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

Interactions between individual ultrasound-stimulated microbubbles and fibrin clots

The use of ultrasound-stimulated microbubbles (USMBs) to promote thrombolysis is well established, but there remains considerable uncertainty about the mechanisms of this process. Here we examine the microscale interactions between individual USMBs and fibrin clots as a function of bubble size, exposure conditions and clot type. Microbubbles (n = 185) were placed adjacent to clot boundaries ("coarse" or "fine") using optical tweezers and exposed to 1-MHz ultrasound as a function of pressure (0.1-0.39 MPa). High-speed (10 kfps) imaging was employed, and clots were subsequently assessed with 2-photon microscopy. For fine clots, 46% of bubbles "embedded" within 10 μm of the clot boundary at pressures of 0.1 and 0.2 MPa, whereas at 0.39 MPa, 53% of bubbles penetrated and transited into the clots with an incidence inversely related to their diameter. A substantial fraction of penetrating bubbles induced fibrin network damage and promoted the uptake of nanobeads. In coarse clots, penetration occurred more readily and at lower pressures than in fine clots. The results therefore provide direct evidence of therapeutically relevant effects of USMBs and indicate their dependence on size, exposure conditions and clot properties.

Cardiovascular drug delivery with ultrasound and microbubbles

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

Cavitation thresholds of contrast agents in an in vitro human clot model exposed to 120-kHz ultrasound

Ultrasound contrast agents (UCAs) can be employed to nucleate cavitation to achieve desired bioeffects, such as thrombolysis, in therapeutic ultrasound applications. Effective methods of enhancing thrombolysis with ultrasound have been examined at low frequencies (<1 MHz) and low amplitudes (<0.5 MPa). The objective of this study was to determine cavitation thresholds for two UCAs exposed to 120-kHz ultrasound. A commercial ultrasound contrast agent (Definity(®)) and echogenic liposomes were investigated to determine the acoustic pressure threshold for ultraharmonic (UH) and broadband (BB) generation using an in vitro flow model perfused with human plasma. Cavitation emissions were detected using two passive receivers over a narrow frequency bandwidth (540-900 kHz) and a broad frequency bandwidth (0.54-1.74 MHz). UH and BB cavitation thresholds occurred at the same acoustic pressure (0.3 ± 0.1 MPa, peak to peak) and were found to depend on the sensitivity of the cavitation detector but not on the nucleating contrast agent or ultrasound duty cycle.

Neurological assessment scores in rabbit embolic stroke models

Background:

Neurological outcomes and behavioral assessments are widely used in animal models of stroke, but assessments in rabbit models are not fully validated. The wryneck model of neurological assessment scores (NAS) was compared to percent infarct volume (%IV) values (infarct volume is a proven clinical indicator of stroke severity) and arterial occlusion localization in three rabbit angiographic stroke models.

Hypothesis:

NAS values will correlate with percent infarct volume values.

Methods:

Anesthetized New Zealand White rabbits (N=131, 4-5 kg) received internal carotid artery emboli by angiographic catheter introduced into the femoral artery and occlusions were characterized. Rabbits were evaluated at 24 hours post embolism using the NAS test of 0 (normal) to 10 (death). Deficit criteria included neck twist, righting reflex, extension reflex in hind paw and forepaw, and posture. Brain sections stained with triphenyltetrazolium chloride (TTC) were analyzed for %IV. Volume of the infarct was measured and calculated as a percent of the total brain volume.

Results:

The aggregate correlation for NAS values vs. %IV values was R=0.61, p<0.0001, a strong positive relationship, while correlations of the NAS components ranged from R=0.28-0.46. Occlusionsof the posterior cerebral artery vs. the middle cerebral artery alone produced significantly greater deficit scores at p<0.0001.

Conclusions:

These positive results validate the NAS system in the rabbit angiographic embolic stroke model.

The use of ultrasound-stimulated contrast agents as an adjuvant for collagenase therapy in chronic total occlusions

AIMS

To investigate the effectiveness of combining collagenase and ultrasound-stimulated microbubble (USMB) treatments in reducing the mechanical force required for crossing a guidewire through CTOs.

METHODS AND RESULTS

Experiments were conducted on ex vivo specimens of a rabbit femoral artery CTO model (n=45 total samples). Four primary groups were employed: control (n=6), collagenase only (n=15), USMB only (1 MHz frequency) (n=5), and collagenase+USMB (n=19). In one set of experiments the force required to puncture through CTO samples was measured and it was found that the puncture force was 2.31-fold lower for the combined treatment group relative to the comparable collagenase-only group (p<0.05). In a second set of experiments, the total protein and hydroxyproline content of the supernatant solution adjacent to the CTO was analysed. Significantly higher hydroxyproline levels were measured in collagenase+USMB treated CTOs (0.065 g/mL) compared to collagenase (0.030 g/mL), USMB (0.003 g/mL) and control (0.004 g/mL) (p<0.05), indicating that the combined treatment augmented collagenase degradation.

CONCLUSIONS

Ultrasound-stimulated microbubbles improved the effectiveness of collagenase in reducing the force required to cross experimental CTOs. This new approach may have the potential to reduce treatment times and improve the success rates of emerging collagenase-based treatments of CTO.

Three variations in rabbit angiographic stroke models

PURPOSE

To develop angiographic models of embolic stroke in the rabbit using pre-formed clot or microspheres to model clinical situations ranging from transient ischemic events to severe ischemic stroke.

MATERIALS AND METHODS

New Zealand White rabbits (N=151) received angiographic access to the internal carotid artery (ICA) from a femoral approach. Variations of emboli type and quantity of emboli were tested by injection into the ICA. These included fresh clots (1.0-mm length, 3-6h), larger aged clots (4.0-mm length, 3 days), and 2 or 3 insoluble microspheres (700-900 μm). Neurological assessment scores (NAS) were based on motor, sensory, balance, and reflex measures. Rabbits were euthanized at 4, 7, or 24h after embolization, and infarct volume was measured as a percent of total brain volume using 2,3,5-triphenyltetrazolium chloride (TTC).

RESULTS

Infarct volume percent at 24 h after stroke was lower for rabbits embolized with fresh clot (0.45±0.14%), compared with aged clot (3.52±1.31%) and insoluble microspheres (3.39±1.04%). Overall NAS (including posterior vessel occlusions) were positively correlated to infarct volume percent measurements in the fresh clot (r=0.50), aged clot (r=0.65) and microsphere (r=0.62) models (p<0.001).

CONCLUSION

The three basic angiographic stroke models may be similar to human transient ischemic attacks (TIA) (fresh clot), major strokes that can be thrombolysed (aged clot), or major strokes with insoluble emboli such as atheromata (microspheres). Model selection can be tailored to specific research needs.

Bioeffects of ultrasound-stimulated microbubbles on endothelial cells: gene expression changes associated with radiation enhancement in vitro

Ultrasound can be used to target endothelial cells in cancer therapy where the destruction of vasculature leads to tumor cell death. Here, we demonstrate ultrasound bioeffects in which the levels of genes in endothelial cells can be significantly altered by ultrasound-stimulated microbubble exposure. These were compared with established effects of radiation on endothelial cells at a gene level. Human-endothelial cells were exposed to ultrasound and microbubbles, radiation or combinations of ultrasound, microbubbles and radiation. Gene expression analyses revealed an up-regulation of genes known to be involved in apoptosis and ceramide-induced apoptotic pathways, including SMPD2, UGT8, COX6B1, Caspase 9 and MAP2K1 with ultrasound-stimulated microbubble exposure but not SMPD1. This was supported by immunohistochemistry and morphologic changes examined with cell microscopy, which showed changes in SMPD1 gene product in cells with microbubble exposure. This supports the hypothesis that ultrasound-stimulated microbubbles can induce significant bioeffect-related changes in gene expression and can affect ceramide signaling pathways in endothelial cells, leading to apoptosis.

Influences of microbubble diameter and ultrasonic parameters on in vitro sonothrombolysis efficacy

PURPOSE

To quantify the effects of microbubble (MB) size, elasticity, and pulsed ultrasonic parameters on in vitro sonothrombolysis (ultrasound [US]-mediated thrombolysis) efficacy.

MATERIALS AND METHODS

Monodispersive MBs with diameters of 1 μm or 3 μm were exposed to pulsed US (1 MHz or 3 MHz) to lyse rabbit blood clots. Sonothrombolysis efficacy (clot mass loss) was measured as functions of MB size and concentration, ultrasonic frequency and intensity, pulse duration (PD), pulse repeat frequency (PRF), and duty factor.

RESULTS

Sonothrombolysis at 1 MHz was more effective using 3-μm MBs and at 3 MHz using 1-μm MBs. Sonothrombolysis was more effective at 1 MHz when≥75% of MBs remained intact, especially for 3-μm MBs; improving sonothrombolysis by increasing PRF from 100 Hz to 400 Hz at 3 MHz was associated with increasing 3-μm MB survival. However, 60% of 1-μm MBs were destroyed during maximal sonothrombolysis at 3 MHz, indicating that considerable MB collapse may be required for sonothrombolysis under these conditions.

CONCLUSIONS

The ability to control MB size and elasticity permits using a wide range of US parameters (eg, frequency, intensity) to produce desired levels of sonothrombolysis. Comparable, maximal sonothrombolysis efficacy was achieved at 20-fold lower intensity with 3-μm MBs (0.1W/cm(2)) than with 1-μm MBs (2.0W/cm(2)), a potential safety issue for in vivo sonothrombolysis. US parameters that maximized MB survival yielded maximal sonothrombolysis efficacy except with 1-μm MBs at 3MHz where most MBs were destroyed.

High-intensity focused ultrasound (HIFU) for dissolution of clots in a rabbit model of embolic stroke

It is estimated that only 2-6% of patients receive thrombolytic therapy for acute ischemic stroke suggesting that alternative therapies are necessary. In this study, we investigate the potential for high intensity focused ultrasound (HIFU) to initiate thrombolysis in an embolic model of stroke. Iron-loaded blood clots were injected into the middle cerebral artery (MCA) of New Zealand White rabbits, through the internal carotid artery and blockages were confirmed by angiography. MRI was used to localize the iron-loaded clot and target the HIFU beam for treatment. HIFU pulses (1.5 MHz, 1 ms bursts, 1 Hz pulse repetition frequency, 20 s duration) were applied to initiate thrombolysis. Repeat angiograms and histology were used to assess reperfusion and vessel damage. Using 275 W of acoustic power, there was no evidence of reperfusion in post-treatment angiograms of 3 rabbits tested. In a separate group of animals, 415 W of acoustic power was applied and reperfusion was observed in 2 of the 4 (50%) animals treated. In the last group of animals, acoustic power was further increased to 550 W, which led to the reperfusion in 5 of 7 (∼70%) animals tested. Histological analysis confirmed that the sonicated vessels remained intact after HIFU treatment. Hemorrhage was detected outside of the sonication site, likely due to the proximity of the target vessel with the base of the rabbit skull. These results demonstrate the feasibility of using HIFU, as a stand-alone method, to cause effective thrombolysis without immediate damage to the targeted vessels. HIFU, combined with imaging modalities used to identify and assess stroke patients, could dramatically reduce the time to achieve flow restoration in patients thereby significantly increasing the number of patients which benefit from thrombolysis treatments.

In vitro sonothrombolysis of human blood clots with BR38 microbubbles

Microbubble-mediated sonothrombolysis is a promising approach for ischemic stroke treatment. The aim of this in vitro study was to evaluate a new microbubble (MB) formulation (BR38) for sonothrombolysis and to investigate the involved mechanisms. Human whole-blood clots were exposed to different combinations of recombinant tissue plasminogen activator (rtPA), ultrasound (US) and MB. Ultrasound at 1.6 MHz was used at 150, 300, 600 and 1000 kPa (peak-negative pressure). Thrombolysis efficacy was assessed by measuring clot diameter changes during 60-min US exposure. The rate of clot diameter loss (RDL) in μm/min was determined and clot lysis profiles were analyzed. The most efficient clot lysis (5.9 μm/min) was obtained at acoustic pressures of 600 and 1000 kPa in combination with MB and a low concentration of rtPA (0.3 μg/mL). This is comparable with the rate obtained with rtPA at 3 μg/mL alone (6.6 μm/min, p > 0.05). Clot lysis profiles were shown to be related to US beam profiles and microbubble cavitation.

In vitro and in vivo high-intensity focused ultrasound thrombolysis

OBJECTIVES

To characterize the ability of high-intensity focused ultrasound to achieve thrombolysis in vitro and investigate the feasibility of this approach as a means of restoring blood flow in thrombus-occluded arteries in vivo.

MATERIALS AND METHODS

All experiments were approved by the Institutional Animal Care Committee. Thrombolysis was performed with a 1.51-MHz focused ultrasound transducer with pulse lengths of 0.1 to 10 milliseconds and acoustic powers up to 300 W. In vitro experiments were performed with blood clots formed from rabbit arterial blood and situated in 2-mm diameter tubing. Both single location and flow bypass recanalization experiments were conducted. In vitro clot erosion was assessed with 30-MHz ultrasound, with debris size measured with filters and a Coulter counter. In vivo clots were initiated in the femoral arteries of rabbits (n = 26). Cavitation signals from bubbles formed during exposure were monitored. In vivo flow restoration was assessed with 23-MHz Doppler ultrasound.

RESULTS

At a single location, in vitro clot erosion volumes increased with exposure power and pulse length, with debris size reducing with increasing pulse length. Flow bypass experiments achieved 99.2% clot erosion with 1.1% of debris above 0.5 mm in size. In vivo, 10 milliseconds pulses were associated with bleeding, but at 1 millisecond, it was feasible to achieve partial flow restoration in 6 of the 10 clots with only 1 of the 10 showing evidence of bleeding. In all cases, thrombolysis occurred only in the presence of cavitation.

CONCLUSION

High-intensity focused ultrasound thrombolysis is feasible as a means of restoring partial blood flow in thrombus-occluded arteries in the absence of thrombolytic agents. The potential for bleeding with this approach requires further investigation.

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.

Dodecafluoropentane emulsion decreases infarct volume in a rabbit ischemic stroke model

PURPOSE

To assess the efficacy of dodecafluoropentane emulsion (DDFPe), a nanodroplet emulsion with significant oxygen transport potential, in decreasing infarct volume in an insoluble-emboli rabbit stroke model.

MATERIALS AND METHODS

New Zealand White rabbits (N = 64; weight, 5.1 ± 0.50 kg) underwent angiography and received embolic spheres in occluded internal carotid artery branches. Rabbits were randomly assigned to groups in 4-hour and 7-hour studies. Four-hour groups included control (n = 7, embolized without treatment) and DDFPe treatment 30 minutes before stroke (n = 7), at stroke onset (n = 8), and 30 minutes (n = 5), 1 hour (n = 7), 2 hours (n = 5), or 3 hours after stroke (n = 6). Seven-hour groups included control (n = 6) and DDFPe at 1 hour (n = 8) and 6 hours after stroke (n = 5). DDFPe dose was a 2% weight/volume intravenous injection (0.6 mL/kg) repeated every 90 minutes as time allowed. After euthanasia, infarct volume was determined by vital stains on brain sections.

RESULTS

At 4 hours, median infarct volume decreased for all DDFPe treatment times (pretreatment, 0.30% [P = .004]; onset, 0.20% [P = .004]; 30 min, 0.35% [P = .009]; 1 h, 0.30% [P = .01]; 2 h, 0.40% [P = .009]; and 3 h, 0.25% [P = .003]) compared with controls (3.20%). At 7 hours, median infarct volume decreased with treatment at 1 hour (0.25%; P = .007) but not at 6 hours (1.4%; P = .49) compared with controls (2.2%).

CONCLUSIONS

Intravenous DDFPe in an animal model decreases infarct volumes and protects brain tissue from ischemia, justifying further investigation.

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.

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.