Targeting mucosal addressin cellular adhesion molecule (MAdCAM)-1 to noninvasively image experimental Crohn's disease

Lactobacillus acidophilus inhibits the TNF-α-induced increase in intestinal epithelial tight junction permeability via a TLR-2 and PI3K-dependent inhibition of NF-κB activation

Background

Defective intestinal epithelial tight junction (TJ), characterized by an increase in intestinal TJ permeability, has been shown to play a critical role in the pathogenesis of inflammatory bowel disease (IBD). Tumor necrosis factor-α (TNF-α) is a key pro-inflammatory cytokine involved in the immunopathology of IBD and has been shown to cause an increase in intestinal epithelial TJ permeability. Although TNF-α antibodies and other biologics have been advanced for use in IBD treatment, these therapies are associated with severe side effects and have limited efficacy, and there is an urgent need for therapies with benign profiles and high therapeutic efficacy. Probiotic bacteria have beneficial effects and are generally safe and represent an important class of potential therapeutic agents in IBD. Lactobacillus acidophilus (LA) is one of the most used probiotics for wide-ranging health benefits, including in gastrointestinal, metabolic, and inflammatory disorders. A specific strain of LA, LA1, was recently demonstrated to have protective and therapeutic effects on the intestinal epithelial TJ barrier. However, the mechanisms of actions of LA1 remain largely unknown.

Methods

The primary aim of this study was to investigate microbial-epithelial interactions and novel signaling pathways that regulate the effect of LA1 on TNF-α-induced increase in intestinal epithelial TJ permeability, using cell culture and animal model systems.

Results and Conclusion

Pre-treatment of filter-grown Caco-2 monolayers with LA1 prevented the TNF-α-induced increase in intestinal epithelial TJ permeability by inhibiting TNF-α-induced activation of NF-κB p50/p65 and myosin light chain kinase (MLCK) gene and kinase activity in a TLR-2-dependent manner. LA1 produced a TLR-2- and MyD88-dependent activation of NF-κB p50/p65 in immune cells; however, LA1, in intestinal cells, inhibited the NF-κB p50/p65 activation in a TLR-2-dependent but MyD88-independent manner. In addition, LA1 inhibition of NF-κB p50/p65 and MLCK gene was mediated by TLR-2 pathway activation of phosphatidylinositol 3-kinase (PI3K) and IKK-α phosphorylation. Our results demonstrated novel intracellular signaling pathways by which LA1/TLR-2 suppresses the TNF-α pathway activation of NF-κB p50/p65 in intestinal epithelial cells and protects against the TNF-α-induced increase in intestinal epithelial TJ permeability.

Aptamer-Functionalized Microbubbles Targeted to P-selectin for Ultrasound Molecular Imaging of Murine Bowel Inflammation

PURPOSE

Our objectives were to develop a targeted microbubble with an anti-P-selectin aptamer and assess its ability to detect bowel inflammation in two murine models of acute colitis.

PROCEDURES

Lipid-shelled microbubbles were prepared using mechanical agitation. A rapid copper-free click chemistry approach (azide-DBCO) was used to conjugate the fluorescent anti-P-selectin aptamer (Fluor-P-Ap) to the microbubble surface. Bowel inflammation was chemically induced using 2,4,6-trinitrobenzenesulfonic acid (TNBS) in both Balb/C and interleukin-10-deficient (IL-10 KO) mice. Mouse bowels were imaged using non-linear contrast mode following an i.v. bolus of 1 × 10⁸ microbubbles. Each mouse received a bolus of aptamer-functionalized and non-targeted microbubbles. Mouse phenotypes and the presence of P-selectin were validated using histology and immunostaining, respectively.

RESULTS

Microbubble labelling of Fluor-P-Ap was complete after 20 min at 37 ̊C. We estimate approximately 300,000 Fluor-P-Ap per microbubble and confirmed fluorescence using confocal microscopy. There was a significant increase in ultrasound molecular imaging signal from both Balb/C (p = 0.003) and IL-10 KO (p = 0.02) mice with inflamed bowels using aptamer-functionalized microbubbles in comparison to non-targeted microbubbles. There was no signal in healthy mice (p = 0.4051) using either microbubble.

CONCLUSIONS

We constructed an aptamer-functionalized microbubble specific for P-selectin using a clinically relevant azide-DBCO click reaction, which could detect bowel inflammation in vivo. Aptamers have potential as a next generation targeting agent for developing cost-efficient and clinically translatable targeted microbubbles.

Contrast Enhanced Ultrasound Molecular Imaging of Spontaneous Chronic Inflammatory Bowel Disease in an Interleukin-2 Receptor α−/− Transgenic Mouse Model Using Targeted Microbubbles

Inflammatory bowel disease (IBD) is a lifelong inflammatory disorder with relapsing–remission cycles, which is currently diagnosed by clinical symptoms and signs, along with laboratory and imaging findings. However, such clinical findings are not parallel to the disease activity of IBD and are difficult to use in treatment monitoring. Therefore, non-invasive quantitative imaging tools are required for the multiple follow-up exams of IBD patients in order to monitor the disease activity and determine treatment regimens. In this study, we evaluated a dual P- and E-selectin-targeted microbubble (MB_Selectin) in an interleukin-2 receptor α deficient (IL-2Rα^−/−) spontaneous chronic IBD mouse model for assessing long-term anti-inflammatory effects with ultrasound molecular imaging (USMI). We used IL-2Rα^−/− (male and female on a C57BL/6 genetic background; n = 39) and C57BL/6 wild-type (negative control; n = 6) mice for the study. USMI of the proximal, middle, and distal colon was performed with MB_Selectin using a small animal scanner (Vevo 2100) up to six times in each IL-2Rα^−/− mouse between 6–30 weeks of age. USMI signals were compared between IL-2Rα^−/− vs. wild-type mice, and sexes in three colonic locations. Imaged colon segments were analyzed ex vivo for inflammatory changes on H&E-stained sections and for selectin expression by immunofluorescence staining. We successfully detected spontaneous chronic colitis in IL-2Rα^−/− mice between 6–30 weeks (onset at 6–14 weeks) compared to wild-type mice. Both male and female IL-2Rα^−/− mice were equally (p = 0.996) affected with the disease, and there was no significant (p > 0.05) difference in USMI signals of colitis between the proximal, middle, and distal colon. We observed the fluctuating USMI signals in IL-2Rα^−/− mice between 6–30 weeks, which might suggest a resemblance of the remission-flare pattern of human IBD. The ex vivo H&E and immunostaining further confirmed the inflammatory changes, and the high expression of P- and E-selectin in the colon. The results of this study highlight the IL-2Rα^−/− mice as a chronic colitis model and are suitable for the long-term assessment of treatment response using a dual P- and E-selectin-targeted USMI.

Intracolic ultrasound molecular imaging: a novel method for assessing colonic tumor necrosis factor-α expression in inflammatory bowel disease

BACKGROUND

While anti-tumor necrosis factor alpha (TNF-α) therapy has been proven effective in inflammatory bowel disease (IBD), approximately 40% of patients lose the response. Transmembrane TNF-α (mTNF-α) expression in the intestinal mucosa is correlated with therapeutic efficacy, and quantification of mTNF-α expression is significant for predicting response. However, conventional intravenous application of microbubbles is unable to assess mTNF-α expression in intestinal mucosa. Herein, we proposed intracolic ultrasound molecular imaging with TNF-α-targeted microbubbles (MBTNF-α) to quantitatively detect mTNF-α expression in the intestinal mucosa.

METHODS

MBTNF-α was synthesized via a biotin-streptavidin bridging method. TNF-α-targeted ultrasound imaging was performed by intracolic application of MBTNF-α to detect mTNF-α expression in surgical specimens from a murine model and patients with IBD. Linear regression analyses were performed to confirm the accuracy of quantitative targeted ultrasound imaging.

RESULTS

On quantitative TNF-α-targeted ultrasound images, a greater signal intensity was observed in the mouse colons with colitis ([1.96 ± 0.45] × 106 a.u.) compared to that of the controls ([0.56 ± 0.21] × 106 a.u., P < 0.001). Targeted US signal intensities and inflammatory lesions were topographically coupled in mouse colons. Linear regression analyses in specimens of mice and patients demonstrated significant correlations between the targeted ultrasound signal intensity and mTNF-α expression (both P < 0.001). Furthermore, TNF-α-targeted ultrasound imaging qualitatively distinguished the varying inflammatory severity in intestinal specimens from IBD patients.

CONCLUSION

Intracolic ultrasound molecular imaging with MBTNF-α enables quantitative assessment of mTNF-α expression. It may be a potential tool for facilitating the implementation of personalized medicine in IBD.

Recent Progress in the Diagnosis and Precise Nanocarrier-Mediated Therapy of Inflammatory Bowel Disease

The effective colon drug delivery remains to be an international frontier research in inflammatory bowel disease (IBD) therapy. The exploration and research of nanocarrier-based nanomedicine with great potential brings new opportunities for IBD therapy and diagnoses. Functional nanocarriers with varying morphology and characteristics can not only effectively avoid the destruction of the complex gastrointestinal (GI) tract microenvironment but also endow drugs with target therapy and improved bioavailability, thus elevating therapeutic efficacy. In this review, we illustrated several challenges in IBD therapy, then emphasis on some latest research progress of nanoparticles based therapy of oral administration, rectal administration and parenteral administration, as well as IBD diagnoses. Finally, we described the future perspective of nanocarriers in the treatment and diagnoses of IBD.

Anti-MAdCAM-1-Conjugated Nanocarriers Delivering Quantum Dots Enable Specific Imaging of Inflammatory Bowel Disease

Purpose

Assessment of inflammatory bowel disease (IBD) currently relies on aspecific clinical signs of bowel inflammation. Specific imaging of the diseased bowel regions is still lacking. Here, we investigate mucosal addressin cell adhesion molecule 1 (MAdCAM-1) as a reliable and specific endothelial target for engineered nanoparticles delivering imaging agents to obtain an exact mapping of diseased bowel foci.

Materials and Methods

We generated a nanodevice composed of PLGA-PEG coupled with anti-MAdCAM-1 antibody half-chains and loaded with quantum dots (P@QD-MdC NPs). Bowel localization and systemic biodistribution of the nanoconjugate were analyzed upon injection in a murine model of chronic IBD obtained through repeated administration of dextran sulfate sodium salt. Specificity for diseased bowel regions was also assessed ex vivo in human specimens from patients with IBD. Potential for development as contrast agent in magnetic resonance imaging was assessed by preliminary study on animal model.

Results

Synthesized nanoparticles revealed good stability and monodispersity. Molecular targeting properties were analyzed in vitro in a cell culture model. Upon intravenous injection, P@QD-MdC NPs were localized in the bowel of colitic mice, with enhanced accumulation at 24 h post-injection compared to untargeted nanoparticles (p<0.05). Nanoparticles injection did not induce histologic lesions in non-target organs. Ex vivo exposure of human bowel specimens to P@QD-MdC NPs revealed specific recognition of the diseased regions vs uninvolved tracts (p<0.0001). After loading with appropriate contrast agent, the nanoparticles enabled localized contrast enhancement of bowel mucosa in the rectum of treated mice.

Conclusion

P@QD-MdC NPs efficiently detected bowel inflammation foci, accurately following the expression pattern of MAdCAM-1. Fine-tuning of this nanoconjugate with appropriate imaging agents offers a promising non-invasive tool for specific IBD diagnosis.

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.

Ultrasensitive molecular imaging of intestinal mucosal inflammation using leukocyte-mimicking particles targeted to MAdCAM-1 in mice

Mucosal tissues play critical roles in health and disease as the primary barrier between the external world and the inner body, lining the digestive, respiratory, urinary, mammary, and reproductive tracts. Clinical evaluation of mucosal tissues is currently performed using endoscopy, such as ileocolonoscopy for the intestinal mucosa, which causes substantial patient discomfort and can lead to organ damage. Here, we developed a contrast agent for molecular magnetic resonance imaging (MRI) that is targeted to mucosal vascular addressin cell adhesion molecule 1 (MAdCAM-1), an adhesion molecule overexpressed by inflamed mucosal tissues. We investigated the diagnostic performance of molecular MRI of MAdCAM-1 to detect mucosal inflammation in several models of acute and chronic intestinal inflammation in mice. We demonstrated that molecular MRI of MAdCAM-1 reveals disease activity and can evaluate the response to inflammatory treatments along the whole intestinal mucosa in clinically relevant models of inflammatory bowel diseases. We also provide evidence that this technique can detect low, subclinical mucosal inflammation. Molecular MRI of MAdCAM-1 has potential applications in early diagnosis, longitudinal follow-up, and therapeutic response monitoring in diseases affecting mucosal tissues, such as inflammatory bowel diseases.

Toward Molecular Imaging of Intestinal Pathology

Inflammatory bowel disease (IBD) is defined by a chronic relapsing and remitting inflammation of the gastrointestinal tract, with intestinal fibrosis being a major complication. The etiology of IBD remains unknown, but it is thought to arise from a dysregulated and excessive immune response to gut luminal microbes triggered by genetic and environmental factors. To date, IBD has no cure, and treatments are currently directed at relieving symptoms and treating inflammation. The current diagnostic of IBD relies on endoscopy, which is invasive and does not provide information on the presence of extraluminal complications and molecular aspect of the disease. Cross-sectional imaging modalities such as computed tomography enterography (CTE), magnetic resonance enterography (MRE), positron emission tomography (PET), single photon emission computed tomography (SPECT), and hybrid modalities have demonstrated high accuracy for the diagnosis of IBD and can provide both functional and morphological information when combined with the use of molecular imaging probes. This review presents the state-of-the-art imaging techniques and molecular imaging approaches in the field of IBD and points out future directions that could help improve our understanding of IBD pathological processes, along with the development of efficient treatments.

Seeing the Invisible-Ultrasound Molecular Imaging

Ultrasound molecular imaging has been developed in the past two decades with the goal of non-invasively imaging disease phenotypes on a cellular level not depicted on anatomic imaging. Such techniques already play a role in pre-clinical research for the assessment of disease mechanisms and drug effects, and are thought to in the future contribute to earlier diagnosis of disease, assessment of therapeutic effects and patient-tailored therapy in the clinical field. In this review, we first describe the chemical composition and structure as well as the in vivo behavior of the ultrasound contrast agents that have been developed for molecular imaging. We then discuss the strategies that are used for targeting of contrast agents to specific cellular targets and protocols used for imaging. Next we describe pre-clinical data on imaging of thrombosis, atherosclerosis and microvascular inflammation and in oncology, including the pathophysiological principles underlying the selection of targets in each area. Where applicable, we also discuss efforts that are currently underway for translation of this technique into the clinical arena.

Chronic Model of Inflammatory Bowel Disease in IL-10-/- Transgenic Mice: Evaluation with Ultrasound Molecular Imaging

Objective: Acute mouse models of inflammatory bowel disease (IBD) fail to mirror the chronic nature of IBD in patients. We sought to develop a chronic mouse IBD model for assessing long-term anti-inflammatory effects with ultrasound molecular imaging (USMI) by using dual P- and E-selectin targeted microbubbles (MB_Selectin). Materials and Methods: Interleukin 10 deficient (IL-10^-/- on a C57BL/6 genetic background; n=55) and FVB (n=16) mice were used. In IL-10^-/-mice, various experimental regimens including piroxicam, 2,4,6-trinitrobenzenesulfonic acid (TNBS) or dextran sulfate sodium (DSS), respectively were used for promoting colitis; colitis was induced with DSS in FVB mice. Using clinical and small animal ultrasound scanners, evolution of inflammation in proximal, middle and distal colon, was monitored with USMI by using MB_Selectin at multiple time points. Imaged colon segments were analyzed ex vivo for inflammatory changes on H&E staining and for P-selectin expression on immunofluorescence staining. Results: Sustained colitis was not detected with USMI in IL-10^-/- or FVB mice with various experimental regimens. USMI signals either gradually decreased after the colitis enhancing/inducing drug/agents were discontinued, or the mortality rate of mice was high. Inflammation was observed on H&E staining in IL-10^-/- mice with piroxicam promotion, while stable overexpression of P-selectin was not found on immunofluorescence staining in the same mice. Conclusion: Sustained colitis in IL-10^-/- mice induced with piroxicam, TNBS or DSS, and in FVB mice induced with DSS, was not detected with USMI using MB_Selectin, and this was verified by immunofluorescence staining for inflammation marker P-selectin. Thus, these models may not be appropriate for long-term monitoring of chronic colitis and subsequent treatment response with dual-selectin targeted USMI.

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.

In Vivo Molecular Imaging Using Low-Boiling-Point Phase-Change Contrast Agents: A Proof of Concept Study

Sub-micron phase-change contrast agents (PCCAs) have been proposed as a tool for ultrasound molecular imaging based on their potential to extravasate and target extravascular markers and also because of the potential to image these contrast agents with a high contrast-to-tissue ratio. We compare in vivo ultrasound molecular imaging with targeted low-boiling-point PCCAs and targeted microbubble contrast agents. Both agents were targeted to the intravascular (endothelial) integrin α_vß₃via a cyclic RGD peptide (cyclo-Arg-Gly-Asp-D-Tyr-Cys) mechanism and imaged in vivo in a rodent fibrosarcoma model, which exhibits angiogenic microvasculature. Signal intensity was measured using two different techniques, conventional contrast-specific imaging (amplitude/phase modulation) and a droplet vaporization imaging sequence, which detects the unique signature of vaporizing PCCAs. Data indicate that PCCA-specific imaging is more sensitive to small numbers of bound agents than conventional contrast imaging. However, data also revealed that contrast from targeted microbubbles was greater than that provided by PCCAs. Both control and targeted PCCAs were observed to be retained in tissue post-vaporization, which was expected for targeted agents but not expected for control agents. The exact mechanism underlying this observation remains unknown.

US Molecular Imaging of Acute Ileitis: Anti-Inflammatory Treatment Response Monitored with Targeted Microbubbles in a Preclinical Model

Purpose To evaluate whether dual-selectin-targeted US molecular imaging allows longitudinal monitoring of anti-inflammatory treatment effects in an acute terminal ileitis model in swine. Materials and Methods The Institutional Animal Care and Use Committee approved all animal studies. Fourteen swine with chemically induced acute terminal ileitis (day 0) were randomized into the following groups: (a) an anti-inflammatory treatment group (n = 8; meloxicam, 0.25 mg per kilogram of body weight; prednisone, 0.5 mg/kg) and (b) a control group (n = 6; saline). US molecular imaging was performed with a clinical US machine after intravenous injection of clinically translatable dual P- and E-selectin-targeted microbubbles (5 × 108/kg). Three inflamed bowel segments per swine were imaged at baseline, as well as on days 1, 3, and 6 after treatment initiation. At day 6, bowel segments were analyzed ex vivo for selectin expression levels by using quantitative immunofluorescence. Results After induction of inflammation, US molecular imaging signal increased at day 1 in both animal groups (P < .001). At day 3, signal in the treatment group decreased (P < .001 vs day 1), while signal in control animals did not significantly change (P = .18 vs day 1) and was higher (P = .001) compared with that in the treatment group. At day 6, signal in the treatment group further decreased and remained lower (P = .02) compared with that in the control group. Immunofluorescence confirmed significant (P ≤ .04) downregulation of both P- and E-selectin expression levels in treated versus control bowel segments. Conclusion Dual-selectin-targeted US molecular imaging allows longitudinal monitoring of anti-inflammatory treatment effects in a large-animal model of acute ileitis. This supports further clinical development of this quantitative and radiation-free technique for monitoring inflammatory bowel disease. © RSNA, 2018 Online supplemental material is available for this article.

Ultrasound Measurement of Vascular Density to Evaluate Response to Anti-Angiogenic Therapy in Renal Cell Carcinoma

BACKGROUND

Functional and molecular changes often precede gross anatomical changes, so early assessment of a tumor's functional and molecular response to therapy can help reduce a patient's exposure to the side effects of ineffective chemotherapeutics or other treatment strategies.

OBJECTIVE

Our intent was to test the hypothesis that an ultrasound microvascular imaging approach might provide indications of response to therapy prior to assessment of tumor size.

METHODS

Mice bearing clear-cell renal cell carcinoma xenograft tumors were treated with antiangiogenic and Notch inhibition therapies. An ultrasound measurement of microvascular density was used to serially track the tumor response to therapy.

RESULTS

Data indicated that ultrasound-derived microvascular density can indicate response to therapy a week prior to changes in tumor volume and is strongly correlated with physiological characteristics of the tumors as measured by histology ([Formula: see text]). Furthermore, data demonstrated that ultrasound measurements of vascular density can determine response to therapy and classify between-treatment groups with high sensitivity and specificity.

CONCLUSION/SIGNIFICANCE

Results suggests that future applications utilizing ultrasound imaging to monitor tumor response to therapy may be able to provide earlier insight into tumor behavior from metrics of microvascular density rather than anatomical tumor size measurements.

A new preclinical ultrasound platform for widefield 3D imaging of rodents

Noninvasive in vivo imaging technologies enable researchers and clinicians to detect the presence of disease and longitudinally study its progression. By revealing anatomical, functional, or molecular changes, imaging tools can provide a near real-time assessment of important biological events. At the preclinical research level, imaging plays an important role by allowing disease mechanisms and potential therapies to be evaluated noninvasively. Because functional and molecular changes often precede gross anatomical changes, there has been a significant amount of research exploring the ability of different imaging modalities to track these aspects of various diseases. Herein, we present a novel robotic preclinical contrast-enhanced ultrasound system and demonstrate its use in evaluating tumors in a rodent model. By leveraging recent advances in ultrasound, this system favorably compares with other modalities, as it can perform anatomical, functional, and molecular imaging and is cost-effective, portable, and high throughput, without using ionizing radiation. Furthermore, this system circumvents many of the limitations of conventional preclinical ultrasound systems, including a limited field-of-view, low throughput, and large user variability.

Ultrasound Molecular Imaging and Drug Delivery

Ultrasound is a rapidly advancing field with many emerging diagnostic and therapeutic applications. For diagnostics, new vascular targets are routinely identified and mature technologies are being translated to humans, while other recent innovations may bring about the creation of acoustic reporter genes and micron-scale resolution with ultrasound. As a cancer therapy, ultrasound is being explored as an adjuvant to immune therapies and to deliver acoustically or thermally active drugs to tumor regions. Ultrasound-enhanced delivery across the blood brain barrier (BBB) could potentially be very impactful for brain cancers and neurodegenerative diseases where the BBB often impedes the delivery of therapeutic molecules. In this minireview, we provide an overview of these topics in the field of ultrasound that are especially relevant to the interests of World Molecular Imaging Society.

Ultrasound Molecular Imaging of Atherosclerosis for Early Diagnosis and Therapeutic Evaluation through Leucocyte-like Multiple Targeted Microbubbles

Cardiovascular diseases resulting from atherosclerosis have become a serious threat to human health. It is well-known that an ongoing inflammatory response is involved during atherosclerosis progression that ultimately results in the accumulation of lipids and formation of plaques. Monitoring the pathological changes during the inflammatory response will be of great significance for early diagnosis and therapeutic evaluation of atherosclerosis. Targeted contrast-enhanced ultrasonography has been shown to be a promising noninvasive imaging technique for evaluating the degree of atherosclerosis and may potentially be translated to clinical imaging in the future. However, inadequate cell adhesion of targeted microbubbles (MBs) in large arterial vessels still remains a great challenge. Methods: By mimicking the leucocytes that are recruited to the vessel wall during the initiation of atherosclerosis through selectin-dependent arrest and cell adhesion molecule-mediated firm cell adhesion, we developed VCAM-1/ICAM-1/P-selectin-targeted MB_VIS by integrating VCAM-1 and ICAM-1 antibodies and synthetic polymeric sialyl Lewis X (sLe^x) onto the MB surface. Results: The resulting MB_VIS had a high affinity to inflammatory bEnd.3 cells in both static and dynamic flow conditions. Significantly enhanced ultrasound imaging signals were achieved by MB_VIS in detecting the atherosclerosis progress when compared with the single- or dual-targeted MBs. Taking advantage of the artificial MB_VIS, less ultrasound imaging signals were found in the atorvastatin-treated, but not placebo-treated, ApoE-deficient mice with atherosclerosis, revealing a potential therapeutic efficacy of atorvastatin for early stage atherosclerosis. This was further confirmed by histologic staining examination. Conclusions: Our study provides a promising ultrasound molecular imaging probe for early-stage diagnosis and therapeutic evaluation 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.

Ultrasound Molecular Imaging of VEGFR-2 in Clear-Cell Renal Cell Carcinoma Tracks Disease Response to Antiangiogenic and Notch-Inhibition Therapy

Metastatic clear-cell renal cell carcinoma (ccRCC) affects thousands of patients worldwide each year. Antiangiogenic therapy has been shown to have beneficial effects initially, but resistance is eventually developed. Therefore, it is important to accurately track the response of cancer to different therapeutics in order to appropriately adjust the therapy to maximize efficacy. Change in tumor volume is the current gold standard for determining efficacy of treatment. However, functional variations can occur much earlier than measurable volume changes. Contrast-enhanced ultrasound (CEUS) is an important tool for assessing tumor progression and response to therapy, since it can monitor functional changes in the physiology. In this study, we demonstrate how ultrasound molecular imaging (USMI) can accurately track the evolution of the disease and molecular response to treatment. Methods A cohort of NSG (NOD/scid/gamma) mice was injected with ccRCC cells and treated with either the VEGF inhibitor SU (Sunitinib malate, Selleckchem, TX, USA) or the Notch pathway inhibitor GSI (Gamma secretase inhibitor, PF-03084014, Pfizer, New York, NY, USA), or started on SU and later switched to GSI (Switch group). The therapies used in the study focus on disrupting angiogenesis and proper vessel development. SU inhibits signaling of vascular endothelial growth factor (VEGF), which is responsible for the sprouting of new vasculature, and GSI inhibits the Notch pathway, which is a key factor in the correct maturation of newly formed vasculature. Microbubble contrast agents targeted to VEGFR-2 (VEGF Receptor) were delivered as a bolus, and the bound agents were imaged in 3D after the free-flowing contrast was cleared from the body. Additionally, the tumors were harvested at the end of the study and stained for CD31. Results The results show that MI can detect changes in VEGFR-2 expression in the group treated with SU within a week of the start of treatment, while differences in volume only become apparent after the mice have been treated for three weeks. Furthermore, USMI can detect response to therapy in 92% of cases after 1 week of treatment, while the detection rate is only 40% for volume measurements. The amount of targeting for the GSI and Control groups was high throughout the duration of the study, while that of the SU and Switch groups remained low. However, the amount of targeting in the Switch group increased to levels similar to those of the Control group after the treatment was switched to GSI. CD31 staining indicates significantly lower levels of patent vasculature for the SU group compared to the Control and GSI groups. Therefore, the results parallel the expected physiological changes in the tumor, since GSI promotes angiogenesis through the VEGF pathway, while SU inhibits it. Conclusion This study demonstrates that MI can track disease progression and assess functional changes in tumors before changes in volume are apparent, and thus, CEUS can be a valuable tool for assessing response to therapy in disease. Future work is required to determine whether levels of VEGFR-2 targeting correlate with eventual survival outcomes.

Anatomical Road Mapping Using CT and MR Enterography for Ultrasound Molecular Imaging of Small Bowel Inflammation in Swine

OBJECTIVES

To evaluate the feasibility and time saving of fusing CT and MR enterography with ultrasound for ultrasound molecular imaging (USMI) of inflammation in an acute small bowel inflammation of swine.

METHODS

Nine swine with ileitis were scanned with either CT (n = 3) or MR (n = 6) enterography. Imaging times to load CT/MR images onto a clinical ultrasound machine, fuse them to ultrasound with an anatomical landmark-based approach, and identify ileitis were compared to the imaging times without anatomical road mapping. Inflammation was then assessed by USMI using dual selectin-targeted (MB_Selectin) and control (MB_Control) contrast agents in diseased and healthy control bowel segments, followed by ex vivo histology.

RESULTS

Cross-sectional image fusion with ultrasound was feasible with an alignment error of 13.9 ± 9.7 mm. Anatomical road mapping significantly reduced (P < 0.001) scanning times by 40%. Localising ileitis was achieved within 1.0 min. Subsequently performed USMI demonstrated significantly (P < 0.001) higher imaging signal using MB_Selectin compared to MB_Control and histology confirmed a significantly higher inflammation score (P = 0.006) and P- and E-selectin expression (P ≤ 0.02) in inflamed vs. healthy bowel.

CONCLUSIONS

Fusion of CT and MR enterography data sets with ultrasound in real time is feasible and allows rapid anatomical localisation of ileitis for subsequent quantification of inflammation using USMI.

KEY POINTS

• Real-time fusion of CT/MRI with ultrasound to localise ileitis is feasible. • Anatomical road mapping using CT/MRI significantly decreases the scanning time for USMI. • USMI allows quantification of inflammation in swine, verified with ex vivo histology.

Nano-targeting of mucosal addressin cell adhesion molecule-1 identifies bowel inflammation foci in murine model

AIM

We investigate MAdCAM-1 as a reliable target to detect active bowel inflammation for selective noninvasive nanodiagnostics.

MATERIALS & METHODS

We coupled anti-MAdCAM-1 antibodies to manganese oxide nanoparticles, and analyzed nanoconjugate biodistribution and safety in murine model of inflammatory bowel disease by imaging and histology.

RESULTS

Nanoparticles were stable and nontoxic. Upon administration in colitic mice, anti-MAdCAM-1 functionalized nanoparticles preferentially localized in the inflamed bowel, whereas untargeted nanoparticles were more rapidly washed out. Nanoparticles did not induce lesions in nontarget organs.

CONCLUSION

Anti-MAdCAM-1 functionalized nanoparticles detected active bowel inflammation foci, accurately following MAdCAM-1 expression pattern. These nanoconjugates could be a promising noninvasive imaging system for an early and accurate follow-up in patients affected by acute colitis.

A robust in vitro model for trans-lymphatic endothelial migration

Trans-endothelial migration (TEM) is essential for leukocyte circulation. While much is known about trans-blood endothelial migration, far less is known about trans-lymphatic endothelial migration. We established an in vitro system to evaluate lymphatic TEM for various cell types across primary mouse and human lymphatic endothelial cells (LEC), and validated the model for the murine LEC cell line SVEC4-10. T cells exhibited enhanced unidirectional migration from the basal (abluminal) to the apical (luminal) surface across LEC, whereas for blood endothelial cells (BEC) they migrated similarly in both directions. This preferential, vectorial migration was chemotactic toward many different chemoattractants and dose-dependent. Stromal protein fibers, interstitial type fluid flow, distribution of chemokines in the stromal layer, and inflammatory cytokines influenced LEC phenotype and leukocyte TEM. Activated and memory CD4 T cells, macrophages, and dendritic cell (DC) showed chemoattractantΔdriven vectorial migration, while CD8 T cell migration across LEC was not. The system was further validated for studying cancer cell transmigration across lymphatic endothelium. This model for lymphatic TEM for various migrating and endothelial cell types possesses the capacity to be high-throughput, highly reproducible and integrate the complexities of lymphatic biology, stromal variability, chemoattractant distribution, and fluid flow.

Uncovering Pathogenic Mechanisms of Inflammatory Bowel Disease Using Mouse Models of Crohn's Disease-Like Ileitis: What is the Right Model?

Crohn's disease and ulcerative colitis, together known as inflammatory bowel disease, are debilitating chronic disorders of unknown cause and cure. Our evolving understanding of these pathologies is enhanced greatly by the use of animal models of intestinal inflammation that allow in vivo mechanistic studies, preclinical evaluation of new therapies, and investigation into the causative factors that underlie disease pathogenesis. Several animal models, most commonly generated in mice, exist for the study of colitis. The appropriateness of their use often can be determined by their mode of generation (ie, chemical induction, T-cell transfer, targeted genetic manipulation, spontaneously occurring, and so forth), the type of investigation (mechanistic studies, pathogenic experiments, preclinical evaluations, and so forth), and the type of inflammation that occurs in the model (acute vs chronic colitis, tissue injury/repair, and so forth). Although most murine models of inflammatory bowel disease develop inflammation in the colon, Crohn's disease most commonly occurs in the terminal ileum, where a very limited number of mouse models manifest disease. This review discusses appropriate experimental applications for different mouse models of colitis, and highlights the particular utility of 2 highly relevant models of Crohn's-like ileitis-the spontaneous SAMP1/YitFc inbred mouse strain and the genetically engineered Tnf^{ΔAU-rich element/+} mouse model of tumor necrosis factor overexpression, both of which bear strong resemblance to the human condition. Similar to patients with Crohn's disease, SAMP1/YitFc ileitis develops spontaneously, without chemical, genetic, or immunologic manipulation, making this model particularly relevant for studies aimed at identifying the primary defect underlying the occurrence of Crohn's ileitis, as well as preclinical testing of novel treatment modalities.

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.

Evaluation of Liver Ischemia-Reperfusion Injury in Rabbits Using a Nanoscale Ultrasound Contrast Agent Targeting ICAM-1

OBJECTIVE

To assess the feasibility of ultrasound molecular imaging in the early diagnosis of liver ischemia-reperfusion injury (IRI) using a nanoscale contrast agent targeting anti-intracellular adhesion molecule-1 (anti-ICAM-1).

METHODS

The targeted nanobubbles containing anti-ICAM-1 antibody were prepared using the avidin-biotin binding method. Human hepatic sinusoidal endothelial cells (HHSECs) were cultured at the circumstances of hypoxia/reoxygenation (H/R) and low temperature. The rabbit liver IRI model (I/R group) was established using the Pringle's maneuver. The time-intensity curve of the liver contrast ultrasonographic images was plotted and the peak intensity, time to peak, and time of duration were calculated.

RESULTS

The size of the targeted nanobubbles were 148.15 ± 39.75 nm and the concentration was 3.6-7.4 × 109/ml, and bound well with the H/R HHSECs. Animal contrast enhanced ultrasound images showed that the peak intensity and time of duration of the targeted nanobubbles were significantly higher than that of common nanobubbles in the I/R group, and the peak intensity and time of duration of the targeted nanobubbles in the I/R group were also significantly higher than that in the SO group.

CONCLUSION

The targeted nanobubbles have small particle size, stable characteristic, and good targeting ability, which can assess hepatic ischemia-reperfusion injury specifically, noninvasively, and quantitatively at the molecular level.

Diagnostic imaging advances in murine models of colitis

Inflammatory bowel diseases (IBD) such as Crohn’s disease and ulcerative colitis are chronic-remittent inflammatory disorders of the gastrointestinal tract still evoking challenging clinical diagnostic and therapeutic situations. Murine models of experimental colitis are a vital component of research into human IBD concerning questions of its complex pathogenesis or the evaluation of potential new drugs. To monitor the course of colitis, to the present day, classical parameters like histological tissue alterations or analysis of mucosal cytokine/chemokine expression often require euthanasia of animals. Recent advances mean revolutionary non-invasive imaging techniques for in vivo murine colitis diagnostics are increasingly available. These novel and emerging imaging techniques not only allow direct visualization of intestinal inflammation, but also enable molecular imaging and targeting of specific alterations of the inflamed murine mucosa. For the first time, in vivo imaging techniques allow for longitudinal examinations and evaluation of intra-individual therapeutic response. This review discusses the latest developments in the different fields of ultrasound, molecularly targeted contrast agent ultrasound, fluorescence endoscopy, confocal laser endomicroscopy as well as tomographic imaging with magnetic resonance imaging, computed tomography and fluorescence-mediated tomography, discussing their individual limitations and potential future diagnostic applications in the management of human patients with IBD.

In-vivo monitoring of acute DSS-Colitis using Colonoscopy, high resolution Ultrasound and bench-top Magnetic Resonance Imaging in Mice

OBJECTIVE

The aim of this study was to establish and evaluate (colour Doppler-) high-resolution-ultrasound (hrUS) and bench-top magnetic resonance imaging (btMRI) as new methods to monitor experimental colitis.

MATERIALS AND METHODS

hrUS, btMRI and endoscopy were performed in mice without colitis (n = 15), in mice with acute colitis (n = 14) and in mice with acute colitis and simultaneous treatment with infliximab (n = 19).

RESULTS

Determination of colon wall thickness using hrUS (32 MHz) and measurement of the cross-sectional colonic areas by btMRI allowed discrimination between the treatment groups (mean a vs. b vs. c - btMRI: 922 vs. 2051 vs. 1472 pixel, hrUS: 0.26 vs. 0.45 vs. 0.31 mm). btMRI, endoscopy, hrUS and colour Doppler-hrUS correlated to histological scoring (p < 0.05), while endoscopy and btMRI correlated to post-mortem colon length (p < 0.05).

CONCLUSIONS

The innovative in vivo techniques btMRI and hrUS are safe and technically feasible. They differentiate between distinct grades of colitis in an experimental setting, and correlate with established post-mortem parameters. In addition to endoscopic procedures, these techniques provide information regarding colon wall thickness and perfusion. Depending on the availability of these techniques, their application increases the value of in vivo monitoring in experimental acute colitis in small rodents.

KEY POINTS

• Improved in vivo monitoring might balance interindividual differences in murine colitis. • In monitoring murine colitis, btMRI and hrUS are safe and technically feasible. • Very short examination times underline the usefulness especially of hrUS. • Results of btMRI and hrUS correlate with endoscopic and post-mortem findings.

Quantitative Assessment of Inflammation in a Porcine Acute Terminal Ileitis Model: US with a Molecularly Targeted Contrast Agent

PURPOSE

To evaluate the feasibility and reproducibility of ultrasonography (US) performed with dual-selectin-targeted contrast agent microbubbles (MBs) for assessment of inflammation in a porcine acute terminal ileitis model, with histologic findings as a reference standard.

MATERIALS AND METHODS

The study had institutional Animal Care and Use Committee approval. Acute terminal ileitis was established in 19 pigs; four pigs served as control pigs. The ileum was imaged with clinical-grade dual P- and E-selectin-targeted MBs (MBSelectin) at increasing doses (0.5, 1.0, 2.5, 5.0, 10, and 20 × 10(8) MB per kilogram of body weight) and with control nontargeted MBs (MBControl). For reproducibility testing, examinations were repeated twice after the MBSelectin and MBControl injections. After imaging, scanned ileal segments were analyzed ex vivo both for inflammation grade (by using hematoxylin-eosin staining) and for expression of selectins (by using quantitative immunofluorescence analysis). Statistical analysis was performed by using the t test, intraclass correlation coefficients (ICCs), and Spearman correlation analysis.

RESULTS

Imaging signal increased linearly (P < .001) between a dose of 0.5 and a dose of 5.0 × 10(8) MB/kg and plateaued between a dose of 10 and a dose of 20 × 10(8) MB/kg. Imaging signals were reproducible (ICC = 0.70), and administration of MBSelectin in acute ileitis resulted in a significantly higher (P < .001) imaging signal compared with that in control ileum and MBControl. Ex vivo histologic grades of inflammation correlated well with in vivo US signal (ρ = 0.79), and expression levels of both P-selectin (37.4% ± 14.7 [standard deviation] of vessels positive; P < .001) and E-selectin (31.2% ± 25.7) in vessels in the bowel wall of segments with ileitis were higher than in control ileum (5.1% ± 3.7 for P-selectin and 4.8% ± 2.3 for E-selectin).

CONCLUSION

Quantitative measurements of inflammation obtained by using dual-selectin-targeted US are reproducible and correlate well with the extent of inflammation at histologic examination in a porcine acute ileitis model as a next step toward clinical translation.

Antibody-based imaging strategies for cancer

Although mainly developed for preclinical research and therapeutic use, antibodies have high antigen specificity, which can be used as a courier to selectively deliver a diagnostic probe or therapeutic agent to cancer. It is generally accepted that the optimal antigen for imaging will depend on both the expression in the tumor relative to normal tissue and the homogeneity of expression throughout the tumor mass and between patients. For the purpose of diagnostic imaging, novel antibodies can be developed to target antigens for disease detection, or current FDA-approved antibodies can be repurposed with the covalent addition of an imaging probe. Reuse of therapeutic antibodies for diagnostic purposes reduces translational costs since the safety profile of the antibody is well defined and the agent is already available under conditions suitable for human use. In this review, we will explore a wide range of antibodies and imaging modalities that are being translated to the clinic for cancer identification and surgical treatment.

Ultrasound molecular imaging: Moving toward clinical translation

Ultrasound is a widely available, cost-effective, real-time, non-invasive and safe imaging modality widely used in the clinic for anatomical and functional imaging. With the introduction of novel molecularly-targeted ultrasound contrast agents, another dimension of ultrasound has become a reality: diagnosing and monitoring pathological processes at the molecular level. Most commonly used ultrasound molecular imaging contrast agents are micron sized, gas-containing microbubbles functionalized to recognize and attach to molecules expressed on inflamed or angiogenic vascular endothelial cells. There are several potential clinical applications currently being explored including earlier detection, molecular profiling, and monitoring of cancer, as well as visualization of ischemic memory in transient myocardial ischemia, monitoring of disease activity in inflammatory bowel disease, and assessment of arteriosclerosis. Recently, a first clinical grade ultrasound contrast agent (BR55), targeted at a molecule expressed in neoangiogenesis (vascular endothelial growth factor receptor type 2; VEGFR2) has been introduced and safety and feasibility of VEGFR2-targeted ultrasound imaging is being explored in first inhuman clinical trials in various cancer types. This review describes the design of ultrasound molecular imaging contrast agents, imaging techniques, and potential future clinical applications of ultrasound molecular imaging.

Ultrasound molecular imaging of vascular endothelial growth factor receptor 2 expression for endometrial receptivity evaluation

Purpose: Ultrasound (US) molecular imaging by examining the expression of vascular endothelial growth factor receptor 2 (VEGFR2) on uterus vascular endothelium was applied to evaluate the endometrial receptivity.

Methods: VEGFR2-targeted ultrasound contrast agents (UCA) and the control UCA (without VEGFR2) were prepared and characterized. Adhesion experiment in vitro was performed with mouse microvascular endothelial cells (bEnd.3) and the ratio of the number of UCA to that of cells at the same field was compared. In vivo study, randomized boluses of targeted or control UCA were injected into the animals of non-pregnancy (D0), pregnancy on day 2 (D2) and day 4 (D4), respectively. Sonograms were acquired by an ultrasound equipment with a 40-MHz high-frequency transducer (Vevo 2100; VisualSonics, Toronto, Canada). The ultrasonic imaging signals were quantified as the video intensity amplitudes generated by the attachment of VEGFR2-targeted UCA. Immunoblotting and immunofluorescence assays were used for confirmation of VEGFR2 expression.

Results: Our results showed that VEGFR2-targeted UCA could bind to bEnd.3 cells with significantly higher affinity than the control UCA (9.8 ± 1.0 bubbles/cell versus 0.7 ± 0.3 bubbles/cell, P < 0.01) in vitro. The mean video intensity from the US backscattering of the retained VEGFR2-targeted UCA was significantly higher than that of the control UCA in D2 and D4 mice (D2, 10.5 ± 2.5 dB versus 1.5 ± 1.1 dB, P < 0.01; D4, 15.7 ± 4.0 dB versus 1.5 ± 1.2 dB, P < 0.01), but not significantly different in D0 mice (1.0 ± 0.8 dB versus 0.9 ± 0.6 dB, P > 0.05). Moreover, D4 mice showed the highest video intensity amplitude, indicating the highest VEGFR2 expression when compared with D2 and D0 mice (P < 0.01). This was further confirmed by our immunoblotting and immunofluorescence experiments.

Conclusion: Ultrasound molecular imaging with VEGFR2-targeted UCA may be used for noninvasive evaluation of endometrial receptivity in murine models.

Contrast-enhanced molecular ultrasound differentiates endoglin genotypes in mouse embryos

Targeted ultrasound contrast imaging has the potential to become a reliable molecular imaging tool. A better understanding of the quantitative aspects of molecular ultrasound technology could facilitate the translation of this technique to the clinic for the purposes of assessing vascular pathology and detecting individual response to treatment. The objective of this study was to evaluate whether targeted ultrasound contrast-enhanced imaging can provide a quantitative measure of endogenous biomarkers. Endoglin, an endothelial biomarker involved in the processes of development, vascular homeostasis, and altered in diseases, including hereditary hemorrhagic telangiectasia type 1 and tumor angiogenesis, was the selected target. We used a parallel plate perfusion chamber in which endoglin-targeted (MBE), rat isotype IgG2 control and untargeted microbubbles were perfused across endoglin wild-type (Eng+/+), heterozygous (Eng+/-) and null (Eng-/-) embryonic mouse endothelial cells and their adhesion quantified. Microbubble binding was also assessed in late-gestation, isolated living transgenic Eng+/- and Eng+/+ embryos. Nonlinear contrast-specific ultrasound imaging performed at 21 MHz was used to collect contrast mean power ratios for all bubble types. Statistically significant differences in microbubble binding were found across genotypes for both in vitro (p<0.05) and embryonic studies (p<0.001); MBE binding was approximately twofold higher in Eng+/+ cells and embryos compared with their Eng+/- counterparts. These results suggest that molecular ultrasound is capable of reliably differentiating between molecular genotypes and relating receptor densities to quantifiable molecular ultrasound levels.

Anti-VEGF therapy reduces intestinal inflammation in Endoglin heterozygous mice subjected to experimental colitis

Chronic intestinal inflammation is associated with pathological angiogenesis that further amplifies the inflammatory response. Vascular endothelial growth factor (VEGF), is a major angiogenic cytokine that has been implicated in chronic colitis and inflammatory bowel diseases. Endoglin (CD105), a transforming growth factor-β superfamily co-receptor expressed on endothelial and some myeloid cells, is a modulator of angiogenesis involved in wound healing and potentially in resolution of inflammation. We showed previously that Endoglin heterozygous (Eng (+/-)) mice subjected to dextran sodium sulfate developed severe colitis, abnormal colonic vessels and high tissue VEGF. We therefore tested in the current study if treatment with a monoclonal antibody to VEGF could ameliorate chronic colitis in Eng (+/-) mice. Tissue inflammation and microvessel density (MVD) were quantified on histological slides. Colonic wall thickness, microvascular hemodynamics and targeted MAdCAM-1(+) inflamed vessels were assessed in vivo by ultrasound. Mediators of angiogenesis and inflammation were measured by Milliplex and ELISA assays. Colitic Eng (+/-) mice showed an increase in intestinal inflammation, MVD, colonic wall thickness, microvascular hemodynamics and the number of MAdCAM-1(+) microvessels relative to colitic Eng (+/+) mice; these parameters were all attenuated by anti-VEGF treatment. Of all factors up-regulated in the inflamed gut, granulocyte colony-stimulating factor (G-CSF) and amphiregulin were further increased in colitic Eng (+/-) versus Eng (+/+) mice. Anti-VEGF therapy decreased tissue VEGF and inflammation-induced endoglin, IL-1β and G-CSF in colitic Eng (+/-) mice. Our results suggest that endoglin modulates intestinal angiogenic and inflammatory responses in colitis. Furthermore, contrast-enhanced ultrasound provides an excellent non-invasive imaging modality to monitor gut angiogenesis, inflammation and responses to anti-angiogenic treatment.

Molecular imaging of inflammation in inflammatory bowel disease with a clinically translatable dual-selectin-targeted US contrast agent: comparison with FDG PET/CT in a mouse model

PURPOSE

To develop and test a molecular imaging approach that uses ultrasonography (US) and a clinically translatable dual-targeted (P- and E-selectin) contrast agent (MBSelectin) in the quantification of inflammation at the molecular level and to quantitatively correlate selectin-targeted US with fluorodeoxyglucose (FDG) combined positron emission tomography (PET) and computed tomography (CT) in terms of visualization and quantification of different levels of inflammation in a murine acute colitis model.

MATERIALS AND METHODS

Animal studies were approved by the Institutional Administrative Panel on Laboratory Animal Care at Stanford University. MBSelectin was developed by covalently binding an analog of the naturally occurring binding ligand P-selectin glycoprotein ligand 1 fused to a human fragment crystallizable(or Fc) domain onto the lipid shell of perfluorobutane and nitrogen-containing MBs. Binding specificity of MBSelectin was assessed in vitro with a flow chamber assay and in vivo with a chemically induced acute colitis murine model. US signal was quantitatively correlated with FDG uptake at PET/CT and histologic grade. Statistical analysis was performed with the Student t test, analysis of variance, and Pearson correlation analysis.

RESULTS

MBSelectin showed strong attachment to both human and mouse P- and E-selectin compared with MBControl in vitro (P ≤ .002). In vivo, US signal was significantly increased (P < .001) in mice with acute colitis (173.8 arbitrary units [au] ± 134.8 [standard deviation]) compared with control mice (5.0 au ± 4.5). US imaging signal strongly correlated with FDG uptake on PET/CT images (ρ = 0.89, P < .001). Ex vivo analysis enabled confirmation of inflammation in mice with acute colitis and high expression levels of P- and E-selectin in mucosal capillaries (P = .014).

CONCLUSION

US with MBSelectin specifically enables detection and quantification of inflammation in a murine acute colitis model, leveraging the natural pathway of leukocyte recruitment in inflammatory tissue. US imaging with MBSelectin correlates well with FDG uptake at PET/CT imaging.

An animal model allowing controlled receptor expression for molecular ultrasound imaging

Reported in this study is an animal model system for evaluating targeted ultrasound (US) contrast agents binding using adenoviral (Ad) vectors to modulate cellular receptor expression. An Ad vector encoding an extracellular hemagglutinin (HA) epitope tag and a green fluorescent protein (GFP) reporter was used to regulate receptor expression. A low and high receptor density (in breast cancer tumor bearing mice) was achieved by varying the Ad dose with a low plaque forming unit (PFU) on day 1 and high PFU on day 2 of experimentation. Targeted US contrast agents, or microbubbles (MB), were created by conjugating either biotinylated anti-HA or IgG isotype control antibodies to the MB surface with biotin-streptavidin linkage. Targeted and control MBs were administered on both days of experimentation and contrast-enhanced US (CEUS) was performed on each mouse using MB flash destruction technique. Signal intensities from MBs retained within tumor vasculature were analyzed through a custom Matlab program. Results showed intratumoral enhancement attributable to targeted MB accumulation was significantly increased from the low Ad vector dosing and the high Ad vector dosing (p = 0.001). Control MBs showed no significant differences between day 1 and day 2 imaging (p = 0.96). Additionally, targeted MBs showed a 10.5-fold increase in intratumoral image intensity on day 1 and an 18.8-fold increase in image intensity on day 2 compared with their control MB counterparts.

Ultrasound-based molecular imaging and specific gene delivery to mesenteric vasculature by endothelial adhesion molecule targeted microbubbles in a mouse model of Crohn's disease

Crohn's disease (CD) is a chronic inflammatory disorder of the gastrointestinal tract (GI) for which treatments with immunosuppressive drugs have significant side-effects. Consequently, there is a clinical need for site-specific and non-toxic delivery of therapeutic genes or drugs for CD and related disorders such as inflammatory bowel disease. The aim of this study was to validate a gene delivery platform based on ultrasound-activated lipid-shelled microbubbles (MBs) targeted to inflamed mesenteric endothelium in the CD-like TNFΔARE mouse model. MBs bearing luciferase plasmid were functionalized with antibodies to MAdCAM-1 (MB-M) or VCAM-1 (MB-V), biomarkers of gut endothelial cell inflammation and evaluated in an in vitro flow chamber assay with appropriate ligands to confirm targeting specificity. Following MB retro-orbital injection in TNFΔARE mice, the mean contrast intensity in the ileocecal region from accumulated MB-M and MB-V was 8.5-fold and 3.6-fold greater, respectively, compared to MB-C. Delivery of luciferase plasmid to the GI tract in TNFΔARE mice was achieved by insonating the endothelial cell-bound agents using a commercial sonoporator. Luciferase expression in the midgut was detected 48 h later by bioluminescence imaging and further confirmed by immunohistochemical staining. The liver, spleen, heart, and kidney had no detectable bioluminescence following insonation. Transfection of the microcirculation guided by a targeted, acoustically-activated platform such as an ultrasound contrast agent microbubble has the potential to be a minimally-invasive treatment strategy to ameliorate CD and other inflammatory conditions.

Molecular body imaging: MR imaging, CT, and US. Part II. Applications

Molecular imaging is expected to have a major impact on the early diagnosis of diseases and disease monitoring in the next decade. Traditionally, nuclear imaging techniques have been the mainstay of molecular imaging in the clinical arena. However, with continued development of molecularly targeted contrast agents for nonnuclear imaging techniques such as magnetic resonance (MR), computed tomography (CT), and ultrasonography (US), the spectrum of clinical molecular imaging applications is expanding. In the second part of this review series, an overview of applications of molecular MR imaging-, CT-, and US-based imaging strategies that show promise for clinical translation is presented, and key challenges that need to be addressed to successfully translate these promising techniques in the future are discussed. © RSNA, 2012.

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.

Development and application of endothelium-targeted microparticles for molecular magnetic resonance imaging

Molecular imaging of disease states can enhance diagnosis allowing for accurate and more effective treatment. By specifically targeting molecules differentially expressed in disease states, researchers and clinicians have a means of disease characterization at a cellular or tissue level. Targeted micron-sized particles of iron oxide (MPIO) have been used as molecule-specific contrast agents for use with magnetic resonance imaging (MRI), and early evidence suggests they may be suitable for use with other imaging modalities. Targeting of MPIO to markers of disease is commonly achieved through the covalent attachment of antibodies to the surface of the particles, providing an imaging agent that is both highly specific and which binds with high affinity. When comparing micron-sized particles with nanometre-sized particles, the former provide substantial signal dropout in MRI and confer the sensitivity to detect low levels of target. Furthermore, larger particles appear to bind to targets more potently than smaller particles. Animal models have also demonstrated favorable blood clearance characteristics of MPIO, which are important in achieving favorable signal over background and to attain clearance and disposal. Although the current generation of commercially available MPIO are not suitable for administration into humans, future work may focus on the development of biodegradable and nonimmunogenic MPIO that may allow the use of these imaging agents in a clinical setting.

Production of uniformly sized serum albumin and dextrose microbubbles

Uniformly-sized preparations with average microbubble (MB) diameters from 1 to 7 μm were produced reliably by sonicating decafluorobutane-saturated solutions of serum albumin and dextrose. Detailed protocols for producing and size-separating the MBs are presented, along with the effects that changing each production parameter (serum albumin concentration, sonication power, sonication time, etc.) had on MB size distribution and acoustic stability. These protocols can be used to produce MBs for experimental applications or serve as templates for developing new protocols that yield MBs with physical and acoustic properties better suited to specific applications. Size stability and ultrasonic performance quality control tests were developed to assure that successive MB preparations perform identically and to distinguish the physical and acoustic properties of identically sized MBs produced with different serum albumin-dextrose formulations and sonication parameters. MBs can be stored at 5 °C for protracted periods (2 weeks to one year depending on formulation).

SAMP1/YitFc mouse strain: a spontaneous model of Crohn's disease-like ileitis

The SAMP1/YitFc mouse strain represents a model of Crohn's disease (CD)-like ileitis that is ideal for investigating the pathogenesis of chronic intestinal inflammation. Different from the vast majority of animal models of colitis, the ileal-specific phenotype characteristic of SAMP1/YitFc mice occurs spontaneously, without genetic, chemical, or immunological manipulation. In addition, SAMP1/YitFc mice possess remarkable similarities to the human condition with regard to disease location, histologic features, incidence of extraintestinal manifestations, and response to conventional therapies. SAMP1/YitFc mice also display a well-defined time course of a predisease state and phases of acute and chronic ileitis. As such, the SAMP1/YitFc model is particularly suitable for elucidating pathways that precede the clinical phenotype that may lead to preventive, and therefore more efficacious, intervention with the natural course of disease, or alternatively, for the development of therapeutic strategies directed against chronic, established ileitis. In this review we summarize important contributions made by our group and others that uncover potential mechanisms in the pathogenesis of CD using this unique murine model of chronic intestinal inflammation.

Quantification and monitoring of inflammation in murine inflammatory bowel disease with targeted contrast-enhanced US

PURPOSE

To evaluate ultrasonography (US) by using contrast agent microbubbles (MBs) targeted to P-selectin (MB(P-selectin)) to quantify P-selectin expression levels in inflamed tissue and to monitor response to therapy in a murine model of chemically induced inflammatory bowel disease (IBD).

MATERIALS AND METHODS

All procedures in which laboratory animals were used were approved by the institutional administrative panel on laboratory animal care. Binding affinity and specificity of MB(P-selectin) were tested in cell culture experiments under flow shear stress conditions and compared with control MBs (MB(Control)). In vivo binding specificity of MB(P-selectin) to P-selectin was tested in mice with trinitrobenzenesulfonic acid-induced colitis (n = 22) and control mice (n = 10). Monitoring of anti-tumor necrosis factor α antibody therapy was performed over 5 days in an additional 30 mice with colitis by using P-selectin-targeted US imaging, by measuring bowel wall thickness and perfusion, and by using a clinical disease activity index score. In vivo targeted contrast material-enhanced US signal was quantitatively correlated with ex vivo expression levels of P-selectin as assessed by quantitative immunofluorescence.

RESULTS

Attachment of MB(P-selectin) to endothelial cells was significantly (P = .0001) higher than attachment of MB(Control) and significantly (ρ = 0.83, P = .04) correlated with expression levels of P-selectin on endothelial cells. In vivo US signal in mice with colitis was significantly higher (P = .0001) with MB(P-selectin) than with MB(Control). In treated mice, in vivo US signal decreased significantly (P = .0001) compared with that in nontreated mice and correlated well with ex vivo P-selectin expression levels (ρ = 0.69; P = .04). Colonic wall thickness (P ≥ .06), bowel wall perfusion (P ≥ .85), and clinical disease activity scoring (P ≥ .06) were not significantly different between treated and nontreated mice at any time.

CONCLUSION

Targeted contrast-enhanced US imaging enables noninvasive in vivo quantification and monitoring of P-selectin expression in inflammation in murine IBD.