An investigation of the physical forces leading to thrombosis disruption by cavitation

Ultrasound and Magnetic Responsive Drug Delivery Systems for Cardiovascular Application

With the increasing insight into molecular mechanisms of cardiovascular disease, a promising solution involves directly delivering genes, cells, and chemicals to the infarcted myocardium or impaired endothelium. However, the limited delivery efficiency after administration fails to reach the therapeutic dose and the adverse off-target effect even causes serious safety concerns. Controlled drug release via external stimuli seems to be a promising method to overcome the drawbacks of conventional drug delivery systems (DDSs). Microbubbles and magnetic nanoparticles responding to ultrasound and magnetic fields respectively have been developed as an important component of novel DDSs. In particular, several attempts have also been made for the design and fabrication of dual-responsive DDS. This review presents the recent advances in the ultrasound and magnetic fields responsive DDSs in cardiovascular application, followed by their current problems and future reformation.

Spatially Specific Liposomal Cancer Therapy Triggered by Clinical External Sources of Energy

This review explores the use of energy sources, including ultrasound, magnetic fields, and external beam radiation, to trigger the delivery of drugs from liposomes in a tumor in a spatially-specific manner. Each section explores the mechanism(s) of drug release that can be achieved using liposomes in conjunction with the external trigger. Subsequently, the treatment's formulation factors are discussed, highlighting the parameters of both the therapy and the medical device. Additionally, the pre-clinical and clinical trials of each triggered release method are explored. Lastly, the advantages and disadvantages, as well as the feasibility and future outlook of each triggered release method, are discussed.

Application of cavitation promoting surfaces in management of acute ischemic stroke

High frequency, low intensity ultrasound has the potential to accelerate the clearance of thrombotic occlusion in the absence of cavitation. At high frequency ultrasound, high acoustic pressures, >5.2MPa, are required to generate cavitation in thrombus. The focus of this study was to reduce the cavitation threshold by applying materials with appropriate nucleation sites at the transducer-thrombus boundary to further augment sonothrombolysis. Heterogeneous and homogenous nucleation sites were generated on the outer surface of a polyimide tube (PI) using microfringed (MPI) and laser induced (LPI) microcavities. The cavitation threshold of these materials was determined using a passive cavitation detection system. Furthermore, the biological impact of both materials was investigated in vitro. The results revealed that both MPI and LPI have the potential to induce cavitation at acoustic pressure levels as low as 2.3MPa. In the presence of cavitation, thrombolysis rate could be enhanced by up to two times without any evidence of hemolysis that is generally associated with cavitation activities in blood. A prototype ultrasonic catheter operating at 1.7MHz frequency and acoustic pressure of 2.3MPa with either of MPI or LPI could be considered as a viable option for treatment of acute ischemic stroke.

Potentiating intra-arterial sonothrombolysis for acute ischemic stroke by the addition of the ultrasound contrast agents (Optison™ & SonoVue(®))

Transcranial ultrasound in combination with intravenously administered ultrasound contrast agents (UCA) in the presence or absence of recombinant tissue plasminogen activator (rt-PA) has been widely evaluated as a new modality for treatment of ischemic stroke. Despite the successful demonstration of accelerated clot lysis there are inherent limitations associated with this modality such as inconsistency in temporal window thickness and/or potential serious cardiopulmonary reactions to intravenous administration of UCA that prevent broad application to ischemic stroke populations. As a complementary modality, we evaluated potential lysis enhancement by intra-arterial ultrasound with concurrent intra-clot delivery of UCA and rt-PA. To this end, clots were formed with average pore diameter similar to clinically retracted clots by adjusting the thrombin concentration. Physical characteristic and retention of UCA after delivery through the catheter as a function of clinically relevant flow rates of 6, 12, 18 ml/h were determined using a microscopic method. The ability of the UCA employed in this study, Optison and SonoVue, to penetrate into the clot was verified using ultrasound B-mode imaging. Clot lysis as a function of rt-PA concentration, 0.009 through 0.5 mg/ml, in the presence and absence of UCA diluted to 1:10, 1:100, and 1:200 v/v at two Peak rarefaction acoustic pressures of 1.3 and 2.1 MPa were evaluated using a weighing method. The study results suggest the addition of only 0.02 ml of 1:100 diluted UCA to rt-PA of 0.009, 0.05, 0.3, and 0.5 mg/ml can enhance the lysis rate by 3.9, 2.6, 1.9 and 1.8 fold in the presence of peak rarefaction acoustic pressure of 1.3 MPa and by 5.1, 3.4, 2.6, 3.1 in the presence of peak rarefaction acoustic pressure of 2.1 MPa, respectively. In addition, Optison and SonoVue demonstrated comparable effectiveness in enhancing the clot lysis rate. Addition of UCA to intra-arterial sonothrombolysis could be considered as a viable treatment option for ischemic stroke patients.

Sonothrombolysis: an emerging modality for the treatment of acute ischemic and hemorrhagic stroke

To date, it is believed that rapid removal of impedances hindering normal blood circulation in the brain would salvage ischemic tissue. Hence, most treatment modalities undergoing clinical evaluation for treatment of stroke are focused on faster recanalization in acute ischemic stroke or faster hematoma mass reduction in hemorrhagic stroke. Therapeutic ultrasound is among the promising emerging modalities being clinically evaluated to meet this purpose. This review provides an overview of existing clinical data in evaluating sonothrombolysis applications in treatment of acute ischemic and hemorrhagic stroke. Furthermore, the present status of clinical evaluation of microbubbles as a potential adjuvant to this modality is reviewed.

Enhancement and control of acoustic cavitation yield by low-level dual frequency sonication: a subharmonic analysis

Evaluation of inertial cavitation is a significant problem where this mechanism of action is responsible for therapeutic applications such as drug delivery. It has shown that using multiple frequencies one is able to enhance and control induced cavitation. In this study, we used different sonication frequencies as 28 kHz, 130 kHz, 1 MHz, 3 MHz and their dual combinations to enhance acoustic cavitation. At each frequency, two different intensities were used and the subharmonic amplitude of each frequency in combinations was measured. It was observed that in combinations which include 28 kHz, the cavitation activity is enhanced. The 28 kHz subharmonic amplitude was used to compare these protocols in their ability to enhance cavitation. Besides, the area of cavitation damage was determined using an aluminum foil. Our results showed that the inertial cavitation activity increased at higher intensities and there is a significant correlation between the subharmonic amplitude and sonication intensity at each frequency (R>0.90). In addition, simultaneous combined dual-frequency orthogonal sonication at 28 kHz with other frequencies used can significantly increase the inertial cavitation activity as compared to the algebraic sum of the individual ultrasound irradiations in 28 kHz subharmonic frequency. The 28 kHz subharmonic amplitude for 28 kHz (0.04 W/cm(2)) and 3 MHz (2 and 1 W/cm(2)) combined dual frequency were about 4.6 and 1.5 times higher than that obtained from the algebraic sum of 28 kHz and 3 MHz irradiation, respectively. Also the 28 kHz subharmonic amplitude for combination of 28 kHz (0.04 W/cm(2)) and 1 MHz (2 and 1 W/cm(2)) were about 2.4 and 1.6 times higher than that obtained with their algebraic sum. Among different combinations, the continuous mode for two ultrasound sources of 28 kHz (0.04 W/cm(2)) and 3 MHz (2 W/cm(2)) is more effective than other combinations (p-value<0.05). The results of effective irradiation area showed no damaged aluminum foil in MHz sonication alone. However, there is significant difference between the effective irradiation area of combined dual frequency 28 kHz and 3 MHz with other irradiation modes (p-value<0.05) and it is limited locally.

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.