Characterisation of Liposome-Loaded Microbubble Populations for Subharmonic Imaging

Development of exosome membrane materials-fused microbubbles for enhanced stability and efficient drug delivery of ultrasound contrast agent

Lipid-coated microbubbles are widely used as an ultrasound contrast agent, as well as drug delivery carriers. However, the two main limitations in ultrasound diagnosis and drug delivery using microbubbles are the short half-life in the blood system, and the difficulty of surface modification of microbubbles for active targeting. The exosome, a type of extracellular vesicle, has a preferentially targeting ability for its original cell. In this study, exosome-fused microbubbles (Exo-MBs) were developed by embedding the exosome membrane proteins into microbubbles. As a result, the stability of Exo-MBs is improved over the conventional microbubbles. On the same principle that under the exposure of ultrasound, microbubbles are cavitated and self-assembled into nano-sized particles, and Exo-MBs are self-assembled into exosome membrane proteins-embedded nanoparticles (Exo-NPs). The Exo-NPs showed favorable targeting properties to their original cells. A photosensitizer, chlorin e6, was loaded into Exo-MBs to evaluate therapeutic efficacy as a drug carrier. Much higher therapeutic efficacy of photodynamic therapy was confirmed, followed by cancer immunotherapy from immunogenic cell death. We have therefore developed a novel ultrasound image-guided drug delivery platform that overcomes the shortcomings of the conventional ultrasound contrast agent and is capable of simultaneous photodynamic therapy and cancer immunotherapy.

Formulation and characterisation of drug-loaded antibubbles for image-guided and ultrasound-triggered drug delivery

The aim of this study was to develop high load-capacity antibubbles that can be visualized using diagnostic ultrasound and the encapsulated drug can be released and delivered using clinically translatable ultrasound. The antibubbles were developed by optimising a silica nanoparticle stabilised double emulsion template. We produced an emulsion with a mean size diameter of 4.23 ± 1.63 µm where 38.9 ± 3.1% of the droplets contained a one or more cores. Following conversion to antibubbles, the mean size decreased to 2.96 ± 1.94 µm where 99% of antibubbles were <10 µm. The antibubbles had a peak attenuation of 4.8 dB/cm at 3.0 MHz at a concentration of 200 × 10³ particles/mL and showed distinct attenuation spikes at frequencies between 5.5 and 13.5 MHz. No increase in subharmonic response was observed for the antibubbles in contrast to SonoVue®. High-speed imaging revealed that antibubbles can release their cores at MIs of 0.6. In vivo imaging indicated that the antibubbles have a long half-life of 68.49 s vs. 40.02 s for SonoVue®. The antibubbles could be visualised using diagnostic ultrasound and could be disrupted at MIs of ≥0.6. The in vitro drug delivery results showed that antibubbles can significantly improve drug delivery (p < 0.0001) and deliver the drug within the antibubbles. In conclusion antibubbles are a viable concept for ultrasound guided drug delivery.

An Open Access Chamber Designed for the Acoustic Characterisation of Microbubbles

Microbubbles are used as contrast agents in clinical ultrasound for Left Ventricular Opacification (LVO) and perfusion imaging. They are also the subject of promising research in therapeutics as a drug delivery mechanism or for sonoporation and co-administration. For maximum efficacy in these applications, it is important to understand the acoustic characteristics of the administered microbubbles. Despite this, there is significant variation in the experimental procedures and equipment used to measure the acoustic properties of microbubble populations. A chamber was designed to facilitate acoustic characterisation experiments and was manufactured using additive manufacturing techniques. The design has been released to allow wider uptake in the research community. The efficacy of the chamber for acoustic characterisation has been explored with an experiment to measure the scattering of SonoVue® microbubbles at the fundamental frequency and second harmonic under interrogation from emissions in the frequency range of 1.6 to 6.4 MHz. The highest overall scattering values were measured at 1.6 MHz and decreased as the frequency increased, a result which is in agreement with previously published measurements. Statistical analysis of the acoustic scattering measurements have been performed and a significant difference, at the 5% significance level, was found between the samples containing contrast agent and the control sample containing only deionised water. These findings validate the proposed design for measuring the acoustic scattering characteristics of ultrasound contrast agents.

Horizon: Microfluidic platform for the production of therapeutic microbubbles and nanobubbles

Microbubbles (MBs) have a multitude of applications including as contrast agents in ultrasound imaging and as therapeutic drug delivery vehicles, with further scope for combining their diagnostic and therapeutic properties (known as theranostics). MBs used clinically are commonly made by mechanical agitation or sonication methods, which offer little control over population size and dispersity. Furthermore, clinically used MBs are yet to be used therapeutically and further research is needed to develop these theranostic agents. In this paper, we present our MB production instrument "Horizon," which is a robust, portable, and user-friendly instrument, integrating the key components for producing MBs using microfluidic flow-focusing devices. In addition, we present the system design and specifications of Horizon and the optimized protocols that have so far been used to produce MBs with specific properties. These include MBs with tailored size and low dispersity (monodisperse); MBs with a diameter of ∼2 μm, which are more disperse but also produced in higher concentration; nanobubbles with diameters of 100-600 nm; and therapeutic MBs with drug payloads for targeted delivery. Multiplexed chips were able to improve production rates up to 16-fold while maintaining production stability. This work shows that Horizon is a versatile instrument with potential for mass production and use across many research facilities, which could begin to bridge the gap between therapeutic MB research and clinical use.

A Voltage-Sensitive Ultrasound Enhancing Agent for Myocardial Perfusion Imaging in a Rat Model

Echocardiographers with specialized expertise sometimes perform myocardial perfusion imaging using U.S. Food and Drug Administration-approved microbubbles in an off-label capacity, correlating microbubble replenishment in the near field with blood flow through the myocardium. This study reports the in vivo clinical feasibility of a voltage-sensitive ultrasound enhancing agent (UEA) for myocardial perfusion imaging. Four UEAs were injected into Sprague-Dawley rats while ultrasound images were collected to quantify brightness in the left ventricular (LV) cavity, septal wall, and posterior wall in systole and diastole. Formulation IV, a phase change agent nested within a negatively charged phospholipid bilayer, increased the tissue-to-cavity ratio in both systole and diastole in the septal wall, 6 dB, and in the posterior wall, 5 dB, while leaving the LV cavity at baseline. This outcome improves the signal of the myocardium relative to the LV cavity and shows promise as a myocardial perfusion UEA.

Ultrasound-triggered therapeutic microbubbles enhance the efficacy of cytotoxic drugs by increasing circulation and tumor drug accumulation and limiting bioavailability and toxicity in normal tissues

Most cancer patients receive chemotherapy at some stage of their treatment which makes improving the efficacy of cytotoxic drugs an ongoing and important goal. Despite large numbers of potent anti-cancer agents being developed, a major obstacle to clinical translation remains the inability to deliver therapeutic doses to a tumor without causing intolerable side effects. To address this problem, there has been intense interest in nanoformulations and targeted delivery to improve cancer outcomes. The aim of this work was to demonstrate how vascular endothelial growth factor receptor 2 (VEGFR2)-targeted, ultrasound-triggered delivery with therapeutic microbubbles (thMBs) could improve the therapeutic range of cytotoxic drugs. Methods: Using a microfluidic microbubble production platform, we generated thMBs comprising VEGFR2-targeted microbubbles with attached liposomal payloads for localised ultrasound-triggered delivery of irinotecan and SN38 in mouse models of colorectal cancer. Intravenous injection into tumor-bearing mice was used to examine targeting efficiency and tumor pharmacodynamics. High-frequency ultrasound and bioluminescent imaging were used to visualise microbubbles in real-time. Tandem mass spectrometry (LC-MS/MS) was used to quantitate intratumoral drug delivery and tissue biodistribution. Finally, 89Zr PET radiotracing was used to compare biodistribution and tumor accumulation of ultrasound-triggered SN38 thMBs with VEGFR2-targeted SN38 liposomes alone. Results: ThMBs specifically bound VEGFR2 in vitro and significantly improved tumor responses to low dose irinotecan and SN38 in human colorectal cancer xenografts. An ultrasound trigger was essential to achieve the selective effects of thMBs as without it, thMBs failed to extend intratumoral drug delivery or demonstrate enhanced tumor responses. Sensitive LC-MS/MS quantification of drugs and their metabolites demonstrated that thMBs extended drug exposure in tumors but limited exposure in healthy tissues, not exposed to ultrasound, by persistent encapsulation of drug prior to elimination. 89Zr PET radiotracing showed that the percentage injected dose in tumors achieved with thMBs was twice that of VEGFR2-targeted SN38 liposomes alone. Conclusions: thMBs provide a generic platform for the targeted, ultrasound-triggered delivery of cytotoxic drugs by enhancing tumor responses to low dose drug delivery via combined effects on circulation, tumor drug accumulation and exposure and altered metabolism in normal tissues.

An Ultrasound Enhancing Agent with Nonlinear Acoustic Activity that Depends on the Presence of an Electric Field

The nonlinear acoustic properties of microbubble ultrasound enhancing agents have allowed for the development of subharmonic, second harmonic, and contrast-pulse sequence ultrasound imaging modes, which enhance the quality, reduce the noise, and improve the diagnostic capabilities of clinical ultrasound. This study details acoustic scattering responses of perfluorobutane (PFB) microbubbles, an un-nested perfluoropentane (PFP) nanoemulsion, and two nested PFP nanoemulsions-one comprising a negatively charged phospholipid bilayer and another comprising a zwitterionic phospholipid bilayer-when excited at 1 or 2.25 MHz over a peak negative pressure range of 200 kPa to 4 MPa in the absence and presence of a 1-Hz, 1-V/cm electric field. The only sample that exhibited an increase in nonlinear activity in the presence of an electric field at both excitation frequencies was the negatively charged nested PFP nanoemulsion; the most pronounced effect was observed at an excitation of 2.25 MHz. Interestingly, the application of an electric field not only increased the nonlinear acoustic activity of the negatively charged nested PFP nanoemulsion but increased it beyond that seen when the nanoemulsion is un-nested and on the same scale as PFB microbubbles.

A dynamical model of oncotripsy by mechanical cell fatigue: selective cancer cell ablation by low-intensity pulsed ultrasound

The method of oncotripsy, first proposed in Heyden & Ortiz (Heyden & Ortiz 2016 J. Mech. Phys. Solids 92, 164-175 (doi:10.1016/j.jmps.2016.04.016)), exploits aberrations in the material properties and morphology of cancerous cells in order to ablate them selectively by means of tuned low-intensity pulsed ultrasound. We propose the dynamical model of oncotripsy that follows as an application of cell dynamics, statistical mechanical theory of network elasticity and 'birth-death' kinetics to describe the processes of damage and repair of the cytoskeleton. We also develop a reduced dynamical model that approximates the three-dimensional dynamics of the cell and facilitates parametric studies, including sensitivity analysis and process optimization. We show that the dynamical model predicts-and provides a conceptual basis for understanding-the oncotripsy effect and other trends in the data of Mittelstein et al. (Mittelstein et al. 2019 Appl. Phys. Lett. 116, 013701 (doi:10.1063/1.5128627)), for cells in suspension, including the dependence of cell-death curves on cell and process parameters.

Nano Meets Micro-Translational Nanotechnology in Medicine: Nano-Based Applications for Early Tumor Detection and Therapy

Nanomaterials have great potential for the prevention and treatment of cancer. Circulating tumor cells (CTCs) are cancer cells of solid tumor origin entering the peripheral blood after detachment from a primary tumor. The occurrence and circulation of CTCs are accepted as a prerequisite for the formation of metastases, which is the major cause of cancer-associated deaths. Due to their clinical significance CTCs are intensively discussed to be used as liquid biopsy for early diagnosis and prognosis of cancer. However, there are substantial challenges for the clinical use of CTCs based on their extreme rarity and heterogeneous biology. Therefore, methods for effective isolation and detection of CTCs are urgently needed. With the rapid development of nanotechnology and its wide applications in the biomedical field, researchers have designed various nano-sized systems with the capability of CTCs detection, isolation, and CTCs-targeted cancer therapy. In the present review, we summarize the underlying mechanisms of CTC-associated tumor metastasis, and give detailed information about the unique properties of CTCs that can be harnessed for their effective analytical detection and enrichment. Furthermore, we want to give an overview of representative nano-systems for CTC isolation, and highlight recent achievements in microfluidics and lab-on-a-chip technologies. We also emphasize the recent advances in nano-based CTCs-targeted cancer therapy. We conclude by critically discussing recent CTC-based nano-systems with high therapeutic and diagnostic potential as well as their biocompatibility as a practical example of applied nanotechnology.

Molecular Effects of Glycerol on Lipid Monolayers at the Gas-Liquid Interface: Impact on Microbubble Physical and Mechanical Properties

The production and stability of microbubbles (MBs) is enhanced by increasing the viscosity of both the formation and storage solution, respectively. Glycerol is a good candidate for biomedical applications of MBs, since it is biocompatible, although the exact molecular mechanisms of its action is not fully understood. Here, we investigate the influence glycerol has on lipid-shelled MB properties, using a range of techniques. Population lifetime and single bubble stability were studied using optical microscopy. Bubble stiffness measured by AFM compression is compared with lipid monolayer behavior in a Langmuir-Blodgett trough. We deduce that increasing glycerol concentrations enhances stability of MB populations through a 3-fold mechanism. First, binding of glycerol to lipid headgroups in the interfacial monolayer up to 10% glycerol increases MB stiffness but has limited impact on shell resistance to gas permeation and corresponding MB lifetime. Second, increased solution viscosity above 10% glycerol slows down the kinetics of gas transfer, markedly increasing MB stability. Third, above 10%, glycerol induces water structuring around the lipid monolayer, forming a glassy layer which also increases MB stiffness and resistance to gas loss. At 30% glycerol, the glassy layer is ablated, lowering the MB stiffness, but MB stability is further augmented. Although the molecular interactions of glycerol with the lipid monolayer modulate the MB lipid shell properties, MB lifetime continually increases from 0 to 30% glycerol, indicating that its viscosity is the dominant effect on MB solution stability. This three-fold action and biocompatibility makes glycerol ideal for therapeutic MB formation and storage and gives new insight into the action of glycerol on lipid monolayers at the gas-liquid interface.

Acoustofluidic Measurements on Polymer-Coated Microbubbles: Primary and Secondary Bjerknes Forces

The acoustically-driven dynamics of isolated particle-like objects in microfluidic environments is a well-characterised phenomenon, which has been the subject of many studies. Conversely, very few acoustofluidic researchers looked at coated microbubbles, despite their widespread use in diagnostic imaging and the need for a precise characterisation of their acoustically-driven behaviour, underpinning therapeutic applications. The main reason is that microbubbles behave differently, due to their larger compressibility, exhibiting much stronger interactions with the unperturbed acoustic field (primary Bjerknes forces) or with other bubbles (secondary Bjerknes forces). In this paper, we study the translational dynamics of commercially-available polymer-coated microbubbles in a standing-wave acoustofluidic device. At increasing acoustic driving pressures, we measure acoustic forces on isolated bubbles, quantify bubble-bubble interaction forces during doublet formation and study the occurrence of sub-wavelength structures during aggregation. We present a dynamic characterisation of microbubble compressibility with acoustic pressure, highlighting a threshold pressure below which bubbles can be treated as uncoated. Thanks to benchmarking measurements under a scanning electron microscope, we interpret this threshold as the onset of buckling, providing a quantitative measurement of this parameter at the single-bubble level. For acoustofluidic applications, our results highlight the limitations of treating microbubbles as a special case of solid particles. Our findings will impact applications where knowing the buckling pressure of coated microbubbles has a key role, like diagnostics and drug delivery.

Combining Acoustic Trapping With Plane Wave Imaging for Localized Microbubble Accumulation in Large Vessels

The capability of accumulating microbubbles using ultrasound could be beneficial for enhancing targeted drug delivery. When microbubbles are used to deliver a therapeutic payload, there is a need to track them, for a localized release of the payload. In this paper, a method for localizing microbubble accumulation with fast image guidance is presented. A linear array transducer performed trapping of microbubble populations interleaved with plane wave imaging, through the use of a composite pulse sequence. The acoustic trap in the pressure field was created parallel with the direction of flow in a model of a vessel section. The acoustic trapping force resultant from the large gradients in the acoustic field was engendered to directly oppose the flowing microbubbles. This was demonstrated numerically with field simulations, and experimentally using an Ultrasound Array Research Platform II. SonoVue microbubbles at clinically relevant concentrations were pumped through a tissue-mimicking flow phantom and exposed to either the acoustic trap or a control ultrasonic field composed of a single-peak acoustic radiation force beam. Under the flow condition at a shear rate of 433 s^-1, the use of the acoustic trap led to lower speed estimations ( ) in the center of the acoustic field, and an enhancement of 71% ± 28%( ) in microbubble image brightness.

Indocyanine green conjugated lipid microbubbles as an ultrasound-responsive drug delivery system for dual-imaging guided tumor-targeted therapy

Herein, a multifunctional traceable and ultrasound-responsive drug targeted delivery system based on indocyanine green (ICG) and folic acid (FA) covalently conjugated lipid microbubbles (ILMBs–FA) is proposed. After encapsulation of the anticancer drug resveratrol (RV), the composite (RILMBs–FA) with fluorescence and ultrasound imaging capacity was studied for highly sensitive dual-imaging guided tumor targeted therapy. The resulting RILMBs–FA with an average particle size of 1.32 ± 0.14 μm exhibited good stability and biocompatibility characteristics. The RILMBs–FA featured a high RV loading ratio and the encapsulated RV has been demonstrated to be released from the microbubbles triggered by ultrasound (US) waves. In addition, it was found that the linked FA could facilitate a high cellular uptake of RILMBs–FA via the FA receptor-mediated endocytosis pathway. Compared to free RV and RILMBs, RILMBs–FA with US irradiation demonstrated a more significant tumor cell-killing efficacy mediated by apoptosis in vitro. Eight hours post intravenous injection of RILMBs–FA, the composites showed maximum accumulation in tumorous tissues according to in vivo fluorescence and US images. This ultimately led to the best tumor inhibition effect among all tested drugs under US irradiation. In vivo biosafety evaluations showed that RILMBs–FA featured high biocompatibility characteristics and no significant systemic toxicity over the course of one month. Taken in concert, these results demonstrate the versatility of this drug delivery system with dual-imaging and ultrasound-triggered drug release characteristics for potential future applications in cancer theranostics.

Schematic representation of the synthesis of RILMBs–FA and application in tumor therapy.