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

Growth and dissolution of an encapsulated contrast microbubble: effects of encapsulation permeability

Gas diffusion from an encapsulated microbubble is modeled using an explicit linear relation for gas permeation through the encapsulation. Both the cases of single gas (air) and multiple gases (perfluorocarbon inside the bubble and air dissolved in surrounding liquid) are considered. An analytical expression for the dissolution time for an encapsulated air bubble is obtained; it showed that for small permeability the dissolution time increases linearly with decreasing permeability. A perfluorocarbon-filled contrast microbubble such as Definity was predicted to experience a transient growth because of air infusion before it dissolves in conformity with previous experimental findings. The growth phase occurs only for bubbles with a critical value of initial mole fraction of perfluorocarbon relative to air. With empirically obtained property values, the dissolution time of a 2.5-micron diameter (same as that of Definity), lipid-coated octafluoropropane bubble, with surface tension 25 mN/m, is predicted to be 42 min in an air-saturated medium. The properties such as shell permeability, surface tension and relative mole fraction of octafluoropropane are varied to investigate their effects on the time scales of bubble growth and dissolution, including their asymptotic scalings where appropriate. The dissolution dynamics scales with permeability, in that when the time is nondimensioanlized with permeability, curves for different permeabilities collapse on a single curve. Investigation of bubbles filled with other gases (nonoctafluoropropane perfluorocarbon and sulfur hexafluoride) indicates longer dissolution time because of lower solubility and lower diffusivity for larger gas molecules. For such micron-size encapsulated bubbles, lifetime of hours is possible only at extremely low surface tension (<1 mN/m) or at extreme oversaturation.

Renal ultrafiltration changes induced by focused US

PURPOSE

To determine if focused ultrasonography (US) combined with a diagnostic microbubble-based US contrast agent can be used to modulate glomerular ultrafiltration and size selectivity.

MATERIALS AND METHODS

The experiments were approved by the animal care committee. The left kidney of 17 healthy rabbits was sonicated by using a 260-kHz focused US transducer in the presence of a microbubble-based US contrast agent. The right kidney served as the control. Three acoustic power levels were applied: 0.4 W (six rabbits), 0.9 W (six rabbits), and 1.7 W (five rabbits). Three rabbits were not treated with focused US and served as control animals. The authors evaluated changes in glomerular size selectivity by measuring the clearance rates of 3000- and 70,000-Da fluorescence-neutral dextrans. The creatinine clearance was calculated for estimation of the glomerular filtration rate. The urinary protein-creatinine ratio was monitored during the experiments. The authors assessed tubular function by evaluating the fractional sodium excretion, tubular reabsorption of phosphate, and gamma-glutamyltransferase-creatinine ratio. Whole-kidney histologic analysis was performed. For each measurement, the values obtained before and after sonication were compared by using the paired t test.

RESULTS

Significant (P < .05) increases in the relative (ratio of treated kidney value/nontreated kidney value) clearance of small- and large-molecule agents and the urine flow rates that resulted from the focused US treatments were observed. Overall, 1.23-, 1.23-, 1.61-, and 1.47-fold enhancement of creatinine clearance, 3000-Da dextran clearance, 70 000-Da dextran clearance, and urine flow rate, respectively, were observed. Focal tubular hemorrhage and transient functional tubular alterations were observed at only the highest (1.7-W) acoustic power level tested.

CONCLUSION

Glomerular ultrafiltration and size selectivity can be temporarily modified with simultaneous application of US and microbubbles. This method could offer new opportunities for treatment of renal disease.

Microbubbles as ultrasound triggered drug carriers

Originally developed as contrast agents for ultrasound imaging and diagnostics, in the past years, microbubbles have made their way back from the patients' bedside to the researcher's laboratory. Microbubbles are currently believed to have great potential as carriers for drugs, small molecules, nucleic acids, and proteins. This review provides insight into this intriguing new frontier from the perspective of the pharmaceutical scientist. First, basic aspects on the application of ultrasound-targeted microbubble destruction for drug delivery will be presented. Next, we will review the recently applied approaches for manufacturing and drug-loading microbubbles. Important quality issues and characterization techniques for advanced microbubble formulation will be discussed. Finally, we will provide an assessment of the prospects for microbubbles in drug and gene therapy, illustrating the problems and requirements for their future development.

Design and evaluation of doxorubicin-containing microbubbles for ultrasound-triggered doxorubicin delivery: cytotoxicity and mechanisms involved

Drug delivery with microbubbles and ultrasound is gaining more and more attention in the drug delivery field due to its noninvasiveness, local applicability, and proven safety in ultrasonic imaging techniques. In this article, we tried to improve the cytotoxicity of doxorubicin (DOX)-containing liposomes by preparing DOX-liposome-containing microbubbles for drug delivery with therapeutic ultrasound. In this way, the DOX release and uptake can be restricted to ultrasound-treated areas. Compared to DOX-liposomes, DOX-loaded microbubbles killed at least two times more melanoma cells after exposure to ultrasound. After treatment of the melanoma cells with DOX-liposome-loaded microbubbles and ultrasound, DOX was mainly present in the nuclei of the cancer cells, whereas it was mainly detected in the cytoplasm of cells treated with DOX-liposomes. Exposure of cells to DOX-liposome-loaded microbubbles and ultrasound caused an almost instantaneous cellular entry of the DOX. At least two mechanisms were identified that explain the fast uptake of DOX and the superior cell killing of DOX-liposome-loaded microbubbles and ultrasound. First, exposure of DOX-liposome-loaded microbubbles to ultrasound results in the release of free DOX that is more cytotoxic than DOX-liposomes. Second, the cellular entry of the released DOX is facilitated due to sonoporation of the cell membranes. The in vitro results shown in this article indicate that DOX-liposome-loaded microbubbles could be a very interesting tool to obtain an efficient ultrasound-controlled DOX delivery in vivo.

Cardiovascular therapeutic uses of targeted ultrasound contrast agents

The therapeutic use of ultrasound contrast agents (UCAs) is an emerging methodology with high potential for enhanced directed therapeutic gene, bioactive gas, drug, and stem cell delivery. Ultrasound-targeted microbubble destruction has already demonstrated feasibility for plasmid DNA delivery. Similarly, therapeutic ultrasound for thrombolysis treatment has been taken into the clinical setting, and the addition of UCAs for therapeutic delivery or enhanced effect through cavitation is a natural progression to this investigation. However, as with any new technique, safety needs to be first demonstrated before translation into clinical practice. This review article will focus on the development of UCAs for cardiac and vascular therapeutics as well as the limitations/concerns for the use of therapeutic ultrasound in clinical medicine in order to lay a foundation for investigators planning to enter this exciting field or for those who want to broaden their understanding.

Effects of encapsulation elasticity on the stability of an encapsulated microbubble

A model for gas transport from an encapsulated microbubble into the surrounding medium is developed and investigated incorporating the effects of encapsulation elasticity. Encapsulation elasticity stabilizes microbubbles against dissolution and explains the long shelf life of microbubble contrast agent. We consider air bubbles as well as bubbles containing perfluorocarbon gas. Analytical conditions between saturation level, surface tension and interfacial dilatational elasticity are determined for attaining non-zero equilibrium radius for these microbubbles. Numerical solution of the equation verifies the stability of the equilibrium radii. In an undersaturated medium all encapsulated bubbles dissolve. In a saturated medium, an encapsulated bubble is found to achieve a long-time stable radius when interfacial dilatational elasticity is larger than equilibrium surface tension. For bubbles with interfacial dilatational elasticity smaller than the equilibrium surface tension, stable bubble of non-zero radius can be achieved only when the saturation level is greater than a critical value. Even if they initially contain a gas other than air, bubbles that reach a stable radius finally become air bubbles. The model is applied to an octafluoropropane filled lipid-coated 2.5 microm bubble, which displayed a transient swelling due to air intake before reaching an equilibrium size. Effects of elasticity, shell permeability, initial mole fraction, initial radius and saturation level are investigated and discussed. Shell permeability and mole fraction do not affect the final equilibrium radius of the microbubble but affect the time scale and the transient dynamics. Similarly, the ratio of equilibrium radius to initial radius remains unaffected by the variation in initial radius.

Microbubble tunneling in gel phantoms

Insonified microbubbles were observed in vessels within a gel with a Young's modulus similar to that of tissue, demonstrating shape instabilities, liquid jets, and the formation of small tunnels. In this study, tunnel formulation occurred in the direction of the propagating ultrasound wave, where radiation pressure directed the contact of the bubble and gel, facilitating the activity of the liquid jets. Combinations of ultrasonic parameters and microbubble concentrations that are relevant for diagnostic imaging and drug delivery and that lead to tunnel formation were applied and the resulting tunnel formation was quantified.

Myocardial injury induced by ultrasound-targeted microbubble destruction: evidence for the contribution of myocardial ischemia

Ultrasound-targeted microbubble destruction (UTMD) can cause left ventricular (LV) dysfunction and tissue alterations in rats when high ultrasound (US) energy and long duration of imaging are used. However, the mechanism underlying these alterations remains unclear. The aim of the present work was to investigate the possible role of ischemia in the pathogenesis of the UTMD-induced LV damages in rats. To address this issue, rat hearts were exposed in situ to perfluorocarbon-enhanced sonicated dextrose albumin (PESDA) and US at peak negative pressures of 0.6, 1.2 or 1.8 MPa for 1, 3, 9, 15 or 30 min. Blood pressure and electrocardiogram were continuously recorded during insonation. LV function was assessed before and immediately after US exposure, as well as at 24 h and 7 d. At each time point, groups of rats were euthanized and their hearts were harvested for morphologic analysis. Rats exposed to either PESDA alone or US alone showed no functional or morphologic abnormalities. By contrast, rats exposed to both PESDA and US exhibited transient LV dysfunction, transient ST-segment elevation, premature ventricular contractions, microvascular ruptures, contraction band necrosis and morphologic tissue damage. These bio-effects were spontaneously and completely reversible by one week, except in the groups exposed to the highest peak negative pressure for the longest duration, in which mild dysfunction persisted and interstitial fibrosis developed. In conclusion, simultaneous exposure of rat hearts to PESDA and US in vivo results in significant bio-effects that are similar to myocardial ischemia, including transient regional LV dysfunction, transient ST-segment elevation and myocyte contraction band necrosis.

Ultrasound triggered image-guided drug delivery

The integration of therapeutic interventions with diagnostic imaging has been recognized as one of the next technological developments that will have a major impact on medical treatments. Important advances in this field are based on a combination of progress in guiding and monitoring ultrasound energy, novel drug classes becoming available, the development of smart delivery vehicles, and more in depth understanding of the mechanisms of the cellular and molecular basis of diseases. Recent research demonstrates that both pressure sensitive and temperature sensitive delivery systems hold promise for local treatment. The use of ultrasound for the delivery of drugs has been demonstrated in particular the field of cardiology and oncology for a variety of therapeutics ranging from small drug molecules to biologics and nucleic acids.

Evaluation and comparison of three novel microbubbles: enhancement of ultrasound-induced cell death and free radicals production

Three novel lipid-shell-type microbubbles (MBs), AS-0100, BG6356A and BG6356B, have been evaluated for their impact on ultrasound (US)-induced cell death and free radicals production. Previously studied and well-characterized US exposure conditions were employed in which human myelomonocytic lymphoma U937 cells were exposed to 1MHz pulsed US beam (0.3W/cm(2), 10% duty factor) for 1min with or without MBs. Three different concentrations of each MB were used. Apoptosis and cell lysis were assessed by examining phosphatidylserine externalization and by counting viable cells, respectively, 6h post-exposure. Free radicals production and scavenging activities were evaluated using electron paramagnetic resonance (EPR)-spin trapping. The results showed that only AS-0100 and BG6356A were able to enhance the US-induced apoptosis, mainly by increasing the secondary necrosis. Apoptosis and cell lysis seemed to depend more on mechanical forces exerted by oscillating MBs while free radicals played a trivial role. BG series MBs exhibited pronounced scavenging activities. Generally, despite the need for further optimization, AS-0100 and BG6356A appear to be promising as adjuncts in cases where US-induced cell death is required.

Ultrasound targeted microbubble destruction for drug and gene delivery

BACKGROUND

Gas-filled microbubbles have been used as ultrasound contrast agents for some decades. More recently, such microbubbles have evolved as experimental tools for organ- and tissue-specific drug and gene delivery. When sonified with ultrasound near their resonance frequency, microbubbles oscillate. With higher ultrasound energies, oscillation amplitudes increase, leading to microbubble destruction. This phenomenon can be used to deliver a substance into a target organ, if microbubbles are co-administered loaded with drugs or gene therapy vectors before i.v. injection.

OBJECTIVE

This review focuses on different experimental applications of microbubbles as tools for drug and gene delivery. Different organ systems and different classes of bioactive substances that have been used in previous studies will be discussed.

METHODS

All the available literature was reviewed to highlight the potential of this non-invasive, organ-specific delivery system.

CONCLUSION

Ultrasound targeted microbubble destruction has been used in various organ systems and in tumours to successfully deliver drugs, proteins, gene therapy vectors and gene silencing constructs. Many proof of principle studies have demonstrated its potential as a non-invasive delivery tool. However, too few large animal studies and studies with therapeutic aims have been performed to see a clinical application of this technique in the near future. Nevertheless, there is great hope that preclinical large animal studies will confirm the successful results already obtained in small animals.

Targeted delivery of nanoparticles bearing fibroblast growth factor-2 by ultrasonic microbubble destruction for therapeutic arteriogenesis

Therapeutic strategies in which recombinant growth factors are injected to stimulate arteriogenesis in patients suffering from occlusive vascular disease stand to benefit from improved targeting, less invasiveness, better growth-factor stability, and more sustained growth-factor release. A microbubble contrast-agent-based system facilitates nanoparticle deposition in tissues that are targeted by 1-MHz ultrasound. This system can then be used to deliver poly(D,L-lactic-co-glycolic acid) nanoparticles containing fibroblast growth factor-2 to mouse adductor muscles in a model of hind-limb arterial insufficiency. Two weeks after treatment, significant increases in both the caliber and total number of collateral arterioles are observed, indicating that the delivery of nanoparticles bearing fibroblast growth factor-2 by ultrasonic microbubble destruction may represent an effective and minimally invasive strategy for the targeted stimulation of therapeutic arteriogenesis.

Arteriolar remodeling following ischemic injury extends from capillary to large arteriole in the microcirculation

OBJECTIVE

Skeletal muscle vasculature undergoes arteriogenesis to restore tissue perfusion and function following loss of blood flow. This process has been shown to occur in large vessels following ischemia, although recent studies suggest this may occur in the microcirculation as well. We tested the hypothesis that ischemia induces microvascular remodeling in the skeletal muscle microcirculation on the scale of capillary to sub-35 mum diameter arterioles.

METHODS

Ligations of a feeding arteriole to the caudal-half of the spinotrapezius muscle were performed on C57BL/6 mice. At 5 days, microvascular remodeling responses were quantified using intravital and whole-mount confocal microscopy. Immunohistochemistry was performed to visualize vessels, incorporated leukocytes, and regions of hypoxia.

RESULTS

Ischemic tissue underwent localized microvascular remodeling characteristic of arteriogenesis, including pronounced vessel tortuosity. In patent microvessels (diameters 15-35 microm), we observed increases in vascular density (38%), branching (90%) and collateral development (36.5%). The formation of new arterioles (diameters 6-35 microm) increased by 24.3%, while chronic hypoxia was absent from all tissues.

CONCLUSIONS

Ischemic injury induces arteriogenesis in skeletal muscle microcirculation. Furthermore, this surgical model enables en face analysis of microcirculatory adaptations with single-cell resolution and can provide investigators with morphometric data on a microscale that is difficult to achieve using other models.

Liposomes in ultrasonic drug and gene delivery

Liposome-based drug and gene delivery systems have potential for significant roles in a variety of therapeutic applications. Recently, liposomes have been used to entrap gas and drugs for ultrasound-controlled drug release and ultrasound-enhanced drug delivery. Echogenic liposomes have been produced by different preparation methods, including lyophilization, pressurization, and biotin-avidin binding. Presently, significant in vivo applications of liposomal ultrasound-based drug and gene delivery are being made in cardiac disease, stroke and tumor therapy. Translation of these vehicles into the clinic will require a better understanding of improved physical properties to avoid rapid clearance, as well as of possible side effects, including those of the ultrasound. The aim of this review is to provide orientation for new researchers in the area of ultrasound-enhanced liposome drug and gene delivery.

Driving delivery vehicles with ultrasound

Therapeutic applications of ultrasound have been considered for over 40 years, with the mild hyperthermia and associated increases in perfusion produced by ultrasound harnessed in many of the earliest treatments. More recently, new mechanisms for ultrasound-based or ultrasound-enhanced therapies have been described, and there is now great momentum and enthusiasm for the clinical translation of these techniques. This dedicated issue of Advanced Drug Delivery Reviews, entitled "Ultrasound for Drug and Gene Delivery," addresses the mechanisms by which ultrasound can enhance local drug and gene delivery and the applications that have been demonstrated at this time. In this commentary, the identified mechanisms, delivery vehicles, applications and current bottlenecks for translation of these techniques are summarized.

Microbubbles in ultrasound-triggered drug and gene delivery

Ultrasound contrast agents, in the form of gas-filled microbubbles, are becoming popular in perfusion monitoring; they are employed as molecular imaging agents. Microbubbles are manufactured from biocompatible materials, they can be injected intravenously, and some are approved for clinical use. Microbubbles can be destroyed by ultrasound irradiation. This destruction phenomenon can be applied to targeted drug delivery and enhancement of drug action. The ultrasonic field can be focused at the target tissues and organs; thus, selectivity of the treatment can be improved, reducing undesirable side effects. Microbubbles enhance ultrasound energy deposition in the tissues and serve as cavitation nuclei, increasing intracellular drug delivery. DNA delivery and successful tissue transfection are observed in the areas of the body where ultrasound is applied after intravascular administration of microbubbles and plasmid DNA. Accelerated blood clot dissolution in the areas of insonation by cooperative action of thrombolytic agents and microbubbles is demonstrated in several clinical trials.

New mechanisms for non-porative ultrasound stimulation of cargo delivery to cell cytosol with targeted perfluorocarbon nanoparticles

The cell membrane constitutes a major barrier for non-endocytotic intracellular delivery of therapeutic molecules from drug delivery vehicles. Existing approaches to breaching the cell membrane include cavitational ultrasound (with microbubbles), electroporation and cell-penetrating peptides. We report the use of diagnostic ultrasound for intracellular delivery of therapeutic bulky cargo with the use of molecularly targeted liquid perfluorocarbon (PFC) nanoparticles. To demonstrate the concept, we used a lipid with a surrogate polar head group, nanogold-DPPE, incorporated into the nanoparticle lipid monolayer. Melanoma cells were incubated with nanogold particles and this was followed by insonication with continuous wave ultrasound (2.25 MHz, 5 min, 0.6 MPa). Cells not exposed to ultrasound showed gold particles partitioned only in the outer bilayer of the cell membrane with no evidence of the intracellular transit of nanogold. However, the cells exposed to ultrasound exhibited numerous nanogold-DPPE components inside the cell that appeared polarized inside intracellular vesicles demonstrating cellular uptake and trafficking. Further, ultrasound-exposed cells manifested no incorporation of calcein or the release of lactate dehydrogenase. These observations are consistent with a mechanism that suggests that ultrasound is capable of stimulating the intracellular delivery of therapeutic molecules via non-porative mechanisms. Therefore, non-cavitational adjunctive ultrasound offers a novel paradigm in intracellular cargo delivery from PFC nanoparticles.

Acoustic attenuation by contrast agent microbubbles in superficial tissue markedly diminishes petechiae bioeffects in deep tissue

OBJECTIVE

To measure how ultrasound attenuation by contrast agent microbubbles (MBs) in superficial tissue affects petechiae creation in underlying deep tissue.

MATERIALS AND METHODS

Studies using Sprague-Dawley rats were approved by the Animal Care and Use Committee. MBs were injected intravenously, and 12 ultrasound pulses (100 sinusoids of 1 MHz ultrasound per pulse) were applied through the skin overlying the hindlimb adductors at intervals of 10 or 60 seconds. In some groups, the skin was resected and immediately returned without re-establishing vascular connections. Muscle petechiae were counted.

RESULTS

Applying ultrasound through unperfused skin after bolus and continuous intravenous MB injection yielded, respectively, 30-fold and 3.5-fold more petechiae than for perfused skin. Surprisingly, petechiae/mm2 decreased with a higher MB dosage [0.12 +/- 0.05 (1 x 10 MBs/g) vs. 0.04 +/- 0.02 (3 x 10 MBs/g)] when ultrasound was applied through perfused skin. In contrast, petechiae/mm2 was approximately proportional to MB dosage for unperfused skin [0.17 +/- 0.10(5) (1 x 10 MBs/g) vs. 0.42 + 0.14 (3 x 10(5) MBs/g)]. In comparison to MB-free controls, MB solutions in this concentration range reduced the peak-negative pressure of ultrasound by 65% to 85%.

CONCLUSIONS

Acoustic attenuation by MBs in skin markedly reduces petechiae creation in deep muscle. Petechiae inhibition is dependent on [MB]2.1 and, therefore, dominates the otherwise proportional relationship between petechiae and [MB] in muscle. The drop of peak-negative pressure below a critical microvessel rupturing threshold is the probable mechanism for petechiae inhibition. These results indicate that high MB doses could, paradoxically, reduce the potential for petechiae creation and may have important bearing on the design of contrast ultrasound-based therapeutics.

Bioeffects considerations for diagnostic ultrasound contrast agents

Diagnostic ultrasound contrast agents have been developed for enhancing the echogenicity of blood and for delineating other structures of the body. Approved agents are suspensions of gas bodies (stabilized microbubbles), which have been designed for persistence in the circulation and strong echo return for imaging. The interaction of ultrasound pulses with these gas bodies is a form of acoustic cavitation, and they also may act as inertial cavitation nuclei. This interaction produces mechanical perturbation and a potential for bioeffects on nearby cells or tissues. In vitro, sonoporation and cell death occur at mechanical index (MI) values less than the inertial cavitation threshold. In vivo, bioeffects reported for MI values greater than 0.4 include microvascular leakage, petechiae, cardiomyocyte death, inflammatory cell infiltration, and premature ventricular contractions and are accompanied by gas body destruction within the capillary bed. Bioeffects for MIs of 1.9 or less have been reported in skeletal muscle, fat, myocardium, kidney, liver, and intestine. Therapeutic applications that rely on these bioeffects include targeted drug delivery to the interstitium and DNA transfer into cells for gene therapy. Bioeffects of contrast-aided diagnostic ultrasound happen on a microscopic scale, and their importance in the clinical setting remains uncertain.

Fetal ultrasound: mechanical effects

In this discussion, any biological effect of ultrasound that is accompanied by temperature increments less than 1 degrees C above normal physiologic levels is called a mechanical effect. However, one should keep in mind that the term mechanical effect also includes processes that are not of a mechanical nature but arise secondary to mechanical interaction between ultrasound and tissues, such as chemical reactions initiated by free oxygen species generated during cavitation and sonoluminescence. Investigations with laboratory animals have documented that pulsed ultrasound can produce damage to biological tissues in vivo through nonthermal mechanisms. The acoustic output used to induce these adverse bio-effects is considerably greater than the output of diagnostic devices when gas bodies are not present. However, low-intensity pulsed ultrasound is used clinically to accelerate the bone fracture repair process and induce healing of nonunions in humans. Low-intensity pulsed ultrasound also has been shown to enhance repair of soft tissue damage and accelerate nerve regeneration in animal models. Although such exposures to low intensity do not appear to cause damage to exposed tissues, they do raise questions about the acoustic threshold that might induce potentially adverse developmental effects in the fetus. To date, bioeffects studies in humans do not substantiate a causal relationship between diagnostic ultrasound exposure during pregnancy and adverse biological effects to the fetus. However, the epidemiologic studies were conducted with commercially available devices predating 1992, having outputs not exceeding a derated spatial-peak temporal-average intensity (ISPTA.3) of 94 mW/cm2. Current limits in the United States allow an ISPTA.3 of 720 mW/cm2 for obstetric modes. At the time of this report, available evidence, experimental or epidemiologic, is insufficient to conclude that there is a causal relationship between obstetric diagnostic ultrasound exposure and adverse nonthermal effects to the fetus. However, low-intensity pulsed ultrasound effects reported in humans and animal models indicate a need for further investigation of potentially adverse developmental effects.

Experimental research on therapeutic angiogenesis induced by hepatocyte growth factor directed by ultrasound-targeted microbubble destruction in rats

OBJECTIVE

The purpose of this study was to explore the feasibility of therapeutic angiogenesis in myocardial infarction induced by hepatocyte growth factor (HGF) mediated by ultrasound-targeted microbubble destruction.

METHODS

Forty Wistar rats were divided into 4 groups after the models of myocardial infarction were prepared: (1) HGF, ultrasound, and microbubbles (HGF+US/MB), (2) HGF and ultrasound, (3) HGF and microbubbles, and (4) surgery alone. Destruction of ultrasound-targeted microbubbles loaded with the HGF gene with an electrocardiographic trigger mode was performed in the HGF+US/MB group. All the rats were killed after being transfected for 14 days. Enhanced green fluorescent protein expression was examined in the myocardium, liver, and kidney in all groups by fluorescence microscopy; CD34 expression was detected by immunohistochemistry, and microvessel density (MVD) was counted in the high-power field on microscopy. Hepatocyte growth factor expression in the myocardium was detected by western blotting and an enzyme-linked immunosorbent assay.

RESULTS

Enhanced green fluorescent protein expression was detected in the myocardium of the HGF+US/MB group, but a few areas of HGF expression were detected only in small vessels and the capillary endothelium, and no expression was found in the surgery-alone and HGF and microbubbles groups. The results of MVD counting by microscopy showed that the MVD in the myocardium of the HGF+US/MB group was the highest among all the groups. The results of western blotting and the enzyme-linked immunosorbent assay showed that the amount of HGF in the myocardium was highest in the HGF+US/MB group.

CONCLUSIONS

Ultrasound-targeted microbubble destruction could deliver HGF into the infracted myocardium and produce an angiogenesis effect, which could provide a novel strategy for gene therapy of myocardial infarction.

High-intensity focused ultrasound: current potential and oncologic applications

OBJECTIVE

The objective of this article is to introduce the reader to the principles and applications of high-intensity focused ultrasound (HIFU).

CONCLUSION

Although a great deal about HIFU physics is understood, its clinical applications are currently limited, and multiple trials are underway worldwide to determine its efficacy.

RNA Interference and MicroRNA Modulation for the Treatment of Cardiac Disorders

The current status and challenges of RNA interference (RNAi) and microRNA modulation strategies for the treatment of myocardial disorders are discussed and related to the classical gene therapeutic approaches of the past decade. Section 2 summarizes the key issues of current vector technologies which determine if they may be suitable for clinical translation of experimental RNAi or microRNA therapeutic protocols. We then present and discuss examples dealing with the potential of cardiac RNAi therapy. First, an approach to block a key early step in the pathogenesis of a virus-induced cardiomyopathy by RNAi targeting of a cellular receptor for cardiopathogenic viruses (Section 3). Second, an approach to improve cardiac function by RNAi targeting of late pathway of heart failure pathogenesis common to myocardial disorders of multiple etiologies. This strategy is directed at myocardial Ca²⁺ homeostasis which is disturbed in heart failure due to coronary heart disease, heart valve dysfunction, cardiac inflammation, or genetic defects (Section 4). Whereas the first type of strategies (directed at early pathogenesis) need to be tailor-made for each different type of pathomechanism, the second type (targeting late common pathways) has a much broader range of application. This advantage of the second type of approaches is of key importance since enormous efforts need to be undertaken before any regulatory RNA therapy enters the stage of possible clinical translation. If then the number of patients eligible for this protocol is large, the actual transformation of the experimental therapy into a new therapeutic option of clinical importance is far more likely to occur.

Ultrasound microbubble contrast agents: fundamentals and application to gene and drug delivery

This review offers a critical analysis of the state of the art of medical microbubbles and their application in therapeutic delivery and monitoring. When driven by an ultrasonic pulse, these small gas bubbles oscillate with a wall velocity on the order of tens to hundreds of meters per second and can be deflected to a vessel wall or fragmented into particles on the order of nanometers. While single-session molecular imaging of multiple targets is difficult with affinity-based strategies employed in some other imaging modalities, microbubble fragmentation facilitates such studies. Similarly, a focused ultrasound beam can be used to disrupt delivery vehicles and blood vessel walls, offering the opportunity to locally deliver a drug or gene. Clinical translation of these vehicles will require that current challenges be overcome, where these challenges include rapid clearance and low payload. The technology, early successes with drug and gene delivery, and potential clinical applications are reviewed.

Cavitation threshold of microbubbles in gel tunnels by focused ultrasound

The investigation of inertial cavitation in micro-tunnels has significant implications for the development of therapeutic applications of ultrasound such as ultrasound-mediated drug and gene delivery. The threshold for inertial cavitation was investigated using a passive cavitation detector with a center frequency of 1 MHz. Micro-tunnels of various diameters (90 to 800 microm) embedded in gel were fabricated and injected with a solution of Optison(trade mark) contrast agent of concentrations 1.2% and 0.2% diluted in water. An ultrasound pulse of duration 500 ms and center frequency 1.736 MHz was used to insonate the microbubbles. The acoustic pressure was increased at 1-s intervals until broadband noise emission was detected. The pressure threshold at which broadband noise emission was observed was found to be dependent on the diameter of the micro-tunnels, with an average increase of 1.2 to 1.5 between the smallest and the largest tunnels, depending on the microbubble concentration. The evaluation of inertial cavitation in gel tunnels rather than tubes provides a novel opportunity to investigate microbubble collapse in a situation that simulates in vivo blood vessels better than tubes with solid walls do.

The application of microbubbles for targeted drug delivery

Interest in microbubbles as vehicles for drug delivery has grown in recent years, due in part to characteristics that make them well suited for this role and in part to the need the for localized delivery of drugs in a number of applications. Microbubbles are inherently small, allowing transvascular passage, they can be functionalized for targeted adhesion, and can be acoustically driven, which facilitates ultrasound detection, production of bioeffects and controlled release of the cargo. This article provides an overview of related microbubble biofluid mechanics and reviews recent developments in the application of microbubbles for targeted drug delivery. Additionally, related advances in non-bubble microparticles for drug delivery are briefly described in the context of targeted adhesion.

DNA and polylysine adsorption and multilayer construction onto cationic lipid-coated microbubbles

We report on a novel application of the layer-by-layer (LbL) assembly technique to attach multiple layers of DNA and poly-l-lysine (PLL) onto preformed lipid-coated microbubbles to increase the DNA loading capacity. We first measured the effects of the cationic lipid fraction and salt concentration on the microbubble stability. Microbubble production and stability were robust up to a cationic lipid fraction of 40 mol % in 10 mM NaCl. DNA adsorption was heterogeneous over the microbubble shell and occurred primarily on the condensed phase domains. The amount of adsorbed DNA, and subsequently adsorbed PLL, increased linearly with the fraction of cationic lipid in the shell. DNA loading was further enhanced by the LbL assembly method to construct polyelectrolyte multilayers (PEMs) of DNA and PLL. PEM buildup was demonstrated by experimental results from zeta potential analysis, fluorescence microscopy, UV spectroscopy, and flow cytometry. The PEMs exhibited two growth stages and were heterogeneously distributed over the microbubble surface. The DNA loading capacity onto the microbubbles was enhanced by over 10-fold by using five paired layers. However, the PEM shell did not prevent oscillation or destruction during ultrasound insonification. These results suggest that the surface can be compartmentalized to make multifunctional, high-payload ultrasound contrast agents for targeted gene therapy.

Enhancement of vascular permeability with low-frequency contrast-enhanced ultrasound in the chorioallantoic membrane model

PURPOSE

To characterize the effect of low-frequency contrast material-enhanced ultrasound on the vascular endothelium and to determine the parameters and techniques required to deliver a therapeutic agent by using the chorioallantoic membrane (CAM) model.

MATERIALS AND METHODS

All in vivo animal procedures were conducted with institutional Animal Care and Use Committee approval. Extravasation of 8.5-nm-diameter fluorescein isothiocyanate-labeled dextran was evaluated in the vasculature of a chick CAM model. Intravital microscopy was performed during contrast-enhanced ultrasound exposure (1.00 or 2.25 MHz); results were compared with results of electron microscopy of the insonated regions. Data acquired after insonation with greater mechanical stress (n = 30 animals) (mechanical index [MI] > 1.3) and with lower mechanical stress (n = 86 animals) (MI < 1.13) were compared with measurements in control conditions (n = 46 animals). The diameter of affected vessels; number of extravasation sites; extravasation rate, area, and location; and changes in endothelial cells and basement membrane were evaluated. Differences were tested with analysis of variance or the Student t test.

RESULTS

After ultrasound application, convective transport of the model drug was observed through micron-sized openings with a mean fluid velocity of 188.6 microm/sec in the low-stress class and 362.5 microm/sec in the high-stress class. Electron microscopy revealed micron-sized focal endothelial gaps and disseminated blebs, vacuoles, and filopodia extending across tens of microns. The threshold pressure for extravasation was 0.5 MPa for a transmitted center frequency of 1.00 MHz (MI = 0.5) and 1.6 MPa for a frequency of 2.25 MHz (MI = 1.06); thus, the frequency dependence of the threshold was not predicted simply by the MI.

CONCLUSION

Low-frequency contrast-enhanced ultrasound can increase vascular permeability and result in convective extravasation of an 8.5-nm-diameter model drug.

Use of ultrasound pulses combined with Definity for targeted blood-brain barrier disruption: a feasibility study

We have developed a method to use low-intensity focused ultrasound pulses combined with an ultrasound contrast agent to produce temporary blood-brain barrier disruption (BBBD). This method could provide a means for the targeted delivery of drugs or imaging agents into the brain. In all our previous work, we used Optison as the ultrasound contrast agent. The purpose of this study was to test the feasibility of using the contrast agent Definity for BBBD. A total of 36 non-overlapping locations were sonicated through a craniotomy in experiments in the brains of nine rabbits (four locations per rabbit; ultrasound [US] frequency: 0.69 MHz; burst: 10 ms; pulse repetition frequency (PRF): 1 Hz; duration: 20 s). The peak negative pressure amplitude ranged from 0.2 to 1.5 MPa. An additional 11 locations were sonicated using Optison at pressure amplitude of 0.5 MPa. Definity and Optison dosages were the same as those used clinically for ultrasound imaging: 10 and 50 microl/kg, respectively. The probability for BBBD (determined using MRI contrast agent enhancement) as a function of pressure amplitude was similar to that found earlier with Optison. For both agents, the probability was estimated to be 50% at 0.4 MPa using probit regression. Histologic examination revealed small, isolated areas of extravasated erythrocytes in some locations. At 0.8 MPa and higher, these areas were sometimes accompanied by tiny (dimensions of 100 microm or less) regions of damaged brain parenchyma. The magnitude of the BBBD was larger with Optison than with Definity at 0.5 MPa (signal enhancement: 13.3% +/- 4.4% vs. 8.4% +/- 4.9%; p = 0.04). In addition, more areas with extravasated erythrocytes were observed with Optison (5.0 +/- 3.5 vs. 1.4 +/- 1.9 areas with extravasation in histology section with largest effect; p = 0.03). We concluded that BBBD is possible using Definity at the dosage of contrast agent and the acoustic parameters tested in this study. The probability for BBBD as a function of pressure amplitude and the type of acute tissue effects were similar to what has been observed using Optison. However, under the experimental conditions used in this study, Optison produced a larger effect for the same acoustic pressure amplitude.

Applications of carrier erythrocytes in delivery of biopharmaceuticals

Carrier erythrocytes, resealed erythrocytes loaded by a drug or other therapeutic agents, have been exploited extensively in recent years for both temporally and spatially controlled delivery of a wide variety of drugs and other bioactive agents owing to their remarkable degree of biocompatibility, biodegradability and a series of other potential advantages. Biopharmaceuticals, therapeutically significant peptides and proteins, nucleic acid-based biologicals, antigens and vaccines, are among the recently focused pharmaceuticals for being delivered using carrier erythrocytes. In this article, the potential applications of erythrocytes in drug delivery have been reviewed with a particular stress on the studies and laboratory experiences on successful erythrocyte loading and characterization of the different classes of biopharmaceuticals.

Microbubble-enhanced ultrasound to deliver an antisense oligodeoxynucleotide targeting the human androgen receptor into prostate tumours

We have shown recently that downregulation of the androgen receptor (AR), one of the key players in prostate tumor cells, with short antisense oligodeoxynucleotides (ODNs) results in inhibition of prostate tumor growth. Particularly with regard to an application of these antisense drugs in vivo, we now investigated the usefulness of microbubble-enhanced ultrasound to deliver these ODNs into prostate cancer cells. Our short antisense AR ODNs were loaded onto the lipid surface of cationic gas-filled microbubbles by ion charge binding, and delivered into the cells by bursting the loaded microbubbles with ultrasound. In vitro experiments were initially performed to show that this kind of delivery system works in principle. In fact, transfection of prostate tumor cells with antisense AR ODNs using microbubble-enhanced ultrasound resulted in 49% transfected cells, associated with a decrease in AR expression compared to untreated controls. In vivo, uptake of a digoxigenin-labelled ODN was found in prostate tumour xenografts in nude mice following intratumoral or intravenous injection of loaded microbubbles and subsequent exposure of the tumour to ultrasound, respectively. Our results show that ultrasound seems to be the driving force of this delivery system. Uptake of the ODN was also observed in tumors after treatment with ultrasound alone, with only minor differences compared to the combined use of microbubbles and ultrasound.

Diffusion of Alexa Fluor 488-conjugated dendrimers in rat aortic tissue

In this study, the distribution of labeled dendrimers in native and aneurysmal rat aortic tissue was examined. Adult male rats underwent infrarenal aorta perfusion with generation 5 (G5) acetylated Alexa Fluor 488-conjugated dendrimers for varying lengths of time. In a second set of experiments, rats underwent aortic elastase perfusion followed by aortic dendrimer perfusion 7 days later. Aortic diameters were measured prior to and postelastase perfusion, and again on the day of harvest. Aortas were harvested 0, 12, or 24 h postperfusion, fixed, and mounted. Native aortas were harvested and viewed as negative controls. Aortic cross-sections were viewed and imaged using confocal microscopy. Dendrimers were quantified (counts/high-powered field). Results were evaluated by repeated measures ANOVA and Student's t-test. We found that in native aortas, dendrimers penetrated the aortic wall in all groups. For all perfusion times, fewer dendrimers were present as time between dendrimer perfusion and aortic harvest increased. Longer perfusion times resulted in increased diffusion of dendrimers throughout the aortic wall. By 24 h, the majority of the dendrimers were through the wall. Dendrimers in aneurysmal aortas, on day 0 postdendrimer perfusion, diffused farther into the aortic wall than controls. In conclusion, this study documents labeled dendrimers delivered intra-arterially to native rat aortas in vivo, and the temporal diffusion of these molecules within the aortic wall. Increasing perfusion time and length of time prior to harvest resulted in continued dendrimer diffusion into the aortic wall. These preliminary data provide a novel mechanism whereby local inhibitory therapy may be delivered locally to aortic tissue.

Ultrasound-induced cavitation: applications in drug and gene delivery

Ultrasound, which has been conventionally used for diagnostics until recently, is now being extensively used for drug and gene delivery. This transformation has come about primarily due to ultrasound-mediated acoustic cavitation - particularly transient cavitation. Acoustic cavitation has been used to facilitate the delivery of small molecules, as well as macromolecules, including proteins and DNA. Controlled generation of cavitation has also been used for targeting drugs to diseased tissues, including skin, brain, eyes and endothelium. Ultrasound has also been employed for the treatment of several diseases, including thromboembolism, arteriosclerosis and cancer. This review provides a detailed account of mechanisms, current status and future prospects of ultrasonic cavitation in drug and gene delivery applications.

Acoustic response of compliable microvessels containing ultrasound contrast agents

The existing models of the dynamics of ultrasound contrast agents (UCAs) have largely been focused on an UCA surrounded by an infinite liquid. Preliminary investigations of a microbubble's oscillation in a rigid tube have been performed using linear perturbation, under the assumption that the tube diameter is significantly larger than the UCA diameter. In the potential application of drug and gene delivery, it may be desirable to fragment the agent shell within small blood vessels and in some cases to rupture the vessel wall, releasing drugs and genes at the site. The effect of a compliant small blood vessel on the UCA's oscillation and the microvessel's acoustic response are unknown. The aim of this work is to propose a lumped-parameter model to study the interaction of a microbubble oscillation and compliable microvessels. Numerical results demonstrate that in the presence of UCAs, the transmural pressure through the blood vessel substantially increases and thus the vascular permeability is predicted to be enhanced. For a microbubble within an 8 to 40 microm vessel with a peak negative pressure of 0.1 MPa and a centre frequency of 1 MHz, small changes in the microbubble oscillation frequency and maximum diameter are observed. When the ultrasound pressure increases, strong nonlinear oscillation occurs, with an increased circumferential stress on the vessel. For a compliable vessel with a diameter equal to or greater than 8 microm, 0.2 MPa PNP at 1 MHz is predicted to be sufficient for microbubble fragmentation regardless of the vessel diameter; however, for a rigid vessel 0.5 MPa PNP at 1 MHz may not be sufficient to fragment the bubbles. For a centre frequency of 1 MHz, a peak negative pressure of 0.5 MPa is predicted to be sufficient to exceed the stress threshold for vascular rupture in a small (diameter less than 15 microm) compliant vessel. As the vessel or surrounding tissue becomes more rigid, the UCA oscillation and vessel dilation decrease; however the circumferential stress is predicted to increase. Decreasing the vessel size or the centre frequency increases the circumferential stress. For the two frequencies considered in this work, the circumferential stress does not scale as the inverse of the square root of the acoustic frequency va as in the mechanical index, but rather has a stronger frequency dependence, 1/va.

Overview of experimental studies of biological effects of medical ultrasound caused by gas body activation and inertial cavitation

Ultrasound exposure can induce bioeffects in mammalian tissue by the nonthermal mechanism of gas body activation. Pre-existing bodies of gas may be activated even at low-pressure amplitudes. At higher-pressure amplitudes, violent cavitation activity with inertial collapse of microbubbles can be generated from latent nucleation sites or from the destabilization of gas bodies. Mechanical perturbation at the activation sites leads to biological effects on nearby cells and structures. Shockwave lithotripsy was the first medical ultrasound application for which significant cavitational bioeffects were demonstrated in mammalian tissues, including hemorrhage and injury in the kidney. Lithotripter shockwaves can also cause hemorrhage in lung and intestine by activation of pre-existing gas bodies in these tissues. Modern diagnostic ultrasound equipment develops pressure amplitudes sufficient for inertial cavitation, but the living body normally lacks suitable cavitation nuclei. Ultrasound contrast agents (UCAs) are suspensions of microscopic gas bodies created to enhance the echogenicity of blood. Ultrasound contrast agent gas bodies also provide nuclei for inertial cavitation. Bioeffects from contrast-aided diagnostic ultrasound depend on pressure amplitude, UCA dose, dosage delivery method and image timing parameters. Microvascular leakage, capillary rupture, cardiomyocyte killing, inflammatory cell infiltration, and premature ventricular contractions have been reported for myocardial contrast echocardiography with clinical ultrasound machines and clinically relevant agent doses in laboratory animals. Similar bioeffects have been reported in intestine, skeletal muscle, fat, lymph nodes and kidney. These microscale bioeffects could be induced unknowingly in diagnostic examinations; however, the medical significance of bioeffects of diagnostic ultrasound with contrast agents is not yet fully understood in relation to the clinical setting.

Effects of shear stress on endothelial cells: possible relevance for ultrasound applications

This review forms part of a series of papers resulting from a workshop on safety of ultrasound applications. The physical effects of ultrasound include generation of steady streaming in large fluid volumes, and micro-streaming around contrast bubbles. Such streaming induces shear stress acting on the vascular endothelium. This review provides a discussion on the levels of endothelial shear stress associated with diagnostic ultrasound applications, and on the biological effects of shear stress acting on the endothelial cells. Depending on vessel size and ultrasound characteristics, shear stresses associated with streaming and micro-streaming may exceed the physiological levels associated with the flow of blood by many orders of magnitude. The resulting biological effects could range anywhere from activation of normal shear stress sensors such as ion channels, damage of the endothelial surface layer, reversible perforation of the membrane, to cell detachment and lysis. The possible presence of such biological effects does not necessarily mean that the effects are harmful for the individual. However, considering the ever-increasing use of ultrasound, a further investigation into these shear stress-related effects, using both experiments and modelling, is desired. Apart from safety concerns, such effects may provide a base for strategies aimed at targeted delivery of drugs.

The influence of acoustic transmit parameters on the destruction of contrast microbubbles in vitro

In this study, the destruction of the contrast agent Sonazoid (GE Healthcare, Oslo, Norway) was measured in vitro as a function of centre frequency (2-3 MHz), acoustic amplitude (0.66-1.6 MPa), pulse length (2-16 cycles) and PRF (0.5-8.0 kHz). Up to 82% of microbubbles were destroyed after exposure to a single 1.6 MPa acoustic pulse (16 cycles, 2.5 MHz and PRF of 1.0 kHz), while at a low amplitude of 0.66 MPa, fractional destruction increased gradually from 0 to 40% after exposure to 9 (identical) pulses. Fractional destruction increased from approximately 8 to 66% as pulse length was changed from 2 to 16 cycles following exposure to a single 2.5 MHz, 1.3 MPa pulse. As the PRF was increased from 0.5 to 8.0 kHz, shorter exposure time intervals (from 4.8 to 1.2 ms) were needed to achieve the same fractional destruction of 80%. Conversely, as the transmit frequency was increased from 2 to 3 MHz the fractional destruction decreased (by more than half within the first 3 pulses). The influence of changes in acoustic pressure and duty cycle on the destruction of Sonazoid microbubbles was highly statistically significant (p < or = 0.01) with a threshold around 0.67 MPa for a duty cycle of 0.0064. In conclusion, the fractional destruction increases with the duty cycle and the acoustic pressure amplitude and decreases with ultrasonic transmit frequency. Better understanding of the influence of the ultrasound transmit parameters on the destruction of contrast microbubbles should help improve existing contrast-assisted imaging modalities and may help develop new techniques for better use of contrast agents.

New technologies and directed agents for applications of cancer imaging

Molecular imaging represents tissue-specific imaging and quantification of physiologic (functional) and molecular events in tumors utilizing new noninvasive imaging modalities, radioligands, and contrast agents. It combines anatomic, physiologic, and metabolic information in a single imaging session. Molecular imaging relies on the ability to target genes and proteins that are linked directly or indirectly to human disease. New imaging biomarkers are being developed. In addition, functional and molecular imaging can potentially replace anatomic longitudinal studies by assessing treatment response earlier. Vascular targeting agents can be evaluated by imaging of tumor angiogenesis using magnetic resonance imaging (MRI), computed tomography and ultrasound, and positron emission tomography (PET). Targeted contrast agents can accomplish site-directed imaging or therapy by a variety of active and passive mechanisms. Furthermore, there is the possibility of combining different modalities such as ultrasonic imaging and MRI or MRI and PET to increase the flexibility unachievable with either modality alone. However, there is a need to standardize these techniques so that longitudinal evaluation of tumor response to treatment is feasible.

Microbubble contrast agents: targeted ultrasound imaging and ultrasound-assisted drug-delivery applications

The use of microbubble contrast agents for general tissue delineation and perfusion enjoys steady interest in ultrasound imaging. Microbubbles as contrast materials require a small dosage and show excellent detection sensitivity. Targeting ligands on the surface of microbubbles permit the selective accumulation of these particles in the areas of interest, which show an up-regulated level of receptor molecules on vascular endothelium. Selective contrast imaging of inflammation, ischemia-reperfusion injury, angiogenesis, and thrombosis has been achieved in animal models. Ultrasound-assisted drug delivery and activation, performed by combining microbubble agent containing drug substances or coadministered with pharmaceutical agents (including plasmid DNA for transfection), has been achieved in multiple model systems in vitro and in vivo. Ultrasound and microbubbles-based targeted acceleration of the thrombolytic enzyme action already have reached clinical trials. Overall, microbubble targeting and ultrasound-assisted microbubble-based drug-delivery systems will offer a step toward the application of targeted personalized diagnostics and therapy.

Bioeffects of albumin-encapsulated microbubbles and real-time myocardial contrast echocardiography in an experimental canine model

Myocardial contrast echocardiography has been used for assessing myocardial perfusion. Some concerns regarding its safety still remain, mainly regarding the induction of microvascular alterations. We sought to determine the bioeffects of microbubbles and real-time myocardial contrast echocardiography (RTMCE) in a closed-chest canine model. Eighteen mongrel dogs were randomly assigned to two groups. Nine were submitted to continuous intravenous infusion of perfluorocarbon-exposed sonicated dextrose albumin (PESDA) plus continuous imaging using power pulse inversion RTMCE for 180 min, associated with manually deflagrated high-mechanical index impulses. The control group consisted of 3 dogs submitted to continuous imaging using RTMCE without PESDA, 3 dogs received PESDA alone, and 3 dogs were sham-operated. Hemodynamics and cardiac rhythm were monitored continuously. Histological analysis was performed on cardiac and pulmonary tissues. No hemodynamic changes or cardiac arrhythmias were observed in any group. Normal left ventricular ejection fraction and myocardial perfusion were maintained throughout the protocol. Frequency of mild and focal microhemorrhage areas in myocardial and pulmonary tissue was similar in PESDA plus RTMCE and control groups. The percentages of positive microscopical fields in the myocardium were 0.4 and 0.7% (P = NS) in the PESDA plus RTMCE and control groups, respectively, and in the lungs they were 2.1 and 1.1%, respectively (P = NS). In this canine model, myocardial perfusion imaging obtained with PESDA and RTMCE was safe, with no alteration in cardiac rhythm or left ventricular function. Mild and focal myocardial and pulmonary microhemorrhages were observed in both groups, and may be attributed to surgical tissue manipulation.

Myocardium-targeted delivery of endothelial progenitor cells by ultrasound-mediated microbubble destruction improves cardiac function via an angiogenic response

Application of ultrasound-mediated destruction of microbubbles (US + Bubble) to skeletal muscle creates capillary ruptures leading to leakage of the cell components. We studied whether US + Bubble combined with bone-marrow-derived mononuclear cells (BM-MNCs) infusion enables the targeted delivery of endothelial-lineage cells into the myocardium and improves cardiac function of the cardiomyopathy model due to the paucity of neocapillary formation. Pulsed US was applied to the anterior chest of BIOTO2 cardiomyopathy hamsters for 90 s after the intravenous injection of microbubble (Optison) followed by infusion of BM-MNCs. Cardiac samples from US + microbubble + BM-MNCs (US + Bubble + BM), US + Bubble, US + BM without Bubble, and saline infusion control groups were analyzed 12 weeks after treatment. Labeled BM-MNCs transplanted by US + Bubble were found to be mainly localized in the microvessels, but not by US stimulation without microbubble (121.2 +/- 24.5 vs. 2.80 +/- 1.30 cells/mm2, P < 0.001). Capillary densities in US + Bubble + BM group were increased 1.7-fold (P < 0.05) over the control, and neither US + Bubble nor US + BM enhanced neocapillary formation. 99mTc-Tetrofosmin scintigraphy revealed that blood perfusion area in the US + Bubble + BM group was 48% greater than the control (P < 0.01). US + Bubble stimulation induces the expression of adhesion molecules (VCAM-1 and ICAM-1) in capillaries, and the US + Bubble-mediated supply of BM-MNCs increased the myocardial content of VEGF and bFGF. The left ventricular wt/body wt, area of cardiac fibrosis, and apoptotic cell numbers in the US + Bubble + BM group significantly (P < 0.05) decreased by 82%, 73%, and 64% relative to the control, respectively. The cardiac function in myopathic hamsters (assessed by fractional shortening) was markedly improved 36% (P < 0.05) by US + Bubble + BM treatment. Targeted delivery of BM-MNCs by US + Bubble to the myocardium of the cardiomyopathic hamster increased the capillary densities and regional blood flow and inhibited cardiac remodeling, resulting in the prevention of heart failure. This non-invasive cell delivery system may be useful as a novel efficient approach for angiogenic cell therapy to the myocardium.