Microbubble potentiated ultrasound as a method of stroke therapy in a pig model: preliminary findings

Effect of Recombinant Tissue Plasminogen Activator and 120-kHz Ultrasound on Porcine Intracranial Thrombus Density

Surgical intervention for the treatment of intracerebral hemorrhage (ICH) has been limited by inadequate lysis of the target thrombus. Adjuvant transcranial ultrasound exposure is hypothesized to improve thrombolysis, expedite hematoma evacuation and improve clinical outcomes. A juvenile porcine intracerebral hemorrhage model was established by direct infusion of autologous blood into the porcine white matter. Thrombi were either not treated (sham) or treated with recombinant tissue plasminogen activator alone (rt-PA only) or in combination with pulsed transcranial 120-kHz ultrasound (sonothrombolysis). After treatment, pigs were euthanized, the heads frozen and sectioned and the thrombi extracted. D-Dimer and thrombus density assays were used to assess degree of lysis. Both porcine and human D-dimer assays tested did not have sufficient sensitivity to detect porcine D-dimer. Thrombi treated with rt-PA with or without 120-kHz ultrasound had a significantly lower density compared with sham-treated thrombi. No enhancement of rt-PA-mediated thrombolysis was noted with the addition of 120-kHz ultrasound (sonothrombolysis). The thrombus density assay revealed thrombolytic efficacy caused by rt-PA in an in vivo juvenile porcine model of intracerebral hemorrhage. Transcranial sonothrombolysis did not enhance rt-PA-induced thrombolysis, likely because of the lack of exogenous cavitation nuclei.

Contrast enhanced sonothrombolysis using streptokinase loaded phase change nano-droplets for potential treatment of deep venous thrombosis

Current thrombolytic therapies for deep venous thrombosis are limited due to the wide side effect profile. Contrast mediated sonothrombolysis is a promising approach for thrombus treatment. The current study examines the effectiveness of in vitro streptokinase (SK) loaded phase-change nanodroplet (PCND) mediated sonothrombolysis at 7 MHz for the diagnosis of deep venous thrombosis. Lecithin shell and perfluorohexane core nanodroplets were prepared via the thin-film hydration method and morphologically characterized. Sonothrombolysis was performed at 7 MHz at different mechanical indexes of samples i.e., only sonothrombolysis, PCND mediated sonothrombolysis, sonothrombolysis with SK and SK loaded PCND mediated sonothrombolysis. Thrombolysis efficacy was assessed by measuring clot weight changes during 30 min US exposure, recording the mean gray intensity from the US images of the clot by computer software ImageJ, and spectrophotometric quantification of the hemoglobin in the clot lysate. In 15 minutes of sonothrombolysis performed at high mechanical index (0.9 and 1.2), SK loaded PCNDs showed a 48.61% and 74.29% reduction of mean gray intensity. At 0.9 and 1.2 MI, 86% and 92% weight loss was noted for SK-loaded PCNDs in confidence with spectrophotometric results. A significant difference (P < 0.05) was noted for SK-loaded PCND mediated sonothrombolysis compared to other groups. Loading of SK inside the PCNDs enhanced the efficacy of sonothrombolysis. An increase in MI and time also increased the efficacy of sonothrombolysis. This in vitro study showed the potential use of SK-loaded perfluorohexane core PCNDs as sonothrombolytic agents for deep venous thrombosis.

Contrast enhanced sonothrombolysis using streptokinase loaded phase change nano-droplets.

Recent ultrasound advancements for the manipulation of nanobiomaterials and nanoformulations for drug delivery

Recent advances in ultrasound (US) have shown its great potential in biomedical applications as diagnostic and therapeutic tools. The coupling of US-assisted drug delivery systems with nanobiomaterials possessing tailor-made functions has been shown to remove the limitations of conventional drug delivery systems. The low-frequency US has significantly enhanced the targeted drug delivery effect and efficacy, reducing limitations posed by conventional treatments such as a limited therapeutic window. The acoustic cavitation effect induced by the US-mediated microbubbles (MBs) has been reported to replace drugs in certain acute diseases such as ischemic stroke. This review briefly discusses the US principles, with particular attention to the recent advancements in drug delivery applications. Furthermore, US-assisted drug delivery coupled with nanobiomaterials to treat different diseases (cancer, neurodegenerative disease, diabetes, thrombosis, and COVID-19) are discussed in detail. Finally, this review covers the future perspectives and challenges on the applications of US-mediated nanobiomaterials.

The promising shadow of microbubble over medical sciences: from fighting wide scope of prevalence disease to cancer eradication

Microbubbles are typically 0.5-10 μm in size. Their size tends to make it easier for medication delivery mechanisms to navigate the body by allowing them to be swallowed more easily. The gas included in the microbubble is surrounded by a membrane that may consist of biocompatible biopolymers, polymers, surfactants, proteins, lipids, or a combination thereof. One of the most effective implementation techniques for tiny bubbles is to apply them as a drug carrier that has the potential to activate ultrasound (US); this allows the drug to be released by US. Microbubbles are often designed to preserve and secure medicines or substances before they have reached a certain area of concern and, finally, US is used to disintegrate microbubbles, triggering site-specific leakage/release of biologically active drugs. They have excellent therapeutic potential in a wide range of common diseases. In this article, we discussed microbubbles and their advantageous medicinal uses in the treatment of certain prevalent disorders, including Parkinson's disease, Alzheimer's disease, cardiovascular disease, diabetic condition, renal defects, and finally, their use in the treatment of various forms of cancer as well as their incorporation with nanoparticles. Using microbubble technology as a novel carrier, the ability to prevent and eradicate prevalent diseases has strengthened the promise of effective care to improve patient well-being and life expectancy.

Hydrogen sulfide-loaded microbubbles combined with ultrasound mediate thrombolysis and simultaneously mitigate ischemia-reperfusion injury in a rat hindlimb model

BACKGROUND

Thromboembolism and subsequent ischemia/reperfusion injury (IRI) remain major clinical challenges.

OBJECTIVES

To investigate whether hydrogen sulfide (H2 S)-loaded microbubbles (hs-Mbs) combined with ultrasound (US) radiation (hs-Mbs+US) dissolve thrombi and simultaneously alleviate tissue IRI through local H2 S release.

METHODS

hs-Mbs were manufactured and US-triggered H2 S release was recorded. White and red thromboembolisms were established ex vivo and in rats left iliac artery. All subjects randomly received control, US, Mbs+US, or hs-Mbs+US treatment for 30 minutes.

RESULTS

H2 S was released from hs-Mbs+US both ex vivo and in vivo. Compared with control and US, hs-Mbs+US and Mbs+US showed comparable substantial decreases in thrombotic area, clot mass, and flow velocity increases for both ex vivo macrothrombi. In vivo, hs-Mbs+US and Mbs+US caused similarly increased recanalization rates, blood flow velocities, and hindlimb perfusion for both thrombi compared with the other treatments, with no obvious influence on hemodynamics, respiration, and macrophage vitality. More importantly, hs-Mbs+US substantially alleviated skeletal muscle IRI by reducing reactive oxygen species, cellular apoptosis, and proapoptotic Bax, caspase-3, and caspase-9 and increasing antiapoptotic Bcl-2 compared with other treatments. In vitro, hypoxia/reoxygenation-predisposed skeletal muscle cells and endothelial cells treated with normal saline solution exhibited similar trends, which were largely reversed by an H2 S scavenger or an inhibitor of Akt phosphorylation.

CONCLUSION

hs-Mbs+US effectively dissolved both white and red macrothrombi and simultaneously alleviated skeletal muscle IRI through the US-triggered, organ-specific release of H2 S. This integrated therapeutic strategy holds promise for treating thromboembolic diseases and subsequent IRI.

Accelerated sonothrombolysis with Definity in a xenographic porcine cerebral thromboembolism model

Adjuvant ultrasound at 2 MHz with or without an ultrasound contrast agent improves the rate of thrombus resolution by recombinant tissue plasminogen activator (rt-PA) in laboratory and clinical studies. A sub-megahertz approach can further expand this therapy to a subset of patients with an insufficient temporal bone window, improving efficacy in unselected patient populations. The aim of this study was to determine if a clinical ultrasound contrast agent (UCA), Definity, and 220 kHz pulsed ultrasound accelerated rt-PA thrombolysis in a preclinical animal model of vascular occlusion. The effect of Definity and ultrasound on thrombus clearance was first investigated in vitro and subsequently tested in a xenographic porcine cerebral thromboembolism model in vivo. Two different microcatheter designs (end-hole, multi-side-hole) were used to infuse rt-PA and Definity at the proximal edge or directly into clots, respectively. Sonothrombolysis with Definity increased clot mass loss relative to saline or rt-PA alone in vitro, only when rt-PA was administered directly into clots via a multi-side-hole microcatheter. Combined treatment with rt-PA, Definity, and ultrasound in vivo increased the rate of reperfusion up to 45 min faster than clots treated with rt-PA or saline. In this porcine cerebral thromboembolism model employing retracted human clots, 220 kHz ultrasound, in conjunction with Definity increased the probability of early successful reperfusion with rt-PA.

Angular dependence of the acoustic signal of a microbubble cloud

Microbubble-mediated ultrasound therapies have a common need for methods that can noninvasively monitor the treatment. One approach is to use the bubbles' acoustic emissions as feedback to the operator or a control unit. Current methods interpret the emissions' frequency content to infer the microbubble activities and predict therapeutic outcomes. However, different studies placed their sensors at different angles relative to the emitter and bubble cloud. Here, it is evaluated whether such angles influence the captured emissions such as the frequency content. In computer simulations, 128 coupled bubbles were sonicated with a 0.5-MHz, 0.35-MPa pulse, and the acoustic emissions generated by the bubbles were captured with two sensors placed at different angles. The simulation was replicated in experiments using a microbubble-filled gel channel (0.5-MHz, 0.19-0.75-MPa pulses). A hydrophone captured the emissions at two different angles. In both the simulation and the experiments, one angle captured periodic time-domain signals, which had high contributions from the first three harmonics. In contrast, the other angle captured visually aperiodic time-domain features, which had much higher harmonic and broadband content. Thus, by placing acoustic sensors at different positions, substantially different acoustic emissions were captured, potentially leading to very different conclusions about the treatment outcome.

Relevance of Porcine Stroke Models to Bridge the Gap from Pre-Clinical Findings to Clinical Implementation

In the search of animal stroke models providing translational advantages for biomedical research, pigs are large mammals with interesting brain characteristics and wide social acceptance. Compared to rodents, pigs have human-like highly gyrencephalic brains. In addition, increasingly through phylogeny, animals have more sophisticated white matter connectivity; thus, ratios of white-to-gray matter in humans and pigs are higher than in rodents. Swine models provide the opportunity to study the effect of stroke with emphasis on white matter damage and neuroanatomical changes in connectivity, and their pathophysiological correlate. In addition, the subarachnoid space surrounding the swine brain resembles that of humans. This allows the accumulation of blood and clots in subarachnoid hemorrhage models mimicking the clinical condition. The clot accumulation has been reported to mediate pathological mechanisms known to contribute to infarct progression and final damage in stroke patients. Importantly, swine allows trustworthy tracking of brain damage evolution using the same non-invasive multimodal imaging sequences used in the clinical practice. Moreover, several models of comorbidities and pathologies usually found in stroke patients have recently been established in swine. We review here ischemic and hemorrhagic stroke models reported so far in pigs. The advantages and limitations of each model are also discussed.

Advances in Sonothrombolysis Techniques Using Piezoelectric Transducers

One of the great advancements in the applications of piezoelectric materials is the application for therapeutic medical ultrasound for sonothrombolysis. Sonothrombolysis is a promising ultrasound based technique to treat blood clots compared to conventional thrombolytic treatments or mechanical thrombectomy. Recent clinical trials using transcranial Doppler ultrasound, microbubble mediated sonothrombolysis, and catheter directed sonothrombolysis have shown promise. However, these conventional sonothrombolysis techniques still pose clinical safety limitations, preventing their application for standard of care. Recent advances in sonothrombolysis techniques including targeted and drug loaded microbubbles, phase change nanodroplets, high intensity focused ultrasound, histotripsy, and improved intravascular transducers, address some of the limitations of conventional sonothrombolysis treatments. Here, we review the strengths and limitations of these latest pre-clincial advancements for sonothrombolysis and their potential to improve clinical blood clot treatments.

The Thrombolytic Effect of Diagnostic Ultrasound-Induced Microbubble Cavitation in Acute Carotid Thromboembolism

BACKGROUND

Acute ischemic stroke is often due to thromboembolism forming over ruptured atherosclerotic plaque in the carotid artery (CA). The presence of intraluminal CA thrombus is associated with a high risk of thromboembolic cerebral ischemic events. The cavitation induced by diagnostic ultrasound high mechanical index (MI) impulses applied locally during a commercially available intravenous microbubble infusion has dissolved intravascular thrombi, especially when using longer pulse durations. The beneficial effects of this in acute carotid thromboembolism is not known.

MATERIALS AND METHODS

An oversized balloon injury was created in the distal extracranial common CA of 38 porcine carotid arteries. After this, a 70% to 80% stenosis was created in the mid common CA proximal to the injury site using partial balloon inflation. Acute thrombotic CA occlusions were created just distal to the balloon catheter by injecting fresh autologous arterial thrombi. After angiographic documentation of occlusion, the common carotid thrombosis was treated with either diagnostic low MI imaging alone (0.2 MI; Philips S5-1) applied through a tissue mimicking phantom (TMP) or intermittent diagnostic high MI stable cavitation (SC)-inducing impulses with a longer pulse duration (0.8 MI; 20 microseconds' pulse duration) or inertial cavitation (IC) impulses (1.2 MI; 20 microseconds' pulse duration). All treatment times were for 30 minutes. Intravenous ultrasound contrast (2% Definity; Lantheus Medical) was infused during the treatment period. Angiographic recanalization in 4 intracranial and extracranial vessels downstream from the CA occlusion (auricular, ascending pharyngeal, buccinator, and maxillary) was assessed with both magnetic resonance 3-dimensional time-of-flight and phase contrast angiography. All magnetic resonance images were interpreted by an independent neuroradiologist using the thrombolysis in cerebral infarction (TICI) scoring system.

RESULTS

By phase contrast angiography, at least mild recanalization (TICI 2a or higher) was seen in 64% of downstream vessels treated with SC impulses compared with 33% of IC treated and 29% of low MI alone treated downstream vessels (P = 0.001), whereas moderate or complete recanalization (TICI 2b or higher) was seen in 39% of SC treated vessels compared with 10% IC treated and 21% of low MI alone treated vessels (P = 0.001).

CONCLUSIONS

High MI 20-microsecond pulse duration impulses during a commercial microbubble infusion can be used to recanalize acutely thrombosed carotid arteries and restore downstream flow without anticoagulants. However, this effect is only seen with SC-inducing impulses and not at higher mechanical indices, when a paradoxical reversal of the thrombolytic effect is observed. Diagnostic ultrasound-induced SC can be a nonsurgical method of dissolving CA thrombi and preventing thromboembolization.

Microbubbles combined with ultrasound therapy in ischemic stroke: A systematic review of in-vivo preclinical studies

BACKGROUND

Microbubbles (MBs) combined with ultrasound sonothrombolysis (STL) appears to be an alternative therapeutic strategy for acute ischemic stroke (IS), but clinical results remain controversial.

OBJECTIVE

The aim of this systematic review is to identify the parameters tested; to assess evidence on the safety and efficacy on preclinical data on STL; and to assess the validity and publication bias.

METHODS

Pubmed® and Web of ScienceTM databases were systematically searched from January 1995 to April 2017 in French and English. We included studies evaluating STL on animal stroke model. This systematic review was conducted in accordance with the PRISMA guidelines. Data were extracted following a pre-defined schedule by two of the authors. The CAMARADES criteria were used for quality assessment. A narrative synthesis was conducted.

RESULTS

Sixteen studies met the inclusion criteria. The result showed that ultrasound parameters and types of MBs were heterogeneous among studies. Numerous positive outcomes on efficacy were found, but only four studies demonstrated superiority of STL versus recombinant tissue-type plasminogen activator on clinical criteria. Data available on safety are limited.

LIMITATIONS

Quality assessment of the studies reviewed revealed a number of biases.

CONCLUSION

Further in vivo studies are needed to demonstrate a better efficacy and safety of STL compared to currently approved therapeutic options.

SYSTEMATIC REVIEW REGISTRATION

http://syrf.org.uk/protocols/.

In vivo evaluation of urokinase-loaded hollow nanogels for sonothrombolysis on suture embolization-induced acute ischemic stroke rat model

The urokinase-type plasminogen activator (uPA) loaded hollow nanogels (nUK) were synthesized by a one-step reaction of glycol chitosan and aldehyde capped poly (ethylene oxide). The resultant formulation is sensitive to diagnostic ultrasound (US) of 2 MHz. Herein, we evaluated the in vivo sonothrombolysis performance of the nUK on acute ischemic stroke rat model which was established by suture embolization of middle cerebral artery (MCA). Via intravenous (i.v.) administration, the experimental data prove a controlled release of the therapeutic protein around the clots under ultrasound stimulation, leading to enhanced thrombolysis efficiency of the nUK, evidenced from smaller infarct volume and better clinical scores when compared to the i.v. dose of free uPA no matter with or without US intervention. Meanwhile, the preservation ability of the nanogels not only prolonged the circulation duration of the protein, but also resulted in the better blood-brain barrier protection of the nUK formulation, showing no increased risk on the hemorrhagic transformation than the controls. This work suggests that the nUK is a safe sonothrombolytic formulation for the treatment of acute ischemic stroke.

•

Ultrasonic responsive urokinase (uPA)-loaded hollow nanogels (nUK) were synthesized for stroke treatment.

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Acute ischemic stroke rat model was established by suture embolization of middle cerebral artery.

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The nUK enhanced the sonothrombolytic efficacy and led to better BBB protection compared to the free uPA.

Combined Low-Frequency Ultrasound and Urokinase-Containing Microbubbles in Treatment of Femoral Artery Thrombosis in a Rabbit Model

This paper aims to study the thrombolytic effect of low-frequency ultrasound combined with targeted urokinase-containing microbubble contrast agents on treatment of thrombosis in rabbit femoral artery; and to determine the optimal combination of parameters for achieving thrombolysis in this model. A biotinylated-avidin method was used to prepare microbubble contrast agents carrying urokinase and Arg-Gly-Asp-Ser (RGDS) peptides. Following femoral artery thrombosis in New Zealand white rabbits, microbubble contrast agents were injected intravenously, and ultrasonic exposure was applied. A 3 × 2 × 2 factorial table was applied to categorize the experimental animals based on different levels of combination of ultrasonic frequencies (Factor A: 1.6 MHz, 2.2 MHz, 2.8 MHz), doses of urokinase (Factor B: 90,000 IU/Kg, 180,000 IU/Kg) and ultrasound exposure time (Factor C: 30 min, 60 min). A total of 72 experimental animals were randomly divided into 12 groups (n = 6/group). Doppler techniques were used to assess blood flow in the distal end of the thrombotic femoral artery during the 120 minutes thrombolysis experiment. The rate of recanalization following thrombolysis was calculated, and thrombolytic efficacy was evaluated and compared. The thrombolytic recanalization rate for all experimental subjects after thrombolytic therapy was 68.1%. The optimal parameters for thrombolysis were determined to be 1) an ultrasound frequency of 2.2 MHz and 2) a 90,000 IU/kg dose of urokinase. Ultrasound exposure time (30 min vs. 60 min) had no significant effect on the thrombolytic effects. The combination of local low-frequency ultrasound radiation, targeted microbubbles, and thrombolytic urokinase induced thrombolysis of femoral artery thrombosis in a rabbit model. The ultrasonic frequency of 2.2 MHz and urokinase dose of 90,000 IU/kg induced optimal thrombolytic effects, while the application of either 30 min or 60 min of ultrasound exposure had similar effects.

Safety of thrombolytic therapy with rt-PA and transcranial color Doppler ultrasound (TCCS) combined with microbubbles: a histopathologic study on rabbit brain tissues

OBJECTIVE

To evaluate effect of thrombolytic therapy with rt-PA (recombinant tissue plasminogen activator) and transcranial color Doppler ultrasound (TCCS) combined with microbubbles on histology of brain tissue.

METHODS

New Zealand rabbits were subjected to TCCS based thrombolytic therapy, in 8 groups depending on dose of rt-PA, exposure duration of TCCS and presence of attenuation by skull bone window, 2 animals/group: (1) skull+1/2 rt-PA+TCCS+MBs, 10 min, (2) skull+rt-PA+TCCS+MBs, 10 min, (3) skull+1/2 rt-PA+TCCS+MBs, 20 min, (4) skull+rt-PA+TCCS+MBs, 20 min, (5) skull+1/2 rt-PA+TCCS+MBs, 30 min, (6) skull+rt-PA+TCCS+MBs, 30 min, (7) 1/2 rt-PA+TCCS+MBs, 10 min, (8) 1/2 rt-PA+TCCS+MBs, 20 min. The brain tissues were harvested after therapies and submitted for microscopic, electronic microscope and immunohistochemical examination. The histological changes were scored.

RESULTS

TCCS caused exposure duration dependent brain tissue damage. With attenuation by bone window, TCCS based therapies for 10-20 min caused minimal tissue damage. However, significant tissue damage was observed upon TCCS for 30 min in presence of skull bone window, presenting as hemorrhage, misdistribution of organelles, demyelination of nerve fibers, and thinning of basement membrane in blood-brain barrier, which was milder than that after 20 min of exposure to TCCS in absence of bone window. Dose of rt-PA did not affect brain histology in all groups.

CONCLUSION

Short treatment of brain tissue with TCCS through a bone window is relative safe. And skull bone window protected brain tissue from TCCS induced damage.

Prehospital stroke diagnosis and treatment in ambulances and helicopters-a concept paper

Stroke is the second common cause of death and the primary cause of early invalidity worldwide. Different from other diseases is the time sensitivity related to stroke. In case of an ischemic event occluding a brain artery, 2000000 neurons die every minute. Stroke diagnosis and treatment should be initiated at the earliest time point possible, preferably at the site or during patient transport. Portable ultrasound has been used for prehospital diagnosis for applications other than stroke, and its acceptance as a valuable diagnostic tool "in the field" is growing. The intrahospital use of transcranial ultrasound for stroke diagnosis has been described extensively in the literature. Beyond its diagnostic use, first clinical trials as well as numerous preclinical work demonstrate that ultrasound can be used to accelerate clot lysis (sonothrombolysis) in presence as well as in absence of tissue plasminogen activator. Hence, the use of transcranial ultrasound for diagnosis and possibly treatment of stroke bares the potential to add to current stroke care paradigms significantly. The purpose of this concept article is to describe the opportunities presented by recent advances in transcranial ultrasound to diagnose and potentially treat large vessel embolic stroke in the prehospital environment.

Pulsed high-intensity-focused US and tissue plasminogen activator (TPA) versus TPA alone for thrombolysis of occluded bypass graft in swine

PURPOSE

Prosthetic arteriovenous or arterial-arterial bypass grafts can thrombose and be resistant to revascularization. A thrombosed bypass graft model was created to evaluate the potential therapeutic enhancement and safety profile of pulsed high-intensity-focused ultrasound (pHIFU) on pharmaceutical thrombolysis.

MATERIALS AND METHODS

In swine, a right carotid-carotid expanded polytetrafluoroethylene bypass graft was surgically constructed, containing a 40% stenosis at its distal end to induce graft thrombosis. The revascularization procedure was performed 7 days after surgery. After model development and dose response experiments (n = 11), two cohorts were studied: pHIFU with tissue plasminogen activator (TPA; n = 4) and sham pHIFU with TPA (n = 3). The experiments were identical in both groups except no energy was delivered in the sham pHIFU group. Serial angiograms were obtained in all cases. The area of graft opacified by contrast medium on angiograms was quantified with digital image processing software. A blinded reviewer calculated the change in the graft area opacified by contrast medium and expressed it as a percentage, representing percentage of thrombolysis.

RESULTS

Combining pHIFU with 0.5 mg of TPA resulted in a 52% ± 4% increase in thrombolysis on angiograms obtained at 30 minutes, compared with a 9% ± 14% increase with sham pHIFU and 0.5 mg TPA (P = .003). Histopathologic examination demonstrated no differences between the groups.

CONCLUSIONS

Thrombolysis of occluded bypass grafts was significantly increased when combining pHIFU and TPA versus sham pHIFU and TPA. These results suggest that application of pHIFU may augment thrombolysis with a reduced time and dose.

Introduction of a rabbit carotid artery model for sonothrombolysis research

The goal of this study was to develop an in vivo sonothrombolysis model for stroke research. The rabbit carotid artery has average vessel diameters similar to human M1/M2 segments and allows generation of a thrombotic occlusion using various kinds of thrombus material as well as thrombus placement under visual control. It further allows real-time monitoring of flow and clot mechanics during the sonothrombolysis procedure using high-frequency diagnostic ultrasound. In the present study, the model will be introduced and first results to show feasibility using diagnostic as well as high-intensity focused ultrasound will be presented.

Thrombolytic efficacy of tissue plasminogen activator-loaded echogenic liposomes in a rabbit thrombus model

INTRODUCTION

Ultrasound (US)-enhanced thrombolytic treatment protocols currently in clinical trials for stroke applications involve systemic administration of tissue plasminogen activator (tPA; Alteplase), which carries a risk of adverse bleeding events. The present study aimed to compare the thrombolytic efficacy of a tPA-loaded echogenic liposome (ELIP) formulation with insonification protocols causing rapid fragmentation or acoustically-driven diffusion.

MATERIALS AND METHODS

Thrombi were induced in the abdominal aortas of male New Zealand white rabbits (2-3kg) using thrombin and a sclerosing agent (sodium ricinoleate) after aortic denudation with a balloon catheter. Thrombolytic and cavitation nucleation agents (200μg of tPA alone, tPA mixed with 50μg of a microbubble contrast agent, or tPA-loaded ELIP) were bolus- injected proximal to the clot through a catheter introduced into the abdominal aorta from the carotid artery. Clots were exposed to transabdominal color Doppler US (6MHz) for 30 minutes at a low mechanical index (MI=0.2) to induce sustained bubble activity (acoustically-driven diffusion), or for 2 minutes at an MI of 0.4 to cause ELIP fragmentation. Degree of recanalization was determined by Doppler flow measurements distal to the clots.

RESULTS

All treatments showed thrombolysis, but tPA-loaded ELIP was the most efficacious regimen. Both US treatment strategies enhanced thrombolytic activity over control conditions.

CONCLUSIONS

The thrombolytic efficacy of tPA-loaded ELIP is comparable to other clinically described effective treatment protocols, while offering the advantages of US monitoring and enhanced thrombolysis from a site-specific delivery agent.

Sonothrombolysis for the treatment of acute stroke: current concepts and future directions

Achieving rapid reperfusion transcranial color-coded duplex is the critical issue in acute stroke treatment. Ultrasound (US) generates negative pressure waves that are associated with an increase in either intrinsic or intravenous tissue plasminogen activator (tPA)-induced fibrinolytic activity. Higher rates of tPA-induced arterial recanalization, associated with a trend towards better functional outcome, have been safely achieved by using high-frequency US. By contrast, the use of low-frequency US and transcranial color-coded duplex has been linked to significant hemorrhagic complications. US-accelerated thrombolysis has been safely enhanced by lowering the amount of energy needed for acoustic cavitation with the administration of microbubbles. Other applications of US are being studied, including its intra-arterial use. Operator-independent devices, which will spread the use of these US techniques further, are also being developed. This article reviews the present status of sonothrombolysis in acute stroke treatment, highlighting both experimental and clinical studies addressing this issue, and discusses its future regarding both efficacy and safety.

Microbubble-augmented ultrasound sonothrombolysis decreases intracranial hemorrhage in a rabbit model of acute ischemic stroke

OBJECTIVES

Increasing evidence confirms that microbubble (MB)-augmented ultrasound (US) thrombolysis enhances clot lysis with or without tissue plasminogen activator (tPA). Intracranial hemorrhage (ICH) is a major complication militating against tPA use in acute ischemic stroke. We quantified the incidence of ICH associated with tPA thrombolysis and MB + US therapy and compared infarct volumes in a rabbit model of acute ischemic stroke.

MATERIALS AND METHODS

Rabbits (n = 158) received a 1.0-mm clot, angiographically injected into the internal carotid artery causing infarcts. Rabbits were randomized to 6 test groups including (1) control (n = 50), embolized without therapy, (2) US (n = 18), (3) tPA only (n = 27), (4) tPA + US (n = 22), (5) MB + US (n = 27), and (6) tPA + MB + US (n = 14). US groups received pulsed wave US (1 MHz, 0.8 W/cm) for 1 hour; rabbits with tPA received intravenous tPA (0.9 mg/kg) over 1 hour. Rabbits with MB received intravenous MB (0.16 mg/kg) given over 30 minutes. Rabbits were killed 24 hours later and infarct volume and incidence, location, and severity of ICH were determined by histology and pathologic examination.

RESULTS

Percentage of rabbits having ICH outside the infarct area was significantly decreased (P = 0.004) for MB + US (19%) rabbits compared with tPA + US (73%), US only (56%), tPA (48%), tPA + MB + US (36%), and control (36%) rabbits. Incidence and severity of ICH within the infarct did not differ (P > 0.39). Infarct volume was significantly greater (P = 0.002) for rabbits receiving US (0.97% ± 0.17%) than for MB + US (0.20% ± 0.14%), tPA + US (0.15% ± 0.16%), tPA (0.14% ± 0.14%), and tPA + MB + US (0.10% ± 20%) rabbits; these treatments collectively, excluding US only, differed (P = 0.03) from control (0.45% ± 0.10%).

CONCLUSIONS

Treatment with MB + US after embolization decreased the incidence of ICH and efficacy was similar to tPA in reducing infarct volume.

Successful microbubble sonothrombolysis without tissue-type plasminogen activator in a rabbit model of acute ischemic stroke

BACKGROUND AND PURPOSE

Microbubbles (MB) combined with ultrasound (US) have been shown to lyse clots without tissue-type plasminogen activator (tPA) both in vitro and in vivo. We evaluated sonothrombolysis with 3 types of MB using a rabbit embolic stroke model.

METHODS

New Zealand White rabbits (n=74) received internal carotid angiographic embolization of single 3-day-old cylindrical clots (0.6 × 4.0 mm). Groups included: (1) control (n=11) embolized without treatment; (2) tPA (n=20); (3) tPA+US (n=10); (4) perflutren lipid MB+US (n=16); (5) albumin 3 μm MB+US (n=8); and (6) tagged albumin 3 μm MB+US (n=9). Treatment began 1 hour postembolization. Ultrasound was pulsed-wave (1 MHz; 0.8 W/cm²) for 1 hour; rabbits with tPA received intravenous tPA (0.9 mg/kg) over 1 hour. Lipid MB dose was intravenous (0.16 mg/kg) over 30 minutes. Dosage of 3 μm MB was 5 × 10⁹ MB intravenously alone or tagged with eptifibatide and fibrin antibody over 30 minutes. Rabbits were euthanized at 24 hours. Infarct volume was determined using vital stains on brain sections. Hemorrhage was evaluated on hematoxylin and eosin sections.

RESULTS

Infarct volume percent was lower for rabbits treated with lipid MB+US (1.0%± 0.6%; P=0.013), 3 μm MB+US (0.7% ± 0.9%; P=0.018), and tagged 3 μm MB+US (0.8% ± 0.8%; P=0.019) compared with controls (3.5%± 0.8%). The 3 MB types collectively had lower infarct volumes (P=0.0043) than controls. Infarct volume averaged 2.2% ± 0.6% and 1.7%± 0.8% for rabbits treated with tPA alone and tPA+US, respectively (P=nonsignificant).

CONCLUSIONS

Sonothrombolysis without tPA using these MB is effective in decreasing infarct volumes. Study of human application and further MB technique development are justified.

Microbubbles improve sonothrombolysis in vitro and decrease hemorrhage in vivo in a rabbit stroke model

INTRODUCTION

Tissue plasminogen activator (tPA) is the thrombolytic standard of care for acute ischemic stroke, but intracerebral hemorrhage (ICH) remains a common and devastating complication. We investigated using ultrasound (US) and microbubble (MB) techniques to reduce required tPA doses and to decrease ICH.

MATERIALS AND METHODS

Fresh blood clots (3-5 hours) were exposed in vitro to tPA (0.02 or 0.1 mg/mL) plus pulsed 1 MHz US (0.1 W/cm²), with or without 1.12 × 10⁸/mL MBs (Definity or albumin/dextrose MBs [adMB]). Clot mass loss was measured to quantify thrombolysis. New Zealand white rabbits (n = 120) received one 3- to 5-hour clot angiographically delivered into the internal carotid artery. All had transcutaneous pulsed 1 MHz US (0.8 W/cm²) for 60 minutes and intravenous tPA (0.1-0.9 mg/kg) with or without Definity MBs (0.16 mL/mg/kg). After killing the animals, the brains were removed for histology 24 hours later.

RESULTS

In vitro, MBs (Definity or adMB) increased US-induced clot loss significantly, with or without tPA (P < 0.0001). At 0 and 0.02 mg/mL, tPA clot loss was greater with adMBs compared with Definity (P ≤ 0.05). With MB, the tPA dose was reduced 5-fold with good efficacy. In vivo, both Definity MB and tPA groups had less infarct volume compared with controls at P < 0.0183 and P = 0.0003, respectively. Definity MB+tPA reduces infarct volume compared with controls (P < 0.0001), and ICH incidence outside of strokes was significantly lower (P = 0.005) compared with no MB. However, infarct volume in Definity MB versus tPA was not different at P = 0.19.

CONCLUSION

Combining tPA and MB yielded effective loss of clot with very low dose or even no dose tPA, and infarct volumes and ICH were reduced in acute strokes in rabbits. The ability of MBs to reduce tPA requirements may lead to lower rates of hemorrhage in human stroke treatment.

Platelet-targeted microbubbles inhibit re-occlusion after thrombolysis with transcutaneous ultrasound and microbubbles

Microbubbles (MBs) can augment the acoustic cavitation' (US), thereby facilitating the thrombolysis of external ultrasound. But we observed re-thrombosis after successful thrombolysis by MBs and transcutaneous ultrasound in an endothelium injury model. This study was designed to explore whether platelet-targeted MBs can prevent the reformation of thrombi. Arterial injury was induced in canine femoral arteries with balloon, and the arteries were completely thrombotically occluded. The arteries were treated with intra-arterial MBs or platelet-targeted MBs (TMB) and transcutaneous low frequency ultrasound (LFUS) to achieve complete thrombolysis. The arterial flow was monitored with angiogram for 4h following treatment. Results showed that both MBs and TMBs produced successful dissolution of clots in the presence of ultrasound. The re-occlusion began to occur 1h after thrombolysis in MB/LFUS treatment, and 7 of 8 arteries were re-occluded within 3h. Most of the arteries (7 of 8) in the TMB/LFUS group remained patent for 4h following treatment. The flow tended to decrease after thrombolysis in MB/LFUS treatment. These results indicated that platelet-targeted microbubbles were beneficial in preventing re-thrombosis in vivo and microbubbles served as good carrier of thrombolytic and anticoagulation drugs.

Sonothrombolysis: an emerging modality for the management of stroke

OBJECTIVE

Ischemic stroke and intracranial hemorrhage remain a persistent scourge in Western civilization. Therefore, novel therapeutic modalities are desperately needed to expand the current limitations of treatment. Sonothrombolysis possesses the potential to fill this void because it has experienced a dramatic evolution from the time of early conceptualization in the 1960s. This process began in the realm of peripheral and cardiovascular disease and has since progressed to encompass intracranial pathologies. Our purpose is to provide a comprehensive review of the historical progression and existing state of knowledge, including underlying mechanisms as well as evidence for clinical application of ultrasound thrombolysis.

METHODS

Using MEDLINE, in addition to cross-referencing existing publications, a meticulous appraisal of the literature was conducted. Additionally, personal communications were used as appropriate.

RESULTS

This appraisal revealed several different technologies close to broad clinical use. However, fundamental questions remain, especially in regard to transcranial high-intensity focused ultrasound. Currently, the evidence supporting low intensity ultrasound's potential in isolation, without tissue plasminogen, remains uncertain; however, possibilities exist in the form of microbubbles to allow for focal augmentation with minimal systemic consequences. Alternatively, the literature clearly demonstrates, the efficacy of high-intensity focused ultrasound for independent thrombolysis.

CONCLUSION

Sonothrombolysis exists as a promising modality for the noninvasive or minimally invasive management of stroke, both ischemic and hemorrhagic. Further research facilitating clinical application is warranted.

Safety and dose-escalation study design of Transcranial Ultrasound in Clinical SONolysis for acute ischemic stroke: the TUCSON Trial

Rationale Transcranial Doppler (TCD) monitoring during intravenous tissue plasminogen activator (i.v.-tPA) infusion increases recanalization rates in acute ischemic stroke. Addition of perflutren-lipid microspheres MRX-801 (microS) may further enhance the process of recanalization. This article describes the design of the Transcranial Ultrasound in Clinical SONolysis (TUCSON) trial. Aims and Design TUCSON is a phase I-II, randomized, placebo-controlled, open-label, safety, dose-escalation clinical trial of microS+TCD ultrasound (sonolysis). Patients with acute ischemic stroke and arterial intracranial occlusions are enrolled within 3 h of symptom onset. All patients receive standard i.v.-tPA and will be randomized to 90 min of continuous 2-MHz TCD+microS or 90 min of saline+brief TCD vessel assessments. The safety profile of four escalating dose tiers will be assessed. Arterial occlusions and recanalization are defined with the Thrombolysis in Brain Ischemia flow grades. Study Outcomes Safety is determined by the rates of symptomatic intracerebral hemorrhage within 36 h. Neurological deficits and outcomes are measured with the National Institute of Health Stroke Scale and modified Rankin Scale (mRS). The signal-of-efficacy is determined by rates of recanalization, dramatic or early clinical recovery within 2 h, clinical recovery at 24-36 h and independent outcome (mRS 0-2) at 90 days.

Effect of combined ultrasound and microbubbles treatment in an experimental model of cerebral ischemia

Combined 2-MHz ultrasound (US) and second-generation, sulfur hexafluoride microbubbles (MB) treatment (US+MB) was performed in a permanent middle cerebral artery (MCA) occlusion model in rats to evaluate possible effects on the ischemic cascade. We used 16 Wistar rats and the MCA occlusion model for stroke induction. Glutamate, pyruvate, lactate and glycerol levels were measured by intracerebral microdialysis before and after stroke induction and after US+MB application (n = 8) for 20 h. After 24 h, brain infarct volume, apoptosis and IL-6 and TNF-alpha levels were evaluated. The infarct volume was significantly reduced (p < 0.05) in the US+MB-treated group compared with control animals. In additional, glutamate levels were significantly lower in US+MB-treated animals, and these animals showed a higher rate of apoptotic cell death in the infarcted area. The levels of IL-6 and TNF-alpha concentrations were not different in both groups, and there was no apoptotic cell death outside the infarction in animals treated with US+MB. The results demonstrate that US+MB with second generation microbubbles does not have a harmful effect on ischemic stroke in an MCA occlusion model of the rat.

Ultrasound-enhanced thrombolysis using Definity as a cavitation nucleation agent

Ultrasound has been shown previously to act synergistically with a thrombolytic agent, such as recombinant tissue plasminogen activator (rt-PA) to accelerate thrombolysis. In this in vitro study, a commercial contrast agent, Definity, was used to promote and sustain the nucleation of cavitation during pulsed ultrasound exposure at 120 kHz. Ultraharmonic signals, broadband emissions and harmonics of the fundamental were measured acoustically by using a focused hydrophone as a passive cavitation detector and used to quantify the level of cavitation activity. Human whole blood clots suspended in human plasma were exposed to a combination of rt-PA, Definity and ultrasound at a range of ultrasound peak-to-peak pressure amplitudes, which were selected to expose clots to various degrees of cavitation activity. Thrombolytic efficacy was determined by measuring clot mass loss before and after the treatment and correlated with the degree of cavitation activity. The penetration depth of rt-PA and plasminogen was also evaluated in the presence of cavitating microbubbles using a dual-antibody fluorescence imaging technique. The largest mass loss (26.2%) was observed for clots treated with 120-kHz ultrasound (0.32-MPa peak-to-peak pressure amplitude), rt-PA and stable cavitation nucleated by Definity. A significant correlation was observed between mass loss and ultraharmonic signals (r = 0.85, p < 0.0001, n = 24). The largest mean penetration depth of rt-PA (222 microm) and plasminogen (241 microm) was observed in the presence of stable cavitation activity. Stable cavitation activity plays an important role in enhancement of thrombolysis and can be monitored to evaluate the efficacy of thrombolytic treatment.

Ultrasound enhanced thrombolysis in acute arterial ischemia

In vitro and animal studies have shown that thrombolysis with intravenous tissue plasminogen activator (tPA) can be enhanced with ultrasound. Ultrasound delivers mechanical pressure waves to the clot, thus exposing more thrombus surface to circulating drug. Moreover, intravenous gaseous microspheres with ultrasound have been shown to be a potential alternative to fibrinolytic agents to recanalize discrete peripheral thrombotic arterial occlusions or acute arteriovenous graft thromboses. Small phase I-II randomized and non-randomized clinical trials have shown promising results concerning the potential applications of ultrasound-enhanced thrombolysis in the setting of acute cerebral ischemia. CLOTBUST was an international four-center phase II trial, which demonstrated that, in patients with acute ischemic stroke, transcranial Doppler (TCD) monitoring augments tPA-induced arterial recanalization (sustained complete recanalization rates: 38% vs. 13%) with a non-significant trend toward an increased rate of clinical recovery from stroke, as compared with placebo. The rates of symptomatic intracerebral hemorrhage (sICH) were similar in the active and placebo group (4.8% vs. 4.8%). Smaller single-center clinical trials using transcranial color-coded sonography (TCCD) reported recanalization rates ranging from 27% to 64% and sICH rates of 0-18%. A separate clinical trial evaluating the safety and efficacy of therapeutic low-frequency ultrasound was discontinued because of a concerning sICH rate of 36% in the active group. To further enhance the ability of tPA to break up thrombi, current ongoing clinical trials include phase II studies of a single beam 2 MHz TCD with perflutren-lipid microspheres. Moreover, potential enhancement of intra-arterial tPA delivery is being clinically tested with 1.7-2.1 MHz pulsed wave ultrasound (EKOS catheter) in ongoing phase II-III clinical trials. Intravenous platelet-targeted microbubbles with low-frequency ultrasound are currently investigated as a rapid noninvasive technique to identify thrombosed intracranial and peripheral vessels. Multi-national dose escalation studies of microspheres and the development of an operator independent ultrasound device are underway.

Cavitational mechanisms in ultrasound-accelerated fibrinolysis

The role of both inertial and stable cavitation was investigated during in vitro ultrasound-accelerated fibrinolysis by recombinant tissue plasminogen activator (rt-PA) in the presence and absence of Optison. A unique treatment configuration applied ultrasound, rt-PA and Optison to the interior of a plasma clot. Lysis efficacy was measured as clot weight reduction. Cavitational mechanisms were investigated by monitoring subharmonic and broadband noise. In the absence of Optison, 1.7 MHz pulsed ultrasound with 1.5 MPa peak-negative pressure applied for 30 min resulted in 45 +/- 19% lysis enhancement relative to rt-PA alone. Cavitation was not detected, indicating a role of noncavitational effects of ultrasound. The addition of Optison increased lysis enhancement to 88 +/- 25%. Inertial cavitation was present only at the start of the exposure, while low-amplitude subharmonic emissions persisted throughout. Additional protocols suggested a possible correlation between the increased lysis in the presence of Optison and the subharmonic emission, indicating a potentially important role of stable rather than inertial cavitation in microbubble-enhanced ultrasound-accelerated rt-PA-mediated thrombolysis.

Hyperoxia potentiated sonothrombolysis as a method of acute ischemic stroke therapy

The main goal in the treatment of acute ischemic stroke is prompt arterial recanalization. Thrombolysis with recombinant tissue plasminogen activator (rtPA) is efficient in humans, but shows significant problems including slow and incomplete recanalization and frequent bleeding complications. Limited therapeutic window (the first three hours after onset) is the major limitation resulting in reach too few patients. Therefore, adjunctive therapies extending the reperfusion time window, increasing efficacy and reducing side effects of rtPA are needed. Ultrasound augmentation of rtPA-mediated thrombolysis is suggested to overcome some of these problems, but low-frequency ultrasound (less than 1 MHz) is not safe and high frequency ultrasound (2 MHz) is not much effective. We suggest that normobaric hyperoxia (NBO) may increase the efficacy of ultrasound and rtPA combination in addition to its own efficacy in acute ischemic stroke. Briefly, NBO increases arterial partial oxygen pressure (pO(2)) significantly up to 6-fold. Increase of pO(2) results in an increase of dissolved oxygen in the blood according to Henry's law. Enhanced dissolved oxygen increases gas nuclei formation around and inside of the clot, and decreases the Blake threshold. Under ultrasound field, these small gas nuclei form nano bubbles which fuel inertial cavitation as substrates, and therefore increase the clot fragmentation and lysis. This hypothesis has not been tested so far. The combination of rtPA, therapeutic ultrasound and NBO may be more efficacious than rtPA alone or its combination with ultrasound as acute stroke treatment modality, because each has different and probably additive mechanism of action.

Continuous monitoring of middle cerebral artery recanalization with transcranial color-coded sonography and Levovist

BACKGROUND AND PURPOSE

Treatment of acute ischemic stroke with intravenous recombinant tissue plasminogen activator (rtPA) is relatively ineffective in patients with large vessel occlusion. Numerous experimental studies have demonstrated that ultrasound (US) can accelerate enzymatic fibrinolysis and acceleration of lysis by US is enhanced in the presence of microbubbles used as echo-contrast agents. The purpose of this study was to evaluate the feasibility of continuous monitoring of middle cerebral artery (MCA) recanalization using transcranial color-coded sonography (TCCS) and intravenously administered microbubbles.

METHODS

Recanalization of middle cerebral artery (MCA) mainstem occlusion was assessed using continuous monitoring with TCCS and intravenously administered galactose-based microbubbles (Levovist) in 8 consecutive patients with acute ischemic stroke treated with intravenous rt-PA within 3 hours of symptom onset.

RESULTS

Recanalization at one hour occurred in 4 of 8 patients. The median NIHSS score was 21 (range 10 to 28) at baseline, 15 (range 0 to 24) at 1 h, and 11 (range 0 to 22) at 24 h. Asymptomatic hemorrhagic transformation (HT) was demonstrated on brain imaging in 6 patients.

CONCLUSION

This study demonstrates the feasibility of continuous monitoring of MCA recanalization using TCCS and Levovist, in acute stroke patients. The findings suggest a high rate of asymptomatic HT in monitored patients. Although all HTs were asymptomatic and did not preclude early clinical improvement, particular attention should be given to the incidence and clinical significance of HT in future studies using these methods.

Transcranial Doppler in Acute Stroke

Transcranial Doppler (TCD) is an evolving neurovascular ultrasound technique that has an established diagnostic and potential therapeutic role in acute stroke management. Angiographically validated criteria for circle-of-Willis occlusion and thrombolysis in brain ischemia classification of residual flow have set the stage for the further development of this technique. TCD has shown its clinical value in thrombolysis monitoring and early emboli detection. The therapeutic effect requires confirmation and may be enhanced further by nanobubble technologies.

Alteplase stability and bioactivity after low-power ultrasonic energy delivery with the OmniSonics resolution system

PURPOSE

Low-power ultrasonic (US) energy is capable of clot dissolution in vivo. The combination of US energy plus alteplase may further accelerate clot lysis; however, the effects of cavitation could potentially denature and inactivate the lytic protein. The purpose of this study was to determine the bioactivity and stability of alteplase when exposed to US energy with use of a novel intravascular US wire in an in vitro model.

MATERIALS AND METHODS

The model consisted of a 6.4-mm-diameter silicone tube closed at one end and filled with alteplase (1 mg/mL) in a water bath (37 degrees C). A 95-cm US wire (0.025-inch diameter, 20 kHz) was inserted into the tube and connected to a variable power generator. The wire delivers low-power acoustic energy 360 degrees around its 20-cm active length and was irrigated by a continuous infusion of purified water. Fresh 6-mL alteplase aliquots were exposed to US energy and tested in duplicates. Zero (control), 1 W, or 2 W of energy was delivered to individual test samples for zero (control), 0.5, 3, or 6 minutes. Alteplase samples were assayed for optical clarity and protein concentration with use of UV spectrophotometry, for percent protein monomer with use of high-performance size-exclusion chromatography, and for in vitro clot lysis activity.

RESULTS

In the control samples, optical clarity was clear or colorless in all samples; protein concentration was 1.02 mg/mL +/- 0; protein monomer was 98%; and clot lysis activity was 108% per mg +/- 1. In the test samples, optical clarity was clear or colorless in all samples; protein concentrations at 0.5, 3, and 6 minutes were 0.98 mg/mL +/- 0.02, 0.93 mg/mL +/- 0.01, and 0.86 mg/mL +/- 0.02, respectively, at 1 W, and 1.00 mg/mL +/- 0.03, 0.94 mg/mL +/- 0.10, and 0.84 mg/mL +/- 0.17, respectively, at 2 W. Protein monomer was 98% for all samples. Clot lysis activity levels at 0.5, 3, and 6 minutes were 111% per mg +/- 1, 110% per mg +/- 1, and 115% per mg +/- 1, respectively, at 1 W, and 110% per mg +/- 0, 111% per mg +/- 1, and 116% per mg +/- 2, respectively, at 2 W.

CONCLUSIONS

Alteplase solutions exposed to low-power US energy for as long as 6 minutes remained fully active and stable as determined by protein assays. Further investigation is warranted with use of combinations of US energy and alteplase.

Can a Commercial Diagnostic Ultrasound Device Accelerate Thrombolysis?

Background and Purpose— Recently, 3 clinical trials revealed encouraging results in recanalization and clinical outcome in acute stroke patients when 2-MHz transcranial Doppler monitoring was applied. This study investigated whether a 1.8-MHz commercial diagnostic ultrasound device has the potential to facilitate thrombolysis using an in vitro stroke model.

Methods— Duplex-Doppler, continuous wave-Doppler, and pulsed wave (PW)-Doppler were compared on their impact on recombinant tissue plasminogen activator (rtPA)–mediated thrombolysis. Blood clots were transtemporally sonicated in a human stroke model. Furthermore, ultrasound attenuation of 5 temporal bones of different thickness was determined.

Results— In comparison, only PW-Doppler accelerated rtPA–mediated thrombolysis significantly. Without temporal bone, PW-Doppler plus rtPA showed a significant enhancement in relative clot weight loss of 23.7% when compared with clots treated with rtPA only (33.9±5.5% versus 27.4±5.2%; P <0.0005). Ultrasound attenuation measurements revealed decreases of the output intensity of 86.8% (8.8 dB) up to 99.2% (21.2 dB), depending on temporal bone thickness (1.91 to 5.01 mm).

Conclusion— Without temporal bone, PW-Doppler significantly enhanced thrombolysis. However, because of a high attenuation of ultrasound by temporal bone, no thrombolytic effect was observed in our in vitro model, although Doppler imaging through the same temporal bone was still possible.

Intracranial Clot Lysis With Intravenous Microbubbles and Transcranial Ultrasound in Swine

Background and Purpose— Destruction of microbubbles by transcutaneous low-frequency ultrasound (LFUS) has been used to lyse adjacent clot and recanalize acutely thrombosed vessels. LFUS with intraarterial microbubbles has been shown to lyse cerebral clot rapidly in pigs without thrombolytic drugs. We hypothesized that intravenous platelet-targeted microbubbles with LFUS may be a rapid noninvasive technique to recanalize thrombosed intracerebral vessels.

Methods— After angiography, 0.5 cc of autogenous thrombus was injected into 1 ascending pharyngeal artery of a pig, occluding it and the rete mirabile. These vessels connect the carotid to the internal carotid and are the main cerebral blood supply. Saline control or intravenous decafluorobutane-sonicated dextrose albumin microbubbles tagged with a subtherapeutic quantity of glycoprotein 2b/3a receptor inhibitor eptifibatide (75 U/kg plus 12 cc of microbubbles administered over 21 minutes), or eptifibatide control, was given with transcutaneous temporal LFUS (1 MHz at 2.0 W/cm 2 ) for 24 minutes. Angiography followed with scoring of declotting and flow. The same protocol was repeated on the contralateral side with the other test fluid so each animal received a saline control and either tagged microbubble or eptifibatide alone.

Results— Fifteen pigs completed the protocol with a mean clot age of 4.6 hours. Using tagged microbubbles, 6 of 8 achieved success compared with 0 of 7 receiving eptifibatide alone ( P =0.007) and 1 of 15 receiving saline alone ( P =0.02).

Conclusions— Intravenous platelet-targeted microbubbles combined with transcranial LFUS can rapidly open acute intracranial thrombotic occlusions. Further development for ischemic stroke therapy is justified.