Targeted ultrasound contrast agent for molecular imaging of inflammation in high-shear flow

In Vivo Validation of Modulated Acoustic Radiation Force-Based Imaging in Murine Model of Abdominal Aortic Aneurysm Using VEGFR-2-Targeted Microbubbles

OBJECTIVES

The objective of this study is to validate the modulated acoustic radiation force (mARF)-based imaging method in the detection of abdominal aortic aneurysm (AAA) in murine models using vascular endothelial growth factor receptor 2 (VEGFR-2)-targeted microbubbles (MBs).

MATERIALS AND METHODS

The mouse AAA model was prepared using the subcutaneous angiotensin II (Ang II) infusion combined with the β-aminopropionitrile monofumarate solution dissolved in drinking water. The ultrasound imaging session was performed at 7 days, 14 days, 21 days, and 28 days after the osmotic pump implantation. For each imaging session, 10 C57BL/6 mice were implanted with Ang II-filled osmotic pumps, and 5 C57BL/6 mice received saline infusion only as the control group. Biotinylated lipid MBs conjugated to either anti-mouse VEGFR-2 antibody (targeted MBs) or isotype control antibody (control MBs) were prepared before each imaging session and were injected into mice via tail vein catheter. Two separate transducers were colocalized to image the AAA and apply ARF to translate MBs simultaneously. After each imaging session, tissue was harvested and the aortas were used for VEGFR-2 immunostaining analysis. From the collected ultrasound image data, the signal magnitude response of the adherent targeted MBs was analyzed, and a parameter, residual-to-saturation ratio ( Rres - sat ), was defined to measure the enhancement in the adherent targeted MBs signal after the cessation of ARF compared with the initial signal intensity. Statistical analysis was performed with the Welch t test and analysis of variance test.

RESULTS

The Rres - sat of abdominal aortic segments from Ang II-challenged mice was significantly higher compared with that in the saline-infused control group ( P < 0.001) at all 4 time points after osmotic pump implantation (1 week to 4 weeks). In control mice, the Rres - sat values were 2.13%, 1.85%, 3.26%, and 4.85% at 1, 2, 3, and 4 weeks postimplantation, respectively. In stark contrast, the Rres - sat values for the mice with Ang II-induced AAA lesions were 9.20%, 20.6%, 22.7%, and 31.8%, respectively. It is worth noting that there was a significant difference between the Rres - sat for Ang II-infused mice at all 4 time points ( P < 0.005), a finding not present in the saline-infused mice. Immunostaining results revealed the VEGFR-2 expression was increased in the abdominal aortic segments of Ang II-infused mice compared with the control group.

CONCLUSIONS

The mARF-based imaging technique was validated in vivo using a murine model of AAA and VEGFR-2-targeted MBs. Results in this study indicated that the mARF-based imaging technique has the ability to detect and assess AAA growth at early stages based on the signal intensity of adherent targeted MBs, which is correlated with the expression level of the desired molecular biomarker. The results may suggest, in very long term, a pathway toward eventual clinical implementation for an ultrasound molecular imaging-based approach to AAA risk assessment in asymptomatic patients.

Contrast-enhanced ultrasound imaging using long-circulating cationic magnetic microbubbles in vitro and in vivo validations

Traditional encapsulated microbubbles are recently used as delivery carriers for drugs and genes, but they have low efficiency. If the local microbubble concentration could be increased, this might be able to improve the therapeutic efficacy of diseases. In this study, we developed novel cationic magnetic microbubbles (MB_M), which could simultaneously realize targeted aggregation under a magnetic field as well as ultrasonographic real-time visualization. Their physicochemical properties, biocompatibility, ultrasonography, magnetic response characteristics, and biodistribution were systematically evaluated. Here, the MB_M were 2.55±0.14µm in size with a positive zeta potential, and had a good biocompatibility. They were able to enhance ultrasonographic contrast both in vitro and in vivo. MB_M could be attracted by an external magnet for directional movement and aggregation in vitro. We confirmed that MB_M also had a great magnetic response in vivo, by means of fluorescence imaging and contrast-enhanced ultrasound imaging. Following intravenous injection into tumor-bearing mice, MB_M showed excellent stability in the internal circulation, and could accumulate in the tumor vasculature through magnetic targeting. With the excellent combination of magnetic response and acoustic properties, cationic magnetic microbubbles (MB_M) have promising potential for use as a new kind of drug/gene carrier for theranostics in the future.

Dynamic Filtering of Adherent and Non-adherent Microbubble Signals Using Singular Value Thresholding and Normalized Singular Spectrum Area Techniques

Ultrasound molecular imaging techniques rely on the separation and identification of three types of signals: static tissue, adherent microbubbles and non-adherent microbubbles. In this study, the image filtering techniques of singular value thresholding (SVT) and normalized singular spectrum area (NSSA) were combined to isolate and identify vascular endothelial growth factor receptor 2-targeted microbubbles in a mouse hindlimb tumor model (n = 24). By use of a Verasonics Vantage 256 imaging system with an L12-5 transducer, a custom-programmed pulse inversion sequence employing synthetic aperture virtual source element imaging was used to collect contrast images of mouse tumors perfused with microbubbles. SVT was used to suppress static tissue signals by 9.6 dB while retaining adherent and non-adherent microbubble signals. NSSA was used to classify microbubble signals as adherent or non-adherent with high accuracy (receiver operating characteristic area under the curve [ROC AUC] = 0.97), matching the classification performance of differential targeted enhancement. The combined SVT + NSSA filtering method also outperformed differential targeted enhancement in differentiating MB signals from all other signals (ROC AUC = 0.89) without necessitating destruction of the contrast agent. The results from this study indicate that SVT and NSSA can be used to automatically segment and classify contrast signals. This filtering method with potential real-time capability could be used in future diagnostic settings to improve workflow and speed the clinical uptake of ultrasound molecular imaging techniques.

Contrast-enhanced ultrasound molecular imaging of activated platelets in the progression of atherosclerosis using microbubbles bearing the von Willebrand factor A1 domain

Platelet-endothelial interactions have been linked to increased inflammatory activation and a prothrombotic state in atherosclerosis. The interaction between von Willebrand factor (vWF)-A1 domain and platelet glycoprotein (GP) Ib/IX plays a significant role in mediating the adhesion of platelets to the injured endothelium. In the present study, contrast-enhanced ultrasound (CEU) molecular imaging with microbubbles bearing the vWF-A1 domain was performed to non-invasively monitor activated platelets on the vascular endothelium in the procession of atherosclerosis. A targeted CEU contrast agent was prepared by attaching the vWF-A1 domain to the shell of microbubbles (Mb_A1). Rat isotype control antibody was used to produce control (Mb_ctrl) microbubbles. The binding of Mb_A1 and Mb_ctrl to activated platelets was assessed in in vitro flow chamber experiments. Apolipoprotein E (ApoE^-/-) deficient mice were studied as a model of atherosclerosis. At 8, 16 and 32 weeks of age, CEU molecular imaging of the proximal aorta with Mb_A1 and Mb_ctrl was performed and the imaging signals from microbubbles were quantified. Atherosclerotic lesion severity and platelets on the endothelial surface were assessed by histology and immunohistochemistry. In in vitro flow chamber studies, attachment of Mb_A1 to activated platelets on culture dishes was significantly greater than that of Mb_ctrl across a range of shear stresses (P<0.05). The attachment of Mb_ctrl was sparse and not related to the aggregated platelets. As lesion development progressed in the ApoE^-/- mice, molecular imaging of activated platelets demonstrated selective signal enhancement of Mb_A1 (P<0.05 vs. Mb_ctrl) at all ages. Selective signal enhancement from Mb_A1 increased from 8 to 32 weeks of age. Immunohistochemistry for GPIIb revealed the presence of platelets on the endothelial cell surface in each group of ApoE^-/- mice and that the degree of platelet deposits was age-dependent. The results of the present study indicated that non-invasive CEU molecular imaging with targeted microbubbles bearing the vWF-A1 domain could not only detect activated platelets on the vascular endothelium but also indicate lesion severity in atherosclerosis.

A Targeted Molecular Localization Imaging Method Applied to Tumor Microvasculature

OBJECTIVES

Ultrasound contrast agents, consisting of gas-filled microbubbles (MBs), have been imaged using several techniques that include ultrasound localization microscopy and targeted molecular imaging. Each of these techniques aims to provide indicators of the disease state but has traditionally been performed independently without co-localization of molecular markers and super-resolved vessels. In this article, we present a new imaging technology: a targeted molecular localization (TML) approach, which uses a single imaging sequence and reconstruction approach to co-localize super-resolved vasculature with molecular imaging signature to provide simultaneous anatomic and biological information for potential multiscale disease evaluation.

MATERIALS AND METHODS

The feasibility of the proposed TML technique was validated in a murine hindlimb tumor model. Targeted molecular localization imaging was performed on 3 groups, which included control tissue (leg), tumor tissue, and tumor tissue after sunitinib an-tivascular treatment. Quantitative measures for vascular index (VI) and molecular index (MITML) were calculated from the microvasculature and TML images, respectively. In addition to these conventional metrics, a new metric unique to the TML technique, reporting the ratio of targeted molecular index to vessel surface, was assessed.

RESULTS

The quantitative resolution results of the TML approach showed resolved resolution of the microvasculature down to 28.8 μm. Vascular index increased in tumors with and without sunitinib compared with the control leg, but the trend was not statistically significant. A decrease in MITML was observed for the tumor after treatment (P < 0.0005) and for the control leg (P < 0.005) compared with the tumor before treatment. Statistical differences in the ratio of molecular index to vessel surface were found between all groups: the control leg and tumor (P < 0.05), the control leg and tumor after sunitinib treatment (P < 0.05), and between tumors with and without sunitinib treatment (P < 0.001).

CONCLUSIONS

These findings validated the technical feasibility of the TML method and pre-clinical feasibility for differentiating between the normal and diseased tissue states.

Molecular Ultrasound Imaging

In the last decade, molecular ultrasound imaging has been rapidly progressing. It has proven promising to diagnose angiogenesis, inflammation, and thrombosis, and many intravascular targets, such as VEGFR2, integrins, and selectins, have been successfully visualized in vivo. Furthermore, pre-clinical studies demonstrated that molecular ultrasound increased sensitivity and specificity in disease detection, classification, and therapy response monitoring compared to current clinically applied ultrasound technologies. Several techniques were developed to detect target-bound microbubbles comprising sensitive particle acoustic quantification (SPAQ), destruction-replenishment analysis, and dwelling time assessment. Moreover, some groups tried to assess microbubble binding by a change in their echogenicity after target binding. These techniques can be complemented by radiation force ultrasound improving target binding by pushing microbubbles to vessel walls. Two targeted microbubble formulations are already in clinical trials for tumor detection and liver lesion characterization, and further clinical scale targeted microbubbles are prepared for clinical translation. The recent enormous progress in the field of molecular ultrasound imaging is summarized in this review article by introducing the most relevant detection technologies, concepts for targeted nano- and micro-bubbles, as well as their applications to characterize various diseases. Finally, progress in clinical translation is highlighted, and roadblocks are discussed that currently slow the clinical translation.

Ultrasound for the treatment of acute kidney injury and other inflammatory conditions: a promising path toward noninvasive neuroimmune regulation

Acute kidney injury (AKI) is an important clinical disorder with high prevalence, serious consequences, and limited therapeutic options. Modulation of neuroimmune interaction by nonpharmacological methods is emerging as a novel strategy for treating inflammatory diseases, including AKI. Recently, pulsed ultrasound (US) treatment was shown to protect from AKI by stimulating the cholinergic anti-inflammatory pathway. Because of the relatively simple, portable, and noninvasive nature of US procedures, US stimulation may be a valuable therapeutic option for treating inflammatory conditions. This review discusses potential impacts of US bioeffects on the nervous system and how this may generate feedback onto the immune system. We also discuss recent evidence supporting the use of US as a means to treat AKI and other inflammatory diseases.

Pharmacokinetic Modeling of Targeted Ultrasound Contrast Agents for Quantitative Assessment of Anti-Angiogenic Therapy: a Longitudinal Case-Control Study in Colon Cancer

PURPOSE

To evaluate quantitative and semi-quantitative ultrasound molecular imaging (USMI) for antiangiogenic therapy monitoring in human colon cancer xenografts in mice.

PROCEDURES

Colon cancer was established in 17 mice by injection of LS174T (N_r = 9) or CT26 (N_n = 8) cancer cells to simulate clinical responders and non-responders, respectively. Antiangiogenic treatment (bevacizumab; N_rt = N_nt = 5) or control treatment (saline; N_rc = 4, N_nc = 3) was administered at days 0, 3, and 7. Three-dimensional USMI was performed by injection at days 0, 1, 3, 7, and 10 of microbubbles targeted to the vascular endothelial growth factor receptor 2 (VEGFR2). Microbubble binding rate (k_b), estimated by first-pass binding model fitting, and semi-quantitative parameters late enhancement (LE) and differential targeted enhancement (dTE) were compared at each day to evaluate their ability to assess and predict the response to therapy. Correlation analysis with the ex-vivo immunohistological quantification of VEGFR2 expression and the percentage blood vessel area was also performed.

RESULTS

Significant changes in the USMI parameters during treatment were observed only in the responders treated with bevacizumab (p-value < 0.05). Prediction of the response to therapy as early as 1 day after treatment was achieved by the quantitative parameter k_b (p-value < 0.01), earlier than possible by tumor volume quantification. USMI parameters could significantly distinguish between clinical responders and non-responders (p-value << 0.01) and correlated well with the ex-vivo quantification of VEGFR2 expression and the percentage blood vessels area (p-value << 0.01).

CONCLUSION

USMI (semi)quantitative parameters provide earlier assessment of the response to therapy compared to tumor volume, permit early prediction of non-responders, and correlate well with ex-vivo angiogenesis biomarkers.

Development of a Translatable Ultrasound Molecular Imaging Agent for Inflammation

This study details the development, characterization and non-clinical efficacy of an ultrasound molecular imaging agent intended for molecular imaging of P-selectin in humans. A targeting ligand based on a recently discovered human selectin ligand was manufactured as fusion protein, and activity for human and mouse P- and E-selectin was evaluated by functional immunoassay. The targeting ligand was covalently conjugated to a lipophilic anchor inserted into a phospholipid microbubble shell. Three lots of the targeted microbubble drug product, TS-07-009, were produced, and assays for size distribution, zeta potential and morphology were established. The suitability of TS-07-009 as a molecular imaging agent was evaluated in vitro in a flow-based adhesion assay and in vivo using a canine model of transient myocardial ischemia. Selectivity for P-selectin over E-selectin was observed in both the human and murine systems. Contrast agent adhesion increased with P-selectin concentration in a dynamic adhesion assay. Significant contrast enhancement was observed on ultrasound imaging with TS-07-009 in post-ischemic canine myocardium at 30 or 90 min of re-perfusion. Negligible enhancement was observed in resting (no prior ischemia) hearts or with a control microbubble 90 min after ischemia. The microbubble contrast agent described here exhibits physiochemical properties and in vivo behavior suitable for development as a clinical imaging agent.

Validation of Normalized Singular Spectrum Area as a Classifier for Molecularly Targeted Microbubble Adherence

Ultrasound molecular imaging is a diagnostic technique wherein molecularly targeted microbubble contrast agents are imaged to reveal disease markers on the blood vessel endothelium. Currently, microbubble adhesion to affected tissue can be quantified using differential targeted enhancement (dTE), which measures the late enhancement of adherent microbubbles through administration of destructive ultrasound pressures. In this study, we investigated a statistical parameter called the normalized singular spectrum area (NSSA) as a means to detect microbubble adhesion without microbubble destruction. We compared the signal differentiation capability of NSSA with matched dTE measurements in a mouse hindlimb tumor model. Results indicated that NSSA-based signal classification performance matches dTE when differentiating adherent microbubble from non-adherent microbubble signals (receiver operating characteristic area under the curve = 0.95), and improves classification performance when differentiating microbubble from tissue signals (p < 0.005). NSSA-based signal classification eliminates the need for destruction of contrast, and may offer better sensitivity, specificity and the opportunity for real-time microbubble detection and classification.

Formation of Microbubbles for Targeted Ultrasound Contrast Imaging: Practical Translation Considerations

For preparation of ligand-decorated microbubbles for targeted ultrasound contrast imaging, it is important to maximize the amount of ligand associated with the bubble shell. We describe optimization of the use of a biocompatible cosurfactant in the microbubble formulation media to maximize the incorporation of targeting ligand-lipid conjugate into the microbubble shell, and thus reduce the fraction of ligand not associated with microbubbles, following amalgamation preparation. The influence of the concentration of a helper cosurfactant propylene glycol (PG) on the efficacy of microbubble preparation by amalgamation and on the degree of association of fluorescent PEG-lipid with the microbubble shell was tested. Three sets of targeted bubbles were then prepared: with VCAM-1-targeting peptide VHPKQHRGGSK(FITC)GC-PEG-DSPE, cyclic RGDfK-PEG-DSPE, selective for α_Vβ₃, and control cRADfK-PEG-DSPE, without such affinity. Microbubbles were prepared by 45 s amalgamation, with DSPC and PEG stearate as the main components of the shell, with 15% PG in aqueous saline. In vitro microbubble targeting was assessed with a parallel plate flow chamber with a recombinant receptor coated surface. In vivo targeting was assessed in MC-38 tumor-bearing mice (subcutaneous tumor in hind leg), 10 min after intravenous bolus of microbubble contrast agent (20 million particles per injection). Ultrasound imaging of the tumor and control nontarget muscle tissue in a contralateral leg was performed with a clinical scanner. Amalgamation technique with PG cosurfactant produced microbubbles at concentrations exceeding 2 × 10⁹ particles/mL, and ∼50-60% or more of the added fluorescein-PEG-DSPE or VCAM-1-targeted fluorescent peptide was associated with microbubbles, about 2 times higher than that in the absence of PG. After intravenous injection, peptide-targeted bubbles selectively accumulated in the tumor vasculature, with negligible accumulation in nontumor contralateral leg muscle, or with control nontargeted microbubbles (assessed by contrast ultrasound imaging). For comparison, administration of RGD-decorated microbubbles prepared by traditional sonication, and purified from free peptide-PEG-lipid by repeated centrifugation, resulted in the same accumulation pattern as for translatable amalgamated microbubbles. Following amalgamation in the presence of PG, efficient transfer of ligand-PEG-lipid to microbubble shell was achieved and quantified. Purification of microbubbles from free peptide-PEG-lipid was not necessary, as proven by in vitro and in vivo targeting studies, so PG cosurfactant amalgamation technique generated peptide-targeted microbubbles are amenable for bedside preparation and clinical translation. The pathway to clinical translation is simplified by the fact that most of the materials used in this study either are on the United States Food and Drug Administration GRAS list or can be procured as pharmaceutical grade substances.

Applications of Ultrasound to Stimulate Therapeutic Revascularization

Many pathological conditions are characterized or caused by the presence of an insufficient or aberrant local vasculature. Thus, therapeutic approaches aimed at modulating the caliber and/or density of the vasculature by controlling angiogenesis and arteriogenesis have been under development for many years. As our understanding of the underlying cellular and molecular mechanisms of these vascular growth processes continues to grow, so too do the available targets for therapeutic intervention. Nonetheless, the tools needed to implement such therapies have often had inherent weaknesses (i.e., invasiveness, expense, poor targeting, and control) that preclude successful outcomes. Approximately 20 years ago, the potential for using ultrasound as a new tool for therapeutically manipulating angiogenesis and arteriogenesis began to emerge. Indeed, the ability of ultrasound, especially when used in combination with contrast agent microbubbles, to mechanically manipulate the microvasculature has opened several doors for exploration. In turn, multiple studies on the influence of ultrasound-mediated bioeffects on vascular growth and the use of ultrasound for the targeted stimulation of blood vessel growth via drug and gene delivery have been performed and published over the years. In this review article, we first discuss the basic principles of therapeutic ultrasound for stimulating angiogenesis and arteriogenesis. We then follow this with a comprehensive cataloging of studies that have used ultrasound for stimulating revascularization to date. Finally, we offer a brief perspective on the future of such approaches, in the context of both further research development and possible clinical translation.

Molecular Imaging of a New Multimodal Microbubble for Adhesion Molecule Targeting

Introduction

Inflammation is an important risk-associated component of many diseases and can be diagnosed by molecular imaging of specific molecules. The aim of this study was to evaluate the possibility of targeting adhesion molecules on inflammation-activated endothelial cells and macrophages using an innovative multimodal polyvinyl alcohol-based microbubble (MB) contrast agent developed for diagnostic use in ultrasound, magnetic resonance, and nuclear imaging.

Methods

We assessed the binding efficiency of antibody-conjugated multimodal contrast to inflamed murine or human endothelial cells (ECs), and to peritoneal macrophages isolated from rats with peritonitis, utilizing the fluorescence characteristics of the MBs. Single-photon emission tomography (SPECT) was used to illustrate ^99mTc-labeled MB targeting and distribution in an experimental in vivo model of inflammation.

Results

Flow cytometry and confocal microscopy showed that binding of antibody-targeted MBs to the adhesion molecules ICAM-1, VCAM-1, or E-selectin, expressed on cytokine-stimulated ECs, was up to sixfold higher for human and 12-fold higher for mouse ECs, compared with that of non-targeted MBs. Under flow conditions, both VCAM-1- and E-selectin-targeted MBs adhered more firmly to stimulated human ECs than to untreated cells, while VCAM-1-targeted MBs adhered best to stimulated murine ECs. SPECT imaging showed an approximate doubling of signal intensity from the abdomen of rats with peritonitis, compared with healthy controls, after injection of anti-ICAM-1-MBs.

Conclusions

This novel multilayer contrast agent can specifically target adhesion molecules expressed as a result of inflammatory stimuli in vitro, and has potential for use in disease-specific multimodal diagnostics in vivo using antibodies against targets of interest.

Klf4 has an unexpected protective role in perivascular cells within the microvasculature

Recent smooth muscle cell (SMC) lineage-tracing studies have revealed that SMCs undergo remarkable changes in phenotype during development of atherosclerosis. Of major interest, we demonstrated that Kruppel-like factor 4 (KLF4) in SMCs is detrimental for overall lesion pathogenesis, in that SMC-specific conditional knockout of the KLF4 gene ( Klf4) resulted in smaller, more-stable lesions that exhibited marked reductions in the numbers of SMC-derived macrophage- and mesenchymal stem cell-like cells. However, since the clinical consequences of atherosclerosis typically occur well after our reproductive years, we sought to identify beneficial KLF4-dependent SMC functions that were likely to be evolutionarily conserved. We tested the hypothesis that KLF4-dependent SMC transitions play an important role in the tissue injury-repair process. Using SMC-specific lineage-tracing mice positive and negative for simultaneous SMC-specific conditional knockout of Klf4, we demonstrate that SMCs in the remodeling heart after ischemia-reperfusion injury (IRI) express KLF4 and transition to a KLF4-dependent macrophage-like state and a KLF4-independent myofibroblast-like state. Moreover, heart failure after IRI was exacerbated in SMC Klf4 knockout mice. Surprisingly, we observed a significant cardiac dilation in SMC Klf4 knockout mice before IRI as well as a reduction in peripheral resistance. KLF4 chromatin immunoprecipitation-sequencing analysis on mesenteric vascular beds identified potential baseline SMC KLF4 target genes in numerous pathways, including PDGF and FGF. Moreover, microvascular tissue beds in SMC Klf4 knockout mice had gaps in lineage-traced SMC coverage along the resistance arteries and exhibited increased permeability. Together, these results provide novel evidence that Klf4 has a critical maintenance role within microvascular SMCs: it is required for normal SMC function and coverage of resistance arteries. NEW & NOTEWORTHY We report novel evidence that the Kruppel-like factor 4 gene ( Klf4) has a critical maintenance role within microvascular smooth muscle cells (SMCs). SMC-specific Klf4 knockout at baseline resulted in a loss of lineage-traced SMC coverage of resistance arteries, dilation of resistance arteries, increased blood flow, and cardiac dilation.

Ultrasound Molecular Imaging of Inflammation in Mouse Abdominal Aorta

OBJECTIVES

The aim of this study was to demonstrate a new clinically translatable ultrasound molecular imaging approach, modulated acoustic radiation force-based imaging, which is capable of rapid and reliable detection of inflammation as validated in mouse abdominal aorta.

MATERIALS AND METHODS

Animal studies were approved by the Institutional Animal Care and Use Committee at the University of Virginia. C57BL/6 mice stimulated with tumor necrosis factor α, or fed with a high-fat diet, were used as inflammation (MInflammation) and diet-induced obesity (DIO) (MDIO) models, respectively. C57BL/6 mice, not exposed to tumor necrosis factor α or DIO, were used as controls (MNormal). P-selectin-targeted (MBP-selectin), vascular cell adhesion molecule (VCAM)-1-targeted (MBVCAM-1), and isotype control (MBControl) microbubbles were synthesized by conjugating anti-P-selectin, anti-VCAM-1, and isotype control antibodies to microbubbles, respectively. The abdominal aortas were imaged for 180 seconds during a constant infusion of microbubbles. A parameter, residual-to-saturation ratio (RSR), was used to assess P-selectin and VCAM-1. Statistical analysis was performed with the Student t test.

RESULTS

For the inflammation model, RSR of the MInflammation + MBP-selectin group was significantly higher (40.9%, P < 0.0005) than other groups. For the DIO model, RSR of the MDIO + MBVCAM-1 group was significantly higher (60.0%, P < 0.0005) than other groups. Immunohistochemistry staining of the abdominal aorta confirmed the expression of P-selectin and VCAM-1.

CONCLUSIONS

A statistically significant assessment of P-selectin and VCAM-1 in mouse abdominal aorta was achieved. This technique yields progress toward rapid targeted molecular imaging in large blood vessels and thus has the potential for early diagnosis, treatment selection, and risk stratification of atherosclerosis.

Targeting of microbubbles: contrast agents for ultrasound molecular imaging

For contrast ultrasound imaging, the most efficient contrast agents comprise highly compressible gas-filled microbubbles. These micrometer-sized particles are typically filled with low-solubility perfluorocarbon gases, and coated with a thin shell, often a lipid monolayer. These particles circulate in the bloodstream for several minutes; they demonstrate good safety and are already in widespread clinical use as blood pool agents with very low dosage necessary (sub-mg per injection). As ultrasound is an ubiquitous medical imaging modality, with tens of millions of exams conducted annually, its use for molecular/targeted imaging of biomarkers of disease may enable wider implementation of personalised medicine applications, precision medicine, non-invasive quantification of biomarkers, targeted guidance of biopsy and therapy in real time. To achieve this capability, microbubbles are decorated with targeting ligands, possessing specific affinity towards vascular biomarkers of disease, such as tumour neovasculature or areas of inflammation, ischaemia-reperfusion injury or ischaemic memory. Once bound to the target, microbubbles can be selectively visualised to delineate disease location by ultrasound imaging. This review discusses the general design trends and approaches for such molecular ultrasound imaging agents, which are currently at the advanced stages of development, and are evolving towards widespread clinical trials.

Biomimetic nanoparticles with enhanced affinity towards activated endothelium as versatile tools for theranostic drug delivery

Activation of the vascular endothelium is characterized by increased expression of vascular adhesion molecules and chemokines. This activation occurs early in the progression of several diseases and triggers the recruitment of leukocytes. Inspired by the tropism of leukocytes, we investigated leukocyte-based biomimetic nanoparticles (i.e., leukosomes) as a novel theranostic platform for inflammatory diseases.

Methods: Leukosomes were assembled by combining phospholipids and membrane proteins from leukocytes. For imaging applications, phospholipids modified with rhodamine and gadolinium were used. Leukosomes incubated with antibodies blocking lymphocyte function-associated antigen 1 (LFA-1) and CD45 were administered to explore their roles in targeting inflammation. In addition, relaxometric assessment of NPs was evaluated.

Results: Liposomes and leukosomes were both spherical in shape with sizes ranging from 140-170 nm. Both NPs successfully integrated 8 and 13 µg of rhodamine and gadolinium, respectively, and demonstrated less than 4% variation in physicochemical features. Leukosomes demonstrated a 16-fold increase in breast tumor accumulation relative to liposomes. Furthermore, quantification of leukosomes in tumor vessels demonstrated a 4.5-fold increase in vessel lumens and a 14-fold increase in vessel walls. Investigating the targeting mechanism of action revealed that blockage of LFA-1 on leukosomes resulted in a 95% decrease in tumor accumulation. Whereas blockage of CD45 yielded a 60% decrease in targeting and significant increases in liver and spleen accumulation. In addition, when administered in mice with atherosclerotic plaques, leukosomes exhibited a 4-fold increase in the targeting of inflammatory vascular lesions. Lastly, relaxometric assessment of NPs demonstrated that the incorporation of membrane proteins into leukosomes did not impact the r₁ and r₂ relaxivities of the NPs, demonstrating 6 and 30 mM^-1s^-1, respectively.

Conclusion: Our study demonstrates the ability of leukosomes to target activated vasculature and exhibit superior accumulation in tumors and vascular lesions. The versatility of the phospholipid backbone within leukosomes permits the incorporation of various contrast agents. Furthermore, leukosomes can potentially be loaded with therapeutics possessing diverse physical properties and thus warrant further investigation toward the development of powerful theranostic agents.

Contrast-Enhanced Ultrasound for Assessing Renal Perfusion Impairment and Predicting Acute Kidney Injury to Chronic Kidney Disease Progression

AIMS

Acute kidney injury (AKI) is increasingly recognized as a major risk factor leading to progression to chronic kidney disease (CKD). However, the diagnostic tools for predicting AKI to CKD progression are particularly lacking. Here, we tested the utility of contrast-enhanced ultrasound (CEUS) for predicting progression to CKD after AKI by using both mild (20-min) and severe (45-min) bilateral renal ischemia-reperfusion injury mice.

RESULTS

Renal perfusion measured by CEUS reduced to 25% ± 7% and 14% ± 6% of the pre-ischemic levels in mild and severe AKI 1 h after ischemia (p < 0.05). Renal perfusion returned to pre-ischemic levels 1 day after mild AKI followed by restoration of kidney function. However, severe AKI caused persistent renal perfusion impairment (60% ± 9% of baseline levels) accompanied by progressive renal fibrosis and sustained decrease in renal function. Renal perfusion at days 1-21 significantly correlated with tubulointerstitial fibrosis 42 days after AKI. For predicting renal fibrosis at day 42, the area under the receiver operating characteristics curve of renal perfusion impairment at day 1 was 0.84. Similar changes in the renal image of CEUS were observed in patients with AKI-CKD progression.

INNOVATION

This study demonstrates that CEUS enables dynamic and noninvasive detection of renal perfusion impairment after ischemic AKI and the perfusion abnormalities shown by CEUS can early predict the progression to CKD after AKI.

CONCLUSIONS

These results indicate that CEUS enables the evaluation of renal perfusion impairment associated with CKD after ischemic AKI and may serve as a noninvasive technique for assessing AKI-CKD progression. Antioxid. Redox Signal. 27, 1397-1411.

Ultrasound molecular imaging of arterial thrombi with novel microbubbles modified by cyclic RGD in vitro and in vivo

Despite immense potential, ultrasound molecular imaging (UMI) of arterial thrombi remains very challenging because the high-shear arterial flow limits binding of site-targeted microbubbles to the thrombi. The linear Arg-Gly-Asp (RGD) peptides have been successfully applied to evaluate venous, atrial, and arteriolar thrombi, but have thus far failed in the detection of arterial thrombi. Cyclic RGD (Arg-Gly-Asp-D-Phe-Cys) is a cyclic conformation of linear RGD peptides, which has much higher binding-affinity and selectivity for binding to the glycoprotein (GP) IIb/IIIa receptor than its linear counterpart and thus is likely to be an optimal targeted molecular probe for ultrasound molecular imaging of arterial thrombi. In this study, we sought to assess the feasibility of a novel microbubble conjugated with cyclic RGD (Mb-cyclic RGD) in UMI of arterial thrombi in vitro and in vivo. As expected, Mb-cyclic RGD had greater GP IIb/IIIa-targeted binding capability in all shear stress conditions. In addition, the shear stress at half-maximal detachment of Mb-cyclic RGD was 5.7-fold higher than that of microbubbles with nonspecific peptide (Mb-CON) (p<0.05). Mb-cyclic RGD enhanced the echogenicity of the platelet-rich thrombus in vitro whereas Mb-CON did not produce enhancement. In the in vivo setting, optimal signal enhancement of the abdominal aortic thrombus was displayed with Mb-cyclic RGD in all cases. Mean video intensity of the abdominal aortic thrombi with Mb-cyclic RGD was 3.2-fold higher than that with Mb-CON (p<0.05). The novel Mb-cyclic RGD facilitated excellent visualisation of arterial thrombi using UMI and showed great promise for clinical applications.

Ultra-Low-Dose Ultrasound Molecular Imaging for the Detection of Angiogenesis in a Mouse Murine Tumor Model: How Little Can We See?

OBJECTIVES

The objective of this study was to evaluate the minimum microbubble dose for ultrasound molecular imaging to achieve statistically significant detection of angiogenesis in a mouse model.

MATERIALS AND METHODS

The preburst minus postburst method was implemented on a Verasonics ultrasound research scanner using a multiframe compounding pulse inversion imaging sequence. Biotinylated lipid (distearoyl phosphatidylcholine-based) microbubbles that were conjugated with antivascular endothelial growth factor 2 (VEGFR2) antibody (MBVEGFR2) or isotype control antibody (MBControl) were injected into mice carrying adenocarcinoma xenografts. Different injection doses ranging from 5 × 10 to 1 × 10 microbubbles per mouse were evaluated to determine the minimum diagnostically effective dose.

RESULTS

The proposed imaging sequence was able to achieve statistically significant detection (P < 0.05, n = 5) of VEGFR2 in tumors with a minimum MBVEGFR2 injection dose of only 5 × 10 microbubbles per mouse (distearoyl phosphatidylcholine at 0.053 ng/g mouse body mass). Nonspecific adhesion of MBControl at the same injection dose was negligible. In addition, the targeted contrast ultrasound signal of MBVEGFR2 decreased with lower microbubble doses, whereas nonspecific adhesion of MBControl increased with higher microbubble doses.

CONCLUSIONS

The dose of 5 × 10 microbubbles per animal is now the lowest injection dose on record for ultrasound molecular imaging to achieve statistically significant detection of molecular targets in vivo. Findings in this study provide us with further guidance for future developments of clinically translatable ultrasound molecular imaging applications using a lower dose of microbubbles.

The Use of Acoustic Radiation Force Decorrelation-Weighted Pulse Inversion for Enhanced Ultrasound Contrast Imaging

OBJECTIVES

The use of ultrasound imaging for cancer diagnosis and screening can be enhanced with the use of molecularly targeted microbubbles. Nonlinear imaging strategies such as pulse inversion (PI) and "contrast pulse sequences" (CPS) can be used to differentiate microbubble signal, but often fail to suppress highly echogenic tissue interfaces. This failure results in false-positive detection and potential misdiagnosis. In this study, a novel acoustic radiation force (ARF)-based approach was developed for superior microbubble signal detection. The feasibility of this technique, termed ARF decorrelation-weighted PI (ADW-PI), was demonstrated in vivo using a subcutaneous mouse tumor model.

MATERIALS AND METHODS

Tumors were implanted in the hindlimb of C57BL/6 mice by subcutaneous injection of MC38 cells. Lipid-shelled microbubbles were conjugated to anti-VEGFR2 antibody and administered via bolus injection. An image sequence using ARF pulses to generate microbubble motion was combined with PI imaging on a Verasonics Vantage programmable scanner. ADW-PI images were generated by combining PI images with interframe signal decorrelation data. For comparison, CPS images of the same mouse tumor were acquired using a Siemens Sequoia clinical scanner.

RESULTS

Microbubble-bound regions in the tumor interior exhibited significantly higher signal decorrelation than static tissue (n = 9, P < 0.001). The application of ARF significantly increased microbubble signal decorrelation (n = 9, P < 0.01). Using these decorrelation measurements, ADW-PI imaging demonstrated significantly improved microbubble contrast-to-tissue ratio when compared with corresponding CPS or PI images (n = 9, P < 0.001). Contrast-to-tissue ratio improved with ADW-PI by approximately 3 dB compared with PI images and 2 dB compared with CPS images.

CONCLUSIONS

Acoustic radiation force can be used to generate adherent microbubble signal decorrelation without microbubble bursting. When combined with PI, measurements of the resulting microbubble signal decorrelation can be used to reconstruct images that exhibit superior suppression of highly echogenic tissue interfaces when compared with PI or CPS alone.

Molecular Ultrasound Imaging for the Detection of Neural Inflammation: A Longitudinal Dosing Pilot Study

Molecular ultrasound imaging provides the ability to detect physiologic processes noninvasively by targeting a variety of biomarkers in vivo. The current study was performed by exploiting an inflammatory biomarker, P-selectin, known to be present following spinal cord injury. Using a murine model (n = 6), molecular ultrasound imaging was performed using contrast microbubbles modified to target and adhere to P-selectin, prior to spinal cord injury (0D), acute stage postinjury (7D), and chronic stage (42D). Additionally, two imaging sessions were performed on each subject at specific time points, using doses of 30 μL and 100 μL. Upon analysis, targeted contrast analysis parameters were appreciably increased during the 7D scan compared with the 42D scan, without statistical significance. When examining the dose levels, the 30-μL dose demonstrated greater values than the 100-μL dose but lacked statistical significance. These findings provide additional preclinical evidence for the use of molecular ultrasound imaging for the possible detection of inflammation.

Assessment of Molecular Acoustic Angiography for Combined Microvascular and Molecular Imaging in Preclinical Tumor Models

PURPOSE

The purposes of the present study is to evaluate a new ultrasound molecular imaging approach in its ability to image a preclinical tumor model and to investigate the capacity to visualize and quantify co-registered microvascular and molecular imaging volumes.

PROCEDURES

Molecular imaging using the new technique was compared with a conventional ultrasound molecular imaging technique (multi-pulse imaging) by varying the injected microbubble dose and scanning each animal using both techniques. Each of the 14 animals was randomly assigned one of three doses; bolus dose was varied, and the animals were imaged for three consecutive days so that each animal received every dose. A microvascular scan was also acquired for each animal by administering an infusion of nontargeted microbubbles. These scans were paired with co-registered molecular images (VEGFR2-targeted microbubbles), the vessels were segmented, and the spatial relationships between vessels and VEGFR2 targeting locations were analyzed. In five animals, an additional scan was performed in which the animal received a bolus of microbubbles targeted to E- and P-selectins. Vessel tortuosity as a function of distance from VEGF and selectin targeting was analyzed in these animals.

RESULTS

Although resulting differences in image intensity due to varying microbubble dose were not significant between the two lowest doses, superharmonic imaging had significantly higher contrast-to-tissue ratio (CTR) than multi-pulse imaging (mean across all doses 13.98 dB for molecular acoustic angiography vs. 0.53 dB for multi-pulse imaging; p = 4.9 × 10^-10). Analysis of registered microvascular and molecular imaging volumes indicated that vessel tortuosity decreases with increasing distance from both VEGFR2- and selectin-targeting sites.

CONCLUSIONS

Molecular acoustic angiography (superharmonic molecular imaging) exhibited a significant increase in CTR at all doses tested due to superior rejection of tissue artifact signals. Due to the high resolution of acoustic angiography molecular imaging, it is possible to analyze spatial relationships in aligned microvascular and molecular superharmonic imaging volumes. Future studies are required to separate the effects of biomarker expression and blood flow kinetics in comparing local tortuosity differences between different endothelial markers such as VEGFR2, E-selectin, and P-selectin.

Molecular Ultrasound Imaging of the Spinal Cord for the Detection of Acute Inflammation

Molecular ultrasound imaging provides the ability to detect physiologic processes non-invasively by targeting a wide variety of biological markers in vivo. The current study investigates the novel application of molecular ultrasound imaging for the detection of neural inflammation. Using a murine model with acutely injured spinal cords (n=31), subjects were divided into four groups, each being administered ultrasound contrast microbubbles bearing antibodies against various known inflammatory molecules (P-selectin, vascular cell adhesion protein 1 [VCAM-1], intercellular adhesion molecule 1 [ICAM-1], and isotype control) during molecular ultrasound imaging. Upon administration of the targeted contrast agent, ultrasound imaging of the injured spinal cord was performed at 40MHz for seven minutes, followed by a bursting pulse. We observed significantly enhanced signals from contrast targeted to P-selectin and VCAM-1, using a variety of outcome measures. These findings provide preclinical evidence that molecular ultrasound imaging could be a useful tool in the detection of neural inflammation.

Molecular imaging of the extracellular matrix in the context of atherosclerosis

This review summarizes the current status of molecular imaging of the extracellular matrix (ECM) in the context of atherosclerosis. Apart from cellular components, the ECM of the atherosclerotic plaque plays a relevant role during the initiation of atherosclerosis and its' subsequent progression. Important structural and signaling components of the ECM include elastin, collagen and fibrin. However, the ECM not only plays a structural role in the arterial wall but also interacts with different cell types and has important biological signaling functions. Molecular imaging of the ECM has emerged as a new diagnostic tool to characterize biological aspects of atherosclerotic plaques, which cannot be characterized by current clinically established imaging techniques, such as X-ray angiography. Different types of molecular probes can be detected in vivo by imaging modalities such as magnetic resonance imaging (MRI), positron emission tomography (PET) and single photon emission computed tomography (SPECT). The modality specific signaling component of the molecular probe provides information about its spatial location and local concentration. The successful introduction of molecular imaging into clinical practice and guidelines could open new pathways for an earlier detection of disease processes and a better understanding of the disease state on a biological level. Quantitative in vivo molecular parameters could also contribute to the development and evaluation of novel cardiovascular therapeutic interventions and the assessment of response to treatment.

Lipid microbubbles as a vehicle for targeted drug delivery using focused ultrasound-induced blood-brain barrier opening

Focused ultrasound in conjunction with lipid microbubbles has fully demonstrated its ability to induce non-invasive, transient, and reversible blood-brain barrier opening. This study was aimed at testing the feasibility of our lipid-coated microbubbles as a vector for targeted drug delivery in the treatment of central nervous system diseases. These microbubbles were labeled with the fluorophore 5-dodecanoylaminfluorescein. Focused ultrasound targeted mouse brains in vivo in the presence of these microbubbles for trans-blood-brain barrier delivery of 5-dodecanoylaminfluorescein. This new approach, compared to previously studies of our group, where fluorescently labeled dextrans and microbubbles were co-administered, represents an appreciable improvement in safety outcome and targeted drug delivery. This novel technique allows the delivery of 5-dodecanoylaminfluorescein at the region of interest unlike the alternative of systemic exposure. 5-dodecanoylaminfluorescein delivery was assessed by ex vivo fluorescence imaging and by in vivo transcranial passive cavitation detection. Stable and inertial cavitation doses were quantified. The cavitation dose thresholds for estimating, a priori, successful targeted drug delivery were, for the first time, identified with inertial cavitation were concluded to be necessary for successful delivery. The findings presented herein indicate the feasibility and safety of the proposed microbubble-based targeted drug delivery and that, if successful, can be predicted by cavitation detection in vivo.

Targeted Contrast-Enhanced Ultrasound for Inflammation Detection

Molecular imaging is a form of nanotechnology that enables the noninvasive examination of biological processes in vivo. Radiopharmaceutical agents are used to target biochemical markers, permitting their detection and evaluation. Early visualization of molecular variations indicative of pathophysiological processes can aid in patient diagnoses and management decisions. Molecular imaging is performed by introducing into the body molecular probes, which are often contrast agents that have been nanoengineered to target and tether to molecules, thus enabling their radiologic identification. Through a nanoengineering process, ultrasound contrast agents can be targeted to specific molecules, extending ultrasound’s capabilities from the tissue to molecular level. Molecular ultrasound, or targeted contrast-enhanced ultrasound (TCEUS), has recently emerged as a popular molecular imaging technique due to its ability to provide real-time anatomic and functional information without ionizing radiation. However, molecular ultrasound represents a novel form of molecular imaging and consequently remains largely preclinical. This review explores the commonalities of TCEUS across several molecular targets and points to the need for standardization of kinetic behavior analysis. The literature underscores evidence gaps and the need for additional research. The application of TCEUS is unlimited but needs further standardization to ensure that future research studies are comparable.

Microbubbles with a Self-Assembled Poloxamer Shell and a Fluorocarbon Inner Gas

The numerous applications of microbubbles in food science and medicine call for a better understanding and control of the effects of the properties of their shells on their stability and ability to resonate at chosen frequencies when submitted to an ultrasound field. We have investigated both millimetric and micrometric bubbles stabilized by an amphiphilic block copolymer, Poloxamer 188 (e.g., Pluronic F-68). Although Pluronic F-68 is routinely being used as a dispersing and foaming agent to facilitate phospholipid-based microbubble preparation, it has never been studied as a shell component per se. First, we investigated the adsorption kinetics of Pluronic F-68 at the interface between water and air, or air saturated with vapors of perfluorohexane (F-hexane), using bubble profile tensiometry analysis. F-Hexane was found to strongly accelerate the adsorption of Pluronic F-68 (at low concentrations) and decrease the interfacial tension values at equilibrium (at all concentrations). We also found that relatively stable microbubbles could unexpectedly be prepared from Pluronic F-68 in the absence of any other surfactant, but only when F-hexane was present. These bubbles showed an only limited volume increase over ∼3 h, while a 10-fold increase in size occurred within 200 s in the absence of a fluorocarbon. Remarkably, their deflation rate decreased when the Pluronic F-68 concentration decreased, suggesting that bubbles with semidilute copolymer coverage are more stable than those more densely covered by copolymer brushes. Single-bubble experiments using laser Doppler vibratometry showed that, by contrast with other surfactant-coated microbubbles, the resonance radius of the Pluronic F-68-coated microbubbles was lower than that of naked microbubbles, meaning that they are less elastic. It was also found that the bubble's vibrational displacement amplitude decreased substantially when the microbubbles were covered with Pluronic F-68, an effect that was further amplified by F-hexane.

Exploring Targeted Contrast-Enhanced Ultrasound to Detect Neural Inflammation

Targeted contrast-enhanced ultrasound (TCEUS) is an innovative method of molecular imaging used for detection of inflammatory biomarkers in vivo. By targeting ultrasound contrast to cell adhesion molecules (CAMs), which are known inflammatory markers within neural tissue, a more direct evaluation of neural inflammation can be made. Due to the novel nature of TCEUS, standardized methods of image analysis do not yet exist. Time intensity curve (TIC) shape analysis is currently used in magnetic resonance contrast imaging to determine temporal behavior of perfusion. Therefore, the presented research attempts to determine TIC shape analysis utility in TCEUS imaging by applying it to TCEUS scans targeted to CAMs present in neural inflammation. This was done in an animal model that underwent a traumatic spinal cord injury to induce inflammation ( n = 31). Subjects were divided into four groups, each receiving a TCEUS targeted to a different CAM seven days after surgery (P-selectin, intracellular adhesion molecule 1 [ICAM-1], vascular cell adhesion molecule 1 [VCAM-1], and control). TICs were generated using average pixel intensity within the injured region of the spinal cord. TIC shape analysis found similar curves were produced while targeting P-selectin and VCAM-1, both demonstrating rapid and sustained enhancement. Control injections demonstrated no enhancement. ICAM-1 injections demonstrated limited enhancement and a shape similar to the control.

Non-Invasive, Focal Disconnection of Brain Circuitry Using Magnetic Resonance-Guided Low-Intensity Focused Ultrasound to Deliver a Neurotoxin

Disturbances in the function of neuronal circuitry contribute to most neurologic disorders. As knowledge of the brain's connectome continues to improve, a more refined understanding of the role of specific circuits in pathologic states will also evolve. Tools capable of manipulating identified circuits in a targeted and restricted manner will be essential not only to expand our understanding of the functional roles of such circuits, but also to therapeutically disconnect critical pathways contributing to neurologic disease. This study took advantage of the ability of low-intensity focused ultrasound (FUS) to transiently disrupt the blood-brain barrier (BBB) to deliver a neurotoxin with poor BBB permeability (quinolinic acid [QA]) in a guided manner to a target region in the brain parenchyma. Ten male Sprague-Dawley rats were divided into two groups receiving the following treatments: (i) magnetic resonance-guided FUS + microbubbles + saline (n = 5), or (ii) magnetic resonance-guided FUS + microbubbles + QA (n = 5). Systemic administration of QA was well tolerated. However, when QA and microbubbles were systemically administered in conjunction with magnetic resonance-guided FUS, the BBB was disrupted and primary neurons were destroyed in the targeted subregion of the hippocampus in all QA-treated animals. Administration of vehicle (saline) together with microbubbles and FUS also disrupted the BBB but did not produce neuronal injury. These findings indicate the feasibility of non-invasively destroying a targeted region of the brain parenchyma using low-intensity FUS together with systemic administration of microbubbles and a neurotoxin. This approach could be of therapeutic value in various disorders in which disturbances of neural circuitry contribute to neurologic disease.

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.

Ultrasound Molecular Imaging of the Breast Cancer Neovasculature using Engineered Fibronectin Scaffold Ligands: A Novel Class of Targeted Contrast Ultrasound Agent

Molecularly-targeted microbubbles (MBs) are increasingly being recognized as promising contrast agents for oncological molecular imaging with ultrasound. With the detection and validation of new molecular imaging targets, novel binding ligands are needed that bind to molecular imaging targets with high affinity and specificity. In this study we assessed a novel class of potentially clinically translatable MBs using an engineered 10^th type III domain of human-fibronectin (MB-FN3_VEGFR2) scaffold-ligand to image VEGFR2 on the neovasculature of cancer. The in vitro binding of MB-FN3_VEGFR2 to a soluble VEGFR2 was assessed by flow-cytometry (FACS) and binding to VEGFR2-expressing cells was assessed by flow-chamber cell attachment studies under flow shear stress conditions. In vivo binding of MB-FN3_VEGFR2 was tested in a transgenic mouse model (FVB/N Tg(MMTV/PyMT634Mul) of breast cancer and control litter mates with normal mammary glands. In vitro FACS and flow-chamber cell attachment studies showed significantly (P<0.01) higher binding to VEGFR2 using MB-FN3_VEGFR2 than control agents. In vivo ultrasound molecular imaging (USMI) studies using MB-FN3_VEGFR2 demonstrated specific binding to VEGFR2 and was significantly higher (P<0.01) in breast cancer compared to normal breast tissue. Ex vivo immunofluorescence-analysis showed significantly (P<0.01) increased VEGFR2-expression in breast cancer compared to normal mammary tissue. Our results suggest that MBs coupled to FN3-scaffolds can be designed and used for USMI of breast cancer neoangiogenesis. Due to their small size, stability, solubility, the lack of glycosylation and disulfide bonds, FN3-scaffolds can be recombinantly produced with the advantage of generating small, high affinity ligands in a cost efficient way for USMI.

Stable Small Composite Microbubbles Decorated with Magnetite Nanoparticles - A Synergistic Effect between Surfactant Molecules and Nanoparticles

Three approaches to preparing iron oxide nanoparticle-decorated microbubbles (NP-decoMBs) have been investigated. The size and stability characteristics of these microbubbles (MBs) were investigated by optical microscopy, laser light scattering and an acoustical method, and compared with those of non-decorated MBs. First, magnetite nanoparticles (Fe3O4NPs) grafted with dimyristoylphosphatidylcholine (DMPC) were synthesized and used to prepare MBs by brief sonication under an atmosphere of air saturated with perfluorohexane. These MBs had a rather large mean radius (r ~ 12 µm), and a moderate volume of encapsulated gas. Remarkably, a second approach that consisted of dispersing unbound DMPC molecules in the aqueous phase along with DMPC-grafted Fe3O4NPs prior to sonication was found to drastically change the situation, allowing the obtaining of monomodal populations of much smaller (r ~ 0.6 µm) NP-decoMBs. The latter were echogenic and stable for at least 10 days at room temperature, without significant variation of their size characteristics. In a third approach, NP-decoMBs were directly prepared from dispersions of naked Fe3O4NPs in the presence of DMPC. The resulting NP-decoMBs suspensions consisted of broadly distributed bubble populations mostly containing two populations (with r ~ 5 and ~ 15 µm). Control microbubbles made of DMPC only were small (r ~ 1.3 µm), although not as small as those formed from DMPC-grafted Fe3O4NPs in the presence of free DMPC, and were less stable, with a room temperature half-life of only ~1 day. These observations imply that there is a synergy between the Fe3O4NPs and the DMPC molecules in the air/water interfacial film stabilization process.

Molecular Acoustic Angiography: A New Technique for High-resolution Superharmonic Ultrasound Molecular Imaging

Ultrasound molecular imaging utilizes targeted microbubbles to bind to vascular targets such as integrins, selectins and other extracellular binding domains. After binding, these microbubbles are typically imaged using low pressures and multi-pulse imaging sequences. In this article, we present an alternative approach for molecular imaging using ultrasound that relies on superharmonic signals produced by microbubble contrast agents. Bound bubbles were insonified near resonance using a low frequency (4 MHz) element and superharmonic echoes were received at high frequencies (25-30 MHz). Although this approach was observed to produce declining image intensity during repeated imaging in both in vitro and in vivo experiments because of bubble destruction, the feasibility of superharmonic molecular imaging was demonstrated for transmit pressures, which are sufficiently high to induce shell disruption in bound microbubbles. This approach was validated using microbubbles targeted to the αvβ3 integrin in a rat fibrosarcoma model (n = 5) and combined with superharmonic images of free microbubbles to produce high-contrast, high-resolution 3-D volumes of both microvascular anatomy and molecular targeting. Image intensity over repeated scans and the effect of microbubble diameter were also assessed in vivo, indicating that larger microbubbles yield increased persistence in image intensity. Using ultrasound-based acoustic angiography images rather than conventional B-mode ultrasound to provide the underlying anatomic information facilitates anatomic localization of molecular markers. Quantitative analysis of relationships between microvasculature and targeting information indicated that most targeting occurred within 50 μm of a resolvable vessel (>100 μm diameter). The combined information provided by these scans may present new opportunities for analyzing relationships between microvascular anatomy and vascular targets, subject only to limitations of the current mechanically scanned system and microbubble persistence to repeated imaging at moderate mechanical indices.

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.

Differential Impact of Plasma Proteins on the Adhesion Efficiency of Vascular-Targeted Carriers (VTCs) in Blood of Common Laboratory Animals

Vascular-targeted carrier (VTC) interaction with human plasma is known to reduce targeted adhesion efficiency in vitro. However, the role of plasma proteins on the adhesion efficiency of VTCs in laboratory animals remains unknown. Here, in vitro blood flow assays are used to explore the effects of plasma from mouse, rabbit, and porcine on VTC adhesion. Porcine blood exhibited a strong negative plasma effect on VTC adhesion while no significant plasma effect was found with rabbit and mouse blood. A brush density poly(ethylene glycol) (PEG) on VTCs was effective at improving adhesion of microsized, but not nanosized, VTCs in porcine blood. Overall, the results suggest that porcine models, as opposed to mouse, can serve as better models in preclinical research for predicting the in vivo functionality of VTCs for use in humans. These considerations hold great importance for the design of various pharmaceutical products and development of reliable drug delivery systems.

Membrane blebbing as a recovery manoeuvre in site-specific sonoporation mediated by targeted microbubbles

Site-specific perforation of the plasma membrane can be achieved through ultrasound-triggered cavitation of a single microbubble positioned adjacent to the cell. However, for this perforation approach (sonoporation), the recovery manoeuvres invoked by the cell are unknown. Here, we report new findings on how membrane blebbing can be a recovery manoeuvre that may take place in sonoporation episodes whose pores are of micrometres in diameter. Each sonoporation site was created using a protocol involving single-shot ultrasound exposure (frequency: 1 MHz; pulse length: 30 cycles; peak negative pressure: 0.45 MPa) which triggered inertial cavitation of a single targeted microbubble (diameter: 1-5 µm). Over this process, live confocal microscopy was conducted in situ to monitor membrane dynamics, model drug uptake kinetics and cytoplasmic calcium ion (Ca(2+)) distribution. Results show that blebbing would occur at a recovering sonoporation site after its resealing, and it may emerge elsewhere along the membrane periphery. The bleb size was correlated with the pre-exposure microbubble diameter, and 99% of blebbing cases at sonoporation sites were inflicted by microbubbles larger than 1.5 µm diameter (analysed over 124 sonoporation episodes). Blebs were not observed at irreversible sonoporation sites or when sonoporation site repair was inhibited via extracellular Ca(2+) chelation. Functionally, the bleb volume was found to serve as a buffer compartment to accommodate the cytoplasmic Ca(2+) excess brought about by Ca(2+) influx during sonoporation. These findings suggest that membrane blebbing would help sonoporated cells restore homeostasis.

Ultrasound in Radiology: From Anatomic, Functional, Molecular Imaging to Drug Delivery and Image-Guided Therapy

During the past decade, ultrasound has expanded medical imaging well beyond the "traditional" radiology setting: a combination of portability, low cost, and ease of use makes ultrasound imaging an indispensable tool for radiologists as well as for other medical professionals who need to obtain imaging diagnosis or guide a therapeutic intervention quickly and efficiently. Ultrasound combines excellent ability for deep penetration into soft tissues with very good spatial resolution, with only a few exceptions (ie, those involving overlying bone or gas). Real-time imaging (up to hundreds and thousands of frames per second) enables guidance of therapeutic procedures and biopsies; characterization of the mechanical properties of the tissues greatly aids with the accuracy of the procedures. The ability of ultrasound to deposit energy locally brings about the potential for localized intervention encompassing the following: tissue ablation, enhancing penetration through the natural barriers to drug delivery in the body and triggering drug release from carrier microparticles and nanoparticles. The use of microbubble contrast agents brings the ability to monitor and quantify tissue perfusion, and microbubble targeting with ligand-decorated microbubbles brings the ability to obtain molecular biomarker information, that is, ultrasound molecular imaging. Overall, ultrasound has become the most widely used imaging modality in modern medicine; it will continue to grow and expand.

Effect of variation in hemorheology between human and animal blood on the binding efficacy of vascular-targeted carriers

Animal models are extensively used to evaluate the in vivo functionality of novel drug delivery systems (DDS). However, many variations likely exist in vivo between the animals and human physiological environment that significantly alter results obtained with animal models relative to human system. To date, it is not clear if the variation in hemorheology and hemodynamics between common animal and human models affect the functionality of DDS. This study investigates the role of hemorheology of humans and various animal models in dictating the binding efficiency of model vascular-targeted carriers (VTCs) to the wall in physiological blood flows. Specifically, the adhesion of sLe^A-coated nano- and micro-spheres to inflamed endothelial cells monolayers were conducted via a parallel plate flow chamber assay with steady and disturbed red blood cells (RBCs)-in-buffer and whole blood flows of common animal models. Our results suggest that the ratio of carrier size to RBC size dictate particle binding in blood flow. Additionally, the presence of white blood cells affects the trend of particle adhesion depending on the animal species. Overall, this work sheds light on some deviation in VTC vascular wall interaction results obtained with in vivo animal experimentation from expected outcome and efficiency in vivo in human.

Quantification of bound microbubbles in ultrasound molecular imaging

Molecular markers associated with diseases can be visualized and quantified noninvasively with targeted ultrasound contrast agent (t-UCA) consisting of microbubbles (MBs) that can bind to specific molecular targets. Techniques used for quantifying t-UCA assume that all unbound MBs are taken out of the blood pool few minutes after injection and only MBs bound to the molecular markers remain. However, differences in physiology, diseases, and experimental conditions can increase the longevity of unbound MBs. In such conditions, unbound MBs will falsely be quantified as bound MBs. We have developed a novel technique to distinguish and classify bound from unbound MBs. In the post-processing steps, first, tissue motion was compensated using block-matching (BM) techniques. To preserve only stationary contrast signals, a minimum intensity projection (MinIP) or 20th-percentile intensity projection (PerIP) was applied. The after-flash MinIP or PerIP was subtracted from the before-flash MinIP or PerIP. In this way, tissue artifacts in contrast images were suppressed. In the next step, bound MB candidates were detected. Finally, detected objects were tracked to classify the candidates as unbound or bound MBs based on their displacement. This technique was validated in vitro, followed by two in vivo experiments in mice. Tumors (n = 2) and salivary glands of hypercholesterolemic mice (n = 8) were imaged using a commercially available scanner. Boluses of 100 μL of a commercially available t-UCA targeted to angiogenesis markers and untargeted control UCA were injected separately. Our results show considerable reduction in misclassification of unbound MBs as bound ones. Using our method, the ratio of bound MBs in salivary gland for images with targeted UCA versus control UCA was improved by up to two times compared with unprocessed images.

Echogenic perfluorohexane-loaded macrophages adhere in vivo to activated vascular endothelium in mice, an explorative study

Background

Macrophages may concentrate ultrasound contrast agents and exhibit selective adhesion to activated endothelium. The present study investigates in mice the potential of perfluorohexane (PFH) loaded macrophages to act as ultrasound contrast agent with high reflectivity and specifically targeted at (atherosclerotic) vascular lesions.

Methods

Lung passage was evaluated with a mouse echo scanner after injection, at a slow pace or as a bolus, of varying doses of PFH-loaded and unloaded bone marrow macrophages (BMM) into the jugular vein. The interaction of PFH-loaded and unloaded BMM with TNF-α stimulated carotid artery endothelium after tail vein injection was assessed by means of intravital microscopy.

Results

High doses of jugular vein injected PFH-loaded BMM were visible with ultrasound in the pulmonary artery and detectable in the carotid artery. At intravital microscopy, tail vein injected BMM exhibited rolling and adhesion behavior at the TNF-α stimulated carotid endothelium, similar to that of native blood leukocytes. Rolling behavior was not different between PFH-loaded and unloaded BMM (p = 0.38).

Conclusion

In vivo, perfluorohexane loaded macrophages pass the pulmonary circulation and appear on the arterial side. Moreover, they roll and adhere selectively to activated endothelium under physiological flow conditions. These findings indicate that perfluorohexane loaded BMM could be used to study processes in vivo where endothelial activation plays a role, such as atherosclerosis.

Dynamics of targeted microbubble adhesion under pulsatile compared with steady flow

Hemodynamic flow variations at low fluid shear stress are thought to play a critical role in local atherosclerotic plaque initiation and development and to affect plaque instability. Targeted microbubbles are being developed as intravascular agents for identifying atherosclerotic lesions using ultrasound. How variations in local hydrodynamic flow influence the adhesiveness of targeted microbubbles is not well understood. We postulated that rates of targeted microbubble binding and accumulation differ when subjected to steady flow (SF) as compared with oscillatory or pulsatile flow (PF), because PF imposes non-uniform blood rheology and periodic acceleration and deceleration of blood velocity, when compared with SF. We assessed the binding rates of targeted microbubbles in seven randomly assigned PF and seven matched SF replicate runs at low (<1 Pa) and intermediate (≥1 and <2.5 Pa) wall shear stress (WSS) by drawing 4.8 × 10(6) microbubbles mL(-1) over streptavidin-coated substrates, immobilized within a parallel plate flow chamber at a calculated density of 81 binding sites μm(-2). Selective binding and accumulation of targeted microbubbles was recorded in a single field of view using real-time video microscopy. Microbubble accumulation was modeled to obtain flow-mediated microbubble binding kinetics (amplitude, A, and rate constant, k). PF elicited higher microbubble accumulation rates, in comparison to SF. The rates of microbubble accumulation differed significantly between PF and SF (p < 0.05) at intermediate WSS but not at low WSS (p > 0.05). The rate of microbubble accumulation decreased as WSS increased.

Stimulus-responsive ultrasound contrast agents for clinical imaging: motivations, demonstrations, and future directions

Microbubble ultrasound contrast agents allow imaging of the vasculature with excellent resolution and signal-to-noise ratios. Contrast in microbubbles derives from their interaction with an ultrasound wave to generate signal at harmonic frequencies of the stimulating pulse; subtracting the elastic echo caused by the surrounding tissue can enhance the specificity of these harmonic signals significantly. The nonlinear acoustic emission is caused by pressure-driven microbubble size fluctuations, which in both theoretical descriptions and empirical measurements was found to depend on the mechanical properties of the shell that encapsulates the microbubble as well as stabilizes it against the surrounding aqueous environment. Thus biochemically induced switching between a rigid 'off' state and a flexible 'on' state provides a mechanism for sensing chemical markers for disease. In our research, we coupled DNA oligonucleotides to a stabilizing lipid monolayer to modulate stiffness of the shell and thereby induce stimulus-responsive behavior. In initial proof-of-principle studies, it was found that signal modulation came primarily from DNA crosslinks preventing the microbubble size oscillations rather than merely damping the signal. Next, these microbubbles were redesigned to include an aptamer sequence in the crosslinking strand, which not only allowed the sensing of the clotting enzyme thrombin but also provided a general strategy for sensing other soluble biomarkers in the bloodstream. Finally, the thrombin-sensitive microbubbles were validated in a rabbit model, presenting the first example of an ultrasound contrast agent that could differentiate between active and inactive clots for the diagnosis of deep venous thrombosis.

Ultrasound molecular imaging of E-selectin in tumor vessels using poly n-butyl cyanoacrylate microbubbles covalently coupled to a short targeting peptide

OBJECTIVES

The purposes of this study were the development and preclinical evaluation of clinically translatable E-selectin-specific ultrasound contrast agents based on a peptide ligand with the recognition sequence IELLQAR.

MATERIALS AND METHODS

The E-selectin-specific peptide was synthesized through solid phase peptide synthesis and covalently attached to poly n-butylcyanoacrylate-stabilized microbubbles with an air core. Quantification of the microbubble surface coverage with peptides was performed through flow cytometry. Targeted adhesion of peptide-coated microbubbles was investigated in vitro using parallel plate flow chamber assays on tumor necrosis factor-α-stimulated human umbilical vein endothelial cells. In vivo imaging was performed in nude mice bearing human ovarian carcinoma xenografts (MLS), followed by ex vivo immunohistochemistry validation of E-selectin expression.

RESULTS

Success of peptide synthesis was validated through preparative reverse phase high-pressure liquid chromatography and electronspray ionization-mass spectrometry. Results of the flow cytometry revealed approximately 4000 E-selectin-specific peptides/microbubble surface. Results of the in vitro experiments demonstrated the specificity of peptide-coated microbubbles to E-selectin (1.10 ± 0.48 vs 0.19 ± 0.09 bound microbubbles per cell, before and after competition respectively; P < 0.01). The in vivo imaging enabled specific assessment of E-selectin expression in MLS carcinoma xenografts (5.21 ± 3.41 vs 1.37 ± 0.67 contrast intensity before and after competition, respectively; P < 0.05).

CONCLUSIONS

Clinically translatable microbubbles that were covalently coupled to the short E-selectin-specific peptide (IELLQAR) enabled specific imaging of the E-selectin expression in tumor vessels in vivo.

Cardiovascular drug delivery with ultrasound and microbubbles

Microbubbles lower the threshold for cavitation of ultrasound and have multiple potential therapeutic applications in the cardiovascular system. One of the first therapeutic applications to enter into clinical trials has been microbubble-enhanced sonothrombolysis. Trials were conducted in acute ischemic stroke and clinical trials are currently underway for sonothrombolysis in treatment of acute myocardial infarction. Microbubbles can be targeted to epitopes expressed on endothelial cells and thrombi by incorporating targeting ligands onto the surface of the microbubbles. Targeted microbubbles have applications as molecular imaging contrast agents and also for drug and gene delivery. A number of groups have shown that ultrasound with microbubbles can be used for gene delivery yielding robust gene expression in the target tissue. Work has progressed to primate studies showing delivery of therapeutic genes to generate islet cells in the pancreas to potentially cure diabetes. Microbubbles also hold potential as oxygen therapeutics and have shown promising results as a neuroprotectant in an ischemic stroke model. Regulatory considerations impact the successful clinical development of therapeutic applications of microbubbles with ultrasound. This paper briefly reviews the field and suggests avenues for further development.