Detection of retained microbubbles in carotid arteries with real-time low mechanical index imaging in the setting of endothelial dysfunction

Role of biomaterials in the diagnosis, prevention, treatment, and study of corona virus disease 2019 (COVID-19)

Recently emerged novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and the resulting corona virus disease 2019 (COVID-19) led to urgent search for methods to prevent and treat COVID-19. Among important disciplines that were mobilized is the biomaterials science and engineering. Biomaterials offer a range of possibilities to develop disease models, protective, diagnostic, therapeutic, monitoring measures, and vaccines. Among the most important contributions made so far from this field are tissue engineering, organoids, and organ-on-a-chip systems, which have been the important frontiers in developing tissue models for viral infection studies. Also, due to low bioavailability and limited circulation time of conventional antiviral drugs, controlled and targeted drug delivery could be applied alternatively. Fortunately, at the time of writing this paper, we have two successful vaccines and new at-home detection platforms. In this paper, we aim to review recent advances of biomaterial-based platforms for protection, diagnosis, vaccination, therapeutics, and monitoring of SARS-CoV-2 and discuss challenges and possible future research directions in this field.

Functional micro/nanobubbles for ultrasound medicine and visualizable guidance

Chemically functionalized gas-filled bubbles with a versatile micro/nano-sized scale have witnessed a long history of developments and emerging applications in disease diagnosis and treatments. In combination with ultrasound and image-guidance, micro/nanobubbles have been endowed with the capabilities of biomedical imaging, drug delivery, gene transfection and disease-oriented therapy. As an external stimulus, ultrasound (US)-mediated targeting treatments have been achieving unprecedented efficiency. Nowadays, US is playing a crucial role in visualizing biological/pathological changes in lives as a reliable imaging technique and a powerful therapeutic tool. This review retrospects the history of ultrasound, the chemistry of functionalized agents and summarizes recent advancements of functional micro/nanobubbles as US contrast agents in preclinical and trans-clinical research. Latest ultrasound-based treatment modalities in association with functional micro/nanobubbles have been highlighted as their great potentials for disease precision therapy. It is believed that these state-of-the-art micro/nanobubbles will become a booster for ultrasound medicine and visualizable guidance to serve future human healthcare in a more comprehensive and practical manner.

Targeted, Site-Specific, Delivery Vehicles of Therapeutics for COVID-19 Patients. Brief Review

Definitive pharmacological therapies for COVID-19 have yet to be identified. Several hundred trials are ongoing globally in the hope of a solution. However, nearly all treatments rely on systemic delivery but COVID-19 damages the lungs preferentially. The use of a targeted delivery approach is reviewed where engineered products are able to reach damaged lung tissue directly, which includes catheter-based and aerosol-based approaches. In this review we have outlined various target directed approaches which include microbubbles, extracellular vesicles including exosomes, adenosine nanoparticles, novel bio-objects, direct aerosol targeted pulmonary delivery and catheter-based drug delivery with reference to their relative effectiveness for the specific lesions. Currently several trials are ongoing to determine the effectiveness of such delivery systems alone and in conjunction with systemic therapies. Such approaches may prove to be very effective in the controlled and localized COVID-19 viral lesions in the lungs and potential sites. Moreover, localized delivery offered a safer delivery mode for such drugs which may have systemic adverse effects.

Microbubble Formulations: Synthesis, Stability, Modeling and Biomedical Applications

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

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 imaging for risk assessment in atherosclerosis

Atherosclerosis and its consequences like acute myocardial infarction or stroke are highly prevalent in western countries, and the incidence of atherosclerosis is rapidly rising in developing countries. Atherosclerosis is a disease that progresses silently over several decades before it results in the aforementioned clinical consequences. Therefore, there is a clinical need for imaging methods to detect the early stages of atherosclerosis and to better risk stratify patients. In this review, we will discuss how ultrasound imaging can contribute to the detection and risk stratification of atherosclerosis by (a) detecting advanced and early plaques; (b) evaluating the biomechanical consequences of atherosclerosis in the vessel wall;

Molecular imaging of carotid plaque vulnerability

BACKGROUND

Carotid endarterectomy (CEA) has been shown to be beneficial in patients with high-grade symptomatic carotid artery stenosis. Patients with high-grade asymptomatic stenosis may only exceptionally benefit from CEA during periods of increased plaque vulnerability. Imaging modalities to characterize unstable, vulnerable plaques are strongly needed for better risk stratification in these patients.

SUMMARY

Contrast-enhanced ultrasound (CEUS) is a novel and noninvasive technique capable to identify several surrogate markers of vulnerable carotid plaques. The use of specific ultrasound microbubbles allows a reliable detection of microulcerations due to an optimized visualization of the plaque-lumen border. As microbubbles are strictly intravascular tracers, the detection of individual microbubbles within the plaque corresponds to intraplaque neovessels. The accuracy of CEUS in the visualization of newly formed microvessels has been confirmed in histological studies on carotid endarterectomy specimens. Together with the formation of adventitial vasa vasorum, intraplaque neovascularization is a strong predictor for symptomatic disease. The phenomenon of late phase contrast enhancement is based on the adherence of microbubble-containing monocytes on inflamed endothelium. Recent studies suggest that late phase contrast enhancement may reflect endothelial inflammation or activation within carotid plaques. The development of conjugated microbubbles that bind to specific ligands such as thrombotic material or neovessels has led to the term 'molecular imaging'. CEUS with microbubbles targeted to P-selectin and VCAM-1, key molecules in leukocyte trafficking, was used to detect an inflammatory plaque phenotype, whereas microbubbles coupled to the VEGF-receptor may allow for a detection of neovascularization. Even though imaging with targeted microbubbles is yet in an experimental stage, this technique can visualize active plaque reorganization with increased vulnerability leading to generation of arterio-arterial embolism. Key Messages: The use of contrast-enhanced ultrasound can be recommended to assess atherosclerotic carotid lesions at risk for rupture. Prospective clinical studies are needed to validate the use of CEUS in patients with high risks of recurrent large artery strokes. In particular, this applies to the detection of intraplaque neovascularization, a well-established marker in preclinical and observational studies, while the clinical significance of late phase contrast enhancement still needs to be determined..

Carotid inflammation is unaltered by exercise in hypercholesterolemic Swine

INTRODUCTION

Reduction of vascular inflammation might contribute to the beneficial effects of exercise. We hypothesized that 1) exercise would reduce carotid endothelial vascular cell adhesion molecule-1 (VCAM-1) and that 2) in vivo detection of carotid inflammation can be achieved in a large animal model using contrast-enhanced ultrasound (CEU) with VCAM-1-targeted microbubbles (MBs).

METHODS

Familial hypercholesterolemic (FH) swine were divided into sedentary (Sed) and exercise-trained (Ex) groups. Ex pigs underwent 16-20 wk of treadmill aerobic exercise. At the end of the study, in vivo CEU with VCAM-1-targeted MBs and assessment of endothelial-dependent dilation (EDD) were performed in carotid arteries. VCAM-1 mRNA and protein expression were compared with markers of atherosclerotic disease and health, and in vitro EDD was assessed in carotid arteries.

RESULTS

Exercise training neither reduced inflammation nor improved EDD in carotid arteries of FH swine. Markers of atherosclerosis including VCAM-1 were prominent in the bifurcation compared with the proximal or distal common carotid artery and inversely associated with phosphorylated and total endothelial nitric oxide synthase. Signal intensity from VCAM-1-to-control MBs positively correlated with carotid VCAM-1 protein expression, validating our technique.

CONCLUSION

These results first demonstrate that aerobic exercise has no effect on carotid endothelial inflammatory markers and EDD in FH swine. Second, our findings indicate that CEU using VCAM-1-targeted MBs can detect inflammation in vivo, providing strong foundations for longitudinal studies examining the effect of therapeutic interventions on the inflammatory status of the endothelium.

Ultrasound-mediated drug delivery for cardiovascular disease

INTRODUCTION

Ultrasound (US) has been developed as both a valuable diagnostic tool and a potent promoter of beneficial tissue bioeffects for the treatment of cardiovascular disease. These effects can be mediated by mechanical oscillations of circulating microbubbles, or US contrast agents, which may also encapsulate and shield a therapeutic agent in the bloodstream. Oscillating microbubbles can create stresses directly on nearby tissue or induce fluid effects that effect drug penetration into vascular tissue, lyse thrombi or direct drugs to optimal locations for delivery.

AREAS COVERED

The present review summarizes investigations that have provided evidence for US-mediated drug delivery as a potent method to deliver therapeutics to diseased tissue for cardiovascular treatment. In particular, the focus will be on investigations of specific aspects relating to US-mediated drug delivery, such as delivery vehicles, drug transport routes, biochemical mechanisms and molecular targeting strategies.

EXPERT OPINION

These investigations have spurred continued research into alternative therapeutic applications, such as bioactive gas delivery and new US technologies. Successful implementation of US-mediated drug delivery has the potential to change the way many drugs are administered systemically, resulting in more effective and economical therapeutics, and less-invasive treatments.

Current status and future developments of contrast-enhanced ultrasound of carotid atherosclerosis

B-mode and Doppler ultrasound are commonly used for the evaluation of atherosclerosis in the carotid arteries. Recently, contrast-enhanced ultrasound (CEUS) has been introduced as a technique to improve the detection of carotid atherosclerosis and evaluate the presence of intraplaque neovascularization, which is considered a marker of plaque vulnerability. The present review focuses on the role of CEUS for the assessment of atherosclerosis and plaque instability. Currently available literature and future developments with CEUS are discussed.

Ultrasound imaging in an animal model of vascular inflammation following balloon injury

Cardiovascular disease is a major cause of morbidity and mortality in the world and better prevention and treatment strategies are needed. Studies from this laboratory have shown that perfluorocarbon exposed sonicated dextrose albumin (PESDA) microbubbles bind to inflamed vasculature through interactions with scavenger receptors (SR). This current study details the use of PESDA as a tool for accessing and quantifying the extent of vascular inflammation. Angioplastied rat aortas were evaluated with low mechanical index microbubble imaging techniques contrast pulse sequencing (CPS); Siemens Acuson Sequoia 15L8, 7-15 MHz ultrasound probe with a mechanical index of 0.09 to detect microbubble binding. Real-time polymerase chain reaction (RT-PCR) analysis of angioplastied aortas demonstrated a significantly (p < 0.01) increased expression of both SRs and Interleukin 6 (IL-6). Vessel wall enhancement was quantified using densitometry of CPS ultrasound images and correlated with the upregulated expression of scavenger receptors, Toll-like receptors and IL-6. This study demonstrates that PESDA, in conjunction with CPS ultrasound, is an effective imaging technique to better detect early vascular inflammation and potential cardiovascular disease.

INJECTABLE OXYGEN DELIVERY BASED ON PROTEIN-SHELLED MICROBUBBLES

Injectable oxygen delivery is an emerging technology that presents an opportunity for improved patient care in a number of medical disciplines. Here, we report on the fabrication and characterization of novel protein-encapsulated oxygen microbubbles (OMBs) designed for intravenous injection. The nanothick albumin encapsulation provided OMBs small enough for transcapillary passage: 99% of the microbubbles were less than 3-μm diameter and less than 1% of the oxygen was encapsulated in microbubbles greater than 8-μm diameter. The protein OMBs were remarkably stable, losing less than 40% of the encapsulated gas over 12 days. Upon injection into an oxygen-depleted saline solution, the protein OMBs rapidly equilibrated by releasing their oxygen core. These results indicate that protein microbubbles may serve as a suitable platform for direct injection of bioactive and therapeutic gases.

Molecular imaging of disease with targeted contrast ultrasound imaging

To enhance clinical care for patients, methods for noninvasive imaging of specific disease-related molecular changes are being developed to expand and improve diagnostic capabilities. These new techniques are used in research programs to characterize pathophysiology and as a surrogate end point for therapeutic efficacy. Molecular imaging with contrast-enhanced ultrasound relies on the detection of microbubbles or other acoustically active particulate agents that are targeted to and retained at sites of disease. This review describes the progress that has been made in the development and testing of methods for contrast ultrasound molecular imaging with a specific focus on cardiovascular disease. Specific topics addressed include probe development, detection methods, and specific biologic processes that are important in clinical cardiovascular medicine and that have been targeted with microbubble contrast agents.

Late-Phase Contrast-Enhanced Ultrasound Reflects Biological Features of Instability in Human Carotid Atherosclerosis

Background and Purpose—

Development of translational functional imaging modalities for atherosclerosis risk stratification is sought for stroke prediction. Our group has developed late-phase contrast-enhanced ultrasound (LP-CEUS) to quantify microbubble contrast retention within carotid atherosclerosis and shown it to separate asymptomatic plaques from those responsible for recent cerebrovascular events. We hypothesized that microbubbles are retained in areas of plaque inflammation, aiming to examine whether LP-CEUS signal reflects plaque biology.

Methods—

Subjects awaiting carotid endarterectomy (n=31) underwent axial LP-CEUS and diseased intimal segments were symmetrically divided in the long axis. Half-specimens underwent quantitative immunohistochemical analysis for CD68 (macrophages) and CD31 (angiogenesis). Half-specimens were processed for atheroma cell culture and supernatant collected at 24 hours for multianalyte profiling for 34 analytes.

Results—

Percentage area immunopositivity was significantly higher in subjects in which normalized plaque late-phase intensity was ≥0 versus <0 (CD68 mean 11.8 versus 6.68, P =0.004; CD31 mean 9.45 versus 4.82, P =0.025). Interleukin-6, matrix metalloproteinase-1, and matrix metalloproteinase-3 were significantly higher by multianalyte profiling when LP-CEUS was ≥0.

Conclusions—

LP-CEUS reflects biological features of inflammation and angiogenesis, key features predisposing to plaque rupture. Further investigation of LP-CEUS as a tissue-specific marker of inflammation for risk stratification of carotid atherosclerosis is warranted.

Emerging diagnostic and therapeutic molecular imaging applications in vascular disease

Assessment of vascular disease has evolved from mere indirect and direct measurements of luminal stenosis to sophisticated imaging methods to depict millimeter structural changes of the vasculature. In the near future, the emergence of multimodal molecular imaging strategies may enable robust therapeutic and diagnostic ('theragnostic') approaches to vascular diseases that comprehensively consider structural, functional, biological and genomic characteristics of the disease in individualized risk assessment, early diagnosis and delivery of targeted interventions.This review presents a summary of recent preclinical and clinical developments in molecular imaging and theragnostic applications covering diverse atherosclerosis events such as endothelial activation, macrophage inflammatory activity, plaque neovascularization and arterial thrombosis. The main focus is on molecular targets designed for imaging platforms commonly used in clinical medicine including magnetic resonance, computed tomography and positron emission tomography. A special emphasis is given to vascular ultrasound applications, considering the important role this imaging platform plays in the clinical and research practice of the vascular medicine specialty.

Imaging of atherosclerosis

It is now well recognized that the atherosclerotic plaques responsible for thrombus formation are not necessarily those that impinge most on the lumen of the vessel. Nevertheless, clinical investigations for atherosclerosis still focus on quantifying the degree of stenosis caused by plaques. Many of the features associated with a high-risk plaque, including a thin fibrous cap, large necrotic core, macrophage infiltration, neovascularization, and intraplaque hemorrhage, can now be probed by novel imaging techniques. Each technique has its own strengths and drawbacks. In this article, we review the various imaging modalities used for the evaluation and quantification of atherosclerosis.

Albumin-based microbubbles bind up-regulated scavenger receptors following vascular injury

We have shown previously that perfluorocarbon-exposed sonicated dextrose albumin (PESDA) microbubbles bind to injured vascular tissue and can be detected with ultrasound imaging techniques. Prior studies have shown that scavenger receptors (SRs) are regulators of innate and adaptive immune responses and are involved in the progression of vascular disease such as atherosclerosis. In this study, we sought to determine the molecular mechanism of PESDA binding to balloon-injured vasculature. RT-PCR analysis of angioplastied aortas demonstrated a significantly (p ≤ 0.01) increased expression of SRs. Binding to SRs was confirmed using SR-expressing CHO cells, and this binding was blocked by competitive inhibition with the SR-binding ligands oxidized LDL and malondialdehyde-acetaldehyde-modified LDL. Confocal imaging confirmed the co-localization of PESDA microbubbles to CD36, SRB-1, and Toll-like receptor 4, but not to monocytes/macrophages. This study demonstrates that PESDA binds to SRs and that this binding is in major part dependent upon the oxidized nature of PESDA microbubble shell proteins. The extent of SR mRNA expression was increased with injury and associated with microbubble retention as defined by scanning electron microscopy and immunohistochemistry. These findings clarify the mechanisms of how albumin-based microbubbles bind to injured and inflamed vasculature and further support the potential of this imaging technique to detect early vascular innate inflammatory pathophysiologic processes.

Inflammation within carotid atherosclerotic plaque: assessment with late-phase contrast-enhanced US

PURPOSE

To determine if the number of nontargeted microbubbles retained in human carotid plaque is sufficient to be detected with ultrasonography (US).

MATERIALS AND METHODS

The study protocol was approved by the local research ethics committee. Informed consent was obtained. A total of 37 subjects with carotid atherosclerosis (mean age, 69.9 years; age range, 49-86 years), of whom 27 (73%) were men (mean age, 69.7 years; age range, 58-86 years) and 10 (27%) were women (mean age, 70.3 years; age range, 49-86 years), were studied between December 2008 and May 2009 with late-phase (LP) contrast material-enhanced US by using flash imaging with a nonlinear mode at an intermediate mechanical index of 0.34 6 minutes after bolus contrast agent injection. Plaques were defined as symptomatic if symptoms consistent with stroke, transient ischemic attack, or amaurosis fugax had occurred in the neurovascular territory of the plaque studied within 12 months prior to entry into the study. Plaques were defined as asymptomatic if no such events had ever occurred within the neurovascular territory. Raw linear data were used to quantify echogenicity of the plaque, which was normalized to lumen echogenicity. Gray-scale median score was also calculated.

RESULTS

Of the 37 subjects, 16 (43%) had symptomatic plaques and 21 (57%) had asymptomatic plaques. All examinations yielded evaluable LP contrast-enhanced US data. Normalized LP plaque echogenicity was greater in the symptomatic group (0.39; 95% confidence interval: -0.11, 0.89) than in the asymptomatic group (0.69; 95% confidence interval: -1.04, -0.34) (P = .0005). There was a moderate (rho = -0.44, P = .016) inverse correlation between normalized LP plaque echogenicity and gray-scale median score.

CONCLUSION

By quantifying microbubble retention within the carotid plaque, LP contrast-enhanced US depicts clear differences between groups of subjects with plaque ipsilateral to symptoms and asymptomatic plaques. This technique has promise as a tissue-specific marker of inflammation and a potential role in the risk stratification of atherosclerotic carotid stenosis.

Molecular imaging of myocardial and vascular disorders with ultrasound

Methods for noninvasive imaging of specific disease-related molecular changes are being developed in order to expand and improve diagnostic capabilities and to enhance therapeutic decision making in the clinical setting. These new techniques have also started to be incorporated into research programs in order to better characterize pathophysiology or evaluate treatment efficacy. Molecular imaging with contrast-enhanced ultrasound relies on the detection of the acoustic signal produced by either microbubbles or other acoustically active particulate agents that are targeted to sites of disease. This review describes the progress that has been made in the development and testing of methods for contrast ultrasound molecular imaging of cardiovascular disease. Specifically, topics that will be addressed include: 1) the bioengineering and detection schemes for targeted probes; 2) specific disease processes (myocardial ischemia, atherosclerosis, and transplant rejection) where molecular imaging may play a role; and 3) the potential role of ultrasound as a molecular imaging technique.

Specific targeting of human inflamed endothelium and in situ vascular tissue transfection by the use of ultrasound contrast agents

OBJECTIVES

We used human umbilical cord segments as an ex vivo model to investigate the possible clinical diagnostic and therapeutic applications of microbubbles (MBs).

BACKGROUND

Microbubbles are commonly used in clinical practice as ultrasound contrast agents. Several studies have addressed the in vivo applications of MBs for specific targeting of vascular dysfunction or sonoporation in animal models, but to date no human tissue model has been established.

METHODS

Primary venular endothelial cell monolayers were targeted with MBs conjugated to an antibody against a highly expressed endothelial marker (tetraspanin CD9), and binding was assessed under increasing flow rates (0.5 to 5 dynes/cm(2)). Furthermore, CD9-coupled MB endothelial targeting was measured under flow conditions by contrast-enhanced ultrasound analysis in an ex vivo human macrovascular model (umbilical cord vein), and the same tissue model was used for the detection of inflamed vasculature with anti-intercellular adhesion molecule (ICAM)-1-coated MBs. Finally, plasmids encoding fluorescent proteins were sonoporated into umbilical cord vessels.

RESULTS

Specific endothelial targeting in the in vitro and ex vivo models described previously was achieved by the use of MBs covered with an anti-CD9. Furthermore, we managed to induce inflammation in umbilical cord veins and detect it with real-time echography imaging using anti-ICAM-1-coupled MBs. Moreover, expression and correct localization of green fluorescent protein and green fluorescent protein-tagged ICAM-1 were assessed in this human ex vivo model without causing vascular damage.

CONCLUSIONS

In the absence of clinical trials to test the benefits and possible applications of ultrasound contrast agents for molecular imaging and therapy, we have developed a novel ex vivo human model using umbilical cords that is valid for the detection of inflammation and for exogenous expression of proteins by sonoporation.

Contrast ultrasound molecular imaging of inflammation in cardiovascular disease

The cellular immune response plays an important role in almost every major form of cardiovascular disease. The ability to image the key aspects of the immune response in the clinical setting could be used to improve diagnostic information, to provide important prognostic or risk information, and to customize therapy according to disease phenotype. Accordingly, targeted imaging probes for assessing inflammation have been developed for essentially all forms of medical imaging. Molecular imaging of inflammation with contrast ultrasound relies on the detection of targeted microbubble or other gas-filled particle contrast agents. These agents are confined to the vascular space and, hence, have been targeted to either activated leucocytes or endothelial cell adhesion molecules that are upregulated in inflammation and mediate leucocyte recruitment and adhesion. This review focuses on the inflammation-targeting strategies for ultrasound contrast agents and how they have been matched to cardiovascular disease states such as myocardial ischaemia, infarction, atherosclerosis, transplant rejection, and arteriogenesis.

Molecular imaging in atherosclerosis, thrombosis, and vascular inflammation

Appreciation of the molecular and cellular processes of atherosclerosis, thrombosis, and vascular inflammation has identified new targets for imaging. The common goals of molecular imaging approaches are to accelerate and refine diagnosis, provide insights that reveal disease diversity, guide specific therapies, and monitor the effects of those therapies. Here we undertake a comparative analysis of imaging modalities that have been used in this disease area. We consider the elements of contrast agents, emphasizing how an understanding of the biology of atherosclerosis and its complications can inform optimal design. We address the potential and limitations of current contrast approaches in respect of translation to clinically usable agents and speculate on future applications.

Ultrasound-mediated release of hydrophilic and lipophilic agents from echogenic liposomes

OBJECTIVE

To achieve ultrasound-controlled drug delivery using echogenic liposomes (ELIPs), we assessed ultrasound-triggered release of hydrophilic and lipophilic agents in vitro using color Doppler ultrasound delivered with a clinical 6-MHz compact linear array transducer.

METHODS

Calcein, a hydrophilic agent, and papaverine, a lipophilic agent, were each separately loaded into ELIPs. Calcein-loaded ELIP (C-ELIP) and papaverine-loaded ELIP (P-ELIP) solutions were circulated in a flow model and treated with 6-MHz color Doppler ultrasound or Triton X-100. Treatment with Triton X-100 was used to release the encapsulated calcein or papaverine content completely. The free calcein concentration in the solution was measured directly by spectrofluorimetry. The free papaverine in the solution was separated from liposome-bound papaverine by spin column filtration, and the resulting papaverine concentration was measured directly by absorbance spectrophotometry. Dynamic changes in echogenicity were assessed with low-output B-mode ultrasound (mechanical index, 0.04) as mean digital intensity.

RESULTS

Color Doppler ultrasound caused calcein release from C-ELIPs compared with flow alone (P < .05) but did not induce papaverine release from P-ELIPs compared with flow alone (P > .05). Triton X-100 completely released liposome-associated calcein and papaverine. Initial echogenicity was higher for C-ELIPs than P-ELIPs. Color Doppler ultrasound and Triton X-100 treatments reduced echogenicity for both C-ELIPs and P-ELIPs (P < .05).

CONCLUSIONS

The differential efficiency of ultrasound-mediated pharmaceutical release from ELIPs for water- and lipid-soluble compounds suggests that water-soluble drugs are better candidates for the design and development of ELIP-based ultrasound-controlled drug delivery systems.

Unilateral versus bilateral middle cerebral artery detection of right-to-left shunt by power M-mode transcranial doppler

BACKGROUND AND PURPOSE

Comparison was performed between unilateral and bilateral power M-mode transcranial Doppler to detect right-to-left circulatory shunt (RLS).

METHODS

Recorded Doppler data from 87 patients with confirmed RLS referred for transcatheter closure of patent foramen ovale were reanalyzed for embolic tracks (ET) counted from left and right temporal bone windows during bubble study. Unilateral counts were obtained by multiplying each side by 2; bilateral counts were obtained by summing left and right ET. Both unilateral and bilateral ET were converted to a 6-point logarithmic grade. Sex and age group subanalyses were performed.

RESULTS

At rest, significantly more ET were detected with bilateral versus unilateral detection (P= .01), but not following Valsalva (P= .13). Unilateral and bilateral detection were equally able to detect large RLS (grades IV or V) following Valsalva (P= 1.00). For the group aged > or =55 years, the right-hand side yielded greater ET than the left-hand side (mean difference 9%+/- 37; 95% confidence interval -3 to 21%) at rest (P= .01), but not following Valsalva (mean difference 1%+/- 25; 95% confidence interval -7 to 9%, P= .10).

CONCLUSIONS

Unilateral detection of ET by power M-mode transcranial Doppler is equivalent to bilateral detection to assess RLS.

Enhanced Detection of Thromboemboli With the Use of Targeted Microbubbles

Background and Purpose— Targeted ultrasound contrast agents have recently been developed to adhere selectively to specific pathogenic materials such as plaque or thrombus. Administration of such microbubbles has potential to aid transcranial Doppler ultrasound (TCD) detection of emboli and to act as markers for distinguishing one embolic material from another. The purpose of this study was to investigate whether TCD detection of circulating thrombus emboli would be enhanced by the addition of targeted microbubbles.

Methods— Binding of microbubbles to the surface of the thrombus was confirmed by scanning electron microscopy. Targeted and control bubbles were then introduced to thrombus and tissue-mimicking material circulated under pulsatile-flow conditions in an in vitro flow rig. Embolic signal intensities before and after introduction of the bubbles were measured by TCD.

Results— Targeted microbubbles enhanced TCD signal intensities from thrombus emboli by up to 13 dB. The bubbles were capable of binding to moving thrombus when injected into the flow circuit in low concentrations (≈36 bubbles per 100 mL) and were retained on the thrombus under pulsatile-flow conditions. Signal intensities from similarly sized pieces of tissue-mimicking material were not enhanced by injection of targeted bubbles.

Conclusions— Injection of appropriately targeted microbubbles significantly enhances TCD detection of circulating thrombus emboli in vitro.

Targeted vascular delivery of antisense molecules using intravenous microbubbles

OBJECTIVE

Perfluorocarbon-exposed sonicated dextrose albumin (PESDA) microbubbles bind the antisense to the c-myc protooncogene (anti-c-myc) which prevents neointimal hyperplasia following vascular endothelial injury. The microbubbles also adhere to sites of damaged vascular endothelium and thus may be a method of systemically targeting delivery of anti-c-myc.

METHODS

Laser scanning microscopy was performed on the aorta of 10 mice (five which were complement depleted) that received intravenous FITC-PESDA following aortic endothelial injury. C-myc expression was quantified following selective intracoronary injury in nine pigs that received intravenous (IV) anti-c-myc bound to PESDA. Finally, neointimal formation was measured following intracoronary stent deployment in 30 pigs that received either IV anti-c-myc alone or the same dose bound to PESDA.

RESULTS

Fluorescent microscopy confirmed selective PESDA microbubble adherence to aortic endothelium in all mice with aortic injury. This binding was nearly abolished when serum complement was depleted prior to injury. C-myc expression at the site of coronary endothelial injury was significantly lower in pigs treated with systemic anti-c-myc bound to PESDA. There was a 33% reduction in % stenosis and a 28% reduction in intimal area at 45 days post-stent deployment in pigs that received IV antisense plus PESDA. The stent margins also had reduced neointimal formation.

CONCLUSION

Systemic administration of anti-c-myc bound to PESDA microbubbles may be a good method for preventing coronary neointimal formation within and around implanted stents.

Case report: Optimizing intraoperative detection of pulmonary embolism using contrast-enhanced echocardiography

PURPOSE

Perioperative pulmonary embolism (PE) is associated with significant morbidity and mortality. Intraoperatively, the clinical management of patients with PE can be enhanced by the use of transesophageal echocardiography (TEE) to visualize emboli, assess pulmonary artery (PA) anatomy, and monitor the function of the right ventricle. However, the sensitivity of intraoperative TEE to detect thromboemboli is reported to be below 50%. In this report, we describe the use of contrast-enhanced TEE (CE-TEE) to improve the visualization of PE.

CLINICAL FEATURES

A 44-yr-old female with chronic thrombo-embolic pulmonary hypertension was scheduled for pulmonary thromboendarterectomy. The precardiopulmonary bypass TEE exam demonstrated signs of PA obstruction and right ventricle dysfunction, but the borders of the thrombus in the right PA were only minimally visualized. Perflutren lipid microspheres, composed of octafluoropropane encapsulated in an outer lipid shell, were injected as a 0.3 mL iv bolus, while visualizing the right PA with harmonic ultrasound imaging. The CE-TEE image clearly visualized a large mobile thrombus along with a distinct pattern consistent with pulmonary flow obstruction. The postcardiopulmonary bypass CE-TEE confirmed thrombus evacuation and absence of PA flow abnormalities.

CONCLUSION

Contrast-enhanced-TEE may decrease operator dependency and increase the sensitivity necessary to detect central, surgically accessible PE.

Novel site-specific systemic delivery of Rapamycin with perfluorobutane gas microbubble carrier reduced neointimal formation in a porcine coronary restenosis model

Earlier studies demonstrated that perfluorobutane gas microbubble carrier (PGMC) adheres to injured arteries and enhances the drug uptake specifically into the cells of the denuded vessel segment. The purpose of this study was to investigate the effect of PGMC-based systemic delivery of Rapamycin on expression of p27 in vascular tissue and restenosis in porcine coronary arteries after stent implantation. Eight pigs underwent coronary stent implantation (three stents per animal). Five pigs were treated with i.v. injection of PGMC with 2 mg of Rapamycin and three animals served as control. Four hours postprocedure, three pigs were sacrificed and stented segments were analyzed by high-performance liquid chromatography (HPLC) and Western blot. In chronic experiments, five pigs (15 stent sites) were sacrificed at 28 days following intervention and vessels were perfusion-fixed. HPLC of the treated arteries demonstrated high drug concentration in the vessel tissue, and Western blot analysis showed elevated expression of p27 at 4 hr postprocedure. Histomorphometry revealed significantly reduced (by 40%) neointimal formation in the PGMC/Rapamycin group compared with controls (1.84 +/- 0.84 vs. 4.77 +/- 1.71 mm2, respectively; P < 0.001). In the porcine coronary model, site-specific systemic delivery of Rapamycin utilizing PGMC resulted in overexpression of p27 and a significant reduction of neointimal formation within the stented segments.

The use of microbubbles to target drug delivery

Ultrasound-mediated microbubbles destruction has been proposed as an innovative method for noninvasive delivering of drugs and genes to different tissues. Microbubbles are used to carry a drug or gene until a specific area of interest is reached, and then ultrasound is used to burst the microbubbles, causing site-specific delivery of the bioactive materials. Furthermore, the ability of albumin-coated microbubbles to adhere to vascular regions with glycocalix damage or endothelial dysfunction is another possible mechanism to deliver drugs even in the absence of ultrasound. This review focuses on the characteristics of microbubbles that give them therapeutic properties and some important aspects of ultrasound parameters that are known to influence microbubble-mediated drug delivery. In addition, current studies involving this novel therapeutical application of microbubbles will be discussed.