Spatiotemporal local and abscopal cell death and immune responses to histotripsy focused ultrasound tumor ablation

Nanostrategies synergize with locoregional interventional therapies for boosting antitumor immunity

Compared with traditional surgical resection, systemic chemotherapy, or radiotherapy, locoregional interventional therapies (LITs) possess their own advantages of minimally invasive procedure and immunomodulatory effects in cancer treatment. Local ablation and intravascular interventional therapy represent excellent LIT candidate to combine with immunotherapy. Diverse nanomaterials with excellent biocompatibility show promises in modulating antitumor immunity. In this review, we summarized several immune-LIT combinations, discussed the following immunomodulatory effects, and presented the novel nanostrategies for synergizing with the combination therapy. With continuous optimization, further promotion of clinical translation will ultimately benefit patients with personalized and tailored cancer strategy.

Ultrasound-assisted immunotherapy for malignant tumour

Malignant tumour represents a significant global public health concern. The advent of immunotherapy has brought about a revolutionary shift in the landscape of tumour treatment, offering a ray of hope to patients across the globe. Immunotherapy strategies have demonstrated considerable promise in clinical trials. However, the immunosuppressive environment within the tumour microenvironment has constituted a significant obstacle to the advancement of immunotherapies. It is therefore imperative to develop more efficacious and personalised approaches. The utilisation of non-invasive ultrasound-assisted immunotherapy represents a promising strategy. Ultrasound has the capacity to induce an immune response and stimulate other drugs to achieve a specific response, thereby reducing the toxic side effects of treatment and enhancing the outcome of immunotherapy. This paper presents a systematic introduction to the various mechanisms related to ultrasound and reviews the recent advancements of ultrasound-assisted tumour immunotherapy, including ultrasonic ablation, combined application with contrast agents, and sonodynamic therapy.

MRI Coregistered Rodent Histotripsy Array for Orthotopic Liver Models

Histotripsy has emerged as a promising therapeutic option for liver tumors, recently gaining food and drug administration (FDA) approval for clinical use in October 2023. Preclinical in vivo histotripsy experiments primarily utilize subcutaneous ectopic murine tumor models, which fail to accurately replicate the complex immunosuppressive tumor microenvironment (TME) of liver tumors. In order to address this gap, we present the design, development, and in vivo demonstration of a miniature, electronically steerable magnetic resonance imaging (MRI)-guided histotripsy array tailored for orthotopic murine liver tumor models. This novel system integrates an 89-element phased array within a 7.0-T small animal MRI scanner, enabling precise targeting through enhanced soft tissue contrast and 3-D visualization. The targeting accuracy of the array was validated in tissue-mimicking red blood cell (RBC) phantoms, exhibiting targeting precision of $0.24~\pm ~0.1$ mm. Subsequent in vivo experiments in naïve mice demonstrated successful liver ablations, confirmed by gross morphology and histological analysis. However, the presence of grating lobes led to undesired collateral damage, highlighted by lung hemorrhages, necessitating future adjustments in the array's design. This study illustrates the foundational steps necessary for translating histotripsy experiments from subcutaneous to orthotopic models.

Histotripsy of Liver Tumors: Patient Selection, Ethical Discussions, and How We Do It

Liver malignancies, both primary and metastatic tumors, are a major cause of cancer-related mortality. Colorectal cancer alone results in liver metastases in nearly 50% of patients, with approximately 85% presenting with unresectable disease. Similarly, hepatocellular carcinoma and intrahepatic cholangiocarcinoma frequently present at advanced stages, limiting curative options. Systemic therapies provide modest survival benefits, underscoring the need for alternative treatments. Locoregional approaches, such as thermal ablation and chemoembolization, while effective, have notable limitations, including invasiveness, peri-procedural risks, and the requirement to interrupt systemic treatments. Histotripsy is a novel, non-invasive method that uses focused ultrasound-induced cavitation to enable precise tumor ablation without heat or radiation. Our institution utilizes a multidisciplinary tumor board approach to evaluate patients for histotripsy, particularly in cases involving unresectable disease, complex surgical candidacy, palliative intent related to disease control and symptom management, or as bridging therapy for transplantation. Early results, including preclinical data and the THERESA and #HOPE4LIVER trials, highlight its efficacy in treating liver tumors with minimal complications. This review outlines institutional protocols for histotripsy, covering pre- and post-procedural management, along with ethical considerations of current treatment paradigms. As a patient-centered approach, histotripsy offers a novel treatment option with a favorable safety profile and compatibility with systemic therapies.

Perfluorohexane nanodroplet-assisted mechanical high intensity focused ultrasound cavitation: A strategy for hepatocellular carcinoma treatment

The activation of immune-stimulatory molecules is critical for effective antitumor immunotherapy. Mechanical high-intensity focused ultrasound (mHIFU) sustains this activation in tumor cell debris through cavitation. To enhance cavitation, perfluorohexane nanodroplets (NDs-PFH) were utilized in this study to lower the cavitation threshold during mHIFU ablation. Our results showed that NDs-PFH combined with mHIFU induced 77.2 % Hepa 1-6 tumor cells death, and activated the release of damage-associated molecular patterns (such as HMGB1, CRT, and ATP), enhancing dendritic cell maturation (20.2 %) and T cell activation (1.8 % of TNF-α+ and 2.7 % of IFN-γ+). In vivo, the combination of NDs-PFH and mHIFU effectively suppressed both primary and distant untreated tumors, reducing the tumor volume by 83.3 % (from 657.4 mm3 to 110.0 mm3) and metastatic tumor volume by 76.6 % (from 365.5 mm3 to 85.6 mm3) through enhanced anticancer immune response and a robust abscopal effect. Furthermore, combining NDs-PFH with mHIFU significantly enhanced the efficacy of immune checkpoint inhibitors in liver cancer. When combined with αPD-1 therapy, tumor inhibition improved by 30 % (from 63.6 mm3 to 19.3 mm3) compared to αPD-1 monotherapy. These results highlight the potential of combining mHIFU with a PFH nano-loaded drug delivery system as a promising strategy for advancing antitumor immunotherapy. STATEMENT OF SIGNIFICANCE: Mechanical high-intensity focused ultrasound (mHIFU) can ablate tumors via cavitation effects, however, achieving these effects typically requires an extremely high cavitation threshold. In this study, we utilized widely used perfluorohexane nanodroplets (NDs-PFH) to effectively lower the cavitation threshold. The tumor cell debris generated by the combination of NDs-PFH and mHIFU not only induced immunogenic cells death but also activated antitumor immune responses within the tumor microenvironment. Additionally, our findings demonstrated that this combination elicited a significant abscopal effect and enhanced the efficacy of immunotherapy.

The Emerging Role of Histotripsy in Liver Cancer Treatment: A Scoping Review

BACKGROUND/OBJECTIVE

Hepatocellular carcinoma (HCC) is an aggressive disease that is known to be resistant to conventional chemotherapy and radiotherapy. While surgical resection and transarterial therapy can improve overall survival, the biological aspects of HCC contribute to the complexity of its management and limit the effectiveness of current treatment options. The purpose of this scoping review is to identify the limitations of the currently available therapies for HCC and explore the emerging role that histotripsy could play in addressing these limitations, with the intent of informing the direction of future research and clinical management.

METHODS

The PRISMA checklist for scoping reviews was followed to structure this review, and a systematic search was conducted in the following online databases: PubMed/MEDLINE (National Library of Medicine), Embase (Elsevier), and Scopus (Elsevier).

RESULTS

The current evidence supports that histotripsy offers several key advantages that address the limitations of the current treatment strategies for HCC. Clinical trials have highlighted the ability of this technology to destroy solid tumors and induce remission with minimal side effects. In addition, current preclinical studies point to the potent immunostimulatory effects of histotripsy, including the induction of abscopal effects. This poses significant promise in treating tumor metastasis as well as improving clinical regimens by combining histotripsy with immunotherapy. Future research should aim to overcome the current limitations of histotripsy and enhance clinical outcomes for patients. This review examines existing treatments for HCC, emphasizing the promising potential of combining histotripsy with immunotherapy to target the metastatic and advanced stages of the disease.

The abscopal effects of sonodynamic therapy in cancer

The abscopal effect is a phenomenon wherein localised therapy on the primary tumour leads to regression of distal metastatic growths. Interestingly, various pre-clinical studies utilising sonodynamic therapy (SDT) have reported significant abscopal effects, however, the mechanism remains largely enigmatic. SDT is an emerging non-invasive cancer treatment that uses focussed ultrasound (FUS) and a sonosensitiser to induce tumour cell death. To expand our understanding of abscopal effects of SDT, we have summarised the preclinical studies that have found SDT-induced abscopal responses across various cancer models, using diverse combination strategies with nanomaterials, microbubbles, chemotherapy, and immune checkpoint inhibitors. Additionally, we shed light on the molecular and immunological mechanisms underpinning SDT-induced primary and metastatic tumour cell death, as well as the role and efficacy of different sonosensitisers. Notably, the observed abscopal effects underscore the need for continued investigation into the SDT-induced 'vaccine-effect' as a potential strategy for enhancing systemic anti-tumour immunity and combating metastatic disease. The results of the first SDT human clinical trials are much awaited and are hoped to enable the further evaluation of the safety and efficacy of SDT, paving the way for future studies specifically designed to explore the potential of translating SDT-induced abscopal effects into clinical reality.

Ethanol inhibits the growth and metastasis of hepatocellular carcinoma by inducing immunogenic cell death

BACKGROUND

Immunogenic cell death (ICD) can elicit an adaptive immune response with significant antitumor effects. Percutaneous ethanol injection therapy has been applied as tumor ablation for small hepatocellular carcinoma (HCC). However, it was not clear whether ethanol can elicit ICD. The aim of this study is to investigate the role of ethanol as an ICD inducer.

METHODS

HCC cell lines were treated with low-concentration ethanol and ICD markers, such as calreticulin, high-mobility group box 1, and ATP were assayed. The mouse vaccination-rechallenge assay was used to further confirm ethanol as an ICD inducer. Western blot and real-time PCR were used to investigate ICD-related endoplasmic reticulum (ER) stress and signaling pathways. The genes with differential expression levels between primary and distant tumors were analyzed by nCounter gene expression. Intratumoral injection of ethanol was done to investigate the abscopal effect.

RESULTS

The low-concentration ethanol could induce ICD in HCC cell lines through unfolded protein responses initiated by ER stress and multiple cell-death pathways. Intratumoral injections of low-concentration ethanol had significantly direct and abscopal antitumor effects in mouse models of both subcutaneous and orthotopic HCC. nCounter gene expression analysis in primary and distant tumors revealed the activation of various immune-response pathways, notably those mediated by CD8 T cells and the interferon pathway. Vaccinating mice with low-concentration ethanol-treated HCC cells successfully inhibited metastasis in both intravenous and intrasplenic metastasis models.

CONCLUSIONS

Our results suggest that low-concentration ethanol could serve as an inducer of ICD. Low-concentration ethanol could potentially improve therapeutic antitumor immunity by inducing substantial ICD.

JR, Xu Z, Wang S. Ultrasound-Guided Histotripsy Triggers the Release of Tumor-Associated Antigens from Breast Cancers

Background/Objectives: There is increasing evidence to indicate that histotripsy treatment can enhance the host anti-tumor immune responses both locally at the targeting tumor site as well as systemically from abscopal effects. Histotripsy is a non-invasive ultrasound ablation technology that mechanically disrupts target tissue via cavitation. A key factor contributing to histotripsy-induced abscopal effects is believed to be the release of tumor-specific antigens (TSAs) or tumor-associated antigens (TAAs) that induce a systemic immune response. In this study, we studied the effect of histotripsy treatment on the release of HER2, a well-defined TAA target for cancer immunotherapy. Methods: A range of doses of histotripsy administered to HER2-postive mammary tumor cells in an in vitro cell culture system and an ex vivo tumor were applied. In addition, a single dose of histotripsy was used for an in vivo murine tumor model. The released proteins, and specifically HER2, in both tumor cell-free supernatants and tumor cell pellets were analyzed by a BCA protein assay, an ultra-performance liquid chromatography (UPLC) assay, and Western blot. Results: Our results showed that histotripsy could significantly trigger the release of HER2 proteins in the current study. The level of HER2 proteins was actually higher in tumor cell-free supernatants than in tumor cell pellets, suggesting that HER2 was released from the intracellular domain into the extracellular compartment. Furthermore, proportionally more HER2 protein was released at higher histotripsy doses, indicating free HER2 was histotripsy-dose-dependent. Conclusions: In conclusion, we have qualitatively and quantitatively demonstrated that histotripsy treatment triggers the release of HER2 from the tumor cells into the extracellular compartment. The histotripsy-mediated release of HER2 antigens provides important insights into the mechanism underlying its immunostimulation and suggests the potential of TSA/TAA-based immunotherapies in numerous cancer types.

Focused Ultrasound in Cancer Immunotherapy: A Review of Mechanisms and Applications

Ultrasound is well-perceived for its diagnostic application. Meanwhile, ultrasound, especially focused ultrasound (FUS), has also demonstrated therapeutic capabilities, such as thermal tissue ablation, hyperthermia, and mechanical tissue ablation, making it a viable therapeutic approach for cancer treatment. Cancer immunotherapy is an emerging cancer treatment approach that boosts the immune system to fight cancer, and it has also exhibited enhanced effectiveness in treating previously considered untreatable conditions. Currently, cancer immunotherapy is regarded as one of the four pillars of cancer treatment because it has fewer adverse effects than radiation and chemotherapy. In recent years, the unique capabilities of FUS in ablating tumors, regulating the immune system, and enhancing anti-tumor responses have resulted in a new field of research known as FUS-induced/assisted cancer immunotherapy. In this work, we provide a comprehensive overview of this new research field by introducing the basics of focused ultrasound and cancer immunotherapy and providing the state-of-the-art applications of FUS in cancer immunotherapy: the mechanisms and preclinical and clinical studies. This review aims to offer the scientific community a reliable reference to the exciting field of FUS-induced/assisted cancer immunotherapy, hoping to foster the further development of related technology and expand its medical applications.

Abscopal effect of focused ultrasound combined immunotherapy in animal solid tumor model: a systematic reviews and meta-analysis

Introduction

The abscopal effect, a systemic anti-tumor response triggered by localized treatment, has gained attention but remains poorly understood. This study evaluates the efficacy and consistency of focused ultrasound (FUS) combined with immunotherapy in inducing the abscopal effect.

Methods

A systematic review and meta-analysis were conducted on preclinical studies using solid tumor models. Data on tumor response, immune modulation, and survival outcomes were analyzed to assess the combination therapy's effectiveness.

Results

FUS combined with immunotherapy enhanced anti-tumor responses at local and distant sites, with evidence of immune activation and increased abscopal effect rates. However, heterogeneity across tumor models and protocols was observed.

Discussion

The findings provide a theoretical basis for FUS-immunotherapy combinations in cancer treatment, while emphasizing the need for standardized protocols and further research to elucidate underlying mechanisms.

Systematic review registration

https://www.crd.york.ac.uk/prospero/, identifier CRD42023460710.

Mechanical HIFU and immune checkpoint inhibition: toward clinical implementation

Objective: Immune checkpoint inhibition (ICI) has significantly advanced the field of immuno-oncology, yet not all patients benefit from this therapy. Combining ICI with other therapeutic modalities, including tumor ablation, is currently being explored as a method to enhance ICI efficacy. Mechanical High-Intensity Focused Ultrasound (M-HIFU) represents a promising tumor ablative therapy, inducing cavitation within the tumor, resulting in tumor cell destruction and the release of danger signals and tumor antigens, two key factors contributing to anti-tumor immune responses.

Methods/Results: Preclinical studies on the impact of M-HIFU on the anti-tumor immune response are guiding the translational application of this technique in the clinical setting. This review provides a comprehensive overview of the current understanding of the effects of M-HIFU on the immune system. We report on the effect of M-HIFU on soluble immune modulators and immune cells in different preclinical models, and potential contributions to the anti-tumor immune response. We discuss clinical studies applying M-HIFU and studies that have combined ICI with other ablative therapies to draw parallels to clinical implementation of M-HIFU. Further, we will highlight essential questions that should be addressed in future clinical trials exploring the combination of M-HIFU and ICI in the clinical setting.

Conclusion: Overall, this review offers guidance for the clinical implementation of combining M-HIFU with ICI and highlights key questions that remain to be addressed in first clinical studies.

Exploiting the mechanical effects of ultrasound for noninvasive therapy

Focused ultrasound is a platform technology capable of eliciting a wide range of biological responses with high spatial precision deep within the body. Although focused ultrasound is already in clinical use for focal thermal ablation of tissue, there has been a recent growth in development and translation of ultrasound-mediated nonthermal therapies. These approaches exploit the physical forces of ultrasound to produce a range of biological responses dependent on exposure conditions. This review discusses recent advances in four application areas that have seen particular growth and have immense clinical potential: brain drug delivery, neuromodulation, focal tissue destruction, and endogenous immune system activation. Owing to the maturation of transcranial ultrasound technology, the brain is a major target organ; however, clinical indications outside the brain are also discussed.

The #HOPE4LIVER Single-Arm Pivotal Trial for Histotripsy of Primary and Metastatic Liver Tumors

Background Histotripsy is a nonthermal, nonionizing, noninvasive, focused US technique that relies on cavitation for mechanical tissue breakdown at the focal point. Preclinical data have shown its safety and technical success in the ablation of liver tumors. Purpose To evaluate the safety and technical success of histotripsy in destroying primary or metastatic liver tumors. Materials and Methods The parallel United States and European Union and England #HOPE4LIVER trials were prospective, multicenter, single-arm studies. Eligible patients were recruited at 14 sites in Europe and the United States from January 2021 to July 2022. Up to three tumors smaller than 3 cm in size could be treated. CT or MRI and clinic visits were performed at 1 week or less preprocedure, at index-procedure, 36 hours or less postprocedure, and 30 days postprocedure. There were co-primary end points of technical success of tumor treatment and absence of procedure-related major complications within 30 days, with performance goals of greater than 70% and less than 25%, respectively. A two-sided 95% Wilson score CI was derived for each end point. Results Forty-four participants (21 from the United States, 23 from the European Union or England; 22 female participants, 22 male participants; mean age, 64 years ± 12 [SD]) with 49 tumors were enrolled and treated. Eighteen participants (41%) had hepatocellular carcinoma and 26 (59%) had non-hepatocellular carcinoma liver metastases. The maximum pretreatment tumor diameter was 1.5 cm ± 0.6 and the maximum post-histotripsy treatment zone diameter was 3.6 cm ± 1.4. Technical success was observed in 42 of 44 treated tumors (95%; 95% CI: 84, 100) and procedure-related major complications were reported in three of 44 participants (7%; 95% CI: 2, 18), both meeting the performance goal. Conclusion The #HOPE4LIVER trials met the co-primary end-point performance goals for technical success and the absence of procedure-related major complications, supporting early clinical adoption. Clinical trial registration nos. NCT04572633, NCT04573881 Published under a CC BY 4.0 license. Supplemental material is available for this article. See also the editorial by Nezami and Georgiades in this issue.

Histotripsy: A Method for Mechanical Tissue Ablation with Ultrasound

Histotripsy is a relatively new therapeutic ultrasound technology to mechanically liquefy tissue into subcellular debris using high-amplitude focused ultrasound pulses. In contrast to conventional high-intensity focused ultrasound thermal therapy, histotripsy has specific clinical advantages: the capacity for real-time monitoring using ultrasound imaging, diminished heat sink effects resulting in lesions with sharp margins, effective removal of the treated tissue, a tissue-selective feature to preserve crucial structures, and immunostimulation. The technology is being evaluated in small and large animal models for treating cancer, thrombosis, hematomas, abscesses, and biofilms; enhancing tumor-specific immune response; and neurological applications. Histotripsy has been recently approved by the US Food and Drug Administration to treat liver tumors, with clinical trials undertaken for benign prostatic hyperplasia and renal tumors. This review outlines the physical principles of various types of histotripsy; presents major parameters of the technology and corresponding hardware and software, imaging methods, and bioeffects; and discusses the most promising preclinical and clinical applications.

Histoplasty Modification of the Tumor Microenvironment in a Murine Preclinical Model of Breast Cancer

PURPOSE

To develop a noninvasive therapeutic approach able to alter the biophysical organization and physiology of the extracellular matrix (ECM) in breast cancer.

MATERIALS AND METHODS

In a 4T1 murine model of breast cancer, histoplasty treatment with a proprietary 700-kHz multielement therapy transducer using a coaxially aligned ultrasound (US) imaging probe was used to target the center of an ex vivo tumor and deliver subablative acoustic energy. Tumor collagen morphology was qualitatively evaluated before and after histoplasty with second harmonic generation. Separately, mice bearing bilateral 4T1 tumors (n = 4; total tumors = 8) were intravenously injected with liposomal doxorubicin. The right flank tumor was histoplasty-treated, and tumors were fluorescently imaged to detect doxorubicin uptake after histoplasty treatment. Next, 4T1 tumor-bearing mice were randomized into 2 treatment groups (sham vs histoplasty, n = 3 per group). Forty-eight hours after sham/histoplasty treatment, tumors were harvested and analyzed using flow cytometry.

RESULTS

Histoplasty significantly increased (P = .002) liposomal doxorubicin diffusion into 4T1 tumors compared with untreated tumors (2.12- vs 1.66-fold increase over control). Flow cytometry on histoplasty-treated tumors (n = 3) demonstrated a significant increase in tumor macrophage frequency (42% of CD45 vs 33%; P = .022) and a significant decrease in myeloid-derived suppressive cell frequency (7.1% of CD45 vs 10.3%; P = .044). Histoplasty-treated tumors demonstrated increased CD8+ (5.1% of CD45 vs 3.1%; P = .117) and CD4+ (14.1% of CD45 vs 11.8%; P = .075) T-cell frequency.

CONCLUSIONS

Histoplasty is a nonablative focused US approach to noninvasively modify the tumor ECM, increase chemotherapeutic uptake, and alter the tumor immune microenvironment.

Development of Convolutional Neural Network to Segment Ultrasound Images of Histotripsy Ablation

OBJECTIVE

Histotripsy is a focused ultrasound therapy that ablates tissue via the action of bubble clouds. It is under investigation to treat a number of ailments, including renal tumors. Ultrasound imaging is used to monitor histotripsy, though there remains a lack of definitive imaging metrics to confirm successful treatment outcomes. In this study, a convolutional neural network (CNN) was developed to segment ablation on ultrasound images.

METHODS

A transfer learning approach was used to replace classification layers of the residual network ResNet-18. Inputs to the classification layers were based on ultrasound images of ablated red blood cell phantoms. Digital photographs served as the ground truth. The efficacy of the CNN was compared to subtraction imaging, and manual segmentation of images by two board-certified radiologists.

RESULTS

The CNN had a similar performance to manual segmentation, though was improved relative to segmentation with subtraction imaging. Predictions of the network improved over the course of treatment, with the Dice similarity coefficient less than 20% for fewer than 500 applied pulses, but 85% for more than 750 applied pulses. The network was also applied to ultrasound images of ex vivo kidney exposed to histotripsy, which indicated a morphological shift in the treatment profile relative to the phantoms. These findings were consistent with histology that confirmed ablation of the targeted tissue.

CONCLUSION

Overall, the CNN showed promise as a rapid means to assess outcomes of histotripsy and automate treatment.

SIGNIFICANCE

Data collected in this study indicate integration of CNN image segmentation to gauge outcomes for histotripsy ablation holds promise for automating treatment procedures.

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.

Investigating cell death responses associated with histotripsy ablation of canine osteosarcoma

BACKGROUND

Osteosarcoma (OS) is the most frequently occurring primary bone tumor in dogs and people and innovative treatment options are profoundly needed. Histotripsy is an emerging tumor ablation modality, and it is essential for the clinical translation of histotripsy to gain knowledge about the outcome of nonablated tumor cells that could remain postablation. The objective of this study was to characterize the cell death genetic signature and proliferation response of canine OS cells post a near complete histotripsy ablation (96% ± 1.5) and to evaluate genetic cell death signatures associated with histotripsy ablation and OS in vivo.

METHODS

In the current study, we ablated three canine OS cell lines with a histotripsy dose that resulted in near complete ablation to allow for a viable tumor cell population for downstream analyses. To assess the in vivo cell death genetic signature, we characterized cell death genetic signature in histotripsy-ablated canine OS tumors collected 24-h postablation.

RESULTS

Differential gene expression changes observed in the 4% viable D17 and D418 cells, and histotripsy-ablated OS tumor samples, but not in Abrams cells, were associated with immunogenic cell death (ICD). The 4% viable OS cells demonstrated significantly reduced proliferation, compared to control OS cells, in vitro.

CONCLUSION

Histotripsy ablation of OS cell lines leads to direct and potentially indirect cell death as evident by, reduced proliferation in remaining viable OS cells and cell death genetic signatures suggestive of ICD both in vitro and in vivo.

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.

A Comparison of Histotripsy and Percutaneous Cryoablation in a Chronic Healthy Swine Kidney Model

PURPOSE

To compare the safety and efficacy of histotripsy with cryoablation in a chronic human-scale normal porcine kidney model.

MATERIALS AND METHODS

Eighteen female domestic swine were divided evenly into histotripsy and cryoablation treatment arms. A planned 2-3 cm diameter treatment was performed under ultrasound (histotripsy) or ultrasound and computed tomography (CT) guidance (cryoablation). Contrast-enhanced CT and serum blood analysis were performed immediately postprocedure and on day 7, with either immediate killing (n = 3) or continued survival to day 30 (n = 6), at which time contrast-enhanced CT, serum blood analysis, and necropsy were performed. Animal welfare, treatment zone appearance, procedure-related adverse events, and histopathology of the treatment zones and surrounding tissues were assessed.

RESULTS

Histotripsy treatment zones (mean ±standard deviation diameters, 2.7 ± 0.12 × 2.4 ± 0.19 × 2.4 ± 0.26 cm; volume, 8.3 ± 1.9 cm3) were larger than cryoablation zones (mean diameters, 2.2 ± 0.19 × 1.9 ± 0.13 × 1.7 ± 0.19 cm; volume, 3.9 ± 0.8 cm3; P < .001). At 30 days, histotripsy and cryoablation treatment zone volumes decreased by 96% and 83% on CT, respectively (P < .001). Perirenal hematomas were present after 8 of 9 (89%) cryoablation (mean volume, 22.2 cm3) and 1 of 9 (11%, P < .001) histotripsy (volume, 0.4 cm3) procedures, with active arterial extravasation in 4 of 9 (44%) cryoablation and no histotripsy animals (P = .206). All 9 histotripsy animals and 5 of 9 (56%) cryoablation animals had collecting system debris (P = .042). Changes in serum creatinine were similar between the groups (P = .321).

CONCLUSIONS

Other than a higher rate of bleeding after cryoablation, the safety and early efficacy of histotripsy and cryoablation were comparable for creating treatment zones in a chronic normal porcine kidney model.

Clinical translation of abdominal histotripsy: a review of preclinical studies in large animal models

Histotripsy is an emerging noninvasive, non-thermal, and non-ionizing focused ultrasound (US) therapy that can be used to destroy targeted tissue. Histotripsy has evolved from early laboratory prototypes to clinical systems which have been comprehensively evaluated in the preclinical environment to ensure safe translation to human use. This review summarizes the observations and results from preclinical histotripsy studies in the liver, kidney, and pancreas. Key findings from these studies include the ability to make a clinically relevant treatment zone in each organ with maintained collagenous architecture, potentially allowing treatments in areas not currently amenable to thermal ablation. Treatments across organ capsules have proven safe, including in anticoagulated models which may expand patients eligible for treatment or eliminate the risk associated with taking patients off anti-coagulation. Treatment zones are well-defined with imaging and rapidly resorb, which may allow improved evaluation of treatment zones for residual or recurrent tumor. Understanding the effects of histotripsy in animal models will help inform physicians adopting histotripsy for human clinical use.

The use of histotripsy as intratumoral immunotherapy beyond tissue ablation-the rationale for exploring the immune effects of histotripsy

Mechanical high-intensity focused ultrasound (M-HIFU), which includes histotripsy, is a non-ionizing, non-thermal ablation technology that can be delivered by noninvasive methods. Because acoustic cavitation is the primary mechanism of tissue disruption, histotripsy is distinct from the conventional HIFU techniques resulting in hyperthermia and thermal injury. Phase I human trials have shown the initial safety and efficacy of histotripsy in treating patients with malignant liver tumors. In addition to tissue ablation, a promising benefit of M-HIFU has been stimulating a local and systemic antitumor immune response in preclinical models and potentially in the Phase I trial. Preclinical studies combining systemic immune therapies appear promising, but clinical studies of combinations have been complicated by systemic toxicities. Consequently, combining M-HIFU with systemic immunotherapy has been demonstrated in preclinical models and may be testing in future clinical studies. An additional alternative is to combine intratumoral M-HIFU and immunotherapy using microcatheter-placed devices to deliver both M-HIFU and immunotherapy intratumorally. The promise of M-HIFU as a component of anti-cancer therapy is promising, but as forms of HIFU are tested in preclinical and clinical studies, investigators should report not only the parameters of the energy delivered but also details of the preclinical models to enable analysis of the immune responses. Ultimately, as clinical trials continue, clinical responses and immune analysis of patients undergoing M-HIFU including forms of histotripsy will provide opportunities to optimize clinical responses and to optimize application and scheduling of M-HIFU in the context of the multi-modality care of the cancer patient.

Impact of immune tolerance mechanisms on the efficacy of immunotherapy in primary and secondary liver cancers

The liver is a functionally unique organ with an immunosuppressive microenvironment. The liver is the sixth most common site of primary cancer in humans and is a frequent site of metastasis from other solid tumors. The development of effective therapies for primary and metastatic liver cancer has been challenging due to the complex metabolic and immune microenvironment of the liver. The liver tumor microenvironment (TME) in primary and secondary (metastatic) liver cancers is heterogenous and consists of unique immune and stromal cell populations. Crosstalk between these cell populations and tumor cells creates an immunosuppressive microenvironment within the liver which potentiates cancer progression. Immune checkpoint inhibitors (ICIs) are now clinically approved for the management of primary and secondary liver cancer and can partially overcome liver immune tolerance, but their efficacy is limited. In this review, we describe the liver microenvironment and the use of immunotherapy in primary and secondary liver cancer. We discuss emerging combination strategies utilizing locoregional and systemic therapy approaches which may enhance efficacy of immunotherapy in primary and secondary liver cancer. A deeper understanding of the immunosuppressive microenvironment of the liver will inform novel therapies and therapeutic combinations in order to improve outcomes of patients with primary and secondary liver cancer.

Magic bubbles: utilizing histotripsy to modulate the tumor microenvironment and improve systemic anti-tumor immune responses

Focused Ultrasound (FUS) is emerging as a promising primary and adjunct therapy for the treatment of cancer. This includes histotripsy, which is a noninvasive, non-ionizing, non-thermal ultrasound guided ablation modality. As histotripsy has progressed from bench-to-bedside, it has become evident that this therapy has benefits beyond local tumor ablation. Specifically, histotripsy has the potential to shift the local tumor microenvironment from immunologically 'cold' to 'hot'. This is associated with the production of damage associated molecular patterns, the release of a selection of proinflammatory mediators, and the induction of inflammatory forms of cell death in cells just outside of the treatment zone. In addition to the induction of this innate immune response, histotripsy can also improve engagement of the adaptive immune system and promote systemic anti-tumor immunity targeting distal tumors and metastatic lesions. These tantalizing observations suggest that, in settings of widely metastatic disease burden, selective histotripsy of a limited number of accessible tumors could be a means of maximizing responsiveness to systemic immunotherapy. More work is certainly needed to optimize treatment strategies that best synergize histotripsy parameters with innate and adaptive immune responses. Likewise, rigorous clinical studies are still necessary to verify the presence and repeatability of these phenomena in human patients. As this technology nears regulatory approval for clinical use, it is our expectation that the insights and immunomodulatory mechanisms summarized in this review will serve as directional guides for rational clinical studies to validate and optimize the potential immunotherapeutic role of histotripsy tumor ablation.