echogenicity and cytotoxicity of this novel NBs. According to the amount ratio of copolymers to surfactant, the NBs were divided into 5 groups (0%, 5%, 10%, 15% and 20%). Group "10%" were the optimum NBs, with a size of 675.6 nm, polydispersity index of 0.39. Moreover, these NBs gave a maximum contrast intensity of 31.0 ± 0.2 dB over baseline and little loss of contrast signal after 10 min. In conclusion, this novel kind of ST68/PLA-PEG NBs which exhibited a high echogenicity and good stability were successfully prepared, and they may offer a potential strategy for drug delivery and tumor-targeted theranostic.

Magnetic Nanomaterials for Advanced Regenerative Medicine: The Promise and Challenges

The recent emergence of numerous nanotechnologies is expected to facilitate the development of regenerative medicine, which is a tissue regeneration technique based on the replacement/repair of diseased tissue or organs to restore the function of lost, damaged, and aging cells in the human body. In particular, the unique magnetic properties and specific dimensions of magnetic nanomaterials make them promising innovative components capable of significantly advancing the field of tissue regeneration. Their potential applications in tissue regeneration are the focus here, beginning with the fundamentals of magnetic nanomaterials. How nanomaterials-both those that are intrinsically magnetic and those that respond to an externally applied magnetic field-can enhance the efficiency of tissue regeneration is also described. Applications including magnetically controlled cargo delivery and release, real-time visualization and tracking of transplanted cells, magnetic regulation of cell proliferation/differentiation, and magnetic activation of targeted ion channels and signal pathways involved in regeneration are highlighted, and comments on the perspectives and challenges in magnetic nanomaterial-based tissue regeneration are given.

Antitumor Effects of Intra-Arterial Delivery of Albumin-Doxorubicin Nanoparticle Conjugated Microbubbles Combined with Ultrasound-Targeted Microbubble Activation on VX2 Rabbit Liver Tumors

Image-guided intra-arterial therapies play a key role in the management of hepatic malignancies. However, limited clinical outcomes suggest the need for new multifunctional drug delivery systems to enhance local drug concentration while reducing systemic adverse reactions. Therefore, we developed the albumin-doxorubicin nanoparticle conjugated microbubble (ADMB) to enhance therapeutic efficiency by sonoporation under exposure to ultrasound. ADMB demonstrated a size distribution of 2.33 ± 1.34 µm and a doxorubicin loading efficiency of 82.7%. The echogenicity of ADMBs was sufficiently generated in the 2–9 MHz frequency range and cavitation depended on the strength of the irradiating ultrasound. In the VX2 rabbit tumor model, ADMB enhanced the therapeutic efficiency under ultrasound exposure, compared to free doxorubicin. The intra-arterial administration of ADMBs sufficiently reduced tumor growth by five times, compared to the control group. Changes in the ADC values and viable tumor fraction supported the fact that the antitumor effect of ADMBs were enhanced by evidence of necrosis ratio (over 70%) and survival tumor cell fraction (20%). Liver toxicity was comparable to that of conventional therapies. In conclusion, this study shows that tumor suppression can be sufficiently maximized by combining ultrasound exposure with intra-arterial ADMB administration.

Ultrasound Contrast Agents and Delivery Systems in Cancer Detection and Therapy

Ultrasound is the second most utilized imaging modality in the world because it is widely accessible, robust, and safe. Aside from its extensive use in diagnostic imaging, ultrasound has also been frequently utilized in therapeutic applications. Particularly, when combined with appropriate delivery systems, ultrasound provides a flexible platform for simultaneous real-time imaging and triggered release, enabling precise, on-demand drug delivery to target sites. This chapter will discuss the basics of ultrasound including its mechanism of action and how it can be used to trigger the release of encapsulated drug either through thermal or cavitation effects. Fundamentals of ultrasound contrast agents, how they enhance ultrasound signals, and how they can be modified to function as carriers for triggered and targeted release of drugs will also be discussed.

Non-viral Gene Delivery

Although viral vectors comprise the majority of gene delivery vectors, their various safety, production, and other practical concerns have left a research gap to be addressed. The non-viral vector space encompasses a growing variety of physical and chemical methods capable of gene delivery into the nuclei of target cells. Major physical methods described in this chapter are microinjection, electroporation, and ballistic injection, magnetofection, sonoporation, optical transfection, and localized hyperthermia. Major chemical methods described in this chapter are lipofection, polyfection, gold complexation, and carbon-based methods. Combination approaches to improve transfection efficiency or reduce immunological response have shown great promise in expanding the scope of non-viral gene delivery.

Model for Microcapsule Drug Release with Ultrasound-Activated Enhancement

Microbubbles and microcapsules of silane-polycaprolactone (SiPCL) have been filled with a fluorescent acridium salt (lucigenin) as a model for a drug-loaded delivery vehicle. The uptake and delivery were studied and compared with similar microbubbles and microcapsules of silica/mercaptosilica (S/M/S). Positively charged lucigenin was encapsulated through an electrostatic mechanism, following a Type I Langmuir isotherm as expected, but with an additional multilayer uptake that leads to a much higher loading for the SiPCL system (∼280 μg/2.4 × 10⁹ microcapsules compared with ∼135 μg/2.4 × 10⁹ microcapsules for S/M/S). Whereas the lucigenin release from the S/M/S bubbles and capsules loaded below the solubility limit is consistent with diffusion from a monolithic structure, the SiPCL structures show distinct release patterns; the Weibull function predicts a general trend for diffusion from normal Euclidean space at short times tending toward diffusion out of fractal spaces with increasing time. As a slow release system, the dissolution time (T_d) increases from 1 to 2 days for the S/M/S and for the low concentration, loaded SiPCl vehicles to ∼10 days for the high loaded microcapsules. However, T_d can be reduced on insonation to 2 days, indicating the potential to gain control over the local enhanced release with ultrasound. This was tested for a docetaxel model and its effect on C4-2B prostate cancer cells, showing improved cell toxicity for concentrations below the normal EC₅₀ in solution.

An Introduction to Contrast-Enhanced Ultrasound for Nephrologists

Contrast-enhanced ultrasound (CEUS) is an emerging technology with no known nephrotoxicity. CEUS has been utilized in cardiac and abdominal imaging for decades in Asia and Europe and has recently received greater attention in the United States with its approval for characterization of indeterminate liver lesions. Emerging data suggest that CEUS has potential as a diagnostic imaging tool among individuals who have contraindications to CT and MRI. Few nephrologists are aware of CEUS and even fewer are aware of its potential applications among individuals with kidney disease. This review introduces CEUS to the nephrology community and provides a basic overview of CEUS technology. Knowledge of the applications, advantages, and disadvantages of CEUS provides the framework for nephrologists to make informed decisions regarding this emerging imaging test in appropriate circumstances. This review focuses on the use of CEUS for the characterization of indeterminate kidney lesions and summarizes the most recent data, some of which specifically includes patients with chronic kidney disease (CKD). The results demonstrate that CEUS has high sensitivity and moderate specificity for detecting malignancy in indeterminate kidney lesions among individuals with and without CKD. In conclusion, CEUS is an emerging imaging technique that may have clinically useful applications for detecting malignant kidney lesions, specifically in patients with CKD. However, most of the current data come from small, single-center studies, and larger, multicenter studies are needed.

Imaging of Brown Adipose Tissue: State of the Art

The rates of diabetes, obesity, and metabolic disease have reached epidemic proportions worldwide. In recent years there has been renewed interest in combating these diseases not only by modifying energy intake and lifestyle factors, but also by inducing endogenous energy expenditure. This approach has largely been stimulated by the recent recognition that brown adipose tissue (BAT)-long known to promote heat production and energy expenditure in infants and hibernating mammals-also exists in adult humans. This landmark finding relied on the use of clinical fluorine 18 fluorodeoxyglucose positron emission tomography/computed tomography, and imaging techniques continue to play a crucial and increasingly central role in understanding BAT physiology and function. Herein, the authors review the origins of BAT imaging, discuss current preclinical and clinical strategies for imaging BAT, and discuss imaging methods that will provide crucial insight into metabolic disease and how it may be treated by modulating BAT activity. (©) RSNA, 2016.

Vibrational Responses of Bound and Nonbound Targeted Lipid-Coated Single Microbubbles

One of the main challenges for ultrasound molecular imaging is acoustically distinguishing nonbound microbubbles from those bound to their molecular target. In this in vitro study, we compared two types of in-house produced targeted lipid-coated microbubbles, either consisting of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, C16:0 (DPPC) or 1,2-distearoyl-sn-glycero-3-phosphocholine, C18:0 (DSPC) as the main lipid, using the Brandaris 128 ultrahigh-speed camera to determine vibrational response differences between bound and nonbound biotinylated microbubbles. In contrast to previous studies that studied vibrational differences upon binding, we used a covalently bound model biomarker (i.e., streptavidin) rather than physisorption, to ensure binding of the biomarker to the membrane. The microbubbles were insonified at frequencies between 1 and 4 MHz at pressures of 50 and 150 kPa. This paper shows lower acoustic stability of bound microbubbles, of which DPPC-based microbubbles deflated most. For DPPC microbubbles with diameters between 2 and [Formula: see text] driven at 50 kPa, resonance frequencies of bound microbubbles were all higher than 1.8 MHz, whereas those of nonbound microbubbles were significantly lower. In addition, the relative radial excursions at resonance were also higher for bound DPPC microbubbles. These differences did not persist when the pressure was increased to 150 kPa, except for the acoustic stability which further decreased. No differences in resonance frequencies were observed between bound and nonbound DSPC microbubbles. Nonlinear responses in terms of emissions at the subharmonic and second harmonic frequencies were similar for bound and nonbound microbubbles at both pressures. In conclusion, we identified differences in vibrational responses of bound DPPC microbubbles with diameters between 2 and [Formula: see text] that distinguish them from nonbound ones.

Next generation ultrasound platforms for theranostics

Microbubbles are a well-established contrast agent which improves diagnostic ultrasound imaging. During the last decade research has focused on expanding their use to include molecular imaging, targeted therapy and imaging modalities other than ultrasound. However, bioadhesion of targeted microbubbles under physiological flow conditions is still difficult to achieve, the main challenge being connected to the poor stability of lipid microbubbles in the body's circulation system. In this article, we investigate the use of polymeric microbubbles based on a poly (vinyl alcohol) shell as an alternative to lipid microbubbles. In particular, we report on the development of microbubble shell modification, using mild reaction conditions, with the aim of designing a multifunctional platform to enable diagnosis and therapy. Superparamagnetic iron oxide nanoparticles and a near infrared fluorescent probe, indocyanine green, are coupled to the bubbles surface in order to support magnetic resonance and fluorescence imaging. Furthermore, anchoring cyclic arginyl-glycyl-aspartic acid (RGD) peptide, and cyclodextrin molecules, allows targeting and drug loading, respectively. Last but not least, shell topography is provided by atomic force microscopy. These applications and features, together with the high echogenicity of poly (vinyl alcohol) microbubbles, may offer a more stable alternative to lipid microbubbles for the development of a multimodal theranostic platform.

Emerging translational research on magnetic nanoparticles for regenerative medicine

Regenerative medicine, which replaces or regenerates human cells, tissues or organs, to restore or establish normal function, is one of the fastest-evolving interdisciplinary fields in healthcare. Over 200 regenerative medicine products, including cell-based therapies, tissue-engineered biomaterials, scaffolds and implantable devices, have been used in clinical development for diseases such as diabetes and inflammatory and immune diseases. To facilitate the translation of regenerative medicine from research to clinic, nanotechnology, especially magnetic nanoparticles have attracted extensive attention due to their unique optical, electrical, and magnetic properties and specific dimensions. In this review paper, we intend to summarize current advances, challenges, and future opportunities of magnetic nanoparticles for regenerative medicine.

On-chip preparation of nanoscale contrast agents towards high-resolution ultrasound imaging

Micron-sized lipid-stabilised bubbles of heavy gas have been utilised as contrast agents for diagnostic ultrasound (US) imaging for many years. Typically bubbles between 1 and 8 μm in diameter are produced to enhance imaging in US by scattering sound waves more efficiently than surrounding tissue. A potential area of interest for Contrast Enhanced Ultrasound (CEUS) are bubbles with diameters <1 μm or 'nanobubbles.' As bubble diameter decreases, ultrasonic resonant frequency increases, which could lead to an improvement in resolution for high-frequency imaging applications when using nanobubbles. In addition, current US contrast agents are limited by their size to the vasculature in vivo. However, molecular-targeted nanobubbles could penetrate into the extra-vascular space of cancerous tissue providing contrast in regions inaccessible to traditional microbubbles. This paper reports a new microfluidic method for the generation of sub-micron sized lipid stabilised particles containing perfluorocarbon (PFC). The nanoparticles are produced in a unique atomisation-like flow regime at high production rates, in excess of 10(6) particles per s and at high concentration, typically >10(11) particles per mL. The average particle diameter appears to be around 100-200 nm. These particles, suspected of being a mix of liquid and gaseous C4F10 due to Laplace pressure, then phase convert into nanometer sized bubbles on the application of US. In vitro ultrasound characterisation from these nanoparticle populations showed strong backscattering compared to aqueous filled liposomes of a similar size. The nanoparticles were stable upon injection and gave excellent contrast enhancement when used for in vivo imaging, compared to microbubbles with an equivalent shell composition.

The effects of low-frequency ultrasound and microbubbles on rabbit hepatic tumors

High-intensity focused ultrasound in combination with microbubbles (MBs) is able to inhibit the growth of VX2 rabbit liver tumors in vivo and prolong the survival time of the animals. In this study, we attempt to investigate the feasibility of VX2 tumor growth inhibition using low-frequency ultrasound (US)-mediated MB disruption. Forty-eight New Zealand rabbits with hepatic VX2 tumors were divided into four groups: control, MBs group, low-frequency US group, and US + MB group. The parameters of the US were 20 kHz, 2 W/cm², 40% duty cycle, 5 min, and once every other day for 2 weeks. At the end of the therapy experiment, 24 rabbits were euthanized, and the cancers were collected and cut into five sections for histological examination, immunohistochemistry, laser confocal microscopy, western blotting assays, and transmission electron microscopy (TEM). Another 24 rabbits were saved, and overall survival time was recorded. The tumor volumes in control, MB, US, and US + MB groups were 6.36 ± 0.58, 5.68 ± 0.42, 5.29 ± 0.26, and 2.04 ± 0.14 cm³, respectively (US + MB versus the other three groups, P < 0.01). Tumor cells manifested coagulation necrosis with internal calcification. Hematoxylin and eosin (H–E) staining revealed interstitial hemorrhage and intravascular thrombosis. The intensity of cyclooxygenase-2 (COX-2), and vascular endothelial growth factor (VEGF) in the US + MB group in the immunohistochemical staining, laser confocal microscopy, and western blotting assays was lower than that of the other three groups (P < 0.05). TEM of the US + MB group revealed vascular endothelial cell wall rupture, widened endothelial cell gaps, interstitial erythrocyte leakage, and microvascular thrombosis, while intact vascular endothelial cells and normal erythrocytes in the tumor vessels were observed in control, MB, and US groups. Rabbits treated with US + MB had a significantly longer overall survival than those in the other three groups (χ2 = 9.328, P = 0.0242). VX2 tumor growth could be inhibited by cavitation induced using low-frequency US and MB.

The influence of blood on targeted microbubbles

The ability to successfully target the delivery of drugs and other therapeutic molecules has been a key goal of biomedical research for many decades. Despite highly promising in vitro results, however, successful translation of targeted drug delivery into clinical use has been extremely limited. This study investigates the significance of the characteristics of whole blood, which are rarely accounted for in vitro assays, as a possible explanation for the poor correlation between in vitro and in vivo experiments. It is shown using two separate model systems employing either biochemical or magnetic targeting that blood causes a substantial reduction in targeting efficiency relative to saline under the same flow conditions. This finding has important implications for the design of targeted drug delivery systems and the assays used in their development.

Targeted ultrasound contrast agents for ultrasound molecular imaging and therapy

Ultrasound contrast agents (UCAs) are used routinely in the clinic to enhance contrast in ultrasonography. More recently, UCAs have been functionalised by conjugating ligands to their surface to target specific biomarkers of a disease or a disease process. These targeted UCAs (tUCAs) are used for a wide range of pre-clinical applications including diagnosis, monitoring of drug treatment, and therapy. In this review, recent achievements with tUCAs in the field of molecular imaging, evaluation of therapy, drug delivery, and therapeutic applications are discussed. We present the different coating materials and aspects that have to be considered when manufacturing tUCAs. Next to tUCA design and the choice of ligands for specific biomarkers, additional techniques are discussed that are applied to improve binding of the tUCAs to their target and to quantify the strength of this bond. As imaging techniques rely on the specific behaviour of tUCAs in an ultrasound field, it is crucial to understand the characteristics of both free and adhered tUCAs. To image and quantify the adhered tUCAs, the state-of-the-art techniques used for ultrasound molecular imaging and quantification are presented. This review concludes with the potential of tUCAs for drug delivery and therapeutic applications.

Contrast-enhanced harmonic endoscopic ultrasound imaging: Basic principles, present situation and future perspectives

Over the last decade, the development of stabilised microbubble contrast agents and improvements in available ultrasonic equipment, such as harmonic imaging, have enabled us to display microbubble enhancements on a greyscale with optimal contrast and spatial resolution. Recent technological advances made contrast harmonic technology available for endoscopic ultrasound (EUS) for the first time in 2008. Thus, the evaluation of microcirculation is now feasible with EUS, prompting the evolution of contrast-enhanced EUS from vascular imaging to images of the perfused tissue. Although the relevant experience is still preliminary, several reports have highlighted contrast-enhanced harmonic EUS (CH-EUS) as a promising noninvasive method to visualise and characterise lesions and to differentiate benign from malignant focal lesions. Even if histology remains the gold standard, the combination of CH-EUS and EUS fine needle aspiration (EUS-FNA) can not only render EUS more accurate but may also assist physicians in making decisions when EUS-FNA is inconclusive, increasing the yield of EUS-FNA by guiding the puncture with simultaneous imaging of the vascularity. The development of CH-EUS has also opened up exciting possibilities in other research areas, including monitoring responses to anticancer chemotherapy or to ethanol-induced pancreatic tissue ablation, anticancer therapies based on ultrasound-triggered drug and gene delivery, and therapeutic adjuvants by contrast ultrasound-induced apoptosis. Contrast harmonic imaging is gaining popularity because of its efficacy, simplicity and non-invasive nature, and many expectations are currently resting on this technique. If its potential is confirmed in the near future, contrast harmonic imaging will become a standard practice in EUS.

Development of therapeutic microbubbles for enhancing ultrasound-mediated gene delivery

Ultrasound (US)-mediated gene delivery has emerged as a promising non-viral method for safe and selective gene delivery. When enhanced by the cavitation of microbubbles (MBs), US exposure can induce sonoporation that transiently increases cell membrane permeability for localized delivery of DNA. The present study explores the effect of generalizable MB customizations on MB facilitation of gene transfer compared to Definity®, a clinically available contrast agent. These modifications are 1) increased MB shell acyl chain length (RN18) for elevated stability and 2) addition of positive charge on MB (RC5K) for greater DNA associability. The MB types were compared in their ability to facilitate transfection of luciferase and GFP reporter plasmid DNA in vitro and in vivo under various conditions of US intensity, MB dosage, and pretreatment MB-DNA incubation. The results indicated that both RN18 and RC5K were more efficient than Definity®, and that the cationic RC5K can induce even greater transgene expression by increasing payload capacity with prior DNA incubation without compromising cell viability. These findings could be applied to enhance MB functions in a wide range of therapeutic US/MB gene and drug delivery approach. With further designs, MB customizations have the potential to advance this technology closer to clinical application.

Increasing the sonoporation efficiency of targeted polydisperse microbubble populations using chirp excitation

The therapeutic use of microbubbles for targeted drug or gene delivery is a highly active area of research. Phospholipid- encapsulated microbubbles typically have a polydisperse size distribution over the 1 to 10 μm range and can be functionalized for molecular targeting and loaded with drugcarrying liposomes. Sonoporation through the generation of shear stress on the cell membrane by microbubble oscillations is one mechanism that results in pore formation in the cell membrane and can improve drug delivery. A microbubble oscillating at its resonant frequency would generate maximum shear stress on a membrane. However, because of the polydisperse nature of phospholipid microbubbles, a range of resonant frequencies would exist in a single population. In this study, the use of linear chirp excitations was compared with equivalent duration and acoustic pressure tone excitations when measuring the sonoporation efficiency of targeted microbubbles on human colorectal cancer cells. A 3 to 7 MHz chirp had the greatest sonoporation efficiency of 26.9 ± 5.6%, compared with 16.4 ± 1.1% for the 1.32 to 3.08 MHz chirp. The equivalent 2.2- and 5-MHz tone excitations have efficiencies of 12.8 ± 2.1% and 15.6 ± 1.1%, respectively, which were all above the efficiency of 4.1 ± 3.1% from the control exposure.

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.