Cardiovascular therapeutic uses of targeted ultrasound contrast agents

From concept to early clinical trials: 30 years of microbubble-based ultrasound-mediated drug delivery research

Ultrasound mediated drug delivery, a promising therapeutic modality, has evolved remarkably over the past three decades. Initially designed to enhance contrast in ultrasound imaging, microbubbles have emerged as a main vector for drug delivery, offering targeted therapy with minimized side effects. This review addresses the historical progression of this technology, emphasizing the pivotal role microbubbles play in augmenting drug extravasation and targeted delivery. We explore the complex mechanisms behind this technology, from stable and inertial cavitation to diverse acoustic phenomena, and their applications in medical fields. While the potential of ultrasound mediated drug delivery is undeniable, there are still challenges to overcome. Balancing therapeutic efficacy and safety and establishing standardized procedures are essential areas requiring attention. A multidisciplinary approach, gathering collaborations between researchers, engineers, and clinicians, is important for exploiting the full potential of this technology. In summary, this review highlights the potential of using ultrasound mediated drug delivery in improving patient care across various medical conditions.

Closer to The Heart: Harnessing the Power of Targeted Extracellular Vesicle Therapies

Extracellular vesicles (EVs) have emerged as novel diagnostic and therapeutic approaches for cardiovascular diseases. EVs derived from various origins exhibit distinct effects on the cardiovascular system. However, the application of native EVs is constrained due to their poor stabilities and limited targeting capabilities. Currently, targeted modification of EVs primarily involves genetic engineering, chemical modification (covalent, non-covalent), cell membrane modification, and biomaterial encapsulation. These techniques enhance the stability, biological activity, target-binding capacity, and controlled release of EVs at specific cells and tissues. The diverse origins of cardioprotective EVs are covered, and the applications of cardiac-targeting EV delivery systems in protecting against cardiovascular diseases are discussed. This review summarizes the current stage of research on the potential of EV-based targeted therapies for addressing cardiovascular disorders.

Highlights in ultrasound-targeted microbubble destruction-mediated gene/drug delivery strategy for treatment of malignancies

Ultrasound is one of the safest and most advanced medical imaging technologies that is widely used in clinical practice. Ultrasound microbubbles, traditionally used for contrast-enhanced imaging, are increasingly applied in Ultrasound-targeted Microbubble Destruction (UTMD) technology which enhances tissue and cell membrane permeability through cavitation and sonoporation, to result in a promising therapeutic gene/drug delivery strategy. Here, we review recent developments in the application of UTMD-mediated gene and drug delivery in the diagnosis and treatment of tumors, including the concept, mechanism of action, clinical application status, and advantages of UTMD. Furthermore, the future perspectives that should be paid more attention to in this field are prospected.

Targeted Drug Delivery of Microbubble to Arrest Abdominal Aortic Aneurysm Development: A Simulation Study Towards Optimized Microbubble Design

Abdominal aortic aneurysm (AAA) is an irreversible bulge in the artery with higher prevalence among the elderlies. Increase of the aneurysm diameter by time is a fatal phenomenon which will lead to its sidewall rupture. Invasive surgical treatments are vital in preventing from AAA development. These approaches however have considerable side effects. Targeted drug delivery using microbubbles (MBs) has been recently employed to suppress the AAA growth. The present study is aimed to investigate the surface adhesion of different types of drug-containing MBs to the inner wall of AAA through ligand-receptor binding, using fluid-structure interaction (FSI) simulation by using a patient CT-scan images of the vascular system. The effect of blood flow through AAA on MBs delivery to the intended surface was also addressed. For this purpose, the adherence of four types of MBs with three different diameters to the inner surface wall of AAA was studied in a patient with 40-mm diameter aneurysm. The effects of the blood mechanical properties on the hematocrit (Hct) percentage of patients suffering from anemia and diabetes were studied. Moreover, the impact of variations in the artery inlet velocity on blood flow was addressed. Simulation results demonstrated the dependency of the surface density of MBs (SDM) adhered on the AAA lumen to the size and the type of MBs. It was observed that the amount of SDM due to adhesion on the AAA lumen for one of the commercially-approved MBs (Optison) with a diameter of 4.5 μm was higher than the other MBs. Furthermore, we have shown that the targeted drug delivery to the AAA lumen is more favorable in healthy individuals (45% Hct) compared to the patients with diabetes and anemia. Also, it was found that the targeted drug delivery method using MBs on the patients having AAA with complicated aneurysm shape and negative inlet blood flow velocity can be severely affected.

Ultrasonic Cavitation-Enabled Treatment for Therapy of Hypertrophic Cardiomyopathy: Proof of Principle

Ultrasound myocardial cavitation-enabled treatment was applied to the SS-16^BN rat model of hypertrophic cardiomyopathy for proof of the principle underlying myocardial reduction therapy. A focused ultrasound transducer was targeted using 10-MHz imaging (10 S, GE Vivid 7) to the left ventricular wall of anesthetized rats in a warmed water bath. Pulse bursts of 4-MPa peak rarefactional pressure amplitude were intermittently triggered 1:8 heartbeats during a 10-min infusion of a microbubble suspension. Methylprednisolone was given to reduce initial inflammation, and Losartan was given to reduce fibrosis in the healing tissue. At 28 d post therapy, myocardial cavitation-enabled treatment significantly reduced the targeted wall thickness by 16.2% (p <0.01) relative to shams, with myocardial strain rate and endocardial displacement reduced by 34% and 29%, respectively, which are sufficient for therapeutic treatment. Premature electrocardiogram complexes and plasma troponin measurements were found to identify optimal and suboptimal treatment cohorts and would aid in achieving the desired impact. With clinical translation, myocardial cavitation-enabled treatment should fill the need for a new non-invasive hypertrophic cardiomyopathy therapy option.

Enhanced effect of nuclear localization signal peptide during ultrasound‑targeted microbubble destruction‑mediated gene transfection

Ultrasound‑targeted microbubble destruction (UTMD) can promote the entry of plasmid DNA (pDNA) into the cell cytoplasm, by increasing the permeability of the cell membrane. But the transfection efficiency remains low due to inability of the pDNA to enter the nucleus. Various methods have been explored to improve the UTMD transfection efficiency, but with little success. In cells, the classic nuclear localization signal (cNLS) peptide is an amino acid sequence that signals proteins that are due for nuclear transport. The present study aimed to investigate whether binding of a cNLS peptide to the pDNA may improve the transfection efficiency of UTMD. Four experimental groups were analyzed: Control group (UTMD + pDNA), group with cNLS (UTMD + pDNA + cNLS), group with mutated NLS (mNLS; UTMD + pDNA + mNLS), and group with cNLS and the nuclear import blocker, wheat germ agglutinin (WGA; UTMD + pDNA + cNLS + WGA). The NLS was labeled by fluorescein isothiocyanate, whereas pDNA was labeled with Cy3. Different molar ratios were tested for the NLS and pDNA combination in order to achieve optimal binding of the two molecules. Human umbilical vein endothelial cells were then transfected using the optimum ultrasonic irradiation parameters and NLS/pDNA molar ratio. At 6 h post‑transfection, the rates of Cy3‑labeled pDNA inside the cells and their nuclei were detected by flow cytometry and laser confocal microscopy, and the cellular vs. nuclear uptake of pDNA was calculated. In order to further evaluate the effect of NLS on UTMD‑mediated gene transfection, the transfection efficiency and relative expression levels of mRNA and protein were detected at 48 h post‑transfection. The results demonstrated that the optimal molar ratio of NLS with pDNA was 104:1. The rates of pDNA successful entry into the cell and nucleus were significantly higher in the cNLS group compared with the control group. The transfection efficiency, and relative expression levels of mRNA and protein from the plasmid were significantly increased in the cNLS group compared with the control group. The mNLS group displayed no significant difference compared with the control group, while the WGA group exhibited significant inhibition in most indicators of transfection efficiency compared to the cNLS group. These results suggest that combining a cNLS peptide with pDNA during UTMD‑mediated transfection significantly improved transfection efficiency. Thus, a cNLS peptide may be an important mediator and a new strategy in enhancing the efficiency of UTMD‑mediated gene transfection.

Maturation of Lesions Induced by Myocardial Cavitation-Enabled Therapy

Myocardial contrast echocardiography at enhanced therapeutic parameters may be a novel means of tissue reduction therapy, as for hypertrophic cardiomyopathy. Dahl/SS rats were anesthetized and treated with high-amplitude pulsed ultrasound guided by 10-MHz ultrasound images. Contrast microbubbles were infused via the tail vein during intermittent pulse-burst exposure at 4 MPa. A sham group, a low-impact group (group A, 5 cycle pulses with Gaussian modulation and 1:4 trigger for 5 min) and a high-impact group (group B, 10 cycle pulses with 4-ms square modulation and 1:8 trigger for 10 min) were tested. The higher exposure used in group B yielded more substantial injury than the lower exposure in group A. Treated rats in both groups A and B had significant increases in wall thickness measured by echocardiography the next day, which returned to normal by the end of 6 wk. Six weeks after ultrasound exposure, heart tissue samples exhibited tissue fibrosis in Masson's trichrome stained histology. Maturation of lesions involved fibrosis replacement, preserving structural tissue integrity. This study indicates that myocardial injury noted previously progresses into permanent loss of myocardial tissue that may be sufficient for possible hypertrophic cardiomyopathy therapy. More research is needed to define the treatment parameters required for symptomatic relief for hypertrophic cardiomyopathy.

Use of Theranostic Strategies in Myocardial Cavitation-Enabled Therapy

The accumulation of microlesions induced by ultrasound interaction with contrast microbubbles in the myocardium potentially represents a new method of tissue reduction therapy. Anesthetized rats were treated in a heated water bath with 1.5-MHz focused ultrasound pulses triggered once every four heartbeats from the electrocardiogram during infusion of microbubble contrast agent. Treatment was guided by an 8-MHz B-mode imaging transducer, which also was used to provide estimates of left ventricular echogenicity as a possible predictor of efficacy during treatment. Strategies to reduce prospective clinical treatment durations were tested, including pulse modulation to simulate a theranostic scanning strategy and an increased agent infusion rate over shorter durations. Sources of variability, including ultrasound path variation and venous catheter placement, also were investigated. Electrocardiographic premature complexes were monitored, and Evans-blue stained cardiomyocyte scores were obtained from frozen sections. Left ventricular echogenicity reflected variations in the infused microbubble concentration, but failed to predict efficacy. Comparison of suspensions of varied microbubble size revealed that left ventricular echogenicity was dominated by larger bubbles, whereas efficacy appeared to be dependent on smaller sizes. Simulated scanning was as effective as the normal fixed-beam treatment, and high agent infusion allowed reduced treatment duration. The success of these theranostic strategies may increase the prospects for realistic clinical translation of myocardial cavitation-enabled therapy.

Improving the efficacy of therapeutic angiogenesis by UTMD-mediated Ang-1 gene delivery to the infarcted myocardium

This study aimed to verify the feasibility and efficacy of ultrasound-targeted microbubble destruction (UTMD)-mediated angiopoietin-1 (Ang-1) gene delivery into the infarcted myocardium. Microbubbles carrying anti-intercellular adhesion molecule-1 (ICAM-1) antibody were prepared and identified. The microbubbles carrying anti-ICAM-1 antibody selectively adhered to the interleukin (IL)-1β-stimulated ECV304 cells and to the ischemic vascular endothelium, and the infarct area was examined to evaluate the targeting ability of ICAM-1 microbubbles in vitro and in vivo. The intravenous administration of the Ang-1 gene was carried out by UTMD in rabbits with acute myocardial infarction (AMI). The rabbits were divided into the control (no treatment), non-targeted microbubble destruction (non-TMB) and the ICAM-1 TMB (TMB) group. Gene delivery by direct intramyocardial injection (IMI) served as a reference. Two weeks later, regional myocardial perfusion and cardiac function were evaluated by echocardiography, and Ang-1 gene-mediated angiogenesis was assessed histologically and biochemically. The results revealed that the ICAM-1-targeted microbubbles selectively adhered to the IL-1β-stimulated ECV304 cells in vitro and to the ischemic vascular endothelium in the infarct area of the rabbits with AMI. Two weeks after the delivery of the Ang-1 gene, compared with the non-TMB group, left ventricular function and myocardial perfusion at the infarct area had improved in the TMB and IMI group (p<0.01). Ang-1 gene expression was detectable in the non-TMB, TMB and IMI group, while its expression was higher in the latter 2 groups (all p<0.01). The microvascular density (MVD) of the infarct area in the non-TMB, TMB and IMI group was 65.6±4.4, 96.7±2.1 and 100.7±3.6, respectively (p<0.01). The findings of our study indicate that UTMD-mediated gene delivery may be used to successfully deliver the Ang-1 gene to the infarcted myocardium, thus improving the efficacy of therapeutic angiogenesis. This may provide a novel strategy for future gene therapy.

Timing of high-intensity pulses for myocardial cavitation-enabled therapy

Background

High-intensity ultrasound pulses intermittently triggered from an ECG signal can interact with circulating contrast agent microbubbles to produce myocardial cavitation microlesions of potential therapeutic value. In this study, the timing of therapy pulses relative to the ECG R wave was investigated to identify the optimal time point for tissue reduction therapy with regard to both the physiological cardiac response and microlesion production.

Methods

Rats were anesthetized, prepared for ultrasound, placed in a heated water bath, and treated with 1.5 MHz focused ultrasound pulses targeted to the left ventricular myocardium with an 8 MHz imaging transducer. Initially, the rats were treated for 1 min at each of six different time points in the ECG while monitoring blood pressure responses to assess cardiac functional effects. Next, groups of rats were treated at three different time points: end diastole, end systole, and mid-diastole to assess the impact of timing on microlesion creation. These rats were pretreated with Evans blue injections and were allowed to recover for 1 day until hearts were harvested for scoring of injured cardiomyocytes.

Results

The initial results showed a wide range of cardiac premature complexes in the ECG, which corresponded with blood pressure pulses for ultrasound pulses triggered during diastole. However, the microlesion experiment did not reveal any statistically significant variations in cardiomyocyte injury.

Conclusion

The end of systole (R + RR/3) was identified as an optimal trigger time point which produced identifiable ECG complexes and substantial cardiomyocyte injury but minimal cardiac functional disruption during treatment.

Optimization of ultrasound parameters of myocardial cavitation microlesions for therapeutic application

Intermittent high intensity ultrasound scanning with contrast microbubbles can induce scattered cavitation microlesions in the myocardium, which may be of value for tissue reduction therapy. Anesthetized rats were treated in a heated water bath with 1.5 MHz focused ultrasound pulses, guided by an 8 MHz imaging transducer. The relative efficacy with 2 or 4 MPa pulses, 1:4 or 1:8 trigger intervals and 5 or 10 cycle pulses was explored in six groups. Electrocardiogram premature complexes (PCs) induced by the triggered pulse bursts were counted, and Evans blue stained cardiomyocyte scores (SCSs) were obtained. The increase from 2 to 4 MPa produced significant increases in PCs and SCSs and eliminated an anticipated decline in the rate of PC induction with time, which might hinder therapeutic efficacy. Increased intervals and pulse durations did not yield significant increases in the effects. The results suggest that cavitation microlesion production can be refined and potentially lead to a clinically robust therapeutic method.

New progress in angiogenesis therapy of cardiovascular disease by ultrasound targeted microbubble destruction

Angiogenesis plays a vital part in the pathogenesis and treatment of cardiovascular disease and has become one of the hotspots that are being discussed in the past decades. At present, the promising angiogenesis therapies are gene therapy and stem cell therapy. Besides, a series of studies have shown that the ultrasound targeted microbubble destruction (UTMD) was a novel gene delivery system, due to its advantages of noninvasiveness, low immunogenicity and toxicity, repeatability and temporal and spatial target specificity; UTMD has also been used for angiogenesis therapy of cardiovascular disease. In this review, we mainly discuss the combination of UTMD and gene therapy or stem cell therapy which is applied in angiogenesis therapy in recent researches, and outline the future challenges and good prospects of these approaches.

Encapsulated microbubbles and echogenic liposomes for contrast ultrasound imaging and targeted drug delivery

Micron- to nanometer-sized ultrasound agents, like encapsulated microbubbles and echogenic liposomes, are being developed for diagnostic imaging and ultrasound mediated drug/gene delivery. This review provides an overview of the current state of the art of the mathematical models of the acoustic behavior of ultrasound contrast microbubbles. We also present a review of the in vitro experimental characterization of the acoustic properties of microbubble based contrast agents undertaken in our laboratory. The hierarchical two-pronged approach of modeling contrast agents we developed is demonstrated for a lipid coated (Sonazoid™) and a polymer shelled (poly D-L-lactic acid) contrast microbubbles. The acoustic and drug release properties of the newly developed echogenic liposomes are discussed for their use as simultaneous imaging and drug/gene delivery agents. Although echogenicity is conclusively demonstrated in experiments, its physical mechanisms remain uncertain. Addressing questions raised here will accelerate further development and eventual clinical approval of these novel technologies.

Broadband attenuation measurements of phospholipid-shelled ultrasound contrast agents

The aim of this study was to characterize the frequency-dependent acoustic attenuation of three phospholipid-shelled ultrasound contrast agents (UCAs): Definity, MicroMarker and echogenic liposomes. A broadband through-transmission technique allowed for measurement over 2 to 25 MHz with a single pair of transducers. Viscoelastic shell parameters of the UCAs were estimated using a linearized model developed by N. de Jong, L. Hoff, T. Skotland and N. Bom (Ultrasonics 1992; 30:95-103). The effect of diluent on the attenuation of these UCA suspensions was evaluated by performing attenuation measurements in 0.5% (w/v) bovine serum albumin and whole blood. Changes in attenuation and shell parameters of the UCAs were investigated at room temperature (25°C) and physiologic temperature (37°C). The attenuation of the UCAs diluted in 0.5% (w/v) bovine serum albumin was found to be identical to the attenuation of UCAs in whole blood. For each UCA, attenuation was higher at 37°C than at 25°C, underscoring the importance of conducting characterization studies at physiologic temperature. Echogenic liposomes exhibited a larger increase in attenuation at 37°C versus 25°C than either Definity or MicroMarker.

Mesenchymal stem cell transplantation enhancement in myocardial infarction rat model under ultrasound combined with nitric oxide microbubbles

OBJECTIVE

This study evaluated the effects of ultrasound combined with the homemade nitric oxide (NO) micro-bubble destruction on the in vitro proliferation, apoptosis, and migration of mesenchymal stem cells (MSCs). Furthermore, we studied whether or not irradiation of the NO micro-bubble combined with bone-marrow derived MSC infusion had a better effect on treating myocardial infarction. The possible mechanism of MSC delivery into the infarcted myocardium was also investigated.

METHODS

The murine bone marrow-derived MSCs were isolated, cultured, irradiated, and combined with different concentrations of NO microbubbles. MTT proliferation assay, annexin V-FITC apoptosis detection, migration assay, and RT-PCR were performed 24 h after the irradiation. The NO micro-bubbles was a intravenously injected, followed by the infusion of MSCs, which were labeled by CM-Dil. Myocardium was harvested 48 h later and the distribution of MSCs was observed by laser scanning confocal microscope after frozen sectioning. Echocardiography, histological examination, RT-PCR, and western blotting were performed four weeks after the cell transplantation.

RESULTS

Ultrasound combined with 1:70 NO micro-bubbles had no significant impact on the proliferation or apoptosis of MSCs. Transwell chamber findings demonstrated that MSCs migrated more efficiently in group that underwent ultrasound combined with 1:70 NO micro-bubbles. The Real-time PCR results indicated that the expression of CXCR4 was much higher in the group undergoing ultrasound combined with 1:70 NO micro-bubbles. The normalized fluorescence intensity greatly increased in the group of US+NO micro-bubbles and the cardiac function was also markedly improved. Immunohistochemical staining showed that the capillary density was much greater in the group of US+NO micro-bubbles as compared to that of the other groups. RT-PCR and western blotting also revealed a higher SDF-1 and VEGF expression in the group of US+NO micro-bubbles.

CONCLUSIONS

NO micro-bubbles could be used in the cell transplantation, which efficiently promoted the MSC homing into the infarcted myocardium.

Ultrasound targeted microbubble destruction promotes angiogenesis and heart function by inducing myocardial microenvironment change

The myocardial microenvironment plays a decisive role in the survival, migration and differentiation of stem cells. We studied myocardial micro-environmental changes induced by ultrasound-targeted microbubble destruction (UTMD) and their influence on the transplantation of mesenchymal stem cells (MSCs). Various intensities of ultrasound were applied to the anterior chest in canines with myocardial infarction after intravenous injection of microbubbles. The expression of cytokines and adhesion molecules in the infarcted area of the myocardium was detected after three sessions of UTMD in 1 wk. Real-time quantitative reverse transcription polymerase chain reaction (RTQ-PCR) showed that the expression of vascular cell adhesion molecule-1 (VCAM-1), stromal cell-derived factor-1 (SDF-1) and vascular endothelial growth factor (VEGF) in the 1.5 W/cm(2) and 1 W/cm(2) groups was markedly increased compared with the 0.5 W/cm(2) or the control groups (3.8- to 4.7-fold, p < 0.01), and the expression of interleukin-1β (IL-1β) in the 1.5 W/cm(2) group was increased twofold over the 1.0 W/cm(2) group, whereas the 0.5 W/cm(2) group experienced no significant changes. UTMD at 1.0 W/cm(2) was performed as previously described before mesenchymal stem cell (MSC) transplantation. Myocardial perfusion, angiogenesis and heart function were investigated before and 1 month after MSC transplantation. Coronary angiography and 99mTc-tetrofosmin scintigraphy revealed that myocardial perfusion was markedly improved after UTMD + MSCs treatment (p < 0.05). At echocardiographic analysis, heart function and the wall motion score index were significantly improved by UTMD + MSCs treatment compared with MSCs or UTMD alone and the control. In a canine model of myocardial infarction, therapeutic effects were markedly enhanced by MSC transplantation after the myocardial micro-environmental changes induced by UTMD; therefore, this novel method may be useful as an efficient approach for cellular therapy.

Relationship between cavitation and loss of echogenicity from ultrasound contrast agents

Ultrasound contrast agents (UCAs) have the potential to nucleate cavitation and promote both beneficial and deleterious bioeffects in vivo. Previous studies have elucidated the pulse-duration-dependent pressure amplitude threshold for rapid loss of echogenicity due to UCA fragmentation. Previous studies have demonstrated that UCA fragmentation was concomitant with inertial cavitation. The purpose of this study was to evaluate the relationship between stable and inertial cavitation thresholds and loss of echogenicity of UCAs as a function of pulse duration. Determining the relationship between cavitation thresholds and loss of echogenicity of UCAs would enable monitoring of cavitation based upon the onscreen echogenicity in clinical applications. Two lipid-shelled UCAs, echogenic liposomes (ELIP) and Definity®, were insonified by a clinical ultrasound scanner in duplex spectral Doppler mode at four pulse durations ('sample volumes') in both a static system and a flow system. Cavitation emissions from the UCAs insonified by Doppler pulses were recorded using a passive cavitation detection system and stable and inertial cavitation thresholds ascertained. Loss of echogenicity from ELIP and Definity® was assessed within regions of interest on B-mode images. A numerical model based on UCA rupture predicted the functional form of the loss of echogenicity from ELIP and Definity®. Stable and inertial cavitation thresholds were found to have a weak dependence on pulse duration. Stable cavitation thresholds were lower than inertial cavitation thresholds. The power of cavitation emissions was an exponential function of the loss of echogenicity over the investigated range of acoustic pressures. Both ELIP and Definity® lost more than 80% echogenicity before the onset of stable or inertial cavitation. Once this level of echogenicity loss occurred, both stable and inertial cavitation were detected in the physiologic flow phantom. These results imply that stable and inertial cavitation are necessary in order to trigger complete loss of echogenicity acoustically from UCAs and this finding can be used when planning diagnostic and therapeutic applications.

Ultrasound-targeted microbubble destruction-mediated gene delivery into canine livers

Ultrasound (US) was applied to a targeted canine liver lobe simultaneously with injection of plasmid DNA (pDNA)/microbubble (MB) complexes into a portal vein (PV) segmental branch and occlusion of the inferior vena cava (IVC) to facilitate DNA uptake. By using a 1.1 MHz, 13 mm diameter transducer, a fivefold increase in luciferase activity was obtained at 3.3 MPa peak negative pressure (PNP) in the treated lobe. For more effective treatment of large tissue volumes in canines, a planar unfocused transducer with a large effective beam diameter (52 mm) was specifically constructed. Its apodized dual element configuration greatly reduced the near-field transaxial pressure variations, resulting in a remarkably uniform field of US exposure for the treated tissues. Together with a 15 kW capacity US amplifier, a 692-fold increase of gene expression was achieved at 2.7 MPa. Transaminase and histology analysis indicated minimal tissue damage. These experiments represent an important developmental step toward US-mediated gene delivery in large animals and clinics.

Ultrasound-mediated drug delivery for cardiovascular disease

INTRODUCTION

Ultrasound (US) has been developed as both a valuable diagnostic tool and a potent promoter of beneficial tissue bioeffects for the treatment of cardiovascular disease. These effects can be mediated by mechanical oscillations of circulating microbubbles, or US contrast agents, which may also encapsulate and shield a therapeutic agent in the bloodstream. Oscillating microbubbles can create stresses directly on nearby tissue or induce fluid effects that effect drug penetration into vascular tissue, lyse thrombi or direct drugs to optimal locations for delivery.

AREAS COVERED

The present review summarizes investigations that have provided evidence for US-mediated drug delivery as a potent method to deliver therapeutics to diseased tissue for cardiovascular treatment. In particular, the focus will be on investigations of specific aspects relating to US-mediated drug delivery, such as delivery vehicles, drug transport routes, biochemical mechanisms and molecular targeting strategies.

EXPERT OPINION

These investigations have spurred continued research into alternative therapeutic applications, such as bioactive gas delivery and new US technologies. Successful implementation of US-mediated drug delivery has the potential to change the way many drugs are administered systemically, resulting in more effective and economical therapeutics, and less-invasive treatments.

Production of uniformly sized serum albumin and dextrose microbubbles

Uniformly-sized preparations with average microbubble (MB) diameters from 1 to 7 μm were produced reliably by sonicating decafluorobutane-saturated solutions of serum albumin and dextrose. Detailed protocols for producing and size-separating the MBs are presented, along with the effects that changing each production parameter (serum albumin concentration, sonication power, sonication time, etc.) had on MB size distribution and acoustic stability. These protocols can be used to produce MBs for experimental applications or serve as templates for developing new protocols that yield MBs with physical and acoustic properties better suited to specific applications. Size stability and ultrasonic performance quality control tests were developed to assure that successive MB preparations perform identically and to distinguish the physical and acoustic properties of identically sized MBs produced with different serum albumin-dextrose formulations and sonication parameters. MBs can be stored at 5 °C for protracted periods (2 weeks to one year depending on formulation).

Emerging diagnostic and therapeutic molecular imaging applications in vascular disease

Assessment of vascular disease has evolved from mere indirect and direct measurements of luminal stenosis to sophisticated imaging methods to depict millimeter structural changes of the vasculature. In the near future, the emergence of multimodal molecular imaging strategies may enable robust therapeutic and diagnostic ('theragnostic') approaches to vascular diseases that comprehensively consider structural, functional, biological and genomic characteristics of the disease in individualized risk assessment, early diagnosis and delivery of targeted interventions.This review presents a summary of recent preclinical and clinical developments in molecular imaging and theragnostic applications covering diverse atherosclerosis events such as endothelial activation, macrophage inflammatory activity, plaque neovascularization and arterial thrombosis. The main focus is on molecular targets designed for imaging platforms commonly used in clinical medicine including magnetic resonance, computed tomography and positron emission tomography. A special emphasis is given to vascular ultrasound applications, considering the important role this imaging platform plays in the clinical and research practice of the vascular medicine specialty.

Ultrasound-mediated tumor imaging and nanotherapy using drug loaded, block copolymer stabilized perfluorocarbon nanoemulsions

Perfluorocarbon nanoemulsions can deliver lipophilic therapeutic agents to solid tumors and simultaneously provide for monitoring nanocarrier biodistribution via ultrasonography and/or (19)F MRI. In the first generation of block copolymer stabilized perfluorocarbon nanoemulsions, perfluoropentane (PFP) was used as the droplet forming compound. Although manifesting excellent therapeutic and ultrasound imaging properties, PFP nanoemulsions were unstable at storage, difficult to handle, and underwent hard to control phenomenon of irreversible droplet-to-bubble transition upon injection. To solve the above problems, perfluoro-15-crown-5-ether (PFCE) was used as a core forming compound in the second generation of block copolymer stabilized perfluorocarbon nanoemulsions. PFCE nanodroplets manifest both ultrasound and fluorine ((19)F) MR contrast properties, which allows using multimodal imaging and (19)F MR spectroscopy for monitoring nanodroplet pharmacokinetics and biodistribution. In the present paper, acoustic, imaging, and therapeutic properties of unloaded and paclitaxel (PTX) loaded PFCE nanoemulsions are reported. As manifested by the (19)F MR spectroscopy, PFCE nanodroplets are long circulating, with about 50% of the injected dose remaining in circulation 2h after the systemic injection. Sonication with 1-MHz therapeutic ultrasound triggered reversible droplet-to-bubble transition in PFCE nanoemulsions. Microbubbles formed by acoustic vaporization of nanodroplets underwent stable cavitation. The nanodroplet size (200nm to 350nm depending on a type of the shell and conditions of emulsification) as well as long residence in circulation favored their passive accumulation in tumor tissue that was confirmed by ultrasonography. In the breast and pancreatic cancer animal models, ultrasound-mediated therapy with paclitaxel-loaded PFCE nanoemulsions showed excellent therapeutic properties characterized by tumor regression and suppression of metastasis. Anticipated mechanisms of the observed effects are discussed.

Biosurfactants for microbubble preparation and application

Biosurfactants can be classified by their chemical composition and their origin. This review briefly describes various classes of biosurfactants based on their origin and introduces a few of the most widely used biosurfactants. The current status and future trends in biosurfactant production are discussed, with an emphasis on those derived from plants. Following a brief introduction of the properties of microbubbles, recent progress in the application of microbubble technology to molecular imaging, wastewater treatment, and aerobic fermentation are presented. Several studies on the preparation, characterization and applications of biosurfactant-based microbubbles are reviewed.