Delivery of colloidal particles and red blood cells to tissue through microvessel ruptures created by targeted microbubble destruction with ultrasound

Enhanced gene delivery into skeletal muscles with ultrasound and microbubble techniques

RATIONALE AND OBJECTIVES

This experiment was directed to explore the effects of ultrasound microbubbles on gene structure in vitro and green fluorescent protein (GFP) plasmid transfer into skeletal muscles in vivo. By establishing a rat ischemic hind limb model, the effects of ultrasound-mediated microbubble destruction on vascular endothelial growth factor (VEGF) gene transfection to skeletal muscles were also studied in vivo.

MATERIALS AND METHODS

Ultrasound irradiation was applied on the mixture of microbubbles and GFP plasmid in vitro. Gel electrophoresis was used to detect the effects of ultrasound and microbubbles on GFP plasmid. For in vivo experiments, ultrasound irradiation was applied on the hind limb after directly injecting microbubbles into the hind limb of Wistar rats. Directly after treatment, the skeletal muscles were harvested to observe the microstructure. We also studied the transfer rate of GFP plasmid DNA into the skeletal muscles of rats by applying ultrasound and microbubble technique. Furthermore, a naked VEGF plasmid was applied to study the feasibility of angiogenesis by using rats ischemia models.

RESULTS

Gel electrophoresis of plasmid DNA showed that there was no difference between the groups. By studying the hematoxylin and eosin stained pictures of the skeletal muscles, we found that ultrasound irradiation of skeletal muscle after injection of microbubbles could cause the exudation of the red blood cells, whereas it had no effects on the microstructure of muscle fibers. In vivo experiments showed that an ultrasound microbubble could enhance the transfer of plasmid DNA to the skeletal muscles.

CONCLUSIONS

The ultrasound-mediated microbubble technique provides an effective noninvasive method for gene therapy.

Magnetic resonance imaging of microvascular leakage induced by myocardial contrast echocardiography in rats

The extent and magnitude of microvascular leakage induced by myocardial contrast echocardiography (MCE) were characterized with contrast-aided magnetic resonance imaging (MRI). Evans blue dye, Definity ultrasound contrast agent and Omniscan magnetic resonance contrast agent were injected intravenously in anesthetized rats suspended in a water bath. Diagnostic ultrasound B mode scans with 1:4 end-systolic triggering were performed at 1.5 MHz using a cardiac phased array scanhead to provide a short axis view of the left ventricle. The in situ peak rarefactional pressure amplitude (PRPA) was 2.0 MPa. Microvascular leakage was characterized by extraction of the dye from tissue samples and by imaging the distribution and concentration of Omniscan within the myocardium. The extracted Evans blue was 2.3 times greater than in shams (P<.05) for heart samples perfused with heparin saline, and 1.6 times greater than shams (not significant) for unperfused samples. The MRI showed the penetration of the ultrasound-induced capillary leakage throughout much of the scan plane. The overall gadolinium content measured by MR showed the same trends as the extracted Evans blue, but was more variable. For pooled data (perfused and unperfused), the exposed samples were significantly increased (P<.05) relative to the sham samples for both Evans blue and gadolinium content. Omniscan leakage was also discernable in two of four MRIs from intact rats (after sacrifice). These results demonstrate a potential for MR mapping of capillary leakage induced by contrast-aided ultrasound, with a possible application to spatial characterization of local drug delivery.

Targeted Therapeutic Applications of Acoustically Active Microspheres in the Microcirculation

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

Hyperoxia potentiated sonothrombolysis as a method of acute ischemic stroke therapy

The main goal in the treatment of acute ischemic stroke is prompt arterial recanalization. Thrombolysis with recombinant tissue plasminogen activator (rtPA) is efficient in humans, but shows significant problems including slow and incomplete recanalization and frequent bleeding complications. Limited therapeutic window (the first three hours after onset) is the major limitation resulting in reach too few patients. Therefore, adjunctive therapies extending the reperfusion time window, increasing efficacy and reducing side effects of rtPA are needed. Ultrasound augmentation of rtPA-mediated thrombolysis is suggested to overcome some of these problems, but low-frequency ultrasound (less than 1 MHz) is not safe and high frequency ultrasound (2 MHz) is not much effective. We suggest that normobaric hyperoxia (NBO) may increase the efficacy of ultrasound and rtPA combination in addition to its own efficacy in acute ischemic stroke. Briefly, NBO increases arterial partial oxygen pressure (pO(2)) significantly up to 6-fold. Increase of pO(2) results in an increase of dissolved oxygen in the blood according to Henry's law. Enhanced dissolved oxygen increases gas nuclei formation around and inside of the clot, and decreases the Blake threshold. Under ultrasound field, these small gas nuclei form nano bubbles which fuel inertial cavitation as substrates, and therefore increase the clot fragmentation and lysis. This hypothesis has not been tested so far. The combination of rtPA, therapeutic ultrasound and NBO may be more efficacious than rtPA alone or its combination with ultrasound as acute stroke treatment modality, because each has different and probably additive mechanism of action.

Stimulus-controlled delivery of drugs and genes

Macromolecular and colloidal systems used for the systemic delivery of drugs and genes promise to improve the way we treat and prevent numerous diseases. New generations of drug and gene delivery systems (DGDS) are being designed to enhance further efficiency by using a range of endogenous and external stimuli. This review focuses on three qualitatively distinct ways a stimulus can improve the efficiency of DGDS; namely, by selectively triggering release of the therapeutic agent from the DGDS, by modulating physical properties of DGDS and by favourably altering physiological properties of tissues to enhance DGDS transport. Recent developments in these areas are discussed to illustrate the potential of stimulus-controlled DGDS in the development of new generations of therapeutics.

Resonance frequency of microbubbles in small blood vessels: a numerical study

Microbubbles are currently used as ultrasound contrast agents. Their potential therapeutic applications are also under investigation. This work is designed to provide some insight into the mechanisms of energy absorption and deposition by a preformed gas bubble in the microvasculature to optimize its efficacy. In the linear regime, the most favourable condition for the transfer of energy from an ultrasonic field to a gas bubble occurs when the centre frequency of the ultrasonic field equals the resonance frequency of the bubble. The resonance frequency of gas microbubbles has been investigated up to now mainly in unbounded liquids; however when bubbles are confined in small regions, their resonance frequency is strongly affected by the surrounding boundaries. A parametric study on how the resonance frequency of microbubbles in blood vessels is affected by the bubble radius, vessel radius and the bubble position in the vessel is presented. The resonance frequency decreases below its free value with decreasing vessel radius for vessels smaller than 200-300 microm depending on the bubble size. This model suggests the possibility of using ultrasound in a range of frequencies that are, in general, lower than the ones used now for therapeutic and diagnostic applications of ultrasound (a few MHz). When microbubbles oscillate at their resonance frequency they absorb and therefore emit more energy. This energy may allow specific blood vessels to be targeted for both diagnostic and therapeutic applications of ultrasound.

Sonoporation using microbubble BR14 promotes pDNA/siRNA transduction to murine heart

Naked plasmid DNA (pDNA) and short interfering RNA (siRNA) duplexes were transduced into adult murine heart by means of sonoporation using the third-generation microbubble, BR14. Plasmid DNAs carrying luciferase, beta-galactosidase (beta-gal), or enhanced green fluorescent protein (EGFP) reporter genes were mixed with BR14 and injected percutaneously into the left ventricular (LV) cavity of C57BL/6 mice while exposed to transthoracic ultrasound at 1MHz for 60s. Sonoporation at an output intensity of 2.0W/cm(2) and a 50% pulse duty ratio resulted in the highest luciferase expression in the heart. Histological examinations revealed significant expression of the beta-gal and EGFP reporters in the subendocardial myocardium of LV. Intraventricular co-injection of siRNA-GFP and BR14 with concomitant ultrasonic exposure resulted in substantial reduction in EGFP expression in the coronary artery in EGFP transgenic mice. The present method may be applicable to gain-of-function and loss-of-function genetic engineering in vivo of adult murine heart.

Ultrasound-microbubble-induced neovascularization in mouse skeletal muscle

Ultrasound-microbubble (US-MB) interactions stimulate neovascularization in rat gracilis muscle (GM). We examined microvascular remodeling (MVR) in GMs of C57BL/6 and balb/C mice following ultrasonic MB destruction. A range of MB dosages were administered IV, and exposed GMs received US. Muscles harvested 3, 7 and 14 d posttreatment were stained for vascular markers and assessed for changes in microvessel number, diameter and length. Muscles receiving a low MB dose (LMBD) and US showed significant increases in microvascular density after 3 d, returning to sham levels after one week. A MB dose producing maximum capillary disruptions was then established. This high MB dose (HMBD) facilitated significant MVR in C57BL/6 mice after one week. Balb/C GMs exhibited neovascularization 3 d, but not 7 or 14 d, following US-HMBD treatment. We conclude that HMBD in C57BL/6 mice induces a more sustained neovascularization response compared to balb/C or LMBD-treated C57BL/6 muscles; however, this response is still impermanent.

Myocardial delivery of colloid nanoparticles using ultrasound-targeted microbubble destruction

AIMS

Ultrasound (US)-targeted microbubble destruction (UTMD) is a promising method for delivering genetic material to the heart. The aim of this study was: (i) to test whether colloid nanoparticles can be delivered to the rat myocardium using UTMD; and (ii) to determine whether tissue damage and contractile dysfunction occurs in hearts exposed to UTMD in vivo.

METHODS AND RESULTS

Hearts from anaesthetized rats were exposed to perfluorocarbon-enhanced sonicated dextrose albumin (PESDA) (at two different microbubble concentrations) and US at peak pressures of 0.6, 1.2, or 1.8 MPa for 1, 3, or 9 min. During US, pairs of 30 and 100 nm fluorescent nanospheres were infused intravenously. Left ventricular function was assessed before and immediately after US, as well as at 24 h and 7 days. At the end of the experiments, the number of ruptured microvessels and the amount of nanospheres deposited were quantified. Rats exposed to PESDA alone or US alone showed no functional abnormalities, no capillary ruptures, and no nanosphere delivery. In contrast, rats exposed to both PESDA and US exhibited microvascular ruptures and nanosphere deposits. They also showed transient contractile dysfunction and premature ventricular contractions. All these changes were time-, US peak pressure-, and PESDA concentration-dependent.

CONCLUSION

UTMD allows colloid nanoparticles to be delivered to the rat myocardium through microvessel rupture sites. The efficacy of delivery depends on the peak pressure applied, the duration of US exposure, and contrast concentration. UTMD also causes time- and peak pressure-dependent contractile dysfunction, and tissue alterations that are spontaneously reversible over time.

Dose-dependent disintegration of human endothelial monolayers by contrast echocardiography

Biological effects on endothelium induced by contrast ultrasound (US) may be relevant for transferring drugs into the tissue. An in vitro tissue-mimicking phantom was developed to simulate clinical precordial echocardiography of three modalities (two-dimensional (2DE), pulsed wave (PW), and Power Doppler echocardiography) with gradual increases of acoustic output (mechanical index (MI) 0.0-1.6 and thermal index soft tissue (TIS) 0.0-1.3, respectively; transmit-frequency 1.8 MHz in second harmonic mode (SHI) by 2DE, 1.8 MHz for PW-Doppler, and 3.2 MHz for Power Doppler) as well as contrast agent (CA) concentrations (0.002-4 mg/mL Levovist). Disintegration of the endothelial monolayer was quantitatively analyzed by counting intercellular gaps in light microscopy. No gaps were observed in CA application without sonication. Only few gaps appeared at sonication without CA application in 2DE at MI=1.6 and in PW- and Power Doppler at TIS > or =0.4 and MI > or =0.4. The number of gaps increased significantly with the gradual increase of US output and to a comparably lesser but also significant extent with CA concentrations. Diagnostic contrast echocardiography may induce endothelial disintegrations dependent on US output as well as on CA concentrations. This aspect might be helpful in further in vivo series on local drug delivery.

Incidence of cardiac arrhythmias with therapeutic versus diagnostic ultrasound and intravenous microbubbles

OBJECTIVE

The purpose of this study was to determine the type of arrhythmias induced with therapeutic versus diagnostic transthoracic low-frequency ultrasound (TLFUS) transducers in the presence of intravenous microbubbles.

METHODS

Intravenous perfluorocarbon-exposed sonicated dextrose albumin (PESDA) microbubbles were infused or given as a bolus injection while TLFUS was applied in the standard parasternal and apical views with either a 1-MHz therapeutic ultrasound transducer or high-mechanical-index diagnostic ultrasound (1.7 MHz).

RESULTS

Significantly more ectopy was produced by the therapeutic transducer, especially at higher-intensity settings in the continuous wave mode after bolus injections of PESDA (P < .001 compared with lower intensities and lower continuous infusion rates). Six patients (15%) had either clinical supraventricular tachycardia or nonsustained ventricular tachycardia after intravenous PESDA with therapeutic TLFUS. In comparison, diagnostic high-mechanical-index ultrasound produced only isolated ventricular ectopy and no sustained ventricular arrhythmias.

CONCLUSIONS

Intravenously injected microbubbles and low-frequency therapeutic transducers operating at longer duty cycles and wide beam widths have the capability of eliciting clinically important arrhythmias in patients at high risk for such events.

Histological characterization of microlesions induced by myocardial contrast echocardiography

BACKGROUND

Myocardial contrast echocardiography (MCE) has been shown to have a potential for apparently reversible side effects related to the interaction of ultrasound with the contrast microbubbles, including premature ventricular contractions and microvascular leakage. We investigated the potential for high-dose MCE to induce histologically definable microlesions.

METHODS

Myocardial contrast echocardiography with 1:4 end-systolic triggering was performed at 1.5 MHz and 1.7 mechanical index in a short axis view of the left ventricle in rats. Two high doses (500 microl/kg) of Optison agent were given 5 minutes apart during 10 minutes of echocardiography. For histology, the hearts were perfused and fixed in 10% neutral-buffered formalin. Slides from rats sacrificed 1 day after MCE were scored blind by a pathologist, and, in addition, photomicrographs in the anterior half were evaluated by digital image analysis.

RESULTS

In rats sacrificed 10 minutes after MCE, microvascular leakage and petechiae were highly significant. However, lesions displaying necrotic debris associated with inflammatory infiltrates were not histologically evident at this time. Heart samples 24 hours after MCE showed microlesions with inflammatory infiltrates scattered primarily over the anterior half of the sections. Pathologically, there was inflammatory cell infiltration in areas of 0.6 +/- 0.5% for shams and 3.6 +/- 3.6% for MCE (P < 0.01). Analysis of photographs from the anterior wall found microlesion areas of 0.5 +/- 0.8% for shams and 7.4 +/- 5.0% for MCE (P < 0.02). For rats sacrificed 1 week and 6 weeks after MCE, the microlesions healed to form small fibrous regions interspersed with normal myocytes.

CONCLUSION

High-dose MCE has a potential for causing microscale lesions in the myocardium and the possibility of therapeutic applications.

Microbubble destruction with ultrasound augments neovascularisation by bone marrow cell transplantation in rat hind limb ischaemia

OBJECTIVE

To examine the effects of microbubble destruction with ultrasound (MB) combined with bone marrow derived mononuclear cell transplantation (BMT) into ischaemic tissues in rat hind limb ischaemia.

METHODS AND RESULTS

Unilateral hind limb ischaemia was surgically induced in Lewis rats. At postoperative day 7, rats were randomly divided into three groups: a vehicle treated group, an ultrasound treated group, and an MB treated group. MB treatment increased vascular endothelial growth factor mRNA as assessed by real time polymerase chain reaction (3.0-fold, p < 0.05). At four weeks, the MB group had increases in laser Doppler blood flow index (LDBFI; 1.2-fold, p < 0.05), angiographically detectable collateral vessels (angiographic score: 1.4-fold, p < 0.01), and capillary to muscle fibre ratio (1.4-fold, p < 0.01) in ischaemic limbs compared with the vehicle treated group. No differences were seen between the vehicle and ultrasound treated groups. Secondly, rats were allocated to vehicle treatment, BMT (5 x 10(6) cells/rat), or a combination of MB and BMT (MB+BMT) at seven days after hind limb ischaemia. BMT treatment significantly increased LDBFI, angiographic score, and capillary to muscle fibre ratio compared with vehicle treatment. Interestingly, MB+BMT treatment produced significantly greater LDBFI (1.2-fold, p < 0.01), angiographic score (1.5-fold, p < 0.01), and capillary to muscle fibre ratio (1.5-fold, p < 0.05) than BMT treatment alone.

CONCLUSIONS

MB may be a useful technique to enhance BMT induced neovascularisation.

An efficient gene transfer method mediated by ultrasound and microbubbles into the kidney

BACKGROUND

Safety issues are of paramount importance in clinical human gene therapy. From this point of view, it would be better to develop a novel non-viral efficient gene transfer method. Recently, it was reported that ultrasound exposure could induce cell membrane permeabilization and enhance gene expression.

METHODS

In this study, we examined the potential of ultrasound for gene transfer into the kidney. First, we transfected rat left kidney with luciferase plasmid mixed with microbubbles, Optison, to optimize the conditions (duration of ultrasound and concentration of Optison). Then, 4, 7, 14 and 21 days after gene transfer, luciferase activity was measured. Next, localization of gene expression was assessed by measuring luciferase activity and green fluorescent protein (GFP) expression. Expression of GFP plasmid was examined under a fluorescence microscope at 4 and 14 days after gene transfer. Finally, to examine the side effects of this gene transfer method, biochemical assays for aspartate aminotransferase (AST), alanine aminotransferase (ALT), blood urea nitrogen (BUN) and creatinine (Cre) were performed.

RESULTS

Optison and/or ultrasound significantly enhanced the efficiency of gene transfer and expression in the kidney. Especially, 70-80% of total glomeruli could be transfected. Also, a significant dose-dependent effect of Optison was observed as assessed by luciferase assay (Optison 25%: 12.5 x 10(5) relative light units (RLU)/g tissue; 50%: 31.3 x 10(5) RLU/g tissue; 100%: 57.9 x 10(5) RLU/g tissue). GFP expression could be observed in glomeruli, tubules and interstitial area. Results of blood tests did not change significantly after gene transfer.

CONCLUSIONS

Overall, an ultrasound-mediated gene transfer method with Optison enhanced the efficiency of gene transfer and expression in the rat kidney. This novel non-viral method may be useful for gene therapy for renal disease.

Augmentation of cardiac protein delivery using ultrasound targeted microbubble destruction

Gas-filled microbubbles have become an important tool as ultrasonic contrast agents. We have previously shown that ultrasound-targeted microbubble destruction (UTMD) can direct plasmids to the heart. The aim of this study was to evaluate UTMD for protein delivery. Six different groups of rats received 1 microg of luciferase protein with varying protocols: (1) luciferase-loaded microbubbles and ultrasound; (2) luciferase only; (3) luciferase and ultrasound; (4) luciferase-loaded microbubbles; (5) unloaded microbubbles incubated with luciferase and ultrasound; (6) unloaded microbubbles with ultrasound followed by luciferase. Relative luminescence units per mg protein per s were determined in hearts and control organs. The rats that received ultrasound and luciferase-loaded bubbles showed a six-fold higher cardiac luciferase uptake compared with control groups that did not include bubbles. None of the other groups significantly augmented cardiac luciferase activity. We conclude that ultrasound-targeted microbubble destruction can substantially and noninvasively augment organ-specific delivery of proteins.

Targeting of VEGF-mediated angiogenesis to rat myocardium using ultrasonic destruction of microbubbles

Myocardial angiogenesis mediated by human vascular endothelial growth factor 165 (hVEGF165) cDNA was promoted in rat myocardium using an in vivo-targeted gene delivery system known as ultrasound-targeted microbubble destruction (UTMD). Microbubbles carrying plasmids encoding hVEGF165, or control solutions were infused intravenously during ultrasonic destruction of the microbubbles within the myocardium. Biochemical and histological assessment of gene expression and angiogenesis were performed 5, 10, and 30 days after UTMD. UTMD-treated myocardium contained hVEGF165 protein and mRNA. The myocardium of UTMD-treated animals showed hypercellular foci associated with hVEGF165 expression and endothelial cell markers. Capillary density in UTMD-treated rats increased 18% at 5 days and 33% at 10 days, returning to control levels at 30 days (P<0.0001). Similarly, arteriolar density increased 22% at 5 days, 86% at 10 days, and 31% at 30 days (P<0.0001). Thus, noninvasive delivery of hVEGF165 to rat myocardium by UTMD resulted in significant increases in myocardial capillary and arteriolar density.

Vascular effects induced by combined 1-MHz ultrasound and microbubble contrast agent treatments in vivo

Previous in vivo studies have demonstrated that microvessel hemorrhages and alterations of endothelial permeability can be produced in tissues containing microbubble-based ultrasound contrast agents when those tissues are exposed to MHz-frequency pulsed ultrasound of sufficient pressure amplitudes. The general hypothesis guiding this research was that acoustic (viz., inertial) cavitation, rather than thermal insult, is the dominant mechanism by which such effects arise. We report the results of testing five specific hypotheses in an in vivo rabbit auricular blood vessel model: (1) acoustic cavitation nucleated by microbubble contrast agent can damage the endothelia of veins at relatively low spatial-peak temporal-average intensities, (2) such damage will be proportional to the peak negative pressure amplitude of the insonifying pulses, (3) damage will be confined largely to the intimal surface, with sparing of perivascular tissues, (4) greater damage will occur to the endothelial cells on the side of the vessel distal to the source transducer than on the proximal side and (5) ultrasound/contrast agent-induced endothelial damage can be inherently thrombogenic, or can aid sclerotherapeutic thrombogenesis through the application of otherwise subtherapeutic doses of thrombogenic drugs. Auricular vessels were exposed to 1-MHz focused ultrasound of variable peak pressure amplitude using low duty factor, fixed pulse parameters, with or without infusion of a shelled microbubble contrast agent. Extravasation of Evans blue dye and erythrocytes was assessed at the macroscopic level. Endothelial damage was assessed via scanning electron microscopy (SEM) image analysis. The hypotheses were supported by the data. We discuss potential therapeutic applications of vessel occlusion, e.g., occlusion of at-risk gastric varices.

Ultrasonographic contrast media: has the time come in obstetrics and gynecology?

OBJECTIVE

The aim of this work was to review the technical aspects and clinical applications of contrast media (microbubbles and nanomolecular agents) in obstetric and gynecologic ultrasonographic imaging.

METHODS

With the use of a computerized database (MEDLINE) and several Web-based search engines (Google Scholar and Copernic), relevant articles on ultrasonographic contrast media were reviewed. References cited in these articles and not obtained via the search engines were also reviewed.

RESULTS

Ultrasonographic contrast media constitute a new and expanding technology. They are frequently used, for example, in adult cardiology. Extensive research in laboratory setups, animals, and human subjects has shown their safety and huge potential as an adjunctive tool in clinical practice. They increase signals returning from insonated tissues and are particularly effective as intravascular agents, enhancing color and Doppler signals, for instance. Preliminary results in tumor imaging are encouraging. The ultrasonographic contrast media permit pharmacokinetic perfusion studies, which may be of enormous clinical importance in the study of early cancer development. Targeted imaging and therapies are becoming a reality. Microbubbles have already brought a new dimension to diagnostic ultrasonographic imaging. Many authors have described the clinical value of these agents in liver, prostate, and breast imaging, among others. Newer types of media, the nanomolecules, are now emerging as the latest in imaging enhancers as well as therapeutic agent carriers.

CONCLUSIONS

Although showing potential in imaging of the uterus and fallopian tubes as well as some obstetric applications, the contrast media, in particular the nanomolecules, seem to be most promising in ovarian cancer.

Ultrasound-mediated microbubble destruction enhances VEGF gene delivery to the infarcted myocardium in rats

OBJECTIVE

To investigate the possibility of improving the delivery of vascular endothelial growth factor (VEGF) gene to the myocardium in rats by using ultrasound-mediated microbubble destruction (UMMD).

METHODS

Fifteen male Wistar rats underwent left anterior descending coronary artery ligation in this study. The rats were divided into three groups 3 days after ligation. Ultrasound microbubble vectors (UMVs) attaching to pcD2VEGF121 gene were injected into the tail vein of rats with or without simultaneous echocardiographic microbubble destruction in two groups. The third group was used as control group. VEGF protein expression and formation of new blood vessels were evaluated by immunohistochemical technique during autopsy on 15 rats at 2 weeks after gene transformation. Microvascular density (MVD) in the area with myocardial infarction was counted under a microscope.

RESULTS

VEGF protein expression and MVD in the ischemic myocardium were higher in the rats receiving UMMD than in the group that did not receive UMMD.

CONCLUSION

UMMD is a noninvasive method to effectively improve the delivery of targeted genes to the heart.

Use of ultrasound contrast agents for gene or drug delivery in cardiovascular medicine

The clinical utility of ultrasound contrast agents has been established in diagnostic echocardiography. Recently, the use of such agents has been promoted for transport and delivery of various bioactive substances, thus providing a technique for non-invasive gene therapy and organ-specific drug delivery. In this review, we give a critical update of published studies using ultrasound contrast agents for therapeutic use. We discuss the potential applications and limitations of this technique and suggest future applications in cardiovascular medicine.

Preparation of human hepatocellular carcinoma-targeted liposome microbubbles and their immunological properties

AIM: To prepare the human hepatocellular carcinoma_-(HCC)-targeted liposome microbubbles and to investigate their immunological properties.

METHODS: Human hepatocarcinoma specific monoclonal antibody HAb18 was attached to the surface of home-made liposome microbubbles by static attraction to prepare the targeted liposome microbubbles. The combination of HAb18 with liposome microbubbles was confirmed by the slide agglutination test and immunofluorescent assay. Their immunological activity was measured by ELISA. Rosette formation test, rosette formation blocking test and immun-ofluorescent assay were used to identify the specific binding of targeted liposome microbubbles to SMMC-7721 hepatoma cells, and cytotoxicity assay was used to detect their effect on human hepatocytes.

RESULTS: The targeted liposome microbubbles were positive in the slide agglutination test and immunofluorescent assay. ELISA indicated that the immunological activity of HAb18 on the liposome microbubbles was similar to that of free HAb18. SMMC-7721 cells were surrounded by the targeting liposome microbubbles to form rosettes, while the control SGC-7901 gastric cancer cells were not. Proliferation of SMMC-7721 cells and normal human hepatocytes was not influenced by the targeted liposome microbubbles.

CONCLUSION: The targeted liposome microbubbles with a high specific biological activity have been successfully prepared, which specifically bind to human hepatocarcinoma cells, and are non-cytotoxic to hepatocytes. These results indicate that the liposome microbubbles can be used as a HCC-targeted ultrasound contrast agent that may enhance ultrasound images and thus improve the diagnosis of HCC, especially at the early stage.

The use of microbubbles to target drug delivery

Ultrasound-mediated microbubbles destruction has been proposed as an innovative method for noninvasive delivering of drugs and genes to different tissues. Microbubbles are used to carry a drug or gene until a specific area of interest is reached, and then ultrasound is used to burst the microbubbles, causing site-specific delivery of the bioactive materials. Furthermore, the ability of albumin-coated microbubbles to adhere to vascular regions with glycocalix damage or endothelial dysfunction is another possible mechanism to deliver drugs even in the absence of ultrasound. This review focuses on the characteristics of microbubbles that give them therapeutic properties and some important aspects of ultrasound parameters that are known to influence microbubble-mediated drug delivery. In addition, current studies involving this novel therapeutical application of microbubbles will be discussed.

Drug and gene delivery and enhancement of thrombolysis using ultrasound and microbubbles

This article reviews some important characteristics of microbubbles that give them therapeutic properties. It discusses the use of microbubbles and ultrasound for targeted delivery of adenovirus and nonviral vectors to myocytes and endothelial cells and for the dissolution of thrombus or potentiation of fibrinolytic agents for acutely thrombosed vessels. Potential applications, such as induction of angiogenesis, inhibition of neointimal hyperplasia, and in the setting of acute myocardial infarction and ischemic stroke,are discussed briefly.

Acoustically active liposomes for drug encapsulation and ultrasound-triggered release

Acoustically active liposomes (AAL), previously developed as ultrasound contrast agents, contain small amounts of air. These AAL have potential to carry pharmaceutics and their acoustic activity could enable them to respond to ultrasound stimulation by releasing their contents. Since liposomes can entrap many kinds of drugs, if such entrapment did not affect their echogenicity, then the release of contents could potentially be controlled by ultrasound stimulation. The aim of this research was to investigate the capacity of acoustically active liposomes for hydrophilic molecule encapsulation and to determine their sensitivity to ultrasound-triggered release. Liposomes, composed of phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, and cholesterol, were made acoustically active by hydrating a lipid film, sonication, freezing in the presence of mannitol, lyophilization, and rehydration. As a test molecule, calcein was added in the hydration step. The procedure for generating acoustically active liposomes was compatible with an encapsulation efficiency of 15% or more. The presence of mannitol during freeze-drying was essential not only for generation of acoustic activity but also for efficient encapsulation. Ultrasound-triggered release was achieved by applying 1 MHz ultrasound at 2 W/cm2 for 10 s. The inclusion of 4% diheptanolyphosphatidylcholine (DHPC) increased the sensitivity of liposomes to ultrasound stimulation and resulted in very efficient stimulated release of contents (1/3 released in 10 s, 2/3 released in six such applications). Release of contents was highly correlated with the loss of air induced either by ultrasound or rapid pressure reduction. These encapsulation and triggered release techniques are highly efficient, and hence may be applicable to drug delivery.

Effect of shell type on the in vivo backscatter from polymer-encapsulated microbubbles

This study compared in vivo enhancement from four different polymer-encapsulated ultrasound (US) contrast agents. The agents were produced with a rigid shell composed of the biodegradable block copolymer poly[D,L-lactide-co-glycolide] (PLGA) with the lactic and glycolic acid ratios 50:50, 75:25, 85:15 and 100:0 (i.e., increasingly hydrophobic shell compositions). Approximately the same bubble diameter (1.2 microm) and concentration (0.4 g/mL) were obtained for each agent. In four rabbits, audio Doppler signals were acquired from a 10 MHz cuff transducer placed around a surgically exposed vessel (contrast dose: 0.0125 to 0.15 mL/kg). In vivo dose responses were calculated off-line (in dB). Nine rabbit kidneys were imaged during contrast administration (0.1 mL/kg) in power Doppler and grey-scale pulse inversion harmonic (PIHI) modes using an HDI 5000 scanner (Philips Medical Systems, Bothell, WA). Time-intensity curves were produced and the time-to-peak, peak intensity, slope, area under the curve (AUC) and total duration of enhancement for each agent were compared. All agents produced marked Doppler enhancement with increasing duration from the 50:50 agent (48 +/- 10 s) to the 75:25 agent (166 +/- 46 s), the 85:15 agent (403 +/- 83 s) and with the 100:0 agent (603 +/- 93 s) lasting longest (p < 0.02). No other parameters changed significantly, except the AUC of the 85:15 agent, which was greater than that of the 50:50 agent (190.75 vs. 61.58; p = 0.02). The in vivo dose-response curves were similar for all agents, with mean enhancement up to 20.6 +/- 1.11 dB (p = 0.17). In conclusion, contrast duration increases by an order of magnitude as the lactic acid component in the polymer-encapsulated bubbles increases and the shell, thus, becomes increasingly hydrophobic.

Treatment of acute myocardial infarction by hepatocyte growth factor gene transfer: the first demonstration of myocardial transfer of a "functional" gene using ultrasonic microbubble destruction

OBJECTIVES

We examined whether ultrasonic microbubble destruction (US/MB) enables therapeutic myocardial gene transfer of hepatocyte growth factor (HGF) for acute myocardial infarction (MI).

BACKGROUND

Hepatocyte growth factor gene transfer provides cardioprotective effects in MI, which requires direct intramyocardial injection or special vectors. Although US/MB was used in myocardial gene transfer, its feasibility in transfer of a therapeutic gene with non-viral vector remains unknown.

METHODS

In a rat model of acute MI, naked plasmid (pVaxl) encoding human HGF (1,500 microg) was infused into the left ventricular (LV) chamber during US/MB (HGF-US/MB) or insonation only (HGF-US) or alone (HGF-alone), while control MI rats received empty pVaxl during US/MB (pVaxl-US/MB). For US/MB, transthoracic intermittent insonation with a diagnostic transducer (1.3 MHz) was performed for 2 min at a peak negative pressure of -2,160 kPa during intravenous 20% Optison.

RESULTS

Baseline risk area was comparable among the groups. Immunohistology seven days after treatment revealed significant myocardial expression of HGF protein only in HGF-US/MB. At three weeks, LV weight in HGF-US/MB (0.89 +/- 0.03 g) was significantly lower than those in HGF-alone (1.09 +/- 0.08 g), HGF-US (1.04 +/- 0.07 g), and pVaxl-US/MB (1.04 +/- 0.05 g). Moreover, scar size was significantly smaller (16 +/- 6% vs. 39 +/- 5%, 41 +/- 6%, and 40 +/- 4% of total myocardial circumferential length, respectively), while capillary density (49 +/- 8 vs. 34 +/- 5, 37 +/- 6, and 36 +/- 4 capillaries/high-power field, respectively) and arterial density (37 +/- 7 vs. 15 +/- 9, 18 +/- 4, and 14 +/- 11 arterioles/high-power field, respectively) in the risk area were higher in HGF-US/MB than the other groups.

CONCLUSIONS

Ultrasound-mediated microbubble destruction may enable myocardial HGF gene transfer with systemic administration of naked plasmid, which enhances angiogenesis, limits infarction size, and prevents LV remodeling after MI.

Microvascular remodeling and accelerated hyperemia blood flow restoration in arterially occluded skeletal muscle exposed to ultrasonic microbubble destruction

We showed previously that microbubble destruction with pulsed 1-MHz ultrasound creates a bioeffect that stimulates arteriogenesis and a chronic increase in hyperemia blood flow in normal rat muscle. Here we tested whether ultrasonic microbubble destruction can be used to create a microvascular remodeling response that restores hyperemia blood flow to rat skeletal muscle affected by arterial occlusion. Pulsed ultrasound (1 MHz) was applied to gracilis muscles in which the lateral feed artery was occluded but the medial feed artery was left intact. Control muscles were similarly occluded but did not receive ultrasound, microbubbles, or both. Hyperemia blood flow and number of smooth muscle (SM) alpha-actin-positive vessels, >30-mum arterioles, and capillaries per fiber were determined 7, 14, and 28 days after treatment. In ultrasound-microbubble-treated muscles, lateral region hyperemia blood flow was increased at all time points and restored to normal at day 28. The number of SM alpha-actin vessels per fiber was increased over control in this region at days 7 and 14 but decreased by day 28, when larger-diameter arterioles became more prevalent in the medial region. The number of capillaries per fiber was increased over control only at day 7 in the lateral region and only at days 7 and 14 in the medial region, indicating that the angiogenesis response was transient and likely did not contribute significantly to flow restoration at day 28. We conclude that ultrasonic microbubble destruction can be tailored to stimulate an arteriogenesis response that restores hyperemia blood flow to skeletal muscle in a rat model of arterial occlusion.

Mechanical Bioeffects of Ultrasound

Ultrasound is used widely in medicine as both a diagnostic and therapeutic tool. Through both thermal and nonthermal mechanisms, ultrasound can produce a variety of biological effects in tissues in vitro and in vivo. This chapter provides an overview of the fundamentals of key nonthermal mechanisms for the interaction of ultrasound with biological tissues. Several categories of mechanical bioeffects of ultrasound are then reviewed to provide insight on the range of ultrasound bioeffects in vivo, the relevance of these effects to diagnostic imaging, and the potential application of mechanical bioeffects to the design of new therapeutic applications of ultrasound in medicine.

Forced linear oscillations of microbubbles in blood capillaries

A theoretical investigation of the forced linear oscillations of a gas microbubble in a blood capillary, whose radius is comparable in size to the bubble radius is presented. The natural frequency of oscillation, the thermal and viscous damping coefficients, the amplitude resonance, the energy resonance, as well as the average energy absorbed by the system, bubble plus vessel, have been computed for different kinds of gas microbubbles, containing air, octafluropropane, and perflurobutane as a function of the bubble radius and applied frequency. It has been found that the bubble behavior is isothermal at low frequencies and for small bubbles and between isothermal and adiabatic for larger bubbles and higher frequencies, with the viscous damping dominating over the thermal damping. Furthermore, the width of the energy resonance is strongly dependent on the bubble size and the natural frequency of oscillation is affected by the presence of the vessel wall and position of the bubble in the vessel. Therefore, the presence of the blood vessel affects the way in which the bubble absorbs energy from the ultrasonic field. The motivation of this study lies in the possibility of using gas microbubbles as an aid to therapeutic focused ultrasound treatments.

Targeted ultrasonic contrast agents for molecular imaging and therapy

Targeted contrast agents are expanding the detectability and diagnosis of pathology from a strict anatomic to biochemical basis. Moreover, these new agents, in their various forms, offer the potential for site-specific drug and gene delivery, i.e., the "magic bullet" first postulated by Paul Erhlich 100 years ago. The ability to direct drugs to the molecular signatures of disease, to confirm noninvasively their presence at the site-of-interest, and to quantify the adequacy of local drug concentration at the time of treatment, ie, rational targeted drug delivery, offers exciting new clinical paradigms in the near future.

Effects of ultrasound-targeted microbubble destruction on cardiac gene expression

Ultrasound (US) contrast agents are increasingly used in diagnostic echocardiography. Recent studies have suggested unanticipated effects of microbubble destruction. This study was designed to evaluate gene regulation caused by US-mediated destruction of microbubbles in the heart. During IV infusion of Optison trade mark, triggered US was applied to rat hearts to destroy microbubbles. A control group received only saline and US. RNA was isolated from hearts 24 and 72 h after treatment. Analysis with a deeply representative murine cardiac-specific microarray was used to identify regulated genes. Real-time polymerase chain reaction (PCR) was then applied to verify regulated genes. Microarray analysis revealed only 5 regulated genes in the 24-h group and 4 in the 72-h group. Of these genes, only carbonic anhydrase was significantly upregulated in the 24-h Optison trade mark group (4.3 fold; p = 0.0005) when examined in individual animals by real-time PCR. By this very sensitive technique, the bioeffects of microbubble destruction are negligible.

Test of the Epstein-Plesset model for gas microparticle dissolution in aqueous media: effect of surface tension and gas undersaturation in solution

The gas from a free air bubble will readily dissolve in water, driven by two main factors: the concentration (undersaturation) of dissolved gas in the aqueous solution and the surface tension of the gas bubble-water interface via a Laplace overpressure in the bubble that this creates. This paper experimentally and theoretically investigates each of these effects individually. To study the effects of surface tension, single- and double-chain surfactants were utilized to control and define interfacial conditions of the microbubble in saturated solution. To study the effect of undersaturation, solid distearoylphosphocholine lipid was utilized to coat the gas microparticle with, essentially, a wax monolayer and to achieve zero tension in the surface. The experimental work was performed using a micromanipulation technique that allows one to create and micromanipulate single air microparticles (5-50 microm radius range) in infinite dilution and to accurately record the size of the particle as it loses volume due to the dissolution process. The micropipet technique has shown to be an improvement over other previous attempts to measure dissolution time with a 3.2% average experimental error in gas microparticle dissolution time. An ability to study a gas microparticle in infinite dilution in an isotropic diffusion field is in line with the theoretical assumptions and conditions of the Epstein-Plesset model. The Epstein-Plesset model on average underpredicted the experimentally determined dissolution time by 8.6%, where the effect of surface tension was considered with a range of surface tensions from 72 down to 25 mN/m. The Epstein-Plesset model on average overpredicted the dissolution time by 8.2%, where the effect of undersaturation was considered for a microparticle with zero tension in the surface (zero Laplace pressure) and a range of gas saturations from 70% to 100%. Compared to previous attempts in the literature, this paper more appropriately and accurately tests the Epstein-Plesset model for the dissolution of a single microbubble and an air-filled microparticle in aqueous solution.

Impact of myocardial contrast echocardiography on vascular permeability: comparison of three different contrast agents

Microvascular permeabilization, petechial hemorrhage and premature ventricular contractions (PVCs) have been demonstrated in an in vivo rat model of myocardial contrast echocardiography (MCE). The purpose of this study was to compare these effects for three US Food and Drug Administration (FDA)-approved ultrasound (US) contrast agents (US CA): Optison, Definity and Imagent. Evans blue dye, an indicator of microvascular permeability, and a contrast agent were injected IV in anesthetized rats suspended in a water bath to mimic scanning depths seen in clinical echocardiology. Diagnostic US B-mode scans with 1:4 end-systolic triggering were performed at 1.7 MHz using a cardiac phased-array scanhead to provide a short axis view of the left ventricle. To elicit readily measurable effects for comparisons, relatively high doses of the agent were used (50 to 500 microL kg(-1) for Optison, 25 to 200 microL kg(-1) for Imagent, 10 to 100 microL kg(-1) for Definity). Microvascular leakage was characterized by the area of Evans blue dye coloration on the hearts and by extraction of the dye from tissue samples. The number of petechia were counted on the epicardial surface of excised hearts. PVCs were counted from ECG traces recorded with the MCE images. Neither evidence of capillary leakage nor PVCs were seen in sham animals. Based on volume dose, Definity MCE produced more microvascular leakage, but there was no apparent difference between the three agents' microvascular damage potential, which increased linearly with dose at low doses, when expressed in terms of the number of stabilized microbubbles. Definity MCE resulted in fewer PVCs than the other agents. The effects increased strongly with peak rarefactional pressure amplitude, with apparent thresholds for petechiae at 0.4 MPa and for PVCs at about 1.0 MPa. These results should be of value for minimizing adverse potential in diagnosis and optimizing efficacy in therapeutic applications.

Gene Transfer with Echo-enhanced Contrast Agents: Comparison between Albunex, Optison, and Levovist in Mice—Initial Results

PURPOSE

To determine if commercially available echo-enhanced microbubble contrast agents could be used to increase gene transfection efficiency by means of relatively low-intensity ultrasound-mediated microbubble destruction in skeletal muscles.

MATERIALS AND METHODS

Three types of ultrasound microbubble contrast agents (0.01 mL of albumin [Albunex] and human albumin [Optison] and 10 mg/mL of SH U 508A [Levovist]) were each separately mixed with the reporter plasmid DNA (25 microg) encoding green fluorescent protein (GFP) prior to intramuscular injection into the quadriceps muscle of a mouse thigh bilaterally (seven mice per contrast agent). One of the muscle sites that was injected with plasmid DNA was irradiated with low-intensity therapeutic ultrasound (1 MHz) at an intensity of 2.0 W/cm2 for 2 minutes. Mice were sacrificed 7 days after ultrasound treatment for gene expression assay. The number of GFP-expressing muscle fibers was counted. Statistical significance was determined with a two-tailed Student t test. P <.05 was considered to indicate statistically significant difference.

RESULTS

Muscle tissue exposed to ultrasound with air-filled Albunex or Levovist microbubbles revealed no difference in the number of GFP-expressing muscle fibers compared with the control non-ultrasound-exposed muscle. Albumin-coated octafluoropropane gas-filled Optison microbubbles showed a 10-fold increase in the number of GFP-expressing fibers (P <.05).

CONCLUSION

Low-intensity ultrasound with echo-enhanced Optison induced efficient gene transfer unlike that with Albunex or Levovist.

Impact of myocardial contrast echocardiography on vascular permeability: an in vivo dose response study of delivery mode, pressure amplitude and contrast dose

An in vivo rat model of myocardial contrast echocardiography (MCE) was defined and used to examine the dose range response of microvascular permeabilization and premature ventricular contractions (PVCs) with respect to method of imaging, peak rarefactional pressure amplitude (PRPA) and agent dose. A left ventricular short axis view was obtained on anesthetized rats at 1.7 MHz using a diagnostic ultrasound system with simultaneous ECG recording. Evans blue dye, a marker for microvascular leakage, and a bolus of Optison were injected i.v. Counts of PVCs were made from video tape during the 3 min of MCE. Hearts were excised 5 min after imaging and petechial hemorrhages, Evans blue colored area and Evans blue content were determined. No PVCs or microvascular leakage were seen in rats imaged without contrast agent followed by contrast agent injection without imaging. When PVCs were detected during MCE, petechial hemorrhages and Evans blue leakage were also found in the myocardium. Triggering 1:4 at end-systole produced the most PVCs per frame and most microvascular leakage, followed by end-systole 1:1, continuous scanning and end-diastole triggering 1:1. All effects increased with increasing Optison dosage in the range 25 to 500 microL kg(-1). Ultrasound PRPA was important, with apparent thresholds for PVCs at 1.0 MPa and for petechiae at 0.54 MPa. PVCs, petechial hemorrhages and microvascular leakage in the myocardium occur as a result of MCE in rats.

Kinetics of a US contrast agent in benign and malignant adnexal tumors

PURPOSE

To evaluate the effects of a microbubble contrast agent on the power Doppler ultrasonographic (US) examination of adnexal tumors, with a special focus on the timing of the transit of the microbubble bolus.

MATERIALS AND METHODS

Seventy patients who were suspected of having ovarian tumors were examined preoperatively with contrast material-enhanced US. Images obtained during a 5-minute examination were stored digitally, and the behavior of the contrast agent was evaluated objectively with measurement of the time-dependent image intensity at the region of interest with a computer program. A time-intensity curve in each case was derived and analyzed. The Mann-Whitney U test was used to compare intensity changes and tumor parameters in benign and malignant adnexal tumors.

RESULTS

Both the baseline and maximum power Doppler intensities, as well as the absolute and relative (percent) rise in intensity, were significantly higher (P <.001) in malignant as compared with benign tumors. The arrival time was shorter (17.5 vs 22.5 seconds; P =.005) and the duration of contrast agent effect was longer (190.4 vs 103.6 seconds; P <.001) in malignant tumors than they were in benign tumors. The area under the time-intensity curve was significantly greater in malignant tumors compared with that in benign tumors (P <.001).

CONCLUSION

After microbubble contrast agent injection, malignant and benign adnexal lesions behave differently in degree, onset, and duration of Doppler US enhancement.

Physical principles of microbubble ultrasound contrast agents

Early contrast agents could not achieve left-sided cardiac opacification because these microbubbles could not traverse the pulmonary circulation and remain intact. The specific shell material and gas used determine the properties of individual microbubbles, including fragility, persistence, and resonance. Persistence, perhaps the most important property of a microbubble, has been achieved by second-generation agents through the use of shells or surfactants and by substituting high-density, high molecular weight gas for air. Today's agents readily achieve opacification, not only of the cardiac chambers but also of the myocardium. Refinements in contrast agents and in the instrumentation for their detection are primarily responsible for these improvements.

In vitro transfer of antisense oligodeoxynucleotides into coronary endothelial cells by ultrasound

Since antisense oligodeoxynucleotides (AS-ODNs) have been recognized as a new generation of putative therapeutic agents, we established a delivery technique that could transfect AS-ODNs, which are designed for endothelin type B receptor (ETB), into cultured human coronary endothelial cells (HCECs) by exposure to ultrasound in the presence of echo contrast microbubbles. Ultrasound offers several advantages such as being nontoxic, nonantigenic and providing rapid gene transfer. We standardized the optimal conditions, which consisted of 2 x 10(6) cells suspended in phosphate buffer with 900nM ODN, 50 microl of echo contrast microbubbles (Optison), and ultrasound exposure (1.0 W/cm(2), 10% duty cycle, and 10s duration). The percentage of transfected cells was 25.2+/-2.0% after ultrasound treatment. This is the first demonstration of the use of the ultrasound exposure technique in conjunction with microbubbles in HCECs.

Does contrast echocardiography with Optison induce myocardial necrosis in humans?

Myocardial contrast echocardiography is a promising diagnostic tool for detecting microvascular integrity. Multiple experimental laboratories have shown that diagnostic combined microbubble contrast and ultrasound exposure can cause vessel rupture and myocardial damage in laboratory animals. This study investigated the phenomenon of contrast ultrasonically induced myocardial damage in human beings. Twenty consecutive patients (mean age of 60 +/- 12 years, 14 men) underwent contrast echocardiography with intravenous Optison using a mechanical index of at least 1.4 (Vivid Five System (GE, Vingmed Ultrasound, Horton, Norway). Creatine kinase (CK), creatine kinase-isoenzyme MB (CK-MB); CK-MB mass, myoglobin, and troponin I were measured before and 2, 4, 8, and 24 hours after contrast echocardiography. There was no significant correlation concerning the response to contrast echocardiography for any pair of parameters at any time after the intervention. Only in 2 patients were there higher values for troponin I before and after contrast echocardiography without an increase of myoglobin, CK, or CK-MB mass and activity. These values were therefore interpreted as false positive because of renal failure and severe heart failure. The use of contrast echocardiography is without demonstrated risk of myocardial damage even in patients with different cardiologic entities.

Targeted imaging using ultrasound

The discipline of medical imaging is expanding to include both traditional anatomic modalities and new techniques for the functional assessment of the presence and extent of disease. Current FDA-approved ultrasound contrast agents are micron-sized bubbles with a stabilizing shell. Microbubble contrast agents can be used to estimate microvascular flow rate in a manner similar to dynamic contrast-enhanced magnetic resonance imaging (MRI). The concentration of these agents within the vasculature, reticulo-endothelial, or lymphatic systems produces an effective passive targeting of these areas. Liquid-filled nanoparticles and liposomes have also demonstrated echogenicity and are under evaluation as ultrasound contrast agents. Actively targeted ultrasound relies on specially designed contrast agents to localize the targeted molecular signature or physiologic system. These agents typically remain within the vascular space, and therefore possible targets include molecular markers on thrombus, endothelial cells, and leukocytes. The purpose of this review is to summarize the requirements, challenges, current progress, and future directions of targeted imaging with ultrasound.

[Target and therapeutic microbubbles]

Because ultrasound microbubbles lower the threshold for cavitation by ultrasound energy, they may be used as cavitation nuclei for drug and gene delivery. By tailoring the physical properties of microbubbles and coating materials, drugs and genetic drugs can be incorporated into ultrasound contrast agents. As the microbubbles enter the region of insonation, the microbubbles cavitate, locally releasing the therapeutic agents.

Contrast ultrasound targeted drug and gene delivery: an update on a new therapeutic modality

The effective delivery of intravascular drugs and genes to regions of pathology is dependent on a number of factors that are often difficult to control. Foremost is the site-specific delivery of the payload to the region of pathology and the subsequent transport of the payload across the endothelial barrier. Ultrasound contrast agent microbubbles, which are typically used for image enhancement, are capable of amplifying both the targeting and transport of drugs and genes to tissue. Microbubble targeting can be achieved by the intrinsic binding properties of the microbubble shells or through the attachment of site-specific ligands. Once microbubbles have been targeted to the region of interest, microvessel walls can be permeabilized by destroying the microbubbles with low-frequency, high-power ultrasound. A second level of targeting specificity can be achieved by carefully controlling the ultrasound field and limiting microbubble destruction to the region of interest. When microbubbles are destroyed, drugs or genes that are housed within them or bound to their shells can be released to the blood stream and then delivered to tissue by convective forces through the permeabilized microvessels. An alternative strategy is to increase payload volume by coinjecting drug- or gene-bearing vehicles, such as liposomes, with the microbubbles. In this manifestation, microbubbles are used for creating sites of microvessel permeabilization that facilitate drug or gene vehicle transport. Recent work in the emerging field of contrast ultrasound-based therapeutics, with particular emphasis on the delivery of drugs and genes to tissue through microvascular networks is reviewed.

DNA-loaded albumin microbubbles enhance ultrasound-mediated transfection in vitro

Ultrasound (US), with or without microbubbles, enhances gene transfer in cultured cells, but the effect is modest. We tested if attaching DNA to albumin microbubbles during bubble synthesis could enhance gene expression. Plasmid DNA was loaded on the albumin shell over a range of concentrations (500 to 10,000 microg/mL). Optimal gene expression occurred with loading doses of 4000 microg DNA/mL (4k-loaded bubbles). These microbubbles had diameters of 2.4 +/- 0.7 microm and carried 40 pg DNA/microbubble. DNA-loaded microbubbles had optimal transfection at higher delivered doses of DNA than unloaded bubbles mixed with plasmid. The 4k-loaded bubbles demonstrated a fivefold (p = 0.0003) increase in luciferase reporter expression over that with unloaded bubbles. Similarly, transfection efficiency was better for 4k-loaded microbubbles than unloaded microbubbles (41 +/- 3% vs. 9 +/- 3%, p < 0.0001). DNA loading of microbubbles enhances gene expression and transfection efficiency in US-targeted transfection in vitro and may represent an improved avenue for therapeutic gene delivery in vivo.

Therapeutic applications of microbubbles

Microbubbles, currently used as contrast agents have potential therapeutic applications. Microbubbles, upon insonation of sufficiently intense ultrasound will cavitate. Cavitation with microbubbles can be used to dissolve blood clots or deliver drugs. Targeting ligands and drugs can be incorporated into microbubbles to make highly specific diagnostic and therapeutic agents for activation with ultrasound. In this paper I will review some of these potential applications and experimental results using such agents for thrombolysis, drug and gene delivery.