Effect of low frequency ultrasound on combined rt-PA and eptifibatide thrombolysis in human clots

Recent advances in micro- and nano-bubbles for atherosclerosis applications

Micro- and nano-bubbles have been developed as powerful multimodal theranostic agents for atherosclerosis treatment.

A novel model for evaluating thrombolytic therapy in dogs with ST-elevation myocardial infarction

Background

There is still no standard large animal model for evaluating the effectiveness of potential thrombolytic therapies. Here, we aimed to develop a new beagle model with ST-elevation myocardial infarction (STEMI) by injecting autologous emboli with similar components of coronary thrombus.

Methods

18 male beagles were included and divided into three groups: red embolus group (n = 6), white embolus group (n = 6) or white embolus + rt-PA group (n = 6). Autologous emboli were infused into the mid-distal region of the left anterior descending coronary artery. The composition of embolus was examined by scanning electron microscope (SEM). Coronary angiography was performed to verify the status of embolism. Myocardial infarct size was measured by 2, 3, 5- triphenyltetrazolium chloride (TTC) staining.

Results

Red thrombus was characteristic of loose reticular structure of erythrocytes under SEM, while the white embolus had compacted structure that mainly consisted of a dense mass of fibrin. Coronary angiography showed the recanalization rate was 2/6 in the red embolus group versus 0/6 in the white embolus group in three hours after occlusion. Arrhythmia, resolution of ST-segment elevation and lower T wave on the electrocardiogram appeared in the red embolus group but not in the white embolus group. Another six dogs with white thrombi were treated with rt-PA. Five out of six dogs exhibited coronary recanalization after two hours of therapy, compared to zero dogs without rt-PA treatment. The size of myocardial infarction in rt-PA group reduced significantly compared with white embolus group using TTC staining method.

Conclusions

The white embolism model was more convenient experimentally and had a higher uniformity, stability and success rate. The major innovation of our study is that we applied fibrin-rich white thrombi to establish beagle model possessing features of clinically observed coronary thrombi in time window of intravenous thrombolysis of STEMI. This model can be used to evaluate new thrombolytic drugs for the treatment of STEMI.

Plasmin-loaded echogenic liposomes for ultrasound-mediated thrombolysis

Plasmin, a direct fibrinolytic, shows a significantly superior hemostatic safety profile compared to recombinant tissue plasminogen activator (rtPA), the only FDA-approved thrombolytic for the treatment of acute ischemic stroke. The improved safety of plasmin is attributed to the rapid inhibition of free plasmin by endogenous plasmin inhibitors present in very high concentrations (1 μM). However, this rapid inhibition prevents the intravenous (IV) administration of plasmin. In emergency situations, catheter-based local administration is not practical. There is a need for an alternative technique for IV administration of plasmin. A possible solution is the encapsulation of plasmin in echogenic liposomes (ELIP) for protection from inhibitors until ultrasound (US)-triggered release at the clot site. ELIP are bilayer phospholipid vesicles with encapsulated gas microbubbles. US induces oscillation and collapse of the gas bubbles, which facilitates ELIP rupture and delivery of the encapsulated contents. Plasmin-loaded ELIP (PELIP) were manufactured and characterized for size, gas and drug encapsulations, and in vitro thrombolytic efficacy using a human whole blood clot model. Clots were exposed to PELIP with and without exposure to US (center frequency 120 kHz, pulse repetition frequency 1667 Hz, peak-to-peak pressure of 0.35 MPa, 50 % duty cycle). Thrombolytic efficacy was calculated by measuring the change in clot width over a 30-min treatment period using an edge detection MATLAB program. The mean clot lysis obtained with PELIP in the presence of US exposure was 31 % higher than that obtained without US exposure and 15 % higher than that obtained with rtPA treatment (p < 0.05).The enhanced clot lysis is attributed to the US-mediated release of plasmin from the liposomes.

Targeted thrombolysis strategies for neuroprotective effect

Stroke is usually treated by systemic thrombolytic therapy if the patient presents within an appropriate time window. There is also widespread interest in the development of thrombolytic agents that can be used in cases of delayed presentation. Current agents that can be used in cases of delayed presentation of nerve damage by thrombus. Current systemic thrombolytic therapy is associated with adverse effects such as fibrinogenolysis and bleeding. In an attempt to increase the efficacy, safety, and specificity of thrombolytic therapy, a number of targeted thrombolytic agents have been studied in recent years. This review focuses on the concepts underlying targeted thrombolytic therapy and describes recent drug developments in this field.

A clinical study of transcranial ultrasound as an adjuvant therapy for progressive cerebral infarction

The present study aimed to investigate the clinical efficacy of transcranial ultrasound as an adjuvant therapy in combination with small doses of urokinase (UK) for the treatment of progressive cerebral infarction. Sixty-one eligible patients with progressive cerebral infarction were successively and randomly assigned into one of the following groups; 30 patients to the treatment group (transcranial ultrasound + small doses of UK) and 31 patients to the control group (single small doses of UK). Based on conventional therapy, patients in the treatment group received transcranial ultrasound. The neural function deficit scale and curative effect scores of the two groups were recorded before treatment and on the 7th and 14th days after treatment. No differences in the neural function deficit scale between the two groups was observed before treatment, however, on the 7th and 14th days after treatment, a significant decrease was evident in the treatment group (P<0.01). The overall response rate was 100% in the treatment group and 74.2% in the control group, with a significant difference (P<0.01). Transcranial ultrasound is able to contribute to the thrombolytic effects of UK and prevent the progression of thrombi, subsequently aiding the recovery of neural functions.

Short-term high-dose effect of lovastatin on thrombolysis by rt-PA in a human whole-blood in vitro clot model

High-dose hydroxymethylglutaryl coenzyme. A reductase inhibitor (statin) administration reduces neuronal injury and improves outcomes in experimental models of acute ischemic stroke, and has been shown to be safe in a phase 1 dose-escalation study using lovastatin at doses higher than currently approved for daily use. Statins also affect the hemostatic system by upregulating t-PA expression and decreasing plasminogen activator inhibitor (PAI-1) expression, platelet adhesion and thrombus formation in animal models. Since a thrombolytic agent, recombinant tissue plasminogen activator (rt-PA), is currently the only FDA-approved therapy for use in ischemic stroke patients, it is important to ascertain whether high statin doses impact the efficacy of rt-PA. The main goal of this study was to evaluate the effect of a high dose of lovastatin and its active form, lovastatin hydroxy acid, on rt-PA thrombolysis in an in vitro model. Percentage clot lysis was measured in the presence and absence of rt-PA in three different treatment groups: lovastatin, lovastatin hydroxy acid, and ethanol. The effect of ethanol on clot lysis was studied since ethanol was used to disperse the highly hydrophobic lovastatin. The decrease in clot width over time was measured using microscopic imaging of an in vitro human whole blood clot model; an approximately 400 μm diameter clot was formed on suture silk, suspended in human fresh frozen plasma (hFFP) and exposed to treatment. In the absence of rt-PA, clot lysis did not show statistically significant differences in the percentage clot lysis between different treatment groups (p=0.103). In the presence of rt-PA, clot lysis was greater than in the absence of rt-PA for all groups, but there were no statistically significant differences between treatment groups (p=0.385). In this in vitro study, high doses of lovastatin neither impaired nor enhanced the lytic efficacy of rt-PA.

Taboos and opportunities in sonothrombolysis for stroke

Systemic thrombolysis with tissue plasminogen activator (tPA) is the only approved treatment for acute ischaemic stroke that improves functional outcome if given up to 4.5 h from symptom onset. At least half of treated patients have unfavourable outcomes long-term though, emphasising the need to amplify the only approved acute stroke therapy. Ultrasound targeting of an intra-arterial occlusive clot and delivering mechanical pressure to its surrounding fluids (referred to as sonothrombolysis) accelerates the thrombolytic effect of tPA. Higher recanalisation rates produce a trend towards better functional outcomes that could be safely achieved with the combination of 2 MHz frequency ultrasound and systemic tPA. To further accelerate the clot-dissolving effect of ultrasound, a variety of frequencies and intensities as well as other adjuvant treatment elements are being studied. However, literature reports argue efficacy and safety of these novel approaches doubting promptly translation into the clinical practice. This review will summarise our current knowledge about potentially harmful (taboos) directions and what we think are promising avenues for these future stroke therapies. We also give a prospect for novel technologies such as operator-independent devices that aim to further spread the use of sonothrombolysis for stroke.

Temperature affects thrombolytic efficacy using rt-PA and eptifibatide, an in vitro study

The potential for hypothermia as a neuroprotectant during stroke has led to its increase in clinical use. At the same time, combination pharmaceutical therapies for ischemic stroke using recombinant tissue plasminogen activator (rt-PA), and GP IIb-IIIa inhibitors, such as Eptifibatide (Epf ), are under study. However, there is little data on how the reactions triggered by these agents are impacted by temperature. Here, clot lysis during exposure to the combination of rt-PA and Epf is measured in an in vitro human clot model at hypothermic temperatures. The hypothesis is that lytic efficacy of rt-PA and Epf decreases with decreasing temperature. Whole blood clots from 31 volunteers were exposed to rt-PA (0.5 μg/mL) and Epf (0.63 μg/mL) in human fresh-frozen plasma (rt-PA+Epf ), rt-PA alone in plasma (rt-PA Alone), or to plasma alone (Control), at temperatures from 30°C to 37°C, for 30 minutes. Clot lysis was measured using a microscopic imaging technique; the mean fractional clot loss (FCL) at 30 minutes was used to determine lytic efficacy. Temperature had a significant impact on FCL in clots exposed to rt-PA+Epf, with the FCL being lower at 30°C to 36°C than at 37°C. The FCL remained significantly higher for rt-PA+Epf–treated clots than Controls regardless of temperature, with the exception of measurements made at 30°C when no significant differences in the FCL were observed between groups. The use of hypothermia as a neuroprotectant may negatively impact the therapeutic benefit of thrombolytic agents.

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.

Combination treatment with rt-PA is more effective than rt-PA alone in an in vitro human clot model

Incidence of intra-cranial hemorrhage linked to treatment of ischemic stroke with recombinant tissue plasminogen activator (rt-PA) has led to interest in adjuvant therapies such as ultrasound (US) or plasminogen, to enhance rt-PA efficacy and improve patient safety. High-frequency US (∼MHz) such as 2-MHz transcranial Doppler (TCD) has demonstrated increased recanalization in situ. Low-frequency US (∼kHz) enhanced thrombolysis (UET) has demonstrated higher lytic capabilities but has been associated with incidence of intracerebral hemorrhage in some clinical trials. In vitro studies using plasminogen have shown enhancement of lysis. This study compared rt-PA-induced lysis using adjuvant therapies, with 120-kHz or 2-MHz pulsed US, or plasminogen, in an in vitro human whole blood clot model. Blood was drawn from 30 subjects after local institutional approval. Clots were exposed to rt-PA at concentrations of 0 to 3.15 μg/ml. Clots were exposed to rt-PA alone (rt-PA) or in combination with plasminogen (Plg), 120-kHz US (120-kHz), or 2-MHz US (2-MHz). Thrombolytic efficacy was determined by assessing the percent fractional clot loss (FCL) at 30 minutes using microscopic imaging. There was no enhancement of lysis for combination therapy with [rt-PA]=0 μg/ml. Adding rt- PA increased lysis for all groups. As [rt-PA] increased, lysis tended to increase for 120-kHz and Plg (FCL: from 50% to 70%, 120-kHz; 65% to 83%, Plg) but not for 2-MHz (58% to 52%). Lytic efficacy in combination therapy depends on rt- PA concentration and the adjuvant therapy type. For non-zero rt-PA concentrations, all combination therapies produced more lysis than rt-PA alone.

Anticoagulant, anti-aggregation and antithrombotic effects of a novel hexapeptide

OBJECTIVES

Hexapeptide is a novel synthetic oligopeptide with a structure similar to that of eptifibatide. This study was designed to investigate the anticoagulant, anti-aggregation, disaggregation and anti-thrombogenesis effects of hexapeptide.

METHODS

The effects of antiplatelet aggregation induced by adenosine diphosphate (ADP), arachidonic acid (AA) and thrombin, and the effect of disaggregation of platelet aggregation induced by ADP were determined. The anticoagulation indexes were determined by different kits.

KEY FINDINGS

Hexapeptide 1 × 10(-5) -1 × 10(-4) m could significantly prolong rabbit blood clotting time, thrombin time, prothrombin time and activated partial thromboplastin enzyme time, and reduce the length, wet weight, dry weight and the index of thrombus in a concentration-dependent manner. Hexapeptide 1 × 10(-4) m decreased platelet adhesion rate by 40.2%. The platelet aggregation inhibition of hexapeptide in dogs and humans was more obvious than in rabbits and rats. The aggregation inhibition rate of 1 × 10(-5) m hexapeptide in dogs, rabbits, rats and humans induced by ADP was 93.9 ± 1.3%, 66.2 ± 1.4%, 76.1 ± 3.2% and 99.8 ± 0.2%, respectively; the 50% inhibitory concentration (IC50) of hexapeptide was 7.24 × 10(-8), 3.24 × 10(-6), 6.61 × 10(-6) and 8.91 × 10(-8) m, respectively. For the aggregation inhibition rate of hexapeptide in dogs, rabbits and humans induced by AA, the IC50 was 1.29 × 10(-9), 1.32 × 10(-6) and 9.33 × 10(-8) m, respectively; the IC50 of aggregation inhibition rates induced by thrombin was 2.88 × 10(-6), >1 × 10(-5) and 4.17 × 10(-6) m, respectively. The disaggregation rate of 1 × 10(-4) m hexapeptide in dog induced by ADP was 68.8 ± 7.4%.

CONCLUSIONS

Hexapeptide has anticoagulant, antiplatelet aggregation, disaggregation and antithrombotic effects in vitro.

Ultrasound-enhanced rt-PA thrombolysis in an ex vivo porcine carotid artery model

Ultrasound is known to enhance recombinant tissue plasminogen activator (rt-PA) thrombolysis. In this study, occlusive porcine whole blood clots were placed in flowing plasma within living porcine carotid arteries. Ultrasonically induced stable cavitation was investigated as an adjuvant to rt-PA thrombolysis. Aged, retracted clots were exposed to plasma alone, plasma containing rt-PA (7.1 ± 3.8 μg/mL) or plasma with rt-PA and Definity® ultrasound contrast agent (0.79 ± 0.47 μL/mL) with and without 120-kHz continuous wave ultrasound at a peak-to-peak pressure amplitude of 0.44 MPa. An insonation scheme was formulated to promote and maximize stable cavitation activity by incorporating ultrasound quiescent periods that allowed for the inflow of Definity®-rich plasma. Cavitation was measured with a passive acoustic detector throughout thrombolytic treatment. Thrombolytic efficacy was measured by comparing clot mass before and after treatment. Average mass loss for clots exposed to rt-PA and Definity® without ultrasound (n = 7) was 34%, and with ultrasound (n = 6) was 83%, which constituted a significant difference (p < 0.0001). Without Definity® there was no thrombolytic enhancement by ultrasound exposure alone at this pressure amplitude (n = 5, p < 0.0001). In the low-oxygen environment of the ischemic artery, significant loss of endothelium occurred but no correlation was observed between arterial tissue damage and treatment type. Acoustic stable cavitation nucleated by an infusion of Definity® enhances rt-PA thrombolysis without apparent treatment-related damage in this ex vivo porcine carotid artery model.

Making the right choice: optimizing rt-PA and eptifibatide lysis, an in vitro study

INTRODUCTION

Recombinant tissue plasminogen activator (rt-PA) is the only FDA approved lytic therapy for acute ischemic stroke. However, there can be complications such as intra-cerebral hemorrhage. This has led to interest in adjuncts such as GP IIb-IIIa inhibitors. However, there is little data on combined therapies. Here, we measure clot lysis for rt-PA and eptifibatide in an in vitro human clot model, and determine the drug concentrations maximizing lysis. A pharmacokinetic model is used to compare drug concentrations expected in clinical trials with those used here. The hypothesis is that there is a range of rt-PA and eptifibatide concentrations that maximize in vitro clot lysis.

MATERIALS AND METHODS

Whole blood clots were made from blood obtained from 28 volunteers, after appropriate institutional approval. Sample clots were exposed to rt-PA and eptifibatide in human fresh-frozen plasma; rt-PA concentrations were 0, 0.5, 1, and 3.15 μg/ml, and eptifibatide concentrations were 0, 0.63, 1.05, 1.26 and 2.31 μg/ml. All exposures were for 30 minutes at 37 C. Clot width was measured using a microscopic imaging technique and mean fractional clot loss (FCL) at 30 minutes was used to determine lytic efficacy. On average, 28 clots (range: 6-148) from 6 subjects (3-24) were used in each group.

RESULTS AND CONCLUSIONS

FCL for control clots was 14% (95% Confidence Interval: 13-15%). FCL was 58% (55-61%) for clots exposed to both drugs at all concentrations, except those at an rt-PA concentration of 3.15 μg/ml, and eptifibatide concentrations of 1.26 μg/ml (Epf) or 2.31 μg/ml. Here, FCL was 43% (36-51) and 35% (32-38) respectively. FCL is maximized at moderate rt-PA and eptifibatide concentration; these values may approximate the average concentrations used in some rt-PA and eptifibatide treatments.

In vitro evaluation of ultrasound-assisted thrombolysis using a targeted ultrasound contrast agent

A thrombus-targeted ultrasound contrast agent bound with tirofiban - a glycoprotein (GP) IIb/IIIa antagonist that can specifically bind to activated platelets in the thrombus - was designed to enhance both the image contrast and thrombolysis effect. In this study, we used 76 canine thrombi for investigation. The targeting ability to thrombi was confirmed by microphotography and high-frequency ultrasound (40 MHz) imaging. The effect of the targeted microbubbles on thrombolysis enhancement was investigated using an in vitro flow system: targeted and nontargeted microbubbles flowed through the clot for 30 seconds with a washing step; the microbubbles remained on the clot that were then cavitated by ultrasound (frequency = 1 MHz, MI = 1.2). The extent of thrombolysis was evaluated by weight reduction and histology analysis. The targeted microbubbles reduced the weight of thrombi by a factor of 1.7 times that of the nontargeted microbubbles. (clot weight reduction: 23.1 +/- 5.3% versus 13.6 +/- 4.9%, p < 0.01 between targeted and nontargeted group), and the signal enhancement was 3.34 +/- 0.30 dB (mean +/- SD, p < 0.01 compared to control). We conclude that targeted microbubbles are applicable not only for molecular imaging of thrombi but also for improving the effectiveness of ultrasound-assisted thrombolysis.

Ultrasound-enhanced thrombolysis with tPA-loaded echogenic liposomes

BACKGROUND AND PURPOSE

Currently, the only FDA-approved therapy for acute ischemic stroke is the administration of recombinant tissue plasminogen activator (tPA). Echogenic liposomes (ELIP), phospholipid vesicles filled with gas and fluid, can be manufactured to incorporate tPA. Also, transcranial ultrasound-enhanced thrombolysis can increase the recanalization rate in stroke patients. However, there is little data on lytic efficacy of combining ultrasound, echogenic liposomes, and tPA treatment. In this study, we measure the effects of pulsed 120-kHz ultrasound on the lytic efficacy of tPA and tPA-incorporating ELIP (t-ELIP) in an in-vitro human clot model. It is hypothesized that t-ELIP exhibits similar lytic efficacy to that of rt-PA.

METHODS

Blood was drawn from 22 subjects after IRB approval. Clots were made in 20-microL pipettes, and placed in a water tank for microscopic visualization during ultrasound and drug treatment. Clots were exposed to combinations of [tPA]=3.15 microg/ml, [t-ELIP]=3.15 microg/ml, and 120-kHz ultrasound for 30 minutes at 37 degrees C in human plasma. At least 12 clots were used for each treatment. Clot lysis over time was imaged and clot diameter was measured over time, using previously developed imaging analysis algorithms. The fractional clot loss (FCL), which is the decrease in mean clot width at the end of lytic treatment, was used as a measure of lytic efficacy for the various treatment regimens.

RESULTS

The fractional clot loss FCL was 31% (95% CI: 26-37%) and 71% (56-86%) for clots exposed to tPA alone or tPA with 120 kHz ultrasound. Similarly, FCL was 48% (31-64%) and 89% (76-100%) for clots exposed to t-ELIP without or with ultrasound.

CONCLUSIONS

The lytic efficacy of tPA containing echogenic liposomes is comparable to that of tPA alone. The addition of 120 kHz ultrasound significantly enhanced lytic treatment efficacy for both tPA and t-ELIP. Liposomes loaded with tPA may be a useful adjunct in lytic treatment with tPA.