Ultrasound-mediated drug delivery for cardiovascular disease

Influence of DNA-Microbubble Coupling on Contrast Ultrasound-Mediated Gene Transfection in Muscle and Liver

BACKGROUND

Contrast ultrasound-mediated gene delivery (CUMGD) is a promising approach for enhancing gene therapy that relies on microbubble (MB) cavitation to augment complementary deoxyribonucleic acid (cDNA) transfection. The aims of this study were to determine optimal conditions for charge-coupling cDNA to MBs and to evaluate the advantages of surface loading for gene transfection in muscle and liver.

METHODS

Charge coupling of fluorescently labeled cDNA to either neutral MBs (MBN) or cationic MBs (MB+) in low- to high-ionic conditions (0.3%-1.8% NaCl) was assessed by flow cytometry. MB aggregation from cDNA coupling was determined by electrozone sensing. Tissue transfection of luciferase in murine hindlimb skeletal muscle and liver was made by CUMGD with MBN or MB+ combined with subsaturated, saturated, or supersaturated cDNA concentrations (2.5, 50, and 200 μg/10(8) MBs).

RESULTS

Charge-coupling of cDNA was detected for MB+ but not MBN. Coupling occurred over almost the entire range of ionic conditions, with a peak at 1.2% NaCl, although electrostatic interference occurred at >1.5% NaCl. DNA-mediated aggregation of MB+ was observed at ≤0.6% NaCl but did not reduce the ability to produce inertial cavitation. Transfection with CUMGD in muscle and liver was low for both MBs at subsaturation concentrations. In muscle, higher cDNA concentrations produced a 10-fold higher degree of transfection with MB+, which was approximately fivefold higher (P < .05) than that for MBN. There was no effect of DNA supersaturation. The same pattern was seen for liver except that supersaturation further increased transfection with MBN equal to that of MB+.

CONCLUSIONS

Efficient charge-coupling of cDNA to MB+ but not MBN occurs over a relatively wide range of ionic conditions without aggregation. Transfection with CUMGD is much more efficient with charge-coupling of cDNA to MBs and is not affected by supersaturation except in the liver, which is specialized for macromolecular and cDNA uptake.

Microfluidic manufacture of rt-PA -loaded echogenic liposomes

Echogenic liposomes (ELIP), loaded with recombinant tissue-type plasminogen activator (rt-PA) and microbubbles that act as cavitation nuclei, are under development for ultrasound-mediated thrombolysis. Conventional manufacturing techniques produce a polydisperse rt-PA-loaded ELIP population with only a small percentage of particles containing microbubbles. Further, a polydisperse population of rt-PA-loaded ELIP has a broadband frequency response with complex bubble dynamics when exposed to pulsed ultrasound. In this work, a microfluidic flow-focusing device was used to generate monodisperse rt-PA-loaded ELIP (μtELIP) loaded with a perfluorocarbon gas. The rt-PA associated with the μtELIP was encapsulated within the lipid shell as well as intercalated within the lipid shell. The μtELIP had a mean diameter of 5 μm, a resonance frequency of 2.2 MHz, and were found to be stable for at least 30 min in 0.5 % bovine serum albumin. Additionally, 35 % of μtELIP particles were estimated to contain microbubbles, an order of magnitude higher than that reported previously for batch-produced rt-PA-loaded ELIP. These findings emphasize the advantages offered by microfluidic techniques for improving the encapsulation efficiency of both rt-PA and perflurocarbon microbubbles within echogenic liposomes.

Dynamic Behavior of Microbubbles during Long Ultrasound Tone-Burst Excitation: Mechanistic Insights into Ultrasound-Microbubble Mediated Therapeutics Using High-Speed Imaging and Cavitation Detection

Ultrasound (US)-microbubble (MB)-mediated therapies have been found to restore perfusion and enhance drug/gene delivery. On the presumption that MBs do not persist during long US exposure under high acoustic pressures, most schemes use short US pulses when a high US pressure is employed. However, we recently observed an enhanced thrombolytic effect using long US pulses at high acoustic pressures. Therefore, we explored the fate of MBs during long tone-burst exposures (5 ms) at various acoustic pressures and MB concentrations via direct high-speed optical observation and passive cavitation detection. MBs first underwent stable or inertial cavitation depending on the acoustic pressure and then formed gas-filled clusters that continued to oscillate, break up and form new clusters. Cavitation detection confirmed continued, albeit diminishing, acoustic activity throughout the 5-ms US excitation. These data suggest that persisting cavitation activity during long tone bursts may confer additional therapeutic effects.

Trans-Stent B-Mode Ultrasound and Passive Cavitation Imaging

Angioplasty and stenting of a stenosed artery enable acute restoration of blood flow. However, restenosis or a lack of re-endothelization can subsequently occur depending on the stent type. Cavitation-mediated drug delivery is a potential therapy for these conditions, but requires that particular types of cavitation be induced by ultrasound insonation. Because of the heterogeneity of tissue and stochastic nature of cavitation, feedback mechanisms are needed to determine whether the sustained bubble activity is induced. The objective of this study was to determine the feasibility of passive cavitation imaging through a metal stent in a flow phantom and an animal model. In this study, an endovascular stent was deployed in a flow phantom and in porcine femoral arteries. Fluorophore-labeled echogenic liposomes, a theragnostic ultrasound contrast agent, were injected proximal to the stent. Cavitation images were obtained by passively recording and beamforming the acoustic emissions from echogenic liposomes insonified with a low-frequency (500 kHz) transducer. In vitro experiments revealed that the signal-to-noise ratio for detecting stable cavitation activity through the stent was greater than 8 dB. The stent did not significantly reduce the signal-to-noise ratio. Trans-stent cavitation activity was also detected in vivo via passive cavitation imaging when echogenic liposomes were insonified by the 500-kHz transducer. When stable cavitation was detected, delivery of the fluorophore into the arterial wall was observed. Increased echogenicity within the stent was also observed when echogenic liposomes were administered. Thus, both B-mode ultrasound imaging and cavitation imaging are feasible in the presence of an endovascular stent in vivo. Demonstration of this capability supports future studies to monitor restenosis with contrast-enhanced ultrasound and pursue image-guided ultrasound-mediated drug delivery to inhibit restenosis.

Sonothrombolysis

Thrombo-occlusive disease is a leading cause of morbidity and mortality. In this chapter, the use of ultrasound to accelerate clot breakdown alone or in combination with thrombolytic drugs will be reported. Primary thrombus formation during cardiovascular disease and standard treatment methods will be discussed. Mechanisms for ultrasound enhancement of thrombolysis, including thermal heating, radiation force, and cavitation, will be reviewed. Finally, in-vitro, in-vivo and clinical evidence of enhanced thrombolytic efficacy with ultrasound will be presented and discussed.

Low Intensity Ultrasound Mediated Liposomal Doxorubicin Delivery Using Polymer Microbubbles

Cardiotoxicity is the major dose-limiting factor in the chemotherapeutic use of doxorubicin (Dox). A delivery vehicle that can be triggered to release its payload in the tumoral microvasculature but not in healthy tissue would help improve the therapeutic window of the drug. Delivery strategies combining liposomal encapsulated Dox (LDox), microbubbles (MBs), and ultrasound (US) have been shown to improve therapeutic efficacy of LDox, but much remains to be known about the mechanisms and the US conditions that maximize cytotoxicity using this approach. In this study, we compared different US pulses in terms of drug release and acute toxicity. Drug uptake and proliferation rates using low-intensity US were measured in squamous cell carcinoma cells exposed to LDox conjugated to or coinjected with polymer MBs. The aims of this study were: (1) to compare the effects of low- and high-pressure US on Dox release kinetics; (2) to evaluate whether conjugating the liposome to the MB surface (DoxLPX) is an important factor for drug release and cytotoxicity; and (3) to determine which US parameters most inhibit cell proliferation and whether this inhibition is mediated by drug release or the MB/US interaction with cells. Low-pressure US (170 kPa) at high duty cycle (stable cavitation) released up to ∼ 70% of the encapsulated Dox from the DoxLPX, thus improving Dox bioavailability and cellular uptake and leading to a significant reduction in cell proliferation at 48 h. Flow cytometry showed that US generating stable oscillations of DoxLPX significantly increased cellular Dox uptake at 4 h after US exposure compared to LDox. Drug uptake was correlated with cytotoxicity at 48 h. Our results demonstrate that Dox-containing liposomes conjugated to polymer MBs can be triggered to release ∼ 70% of their payload using noninertial US. Following release, Dox became bioavailable to the cells and induced significantly higher cytotoxicity compared to nonreleased encapsulated drug. Our findings show promise for targeted drug delivery using this theranostic delivery platform at low US intensities.

In vivo thrombolysis with targeted microbubbles loading tissue plasminogen activator in a rabbit femoral artery thrombus model

The increasingly high incidence of ischemic stroke caused by thrombosis of the arterial vessels is one of the major factors that threaten people's health and lives in the world. The present treatments for thrombosis are unsatisfactory yet. We developed the microbubbles loading tissue plasminogen activator (tPA) and their in vitro thrombolysis efficacy under ultrasound exposure has been proved previously. We tried to investigate their thrombolysis effect in vivo in this present study. Thrombus model was made by clamping bilateral femoral arteries in 70 arteries of 40 rabbits. The targeted tPA-loaded microbubbles were made by lyophilization, taking arginine-glycine-aspartic acid-serine peptide as the targeting ligand. Its thrombolysis efficacy, calculated as count rate and efficiency rate of recanalization, was evaluated by Pearson's χ(2) and One-way ANOVA, respectively. The count rate of recanalization of the targeted tPA-loaded microbubbles under ultrasound exposure (70%) was similar to that of the combination of tPA, microbubbles and ultrasound exposure (80%) (P = 0.61), while its tPA dosage (0.06 mg/kg) was much less than that of latter (0.9 mg/kg). Its efficiency rate of recanalization was the highest among all groups (53.22 ± 40.39%) (P < 0.01). Ultrasound-induced targeted tPA-loaded microbubbles release is a promising thrombolytic method with satisfactory thrombolytic efficacy, lowered tPA dose and potentially decreased hemorrhagic risk.

Shaken and stirred: mechanisms of ultrasound-enhanced thrombolysis

The use of ultrasound and microbubbles as an effective adjuvant to thrombolytics has been reported in vitro, ex vivo and in vivo. However, the specific mechanisms underlying ultrasound-enhanced thrombolysis have yet to be elucidated. We present visual observations illustrating two mechanisms of ultrasound-enhanced thrombolysis: acoustic cavitation and radiation force. An in vitro flow model was developed to observe human whole blood clots exposed to human fresh-frozen plasma, recombinant tissue-type plasminogen activator (0, 0.32, 1.58 or 3.15 μg/mL) and the ultrasound contrast agent Definity (2 μL/mL). Intermittent, continuous-wave ultrasound (120 kHz, 0.44 MPa peak-to-peak pressure) was used to insonify the perfusate. Ultraharmonic emissions indicative of stable cavitation were monitored with a passive cavitation detector. The clot was observed with an inverted microscope, and images were recorded with a charge-coupled device camera. The images were post-processed to determine the time-dependent clot diameter and root-mean-square velocity of the clot position. Clot lysis occurred preferentially surrounding large, resonant-sized bubbles undergoing stable oscillations. Ultraharmonic emissions from stable cavitation were found to correlate with the lytic rate. Clots were observed to translate synchronously with the initiation and cessation of the ultrasound exposure. The root-mean-square velocity of the clot correlated with the lytic rate. These data provide visual documentation of stable cavitation activity and radiation force during sub-megahertz sonothrombolysis. The observations of this study suggest that the process of clot lysis is complex, and both stable cavitation and radiation force are mechanistically responsible for this beneficial bio-effect in this in vitro model.

Loss of echogenicity and onset of cavitation from echogenic liposomes: pulse repetition frequency independence

Echogenic liposomes (ELIP) are being developed for the early detection and treatment of atherosclerotic lesions. An 80% loss of echogenicity of ELIP has been found to be concomitant with the onset of stable and inertial cavitation. The ultrasound pressure amplitude at which this occurs is weakly dependent on pulse duration. It has been reported that the rapid fragmentation threshold of ELIP (based on changes in echogenicity) is dependent on the insonation pulse repetition frequency (PRF). The study described here evaluates the relationship between loss of echogenicity and cavitation emissions from ELIP insonified by duplex Doppler pulses at four PRFs (1.25, 2.5, 5 and 8.33 kHz). Loss of echogenicity was evaluated on B-mode images of ELIP. Cavitation emissions from ELIP were recorded passively on a focused single-element transducer and a linear array. Emissions recorded by the linear array were beamformed, and the spatial widths of stable and inertial cavitation emissions were compared with the calibrated azimuthal beamwidth of the Doppler pulse exceeding the stable and inertial cavitation thresholds. The inertial cavitation thresholds had a very weak dependence on PRF, and stable cavitation thresholds were independent of PRF. The spatial widths of the cavitation emissions recorded by the passive cavitation imaging system agreed with the calibrated Doppler beamwidths. The results also indicate that 64%-79% loss of echogenicity can be used to classify the presence or absence of cavitation emissions with greater than 80% accuracy.

Pulsed ultrasound enhances the delivery of nitric oxide from bubble liposomes to ex vivo porcine carotid tissue

Ultrasound-mediated drug delivery is a novel technique for enhancing the penetration of drugs into diseased tissue beds noninvasively. By encapsulating drugs into microsized and nanosized liposomes, the therapeutic can be shielded from degradation within the vasculature until delivery to a target site by ultrasound exposure. Traditional in vitro or ex vivo techniques to quantify this delivery profile include optical approaches, cell culture, and electrophysiology. Here, we demonstrate an approach to characterize the degree of nitric oxide (NO) delivery to porcine carotid tissue by direct measurement of ex vivo vascular tone. An ex vivo perfusion model was adapted to assess ultrasound-mediated delivery of NO. This potent vasodilator was coencapsulated with inert octafluoropropane gas to produce acoustically active bubble liposomes. Porcine carotid arteries were excised post mortem and mounted in a physiologic buffer solution. Vascular tone was assessed in real time by coupling the artery to an isometric force transducer. NO-loaded bubble liposomes were infused into the lumen of the artery, which was exposed to 1 MHz pulsed ultrasound at a peak-to-peak acoustic pressure amplitude of 0.34 MPa. Acoustic cavitation emissions were monitored passively. Changes in vascular tone were measured and compared with control and sham NO bubble liposome exposures. Our results demonstrate that ultrasound-triggered NO release from bubble liposomes induces potent vasorelaxation within porcine carotid arteries (maximal relaxation 31%±8%), which was significantly stronger than vasorelaxation due to NO release from bubble liposomes in the absence of ultrasound (maximal relaxation 7%±3%), and comparable with relaxation due to 12 μM sodium nitroprusside infusions (maximal relaxation 32%±3%). This approach is a valuable mechanistic tool for assessing the extent of drug release and delivery to the vasculature caused by ultrasound.

Targeted microbubble mediated sonoporation of endothelial cells in vivo

Ultrasound contrast agents as drug-delivery systems are an emerging field. Recently, we reported that targeted microbubbles are able to sonoporate endothelial cells in vitro. In this study, we investigated whether targeted microbubbles can also induce sonoporation of endothelial cells in vivo, thereby making it possible to combine molecular imaging and drug delivery. Live chicken embryos were chosen as the in vivo model. αvß3-targeted microbubbles attached to the vessel wall of the chicken embryo were insonified at 1 MHz at 150 kPa (1 × 10,000 cycles) and at 200 kPa (1 × 1000 cycles) peak negative acoustic pressure. Sonoporation was studied by intravital microscopy using the model drug propidium iodide (PI). Endothelial cell PI uptake was observed in 48% of microbubble-vessel-wall complexes at 150 kPa (n = 140) and in 33% at 200 kPa (n = 140). Efficiency of PI uptake depended on the local targeted microbubble concentration and increased up to 80% for clusters of 10 to 16 targeted microbubbles. Ultrasound or targeted microbubbles alone did not induce PI uptake. This intravital microscopy study reveals that sonoporation can be visualized and induced in vivo using targeted microbubbles.

Recent advancements in the cardiovascular drug carriers

Cardiovascular disease is the disease that affects the cardiovascular system, vascular diseases of the brain and kidney, and peripheral arterial disease. Despite of all advances in pharmacological and clinical treatment, heart failure is a leading cause of morbidness and mortality worldwide. Many new therapeutic advance strategies, including cell transplantation, gene delivery or therapy, and cytokines or other small molecules, have been research to treat heart failure. The main aim of this review article is to focus on nano carriers advancement and addressing the problems associated with old and modern therapeutics such as nonspecific effects and poor stability.

The delayed onset of subharmonic and ultraharmonic emissions from a phospholipid-shelled microbubble contrast agent

Characterizing the non-linear response of microbubble contrast agents is important for their efficacious use in imaging and therapy. In this article, we report that the subharmonic and ultraharmonic response of lipid-shelled microbubble contrast agents exhibits a strong temporal dependence. We characterized non-linear emissions from Targestar-p microbubbles (Targeson Inc., San Diego, CA, USA) periodically for 60 min, at 10 MHz excitation frequency. The results revealed a considerable increase in the subharmonic and ultraharmonic response (nearly 12-15 and 5-8 dB) after 5-10 min of agent preparation. However, the fundamental and the harmonic response remained almost unchanged in this period. During the next 50 min, the subharmonic, fundamental, ultraharmonic, and harmonic responses decreased steadily by 2-5 dB. The temporal changes in the non-linear behavior of the agent appeared to be primarily mediated by gas-exchange through the microbubble shell; temperature and prior acoustic excitation based mechanisms were ruled out. Further, there was no measurable change in the agent size distribution by static diffusion. We envisage that these findings will help obtain reproducible measurements from agent characterization, non-linear imaging, and fluid-pressure sensing. These findings also suggest the possibility for improving non-linear imaging by careful design of ultrasound contrast agents.

Advance of molecular imaging technology and targeted imaging agent in imaging and therapy

Molecular imaging is an emerging field that integrates advanced imaging technology with cellular and molecular biology. It can realize noninvasive and real time visualization, measurement of physiological or pathological process in the living organism at the cellular and molecular level, providing an effective method of information acquiring for diagnosis, therapy, and drug development and evaluating treatment of efficacy. Molecular imaging requires high resolution and high sensitive instruments and specific imaging agents that link the imaging signal with molecular event. Recently, the application of new emerging chemical technology and nanotechnology has stimulated the development of imaging agents. Nanoparticles modified with small molecule, peptide, antibody, and aptamer have been extensively applied for preclinical studies. Therapeutic drug or gene is incorporated into nanoparticles to construct multifunctional imaging agents which allow for theranostic applications. In this review, we will discuss the characteristics of molecular imaging, the novel imaging agent including targeted imaging agent and multifunctional imaging agent, as well as cite some examples of their application in molecular imaging and therapy.

Broadband attenuation measurements of phospholipid-shelled ultrasound contrast agents

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

Single-particle optical sizing of microbubbles

Single-particle optical sizing techniques are being used to determine the size distributions of microbubble ultrasound contrast agents and to study the dynamics of individual microbubbles during ultrasound stimulation. The goal of this study was to compare experimental light obscuration and scattering measurements of microbubble size distributions with predictions from generalized Lorenz-Mie scattering theory (GLMT). First, we illustrate that a mono-modal size distribution can be misrepresented by single-particle light obscuration measurements as multi-modal peaks because of non-linearities in the extinction cross section-versus-diameter curve. Next, polymer bead standards are measured to provide conversion factors between GLMT calculations and experimental flow cytometry scatter plots. GLMT calculations with these conversion factors accurately predict the characteristic Lissajous-like serpentine scattering plot measured by flow cytometry for microbubbles. We conclude that GLMT calculations can be combined with optical forward and side scatter measurements to accurately determine microbubble size.

Silver nanoparticle-embedded microbubble as a dual-mode ultrasound and optical imaging probe

Microbubbles (MBs) coupled with nanoparticles represent a new class of multifunctional probe for multiscale biomedical imaging and drug delivery. In this study, we describe the development of multifunctional, microscale microbubble probes that are composed of a nitrogen gas core and a biocompatible polymer shell harboring silver nanoparticles (AgNPs). Ultrasound imaging studies show that the presence of AgNPs in the MB significantly improves the contrast of ultrasound images. The AgNPs within individual MB can be also imaged by using dark-field microscopy (DFM), which suggests that AgNPs in the polymer shell adopt multiple structural forms. AgNPs are released from the polymer shell following a brief exposure to an ultrasonic field and are subsequently taken up by living cells. AgNPs within labeled cells are imaged by DFM, while surface-enhanced Raman scattering is used to identify specific cytoplasmic biomolecules that bind to the surface of the AgNP. Collectively, these studies demonstrate the application of multifunctional MBs for micrometer scale contrast-enhanced ultrasound imaging, as vehicles for the ultrasound-based delivery of optical probes and drugs to cells, and for imaging of chemical sensing of individual nanopartiles within cells and tissue.

Relationship between cavitation and loss of echogenicity from ultrasound contrast agents

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