Fibrin degradation during sonothrombolysis – Effect of ultrasound, microbubbles and tissue plasminogen activator

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.

Surface Wettability of Nanoparticle Modulated Sonothrombolysis

Sonodynamic therapy (SDT) is a non-invasive and highly penetrating treatment strategy under ultrasound irradiation. However, uncertainty in the mechanism of SDT has seriously hindered its future clinical application. Here, the mechanism of SDT enhanced by the wettability of nanoparticles is investigated. Nanoparticles can adsorb and stabilize nanobubbles in liquid, thus enhancing SDT efficiency. The stability of the nanobubbles is positively correlated with the desorption energy of the nanoparticles, which is determined by the wettability of the nanoparticles. This conclusion is verified for mesoporous silica and polystyrene nanoparticles and it is found that nanoparticles with a water contact angle of about 90° possess the largest desorption energy. To further apply this conclusion, thrombus models are constructed on rats and the experimental results demonstrate that nanoparticles with the largest desorption energy have the highest thrombolytic efficiency. It is believed that these findings will help to better understand the SDT mechanism and guide new strategies for rational design of nanoparticles adopted in SDT.

Functional micro/nanobubbles for ultrasound medicine and visualizable guidance

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

Thrombolytic Agents: Nanocarriers in Controlled Release

Thrombosis is a life-threatening pathological condition in which blood clots form in blood vessels, obstructing or interfering with blood flow. Thrombolytic agents (TAs) are enzymes that can catalyze the conversion of plasminogen to plasmin to dissolve blood clots. The plasmin formed by TAs breaks down fibrin clots into soluble fibrin that finally dissolves thrombi. Several TAs have been developed to treat various thromboembolic diseases, such as pulmonary embolisms, acute myocardial infarction, deep vein thrombosis, and extensive coronary emboli. However, systemic TA administration can trigger non-specific activation that can increase the incidence of bleeding. Moreover, protein-based TAs are rapidly inactivated upon injection resulting in the need for large doses. To overcome these limitations, various types of nanocarriers have been introduced that enhance the pharmacokinetic effects by protecting the TA from the biological environment and targeting the release into coagulation. The nanocarriers show increasing half-life, reducing side effects, and improving overall TA efficacy. In this work, the recent advances in various types of TAs and nanocarriers are thoroughly reviewed. Various types of nanocarriers, including lipid-based, polymer-based, and metal-based nanoparticles are described, for the targeted delivery of TAs. This work also provides insights into issues related to the future of TA development and successful clinical translation.

In Vitro Thrombolytic Efficacy of Single- and Five-Cycle Histotripsy Pulses and rt-PA

Although primarily known as an ablative modality, histotripsy can increase the efficacy of lytic therapy in a retracted venous clot model. Bubble cloud oscillations are the primary mechanism of action for histotripsy, and the type of bubble activity is dependent on the pulse duration. A retracted human venous clot model was perfused with and without the thrombolytic recombinant tissue plasminogen activator (rt-PA). The clot was exposed to histotripsy pulses of single- or five-cycle duration and peak negative pressures of 0-30 MPa. Bubble activity within the clot was monitored via passive cavitation imaging. The combination of histotripsy and rt-PA was more efficacious than rt-PA alone for single- and five-cycle pulses with peak negative pressures of 25 and 20 MPa, respectively. For both excitation schemes, the detected acoustic emissions correlated with the degree of thrombolytic efficacy. These results indicate that rt-PA and single- or multicycle histotripsy pulses enhance thrombolytic therapy.

Low-Intensity Focused Ultrasound-Responsive Phase-Transitional Nanoparticles for Thrombolysis without Vascular Damage: A Synergistic Nonpharmaceutical Strategy

Multimodal molecular imaging has shown promise as a complementary approach to thrombus detection. However, the simultaneous noninvasive detection and lysis of thrombi for cardiovascular diseases remain challenging. Herein, a perfluorohexane (PFH)-based biocompatible nanostructure was fabricated, namely, as-prepared Fe₃O₄-poly(lactic- co-glycolic acid)-PFH-CREKA nanoparticles (NPs), which combine phase transition (PT) thrombolysis capabilities with properties conducive to multimodal imaging. This well-developed PT agent responded effectively to low-intensity focused ultrasound (LIFU) by triggering the vaporization of liquid PFH to achieve thrombolysis. The presence of the CREKA peptide, which binds to the fibrin of the thrombus, allows targeted imaging and efficacious thrombolysis. Then, we found that, compared with thrombolysis using a non-phase-transition agent, PT thrombolysis can produce a robust decrease in the thrombus burden regardless of the acoustic power density of LIFU. In particular, the reduced energy for LIFU-responsive PT during the lysis process guarantees the superior safety of PT thrombolysis. After injecting the NPs intravenously, we demonstrated that this lysis process can be monitored with ultrasound and photoacoustic imaging in vivo to evaluate its efficacy. Therefore, this nonpharmaceutical strategy departs from routine methods and reveals the potential use of PT thrombolysis as an effective and noninvasive alternative to current thrombolytic therapy.

Recent strategies on targeted delivery of thrombolytics

Thrombus formed in blood vessel is a progressive process, which would lead to life-threatening thrombotic diseases such as ischemic stroke. Unlike other diseases, the recognition of thrombus is usually in the late stage where blood vessels are largely blocked. So acute thrombotic diseases have a narrow therapeutic window, and remain leading causes of morbidity and mortality, whereas current thrombolysis therapy has limited therapeutic effects and bleeding complications. Thrombolytic agents in unwanted sites would cause hemorrhage due to the activation of plasminogen. Moreover, untargeted thrombolysis therapy require large amounts of thrombolytic agents, which in return would enhance hemorrhage risk. To improve the efficiency while minimizing the adverse effects of traditional thrombolysis therapy, novel drug delivery systems have been investigated. Various targeting strategies including ultrasound and magnetic field directed targeting, and specific binding, have been designed to deliver thrombolytic drugs to the thrombotic sites. These strategies demonstrate promising results in reducing bleeding risk as well as allowing less dosage of thrombolytic drugs with lowered clot lysis time. In this review, we discuss recent progress on targeted delivery of thrombolytics, and summarize treatment advantages and shortcomings, potentially helping to further promote the development of targeted thrombolysis.

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.

Evolution of contrast agents for ultrasound imaging and ultrasound-mediated drug delivery

Ultrasound (US) is one of the most frequently used diagnostic methods. It is a non-invasive, comparably inexpensive imaging method with a broad spectrum of applications, which can be increased even more by using bubbles as contrast agents (CAs). There are various different types of bubbles: filled with different gases, composed of soft- or hard-shell materials, and ranging in size from nano- to micrometers. These intravascular CAs enable functional analyses, e.g., to acquire organ perfusion in real-time. Molecular analyses are achieved by coupling specific ligands to the bubbles' shell, which bind to marker molecules in the area of interest. Bubbles can also be loaded with or attached to drugs, peptides or genes and can be destroyed by US pulses to locally release the entrapped agent. Recent studies show that US CAs are also valuable tools in hyperthermia-induced ablation therapy of tumors, or can increase cellular uptake of locally released drugs by enhancing membrane permeability. This review summarizes important steps in the development of US CAs and introduces the current clinical applications of contrast-enhanced US. Additionally, an overview of the recent developments in US probe design for functional and molecular diagnosis as well as for drug delivery is given.

Sonothrombolysis: the contribution of stable and inertial cavitation to clot lysis

Microbubble-mediated sonothrombolysis (STL) is a remarkable approach to vascular occlusion therapy. However, STL remains a complex process with multiple interactions between clot, ultrasound (US), microbubbles (MB) and thrombolytic drug. The aim of this study was to evaluate the ability of combining US and MB to degrade fibrin and, more specifically, to assess the roles of both stable (SC) and inertial (IC) cavitation. Human blood clots containing radiolabeled fibrin were exposed to different combinations of recombinant tissue plasminogen activator (rtPA), US (1 MHz) and phospholipid MB. Three acoustic pressures were tested: 200, 350 and 1,300 kPa (peak-negative pressure). Clot lysis was assessed by diameter loss and release of radioactive fibrin degradation products. The combination rtPA + US + MB clearly revealed that IC (1,300 kPa) was able to enhance fibrin degradation significantly (66.3 ± 1.8%) compared with rtPA alone (51.7 ± 2.0%, p < 0.001). However, SC failed to enhance fibrin degradation at an acoustic pressure of 200 kPa. At 350 kPa, a synergistic effect between rtPA and US + MB was observed with an absolute increase of 6% compared to rtPA alone (p < 0.001). Conversely, without rtPA, the combination of US + MB was unable to degrade the fibrin network (0.3 ± 0.1%, p > 0.05 vs. control), but induced a distinct loss of red blood cells throughout the entire thickness of the clot, implying that MB were able to penetrate and cavitate inside the clot.