Enhancement of fibrinolysis with ultrasound energy

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.

Microbubbles and Ultrasound Accelerated Thrombolysis for Peripheral Arterial Occlusions: The Outcomes of a Single Arm Phase II Trial

OBJECTIVE

Acute peripheral arterial occlusions can be treated by catheter directed thrombolysis (CDT). However, CDT is time consuming and accompanied by the risk of bleeding complications. The addition of contrast enhanced ultrasound and microbubbles could improve thrombus susceptibility to thrombolytic agents and potentially shorten treatment time with a lowered risk of bleeding complications. This article reports the outcomes of the safety and feasibility of this novel technique.

METHODS

In this single arm phase II trial, 20 patients with acute lower limb ischaemia received CDT combined with an intravenous infusion of microbubbles and locally applied ultrasound during the first hour of standard intra-arterial thrombolytic therapy. The primary endpoint was safety, i.e., occurrence of serious adverse events (haemorrhagic complications and/or amputation) and death within one year. Secondary endpoints included angiographic and clinical success, thrombolysis duration, additional interventions, conversion, and quality of life.

RESULTS

The study included 20 patients (16 men; median age 68.0 years; range, 50.0 - 83.0; and 40% native artery and 60% bypass graft). In all patients, the use of microbubble contrast enhanced sonothrombolysis could be applied successfully. There were no serious adverse events related to the experimental treatment. Duplex examination showed flow distal from the occlusion after 23.1 hours (range 3.1 - 46.5) with a median thrombolysis time of 47.5 hours (range 6.0 - 81.0). The short term ABI and pain scores significantly improved; however, no changes were observed before or after thrombolysis in the microcirculation. Overall mortality and amputation rates were both 2% within one year. The one year patency rate was 55%.

CONCLUSION

Treatment of patients with acute peripheral arterial occlusions with contrast enhanced sonothrombolysis is feasible and safe to perform in patients. Further research is necessary to investigate the superiority of this new treatment over standard treatment.

Cavitation Emissions Nucleated by Definity Infused through an EkoSonic Catheter in a Flow Phantom

The EkoSonic endovascular system has been cleared by the U.S. Food and Drug Administration for the controlled and selective infusion of physician specified fluids, including thrombolytics, into the peripheral vasculature and the pulmonary arteries. The objective of this study was to explore whether this catheter technology could sustain cavitation nucleated by infused Definity, to support subsequent studies of ultrasound-mediated drug delivery to diseased arteries. The concentration and attenuation spectroscopy of Definity were assayed before and after infusion at 0.3, 2.0 and 4.0 mL/min through the EkoSonic catheter. PCI was used to map and quantify stable and inertial cavitation as a function of Definity concentration in a flow phantom mimicking the porcine femoral artery. The 2.0 mL/min infusion rate yielded the highest surviving Definity concentration and acoustic attenuation. Cavitation was sustained throughout each 15 ms ultrasound pulse, as well as throughout the 3 min infusion. These results demonstrate a potential pathway to use cavitation nucleation to promote drug delivery with the EkoSonic endovascular system.

In-vitro assessment of the thrombolytic efficacy of therapeutic ultrasound

BACKGROUND

Ultrasound is mainly used as a diagnostic tool. Several studies demonstrated that therapeutic ultrasound (TUS) can enhance thrombolysis, but the optimal mechanical parameters to achieve this biological effect are still unknown.

METHODS

We assembled 46 blood clots in a closed in-vitro circulatory model. Clots were randomly divided into 7 groups, control group and six TUS groups of three frequencies (0.3, 0.5, 0.7 MHz) and six intensities (0.75, 1.5, 3, 237.7, 475, 950 W/cm²). Treatment was composed of 12 repetitions, 5 min US application and 3 min pause, lasting 93 min in total. Clots' weight and flow rate were measured before and after the treatment.

RESULTS

Mean initial clot weight (0.318 ± 0.129 g) and flow (0.53 ± 0.31 ml/min) were comparable among the experimental groups. We found a final clot weights reduction (0.15 ± 0.05, 0.16 ± 0.06, 0.09 ± 0.07, 0.21 ± 0.09, 0.17 ± 0.09, 0.17 ± 0.07 and 0.18 ± 0.02 g in groups 1 through 6, respectively) and a flow increase (30.61 ± 19.76, 52.1 ± 25.44, 28.78 ± 8.15, 43.93 ± 20.03, 40.86 ± 18.25 and 45.10 ± 22.20 ml/min in groups 1-6, respectively) in all TUS groups. Clot weight change (%) and flow increase reveals that the TUS profile f = 0.5 MHz I = 1.5 W/cm² was most efficacious. In the control group, clot weight change was +6.3% of baseline and flow increase of 4.4% of baseline, whereas -75.4% of baseline and 209.3% of baseline in the f = 0.5 MHz I = 1.5 W/cm² profile were noted, respectively.

CONCLUSIONS

Our study proved that TUS at low frequency (0.5 MHz) is most effective, whereas changing the intensity of TUS has only a minor effect on clot lysis magnitude.

Review of Protocols Used in Ultrasound Thrombolysis

OBJECTIVES

This paper focuses on the review of protocols used in thrombolysis studies with ultrasound.

MATERIALS AND METHODS

Data from peer-review articles were acquired.

RESULTS

The protocols of several published reports are summarized in 3 tables (in vitro, in vivo, and clinical), providing detailed information concerning clot model, thrombolytic drug, treatment mode, sonication parameters, evaluation method, thrombolysis outcome, side effects, and conclusions.

CONCLUSIONS

The aim of this review was to give an overview of the different protocols used so far in the field of sonothrombolysis and investigate the impact of several aspects involved on sonothrombolysis outcome.

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.

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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.

An In Vitro Assay for Sonothrombolysis Based on the Spectrophotometric Measurement of Clot Thickness

OBJECTIVES

For improved thrombolysis therapy based on ultrasound irradiation, researchers and practitioners would strongly benefit from an easy and efficient in vitro assay system of thrombolysis activity involving irradiated ultrasound. For the present study, we designed a new in vitro sonothrombolysis assay system using a sheet-type clot.

METHODS

We designed a cell for clot assay, and we confirmed that this clot cell did not significantly intervene in the acoustic field. Using human plasma, we made a sheet-type clot in the cell. Clot thicknesses at 100 points along 4 directions were measured photometrically at a rate of approximately 4 points/s.

RESULTS

The sonothrombolysis effects at 13 levels of ultrasonic intensity were obtained with only one sheet-type clot. With this method, we used a clinically oriented probe at 0.7 and 0.3 W/cm² to confirm that sonothrombolysis took place.

CONCLUSIONS

We successfully established a new, easy, and efficient method for conducting in vitro sonothrombolysis assays. This method involves little intervention of either ultrasound reflection or standing waves in the clot cell. We believe that this new assay method is very useful for fundamental analyses of ultrasound's thrombolysis effects.

In Vitro Evaluation of Focused Ultrasound-Enhanced TNK-Tissue Plasminogen Activator-Mediated Thrombolysis

INTRODUCTION

The low and incomplete recanalization performance of thrombolytic therapy in patients with acute ischemic stroke has created the need to use focused ultrasound (FUS) energy as a way to enhance thrombolysis efficiency (sonothrombolysis). Using an in vitro flow model, the role of various parameters involved in FUS-enhanced tenecteplase (TNK-tPA [tissue plasminogen activator])-mediated thrombolysis was evaluated.

MATERIALS AND METHODS

Fully retracted porcine blood clots were used for the proposed parametric studies. A spherically FUS transducer (4 cm diameter), focusing at 10 cm and operating at 1 MHz, was used. Pulsed ultrasound protocols were applied that maintained temperature elevation at the focus that never exceeded 1°C. Thrombolysis efficiency was measured as the relative reduction in the mass of the clot.

RESULTS

The role of various properties on thrombolysis efficacy was examined. These various properties are the acoustic power, the TNK-tPA concentration, the flow rate, the exposure time, the pulse length, the pulse repetition frequency, the duty factor, the formation of standing waves, the acoustic medium, and the administration of microbubbles. Study results have demonstrated that the parameters examined influenced thrombolysis efficacy and the degree of thrombolysis achieved by each parameter was measured.

CONCLUSIONS

Study findings helped us to optimize the treatment protocol for 1 MHz pulsed FUS that maximizes the thrombolytic efficacy of TNK-tPA, which potentially could be applied for therapeutic purposes. The outcome of the study showed poor thrombolysis efficacy, as with 30 minutes of FUS treatment only 370 mg of clot was removed.

Treatment of Critical Limb Ischemia Using Ultrasound-Enhanced Thrombolysis (PARES Trial): Final Results

Purpose:

To evaluate the safety and performance of ultrasound-enhanced thrombolysis in the treatment of acute thrombotic or embolic occlusion of the lower limb arteries.

Methods:

From April 2005 to July 2006, 25 patients (15 men; mean age 64.1 years, range 37–82) presenting with acute (<14 days old) occlusions of the lower limb arteries were treated with local thrombolysis [recombinant tissue plasminogen activator (rtPA)] in a dosage of 1.0 mg/h using the EKOS Lysus Peripheral Catheter System with an ultrasound core. No bolus injection of rtPA was given. The mean occlusion length was 25.1 cm (range 2–70).

Results:

The technical success rate was 100%. Total clot removal was achieved in 22 (88%) patients after 16.9 hours (range 5–24) using a mean 17 mg (range 5–25) of rtPA. In 8 cases, total clot removal of the main lesion was achieved after 6 hours (6 mg of rtPA). In 1 patient, lysis was stopped after 2.5 hours because of bleeding due a dislocation of the introducer sheath. In 2 cases, total clot removal could not be achieved; these patients were successfully treated with thromboaspiration. At the 1-month follow-up, the treated vessel was still patent in 20 patients. Two reocclusions occurred; 1 was treated with a bypass graft and the other with conservative therapy. There were no cases of amputation or death during follow-up. There were no side effects related to rtPA or the catheter system.

Conclusion:

This study demonstrates that local lysis of acute arterial occlusions using the Lysus Peripheral Catheter System is safe and effective. Blood flow is restored quickly.

EkoSonic Thrombolysis as a Therapeutic Adjunct in Venous Occlusive Disease

The use of ultrasound waves in conjunction with local thrombolysis may accelerate clot resolution and serve as an important therapeutic adjunct in the treatment of venous occlusive disease. Our goal was to provide a larger sample population over a 5-year period to evaluate our experience with the EkoSonic endovascular system (EKOS, EKOS Corporation, Bothell, WA). We suspected that ultrasound-accelerated thrombolysis (UAT) using EKOS would provide excellent thrombolysis and midterm patency rates with minimal thrombolytic complications. A retrospective study was conducted to provide a case series with UAT using EKOS. Data were collected over a 5-year period. Primary end points included degree of thrombolysis. Secondarily, we analyzed thrombolytic usage, complication rates, and midterm patency, over a 1-year period. A total of 48 limbs were treated with UAT. Forty cases were diagnosed as acute, whereas the remaining 8 were chronic. Complete thrombolysis was successful in 38/48 (79%) of patients, and partial thrombolysis was accomplished in 10/48 (21%) of patients. Overall mean infusion time was 22.4 hours ±3.6. There were a total of three complications (6%), all of which were minor bleeding. One-year patency was shown to be 87% with no signs of valvular reflux. UAT using EKOS demonstrated effective rates of thrombolysis with very few complications. In addition, our 1-year patency rates were comparable to published data using conventional catheter-directed thrombolysis. UAT provides lytic therapy by utilizing the benefits of ultrasonic waves to help augment the fibrinolytic process. Our institution currently favors the use of EKOS as the treatment of choice in patients that are appropriate thrombolytic candidates.

In vitro demonstration of focused ultrasound thrombolysis using bifrequency excitation

Focused ultrasound involving inertial cavitation has been shown to be an efficient method to induce thrombolysis without any pharmacological agent. However, further investigation of the mechanisms involved and further optimization of the process are still required. The present work aims at studying the relevance of a bifrequency excitation compared to a classical monofrequency excitation to achieve thrombolysis without any pharmacological agent. In vitro human blood clots were placed at the focus of a piezoelectric transducer. Efficiency of the thrombolysis was assessed by weighing each clot before and after sonication. The efficiencies of mono- (550 kHz) and bifrequency (535 and 565 kHz) excitations were compared for peak power ranging from 70 W to 220 W. The thrombolysis efficiency appears to be correlated to the inertial cavitation activity quantified by passive acoustic listening. In the conditions of the experiment, the power needed to achieve 80% of thrombolysis with a monofrequency excitation is reduced by the half with a bifrequency excitation. The thermal effects of bifrequency and monofrequency excitations, studied using MR thermometry measurements in turkey muscle samples where no cavitation occurred, did not show any difference between both types of excitations when using the same power level.

Is sonothrombolysis an effective stroke treatment?

New therapeutic strategies under development aim to improve recanalization rates and clinical outcomes after ischemic stroke. One such approach is ultrasound (US)-enhanced thrombolysis, or sonothrombolysis, which can improve thrombolytic drug actions and even intrinsic fibrinolysis. Although the mechanisms are not fully understood, it is postulated that thrombolysis enhancement is related to nonthermal mechanical effects of US. Recent results indicate that US with or without microbubbles may be effective in clot lysis of ischemic stroke even without additional thrombolytic drugs. Sonothrombolysis is a promising tool for treating acute ischemic stroke, but its efficacy, safety, and technical details have not been elucidated and proved yet in stroke treatment.

Sonothrombolysis in ischemic stroke

OPINION STATEMENT

Acute ischemic stroke remains one of the most devastating diseases when it comes to morbidity and mortality, not to mention the personal and economic burden that occurs in long-term. Intravenous thrombolysis with tissue plasminogen activator (tPA) is the only effective acute stroke therapy that improves outcome if given up to 4.5 hours from symptom onset. However, recanalization rates are meager and the majority of treated patients still have residual disability after stroke, emphasizing the need for further treatment options that may facilitate or even rival the only approved therapy. Sonothrombolysis, the adjuvant continuous ultrasound sonication of an intra-arterial occlusive thrombus during thrombolysis, enhances the clot-dissolving capabilities of intravenous tPA presumably by delivering acoustic pressure to the target brain vessel. Higher recanalization rates produce a trend towards better functional outcomes that could be safely achieved with the combination of high-frequency ultrasound and intravenous tPA. However, data on ultrasound targeting of intracranial proximal occlusive lesions other than those in the middle cerebral arteries are sparse. Moreover, recent sonothrombolysis trials were exclusively conducted with operator-dependent hand-held technology hindering its further testing in clinical sonothrombolysis trials. An operator-independent 2-MHz transcranial Doppler device has been developed allowing health care professionals not formally trained in ultrasound apparatus to provide therapeutic ultrasound as needed. Currently, this operator-independent device covering 12 proximal intracranial segments that most commonly contain thrombo-embolic occlusions enters testing in a pivotal multicenter sonothrombolysis efficacy trial. If this trial demonstrates safety and efficacy, adjuvants, such as gaseous microbubbles that further potentiate the thrombolytic effect of intravenous tPA, could be tested along with this device.

Combining radiation force with cavitation for enhanced sonothrombolysis

The use of acoustic radiation force has been suggested for enhancing the delivery of therapeutic substances, whereas sonothrombolysis has been developed for years as treatment by itself, or in combination with thrombolytic agents or ultrasound contrast agents. We have examined the efficacy of using acoustic radiation force to enhance the targeting of microbubbles in cavitation-induced sonothrombolysis in a flow phantom system. A clot was targeted by microbubbles using avidin-biotin binding, and the process was observed using a confocal microscope. We found that the experimental group in which radiation force was combined with cavitation showed an additional 3% to 9% weight reduction of the thrombus relative to the cavitation group. We also found that the fluorescence intensity of the clot increased with the microbubble concentration at each acoustic setting. Microbubbles traveled 10 to 20 μm further than the control group after being exposed to radiation force, cavitation, or both. These observations confirm that radiation force helps microbubbles to distribute into a clot (as does cavitation). Therefore, combining radiation force with cavitation would provide additional thrombolysis effects (based on clot weight measurements) relative to cavitation alone. A local delivery method based on acoustic radiation force has the potential to improve the safety and efficacy of sonothrombolysis.

Combined low-frequency ultrasound and streptokinase intravascular destruction of arterial thrombi in vivo

To prevent a distal embolization in the course of ultrasound (US) angioplasty, we combined US thrombus disruption in peripheral artery in vivo with simultaneous administration of streptokinase (SK). Acute thrombosis was induced in the femoral arteries of 23 dogs. Two hours after thrombus formation, thrombus destruction was performed using US (36 kHz) and by a combined US+SK (75,000 U/kg) administration. The results showed that thrombi were disrupted completely by 1.5 ± 0.5 min US. A combined US+SK action resulted in activation of fibrinolysis, as indicated by the increase in the content of fibrinogen and fibrin degradation products and D-dimers by a factor of 1.5-2.0 after 120 min from start of treatment compared with the SK lysis. The duration of clot destruction did not change; the distal embolization was not indicated; platelet aggregation activity dropped after thrombus destruction. In summary, intravascular thrombus destruction by a combined US and SK action in vivo is accompanied by enhancing the enzymatic fibrinolysis and lowering the platelet aggregation activity that assists in preventing the distal embolization of the resulting clot debris.

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.

Ultrasound-Enhanced Thrombolysis: EKOS EndoWave Infusion Catheter System

The purpose of the EKOS EndoWave Infusion Catheter System is to enhance catheter-directed thrombolysis by accelerating the fibrinolytic process with the application of ultrasound. Improving the efficiency of the thrombolytic process reduces the treatment time and total lytic dose delivered, thereby lowering the overall cost of therapy and the risk of an associated bleeding complication.

Noninvasive treatment of deep venous thrombosis using pulsed ultrasound cavitation therapy (histotripsy) in a porcine model

PURPOSE

This study evaluated histotripsy as a noninvasive, image-guided method of thrombolysis in a porcine model of deep vein thrombosis. Histotripsy therapy uses short, high-intensity, focused ultrasound pulses to cause mechanical breakdown of targeted soft tissue by acoustic cavitation, which is guided by real-time ultrasound imaging. This is an in vivo feasibility study of histotripsy thrombolysis.

METHODS AND MATERIALS

Acute thrombi were formed in the femoral vein of juvenile pigs weighing 30-40 kg by balloon occlusion with two catheters and thrombin infusion. A 10-cm-diameter 1-MHz focused transducer was used for therapy. An 8-MHz ultrasound imager was used to align the clot with the therapy focus. Therapy consisted of five cycle pulses delivered at a rate of 1 kHz and peak negative pressure between 14 and 19 MPa. The focus was scanned along the long axis of the vessel to treat the entire visible clot during ultrasound exposure. The targeted region identified by a hyperechoic cavitation bubble cloud was visualized via ultrasound during treatment.

RESULTS

Thrombus breakdown was apparent as a decrease in echogenicity within the vessel in 10 of 12 cases and in 7 cases improved flow through the vein as measured by color Doppler. Vessel histology found denudation of vascular endothelium and small pockets of hemorrhage in the vessel adventitia and underlying muscle and fatty tissue, but perforation of the vessel wall was never observed.

CONCLUSIONS

The results indicate histotripsy has potential for development as a noninvasive treatment for deep vein thrombosis.

Sonothrombolysis in the management of acute ischemic stroke

Multiple in vitro and animal models have demonstrated the efficacy of ultrasound to enhance fibrinolysis. Mechanical pressure waves produced by ultrasound energy improve the delivery and penetration of alteplase (recombinant tissue plasminogen activator [tPA]) inside the clot. In human stroke, the CLOTBUST phase II trial showed that the combination of alteplase plus 2 hours of continuous transcranial Doppler (TCD) increased recanalization rates, producing a trend toward better functional outcomes compared with alteplase alone. Other small clinical trials also showed an improvement in clot lysis when transcranial color-coded sonography was combined with alteplase. In contrast, low-frequency ultrasound increased the symptomatic intracranial hemorrhage rate in a clinical trial. Administration of microbubbles (MBs) may further enhance the effect of ultrasound on thrombolysis by lowering the ultrasound-energy threshold needed to induce acoustic cavitation. Initial clinical trials have been encouraging, and a multicenter international study, TUCSON, determined a dose of newly developed MBs that can be safely administered with alteplase and TCD. Even in the absence of alteplase, the ultrasound energy, with or without MBs, could increase intrinsic fibrinolysis. The intra-arterial administration of ultrasound with the EKOS NeuroWave catheter is another ultrasound application for acute stroke that is currently being studied in the IMS III trial. Operator-independent devices, different MB-related techniques, and other ultrasound parameters for improving and spreading sonothrombolysis are being tested.

Effects of ultrasound-induced inertial cavitation on enzymatic thrombolysis

Cavitation induced by ultrasound enhances enzymatic fibrinolysis by increasing the transport of reactants. However, the effects of cavitation need to be fully understood before sonothrombolysis can be applied clinically. In order to understand the underlying mechanisms, we examined the effects of combining ultrasound, microbubbles and thrombolytic enzymes on thrombolysis. First, we evaluated the relations between inertial cavitation and the reduction in the weight of a blood clot. Inertial cavitation was varied by changing the amplitude and duration of the transmitted acoustic wave as well as the concentration of microbubbles used to induce cavitation. Second, we studied the combined effects of streptokinase and inertial cavitation on thrombolysis. The results show that inertial cavitation increases the weight reduction of a blood clot by up to 33.9%. With linear regression fitting, the measured differential inertial cavitation dose and the weight reduction had a correlation coefficient of 0.66. Microscopically, enzymatic thrombolysis effects manifest as multiple large cavities within the clot that are uniformly distributed on the side exposed to ultrasound. This suggests that inertial cavitation plays an important role in producing cavities, while microjetting of the microbubbles induces pits on the clot surface. These observations preliminarily demonstrate the clinical potential of sonothrombolysis. The use of the differential inertial cavitation dose as an indicator of blood clot weight loss for controlled sonothrombolysis is also possible and will be further explored.

Sonothrombolysis for intraocular fibrin formation in an animal model

Vascular diseases such as diabetic retinopathy or retinal arterial occlusion are always associated with retinal and/or choroidal vasculopathy and intravascular thrombosis is commonly found. The ultrasound (US) therapy is a recently developed technique to accelerate fibrinolysis and it is being applied to some clinical fields. The present study was to observe the effects of extraocular US exposure on intraocular fibrin, which is a deteriorating factor in various ocular diseases. Tubes containing human blood (2 mL) in the following groups were irradiated with US; US alone, US with tissue plasminogen activator (tPA), tPA alone, and saline (control). Fibrinolysis was quantified by measuring D-dimer after 2h. In rat eyes, intracameral fibrin (fibrin formation in the anterior chamber of the eye) was induced by YAG-laser-induced iris bleeding. Then, eyes in the following groups were irradiated with US; US alone, subconjunctival tPA alone, US and subconjunctival tPA, control. Intracameral fibrin was scored on day 3 (3+ maximum to 0). The temperatures of rat eyes were measured by infrared thermography. Histologic evaluation was also performed. D-dimer was increased by US with statistical significance (p <0.05) or tPA (p <0.01). D-dimer in US with tPA group was significantly higher than either US alone or tPA alone group (p <0.01). In rat eyes, the average intracameral fibrin score on day 3 was 1.4 in control group and 1.2 in subconjunctival tPA alone group; however, it decreased significantly in the US alone group (0.75; p <0.05, vs. control), US and subconjunctival tPA group (0.71; p <0.01, vs. control). The temperature was less than 34 degrees C after US exposure. No histologic damage was observed. US irradiation from outside accelerated intracameral fibrinolysis without causing apparent tissue damage. This noninvasive method might have therapeutic value for intraocular fibrin.

Blood clot disruption in vitro using shockwaves delivered by an extracorporeal generator after pre-exposure to lytic agent

The standard methods for recanalyzing thrombosed vessels are vascular stenting or administration of thrombolytic drugs. However, these methods suffer from uncertain success rate and side-effects. Therefore, minimally-invasive ultrasound methods have been investigated. In this article, we propose to use shockwaves after pre-exposure to fibrinolytic agent for disrupting thrombus. Shockwaves were delivered by an extracorporeal piezocomposite generator (120 mm in diameter, focused at 97 mm, pulse length = 1.4 micros). In vitro blood clots, made from human blood, were placed at the focal point of the generator. The clots were exposed to shockwaves either with or without prior immersion in a solution of streptokinase. The percentage of lysed clot was determined by weighing the clot before and after treatment. The proportion of lysed clot increased with the pressure at the focus and with the number of shocks. A mean clot reduction of 91% was obtained for 42 MPa in 4-min treatment duration only, without using streptokinase. For a treatment of 2 min at 29 MPa, the clot reduction increased significantly (p < 0.01) from 47% without streptokinase to 82% when streptokinase was used prior to shockwaves. These results also showed no significant damage to streptokinase due to exposure to shockwaves. This study suggests that extracorporeal shockwaves combined with streptokinase is a promising pharmaco-mechanical method for treating occlusive thrombus, and should be confirmed by in vivo trials. Additional studies must also be conducted with other fibrinolytic agents, whose abilities to penetrate clots are different.

Pulsed-high intensity focused ultrasound enhanced tPA mediated thrombolysis in a novel in vivo clot model, a pilot study

INTRODUCTION

Thrombotic disease continues to account for significant morbidity and mortality. Ultrasound energy has been investigated as a potential primary and adjunctive treatment for thrombotic disease. We have previously shown that pulsed-high intensity focused ultrasound (HIFU) enhances thrombolysis induced by tissue plasminogen activator (tPA) in vitro, including describing the non-destructive mechanism by which tPA availability and consequent activity are increased. In this study we aimed to determine if the same effects could be achieved in vivo.

MATERIALS AND METHODS

In this study, pulsed-HIFU exposures combined with tPA boluses were compared to treatment with tPA alone, HIFU alone and control in a novel in vivo clot model. Clots were formed in the rabbit marginal ear vein and verified using venography and infrared imaging. The efficacy of thrombolytic treatment was monitored via high resolution ultrasonography for 5 h post-treatment. The cross-sectional area of clots at 4 points along the vein was measured and normalized to the pre-treatment size.

RESULTS

At 5 h the complete recanalization of clots treated with pulsed-HIFU and tPA was significantly different from the partial recanalization seen with tPA treatment alone. tPA treatment alone showed a significant decrease in clot versus control, where HIFU was not significantly different than control. Histological analysis of the vessel walls in the treated veins showed no apparent irreversible damage to endothelial cells or extravascular tissue.

CONCLUSIONS

This study demonstrates that tPA mediated thrombolysis can be significantly enhanced when combined with non-invasive pulsed-HIFU exposures.

Ultrasound-facilitated thrombolysis using tissue-plasminogen activator-loaded echogenic liposomes

INTRODUCTION

Targeted delivery of thrombolytics to the site of occlusion is an attractive concept, with implications for the treatment of many thrombo-occlusive diseases. Ultrasound enhances thrombolysis, which can be augmented by the addition of a contrast agent. We have previously reported development of echogenic liposomes (ELIP) for targeted highlighting of structures with potential for drug and gene delivery. This study evaluated the potential of ELIP for thrombolytic loading, and the effect of ultrasound exposure of thrombolytic-loaded ELIP on thrombolytic efficacy.

MATERIALS AND METHODS

Tissue-plasminogen activator (tPA) was loaded into ELIP. Echogenicity was assessed and reported as mean grayscale values. Whole porcine clots were treated with plasma, free tPA, tPA+Optison (echocontrast agent), or tPA-loaded ELIP, with and without ultrasound (1 MHz, continuous wave, 2 W/cm(2), for 2 min). Clots were weighed before and after a 30-min treatment period, and results reported as percent clot mass loss.

RESULTS

tPA entrapment into ELIP was feasible with 50% entrapment, and retention of echogenicity. Treatment with tPA-loaded ELIP resulted in effective clot lysis with an effect similar to treatment with free tPA. Ultrasound exposure of tPA-loaded ELIP resulted in enhanced thrombolysis (49.5% relative improvement vs. no ultrasound). Much of the ultrasound effect appeared to be related to drug release from the tPA-ELIP complex.

CONCLUSIONS

We have demonstrated entrapment of tPA into ELIP with effective clot lysis and drug release using ultrasound. Our tPA-loaded ELIP has potential for specific highlighting of clots to confirm agent delivery and help focus ultrasound therapy for targeted ultrasound-facilitated thrombolysis.

Encapsulated ultrasound microbubbles: Therapeutic application in drug/gene delivery

Encapsulated gas microbubbles are well known as ultrasound contrast agents for medical ultrasound imaging. Nonetheless, not only do these microbubbles help to image, but they can also be used as drug/gene carriers. The microbubbles as drug/gene carriers have an average size less than that of red blood cells, i.e. they are capable of penetrating even into the small blood capillaries and releasing drug and genes under the action of ultrasound field. The application of ultrasound and microbubbles to targeted drug and gene delivery has been the subject of intense experimental research. Under exposure of sufficiently high-amplitude ultrasound, these targeted microbubbles would rupture, spewing drugs or genes, which are contained in its encapsulating layer, to targeted cells or tissues. Recently, targeting ligands are attached to the surface of the microbubbles (i.e. targeted-microbubbles), which have been widely used in cardiovascular system and tumor diagnosis and therapy. In this paper, the characterization of novel targeted ultrasonic contrast agents or microbubbles and their potential applications in drug delivery or gene therapy are reviewed.

Pulsed high-intensity focused ultrasound enhances thrombolysis in an in vitro model

PURPOSE

To evaluate the use of pulsed high-intensity focused ultrasound exposures to improve tissue plasminogen activator (tPA)-mediated thrombolysis in an in vitro model.

MATERIALS AND METHODS

All experimental work was compliant with institutional guidelines and HIPAA. Clots were formed by placing 1 mL of human blood in closed-off sections of pediatric Penrose tubes. Four experimental groups were evaluated: control (nontreated) clots, clots treated with pulsed high-intensity focused ultrasound only, clots treated with tPA only, and clots treated with pulsed high-intensity focused ultrasound plus tPA. The focused ultrasound exposures (real or sham) were followed by incubations of the clots in tPA with saline or in saline only. Thrombolysis was measured as the relative reduction in the mass of the clot. D-Dimer assays also were performed. Two additional experiments were performed and yielded dose-response curves for two exposure parameters: number of pulses per raster point and total acoustic power. Radiation force-induced displacements caused by focused ultrasound exposures were simulated in the clots. A Tukey-Kramer honestly significant difference test was performed for comparisons between all pairs of experimental groups.

RESULTS

The clots treated with focused ultrasound alone did not show significant increases in thrombolysis compared with the control clots. The clots treated with focused ultrasound plus tPA showed a 50% ([30.2/20.1]/20.1) increase in the degree of thrombolysis compared with the clots treated with tPA only (P < .001), further corroborating the d-dimer assay results (P < .001). Additional experiments revealed how increasing both the number of pulses per raster point and the total acoustic power yielded corresponding increases in the thrombolysis rate. In the latter experiment, simulations performed at a range of power settings revealed a direct correlation between increased displacement and observed thrombolysis rate.

CONCLUSION

The rate of tPA-mediated thrombolysis can be enhanced by using pulsed high-intensity focused ultrasound exposure in vitro.

Effects of low-intensity focused ultrasound on the mouse submandibular gland

Ultrasound is expected to make a considerable contribution to drug delivery systems (DDSs). We tested the hypothesis that low-intensity focused ultrasound (LIFU) increases vessel permeability in the mouse submandibular gland without causing parenchymal damage. In a preliminary study, LIFU at 3 W/cm2 with a 50% duty cycle for 2 minutes did not cause histologic damage. We therefore applied LIFU to mouse submandibular gland at these conditions before and after injecting horseradish peroxidase. Single labeling laser scanning confocal microscopy revealed positive horseradish peroxidase staining around the excretory ducts in the mucous-producing part of the gland, but absence of staining in control glands. Immunostaining for fibrinogen was positive in the same region. Fibrinogen is an intravascular protein that does not pass through intact vessels. These findings suggest that LIFU increases vessel permeability and disruption without destruction. It is anticipated that this process will be useful in establishing a DDS that uses LIFU.

Vascular effects induced by combined 1-MHz ultrasound and microbubble contrast agent treatments in vivo

Previous in vivo studies have demonstrated that microvessel hemorrhages and alterations of endothelial permeability can be produced in tissues containing microbubble-based ultrasound contrast agents when those tissues are exposed to MHz-frequency pulsed ultrasound of sufficient pressure amplitudes. The general hypothesis guiding this research was that acoustic (viz., inertial) cavitation, rather than thermal insult, is the dominant mechanism by which such effects arise. We report the results of testing five specific hypotheses in an in vivo rabbit auricular blood vessel model: (1) acoustic cavitation nucleated by microbubble contrast agent can damage the endothelia of veins at relatively low spatial-peak temporal-average intensities, (2) such damage will be proportional to the peak negative pressure amplitude of the insonifying pulses, (3) damage will be confined largely to the intimal surface, with sparing of perivascular tissues, (4) greater damage will occur to the endothelial cells on the side of the vessel distal to the source transducer than on the proximal side and (5) ultrasound/contrast agent-induced endothelial damage can be inherently thrombogenic, or can aid sclerotherapeutic thrombogenesis through the application of otherwise subtherapeutic doses of thrombogenic drugs. Auricular vessels were exposed to 1-MHz focused ultrasound of variable peak pressure amplitude using low duty factor, fixed pulse parameters, with or without infusion of a shelled microbubble contrast agent. Extravasation of Evans blue dye and erythrocytes was assessed at the macroscopic level. Endothelial damage was assessed via scanning electron microscopy (SEM) image analysis. The hypotheses were supported by the data. We discuss potential therapeutic applications of vessel occlusion, e.g., occlusion of at-risk gastric varices.

Perspectives on the role of ultrasonic devices in thrombolysis

Reperfusion strategies in acute myocardial infarction and thrombotic vascular occlusion are focused on rapid and complete restoration of antegrade flow in the infarct-related artery in order to maximize myocardial salvage. Due to the limitations of fibrinolytic agents in restoration of vascular flow, ultrasonic clot dissolution alone and concomitantly with fibrinolytic, anti-thrombotic and echocardiographic contrast agents has been intensively studied during the last 2 decades. Ultrasound thrombolysis has been tested in-vitro and in-vivo as well as in patients with acute thrombotic occlusions. We review currently available techniques and methods of ultrasonic thrombolysis and present recent clinical and experimental data. The future role of ultrasonic thrombolysis and the strategy of "power thrombectomy" for treatment of acute coronary syndromes is also discussed.

Emerging technologies in therapeutic ultrasound: thermal ablation to gene delivery

Ultrasound is used today in medicine as a modality for diagnostic imaging. Recently, there have been numerous reports on the application of thermal and nonthermal ultrasound energy for treating various diseases. In addition to thermal ablation of tumors, non-thermal ultrasound combined with drugs and genes have led to much excitement especially for cancer treatment, vascular diseases, and regenerative medicine. Ultrasound energy can enhance the effects of thrombolytic agents such as urokinase for treatment of stroke and acute myocardial infarction. New ultrasound technologies have resulted in advanced devices such as a) ultrasound catheters, b) Non-invasive methods as high intensity focused ultrasound (HIFU) in conjunction with MRI and CT is already being applied in the clinical field, c) Chemical activation of drugs by ultrasound energy for treatment of tumors is another new field recently termed "Sonodynamic Therapy", and d) Combination of genes and microbubble have induced great hopes for ideal gene therapy (sonoporation). Various examples of ultrasound combined modalities are under investigation which could lead to revolutionary therapy.

Mechanical Bioeffects of Ultrasound

Ultrasound is used widely in medicine as both a diagnostic and therapeutic tool. Through both thermal and nonthermal mechanisms, ultrasound can produce a variety of biological effects in tissues in vitro and in vivo. This chapter provides an overview of the fundamentals of key nonthermal mechanisms for the interaction of ultrasound with biological tissues. Several categories of mechanical bioeffects of ultrasound are then reviewed to provide insight on the range of ultrasound bioeffects in vivo, the relevance of these effects to diagnostic imaging, and the potential application of mechanical bioeffects to the design of new therapeutic applications of ultrasound in medicine.

Low-intensity ultrasound increases endothelial cell nitric oxide synthase activity and nitric oxide synthesis

Low-intensity ultrasound (US) increases tissue perfusion in ischemic muscle through a nitric oxide (NO)-dependent mechanism. We have developed a model to expose endothelial cells to well-characterized acoustic fields in vitro and investigate the physical and biological mechanisms involved. Human umbilical vein endothelial cells (HUVEC) or bovine aortic endothelial cells (BAEC) were grown in tissue culture plates suspended in a temperature-controlled water bath and exposed to US. Exposure to 27 kHz continuous wave US at 0.25 W cm(-2) for 10 min increased HUVEC media NO by 102 +/- 19% (P < 0.05) and BAEC by 117 +/- 23% (P < 0.01). Endothelial cell NO synthase activity increased by 27 +/- 24% in HUVEC and by 32 +/- 16% in BAEC (P < 0.05 for each). The cell response was rapid with a significant increase in NO synthesis by 10 s and a maximum increase after exposure for 1 min. By 30 min post-exposure NO synthesis declined to baseline, indicating that the response was transient. Unexpectedly, pulsing at a 10% duty cycle resulted in a 46% increase in NO synthesis over the response seen with continuous wave US, resulting in an increase of 147 +/- 18%. Cells responded to very low intensity US, with a significant increase at 0.075 W cm(-2) (P < 0.01) and a maximum response at 0.125 W cm(-2). US caused minor reversible changes in cell morphology but did not alter proliferative capacity, indicating absence of injury. We conclude that exposure of endothelial cells to low-intensity, low-frequency US increases NO synthase activity and NO production, which could be used to induce vasodilatation experimentally or therapeutically.

Ultrasound in the treatment of ischaemic stroke

Intravenous alteplase (recombinant tissue plasminogen activator) has been shown to be beneficial within a short 3 h window after stroke. Ultrasound has a thrombolytic capacity that can be used for pure mechanical thrombolysis or improvement of enzyme-mediated thrombolysis. Mechanical thrombolysis with ultrasound needs high intensities at the clot (>2 W/cm2) that may have unwanted side-effects, whereas improvement of enzymatic thrombolysis can be done at the safer energy levels used in diagnostic ultrasound. Methods of improving enzymatic thrombolysis with ultrasound include intra-arterial delivery of thrombolytic agents with an ultrasound-emitting catheter and targeted and non-targeted non-invasive transcranial ultra sound delivery during intravenous thrombolytic infusion. Animal and clinical studies of sonothrombolysis have shown clot lysis and accelerated recanalisation of arterial occlusion has been seen in in vitro flow models, occluded peripheral and coronary arteries, and intracerebral arteries. Controlled clinical trials to test safety management and effectiveness of both strategies are in progress.

Adjunctive techniques in percutaneous mechanical thrombectomy

Percutaneous mechanical thrombectomy is an established method in interventional radiology and refers to the removal of acute embolic or thrombotic occlusive material in arteries, veins, or vascular grafts using percutaneous transluminal methods. However, initial complete removal of occlusive material can be achieved only in a minority of patients. The amount of removed material varies with the age and composition of the occlusive material. To achieve sufficient revascularization, adjunctive use of a variety of percutaneous endovascular recanalization techniques is necessitated. Additional treatment with local intra-arterial fibrinolysis, balloon angioplasty, stent implantation, endoluminal atherectomy, and other measures results in primary technical success rates of 70% to 100% for revascularization of acutely occluded vessels. The above-mentioned different techniques should not be viewed as competitive treatment modalities, rather a synergistic approach should be offered. The aim of this report is to review different adjunctive techniques in percutaneous mechanical thrombectomy with emphasis on techniques, mechanisms of action, experimental and clinical results, potential complications, and their potential role in view of clinical pathways to treat acute limb ischemia.

Therapeutic ultrasound in ischemic stroke treatment: experimental evidence

Re-opening of the occluded artery is the primary therapeutic goal in hyper-acute ischemic stroke. Systemic treatment with IV rt-PA has been shown to be beneficial at least in a 3 h 'door to needle' window and is approved within that interval in many countries. Trials of thrombolytic therapy with rt-PA demonstrated a small, but significant improvement in neurological outcome in selected patients. As recently shown, intra-arterial application of rt-PA is effective and opens the therapeutical window to 6 h, but requires invasive intra-arterial angiographic intervention in a high number of patients, who do not finally achieve thrombolysis. Ultrasound (US) is known to have several biological effects depending on the emission characteristics. At higher energy levels US alone has a thrombolytic effect. That effect is already used for clinical purposes in interventional therapy using US catheters. Recently, there is growing evidence that US at lower energy levels (<2 W/cm(2)) facilitates enzymatic mediated thrombolysis, most probably by breaking molecular linkages of fibrin polymers and therefore, increasing the working surface for the thrombolytic drug. Different in-vitro and in-vivo experiments have shown increased clot lysis as well as accelerated recanalization of occluded peripheral, coronary vessels and most recently also intracerebral arteries. Sonothrombolysis at low energy levels, however, is of great interest because of the low risk for collateral tissue damage, enabling external insonation without the need for local catheterization. Whereas little or no attenuation of US can be expected through skin and chest, intensity will be significantly attenuated if penetration of bones, particularly the skull, is required. That effect, however, is frequency dependent. Whereas >90% of intracerebral US intensity is lost (of the output power) in frequencies currently used for diagnostic purposes (mostly 2 MHz and up), that ratio is nearly reversed in the lower KHz range (<300 kHz). US at these low frequencies, however, is efficient for accelerating enzymatic thrombolysis in-vitro as well as in vivo within a wide range of intensities, from 0.5 W/cm(2) (MI approximately 0.3) to several W/cm(2). Since the emitted US beam widens with decreasing frequency, low-frequency US can insonate the entire intracerebral vasculature. That may overcome the limitation of US in the MHz range being restricted to insonation of the MCA mainstem. There are no reports in the preclinical literature about intracerebral bleeding or relevant cerebral cellular damage (either signs of necrosis or apoptosis) for US energy levels up to 1 W/cm(2). Moreover, recent investigations showed no break-down of the blood brain barrier. Safety of US exposure of the brain for therapeutic purposes has to address heating. Heating depends critically on the characteristics of the US. The most significant heating of the brain tissue itself is >1 degrees C/h using a continuous wave (CW) 2 W/cm(2) probe, whereas no significant heating could be found when using an intermittent (pulsed) emission protocol. The experimental data so far help to characterize the optimal US settings for sonothrombolysis and support the hypothesis that this combined treatment is a prospective advance in optimizing thrombolytic therapy in acute stroke.

Ultrasound and thrombolysis

Thrombolytic therapy and mechanical interventions are frequently used in the treatment of both arterial and venous thrombotic disease. Limitations to these approaches include failure to achieve reperfusion and complications including bleeding and vessel wall damage. Increasing evidence indicates that the use of ultrasound offers potential therapeutic advantages. This review considers two distinct approaches which include the use of high intensity ultrasound to mechanically fragment clots and also the use of low intensity ultrasound to augment enzymatic fibrinolysis. High intensity ultrasound can be delivered via catheter or transcutaneously to disrupt clots in vitro or in animal models into small fragments. Initial clinical studies demonstrate potential clinical value in peripheral and coronary arterial thrombosis and occluded saphenous vein bypass grafts treated with the catheter approach. Studies in vitro indicate that low intensity ultrasound accelerates enzymatic thrombolysis through non-thermal mechanisms involving improvement in drug transport. The effect is larger at low frequencies, which also offer better tissue penetration and less heating. The ability to accelerate thrombolysis has been confirmed in animal models demonstrating markedly increased reperfusion and minimal toxicity. The use of ultrasound to mechanically disrupt occlusive thrombi or to accelerate enzymatic thrombolysis offers a new approach to treating occlusive thrombotic disease.

Therapeutic ultrasound: its application in drug delivery

Ultrasound is best known for its imaging capability in diagnostic medicine. However, there have been considerable efforts recently to develop therapeutic uses for it. The purpose of this review is to summarize some of the recent advances made in the area of therapeutic ultrasound as they relate to drug delivery. In particular, this review will focus on the applications of ultrasound to enhance the delivery and effect of three distinctive therapeutic drug classes: chemotherapeutic, thrombolytic, and gene-based drugs. In addition, ultrasound contrast agents have been recently developed for diagnostic ultrasound. New experimental evidence suggests that these contrast agents can be used as exogenous cavitation nuclei for enhancement of drug and gene delivery. Thus, brief review of this new class of agents and their roles in drug delivery will also be provided. By comparison to diagnostic ultrasound, progress in therapeutic use of ultrasound has been somewhat limited. The recent successes in ultrasound-related drug delivery research positions ultrasound as therapeutic tool for drug delivery in the future.

Engineering design of optimal strategies for blood clot dissolution

Blood clots form under hemodynamic conditions and can obstruct flow during angina, acute myocardial infarction, stroke, deep vein thrombosis, pulmonary embolism, peripheral thrombosis, or dialysis access graft thrombosis. Therapies to remove these clots through enzymatic and/or mechanical approaches require consideration of the biochemistry and structure of blood clots in conjunction with local transport phenomena. Because blood clots are porous objects exposed to local hemodynamic forces, pressure-driven interstitial permeation often controls drug penetration and the overall lysis rate of an occlusive thrombus. Reaction engineering and transport phenomena provide a framework to relate dosage of a given agent to potential outcomes. The design and testing of thrombolytic agents and the design of therapies must account for (a) the binding, catalytic, and systemic clearance properties of the therapeutic enzyme; (b) the dose and delivery regimen; (c) the biochemical and structural aspects of the thrombotic occlusion; (d) the prevailing hemodynamics and anatomical location of the thrombus; and (e) therapeutic constraints and risks of side effects. These principles also impact the design and analysis of local delivery devices.

Ultrasound thrombolysis in hemodialysis access: in vitro investigation

PURPOSE

To evaluate the effectiveness of ultrasound thrombolysis in occluded hemodialysis access shunts using an in vitro model.

METHODS

Thrombosed hemodialysis accesses were simulated by clotted bovine blood in a flow model (silicone tubing; inner diameters 4, 6, and 9 mm). After retrograde and antegrade sheath placement (7 Fr), mechanical thrombolysis was performed using an ultrasound probe (Acolysis, Angiosonics, Morrisville, NC, USA). The tip of the device measured 2.2 mm in diameter. During sonication, the catheter was moved slowly back and forth using an over-the-wire system. Thirty complete occlusions [tubing diameters 4 mm (n = 12), 6 mm (n = 12), 9 mm (n = 6)] were treated. Initial thrombus weights were 3.5 (+/- 0.76) g, 7.7 (+/- 1.74) g, and 19.4 (+/- 2.27) g for the three diameters. Maximum sonication time was 15 min for each probe.

RESULTS

With this device, we were able to restore a continuous lumen in all 12 occluded 4 approximately mm silicone tubes. No wall-adherent thrombi remained after sonication for 3.5--9.6 min. In hemodialysis access models with diameters of 6 mm, thrombus fragments persisted in 25% (3/12 accesses). These were located in the medial portion of the access loop and near to the puncture sites. However, flow was re-established after 5.0--13.0 min of treatment in all settings. Mechanical dissolution of thrombus material failed in five of six access models with diameters of 9 mm, even though ultrasound energy was applied for the maximum of 15 min.

CONCLUSION

In a clotted hemodialysis shunt model, successful ultrasound thrombolysis was limited to small access diameters and small amounts of thrombus.

Ultrasound-enhanced thrombolysis at 20 kHz with air-filled and perfluorocarbon-filled contrast bispheres

BACKGROUND

Ultrasound (US) at low frequencies has been shown to enhance clot lysis by itself and in the presence of urokinase (UK). The comparative effects of air-filled versus perfluorocarbon-filled polymer bispheres in enhancing this effect have not been previously demonstrated.

METHODS

Freshly drawn human blood was incubated at 37 degrees C for 2 hours, and the subsequent formed clot was dried and weighed. It was then exposed to saline control, saline + UK (10,000 IU), saline + UK + US, saline + UK + US + low shell-strength polymer bispheres (PB1), saline + UK + US + high shell-strength polymer bispheres (PB2), and perfluorocarbon-filled high shell-strength polymer bipsheres (PB3) for a total of 6 minutes. Clots were removed and weighed to determine the percentage of thrombolysis.

RESULTS

The percentage of clot lysis for each study group was as follows: saline 18.5% +/- 4%, US alone 22.2% +/- 5%, UK alone 21.9% +/- 4%, US+UK 32.2% +/- 8% (P <.05 compared with UK alone), US+UK+PB1 36.9% +/- 8%, US+UK+PB2 34.3% +/- 8%, and US+UK+PB3 45.0% +/- 11% (P <.05 compared with US+UK, P <.05 compared with US+UK+PB2).

CONCLUSION

Ultrasound at 20 kHz significantly enhances clot lysis. The addition of perfluorocarbon-filled bispheres increased this effect more significantly than did the addition of air-filled polymer bispheres.

Cavitational mechanisms in ultrasound-accelerated thrombolysis at 1 MHz

Inertial cavitation is hypothesized to be a mechanism by which ultrasound (US) accelerates the dissolution of human blood clots when the clot is exposed to a thrombolytic agent such as tissue plasminogen activator (t-PA). To test this hypothesis, radiolabeled fibrin clots were exposed or sham-exposed in vitro to 1 MHz c.w. US in a rotating sample holder immersed in a water-filled tank at 37 degrees C. Percent clot dissolution after 60 min of US exposure was assessed by removing the samples, centrifuging, and measuring the radioactivity of the supernatant fluid relative to the pelletized material. To suppress acoustic cavitation, the exposure tank was contained within a hyperbaric chamber capable of pneumatic pressurization to 10 atmospheres (gauge). Various combinations of static pressure (0, 2, 5, and 7.5 atm gauge), US (0 or 4 W/cm(2) SATA), and t-PA (0 or 10 microg/mL) were employed, showing statistically significant reductions in thrombolytic activity as static pressure increased. To gain further insight, an active cavitation detection scheme was employed in which 1-micros duration tonebursts of 20-MHz US (< 1 kPa peak negative pressure, 1 Hz PRF) were used to interrogate clots subjected to US and static pressure. Results of this cavitation detection scheme showed that scattering from within the clot and broadband acoustic emissions that were both present during insonification were significantly reduced with application of static pressure. However, only about half of the acceleration of thrombolysis due to US could be removed by static pressure, suggesting the possibility of other mechanisms in addition to inertial cavitation.

Ultrasound imaging-guided noninvasive ultrasound thrombolysis: preclinical results

BACKGROUND

Catheter-based therapeutic ultrasound thrombolysis was recently shown to be effective and safe. The purpose of this work was to study the safety and efficacy of external high-intensity focused ultrasound thrombolysis guided by ultrasound imaging in experimental settings.

METHODS AND RESULTS

A therapeutic transducer was constructed from an acoustic lens and integrated with an ultrasound imaging transducer. In vitro clots were inserted into bovine arterial segments and sonicated under real-time ultrasound imaging guidance in a water tank. With pulsed-wave (PW) ultrasound, the total sonication time correlated with thrombolysis efficiency (r(2)=0.7666). A thrombolysis efficiency of 91% was achieved with optimal PW parameters (1:25 duty cycle, 200-micros pulse length) at an intensity (I(spta)) of >35+/-5 W/cm(2). Ultrasound imaging during sonication showed the cavitation field as a spherical cloud of echo-dense material. Within <2 minutes, the vessel lumen evidenced neither residual clot nor damage to the arterial wall. On serial filtration, 93+/-1% of the lysed clot became subcapillary in size (<8 microm). In vitro safety studies, however, showed arterial damage when an I(spta) of 45 W/cm(2) was used for periods of >/=300 seconds.

CONCLUSIONS

External high-intensity focused ultrasound thrombolysis using optimal PW parameters for periods of </=300 seconds appears to be a safe and effective method to induce thrombolysis. The procedure can be guided by ultrasound imaging, thereby allowing the monitoring of therapy.

Effect of 40-kHz ultrasound on acute thrombotic ischemia in a rabbit femoral artery thrombosis model: enhancement of thrombolysis and improvement in capillary muscle perfusion

BACKGROUND

We have shown previously that 40-kHz ultrasound (US) at low intensity accelerates fibrinolysis in vitro with little heating and good tissue penetration. These studies have now been extended to examine the effects of 40-kHz US on thrombolysis and tissue perfusion in a rabbit model.

METHODS AND RESULTS

Treatment was administered with either US alone at 0.75 W/cm(2), streptokinase alone, or the combination of US and streptokinase. US or streptokinase resulted in minimal thrombolysis, but reperfusion was nearly complete with the combination after 120 minutes. US also reversed the ischemia in nonperfused muscle in the absence of arterial flow. Tissue perfusion decreased after thrombosis from 13. 7+/-0.2 to 6.6+/-0.8 U and then declined further to 4.5+/-0.4 U after 240 minutes. US improved perfusion to 10.6+/-0.5 and 12.1+/-0. 5 U after 30 and 60 minutes, respectively. This effect was reversible and declined to pretreatment values after US was discontinued. Similarly, tissue pH declined from normal to 7.05+/-0. 02 after thrombosis, but US improved pH to 7.34+/-0.03 after 60 minutes. US-induced improvement in tissue perfusion and pH also occurred after femoral artery ligation, indicating that thrombolysis did not cause these effects.

CONCLUSIONS

40-kHz US at low intensity markedly accelerates fibrinolysis and also improves tissue perfusion and reverses acidosis, effects that would be beneficial in treatment of acute thrombosis.