Mechanical High-Intensity Focused Ultrasound (Histotripsy) in Dogs With Spontaneously Occurring Soft Tissue Sarcomas

Investigation of Optimum Production Conditions and the Stability of β-Cyclodextrin-Perfluorocarbon Nanocone Clusters for Histotripsy Applications

Nanocone clusters (NCCs) have been developed as clusters with inclusion complexes of FDA-approved β-cyclodextrin (βCD) and perfluorocarbons (PFC) (i.e., perfluoropentane (PFP) and perfluorohexane (PFH)) and have shown promise in nanoparticle-mediated histotripsy (NMH) applications owing to their lowered cavitation threshold, ease of production, and fluorocarbon quantification. However, there is still a lack of information on the best conditions of the synthesis of NCCs as a product that can have a maximum determinable fluorocarbon content and maintain the stability of the NCC during synthesis and when used as histotripsy agents or exposed to physiological conditions. These concerns about the stability of the clusters and the best possible formulation are investigated in the current work. The cluster formation potential was tested taking into consideration the nature of both PFCs and βCD by employing different synthesis conditions in terms of solution and environmental parameters such as concentration of solvent, stoichiometry between βCD and PFCs, temperature, pH, solvent type, etc. The best route of synthesis was then translated into various batch sizes and investigated in terms of the PFC loading and yield. These studies revealed that preparing NCCs in double-distilled water in an ice bath at the optimized solution concentration gave the highest yields and optimal PFC loading, as determined from gas chromatography. Furthermore, the stability of the clusters with different stoichiometries was scrutinized in varying concentrations, mechanical disruption times, pH levels, and temperature conditions, showing effects on each cluster's particle size in dynamic light scattering, visualized in transmission electron microscopy, and cavitation behavior in agarose gel tissue phantoms. These studies revealed stable clusters for all formulations, with PFH-containing NCCs emerging to be the most stable in terms of their cluster size and bubble formation potential in histotripsy. Finally, the shelf life of these clusters was investigated using DLS, which revealed a stable cluster. In conclusion, NCCs have shown high stability in terms of both synthesis, which can be replicated in gram-level production, and the cluster itself, which can be exposed to harsher conditions and still form stable bubbles in histotripsy.

Advances in Tumor Management: Harnessing the Potential of Histotripsy

Tissue ablation techniques have long been used in clinical settings to treat various oncologic diseases. However, many of these techniques are invasive and can cause substantial adverse effects. Histotripsy is a noninvasive, nonionizing, nonthermal tissue ablation technique that has the potential to replace surgical interventions in various clinical settings. Histotripsy works by delivering high-intensity focused ultrasound waves to target tissue. These waves create cavitation bubbles within tissues that rapidly expand and collapse, thereby mechanically fractionating the tissue into acellular debris that is subsequently absorbed by the body's immune system. Preclinical and clinical studies have demonstrated the efficacy of histotripsy in treating a range of diseases, including liver, pancreatic, renal, and prostate tumors. Safety outcomes of histotripsy have been generally favorable, with minimal adverse effects reported. However, further studies are needed to optimize the technique and understand its long-term effects. This review aims to discuss the importance of histotripsy as a noninvasive tissue ablation technique, the preclinical and clinical literature on histotripsy and its safety, and the potential applications of histotripsy in clinical practice. Keywords: Tumor Microenvironment, Ultrasound-High-Intensity Focused (HIFU), Ablation Techniques, Abdomen/GI, Genital/Reproductive, Nonthermal Tissue Ablation, Histotripsy, Clinical Trials, Preclinical Applications, Focused Ultrasound © RSNA, 2024.

Insights from in vivo preclinical cancer studies with histotripsy

Histotripsy is the first noninvasive, non-ionizing, and non-thermal ablation technique that mechanically fractionates target tissue into acellular homogenate via controlled acoustic cavitation. Histotripsy has been evaluated for various preclinical applications requiring noninvasive tissue removal including cancer, brain surgery, blood clot and hematoma liquefaction, and correction of neonatal congenital heart defects. Promising preclinical results including local tumor suppression, improved survival outcomes, local and systemic anti-tumor immune responses, and histotripsy-induced abscopal effects have been reported in various animal tumor models. Histotripsy is also being investigated in veterinary patients with spontaneously arising tumors. Research is underway to combine histotripsy with immunotherapy and chemotherapy to improve therapeutic outcomes. In addition to preclinical cancer research, human clinical trials are ongoing for the treatment of liver tumors and renal tumors. Histotripsy has been recently approved by the FDA for noninvasive treatment of liver tumors. This review highlights key learnings from in vivo shock-scattering histotripsy, intrinsic threshold histotripsy, and boiling histotripsy cancer studies treating cancers of different anatomic locations and discusses the major considerations in planning in vivo histotripsy studies regarding instrumentation, tumor model, study design, treatment dose, and post-treatment tumor monitoring.

Effects of pulse repetition frequency on bubble cloud characteristics and ablation in single-cycle histotripsy

Objective. Histotripsy is a cavitation-based ultrasound ablation method in development for multiple clinical applications. This work investigates the effects of pulse repetition frequency (PRF) on bubble cloud characteristics and ablative capabilities for histotripsy using single-cycle pulsing methods.Approach.Bubble clouds produced by a 500 kHz histotripsy system at PRFs from 0.1 to 1000 Hz were visualized using high-speed optical imaging in 1% agarose tissue phantoms at peak negative pressures,p-, of 2-36 MPa.Main results.Results showed a decrease in the cavitation cloud threshold with increasing PRF, ranging from 26.7 ± 0.5 MPa at 0.1 Hz to 15.0 ± 1.9 MPa at 1000 Hz. Bubble cloud analysis showed cavitation clouds generated at low PRFs (0.1-1 Hz) were characterized by consistently dense bubble clouds (41.7 ± 2.8 bubbles mm-2at 0.1 Hz), that closely matched regions of the focus above the histotripsy intrinsic threshold. Bubble clouds formed at higher PRFs measured lower cloud densities (23.1 ± 4.0 bubbles mm-2at 1000 Hz), with the lowest density measured for 10 Hz (8.8 ± 4.1 bubbles mm-2). Furthermore, higher PRFs showed increased pulse-to-pulse correlation, characteristic of cavitation memory effects; however, bubble clouds still filled the entire volume of the focus due to their initial density and enhanced bubble expansion from the restimulation of residual nuclei at the higher PRFs. Histotripsy ablation assessed through lesion analysis in red blood cell (RBC) phantoms showed higher PRFs generated lesions with lower adherence to the initial focal region compared to low PRF ablations; however, no trend of decreasing ablation efficiency with PRF was observed, with similar efficiencies observed for all the PRFs tested in this study.Significance.Notably, this result is different than what has previously been shown for shock-scattering histotripsy, which has shown decreased ablation efficiencies at higher PRFs. Overall, this study demonstrates the essential effects of PRF on single-cycle histotripsy procedures that should be considered to help guide future histotripsy pulsing strategies.

A review of the development of histotripsy for extremity tumor ablation with a canine comparative oncology model to inform human treatments

Cancer is a devasting disease resulting in millions of deaths worldwide in both humans and companion animals, including dogs. Treatment of cancer is complex and challenging and therefore often multifaceted, as in the case of osteosarcoma (OS) and soft tissue sarcoma (STS). OS predominantly involves the appendicular skeleton and STS commonly develops in the extremities, resulting in treatment challenges due to the need to balance wide-margin resections to achieve local oncological control against the functional outcomes for the patient. To achieve wide tumor resection, invasive limb salvage surgery is often required, and the patient is at risk for numerous complications which can ultimately lead to impaired limb function and mobility. The advent of tumor ablation techniques offers the exciting potential of developing noninvasive or minimally invasive treatment options for extremity tumors. One promising innovative tumor ablation technique with strong potential to serve as a noninvasive limb salvage treatment for extremity tumor patients is histotripsy. Histotripsy is a novel, noninvasive, non-thermal, and non-ionizing focused ultrasound technique which uses controlled acoustic cavitation to mechanically disintegrate tissue with high precision. In this review, we present the ongoing development of histotripsy as a non-surgical alternative for extremity tumors and highlight the value of spontaneously occurring OS and STS in the pet dog as a comparative oncology research model to advance this field of histotripsy research.

Ablative and Immunostimulatory Effects of Histotripsy Ablation in a Murine Osteosarcoma Model

Background: Osteosarcoma (OS) is the most frequently occurring malignant bone tumor in humans, primarily affecting children and adolescents. Significant advancements in treatment options for OS have not occurred in the last several decades, and the prognosis remains grim with only a 70% rate of 5-year survival. The objective of this study was to investigate the focused ultrasound technique of histotripsy as a novel, noninvasive treatment option for OS. Methods: We utilized a heterotopic OS murine model to establish the feasibility of ablating OS tumors with histotripsy in a preclinical setting. We investigated the local immune response within the tumor microenvironment (TME) via immune cell phenotyping and gene expression analysis. Findings: We established the feasibility of ablating heterotopic OS tumors with ablation characterized microscopically by loss of cellular architecture in targeted regions of tumors. We observed greater populations of macrophages and dendritic cells within treated tumors and the upregulation of immune activating genes 72 h after histotripsy ablation. Interpretation: This study was the first to investigate histotripsy ablation for OS in a preclinical murine model, with results suggesting local immunomodulation within the TME. Our results support the continued investigation of histotripsy as a novel noninvasive treatment option for OS patients to improve clinical outcomes and patient prognosis.

Surgical Management of Brain Tumors with Focused Ultrasound

Focused ultrasound is a novel technique for the treatment of aggressive brain tumors that uses both mechanical and thermal mechanisms. This non-invasive technique can allow for both the thermal ablation of inoperable tumors and the delivery of chemotherapy and immunotherapy while minimizing the risk of infection and shortening the time to recovery. With recent advances, focused ultrasound has been increasingly effective for larger tumors without the need for a craniotomy and can be used with minimal surrounding soft tissue damage. Treatment efficacy is dependent on multiple variables, including blood-brain barrier permeability, patient anatomical features, and tumor-specific features. Currently, many clinical trials are currently underway for the treatment of non-neoplastic cranial pathologies and other non-cranial malignancies. In this article, we review the current state of surgical management of brain tumors using focused ultrasound.

Characterizing the Ablative Effects of Histotripsy for Osteosarcoma: In Vivo Study in Dogs

Osteosarcoma (OS) is a malignant bone tumor treated by limb amputation or limb salvage surgeries and chemotherapy. Histotripsy is a non-thermal, non-invasive focused ultrasound therapy using controlled acoustic cavitation to mechanically disintegrate tissue. Recent ex vivo and in vivo pilot studies have demonstrated the ability of histotripsy for ablating OS but were limited in scope. This study expands on these initial findings to more fully characterize the effects of histotripsy for bone tumors, particularly in tumors with different compositions. A prototype 500 kHz histotripsy system was used to treat ten dogs with suspected OS at an intermediate treatment dose of 1000 pulses per location. One day after histotripsy, treated tumors were resected via limb amputation, and radiologic and histopathologic analyses were conducted to determine the effects of histotripsy for each patient. The results of this study demonstrated that histotripsy ablation is safe and feasible in canine patients with spontaneous OS, while offering new insights into the characteristics of the achieved ablation zone. More extensive tissue destruction was observed after histotripsy compared to that in previous reports, and radiographic changes in tumor size and contrast uptake following histotripsy were reported for the first time. Overall, this study significantly expands our understanding of histotripsy bone tumor ablation and informs future studies for this application.

The histotripsy spectrum: differences and similarities in techniques and instrumentation

Since its inception about two decades ago, histotripsy - a non-thermal mechanical tissue ablation technique - has evolved into a spectrum of methods, each with distinct potentiating physical mechanisms: intrinsic threshold histotripsy, shock-scattering histotripsy, hybrid histotripsy, and boiling histotripsy. All methods utilize short, high-amplitude pulses of focused ultrasound delivered at a low duty cycle, and all involve excitation of violent bubble activity and acoustic streaming at the focus to fractionate tissue down to the subcellular level. The main differences are in pulse duration, which spans microseconds to milliseconds, and ultrasound waveform shape and corresponding peak acoustic pressures required to achieve the desired type of bubble activity. In addition, most types of histotripsy rely on the presence of high-amplitude shocks that develop in the pressure profile at the focus due to nonlinear propagation effects. Those requirements, in turn, dictate aspects of the instrument design, both in terms of driving electronics, transducer dimensions and intensity limitations at surface, shape (primarily, the F-number) and frequency. The combination of the optimized instrumentation and the bio-effects from bubble activity and streaming on different tissues, lead to target clinical applications for each histotripsy method. Here, the differences and similarities in the physical mechanisms and resulting bioeffects of each method are reviewed and tied to optimal instrumentation and clinical applications.

Histotripsy ablation for the treatment of feline injection site sarcomas: a first-in-cat in vivo feasibility study

PURPOSE

Feline soft tissue sarcoma (STS) and injection site sarcoma (fISS) are rapidly growing tumors with low metastatic potential, but locally aggressive behavior. Histotripsy is a non-invasive focused ultrasound therapy using controlled acoustic cavitation to mechanically disintegrate tissue. In this study, we investigated the in vivo safety and feasibility of histotripsy to treat fISS using a custom 1 MHz transducer.

MATERIALS AND METHODS

Three cats with naturally-occurring STS were treated with histotripsy before surgical removal of the tumor 3 to 6 days later. Gross and histological analyses were used to characterize the ablation efficacy of the treatment, and routine immunohistochemistry and batched cytokine analysis were used to investigate the acute immunological effects of histotripsy.

RESULTS

Results showed that histotripsy ablation was achievable and well-tolerated in all three cats. Precise cavitation bubble clouds were generated in all patients, and hematoxylin & eosin stained tissues revealed ablative damage in targeted regions. Immunohistochemical results identified an increase in IBA-1 positive cells in treated tissues, and no significant changes in cytokine concentrations were identified post-treatment.

CONCLUSIONS

Overall, the results of this study demonstrate the safety and feasibility of histotripsy to target and ablate superficial feline STS and fISS tumors and guide the clinical development of histotripsy devices for this application.