Optimization of ultrasound parameters of myocardial cavitation microlesions for therapeutic application

Targeted microRNA delivery by lipid nanoparticles and gas vesicle-assisted ultrasound cavitation to treat heart transplant rejection

Despite exquisite immune response modulation, the extensive application of microRNA therapy in treating heart transplant rejection is still impeded by poor stability and low target efficiency. Here we have developed a low-intensity pulsed ultrasound (LIPUS) cavitation-assisted genetic therapy after executing the heart transplantation (LIGHT) strategy, facilitating microRNA delivery to target tissues through the LIPUS cavitation of gas vesicles (GVs), a class of air-filled protein nanostructures. We prepared antagomir-155 encapsulated liposome nanoparticles to enhance the stability. Then the murine heterotopic transplantation model was established, and antagomir-155 was delivered to murine allografted hearts via the cavitation of GVs agitated by LIPUS, which reinforced the target efficiency while guaranteeing safety owing to the specific acoustic property of GVs. This LIGHT strategy significantly depleted miR-155, upregulating the suppressors of cytokine signaling 1 (SOCS1), leading to reparative polarization of macrophages, decrease of T lymphocytes and reduction of inflammatory factors. Thereby, rejection was attenuated and the allografted heart survival was markedly prolonged. The LIGHT strategy achieves targeted delivery of microRNA with minimal invasiveness and great efficiency, paving the way towards novel ultrasound cavitation-assisted strategies of targeted genetic therapy for heart transplantation rejection.

Review on Polymeric Citrate Precursor and Sono-chemical Methods for the Synthesis of Nanomaterials

Background:

he properties of materials depend on the way of construction and the arrangement of atoms and molecules. Therefore, it is very important to know synthesis methods for the preparation of novel materials as per their desired structure. The low-temperature synthesis methods, such as polymeric citrate precursor and sonochemical methods are efficient enough to control the preparation of novel nanoparticles with morphological differences that leads to the novel devices with desired technological performances. These methods are simple, very less expensive and are easy to handle to operate for the synthesis of nanoparticles as per the expected morphology and dimensions.

Methods:

Polymeric citrate precursor method, a chelate-based method involves the reaction between mixed cations with citric acid, and then these cations are cross-linked with the help of ethylene glycol for the esterification process. Gel composites were heated which burns the organic moieties leaving behind the nanoparticles, and burning gels becomes essential for the reduction of nanoparticles. The sonochemical method, on the other hand, uses ultrasonic the irradiation results. The acoustic cavitation and high intensity ultrasound has been exploited for the preparation of different series of nanoparticles.

Results:

Commonly known for polymeric citrate method as Pechini gel pyrolysis method gives the evidence of versatile and elegant method for the synthesis of nanoparticles. The sonochemical method provides an unusual route of nanoparticle fabrication without bulk and that too with low temperature and pressure or less reaction time. These two methods have better control for the desired shape morphology and size and provide many opportunities for the use of these prepared nanoparticles in various aspects of science and technology.

Conclusion:

Polymeric citrate precursor and sonochemical methods are efficient to reduce to promote desirable reaction conditions and reduce the metal ions for the fabrication of nanoparticles. The prepared nanoparticles by using such low-cost elegant methods are uniform with a small size distribution, reproducible with good yield as per the demanded applications.

Cardiac Mechano-Electric Coupling: Acute Effects of Mechanical Stimulation on Heart Rate and Rhythm

The heart is vital for biological function in almost all chordates, including humans. It beats continually throughout our life, supplying the body with oxygen and nutrients while removing waste products. If it stops, so does life. The heartbeat involves precise coordination of the activity of billions of individual cells, as well as their swift and well-coordinated adaption to changes in physiological demand. Much of the vital control of cardiac function occurs at the level of individual cardiac muscle cells, including acute beat-by-beat feedback from the local mechanical environment to electrical activity (as opposed to longer term changes in gene expression and functional or structural remodeling). This process is known as mechano-electric coupling (MEC). In the current review, we present evidence for, and implications of, MEC in health and disease in human; summarize our understanding of MEC effects gained from whole animal, organ, tissue, and cell studies; identify potential molecular mediators of MEC responses; and demonstrate the power of computational modeling in developing a more comprehensive understanding of ‟what makes the heart tick.ˮ.

Ultrasonic Cavitation-Enabled Treatment for Therapy of Hypertrophic Cardiomyopathy: Proof of Principle

Ultrasound myocardial cavitation-enabled treatment was applied to the SS-16^BN rat model of hypertrophic cardiomyopathy for proof of the principle underlying myocardial reduction therapy. A focused ultrasound transducer was targeted using 10-MHz imaging (10 S, GE Vivid 7) to the left ventricular wall of anesthetized rats in a warmed water bath. Pulse bursts of 4-MPa peak rarefactional pressure amplitude were intermittently triggered 1:8 heartbeats during a 10-min infusion of a microbubble suspension. Methylprednisolone was given to reduce initial inflammation, and Losartan was given to reduce fibrosis in the healing tissue. At 28 d post therapy, myocardial cavitation-enabled treatment significantly reduced the targeted wall thickness by 16.2% (p <0.01) relative to shams, with myocardial strain rate and endocardial displacement reduced by 34% and 29%, respectively, which are sufficient for therapeutic treatment. Premature electrocardiogram complexes and plasma troponin measurements were found to identify optimal and suboptimal treatment cohorts and would aid in achieving the desired impact. With clinical translation, myocardial cavitation-enabled treatment should fill the need for a new non-invasive hypertrophic cardiomyopathy therapy option.

Cardiomyocyte-targeted and 17β-estradiol-loaded acoustic nanoprobes as a theranostic platform for cardiac hypertrophy

Background

Theranostic perfluorocarbon nanoprobes have recently attracted attention due to their fascinating versatility in integrating diagnostics and therapeutics into a single system. Furthermore, although 17β-estradiol (E2) is a potential anti-hypertrophic drug, it has severe non-specific adverse effects in various organs. Therefore, we have developed cardiomyocyte-targeted theranostic nanoprobes to achieve concurrent targeted imaging and treatment of cardiac hypertrophy.

Results

We had successfully synthesized E2-loaded, primary cardiomyocyte (PCM) specific peptide-conjugated nanoprobes with perfluorocarbon (PFP) as a core (PCM-E2/PFPs) and demonstrated their stability and homogeneity. In vitro and in vivo studies confirmed that when exposed to low-intensity focused ultrasound (LIFU), these versatile PCM-E2/PFPs can be used as an amplifiable imaging contrast agent. Furthermore, the significantly accelerated release of E2 enhanced the therapeutic efficacy of the drug and prevented systemic side effects. PCM-E2/PFPs + LIFU treatment also significantly increased cardiac targeting and circulation time. Further therapeutic evaluations showed that PCM-E2/PFPs + LIFU suppressed cardiac hypertrophy to a greater extent compared to other treatments, revealing high efficiency in cardiac-targeted delivery and effective cardioprotection.

Conclusion

Our novel theranostic nanoplatform could serve as a potential theranostic vector for cardiac diseases.

Electronic supplementary material

The online version of this article (10.1186/s12951-018-0360-3) contains supplementary material, which is available to authorized users.

Multiple ultrasound cavitation-enabled treatments for myocardial reduction

Background

Ultrasound myocardial cavitation enabled treatment (MCET) is an image-guided method for tissue reduction. In this study, a strategy of fractionated (multiple) treatments was tested for efficacy.

Methods

Dahl SS rats were anesthetized and prepared for treatment with a focused ultrasound transducer in a warm water bath. Aiming at the anterior left ventricular wall was facilitated by imaging with a 10 MHz phased array (10S, GE Vivid 7, GE Vingmed Ultrasound, Horten, Norway). MCET was accomplished at 1.5 MHz by pulse bursts of 4 MPa peak rarefactional pressure amplitude, which were intermittently triggered 1:8 from the ECG during infusion of a microbubble suspension for cavitation nucleation. Test groups were sham, a 200 s treatment, three 200 s treatments a week apart, and a 600 s treatment. Treatment outcome was observed by plasma troponin after 4 h, echocardiographic monitoring and histology at 6 wk.

Results

The impacts of the fractionated treatments summed to approximately the same as the long treatment; e. g. the troponin result was 10.5 ± 3.2 for 200 s, 22.7 ± 5.4 (p < 0.001) for the summed fractionated treatments and 29.9 ± 6.4 for 600 s (p = 0.06 relative to the summed fractionated). While wall thickness was not reduced for the fractionated treatment, tissue strain was reduced by 35% in the target area relative sham (p < 0.001).

Conclusion

The ability to fractionate treatment may be advantageous for optimizing patient outcome relative to all-or nothing therapy by surgical myectomy or alcohol ablation.

Effect of Low-Intensity Cavitation on the Isolated Human Thoracic Artery In Vitro

Reported here are the results of an experimental study on the response to low-intensity cavitation induced by low-frequency (4-6 W/cm², 20 kHz and 32.6 kHz) ultrasound of isolated human arterial samples taken during conventional myocardial revascularization operations. Studies have found that low-frequency ultrasound results in a significant (48%-54%) increase in isometric contraction force and does not depend on the number of exposures (10 or 20) or the time passed since the start of ultrasound (0, 10 and 20 min), but does depend on the frequency and location (internal or external) of the blood vessels for the application of ultrasound. Diltiazem (an inhibitor of slow calcium channels) and carbachol (an agonist of muscarinic receptors) used in a concentration-dependent manner did not modify the relaxation dynamics of smooth muscle affected by ultrasound. Thus, ultrasound conditioned to the augmentation of the isometric contraction force the smooth muscle of blood vessels and did not improve endothelial- and calcium channel blocker-dependent relaxation.

Frequency Dependence of Petechial Hemorrhage and Cardiomyocyte Injury Induced during Myocardial Contrast Echocardiography

Myocardial contrast echocardiography (MCE) for perfusion imaging can induce microscale bio-effects during intermittent high-Mechanical Index scans. The dependence of MCE-induced bio-effects on the ultrasonic frequency was examined in rats at 1.6, 2.5 and 3.5 MHz. Premature complexes were counted in the electrocardiogram, petechial hemorrhages with microvascular leakage on the heart surface were observed at the time of exposure, plasma troponin elevation was measured after 4 h and cardiomyocyte injury was detected at 24 h. Increasing response to exposure above an apparent threshold was observed for all endpoints at each frequency. The effects decreased with increasing ultrasonic frequency, and the thresholds increased. Linear regressions for frequency-dependent thresholds indicated coefficients and exponents of 0.6 and 1.07 for petechial hemorrhages, respectively, and 1.02 and 0.8 for cardiomyocyte death, compared with 1.9 and 0.5 (square root) for the guideline limit of the mechanical index. The results clarify the dependence of cardiac bio-effects on frequency, and should allow development of theoretical descriptions of the phenomena and improved safety guidance for MCE.

Maturation of Lesions Induced by Myocardial Cavitation-Enabled Therapy

Myocardial contrast echocardiography at enhanced therapeutic parameters may be a novel means of tissue reduction therapy, as for hypertrophic cardiomyopathy. Dahl/SS rats were anesthetized and treated with high-amplitude pulsed ultrasound guided by 10-MHz ultrasound images. Contrast microbubbles were infused via the tail vein during intermittent pulse-burst exposure at 4 MPa. A sham group, a low-impact group (group A, 5 cycle pulses with Gaussian modulation and 1:4 trigger for 5 min) and a high-impact group (group B, 10 cycle pulses with 4-ms square modulation and 1:8 trigger for 10 min) were tested. The higher exposure used in group B yielded more substantial injury than the lower exposure in group A. Treated rats in both groups A and B had significant increases in wall thickness measured by echocardiography the next day, which returned to normal by the end of 6 wk. Six weeks after ultrasound exposure, heart tissue samples exhibited tissue fibrosis in Masson's trichrome stained histology. Maturation of lesions involved fibrosis replacement, preserving structural tissue integrity. This study indicates that myocardial injury noted previously progresses into permanent loss of myocardial tissue that may be sufficient for possible hypertrophic cardiomyopathy therapy. More research is needed to define the treatment parameters required for symptomatic relief for hypertrophic cardiomyopathy.

Quantitative assessment of damage during MCET: a parametric study in a rodent model

Background

Myocardial cavitation-enabled therapy (MCET) has been proposed as a means to achieve minimally invasive myocardial reduction using ultrasound to produce scattered microlesions by cavitating contrast agent microbubbles.

Methods

Rats were treated using burst mode focused ultrasound at 1.5 MHz center frequency and varying envelope and pressure amplitudes. Evans blue staining indicated lethal cardiomyocytic injury. A previously developed quantitative scheme, evaluating the histologic treatment results, provides an insightful analysis for MCET treatment parameters. Such include ultrasound exposure amplitude and pulse modulation, contrast agent dose, and infusion rate.

Results

The quantitative method overcomes the limitation of visual scoring and works for a large dynamic range of treatment impact. Macrolesions are generated as an accumulation of probability driven microlesion formations. Macrolesions grow radially with radii from 0.1 to 1.6 mm as the ultrasound exposure amplitude (peak negative) increases from 2 to 4 MPa. To shorten treatment time, a swept beam was investigated and found to generate an acceptable macrolesion volume of about 40 μL for a single beam position.

Conclusions

Ultrasound parameters and administration of microbubbles directly influence lesion characteristics such as microlesion density and macrolesion dimension. For lesion generation planning, control of MCET is crucial, especially when targeting larger pre-clinical models.

Pulmonary Capillary Hemorrhage Induced by Fixed-Beam Pulsed Ultrasound

The induction of pulmonary capillary hemorrhage (PCH) by pulsed ultrasound was discovered 25 y ago, but early research used fixed-beam systems rather than actual diagnostic ultrasound machines. In this study, results of exposure of rats to fixed-beam focused ultrasound for 5 min at 1.5 and 7.5 MHz were compared with recent research on diagnostic ultrasound. One exposure condition at each frequency used 10-μs pulses delivered at 25-ms intervals. Three conditions involved Gaussian modulation of the pulse amplitudes at 25-ms intervals to simulate diagnostic scanning: 7.5 MHz with 0.3- and 1.5-μs pulses at 100- and 500-μs pulse repetition periods, respectively, and 1.5 MHz with 1.7-μs pulses at 500-μs repetition periods. Four groups were tested for each condition to assess PCH areas at different exposure levels and to determine occurrence thresholds. The conditions with identical pulse timing resulted in smaller PCH areas for the smaller 7.5-MHz beam, but both had thresholds of 0.69-0.75 MPa in situ peak rarefactional pressure amplitude. The Gaussian modulation conditions for both 7.5 MHz with 0.3-μs pulses and 1.5 MHz with 1.7-μs pulses had thresholds of 1.12-1.20 MPa peak rarefactional pressure amplitude, although the relatively long 1.5-μs pulses at 7.5 MHz yielded a threshold of 0.75 MPa. The fixed-beam pulsed ultrasound exposures produced lower thresholds than diagnostic ultrasound. There was no clear tendency for thresholds to increase with increasing ultrasonic frequency when pulse timing conditions were similar.

Use of Theranostic Strategies in Myocardial Cavitation-Enabled Therapy

The accumulation of microlesions induced by ultrasound interaction with contrast microbubbles in the myocardium potentially represents a new method of tissue reduction therapy. Anesthetized rats were treated in a heated water bath with 1.5-MHz focused ultrasound pulses triggered once every four heartbeats from the electrocardiogram during infusion of microbubble contrast agent. Treatment was guided by an 8-MHz B-mode imaging transducer, which also was used to provide estimates of left ventricular echogenicity as a possible predictor of efficacy during treatment. Strategies to reduce prospective clinical treatment durations were tested, including pulse modulation to simulate a theranostic scanning strategy and an increased agent infusion rate over shorter durations. Sources of variability, including ultrasound path variation and venous catheter placement, also were investigated. Electrocardiographic premature complexes were monitored, and Evans-blue stained cardiomyocyte scores were obtained from frozen sections. Left ventricular echogenicity reflected variations in the infused microbubble concentration, but failed to predict efficacy. Comparison of suspensions of varied microbubble size revealed that left ventricular echogenicity was dominated by larger bubbles, whereas efficacy appeared to be dependent on smaller sizes. Simulated scanning was as effective as the normal fixed-beam treatment, and high agent infusion allowed reduced treatment duration. The success of these theranostic strategies may increase the prospects for realistic clinical translation of myocardial cavitation-enabled therapy.

Emerging non-cancer applications of therapeutic ultrasound

Ultrasound therapy has been investigated for over half a century. Ultrasound can act on tissue through a variety of mechanisms, including thermal, shockwave and cavitation mechanisms, and through these can elicit different responses. Ultrasound therapy can provide a non-invasive or minimally invasive treatment option, and ultrasound technology has advanced to the point where devices can be developed to investigate a wide range of applications. This review focuses on non-cancer clinical applications of therapeutic ultrasound, with an emphasis on treatments that have recently reached clinical investigations, and preclinical research programmes that have great potential to impact patient care.

Characterization of macrolesions induced by myocardial cavitation-enabled therapy

Intermittent high intensity ultrasound pulses with circulating contrast agent microbubbles can induce scattered cavitation caused myocardial microlesions of potential value for tissue reduction therapy. Here, computer-aided histological evaluation of the effective treated volume was implemented to optimize ultrasound pulse parameters, exposure duration, and contrast agent dose. Rats were treated with 1.5 MHz focused ultrasound bursts and Evans blue staining indicates lethal cardiomyocytic injury. Each heart was sectioned to provide samples covering the entire exposed myocardial volume. Both brightfield and fluorescence images were taken for up to 40 tissue sections. Tissue identification and microlesion detection were first done based on 2-D images to form microlesion masks containing the outline of the heart and the stained cell regions. Image registration was then performed on the microlesion masks to reconstruct a volume-based model according to the morphology of the heart. The therapeutic beam path was estimated from the 3-D stacked microlesions, and finally the total microlesion volume, here termed macrolesion, was characterized along the therapeutic beam axis. Radially symmetric fractional macrolesions were characterized via stepping disks of variable radius determined by the local distribution of microlesions. Treated groups showed significant macrolesions of a median volume of 87.3 μL, 2.7 mm radius, 4.8 mm length, and 14.0% lesion density compared to zero radius, length, and lesion density for sham. The proposed radially symmetric lesion model is a robust evaluation for myocardial cavitation-enabled therapy. Future work will include validating the proposed method with varying acoustic exposures and optimizing involved parameters to provide macrolesion characterization.

Timing of high-intensity pulses for myocardial cavitation-enabled therapy

Background

High-intensity ultrasound pulses intermittently triggered from an ECG signal can interact with circulating contrast agent microbubbles to produce myocardial cavitation microlesions of potential therapeutic value. In this study, the timing of therapy pulses relative to the ECG R wave was investigated to identify the optimal time point for tissue reduction therapy with regard to both the physiological cardiac response and microlesion production.

Methods

Rats were anesthetized, prepared for ultrasound, placed in a heated water bath, and treated with 1.5 MHz focused ultrasound pulses targeted to the left ventricular myocardium with an 8 MHz imaging transducer. Initially, the rats were treated for 1 min at each of six different time points in the ECG while monitoring blood pressure responses to assess cardiac functional effects. Next, groups of rats were treated at three different time points: end diastole, end systole, and mid-diastole to assess the impact of timing on microlesion creation. These rats were pretreated with Evans blue injections and were allowed to recover for 1 day until hearts were harvested for scoring of injured cardiomyocytes.

Results

The initial results showed a wide range of cardiac premature complexes in the ECG, which corresponded with blood pressure pulses for ultrasound pulses triggered during diastole. However, the microlesion experiment did not reveal any statistically significant variations in cardiomyocyte injury.

Conclusion

The end of systole (R + RR/3) was identified as an optimal trigger time point which produced identifiable ECG complexes and substantial cardiomyocyte injury but minimal cardiac functional disruption during treatment.