High-speed optical observations of contrast agent destruction

The Compression-Dominated Ultrasound Response of Poly(n-butyl cyanoacrylate) Hard-Shelled Microbubbles Induces Significant Sonoporation and Sonopermeation Effects In Vitro

The process of locally increasing the permeability of cell membranes or cell layers is referred to as sonoporation or sonopermeation, respectively, and opens up perspectives for drug delivery in cancer treatment by facilitating enhanced local drug accumulation. These effects are mediated by ultrasound-activated microbubbles in close proximity to cells. Here, the selection of ultrasound settings according to the intended effect on the biological tissue remains a challenge, especially for broadly size-distributed microbubbles, which show a heterogeneous response to ultrasound. For this purpose, we have analyzed the general response of narrower size-distributed poly(n-butyl cyanoacrylate) hard-shelled microbubbles to ultrasound via ultra-high-speed imaging and evaluated their ability to stimulate sonoporation and sonopermeation in vitro compared to lipid soft-shelled microbubbles. Ultra-high-speed imaging of hard-shelled microbubbles revealed either a compression-dominated or compression-only response at peak negative acoustic pressures higher than 165 kPa and an onset of bursting at 500 kPa. The in vitro experiments demonstrated that the hard-shelled microbubbles induced significant sonoporation and sonopermeation effects, also when only compressing at 300 kPa peak neagtive pressure. Compared to soft-shelled microbubbles, the effects were less prominent, which was attributed to differences in their ultrasound responses and size distributions. This in vitro validation of hard-shelled microbubbles qualifies them for future in vivo applications, which would benefit from their narrow size distribution, thereby allowing more control of their therapeutic effect by suitably adjusting the ultrasound parameters.

High-Speed Optical Characterization of Protein-and-Nanoparticle-Stabilized Microbubbles for Ultrasound-Triggered Drug Release

OBJECTIVE

Ultrasound-triggered bubble-mediated local drug delivery has shown potential to increase therapeutic efficacy and reduce systemic side effects, by loading drugs into the microbubble shell and triggering delivery of the payload on demand using ultrasound. Understanding the behavior of the microbubbles in response to ultrasound is crucial for efficient and controlled release.

METHODS

In this work, the response of microbubbles with a coating consisting of poly(2-ethyl-butyl cyanoacrylate) (PEBCA) nanoparticles and denatured casein was characterized. High-speed recordings were taken of single microbubbles, in both bright field and fluorescence.

RESULTS

The nanoparticle-loaded microbubbles show resonance behavior, but with a large variation in response, revealing a substantial interbubble variation in mechanical shell properties. The probability of shell rupture and the probability of nanoparticle release were found to strongly depend on microbubble size, and the most effective size was inversely proportional to the driving frequency. The probabilities of both rupture and release increased with increasing driving pressure amplitude. Rupture of the microbubble shell occurred after fewer cycles of ultrasound as the driving pressure amplitude or driving frequency was increased.

CONCLUSION

The results highlight the importance of careful selection of the driving frequency, driving pressure amplitude and duration of ultrasound to achieve the most efficient ultrasound-triggered shell rupture and nanoparticle release of protein-and-nanoparticle-stabilized microbubbles.

Determining Ultrasound Parameters for Bursting Polymer Microbubbles for Future Use in Spinal Cord Injury

OBJECTIVE

We believe our poly(lactic acid) (PLA) microbubbles are well suited for therapeutic delivery to spinal cord injury (SCI) using ultrasound-triggered bursting. We investigated the feasibility of clinical ultrasound bursting in situ, the optimal bursting parameters in vitro and the loading and release of a model bio-active DNA.

METHODS

Microbubbles were tested using clinical ultrasound in a rat cadaver SCI model. Burst pressure thresholds were determined using the change in enhancement after ultrasound exposure. Resonance frequency, acoustic enhancement, sizing and morphology were evaluated by comparing two microbubble porogens, ammonium carbonate and ammonium carbamate. Oligonucleotides were loaded into the shell and released using the found optimized ultrasound bursting parameters.

RESULTS

In situ imaging and bursting were successful. In vitro bursting thresholds using frequencies 1, 2.25 and 5 MHz were identified between peak negative pressures 0.2 and 0.5 MPa, believed to be safe for spinal cord. The pressure threshold decreased with decreasing frequencies. PLA bursting was optimized near the resonance frequency of 2.5 to 3.0 MHz using 2.25 MHz and not at lower frequencies. PLA microbubbles, initially with a mean size of approximately 2 µm, remained in one piece, collapsed to between 0.5 and 1 µm and did not fragment. Significantly more oligonucleotide was released after ultrasound bursting of loaded microbubbles. Microbubble-sized debris was detected when using ammonium carbamate, leading to inaccurate microbubble concentration measurements.

CONCLUSION

PLA microbubbles made with ammonium carbonate and burst at appropriate parameters have the potential to safely improve intrathecal therapeutic delivery to SCI using targeted ultrasound.

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

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

Quantifying the Effect of Acoustic Parameters on Temporal and Spatial Cavitation Activity: Gauging Cavitation Dose

OBJECTIVE

Cavitation-enhanced delivery of therapeutic agents is under development for the treatment of cancer and neurodegenerative and cardiovascular diseases, including sonothrombolysis for deep vein thrombosis. The objective of this study was to quantify the spatial and temporal distribution of cavitation activity nucleated by Definity infused through the EKOS catheter over a range of acoustic parameters controlled by the EKOS endovascular system.

METHODS

Three insonation protocols were compared in an in vitro phantom mimicking venous flow to measure the effect of peak rarefactional pressure, pulse duration and pulse repetition frequency on cavitation activity energy, location and duration. Inertial and stable cavitation activity was quantified using passive cavitation imaging, and a metric of cavitation dose based on energy density was defined.

RESULTS

For all three insonation protocols, cavitation was sustained for the entire 30 min Definity infusion. The evolution of cavitation energy during each pulse duration was similar for all three protocols. For insonation protocols with higher peak rarefactional acoustic pressures, inertial and stable cavitation doses also increased. A complex relationship between the temporal behavior of cavitation energy within each pulse and the pulse repetition frequency affected the cavitation dose for the three insonation protocols. The relative predominance of stable or inertial cavitation dose varied according to insonation schemes. Passive cavitation images revealed the spatial distribution of cavitation activity.

CONCLUSION

Our cavitation dose metric based on energy density enabled the impact of different acoustic parameters on cavitation activity to be measured. Depending on the type of cavitation to be promoted or suppressed, particular pulsing schemes could be employed in future studies, for example, to correlate cavitation dose with sonothrombolytic efficacy.

Buckling of lipidic ultrasound contrast agents under quasi-static load

Collapse of lipidic ultrasound contrast agents under high-frequency compressive load has been historically interpreted by the vanishing of surface tension. By contrast, buckling of elastic shells is known to occur when costly compressible stress is released through bending. Through quasi-static compression experiments on lipidic shells, we analyse the buckling events in the framework of classical elastic buckling theory and deduce the mechanical characteristics of these shells. They are then compared with that obtained through acoustic characterization. This article is part of the theme issue 'Probing and dynamics of shock sensitive shells'.

Enhancing Targeted Therapy in Breast Cancer by Ultrasound-Responsive Nanocarriers

Currently, the response to cancer treatments is highly variable, and severe side effects and toxicity are experienced by patients receiving high doses of chemotherapy, such as those diagnosed with triple-negative breast cancer. The main goal of researchers and clinicians is to develop new effective treatments that will be able to specifically target and kill tumor cells by employing the minimum doses of drugs exerting a therapeutic effect. Despite the development of new formulations that overall can increase the drugs’ pharmacokinetics, and that are specifically designed to bind overexpressed molecules on cancer cells and achieve active targeting of the tumor, the desired clinical outcome has not been reached yet. In this review, we will discuss the current classification and standard of care for breast cancer, the application of nanomedicine, and ultrasound-responsive biocompatible carriers (micro/nanobubbles, liposomes, micelles, polymeric nanoparticles, and nanodroplets/nanoemulsions) employed in preclinical studies to target and enhance the delivery of drugs and genes to breast cancer.

Ultrafast Microscopy Imaging of Acoustic Cluster Therapy Bubbles: Activation and Oscillation

Acoustic Cluster Therapy (ACT®) is a platform for improving drug delivery and has had promising pre-clinical results. A clinical trial is ongoing. ACT® is based on microclusters of microbubbles-microdroplets that, when sonicated, form a large ACT® bubble. The aim of this study was to obtain new knowledge on the dynamic formation and oscillations of ACT® bubbles by ultrafast optical imaging in a microchannel. The high-speed recordings revealed the microbubble-microdroplet fusion, and the gas in the microbubble acted as a vaporization seed for the microdroplet. Subsequently, the bubble grew by gas diffusion from the surrounding medium and became a large ACT® bubble with a diameter of 5-50 μm. A second ultrasound exposure at lower frequency caused the ACT® bubble to oscillate. The recorded oscillations were compared with simulations using the modified Rayleigh-Plesset equation. A term accounting for the physical boundary imposed by the microchannel wall was included. The recorded oscillation amplitudes were approximately 1-2 µm, hence similar to oscillations of smaller contrast agent microbubbles. These findings, together with our previously reported promising pre-clinical therapeutic results, suggest that these oscillations covering a large part of the vessel wall because of the large bubble volume can substantially improve therapeutic outcome.

Ultrasound-Mediated Drug Delivery: Sonoporation Mechanisms, Biophysics, and Critical Factors

Sonoporation, or the use of ultrasound in the presence of cavitation nuclei to induce plasma membrane perforation, is well considered as an emerging physical approach to facilitate the delivery of drugs and genes to living cells. Nevertheless, this emerging drug delivery paradigm has not yet reached widespread clinical use, because the efficiency of sonoporation is often deemed to be mediocre due to the lack of detailed understanding of the pertinent scientific mechanisms. Here, we summarize the current observational evidence available on the notion of sonoporation, and we discuss the prevailing understanding of the physical and biological processes related to sonoporation. To facilitate systematic understanding, we also present how the extent of sonoporation is dependent on a multitude of factors related to acoustic excitation parameters (ultrasound frequency, pressure, cavitation dose, exposure time), microbubble parameters (size, concentration, bubble-to-cell distance, shell composition), and cellular properties (cell type, cell cycle, biochemical contents). By adopting a science-backed approach to the realization of sonoporation, ultrasound-mediated drug delivery can be more controllably achieved to viably enhance drug uptake into living cells with high sonoporation efficiency. This drug delivery approach, when coupled with concurrent advances in ultrasound imaging, has potential to become an effective therapeutic paradigm.

Applications of Ultrasound-Mediated Drug Delivery and Gene Therapy

Gene therapy has continuously evolved throughout the years since its first proposal to develop more specific and effective transfection, capable of treating a myriad of health conditions. Viral vectors are some of the most common and most efficient vehicles for gene transfer. However, the safe and effective delivery of gene therapy remains a major obstacle. Ultrasound contrast agents in the form of microbubbles have provided a unique solution to fulfill the need to shield the vectors from the host immune system and the need for site specific targeted therapy. Since the discovery of the biophysical and biological effects of microbubble sonification, multiple developments have been made to enhance its applicability in targeted drug delivery. The concurrent development of viral vectors and recent research on dual vector strategies have shown promising results. This review will explore the mechanisms and recent advancements in the knowledge of ultrasound-mediated microbubbles in targeting gene and drug therapy.

Tailor-Made Single-Core PLGA Microbubbles as Acoustic Cavitation Enhancers for Therapeutic Applications

Microbubbles (MBs), being gas bubbles encapsulated inside a solid shell, have been investigated extensively in the field of therapeutic ultrasound as acoustic cavitation enhancers. Hard-shell MBs have an advantage over soft-shell MBs due to their improved stability. Poly(lactic-co-glycolic acid) (PLGA) is one of the most attractive polymers for hard-shell MB synthesis; however, very little is known regarding the effect of synthesis parameters on the acoustic cavitation activity of PLGA MBs and the tunability of this activity. In this study, by manipulating the synthesis parameters, we were able to control the characteristics of the MBs, such as their internal structure, gas core, size distribution, and shell thickness, which significantly affect the total acoustic cavitation activity that they exhibit (i.e., their cavitation dose). We showed that single-core MBs filled with C₃F₈ gas can produce cavitation effects for extended periods under continuous circulation. These MBs exhibited high stability, and their cavitation activity was not affected by prior circulation in the system. Preliminary in vivo results demonstrated that intravenously injected MBs did not cause adverse effects and produced cavitation activity that increased the permeability of the pig blood-brain barrier. Although more tests should be performed to evaluate the MB long-term safety and activity in vivo, these encouraging results suggest that our PLGA MBs have potential for future therapeutic applications as cavitation enhancers.

Sonoporation Using Nanoparticle-Loaded Microbubbles Increases Cellular Uptake of Nanoparticles Compared to Co-Incubation of Nanoparticles and Microbubbles

Therapeutic agents can benefit from encapsulation in nanoparticles, due to improved pharmacokinetics and biodistribution, protection from degradation, increased cellular uptake and sustained release. Microbubbles in combination with ultrasound have been shown to improve the delivery of nanoparticles and drugs to tumors and across the blood-brain barrier. Here, we evaluate two different microbubbles for enhancing the delivery of polymeric nanoparticles to cells in vitro: a commercially available lipid microbubble (Sonazoid) and a microbubble with a shell composed of protein and nanoparticles. Various ultrasound parameters are applied and confocal microscopy is employed to image cellular uptake. Ultrasound enhanced cellular uptake depending on the pressure and duty cycle. The responsible mechanisms are probably sonoporation and sonoprinting, followed by uptake, and to a smaller degree enhanced endocytosis. The use of commercial Sonazoid microbubbles leads to significantly lower uptake than when using nanoparticle-loaded microbubbles, suggesting that proximity between cells, nanoparticles and microbubbles is important, and that mainly nanoparticles in the shell are taken up, rather than free nanoparticles in solution.

Modifications of Plasma Membrane Organization in Cancer Cells for Targeted Therapy

Modifications of the composition or organization of the cancer cell membrane seem to be a promising targeted therapy. This approach can significantly enhance drug uptake or intensify the response of cancer cells to chemotherapeutics. There are several methods enabling lipid bilayer modifications, e.g., pharmacological, physical, and mechanical. It is crucial to keep in mind the significance of drug resistance phenomenon, ion channel and specific receptor impact, and lipid bilayer organization in planning the cell membrane-targeted treatment. In this review, strategies based on cell membrane modulation or reorganization are presented as an alternative tool for future therapeutic protocols.

Utilizing polymer-conjugate albumin-based ultrafine gas bubbles in combination with ultra-high frequency radiations in drug transportation and delivery

Ultrafine bubbles stabilized by human serum albumin conjugate polyethylene glycol ameliorates the stability of complex as well as the drug payload. Polyethylene glycol presents the crucial role in releasing drug by means of acoustic sound.

Microbubble-Facilitated Ultrasound Catheter Ablation Causes Microvascular Damage and Fibrosis

High-intensity ultrasound (US) ablation produces deeper myocardial lesions than radiofrequency ablation. The presence of intravascular microbubble (MB) contrast agents enhances pulsed-wave US ablation via cavitation-related histotripsy, potentially facilitating ablation in persistently perfused/conducting myocardium. US ablation catheters were developed and tested in the presence of MBs using ex vivo and in vivo models. High-frame-rate videomicroscopy and US imaging of gel phantom models confirmed MB destruction by inertial cavitation. MB-facilitated US ablation in an ex vivo perfused myocardium model generated shallow (2 mm) lesions and, in an in vivo murine hindlimb model, reduced perfusion by 42% with perivascular hemorrhage and inflammation, but no myonecrosis.

Mechanisms underlying sonoporation: Interaction between microbubbles and cells

The past several decades have witnessed great progress in "smart drug delivery", an advance technology that can deliver genes or drugs into specific locations of patients' body with enhanced delivery efficiency. Ultrasound-activated mechanical force induced by the interactions between microbubbles and cells, which can stimulate so-called "sonoporation" process, has been regarded as one of the most promising candidates to realize spatiotemporal-controllable drug delivery to selected regions. Both experimental and numerical studies were performed to get in-depth understanding on how the microbubbles interact with cells during sonoporation processes, under different impact parameters. The current work gives an overview of the general mechanism underlying microbubble-mediated sonoporation, and the possible impact factors (e.g., the properties of cavitation agents and cells, acoustical driving parameters and bubble/cell micro-environment) that could affect sonoporation outcomes. Finally, current progress and considerations of sonoporation in clinical applications are reviewed also.

Concurrent visual and acoustic tracking of passive and active delivery of nanobubbles to tumors

Background: There has been growing interest in nanobubbles for their potential to extend bubble-mediated ultrasound approaches beyond that of their larger microbubble counterparts. In particular, the smaller scale of nanobubbles may enable them to access the tumor extravascular compartment for imaging and therapy in closer proximity to cancer cells. Compelling preliminary demonstrations of the imaging and therapeutic abilities of nanobubbles have thus emerged, with emphasis on their ability to extravasate. However, studies to date rely on indirect histologic evidence that cannot confirm whether the structures remain intact beyond the vasculature - leaving their extravascular potential largely untapped.

Methods: Nanobubble acoustic scattering was assessed using a recently reported ultra-stable formulation at low concentration (10⁶ mL^-1) and frequency (1 MHz), over a range of pressures (100-1500 kPa) in a channel phantom. The pressure-dependent response was utilized as a basis for in vivo experiments where ultrasound transmitters and receivers were integrated into a window chamber for simultaneous intravital multiphoton microscopy and acoustic monitoring in tumor-affected microcirculation. Microscopy and acoustic data were utilized to assess passive and active delivery of nanobubbles and determine whether they remained intact beyond the vasculature.

Results: Nanobubbles exhibit pressure-dependent nonlinear acoustic scattering. Nanobubbles are also found to have prolonged acoustic vascular pharmacokinetics, and passively extravasate intact into tumors. Ultrasound stimulation of nanobubbles is shown to actively enhance the delivery of both intact nanobubbles and shell material, increasing their spatial bioavailability deeper into the extravascular space. A range of acute vascular effects were also observed.

Conclusion: This study presents the first direct evidence that nanobubbles passively and actively extravasate intact in tumor tissue, and is the first to directly capture acute vascular events from ultrasound-stimulation of nanobubbles. The insights gained here demonstrate an important step towards unlocking the potential of nanobubbles and extending ultrasound-based applications.

Ultrasound Contrast Agent Modeling: A Review

Ultrasound is extensively used in medical imaging, being safe and inexpensive and operating in real time. Its scope of applications has been widely broadened by the use of ultrasound contrast agents (UCAs) in the form of microscopic bubbles coated by a biocompatible shell. Their increased use has motivated a large amount of research to understand and characterize their physical properties as well as their interaction with the ultrasound field and their surrounding environment. Here we review the theoretical models that have been proposed to study and predict the behavior of UCAs. We begin with a brief introduction on the development of UCAs. We then present the basics of free-gas-bubble dynamics upon which UCA modeling is based. We review extensively the linear and non-linear models for shell elasticity and viscosity and present models for non-spherical and asymmetric bubble oscillations, especially in the presence of surrounding walls or tissue. Then, higher-order effects such as microstreaming, shedding and acoustic radiation forces are considered. We conclude this review with promising directions for the modeling and development of novel agents.

Numerical and experimental investigation of impacts of nonlinear scattering encapsulated microbubbles on Nakagami distribution

PURPOSE

The Nakagami statistical model and Nakagami shape parameter m have been widely used in linear tissue characterization and preliminarily characterized the envelope distributions of nonlinear encapsulated microbubbles (EMBs). However, the Nakagami distribution of nonlinear scattering EMBs lacked a systematical investigation. Thus, this study aimed to investigate the Nakagami distribution of EMBs and illustrate the impact of EMBs' nonlinearity on the Nakagami model.

METHOD

A group of simulated EMB phantoms and in vitro EMB dilutions with an increasing concentration distribution under various EMB nonlinearities, as regulated by acoustic parameters, were characterized by using the window-modulated compounding Greenwood-Durand estimator.

RESULTS

Raw envelope histograms of simulated and in vitro EMBs were well matched with the Nakagami distribution with a high correlation coefficient of 0.965 ± 0.021 (P < 0.005). The mean values and gradients of m parameters of simulated and in vitro EMBs were smaller than those of linear scatterers due to the stronger nonlinearity. These m values exhibited a quasi-linear improvement with the increase in second harmonic nonlinear-to-linear component ratio regulated by pulse lengths and excitation frequencies at low- and high-concentration conditions.

CONCLUSIONS

The Nakagami distribution was suitable for the EMBs characterization but the corresponding m parameter was affected by the EMBs' nonlinearity. These validations provided support and nonlinear impact assessment for the EMBs' characterization using the Nakagami statistical model in the future.

Delivering Focused Ultrasound to Intervertebral Discs Using Time-Reversal

Chronic low back pain causes more disability worldwide than any other condition and is thought to arise in part through loss of biomechanical function of degenerate intervertebral discs (IVDs). Current treatments can involve replacing part or all of the degenerate IVDs by invasive surgery. Our vision is to develop a minimally invasive approach in which high intensity focused ultrasound (HIFU) is used to mechanically fractionate degenerate tissue in an IVD; a fine needle is then used to first remove the fractionated tissue and then inject a biomaterial able to restore normal physiologic function. The goal of this manuscript is to demonstrate the feasibility of trans-spinal HIFU delivery using simulations of 3-D ultrasound propagation in models derived from patient computed tomography (CT) scans. The CT data were segmented into bone, fat and other soft tissue for three patients. Ultrasound arrays were placed around the waist of each patient model, and time-reversal was used to determine the source signals necessary to create a focus in the center of the disc. The simulations showed that for 0.5 MHz ultrasound, a focus could be created in most of the lumbar IVDs, with the pressure focal gain ranging from 3.2-13.7. In conclusion, it is shown that with patient-specific planning, focusing ultrasound into an IVD is possible in the majority of patients despite the complex acoustic path introduced by the bony structures of the spine.

Minireview: Biophysical Mechanisms of Cell Membrane Sonopermeabilization. Knowns and Unknowns

Microbubble-assisted ultrasound has emerged as a promising method for the delivery of low-molecular-weight chemotherapeutic molecules, nucleic acids, therapeutic peptides, and antibodies in vitro and in vivo. Its clinical applications are under investigation for local delivery drug in oncology and neurology. However, the biophysical mechanisms supporting the acoustically mediated membrane permeabilization are not fully established. This review describes the present state of the investigations concerning the acoustically mediated stimuli (i.e., mechanical, chemical, and thermal stimuli) as well as the molecular and cellular actors (i.e., membrane pores and endocytosis) involved in the reversible membrane permeabilization process. The different hypotheses, which were proposed to give a biophysical description of the membrane permeabilization, are critically discussed.

The effect of resonance on transient microbubble acoustic response: Experimental observations and numerical simulations

A large number of acoustic signals from single lipid-shelled Definity^® (Lantheus Medical Imaging, N. Billerica, MA) microbubbles have been measured using a calibrated microacoustic system, and a unique transient characteristic of resonance has been identified in the onset of scatter. Comparison of the numerically obtained response of microbubbles with acoustic measurements provides good agreement for a soft shell that is characterized by small area dilatation modulus and strain softening behavior, and identifies time to maximum radial excursion and scatter as a robust marker of resonance during transient response. As the sound amplitude increases a two-population pattern emerges in the time delay vs the fundamental acoustic scatter plots, consisting of an initial part pertaining to microbubbles with less than resonant rest radii, which corresponds to the weaker second harmonic resonance, and the dominant resonant envelope pertaining to microbubbles with resonant and greater than resonant rest radii, which corresponds to the primary and subharmonic resonances. Consequently, a wider resonant spectrum is observed. It is a result of the strain softening nature of soft lipid shells, based on which the microbubble sizes corresponding to the above resonances decrease as the sound amplitude increases. This bares an impact on the selection of an optimal microbubble size pertaining to subharmonic imaging.

Combining ultrasound and intratumoral administration of doxorubicin-loaded microspheres to enhance tumor cell killing

Melanoma is an incurable disease for which alternative treatments to chemotherapy alone are sought. Here, using a melanoma model, we investigated the antitumor potential of combining ultrasound (US) with poly(lactic-co-glycolic acid) (PLGA) microspheres loaded with doxorubicin (DOX). The aim was to achieve synergistic tumoricidal activity through direct and indirect US-mediated damage of tumor cells combined with sustained and potentially controllable release (when combined with US) of DOX from microspheres. An in vitro release assay demonstrated an ability of US to affect the release kinetics of DOX from DOX-loaded PLGA microspheres by inducing a 12% increase in the rate of release. In vitro viability assays demonstrated that combining US with DOX-loaded PLGA microspheres resulted in synergistic tumor cell (B16-F10 melanoma cells) killing. Melanoma-bearing mice were treated intratumorally with DOX (8 µg)-loaded microspheres and subjected to US treatment at the tumor site. This treatment could significantly extend survival (mean survival (MS) = 22.1 days) compared to untreated mice (MS = 10.4 days) and most other treatments, such as blank microspheres plus US (MS = 11.5 days) and DOX (8 µg)-loaded microspheres alone (MS = 13 days). The findings that immune checkpoint blockade did not significantly extend survival of mice treated with DOX (8 µg)-loaded microspheres plus US, and that tumor-free ("cured") mice were not protected from subsequent tumor rechallenge suggests minimal involvement of the adaptive immune response in the observed antitumor activity. Nevertheless, the synergistic increase in survival of melanoma-challenged mice treated with the combination of US and DOX-loaded microspheres implicates such a treatment methodology as a promising additional tool for combatting otherwise currently incurable cancers.

Mechanistic Insight into Sonoporation with Ultrasound-Stimulated Polymer Microbubbles

Sonoporation is emerging as a feasible, non-viral gene delivery platform for the treatment of cardiovascular disease and cancer. Despite promising results, this approach remains less efficient than viral methods. The objective of this work is to help substantiate the merit of polymeric microbubble sonoporation as a non-viral, localized cell permeation and payload delivery strategy by taking a ground-up approach to elucidating the fundamental mechanisms at play. In this study, we apply simultaneous microscopy of polymeric microbubble sonoporation over its intrinsic biophysical timescales-with sub-microsecond resolution to examine microbubble cavitation and millisecond resolution over several minutes to examine local macromolecule uptake through enhanced endothelial cell membrane permeability-bridging over six orders of magnitude in time. We quantified microbubble behavior and resulting sonoporation thresholds at transmit frequencies of 0.5, 1 and 2 MHz, and determined that sonic cracking is a necessary but insufficient condition to induce sonoporation. Further, sonoporation propensity increases with the extent of sonic cracking, namely, from partial to complete gas escape from the polymeric encapsulation. For the subset that exhibited complete gas escape from sonic cracking, a proportional relationship between the maximum projected gas area and resulting macromolecule uptake was observed. These results have revealed one aspect of polymeric bubble activity on the microsecond time scale that is associated with eliciting sonoporation in adjacent endothelial cells, and contributes toward an understanding of the physical rationale for sonoporation with polymer-encapsulated microbubble contrast agents.

Cryo-EM Visualization of Lipid and Polymer-Stabilized Perfluorocarbon Gas Nanobubbles - A Step Towards Nanobubble Mediated Drug Delivery

Gas microbubbles stabilized with lipids, surfactants, proteins and/or polymers are widely used clinically as ultrasound contrast agents. Because of their large 1-10 µm size, applications of microbubbles are confined to the blood vessels. Accordingly, there is much interest in generating nanoscale echogenic bubbles (nanobubbles), which can enable new uses of ultrasound contrast agents in molecular imaging and drug delivery, particularly for cancer applications. While the interactions of microbubbles with ultrasound have been widely investigated, little is known about the activity of nanobubbles under ultrasound exposure. In this work, we demonstrate that cryo-electron microscopy (cryo-EM) can be used to image nanoscale lipid and polymer-stabilized perfluorocarbon gas bubbles before and after their destruction with high intensity ultrasound. In addition, cryo-EM can be used to observe electron-beam induced dissipation of nanobubble encapsulated perfluorocarbon gas.

Ultrasound‑targeted microbubbles combined with a peptide nucleic acid binding nuclear localization signal mediate transfection of exogenous genes by improving cytoplasmic and nuclear import

The development of an efficient delivery system is critical for the successful treatment of cardiovascular diseases using non-viral gene therapies. Cytoplasmic and nuclear membrane barriers reduce delivery efficiency by impeding the transfection of foreign genes. Thus, a gene delivery system capable of transporting exogenous genes may improve gene therapy. The present study used a novel strategy involving ultrasound-targeted microbubbles and peptide nucleic acid (PNA)-binding nuclear localization signals (NLS). Ultrasound-targeted microbubble destruction (UTMD) and PNA-binding NLS were used to improve the cytoplasmic and nuclear importation of the plasmid, respectively. Experiments were performed using antibody-targeted microbubbles (AT-MCB) that specifically recognize the SV40T antigen receptor expressed on the membranes of 293T cells, resulting in the localization of ultrasound microbubbles to 293T cell membranes. Furthermore, PNA containing NLS was inserted into the enhanced green fluorescent protein (EGFP)-N3 plasmid DNA (NLS-PNA-DNA), which increased nuclear localization. The nuclear import and gene expression efficiency of the AT-MCB with PNA-binding NLS were compared with AT-MCB alone or a PNA-binding NLS. The effect of the AT-MCB containing PNA-binding NLS on transfection was investigated. The ultrasound and AT-MCB delivery significantly enhanced the cytoplasmic intake of exogenous genes and maintained high cell viability. The nuclear import and gene expression of combined microbubble- and PNA-transfected cells were significantly greater compared with cells that were transfected with AT-MCB or DNA with only PNA-binding NLS. The quantity of EGFP-N3 plasmids in the nuclei was increased by >5.0-fold compared with control microbubbles (CMCB) and NLS-free plasmids. The gene expression was ~1.7-fold greater compared with NLS-free plasmids and 1.3-fold greater compared with control microbubbles. In conclusion, UTMD combined with AT-MCB and a PNA-binding NLS plasmid significantly improved transfection efficiency by increasing cytoplasmic and nuclear DNA import. This method is a promising strategy for the noninvasive and effective delivery of target genes or drugs for the treatment of cardiovascular diseases.

Characterization of Contrast Agent Microbubbles for Ultrasound Imaging and Therapy Research

The high efficiency with which gas microbubbles can scatter ultrasound compared with the surrounding blood pool or tissues has led to their widespread employment as contrast agents in ultrasound imaging. In recent years, their applications have been extended to include super-resolution imaging and the stimulation of localized bio-effects for therapy. The growing exploitation of contrast agents in ultrasound and in particular these recent developments have amplified the need to characterize and fully understand microbubble behavior. The aim in doing so is to more fully exploit their utility for both diagnostic imaging and potential future therapeutic applications. This paper presents the key characteristics of microbubbles that determine their efficacy in diagnostic and therapeutic applications and the corresponding techniques for their measurement. In each case, we have presented information regarding the methods available and their respective strengths and limitations, with the aim of presenting information relevant to the selection of appropriate characterization methods. First, we examine methods for determining the physical properties of microbubble suspensions and then techniques for acoustic characterization of both suspensions and single microbubbles. The next section covers characterization of microbubbles as therapeutic agents, including as drug carriers for which detailed understanding of their surface characteristics and drug loading capacity is required. Finally, we discuss the attempts that have been made to allow comparison across the methods employed by various groups to characterize and describe their microbubble suspensions and promote wider discussion and comparison of microbubble behavior.

Optical Verification of Microbubble Response to Acoustic Radiation Force in Large Vessels With In Vivo Results

OBJECTIVE

The objective of this study was to optically verify the dynamic behaviors of adherent microbubbles in large blood vessel environments in response to a new ultrasound technique using modulated acoustic radiation force.

MATERIALS AND METHODS

Polydimethylsiloxane (PDMS) flow channels coated with streptavidin were used in targeted groups to mimic large blood vessels. The custom-modulated acoustic radiation force beam sequence was programmed on a Verasonics research scanner. In vitro experiments were performed by injecting a biotinylated lipid-perfluorobutane microbubble dispersion through flow channels. The dynamic response of adherent microbubbles was detected acoustically and simultaneously visualized using a video camera connected to a microscope. In vivo verification was performed in a large abdominal blood vessel of a murine model for inflammation with injection of biotinylated microbubbles conjugated with P-selectin antibody.

RESULTS

Aggregates of adherent microbubbles were observed optically under the influence of acoustic radiation force. Large microbubble aggregates were observed solely in control groups without targeted adhesion. Additionally, the dispersion of microbubble aggregates were demonstrated to lead to a transient acoustic signal enhancement in control groups (a new phenomenon we refer to as "control peak"). In agreement with in vitro results, the control peak phenomenon was observed in vivo in a murine model.

CONCLUSIONS

This study provides the first optical observation of microbubble-binding dynamics in large blood vessel environments with application of a modulated acoustic radiation force beam sequence. With targeted adhesion, secondary radiation forces were unable to produce large aggregates of adherent microbubbles. Additionally, the new phenomenon called control peak was observed both in vitro and in vivo in a murine model for the first time. The findings in this study provide us with a better understanding of microbubble behaviors in large blood vessel environments with application of acoustic radiation force and could potentially guide future beam sequence designs or signal processing routines for enhanced ultrasound molecular imaging.

Fluid Viscosity Affects the Fragmentation and Inertial Cavitation Threshold of Lipid-Encapsulated Microbubbles

Ultrasound and microbubble optimization studies for therapeutic applications are often conducted in water/saline, with a fluid viscosity of 1 cP. In an in vivo context, microbubbles are situated in blood, a more viscous fluid (∼4 cP). In this study, ultrahigh-speed microscopy and passive cavitation approaches were employed to investigate the effect of fluid viscosity on microbubble behavior at 1 MHz subject to high pressures (0.25-2 MPa). The propensity for individual microbubble (n = 220) fragmentation was found to significantly decrease in 4-cP fluid compared with 1-cP fluid, despite achieving similar maximum radial excursions. Microbubble populations diluted in 4-cP fluid exhibited decreased wideband emissions (up to 10.2 times), and increasingly distinct harmonic emission peaks (e.g., ultraharmonic) with increasing pressure, compared with those in 1-cP fluid. These results suggest that in vitro studies should consider an evaluation using physiologic viscosity perfusate before transitioning to in vivo evaluations.

In vitro methods to study bubble-cell interactions: Fundamentals and therapeutic applications

Besides their use as contrast agents for ultrasound imaging, microbubbles are increasingly studied for a wide range of therapeutic applications. In particular, their ability to enhance the uptake of drugs through the permeabilization of tissues and cell membranes shows great promise. In order to fully understand the numerous paths by which bubbles can interact with cells and the even larger number of possible biological responses from the cells, thorough and extensive work is necessary. In this review, we consider the range of experimental techniques implemented in in vitro studies with the aim of elucidating these microbubble-cell interactions. First of all, the variety of cell types and cell models available are discussed, emphasizing the need for more and more complex models replicating in vivo conditions together with experimental challenges associated with this increased complexity. Second, the different types of stabilized microbubbles and more recently developed droplets and particles are presented, followed by their acoustic or optical excitation methods. Finally, the techniques exploited to study the microbubble-cell interactions are reviewed. These techniques operate over a wide range of timescales, or even off-line, revealing particular aspects or subsequent effects of these interactions. Therefore, knowledge obtained from several techniques must be combined to elucidate the underlying processes.

Ultrasound-Propelled Nanocups for Drug Delivery

Ultrasound-induced bubble activity (cavitation) has been recently shown to actively transport and improve the distribution of therapeutic agents in tumors. However, existing cavitation-promoting agents are micron-sized and cannot sustain cavitation activity over prolonged time periods because they are rapidly destroyed upon ultrasound exposure. A novel ultrasound-responsive single-cavity polymeric nanoparticle (nanocup) capable of trapping and stabilizing gas against dissolution in the bloodstream is reported. Upon ultrasound exposure at frequencies and intensities achievable with existing diagnostic and therapeutic systems, nanocups initiate and sustain readily detectable cavitation activity for at least four times longer than existing microbubble constructs in an in vivo tumor model. As a proof-of-concept of their ability to enhance the delivery of unmodified therapeutics, intravenously injected nanocups are also found to improve the distribution of a freely circulating IgG mouse antibody when the tumor is exposed to ultrasound. Quantification of the delivery distance and concentration of both the nanocups and coadministered model therapeutic in an in vitro flow phantom shows that the ultrasound-propelled nanocups travel further than the model therapeutic, which is itself delivered to hundreds of microns from the vessel wall. Thus nanocups offer considerable potential for enhanced drug delivery and treatment monitoring in oncological and other biomedical applications.

On the relationship between microbubble fragmentation, deflation and broadband superharmonic signal production

Acoustic angiography imaging of microbubble contrast agents uses the superharmonic energy produced from excited microbubbles and enables high-contrast, high-resolution imaging. However, the exact mechanism by which broadband harmonic energy is produced is not fully understood. To elucidate the role of microbubble shell fragmentation in superharmonic signal production, simultaneous optical and acoustic measurements were performed on individual microbubbles at transmit frequencies from 1.75 to 3.75 MHz and pressures near the shell fragmentation threshold for microbubbles of varying diameter. High-amplitude, broadband superharmonic signals were produced with shell fragmentation, whereas weaker signals (approximately 25% of peak amplitude) were observed in the presence of shrinking bubbles. Furthermore, when populations of stationary microbubbles were imaged with a dual-frequency ultrasound imaging system, a sharper decline in image intensity with respect to frame number was observed for 1-μm bubbles than for 4-μm bubbles. Finally, in a study of two rodents, increasing frame rate from 4 to 7 Hz resulted in decreases in mean steady-state image intensity of 27% at 1000 kPa and 29% at 1300 kPa. Although the existence of superharmonic signals when bubbles shrink has the potential to prolong the imaging efficacy of microbubbles, parameters such as frame rate and peak pressure must be balanced with expected re-perfusion rate to maintain adequate contrast during in vivo imaging.

Unique pumping-out fracturing mechanism of a polymer-shelled contrast agent: an acoustic characterization and optical visualization

This work describes the fracturing mechanism of air-filled microbubbles (MBs) encapsulated by a cross-linked poly(vinyl alcohol) (PVA) shell. The radial oscillation and fracturing events following the ultrasound exposure were visualized with an ultrahigh-speed camera, and backscattered timedomain signals were acquired with the acoustic setup specific for harmonic detection. No evidence of gas emerging from defects in the shell with the arrival of the first insonation burst was found. In optical recordings, more than one shell defect was noted, and the gas core was drained without any sign of air extrusion when several consecutive bursts of 1 MPa amplitude were applied. In acoustic tests, the backscattered peak-to-peak voltage gradually reached its maximum and exponentially decreased when the PVA-based MB suspension was exposed to approximately 20 consecutive bursts arriving at pulse repetition frequencies of 100 and 500 Hz. Taking into account that the PVA shell is porous and possibly contains large air pockets between the cross-linked PVA chains, the aforementioned acoustic behavior might be attributed to pumping gas from these pockets in combination with gas release from the core through shell defects. We refer to this fracturing mechanism as pumping-out behavior, and this behavior could have potential use for the local delivery of therapeutic gases, such as nitric oxide.

Correlation of rupture dynamics to the nonlinear backscatter response from polymer-shelled ultrasound contrast agents

Polymer-shelled ultrasound contrast agents (UCAs) may expel their encapsulated gas subject to ultrasound-induced shell buckling or rupture. Nonlinear oscillations of this gas bubble can produce a subharmonic component in the ultrasound backscatter. This study investigated the relationship between this gas-release mechanism and shell-thickness-to-radius ratios (STRRs) of polymer-shelled UCAs. Three types of polylactide-shelled UCAs with STRRs of 7.5, 40, and 100 nm/μm were studied. Each UCA population had a nominal mean diameter of 2 μm. UCAs were subjected to increasing static overpressure ranging from 2 to 330 kPa over a duration of 2 h in a custom-designed test chamber while being imaged using a 200× magnification video microscope at a frame rate of 5 frames/s. Digitized video images were binarized and processed to obtain the cross-sectional area of individual UCAs. Integration of the normalized cross-sectional area over normalized time, defined as buckling factor (Bf), provided a dimensionless parameter for quantifying and comparing the degree of pre-rupture buckling exhibited by the UCAs of different STRRs in response to overpressure. The UCAs with an STRR of 7.5 nm/μm exhibited a distinct shell-buckling phase before shell rupture (Bf < 1), whereas the UCAs with higher STRRs (40 and 100 nm/μm) did not undergo significant prerupture buckling (Bf ≈ 1). The difference in the overpressure response was correlated with the subharmonic response produced by these UCAs. When excited using 20-MHz ultrasound, individual UCAs (N = 3000) in populations that did not exhibit a buckling phase produced a subharmonic response that was an order of magnitude greater than the UCA population with a prominent pre-rupture buckling phase. These results indicate the mechanism of gas expulsion from these UCAs might be a relevant factor in determining the level of subharmonic response in response to high-frequency ultrasound.

Trigger responsive polymeric nanocarriers for cancer therapy

Conventional chemotherapy for the treatment of cancer has limited specificity when administered systemically and is often associated with toxicity issues. Enhanced accumulation of polymeric nanocarriers at a tumor site may be achieved by passive and active targeting. Incorporation of trigger responsiveness into these polymeric nanocarriers improves the anticancer efficacy of such systems by modulating the release of the drug according to the tumor environment. Triggers used for tumor targeting include internal triggers such as pH, redox and enzymes and external triggers such as temperature, magnetic field, ultrasound and light. While internal triggers are specific cues of the tumor microenvironment, external triggers are those which are applied externally to control the release. This review highlights the various strategies employed for the preparation of such trigger responsive polymeric nanocarriers for cancer therapy and provides an overview of the state of the art in this field.

Ultrasound-Targeted Microbubble Destruction Enhances Gene Expression of microRNA-21 in Swine Heart via Intracoronary Delivery

BACKGROUND

Ultrasound-targeted microbubble destruction (UTMD) has proved to be a promising method for gene delivery. However, the feasibility and efficacy of UTMD-mediated gene delivery to the heart of large animals remain unclear. The present study was to explore the probability of increasing the transfection of microRNA-21 (miR-21) in swine heart by UTMD, and to search for the most suitable transfection conditions.

METHODS

We first optimized ultrasound intensity for successful miR-21 delivery. After intravenous injection of miR-21/microbubble mixture (miR-21/MB), transthoracic ultrasound irradiation (US) was applied from the left anterior chest using different intensities (1, 2, and 3 W/cm(2)). Then the efficacy of UTMD-mediated miR-21 delivery into myocardium via intracoronary injection was explored. Solution of miR-21/MB was infused intravenously or intracoronarily with US over the heart. Swine undergoing phosphate-buffered saline (PBS) injection, miR-21/MB injection via ear vein or coronary artery without US served as the control. The dynamic changes of left ventricular ejection fraction (LVEF) and serum troponin I (cTnI) after UTMD were detected, then the left ventricular myocardium was harvested for hematoxylin and eosin (H&E) staining 4 days later; the expression levels of miR-21 and programmed cell death 4 (PDCD4) were detected by quantitative real time polymerase chain reaction (qRT-PCR) and Western blot, respectively.

RESULTS

Results showed that pulse ultrasound at an intensity of 2 W/cm(2) and a 50% duty ratio for 20 minutes, there was no increase in serum cTnI, no histological sign of myocardial damage, and no noted cardiac dysfunction with relatively higher miR-21 expression (P < 0.05). Compared to miR-21/MB alone, UTMD significantly increased gene expression in myocardium regardless of the delivery routes (P < 0.05). Interestingly, the transfection efficiency was found to be a little bit higher with intracoronary injection than that with intravenous injection, though the dose for intracoronary injection was half of the intravenous injection (P < 0.05).

CONCLUSION

Under suitable conditions, UTMD can efficiently enhance gene expression in swine heart regardless of the delivery routes. The intravenous injection might be superior to intracoronary injection with less invasiveness and lower requirement of the technique. And for those undergoing percutaneous coronary intervention, intracoronary injection seems to be another alternative.

A setup for the assessment of the effect of tubular confinement on the acoustic response of microbubbles

Ultrasound contrast agents are gas filled microbubbles which produced enhanced echoes in ultrasound imaging thus allowing the acquisition of detailed information on the path of blood. It is theoretically known that the size of a vessel affects the behavior of a microbubble, which could potentially be used to discriminate different sized vessels. This information would be useful in the monitoring of neovascularization in tumor growth or treatment. However, currently it is not possible to identify the vessel diameter by any means of signal processing of microbubble echoes. In order to assess microbubble behavior when confined in tubes we compared the acoustic backscatter from biSphere™ microbubbles both free in water and flowing in 200 μm diameter tubes that are similar in size to arterioles. Experimental systems that allow the interrogation of individual microbubbles were designed and modified to allow investigation of both free microbubbles and those in tubes. Unprocessed single microbubble RF data were collected, allowing the calculation of both the fundamental and second harmonic components of the backscattered signal. Microbubbles confined in tubes had lower amplitude response compared to unconfined microbubbles. On consecutive insonations of the same microbubble, free microbubbles produced echoes above noise more often than confined microbubbles. This setup may be used to investigate microbubble behavior in a range of smaller tubes with diameters similar to capillaries thus enabling signal processing design for vessel differentiation.

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.

The polydisperse acoustic signature of rigid microbubbles

Microbubbles are used in medical ultrasound imaging as contrast agents to image the vascular bed under the mode of Ultrasound Contrast Imaging (UCI). The microbubble shell determines the acoustic response and hence the signal that is utilized to form the images in UCI. Single microbubble signals from BiSphere™ (POINT Biomedical, San Carlos, CA, USA) microbubbles were captured using a clinical ultrasound system. Three main typical responses of microbubbles were identified, a) full duration echo, b) echo with duration shorter than the incident pulse and c) echo that in part resembles that in (b) and in addition prior to that another short duration initial lower amplitude signal. These data corroborate that the shell structural and nanomechanical property provide the different responses at different microbubble sizes. These different signals present an opportunity for tracking the movement of well differentiated single microbubbles particularly with novel super-resolution imaging methods that require sparse microbubble populations.

Ultrasound-based measurement of molecular marker concentration in large blood vessels: a feasibility study

Ultrasound molecular imaging has demonstrated efficacy in pre-clinical studies for cancer and cardiovascular inflammation. However, these techniques often require lengthy protocols because of waiting periods or additional control microbubble injections. Moreover, they are not capable of quantifying molecular marker concentration in human tissue environments that exhibit variable attenuation and propagation path lengths. Our group recently investigated a modulated acoustic radiation force-based imaging sequence, which was found to detect targeted adhesion independent of control measurements. In the present study, this sequence was tested against various experimental parameters to determine its feasibility for quantitative measurements of molecular marker concentration. Results indicated that measurements obtained from the sequence (residual-to-saturation ratio, Rresid) were independent of acoustic pressure and attenuation (p > 0.13, n = 10) when acoustic pressures were sufficiently low. The Rresid parameter exhibited a linear relationship with measured molecular marker concentration (R(2) > 0.94). Consequently, feasibility was illustrated in vitro, for quantification of molecular marker concentration in large vessels using a modulated acoustic radiation force-based sequence. Moreover, these measurements were independent of absolute acoustic reflection amplitude and used short imaging protocols (3 min) without control measurements.

Microbubbles for Medical Applications

Ultrasound contrast agent (UCA) suspensions contain encapsulated microbubbles with radii ranging from 1 to 10 micrometers. The bubbles oscillate to the driving ultrasound pulse generating harmonics of the driving ultrasound frequency. This feature allows for the discrimination of non-linear bubble echoes from linear tissue echoes facilitating the visualization and quantification of blood perfusion in organs. Targeting the microbubbles to specific receptors in the body has led to molecular imaging application with ultrasound and targeted drug delivery with drug-loaded microbubbles. Traditional UCA production methods offer high yield but poor control over the microbubble size and uniformity. Medical ultrasound transducers typically operate at a single frequency, therefore only a small selection of bubbles resonates to the driving ultrasound pulse. Here we discuss recent lab-on-a-chip based production and sorting methods that have been shown to produce highly monodisperse bubbles, thereby improving the sensitivity of contrast-enhanced ultrasound imaging and molecular imaging with microbubbles. Moreover, monodisperse UCA show great potential for targeted drug delivery by the well-controlled bubble response.

Multi-modal detection of colon malignancy by NIR-tagged recognition polymers and ultrasound contrast agents

To increase colonoscopy capability to discriminate benign from malignant polyps, we suggest combining two imaging approaches based on targeted polymeric platforms. Water-soluble cationized polyacrylamide (CPAA) was tagged with the near infrared (NIR) dye IR-783-S-Ph-COOH to form Flu-CPAA. The recognition peptide VRPMPLQ (reported to bind specifically to CRC tissues) was then conjugated with the Flu-CPAA to form Flu-CPAA-Pep which was then incorporated into echogenic microbubbles (MBs) made of polylactic acid (PLA) that are highly responsive to ultrasound. The ultimate design includes intravenous administration combined with local ultrasound and intra-colon inspection at the NIR range. In this proof of principle study PLA MBs were prepared by the double emulsion technique and loaded with several types of Flu-CPAA-Pep polymers. After insonation the submicron PLA fragments (SPF)-containing Flu-CPAA-Pep were examined in vitro for their ability to attach to colon cancer cells and in vivo (DMH induced rat model) for their ability to attach to colon malignant tissues and compared to the specific attachment of the free Flu-CPAA-Pep. The generation of SPF-containing Flu-CPAA-Pep resulted in a tissue attachment similar to that of the free, unloaded Flu-CPAA-Pep. The addition of VRPMPLQ to the polymeric backbone of the Flu-CPAA reduced cytotoxicity and improved the specific binding.

Ultrasound and microbubble guided drug delivery: mechanistic understanding and clinical implications

Ultrasound mediated drug delivery using microbubbles is a safe and noninvasive approach for spatially localized drug administration. This approach can create temporary and reversible openings on cellular membranes and vessel walls (a process called "sonoporation"), allowing for enhanced transport of therapeutic agents across these natural barriers. It is generally believed that the sonoporation process is highly associated with the energetic cavitation activities (volumetric expansion, contraction, fragmentation, and collapse) of the microbubble. However, a thorough understanding of the process was unavailable until recently. Important progress on the mechanistic understanding of sonoporation and the corresponding physiological responses in vitro and in vivo has been made. Specifically, recent research shed light on the cavitation process of microbubbles and fluid motion during insonation of ultrasound, on the spatio-temporal interactions between microbubbles and cells or vessel walls, as well as on the temporal course of the subsequent biological effects. These findings have significant clinical implications on the development of optimal treatment strategies for effective drug delivery. In this article, current progress in the mechanistic understanding of ultrasound and microbubble mediated drug delivery and its implications for clinical translation is discussed.

Ultrasound induced cancer immunotherapy

Recently, the use of ultrasound (US) has been shown to have potential in cancer immunotherapy. High intensity focused US destruction of tumors may lead to immunity forming in situ in the body by immune cells being exposed to the tumor debris and immune stimulatory substances that are present in the tumor remains. Another way of achieving anti-cancer immune responses is by using US in combination with microbubbles and nanobubbles to deliver genes and antigens into cells. US leads to bubble destruction and the forces released to direct delivery of the substances into the cytoplasm of the cells thus circumventing the natural barriers. In this way tumor antigens and antigen-encoding genes can be delivered to immune cells and immune response stimulating genes can be delivered to cancer cells thus enhancing immune responses. Combination of bubbles with cell-targeting ligands and US provides an even more sophisticated delivery system whereby the therapy is not only site specific but also cell specific. In this review we describe how US has been used to achieve immunity and discuss the potential and possible obstacles in future development.

Acoustic behavior of microbubbles and implications for drug delivery

Ultrasound contrast agents are valuable in diagnostic ultrasound imaging, and they increasingly show potential for drug delivery. This review focuses on the acoustic behavior of flexible-coated microbubbles and rigid-coated microcapsules and their contribution to enhanced drug delivery. Phenomena relevant to drug delivery, such as non-spherical oscillations, shear stress, microstreaming, and jetting will be reviewed from both a theoretical and experimental perspective. Further, the two systems for drug delivery, co-administration and the microbubble as drug carrier system, are reviewed in relation to the microbubble behavior. Finally, future prospects are discussed that need to be addressed for ultrasound contrast agents to move from a pre-clinical tool into a clinical setting.

Understanding ultrasound induced sonoporation: definitions and underlying mechanisms

In the past two decades, research has underlined the potential of ultrasound and microbubbles to enhance drug delivery. However, there is less consensus on the biophysical and biological mechanisms leading to this enhanced delivery. Sonoporation, i.e. the formation of temporary pores in the cell membrane, as well as enhanced endocytosis is reported. Because of the variety of ultrasound settings used and corresponding microbubble behavior, a clear overview is missing. Therefore, in this review, the mechanisms contributing to sonoporation are categorized according to three ultrasound settings: i) low intensity ultrasound leading to stable cavitation of microbubbles, ii) high intensity ultrasound leading to inertial cavitation with microbubble collapse, and iii) ultrasound application in the absence of microbubbles. Using low intensity ultrasound, the endocytotic uptake of several drugs could be stimulated, while short but intense ultrasound pulses can be applied to induce pore formation and the direct cytoplasmic uptake of drugs. Ultrasound intensities may be adapted to create pore sizes correlating with drug size. Small molecules are able to diffuse passively through small pores created by low intensity ultrasound treatment. However, delivery of larger drugs such as nanoparticles and gene complexes, will require higher ultrasound intensities in order to allow direct cytoplasmic entry.

Binding dynamics of targeted microbubbles in response to modulated acoustic radiation force

Detection of molecular targeted microbubbles plays a foundational role in ultrasound-based molecular imaging and targeted gene or drug delivery. In this paper, an empirical model describing the binding dynamics of targeted microbubbles in response to modulated acoustic radiation forces in large vessels is presented and experimentally verified using tissue-mimicking flow phantoms. Higher flow velocity and microbubble concentration led to faster detaching rates for specifically bound microbubbles (p < 0.001). Higher time-averaged acoustic radiation force intensity led to faster attaching rates and a higher saturation level of specifically bound microbubbles (p < 0.05). The level of residual microbubble signal in targeted experiments after cessation of radiation forces was the only response parameter that was reliably different between targeted and control experiments (p < 0.05). A related parameter, the ratio of residual-to-saturated microbubble signal (Rresid), is proposed as a measurement that is independent of absolute acoustic signal magnitude and therefore able to reliably detect targeted adhesion independently of control measurements (p < 0.01). These findings suggest the possibility of enhanced detection of specifically bound microbubbles in real-time, using relatively short imaging protocols (approximately 3 min), without waiting for free microbubble clearance.

Relationship between cavitation and loss of echogenicity from ultrasound contrast agents

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

Ultrasound contrast materials in cardiovascular medicine: from perfusion assessment to molecular imaging

Ultrasound imaging is widely used in cardiovascular diagnostics. Contrast agents expand the range of tasks that ultrasound can perform. In the clinic in the USA, endocardial border delineation and left ventricle opacification have been an approved indication for more than a decade. However, myocardial perfusion contrast ultrasound studies are still at the clinical trials stage. Blood pool contrast and perfusion in other tissues might be an easier indication to achieve: general blood pool ultrasound contrast is in wider use in Europe, Canada, Japan, and China. Targeted (molecular) contrast microbubbles will be the next generation of ultrasound imaging probes, capable of specific delineation of the areas of disease by adherence to molecular targets. The shell of targeted microbubbles (currently in the preclinical research and early stage clinical trials) is decorated with the ligands (antibodies, peptides or mimetics, hormones, and carbohydrates) that ensure firm binding to the molecular markers of disease.

Ultra-fast bright field and fluorescence imaging of the dynamics of micrometer-sized objects

High speed imaging has application in a wide area of industry and scientific research. In medical research, high speed imaging has the potential to reveal insight into mechanisms of action of various therapeutic interventions. Examples include ultrasound assisted thrombolysis, drug delivery, and gene therapy. Visual observation of the ultrasound, microbubble, and biological cell interaction may help the understanding of the dynamic behavior of microbubbles and may eventually lead to better design of such delivery systems. We present the development of a high speed bright field and fluorescence imaging system that incorporates external mechanical waves such as ultrasound. Through collaborative design and contract manufacturing, a high speed imaging system has been successfully developed at the University of Pittsburgh Medical Center. We named the system "UPMC Cam," to refer to the integrated imaging system that includes the multi-frame camera and its unique software control, the customized modular microscope, the customized laser delivery system, its auxiliary ultrasound generator, and the combined ultrasound and optical imaging chamber for in vitro and in vivo observations. This system is capable of imaging microscopic bright field and fluorescence movies at 25 × 10(6) frames per second for 128 frames, with a frame size of 920 × 616 pixels. Example images of microbubble under ultrasound are shown to demonstrate the potential application of the system.