Effect of surface architecture on in vivo ultrasound contrast persistence of targeted size-selected microbubbles

Effect of Poly(ethylene glycol) Configuration on Microbubble Pharmacokinetics

Microbubbles (MBs) hold substantial promise for medical imaging and therapy; nonetheless, knowledge gaps persist between composition, structure, and in vivo performance, especially with respect to pharmacokinetics. Of particular interest is the role of the poly(ethylene glycol) (PEG) layer, which is thought to shield the MB against opsonization and rapid clearance but is also known to cause an antibody response upon multiple injections. The goal of this study was, therefore, to elucidate the role of the PEG layer in circulation persistence of MBs in the naïve animal (prior to an adaptive immune response). Here, we directly observe the number and size of individual MBs obtained from blood samples, unifying size and concentration into the microbubble volume dose (MVD) parameter. This approach enables direct evaluation of the pharmacokinetics of intact MBs, comprising both the lipid shell and gaseous core, rather than separately assessing the lipid or gas components. We examined the in vivo circulation persistence of 3 μm diameter phospholipid-coated MBs with three different mPEG2000 content: 2 mol % (mushroom), 5 mol % (intermediate), and 10 mol % (brush). MB size and concentration in the blood were evaluated by a hemocytometer analysis over 30 min following intravenous injections of 20 and 40 μL/kg MVD in Sprague-Dawley rats. Interestingly, pharmacokinetic analysis demonstrated that increasing PEG concentration on the MB surface resulted in faster clearance. This was evidenced by a 1.6-fold reduction in half-life and area under the curve (AUC) (p < 0.05) in the central compartment. Conversely, the AUC in the peripheral compartment increased with PEG density, suggesting enhanced MB trapping by the mononuclear phagocyte system. This was supported by an in vitro assay, which showed a significant rise in complement C3a activation with a higher PEG content. In conclusion, a minimal PEG concentration on the MB shell (mushroom configuration) was found to prolong circulation and mitigate immunogenicity.

Effect of Anesthetic Carrier Gas on In Vivo Circulation Times of Intravenously Administered Phospholipid Oxygen Microbubbles in Rats

OBJECTIVE

For the treatment of tumor hypoxia, microbubbles comprising oxygen as a majority component of the gas core with a stabilizing shell may be used to deliver and release oxygen locally at the tumor site through ultrasound destruction. Previous work has revealed differences in circulation half-life in vivo for perfluorocarbon-filled microbubbles, typically used as ultrasound imaging contrast agents, as a function of anesthetic carrier gas. These differences in circulation time in vivo were likely due to gas diffusion as a function of anesthetic carrier gas, among other variables. This work has motivated studies to evaluate the effect of anesthetic carrier gas on oxygen microbubble circulation dynamics.

METHODS

Circulation time for oxygen microbubbles was derived from ultrasound image intensity obtained during longitudinal kidney imaging. Studies were constructed for rats anesthetized on inhaled isoflurane with either pure oxygen or medical air as the anesthetic carrier gas.

RESULTS

Results indicated that oxygen microbubbles were highly visible via contrast-specific imaging. Marked signal enhancement and duration differences were observed between animals breathing air and oxygen. Perhaps counterintuitively, oxygen microbubbles disappeared from circulation significantly faster when the animals were breathing pure oxygen compared with medical air. This may be explained by nitrogen counterdiffusion from blood into the bubble, effectively changing the gas composition of the core, as has been observed in perfluorocarbon core microbubbles.

CONCLUSION

Our findings suggest that the apparent longevity and persistence of oxygen microbubbles in circulation may not be reflective of oxygen delivery when the animal is anesthetized breathing air.

Targeted Microbubbles for Drug, Gene, and Cell Delivery in Therapy and Immunotherapy

Microbubbles are 1-10 μm diameter gas-filled acoustically-active particles, typically stabilized by a phospholipid monolayer shell. Microbubbles can be engineered through bioconjugation of a ligand, drug and/or cell. Since their inception a few decades ago, several targeted microbubble (tMB) formulations have been developed as ultrasound imaging probes and ultrasound-responsive carriers to promote the local delivery and uptake of a wide variety of drugs, genes, and cells in different therapeutic applications. The aim of this review is to summarize the state-of-the-art of current tMB formulations and their ultrasound-targeted delivery applications. We provide an overview of different carriers used to increase drug loading capacity and different targeting strategies that can be used to enhance local delivery, potentiate therapeutic efficacy, and minimize side effects. Additionally, future directions are proposed to improve the tMB performance in diagnostic and therapeutic applications.

Monodispersity Increases Adhesion Efficiency and Specificity for Ultrasound-Targeted Microbubbles

Ultrasound molecular imaging with targeted microbubbles (MBs) can be used to noninvasively diagnose, monitor, and study the progression of different endothelial-associated diseases. Acoustic radiation force (F_rad) can initiate and enhance MB adhesion at the target site. The goal of this study was to elucidate the effects of various MB parameters on F_rad targeting. Monodisperse or polydisperse MBs with the immune-stealth cloaked (buried)-ligand architecture were conjugated with targeting RGD or nonspecific isotype control RAD peptides and then pumped through an α_vβ₃ integrin-coated microvessel phantom at a wall shear stress of 3.5 dyn/cm². Targeting was assessed by measuring MB attachment for varying F_rad time and frequency, as well as MB concentration and size distribution. We first confirmed that primary F_rad is necessary to target the cloaked-ligand MBs. MB targeting increased monotonically with α_vβ₃ integrin density and F_rad time. MB attachment and, to a lesser extent specificity, also increased when driven by F_rad near resonance. MB targeting increased with MB concentration, although a shift in behavior was observed with increasing MB-MB interactions and aggregations forming from secondary F_rad effects as MB concentration was increased. These secondary F_rad effects reduced targeting specificity. Finally, after having validated our approach by testing different parameters with the appropriate controls, we then determined the effects of monodispersity on adhesion efficiency and specific targeting. We observed that both MB targeting efficiency and specificity were greatly enhanced for monodisperse vs polydisperse MBs. Analysis of videomicroscopy images indicated that secondary F_rad effects may have disproportionally inhibited targeting of polydisperse MBs. In conclusion, our in vitro results indicate that monodisperse MBs driven near resonance and at a low concentration (∼10⁶ MB/mL) can be used to maximize the adhesion efficiency (up to 88%) and specificity of RGD-MB targeting.

Slow-Flow Ultrasound Localization Microscopy Using Recondensation of Perfluoropentane Nanodroplets

Ultrasound localization microscopy (ULM) is an emerging, super-resolution imaging technique for detailed mapping of the microvascular structure and flow velocity via subwavelength localization and tracking of microbubbles. Because microbubbles rely on blood flow for movement throughout the vascular space, acquisition times can be long in the smallest, low-flow microvessels. In addition, detection of microbubbles in low-flow regions can be difficult because of minimal separation of microbubble signal from tissue. Nanoscale, phase-change contrast agents (PCCAs) have emerged as a switchable, intermittent or persisting contrast agent for ULM via acoustic droplet vaporization (ADV). Here, the focus is on characterizing the spatiotemporal contrast properties of less volatile perfluoropentane (PFP) PCCAs. The results indicate that at physiological temperature, nanoscale PFP PCCAs with diameters less than 100 nm disappear within microseconds after ADV with high-frequency ultrasound (16 MHz, 5- to 6-MPa peak negative pressure) and that nanoscale PFP PCCAs have an inherent deactivation mechanism via immediate recondensation after ADV. This "blinking" on-and-off contrast signal allowed separation of flow in an in vitro flow phantom, regardless of flow conditions, although with a need for some replenishment at very low flow conditions to maintain count rate. This blinking behavior allows for rapid spatial mapping in areas of low or no flow with ULM, but limits velocity tracking because there is no stable bubble formation with nanoscale PFP PCCAs.

Microbubble Size and Dose Effects on Pharmacokinetics

Optimization of contrast-enhanced imaging and focused ultrasound therapy requires a comprehensive understanding of in vivo microbubble (MB) pharmacokinetics. Prior studies have focused pharmacokinetic analysis on indirect techniques, such as ultrasound imaging of the blood pool and gas chromatography of exhaled gases. The goal of this work was to measure the MB concentration directly in blood and correlate the pharmacokinetic parameters with the MB size and dose. MB volume dose (MVD) was chosen to combine the size distribution and number into a single-dose parameter. Different MB sizes (2, 3, and 5 μm diameter) at 5-40 μL/kg MVD were intravenously injected. Blood samples were withdrawn at different times (1-10 min) and analyzed by image processing. We found that for an MVD threshold < 40 μL/kg for 2 and 3 μm and <10 μL/kg for 5 μm, MB clearance followed first-order kinetics. When matching MVD, MBs of different sizes had comparable half-lives, indicating that gas dissolution and elimination by the lungs are the primary mechanisms for elimination. Above the MVD threshold, MB clearance followed biexponential kinetics, suggesting a second elimination mechanism mediated by organ retention, possibly in the lung, liver, and spleen. In conclusion, we present the first direct MB pharmacokinetic study, demonstrate the utility of MVD as a unified dose metric, and provide insights into the mechanisms of MB clearance from circulation.

Multivariable Dependence of Acoustic Contrast of Fluorocarbon and Xenon Microbubbles under Flow

Microbubbles (MBs) are 1 to 10 µm gas particles stabilized by an amphiphilic shell capable of responding to biomedical ultrasound with strong acoustic signals, allowing them to be commonly used in ultrasound imaging and therapy. The composition of both the shell and the core determines their stability and acoustic properties. While there has been extensive characterization of the dissolution, oscillation, cavitation, collapse and therefore, ultrasound contrast of MBs under static conditions, few reports have examined such behavior under hydrodynamic flow. In this study, we evaluate the interplay of ultrasound parameters (five different mechanical indices [MIs]), MB shell parameter (shell stiffness), type of gas (perfluorocarbon for diagnostic imaging and xenon as a therapeutic gas), and a flow parameter (flow rate) on the ultrasound signal of phospholipid-stabilized MBs flowing through a latex tube embedded in a tissue-mimicking phantom. We find that the contrast gradient (CG), a metric of the rate of decay of contrast along the length of the tube, and the contrast peak (CP), the location where the maximum contrast is reached, depend on the conditions of flow, imaging, and MB material. For instance, while the contrast near the flow inlet of the field of view is highest for a softer shell (dipalmitoylphosphatidylcholine [DPPC], C16) than for stiffer shells (distearoylphosphatidylcholine [DSPC], C18, and dibehenoylphosphatidylcholine [DBPC], C22), the contrast decay is also faster; stiffer shells provide more resistance and hence lead to slower MB dissolution/destruction. At higher flow rates, the CG is low for a fixed length of time because each MB is exposed to ultrasound for a shorter period. The CG becomes high for low flow rates, especially at high incident pressures (high MI), causing more MB destruction closer to the inlet of the field of view. Also, the CP shifts toward the inlet at low flow rates, high MIs, and low shell stiffness. We also report the first demonstration of sustained ultrasound flow imaging of a water-soluble, therapeutic gas MB (xenon). We find that an increased MB concentration is necessary for obtaining the same signal magnitude for xenon MBs. In summary, this study builds a framework depicting how multiple variables simultaneously affect the evolution of MB ultrasound contrast under flow. Depending on the MB composition, imaging conditions, transducer positioning, and image processing, building on such a framework could potentially allow for extraction of additional diagnostic information than is commonly analyzed for physiological flow.

Opening doors with ultrasound and microbubbles: Beating biological barriers to promote drug delivery

Apart from its clinical use in imaging, ultrasound has been thoroughly investigated as a tool to enhance drug delivery in a wide variety of applications. Therapeutic ultrasound, as such or combined with cavitating nuclei or microbubbles, has been explored to cross or permeabilize different biological barriers. This ability to access otherwise impermeable tissues in the body makes the combination of ultrasound and therapeutics very appealing to enhance drug delivery in situ. This review gives an overview of the most important biological barriers that can be tackled using ultrasound and aims to provide insight on how ultrasound has shown to improve accessibility as well as the biggest hurdles. In addition, we discuss the clinical applicability of therapeutic ultrasound with respect to the main challenges that must be addressed to enable the further progression of therapeutic ultrasound towards an effective, safe and easy-to-use treatment tailored for drug delivery in patients.

Microbubble Agents: New Directions

Microbubble ultrasound contrast agents have now been in use for several decades and their safety and efficacy in a wide range of diagnostic applications have been well established. Recent progress in imaging technology is facilitating exciting developments in techniques such as molecular, 3-D and super resolution imaging and new agents are now being developed to meet their specific requirements. In parallel, there have been significant advances in the therapeutic applications of microbubbles, with recent clinical trials demonstrating drug delivery across the blood-brain barrier and into solid tumours. New agents are similarly being tailored toward these applications, including nanoscale microbubble precursors offering superior circulation times and tissue penetration. The development of novel agents does, however, present several challenges, particularly regarding the regulatory framework. This article reviews the developments in agents for diagnostic, therapeutic and "theranostic" applications; novel manufacturing techniques; and the opportunities and challenges for their commercial and clinical translation.

Contrast-Enhanced Ultrasound Quantification: From Kinetic Modeling to Machine Learning

Ultrasound contrast agents (UCAs) have opened up immense diagnostic possibilities by combined use of indicator dilution principles and dynamic contrast-enhanced ultrasound (DCE-US) imaging. UCAs are microbubbles encapsulated in a biocompatible shell. With a rheology comparable to that of red blood cells, UCAs provide an intravascular indicator for functional imaging of the (micro)vasculature by quantitative DCE-US. Several models of the UCA intravascular kinetics have been proposed to provide functional quantitative maps, aiding diagnosis of different pathological conditions. This article is a comprehensive review of the available methods for quantitative DCE-US imaging based on temporal, spatial and spatiotemporal analysis of the UCA kinetics. The recent introduction of novel UCAs that are targeted to specific vascular receptors has advanced DCE-US to a molecular imaging modality. In parallel, new kinetic models of increased complexity have been developed. The extraction of multiple quantitative maps, reflecting complementary variables of the underlying physiological processes, requires an integrative approach to their interpretation. A probabilistic framework based on emerging machine-learning methods represents nowadays the ultimate approach, improving the diagnostic accuracy of DCE-US imaging by optimal combination of the extracted complementary information. The current value and future perspective of all these advances are critically discussed.

Contrast enhanced ultrasound imaging by nature-inspired ultrastable echogenic nanobubbles

Advancement of ultrasound molecular imaging applications requires not only a reduction in size of the ultrasound contrast agents (UCAs) but also a significant improvement in the in vivo stability of the shell-stabilized gas bubble. The transition from first generation to second generation UCAs was marked by an advancement in stability as air was replaced by a hydrophobic gas, such as perfluoropropane and sulfur hexafluoride. Further improvement can be realized by focusing on how well the UCAs shell can retain the encapsulated gas under extreme mechanical deformations. Here we report the next generation of UCAs for which we engineered the shell structure to impart much better stability under repeated prolonged oscillation due to ultrasound, and large changes in shear and turbulence as it circulates within the body. By adapting an architecture with two layers of contrasting elastic properties similar to bacterial cell envelopes, our ultrastable nanobubbles (NBs) withstand continuous in vitro exposure to ultrasound with minimal signal decay and have a significant delay on the onset of in vivo signal decay in kidney, liver, and tumor. Development of ultrastable NBs can potentially expand the role of ultrasound in molecular imaging, theranostics, and drug delivery.

Effect of Bubble Concentration on the in Vitro and in Vivo Performance of Highly Stable Lipid Shell-Stabilized Micro- and Nanoscale Ultrasound Contrast Agents

Ultrasound (US) is a widely used diagnostic imaging tool because it is inexpensive, safe, portable, and broadly accessible. Ultrasound contrast agents (UCAs) are employed to enhance backscatter echo and improve imaging contrast. The most frequently utilized UCAs are echogenic bubbles made with a phospholipid or protein-stabilized hydrophobic gas core. While clinically utilized, applications of UCAs are often limited by rapid signal decay (<5 min) in vivo under typical ultrasound imaging protocols. Here, we report on a formulation of lipid shell-stabilized perfluoropropane (C₃F₈) microbubbles and nanobubbles with a significantly prolonged in vivo stability. Microbubbles (875 ± 280 nm) of the target size were prepared by utilizing a multiple-step centrifugation cycle, while nanobubbles (299 ± 189 nm) were isolated from the activated vial using a single centrifugation step. To provide in-depth acoustic characterization of the new construct we evaluated the effect of size and concentration on their in vitro and in vivo performance. In vitro and in vivo characterization were carried out for a range of bubble concentrations normalized by total gas volume quantified via headspace gas chromatography/mass spectrometry (GC/MS). In vitro characterization revealed that nanobubbles at different concentrations are more consistently stable over time with the highest and lowest dilutions (50-fold decrease) only differing in US signal after 8 min exposure by 10.34%, while for microbubbles the difference was 86.46%. As expected, due to the difference in hydrodynamic diameter and scattering cross section difference, nanobubbles showed lower overall initial signal intensity. In vivo experiments showed that both microbubbles and nanobubbles with similar initial peak signal intensity are comparably stable over time with 66.8% and 60.6% remaining signal after 30 min, respectively. This study demonstrates that bubble concentration has significant effects on the persistence of both microbubbles and nanobubbles in vitro and in vivo, but the effects are more pronounced in larger bubbles. These effects should be taken into account when selecting the appropriate bubble parameters for future imaging applications.

Colloids, nanoparticles, and materials for imaging, delivery, ablation, and theranostics by focused ultrasound (FUS)

This review focuses on different materials and contrast agents that sensitize imaging and therapy with Focused Ultrasound (FUS). At high intensities, FUS is capable of selectively ablating tissue with focus on the millimeter scale, presenting an alternative to surgical intervention or management of malignant growth. At low intensities, FUS can be also used for other medical applications such as local delivery of drugs and blood brain barrier opening (BBBO). Contrast agents offer an opportunity to increase selective acoustic absorption or facilitate destructive cavitation processes by converting incident acoustic energy into thermal and mechanical energy. First, we review the history of FUS and its effects on living tissue. Next, we present different colloidal or nanoparticulate approaches to sensitizing FUS, for example using microbubbles, phase-shift emulsions, hollow-shelled nanoparticles, or hydrophobic silica surfaces. Exploring the science behind these interactions, we also discuss ways to make stimulus-responsive, or "turn-on" contrast agents for improved selectivity. Finally, we discuss acoustically-active hydrogels and membranes. This review will be of interest to those working in materials who wish to explore new applications in acoustics and those in acoustics who are seeking new agents to improve the efficacy of their approaches.

Microbubble Formulations: Synthesis, Stability, Modeling and Biomedical Applications

Microbubbles are increasingly being used in biomedical applications such as ultrasonic imaging and targeted drug delivery. Microbubbles typically range from 0.1 to 10 µm in size and consist of a protective shell made of lipids or proteins. The shell encapsulates a gaseous core containing gases such as oxygen, sulfur hexafluoride or perfluorocarbons. This review is a consolidated account of information available in the literature on research related to microbubbles. Efforts have been made to present an overview of microbubble synthesis techniques; microbubble stability; microbubbles as contrast agents in ultrasonic imaging and drug delivery vehicles; and side effects related to microbubble administration in humans. Developments related to the modeling of microbubble dissolution and stability are also discussed.

Insonation of Systemically Delivered Cisplatin-Loaded Microbubbles Significantly Attenuates Nephrotoxicity of Chemotherapy in Experimental Models of Head and Neck Cancer

The use of cisplatin (CDDP), the most common chemotherapy drug for head and neck cancer, is limited by its undesirable side effects, especially nephrotoxicity. We investigated ultrasound microbubbles (USMB) as a tool to increase the local intra-tumoral CDDP level while decreasing systemic CDDP cytotoxicity. We allowed CDDP to interact with human serum albumin and then sonicated the resulting CDDP‒albumin complex to generate CDDP-loaded MBs (CDDP-MBs). We then established a head-and-neck tumor-bearing mouse model by implanting FaDu-fLuc/GFP cells into severe combined immunodeficiency mice and used IVIS^® bioluminescence imaging to determine the tumor xenograft formation and size. Twice weekly (until Day 33), we administered CDDP only, CDDP + MBs + US, CDDP-MBs, or CDDP-MBs + US intravenously by tail-vein injection. The US treatment was administered at the tumor site immediately after injection. The in vivo systemic distribution of CDDP indicated that the kidney was the most vulnerable organ, followed by the liver, and then the inner ear. However, CDDP uptake into the kidney and liver was significantly decreased in both the CDDP-MBs and CDDP-MBs + US groups, suggesting that MB binding significantly reduced the systemic toxicity of CDDP. The CDDP-MBs + US treatment reduced the tumor size as effectively as conventional CDDP-only chemotherapy. Therefore, the combination of CDDP-MBs with ultrasound is effective and significantly attenuates CDDP-associated nephrotoxicity, indicating a promising clinical potential for this approach.

Accelerated Clearance of Ultrasound Contrast Agents Containing Polyethylene Glycol is Associated with the Generation of Anti-Polyethylene Glycol Antibodies

Emerging evidence suggests that the immune system can recognize polyethylene glycol (PEG), leading to the accelerated blood clearance (ABC) of PEGylated particles. Our aim here was to study the generation of anti-PEG immunity and changes in PEGylated microbubble pharmacokinetics during repeated contrast-enhanced ultrasound imaging in rats. We administered homemade PEGylated microbubbles multiple times over a 28-d period and observed dramatically accelerated clearance (4.2 × reduction in half-life), which was associated with robust anti-PEG IgM and anti-PEG IgG antibody production. Dosing animals with free PEG as a competition agent before homemade PEGylated microbubble administration significantly prolonged microbubble circulation, suggesting that ABC was largely driven by circulating anti-PEG antibodies. Experiments with U.S. Food and Drug Administration-approved Definity microbubbles similarly resulted in ABC and the generation of anti-PEG antibodies. Experiments repeated with non-PEGylated Optison microbubbles revealed a slight shift in clearance, indicating that immunologic factors beyond anti-PEG immunity may play a role in ABC, especially of non-PEGylated agents.

Circulating Magnetic Microbubbles for Localized Real-Time Control of Drug Delivery by Ultrasonography-Guided Magnetic Targeting and Ultrasound

Image-guided and target-selective modulation of drug delivery by external physical triggers at the site of pathology has the potential to enable tailored control of drug targeting. Magnetic microbubbles that are responsive to magnetic and acoustic modulation and visible to ultrasonography have been proposed as a means to realize this drug targeting strategy. To comply with this strategy in vivo, magnetic microbubbles must circulate systemically and evade deposition in pulmonary capillaries, while also preserving magnetic and acoustic activities in circulation over time. Unfortunately, challenges in fabricating magnetic microbubbles with such characteristics have limited progress in this field. In this report, we develop magnetic microbubbles (MagMB) that display strong magnetic and acoustic activities, while also preserving the ability to circulate systemically and evade pulmonary entrapment.

Methods: We systematically evaluated the characteristics of MagMB including their pharmacokinetics, biodistribution, visibility to ultrasonography and amenability to magneto-acoustic modulation in tumor-bearing mice. We further assessed the applicability of MagMB for ultrasonography-guided control of drug targeting.

Results: Following intravenous injection, MagMB exhibited a 17- to 90-fold lower pulmonary entrapment compared to previously reported magnetic microbubbles and mimicked circulation persistence of the clinically utilized Definity microbubbles (>10 min). In addition, MagMB could be accumulated in tumor vasculature by magnetic targeting, monitored by ultrasonography and collapsed by focused ultrasound on demand to activate drug deposition at the target. Furthermore, drug delivery to target tumors could be enhanced by adjusting the magneto-acoustic modulation based on ultrasonographic monitoring of MagMB in real-time.

Conclusions: Circulating MagMB in conjunction with ultrasonography-guided magneto-acoustic modulation may provide a strategy for tailored minimally-invasive control over drug delivery to target tissues.

Optimizing ultrasound molecular imaging of secreted frizzled related protein 2 expression in angiosarcoma

Secreted frizzled related protein 2 (SFRP2) is a tumor endothelial marker expressed in angiosarcoma. Previously, we showed ultrasound molecular imaging with SFRP2-targeted contrast increased average video pixel intensity (VI) of angiosarcoma vessels by 2.2 ± 0.6 VI versus streptavidin contrast. We hypothesized that redesigning our contrast agents would increase imaging performance. Improved molecular imaging reagents were created by combining NeutrAvidin™-functionalized microbubbles with biotinylated SFRP2 or IgY control antibodies. When angiosarcoma tumors in nude mice reached 8 mm, time-intensity, antibody loading, and microbubble dose experiments optimized molecular imaging. 10 minutes after injection, the control-subtracted time-intensity curve (TIC) for SFRP2-targeted contrast reached a maximum, after subtracting the contribution of free-flowing contrast. SFRP2 antibody-targeted VI was greater when contrast was formulated with 10-fold molar excess of maleimide-activated NeutrAvidin™ versus 3-fold (4.5 ± 0.18 vs. 0.32 ± 0.15, VI ± SEM, 5 x 106 dose, p < 0.001). Tumor vasculature returned greater average video pixel intensity using 5 x 107 versus 5 x 106 microbubbles (21.2 ± 2.5 vs. 4.5 ± 0.18, p = 0.0011). Specificity for tumor vasculature was confirmed by low VI for SFRP2-targeted, and control contrast in peri-tumoral vasculature (3.2 ± 0.52 vs. 1.6 ± 0.71, p = 0.92). After optimization, average video pixel intensity of tumor vasculature was 14.2 ± 3.0 VI units higher with SFRP2-targeted contrast versus IgY-targeted control (22.1 ± 2.5 vs. 7.9 ± 1.6, p < 0.001). After log decompression, 14.2 ΔVI was equal to ~70% higher signal, in arbitray acoustic units (AU), for SFRP2 versus IgY. This provided ~18- fold higher acoustic signal enhancement than provided previously by 2.2 ΔVI. Basing our targeted contrast on NeutrAvidin™-functionalized microbubbles, using IgY antibodies for our control contrast, and optimizing our imaging protocol significantly increased the SFRP2-specific signal returned from angiosarcoma vasculature, and may provide new opportunities for targeted molecular imaging.

Stable Encapsulation of Air in Mesoporous Silica Nanoparticles: Fluorocarbon-Free Nanoscale Ultrasound Contrast Agents

While gas-filled micrometer-sized ultrasound contrast agents vastly improve signal-to-noise ratios, microbubbles have short circulation lifetimes and poor extravasation from the blood. Previously reported fluorocarbon-based nanoscale contrast agents are more stable but their contrast is generally lower owing to their size and dispersity. The contrast agents reported here are composed of silica nanoparticles of ≈100 nm diameter that are filled with ≈3 nm columnar mesopores. Functionalization of the silica surface with octyl groups and resuspension with Pluronic F127 create particles with pores that remain filled with air but are stable in buffer and serum. Administration of high intensity focused ultrasound (HIFU) allows sensitive imaging of the silica nanoparticles down to 10(10) particles mL(-1) , with continuous imaging for at least 20 min. Control experiments with different silica particles supported the hypothesis that entrapped air could be pulled into bubble nuclei, which can then in turn act as acoustic scatterers. This process results in very little hemolysis in whole blood, indicating potential for nontoxic blood pool imaging. Finally, the particles are lyophilized and reconstituted or stored in PBS (phosphate-buffered saline, at least for four months) with no loss in contrast, indicating stability to storage and reformulation.

Quantitative ultrasound molecular imaging

Ultrasound molecular imaging using targeting microbubbles is predominantly a semi-quantitative tool, thus limiting its potential diagnostic power and clinical applications. In the work described here, we developed a novel method for acoustic quantification of molecular expression. E-Selectin expression in the mouse heart was induced by lipopolysaccharide. Real-time ultrasound imaging of E-selectin expression in the heart was performed using E-selectin-targeting microbubbles and a clinical ultrasound scanner in contrast pulse sequencing mode at 14 MHz, with a mechanical index of 0.22-0.26. The level of E-selectin expression was quantified using a novel time-signal intensity curve analytical method based on bubble elimination, which consisted of curve-fitting the bi-exponential equation [Formula: see text] to the elimination phase of the myocardial time-signal intensity curve. Ar and Af represent the maximum signal intensities of the retained and freely circulating bubbles in the myocardium, respectively; λr and λf represent the elimination rate constants of the retained and freely circulating bubbles in the myocardium, respectively. Ar correlated strongly with the level of E-selectin expression (|r|>0.8), determined using reverse transcriptase real-time quantitative polymerase chain reaction, and the duration of post-lipopolysaccharide treatment-both linearly related to cell surface E-selectin protein (actual bubble target) concentration in the expression range imaged. Compared with a conventional acoustic quantification method (which used retained bubble signal intensity at 20 min post-bubble injection), this new approach exhibited greater dynamic range and sensitivity and was able to simultaneously quantify other useful characteristics (e.g., the microbubble half-life). In conclusion, quantitative determination of the level of molecular expression is feasible acoustically using a time-signal intensity curve analytical method based on bubble elimination.

Micro- and nanobubbles: a versatile non-viral platform for gene delivery

Micro- and nanobubbles provide a promising non-viral strategy for ultrasound mediated gene delivery. Microbubbles are spherical gas-filled structures with a mean diameter of 1-8 μm, characterised by their core-shell composition and their ability to circulate in the bloodstream following intravenous injection. They undergo volumetric oscillations or acoustic cavitation when insonified by ultrasound and, most importantly, they are able to resonate at diagnostic frequencies. It is due to this behaviour that microbubbles are currently being used as ultrasound contrast agents, but their use in therapeutics is still under investigation. For example, microbubbles could play a role in enhancing gene delivery to cells: when combined with clinical ultrasound exposure, microbubbles are able to favour gene entry into cells by cavitation. Two different delivery strategies have been used to date: DNA can be co-administered with the microbubbles (i.e. the contrast agent) or 'loaded' in purposed-built bubble systems - indeed a number of different technological approaches have been proposed to associate genes within microbubble structures. Nanobubbles, bubbles with sizes in the nanometre order of magnitude, have also been developed with the aim of obtaining more efficient gene delivery systems. Their small sizes allow the possibility of extravasation from blood vessels into the surrounding tissues and ultrasound-targeted site-specific release with minimal invasiveness. In contrast, microbubbles, due to their larger sizes, are unable to extravasate, thus and their targeting capacity is limited to specific antigens present within the vascular lumen. This review provides an overview of the use of microbubbles as gene delivery systems, with a specific focus on recent research into the development of nanosystems. In particular, ultrasound delivery mechanisms, formulation parameters, gene-loading approaches and the advantages of nanometric systems will be described.

In vivo demonstration of cancer molecular imaging with ultrasound radiation force and buried-ligand microbubbles

In designing targeted contrast agent materials for imaging, the need to present a targeting ligand for recognition and binding by the target is counterbalanced by the need to minimize interactions with plasma components and to avoid recognition by the immune system. We have previously reported on a microbubble imaging probe for ultrasound molecular imaging that uses a buried-ligand surface architecture to minimize unwanted interactions and immunogenicity. Here we examine for the first time the utility of this approach for in vivo molecular imaging. In accordance with previous results, we showed a threefold increase in circulation persistence through the tumor of a fibrosarcoma model in comparison with controls. The buried-ligand microbubbles were then activated for targeted adhesion through the application of noninvasive ultrasound radiation forces applied specifically to the tumor region. Using a clinical ultrasound scanner, microbubbles were activated, imaged, and silenced. The results showed visually conspicuous images of tumor neovasculature and a twofold increase in ultrasound radiation force enhancement of acoustic contrast intensity for buried-ligand microbubbles, whereas no such increase was found for exposed-ligand microbubbles. We therefore conclude that the use of acoustically active buried-ligand microbubbles for ultrasound molecular imaging bridges the demand for low immunogenicity with the necessity of maintaining targeting efficacy and imaging conspicuity in vivo.

The effect of lipid monolayer in-plane rigidity on in vivo microbubble circulation persistence

The goal of this study was to increase in vivo microbubble circulation persistence for applications in medical imaging and targeted drug delivery. Our approach was to investigate the effect of lipid monolayer in-plane rigidity to reduce the rate of microbubble dissolution, while holding constant the microbubble size, concentration and surface architecture. We first estimated the impact of acyl chain length of the main diacyl phosphatidylcholine (PC) lipid and inter-lipid distance on the cohesive surface energy and, based on these results, we hypothesized that microbubble stability and in vivo ultrasound contrast persistence would increase monotonically with increasing acyl chain length. We therefore measured microbubble in vitro stability to dilution with and without ultrasound exposure, as well as in vivo ultrasound contrast persistence. All measurements showed a sharp rise in stability between DPPC (C16:0) and DSPC (C18:0), which correlates to the wrinkling transition, signaling the onset of significant surface shear and gas permeation resistance, observed in prior single-bubble dissolution studies. Further evidence for the effect of the wrinkling transition came from an in vitro and in vivo stability comparison of microbubbles coated with pure DPPC with those of lung surfactant extract. Microbubble stability against dilution without ultrasound and in vivo ultrasound contrast persistence showed a monotonic increase with acyl chain length from DSPC to DBPC (C22:0). However, we also observed that stability dropped precipitously for all measurements on further increasing lipid acyl chain length from DBPC to DLiPC (C24:0). This result suggests that hydrophobic mismatch between the main PC lipid and the lipopolymer emulsifier, DSPE-PEG5000, may drive a less stable surface microstructure. Overall, these results support our general hypothesis of the role of in-plane rigidity for increasing the lifetime of microbubble circulation.

Multimodality imaging using SPECT/CT and MRI and ligand functionalized 99mTc-labeled magnetic microbubbles

Background

In the present study, we used multimodal imaging to investigate biodistribution in rats after intravenous administration of a new ^99mTc-labeled delivery system consisting of polymer-shelled microbubbles (MBs) functionalized with diethylenetriaminepentaacetic acid (DTPA), thiolated poly(methacrylic acid) (PMAA), chitosan, 1,4,7-triacyclononane-1,4,7-triacetic acid (NOTA), NOTA-super paramagnetic iron oxide nanoparticles (SPION), or DTPA-SPION.

Methods

Examinations utilizing planar dynamic scintigraphy and hybrid imaging were performed using a commercially available single-photon emission computed tomography (SPECT)/computed tomography (CT) system. For SPION containing MBs, the biodistribution pattern of ^99mTc-labeled NOTA-SPION and DTPA-SPION MBs was investigated and co-registered using fusion SPECT/CT and magnetic resonance imaging (MRI). Moreover, to evaluate the biodistribution, organs were removed and radioactivity was measured and calculated as percentage of injected dose.

Results

SPECT/CT and MRI showed that the distribution of ^99mTc-labeled ligand-functionalized MBs varied with the type of ligand as well as with the presence of SPION. The highest uptake was observed in the lungs 1 h post injection of ^99mTc-labeled DTPA and chitosan MBs, while a similar distribution to the lungs and the liver was seen after the administration of PMAA MBs. The highest counts of ^99mTc-labeled NOTA-SPION and DTPA-SPION MBs were observed in the lungs, liver, and kidneys 1 h post injection. The highest counts were observed in the liver, spleen, and kidneys as confirmed by MRI 24 h post injection. Furthermore, the results obtained from organ measurements were in good agreement with those obtained from SPECT/CT.

Conclusions

In conclusion, microbubbles functionalized by different ligands can be labeled with radiotracers and utilized for SPECT/CT imaging, while the incorporation of SPION in MB shells enables imaging using MR. Our investigation revealed that biodistribution may be modified using different ligands. Furthermore, using a single contrast agent with fusion SPECT/CT/MR multimodal imaging enables visualization of functional and anatomical information in one image, thus improving the diagnostic benefit for patients.

Advances in ultrasound mediated gene therapy using microbubble contrast agents

Microbubble ultrasound contrast agents have the potential to dramatically improve gene therapy treatments by enhancing the delivery of therapeutic DNA to malignant tissue. The physical response of microbubbles in an ultrasound field can mechanically perturb blood vessel walls and cell membranes, enhancing drug permeability into malignant tissue. In this review, we discuss literature that provided evidence of specific mechanisms that enhance in vivo gene delivery utilizing microbubble contrast agents, namely their ability to 1) improving cell membrane permeability, 2) modulate vascular permeability, and 3) enhance endocytotic uptake in cells. Additionally, we review novel microbubble vectors that are being developed in order to exploit these mechanisms and deliver higher gene payloads with greater target specificity. Finally, we discuss some future considerations that should be addressed in the development of next-generation microbubbles in order to improve in vivo microbubble gene delivery. Overall, microbubbles are rapidly gaining popularity as efficient gene carriers, and combined with their functionality as imaging contrast agents, they represent powerful theranostic tools for image guided gene therapy applications.

Microbubbles as ultrasound contrast agents for molecular imaging: preparation and application

OBJECTIVE

The purpose of this review is to describe trends in microbubble application in molecular imaging.

CONCLUSION

Microbubbles are used for contrast ultrasound imaging as blood-pool agents in cardiology and radiology. Their promise as targeted agents for molecular imaging is now being recognized. Microbubbles can be functionalized with ligand molecules that bind to molecular markers of disease. Potential clinical applications of molecular imaging with microbubble-based ultrasound contrast agents are in the monitoring of the biomarker status of vascular endothelium, visualizing tumor vasculature, and imaging inflammation and ischemia-reperfusion injury zones and thrombi.