Antivascular ultrasound therapy: magnetic resonance imaging validation and activation of the immune response in murine melanoma

Ultrasound and Microbubble-Induced Reduction of Functional Vasculature Depends on the Microbubble, Tumor Type and Time After Treatment

OBJECTIVE

Ultrasound in combination with microbubbles can enhance accumulation and improve the distribution of various therapeutic agents in tumor tissue, leading to improved efficacy. Understanding the impact of treatment on the tumor microenvironment, concurrently with how microenvironment attributes affect treatment outcome, will be important for selecting appropriate patient cohorts in future clinical trials. The main aim of this work was to investigate the influence of ultrasound and microbubbles on the functional vasculature of cancer tissue.

METHODS

Four different tumor models in mice (bone, pancreatic, breast and colon cancer) were characterized with respect to vascular parameters using contrast-enhanced ultrasound imaging. The effect of treatment with microbubbles and ultrasound was then investigated using immunohistochemistry and confocal microscopy, quantifying the total amount of vasculature and fraction of functional vessels. Two different microbubbles were used, the clinical contrast agent SonoVue and the large bubbles generated by Acoustic Cluster Therapy (ACT), tailored for therapeutic purposes.

RESULTS

The colon cancer model displayed slower flow but a higher vascular volume than the other models. The pancreatic model showed the fastest flow but also the lowest vascular volume. Ultrasound and SonoVue transiently reduced the amount of functional vasculature in breast and colon tumors immediately after treatment. No reduction was observed for ACT, likely due to shorter ultrasound pulses and lower pressures applied.

CONCLUSION

Variation between tumor models due to tissue characteristics emphasizes the importance of evaluating treatment suitability in the specific tissue of interest, as altered perfusion could have a large impact on drug delivery and therapeutic outcome.

Ultrasound-induced immune responses in tumors: A systematic review and meta-analysis

Ultrasound is widely used in the diagnosis and therapy of cancer. Tumors can be treated by thermal or mechanical tissue ablation. Furthermore, tumors can be manipulated by hyperthermia, sonodynamic therapy and sonoporation, e.g., by increasing tumor perfusion or the permeability of biological barriers to enhance drug delivery. These treatments induce various immune responses in tumors. However, conflicting data and high heterogeneity between experimental settings make it difficult to generalize the effects of ultrasound on tumor immunity. Therefore, we performed a systematic review to answer the question: "Does ultrasound alter the immune reaction of peripheral solid tumors in humans and animals compared to control conditions without ultrasound?" A systematic literature search was performed in PubMed, EMBASE, and Web of Science and 24,401 potentially relevant publications were identified. Of these, 96 publications were eligible for inclusion in the systematic review. Experiments were performed in humans, rats, and mice and focused on different tumor types, primarily breast and melanoma. We collected data on thermal and non-thermal ultrasound settings, the use of sono-sensitizers or sono-enhancers, and anti-tumor therapies. Six meta-analyses were performed to quantify the effect of ultrasound on tumor infiltration by T cells (cytotoxic, helper, and regulatory T cells) and on blood cytokines (interleukin-6, interferon-γ, tumor necrosis factor-α). We provide robust scientific evidence that ultrasound alters T cell infiltration into tumors and increases blood cytokine concentrations. Furthermore, we identified significant differences in immune cell infiltration based on tumor type, ultrasound settings, and mouse age. Stronger effects were observed using hyperthermia in combination with sono-sensitizers and in young mice. The latter may impair the translational impact of study results as most cancer patients are older. Thus, our results may help refining ultrasound parameters to enhance anti-tumor immune responses for therapeutic use and to minimize immune effects in diagnostic applications.

A synergistic approach for modulating the tumor microenvironment to enhance nano-immunotherapy in sarcomas

The lack of properly perfused blood vessels within tumors can significantly hinder the distribution of drugs, leading to reduced treatment effectiveness and having a negative impact on the quality of life of patients with cancer. This problem is particularly pronounced in desmoplastic cancers, where interactions between cancer cells, stromal cells, and the fibrotic matrix lead to tumor stiffness and the compression of most blood vessels within the tumor. To address this issue, two mechanotherapy approaches-mechanotherapeutics and ultrasound sonopermeation-have been employed separately to treat vascular abnormalities in tumors and have reached clinical trials. Here, we performed in vivo studies in sarcomas, to explore the conditions under which these two mechanotherapy strategies could be optimally combined to enhance perfusion and the efficacy of nano-immunotherapy. Our findings demonstrate that combination of the anti-histamine drug ketotifen, as a mechanotherapeutic, and sonopermeation effectively alleviates mechanical forces by decreasing 50 % collagen and hyaluronan levels and thus, reshaping the tumor microenvironment. Furthermore, the combined therapy normalizes the tumor vasculature by increasing two-fold the pericytes coverage. This combination not only improves six times tumor perfusion but also enhances drug delivery. As a result, blood vessel functionality is enhanced, leading to increased infiltration by 40 % of immune cells (CD4+ and CD8+ T-cells) and improving the antitumor efficacy of Doxil nanomedicine and anti-PD-1 immunotherapy. In conclusion, our research underscores the unique and synergistic potential of combining mechanotherapeutics and sonopermeation. Both approaches are undergoing clinical trials to enhance cancer therapy and have the potential to significantly improve nano-immunotherapy in sarcomas.

Ultrasound-augmented cancer immunotherapy

Cancer is a growing worldwide health problem with the most broadly studied treatments, in which immunotherapy has made notable advancements in recent years. However, innumerable patients have presented a poor response to immunotherapy and simultaneously experienced immune-related adverse events, with failed therapeutic results and increased mortality rates. Consequently, it is crucial to develop alternate tactics to boost therapeutic effects without producing negative side effects. Ultrasound is considered to possess significant therapeutic potential in the antitumor field because of its inherent characteristics, including cavitation, pyrolysis, and sonoporation. Herein, this timely review presents the comprehensive and systematic research progress of ultrasound-enhanced cancer immunotherapy, focusing on the various ultrasound-related mechanisms and strategies. Moreover, this review summarizes the design and application of current sonosensitizers based on sonodynamic therapy, with an attempt to provide guidance on new directions for future cancer therapy.

Ultrasound combined with microbubble mediated immunotherapy for tumor microenvironment

The tumor microenvironment (TME) plays an important role in dynamically regulating the progress of cancer and influencing the therapeutic results. Targeting the tumor microenvironment is a promising cancer treatment method in recent years. The importance of tumor immune microenvironment regulation by ultrasound combined with microbubbles is now widely recognized. Ultrasound and microbubbles work together to induce antigen release of tumor cell through mechanical or thermal effects, promoting antigen presentation and T cells' recognition and killing of tumor cells, and improve tumor immunosuppression microenvironment, which will be a breakthrough in improving traditional treatment problems such as immune checkpoint blocking (ICB) and himeric antigen receptor (CAR)-T cell therapy. In order to improve the therapeutic effect and immune regulation of TME targeted tumor therapy, it is necessary to develop and optimize the application system of microbubble ultrasound for organs or diseases. Therefore, the combination of ultrasound and microbubbles in the field of TME will continue to focus on developing more effective strategies to regulate the immunosuppression mechanisms, so as to activate anti-tumor immunity and/or improve the efficacy of immune-targeted drugs, At present, the potential value of ultrasound combined with microbubbles in TME targeted therapy tumor microenvironment targeted therapy has great potential, which has been confirmed in the experimental research and application of breast cancer, colon cancer, pancreatic cancer and prostate cancer, which provides a new alternative idea for clinical tumor treatment. This article reviews the research progress of ultrasound combined with microbubbles in the treatment of tumors and their application in the tumor microenvironment.

Synchronous Intravital Imaging and Cavitation Monitoring of Antivascular Focused Ultrasound in Tumor Microvasculature Using Monodisperse Low Boiling Point Nanodroplets

Focused ultrasound-stimulated microbubbles can induce blood flow shutdown and ischemic necrosis at higher pressures in an approach termed antivascular ultrasound. Combined with conventional therapies of chemotherapy, immunotherapy, and radiation therapy, this approach has demonstrated tumor growth inhibition and profound synergistic antitumor effects. However, the lower cavitation threshold of microbubbles can potentially yield off-target damage that the polydispersity of clinical agent may further exacerbate. Here we investigate the use of a monodisperse nanodroplet formulation for achieving antivascular effects in tumors. We first develop stable low boiling point monodisperse lipid nanodroplets and examine them as an alternative agent to mediate antivascular ultrasound. With synchronous intravital imaging and ultrasound monitoring of focused ultrasound-stimulated nanodroplets in tumor microvasculature, we show that nanodroplets can trigger blood flow shutdown and do so with a sharper pressure threshold and with fewer additional events than conventionally used microbubbles. We further leverage the smaller size and prolonged pharmacokinetic profile of nanodroplets to allow for potential passive accumulation in tumor tissue prior to antivascular ultrasound, which may be a means by which to promote selective tumor targeting. We find that vascular shutdown is accompanied by inertial cavitation and complex-order sub- and ultraharmonic acoustic signatures, presenting an opportunity for effective feedback control of antivascular ultrasound.

Cavitation-Mediated Immunomodulation and Its Use with Checkpoint Inhibitors

The promotion of anti-tumour immune responses can be an effective route to the complete remission of primary and metastatic tumours in a small proportion of patients. Hence, researchers are currently investigating various methods to further characterise and enhance such responses to achieve a beneficial impact across a wider range of patients. Due to its non-invasive, non-ionising, and targetable nature, the application of ultrasound-mediated cavitation has proven to be a popular method to enhance the delivery and activity of immune checkpoint inhibitors. However, to optimise this approach, it is important to understand the biological and physical mechanisms by which cavitation may promote anti-tumour immune responses. Here, the published literature relating to the role that cavitation may play in modulating anti-tumour immunity is therefore assessed.

An optical and acoustic investigation of microbubble cavitation in small channels under therapeutic ultrasound conditions

Therapeutic focused ultrasound in combination with encapsulated microbubbles is being widely investigated for its ability to elicit bioeffects in the microvasculature, such as transient permeabilization for drug delivery or at higher pressures to achieve 'antivascular' effects. While it is well established that the behaviors of microbubbles are altered when they are situated within sufficiently small vessels, there is a paucity of data examining how the bubble population dynamics and emissions change as a function of channel (vessel) diameter over a size range relevant to therapeutic ultrasound, particularly at pressures relevant to antivascular ultrasound. Here we use acoustic emissions detection and high-speed microscopy (10 kframes/s) to examine the behavior of a polydisperse clinically employed agent (Definity®) in wall-less channels as their diameters are scaled from 1200 to 15 µm. Pressures are varied from 0.1 to 3 MPa using either a 5 ms pulse or a sequence of 0.1 ms pulses spaced at 1 ms, both of which have been previously employed in an in vivo context. With increasing pressure, the 1200 µm channel - on the order of small arteries and veins - exhibited inertial cavitation, 1/2 subharmonics and 3/2 ultraharmonics, consistent with numerous previous reports. The 200 and 100 µm channels - in the size range of larger microvessels less affected by therapeutic focused ultrasound - exhibited a distinctly different behavior, having muted development of 1/2 subharmonics and 3/2 ultraharmonics and reduced persistence. These were associated with radiation forces displacing bubbles to the distal wall and inducing clusters that then rapidly dissipated along with emissions. As the diameter transitioned to 50 and then 15 µm - a size regime that is most relevant to therapeutic focused ultrasound - there was a higher threshold for the onset of inertial cavitation as well as subharmonics and ultraharmonics, which importantly had more complex orders that are not normally reported. Clusters also occurred in these channels (e.g. at 3 MPa, the mean lateral and axial sizes were 23 and 72 µm in the 15 µm channel; 50 and 90 µm in the 50 µm channel), however in this case they occupied the entire lumens and displaced the wall boundaries. Damage to the 15 µm channel was observed for both pulse types, but at a lower pressure for the long pulse. Experiments conducted with a 'nanobubble' (<0.45 µm) subpopulation of Definity followed broadly similar features to 'native' Definity, albeit at a higher pressure threshold for inertial cavitation. These results provide new insights into the behavior of microbubbles in small vessels at higher pressures and have implications for therapeutic focused ultrasound cavitation monitoring and control.

Low-intensity focused ultrasound targeted microbubble destruction reduces tumor blood supply and sensitizes anti-PD-L1 immunotherapy

Immune checkpoint blockade (ICB) typified by anti-PD-1/PD-L1 antibodies as a revolutionary treatment for solid malignancies has been limited to a subset of patients due to poor immunogenicity and inadequate T cell infiltration. Unfortunately, no effective strategies combined with ICB therapy are available to overcome low therapeutic efficiency and severe side effects. Ultrasound-targeted microbubble destruction (UTMD) is an effective and safe technique holding the promise to decrease tumor blood perfusion and activate anti-tumor immune response based on the cavitation effect. Herein, we demonstrated a novel combinatorial therapeutic modality combining low-intensity focused ultrasound-targeted microbubble destruction (LIFU-TMD) with PD-L1 blockade. LIFU-TMD caused the rupture of abnormal blood vessels to deplete tumor blood perfusion and induced the tumor microenvironment (TME) transformation to sensitize anti-PD-L1 immunotherapy, which markedly inhibited 4T1 breast cancer's growth in mice. We discovered immunogenic cell death (ICD) in a portion of cells induced by the cavitation effect from LIFU-TMD, characterized by the increased expression of calreticulin (CRT) on the tumor cell surface. Additionally, flow cytometry revealed substantially higher levels of dendritic cells (DCs) and CD8⁺ T cells in draining lymph nodes and tumor tissue, as induced by pro-inflammatory molecules like IL-12 and TNF-α. These suggest that LIFU-TMD as a simple, effective, and safe treatment option provides a clinically translatable strategy for enhancing ICB therapy.

Contrast-Enhanced Ultrasound Tumor Therapy With Abdominal Imaging Transducer

A diagnostic ultrasound machine add-on module (AOM) was created to enable an off-the-shelf abdominal imaging transducer to perform contrast-enhanced therapeutic ultrasound. The AOM creates plane-wave ultrasound through an abdominal imaging transducer targeting intravascular microbubbles within tumors. This therapeutic antivascular ultrasound (AVUS) causes heating and cavitation effects that destroy tumor vasculature and starves it of nutrients. The AOM can switch between therapeutic and imaging modes for monitoring AVUS treatment. The therapeutic capability of the AOM was validated in murine hepatocellular carcinomas (HCC) grown in adult mice. Contrast-enhanced ultrasound imaging performed before and after the therapeutic treatment evaluated the AVUS response to the treatment. The peak enhancement (PE), perfusion index (PI), and area under the curve (AUC) were measured for the control and AOM treatment groups. The AOM group showed a substantial decrease in these parameters compared to the control group. The difference between the pre- and post-therapy was significant, (p < 0.001) for the AOM group and not significant (p > 0.5) for the control group. Tumor temperatures increased markedly for the AOM group with a thermal dose (CEM43) of 124.8 (±2.5). Histochemical analysis of the excised HCC samples revealed several hemorrhagic pools in tumors from the AOM group, absent in the tumors of the control group. These results demonstrate the theranostic potential of the AOM to induce and monitor vascular disruption within murine tumors.

The role of anti-tumor immunity of focused ultrasound for the malignancies: depended on the different ablation categories

Improving anti-tumor immunity has promising outcomes in eradicating malignant tumors. Tumor cells can escape from immune surveillance and killing; therefore, various strategies are continuously developing to inhibit immune escape. Focused ultrasound (FUS) has recently emerged to play an important role in immune modulation. After FUS therapy, various tumor antigens and related signals are released. The non-thermal effect of FUS strengthens the blood and lymph circulation, increases cell permeability, and helps in crossing the physical barrier like the blood-brain barrier and blood-tumor barrier. However, the different ablation of FUS is proposed to have a different anti-tumor immune effect. Therefore, we categorized the FUS ablation into thermal and non-thermal ablation and summarized possible anti-tumor immunity mechanisms.

Ultrasound and microbubble-mediated drug delivery and immunotherapy

Ultrasound induces the oscillation and collapse of microbubbles such as those of an ultrasound contrast agent, where these behaviors generate mechanical and thermal effects on cells and tissues. These, in turn, induce biological responses in cells and tissues, such as cellular signaling, endocytosis, or cell death. These physiological effects have been used for therapeutic purposes. Most pharmaceutical agents need to pass through the blood vessel walls and reach the parenchyma cells to produce therapeutic effects in drug delivery. Therefore, the blood vessel walls act as an obstacle to drug delivery. The combination of ultrasound and microbubbles is a promising strategy to enhance vascular permeability, improving drug transport from blood to tissues. This combination has also been applied to gene and protein delivery, such as cytokines and antigens for immunotherapy. Immunotherapy, in particular, is an attractive technique for cancer treatment as it induces a cancer cell-specific response. However, sufficient anti-tumor effects have not been achieved with the conventional cancer immunotherapy. Recently, new therapies based on immunomodulation with immune checkpoint inhibitors have been reported. Immunomodulation can be regarded as a new strategy for cancer immunotherapy. It was also reported that mechanical and thermal effects induced by the combination of ultrasound and microbubbles could suppress tumor growth by promoting the cancer-immunity cycle via immunomodulation in the tumor microenvironment. In this review, we provide an overview of the application of ultrasound and microbubble combination for drug delivery and activation of the immune system in the microenvironment of tumor tissue.

Synergies between therapeutic ultrasound, gene therapy and immunotherapy in cancer treatment

Due to the ease of use and excellent safety profile, ultrasound is a promising technique for both diagnosis and site-specific therapy. Ultrasound-based techniques have been developed to enhance the pharmacokinetics and efficacy of therapeutic agents in cancer treatment. In particular, transfection with exogenous nucleic acids has the potential to stimulate an immune response in the tumor microenvironment. Ultrasound-mediated gene transfection is a growing field, and recent work has incorporated this technique into cancer immunotherapy. Compared with other gene transfection methods, ultrasound-mediated gene transfection has a unique opportunity to augment the intracellular uptake of nucleic acids while safely and stably modulating the expression of immunostimulatory cytokines. The development and commercialization of therapeutic ultrasound systems further enhance the potential translation. In this Review, we introduce the underlying mechanisms and ongoing preclinical studies of ultrasound-based techniques in gene transfection for cancer immunotherapy. Furthermore, we expand on aspects of therapeutic ultrasound that impact gene therapy and immunotherapy, including tumor debulking, enhancing cytokines and chemokines and altering nanoparticle pharmacokinetics as these effects of ultrasound cannot be fully dissected from targeted gene therapy. We finally explore the outlook for this rapidly developing field.

Ultrasound and microbubbles to beat barriers in tumors: Improving delivery of nanomedicine

Successful delivery of drugs and nanomedicine to tumors requires a functional vascular network, extravasation across the capillary wall, penetration through the extracellular matrix, and cellular uptake. Nanomedicine has many merits, but penetration deep into the tumor interstitium remains a challenge. Failure of cancer treatment can be caused by insufficient delivery of the therapeutic agents. After intravenous administration, nanomedicines are often found in off-target organs and the tumor extracellular matrix close to the capillary wall. With circulating microbubbles, ultrasound exposure focused toward the tumor shows great promise in improving the delivery of therapeutic agents. In this review, we address the impact of focused ultrasound and microbubbles to overcome barriers for drug delivery such as perfusion, extravasation, and transport through the extracellular matrix. Furthermore, we discuss the induction of an immune response with ultrasound and delivery of immunotherapeutics. The review discusses mainly preclinical results and ends with a summary of ongoing clinical trials.

Therapeutic oxygen delivery by perfluorocarbon-based colloids

After the protocol-related indecisive clinical trial of Oxygent, a perfluorooctylbromide/phospholipid nanoemulsion, in cardiac surgery, that often unduly assigned the observed untoward effects to the product, the development of perfluorocarbon (PFC)-based O₂ nanoemulsions ("blood substitutes") has come to a low. Yet, significant further demonstrations of PFC O₂-delivery efficacy have continuously been reported, such as relief of hypoxia after myocardial infarction or stroke; protection of vital organs during surgery; potentiation of O₂-dependent cancer therapies, including radio-, photodynamic-, chemo- and immunotherapies; regeneration of damaged nerve, bone or cartilage; preservation of organ grafts destined for transplantation; and control of gas supply in tissue engineering and biotechnological productions. PFC colloids capable of augmenting O₂ delivery include primarily injectable PFC nanoemulsions, microbubbles and phase-shift nanoemulsions. Careful selection of PFC and other colloid components is critical. The basics of O₂ delivery by PFC nanoemulsions will be briefly reminded. Improved knowledge of O₂ delivery mechanisms has been acquired. Advanced, size-adjustable O₂-delivering nanoemulsions have been designed that have extended room-temperature shelf-stability. Alternate O₂ delivery options are being investigated that rely on injectable PFC-stabilized microbubbles or phase-shift PFC nanoemulsions. The latter combine prolonged circulation in the vasculature, capacity for penetrating tumor tissues, and acute responsiveness to ultrasound and other external stimuli. Progress in microbubble and phase-shift emulsion engineering, control of phase-shift activation (vaporization), understanding and control of bubble/ultrasound/tissue interactions is discussed. Control of the phase-shift event and of microbubble size require utmost attention. Further PFC-based colloidal systems, including polymeric micelles, PFC-loaded organic or inorganic nanoparticles and scaffolds, have been devised that also carry substantial amounts of O₂. Local, on-demand O₂ delivery can be triggered by external stimuli, including focused ultrasound irradiation or tumor microenvironment. PFC colloid functionalization and targeting can help adjust their properties for specific indications, augment their efficacy, improve safety profiles, and expand the range of their indications. Many new medical and biotechnological applications involving fluorinated colloids are being assessed, including in the clinic. Further uses of PFC-based colloidal nanotherapeutics will be briefly mentioned that concern contrast diagnostic imaging, including molecular imaging and immune cell tracking; controlled delivery of therapeutic energy, as for noninvasive surgical ablation and sonothrombolysis; and delivery of drugs and genes, including across the blood-brain barrier. Even when the fluorinated colloids investigated are designed for other purposes than O₂ supply, they will inevitably also carry and deliver a certain amount of O₂, and may thus be considered for O₂ delivery or co-delivery applications. Conversely, O₂-carrying PFC nanoemulsions possess by nature a unique aptitude for ¹⁹F MR imaging, and hence, cell tracking, while PFC-stabilized microbubbles are ideal resonators for ultrasound contrast imaging and can undergo precise manipulation and on-demand destruction by ultrasound waves, thereby opening multiple theranostic opportunities.

Subsequent Ultrasound Vascular Targeting Therapy of Hepatocellular Carcinoma Improves the Treatment Efficacy

The response of hepatocellular carcinoma (HCC) to anti-vascular ultrasound therapy (AVUS) can be affected by the inherent differences in tumor vascular structure, and the functionality of tumor vessels at the time of treatment. In this study, we evaluate the hypothesis that repeated subsequent AVUS therapies are a possible approach to overcome these factors and improve the therapeutic efficacy of AVUS. HCC was induced in 30 Wistar rats by oral ingestion of diethylnitrosamine (DEN) for 12 weeks. A total of 24 rats received treatment with low intensity, 2.8 MHz ultrasound with an intravenous injection of microbubbles. Treated rats were divided into three groups: single therapy group (ST), 2-days subsequent therapy group (2DST), and 7-days subsequent therapy group (7DST). A sham control group did not receive ultrasound therapy. Tumor perfusion was measured by quantitative contrast-enhanced ultrasound (CEUS) nonlinear and power-Doppler imaging. Tumors were harvested for histologic evaluation of ultrasound-induced vascular changes. ANOVA was used to compare the percent change of perfusion parameters between the four treatment arms. HCC tumors treated with 2DST showed the largest reduction in tumor perfusion, with 75.3% reduction on average for all perfusion parameters. The ST group showed an average decrease in perfusion of 54.3%. The difference between the two groups was significant p < 0.001. The 7DST group showed a reduction in tumor perfusion of 45.3%, which was significant compared to the 2DST group (p < 0.001) but not different from the ST group (p = 0.2). The use of subsequent targeted AVUS therapies applied shortly (two days) after the first treatment enhanced the anti-vascular effect of ultrasound. This gain, however, was lost for a longer interval (1 week) between the therapies, possibly due to tumor necrosis and loss of tumor viability. These findings suggest that complex interplay between neovascularization and tumor viability plays a critical role in treatment and, therefore, must be actively monitored following treatment by CEUS for optimizing sequential treatment.

Dose-dependent effects of ultrasound therapy on hepatocellular carcinoma

Non-invasive ischemic cancer therapy requires reduced blood flow whereas drug delivery and radiation therapy require increased tumor perfusion for a better response. In this study we investigate the hypothesis that different dose models of antivascular ultrasound therapy (AVUS) can have opposite effects on hepatocellular carcinoma (HCC) tumor blood flow. HCC was induced in 22 Wistar rats by ingestion of diethylnitrosamine (DEN) for 12 weeks. Rats received AVUS treatment at low and high doses. Low dose group received 1 watt/cm2 ultrasound for 1 min with 0.2 mL microbubbles injected IV. High dose group received 2 watts/cm2 for 2 min with 0.7 mL microbubbles IV. A sham group did not receive any treatment. Tumor perfusion was measured before and after AVUS with contrast-enhanced ultrasound. Quantitative perfusion measures: perfusion index (PI) and peak enhancement (PE) were obtained from each AVUS dose. After high-dose AVUS, PE and PI decreased by an average of 58.1 ± 4.9% and 49.1 ± 6.5 % respectively. Conversely, following low dose AVUS, PE and PI increased from baseline by an average of 47.8 ± 4.5% % and 20.3 ± 2.4 %, respectively. The high-dose AVUS therapy decreased tumoral perfusion, an effect that could be used for noninvasive ischemic therapy. Conversely, low-dose therapy increased tumor perfusion, which may improve drug delivery or radiation therapy. These opposite therapy effects can support multiple roles for AVUS in cancer therapy by tunable modulation of blood flow in tumors.

Ultrasound-Responsive Cavitation Nuclei for Therapy and Drug Delivery

Therapeutic ultrasound strategies that harness the mechanical activity of cavitation nuclei for beneficial tissue bio-effects are actively under development. The mechanical oscillations of circulating microbubbles, the most widely investigated cavitation nuclei, which may also encapsulate or shield a therapeutic agent in the bloodstream, trigger and promote localized uptake. Oscillating microbubbles can create stresses either on nearby tissue or in surrounding fluid to enhance drug penetration and efficacy in the brain, spinal cord, vasculature, immune system, biofilm or tumors. This review summarizes recent investigations that have elucidated interactions of ultrasound and cavitation nuclei with cells, the treatment of tumors, immunotherapy, the blood-brain and blood-spinal cord barriers, sonothrombolysis, cardiovascular drug delivery and sonobactericide. In particular, an overview of salient ultrasound features, drug delivery vehicles, therapeutic transport routes and pre-clinical and clinical studies is provided. Successful implementation of ultrasound and cavitation nuclei-mediated drug delivery has the potential to change the way drugs are administered systemically, resulting in more effective therapeutics and less-invasive treatments.

Ultrasound in tumor immunotherapy: Current status and future developments

Immunotherapy has considerable potential in eliminating cancers by activating the host's own immune system, while the thermal and mechanical effects of ultrasound have various applications in tumor therapy. Hyperthermia, ablation, histotripsy, and microbubble stable/inertial cavitation can alter the tumor microenvironment to enhance immunoactivation to inhibit tumor growth. Microbubble cavitation can increase vessel permeability and thereby improve the delivery of immune cells, cytokines, antigens, and antibodies to tumors. Violent microbubble cavitation can disrupt tumor cells and efficiently expose them to numerous antigens so as to promote the maturity of antigen-presenting cells and subsequent adaptive immune-cell activation. This review provides an overview and compares the mechanisms of ultrasound-induced immune modulation for peripheral and brain tumor therapy, even degenerative brain diseases therapy. The possibility of reversing tumors to an immunoactive microenvironment by utilizing the cavitation of microbubbles loaded with therapeutic gases is also proposed as another potential pathway for immunotherapy. Finally, we disuss the challenges and opportunities of ultrasound in immunotherapy for future development.

Microbubble-enhanced ultrasound for the antivascular treatment and monitoring of hepatocellular carcinoma

Background and Objective: Hepatocellular carcinoma (HCC) is the most common primary liver malignancy, and its current management relies heavily on locoregional therapy for curative therapy, bridge to transplant, and palliative therapy. Locoregional therapies include ablation and hepatic artery therapies such as embolization and radioembolization. In this study we evaluate the feasibility of using novel antivascular ultrasound (AVUS) as a noninvasive locoregional therapy to reduce perfusion in HCC lesions in a rat model and, monitor the effect with contrast-enhanced ultrasound imaging. Methods: HCC was induced in 36 Wistar rats by the ingestion of 0.01% diethylnitrosamine (DEN) for 12 weeks. Two therapy regimens of AVUS were evaluated. A primary regimen (n = 19) utilized 2-W/cm2, 3-MHz ultrasound (US) for 6 minutes insonation with 0.7 ml of microbubbles administered as an intravenous bolus. An alternate dose at half the primary intensity, sonication time, and contrast concentration was evaluated in 11 rats to assess the efficacy of a reduced dose. A control group (n = 6) received a sham therapy. Tumor perfusion was measured before and after AVUS with nonlinear contrast ultrasound (NLC) and power Doppler (PD). The quantitative perfusion measures included perfusion index (PI), peak enhancement (PE), time to peak (TTP), and perfusion area from NLC and PD scans. Total tumor area perfused during the scan was measured by a postprocessing algorithm called delta projection. Tumor histology was evaluated for signs of tissue injury and for vascular changes using CD31 immunohistochemistry. Results: DEN exposure induced autochthonous hepatocellular carcinoma lesions in all rats. Across all groups prior to therapy, there were no significant differences in the nonlinear contrast observations of peak enhancement and perfusion index. In the control group, there were no significant differences in any of the parameters after sham treatment. After the primary AVUS regimen, there were significant changes in all parameters (p ≤ 0.05) indicating substantial decreases in tumor perfusion. Peak enhancement in nonlinear contrast scans showed a 37.9% ± 10.1% decrease in tumor perfusion. Following reduced-dose AVUS, there were no significant changes in perfusion parameters, although there was a trend for the nonlinear contrast observations of peak enhancement and perfusion index to increase. Conclusion: This study translated low-intensity AVUS therapy into a realistic in vivo model of HCC and evaluated its effects on the tumor vasculature. The primary dose of AVUS tested resulted in significant vascular disruption and a corresponding reduction in tumor perfusion. A reduced dose of AVUS, on the other hand, was ineffective at disrupting perfusion but demonstrated the potential for enhancing tumor blood flow. Theranostic ultrasound, where acoustic energy and microbubbles are used to monitor the tumor neovasculature as well as disrupt the vasculature and treat lesions, could serve as a potent tool for delivering noninvasive, locoregional therapy for hepatocellular carcinoma.

Fluid flow rate dictates the efficacy of low-intensity anti-vascular ultrasound therapy in a microfluidic model

OBJECTIVE

Low-intensity anti-vascular ultrasound therapy is an effective means of disrupting the blood supply in the tumor microenvironment. Its diminished effect on the surrounding vasculature is thought to be due to higher blood flow rates outside the tumor that decreases the interaction time between the endothelial lining and the microbubbles, which transduce acoustic energy to thermal heat. However, investigating the effect of circulation rate on the response to low-intensity ultrasound is complicated by the heterogeneity of the in vivo vascular microenvironment. Here, a 3D microfluidic model is used to directly interrogate the dynamics of ultrasound stimulation.

METHODS

A 3D in vitro vessel consisting of LifeACT transfected endothelial cells facilitate real-time analysis of actin dynamics during ultrasound treatment. Using an integrated testing platform, both the flow rate of microbubbles within the vessel and the magnitude of insonation can be varied.

RESULTS

Morphological measurements and dextran transport assays indicate that lower flow rates exacerbate the effect of low-intensity ultrasound on vessel integrity. Additionally, immunostaining for VE-cadherin and transmission electron microscopy provide further insight into structural changes in cell-cell junctions following insonation.

CONCLUSIONS

Overall, these results reveal that blood flow rate is an important parameter to consider during the refinement of anti-vascular low-intensity ultrasound therapies.

Enhancing Checkpoint Inhibitor Therapy with Ultrasound Stimulated Microbubbles

Checkpoint inhibitor (CI) immunotherapy is playing an increasingly prominent role in the treatment of cancer but is effective and durable in only a subset of patients. There are concerted efforts to improve CI therapy through the use of multiple CIs or use of CIs in combination with other anti-cancer agents. Here we investigate the use of "anti-vascular" ultrasound-stimulated microbubble (USMB) treatments in combination with anti-PD-1 CI therapy. The colorectal cancer cell line CT26 was used to conduct longitudinal growth studies along with acute experiments to assess ultrasound-induced anti-tumor immune responses using flow cytometry and enzyme-linked immunospot (ELISPOT) analysis. Longitudinal experiments indicated that USMB + anti-PD-1 treatments significantly enhanced tumor growth inhibition and animal survival relative to monotherapies. Flow cytometry and ELISPOT data did not clearly support a T cell-dependent mechanism for the enhancement. Therefore, the results indicate the ability of anti-vascular USMBs to increase the anti-tumor effects of CI therapy; the specific mechanisms of enhancement remain to be established.

Engineering Theranostic Microbubbles Using Microfluidics for Ultrasound Imaging and Therapy: A Review

Microbubbles interact with ultrasound in various ways to enable their applications in ultrasound imaging and diagnosis. To generate high contrast and maximize therapeutic efficacy, microbubbles of high uniformity are required. Microfluidic technology, which enables precise control of small volumes of fluid at the sub-millimeter scale, has provided a versatile platform on which to produce highly uniform microbubbles for potential applications in ultrasound imaging and diagnosis. Here, we describe fundamental microfluidic principles and the most common types of microfluidic devices used to produce sub-10 μm microbubbles, appropriate for biomedical ultrasound. Bubbles can be engineered for specific applications by tailoring the bubble size, inner gas and shell composition and by functionalizing for additional imaging modalities, therapeutics or targeting ligands. To translate the laboratory-scale discoveries to widespread clinical use of these microfluidic-based microbubbles, increased bubble production is needed. We present various strategies recently developed to improve scale-up. We conclude this review by describing some outstanding problems in the field and presenting areas for future use of microfluidics in ultrasound.

Focused Ultrasound Immunotherapy for Central Nervous System Pathologies: Challenges and Opportunities

Immunotherapy is rapidly emerging as the cornerstone for the treatment of several forms of metastatic cancer, as well as for a host of other pathologies. Meanwhile, several new high-profile studies have uncovered remarkable linkages between the central nervous and immune systems. With these recent developments, harnessing the immune system for the treatment of brain pathologies is a promising strategy. Here, we contend that MR image-guided focused ultrasound (FUS) represents a noninvasive approach that will allow for favorable therapeutic immunomodulation in the setting of the central nervous system. One obstacle to effective immunotherapeutic drug delivery to the brain is the blood brain barrier (BBB), which refers to the specialized structure of brain capillaries that prevents transport of most therapeutics from the blood into brain tissue. When applied in the presence of circulating microbubbles, FUS can safely and transiently open the BBB to facilitate the delivery of immunotherapeutic agents into the brain parenchyma. Furthermore, it has been demonstrated that physical perturbations of the tissue microenvironment via FUS can modulate immune response in both normal and diseased tissue. In this review article, we provide an overview of FUS energy regimens and corresponding tissue bioeffects, followed by a review of the literature pertaining to FUS for therapeutic antibody delivery in normal brain and preclinical models of brain disease. We provide an overview of studies that demonstrate FUS-mediated immune modulation in both the brain and peripheral settings. Finally, we provide remarks on challenges facing FUS immunotherapy and opportunities for future expansion in this area.

Dynamic contrast enhanced MRI detects changes in vascular transport rate constants following treatment with thermally-sensitive liposomal doxorubicin

Temperature-sensitive liposomal formulations of chemotherapeutics, such as doxorubicin, can achieve locally high drug concentrations within a tumor and tumor vasculature while maintaining low systemic toxicity. Further, doxorubicin delivery by temperature-sensitive liposomes can reliably cure local cancer in mouse models. Histological sections of treated tumors have detected red blood cell extravasation within tumors treated with temperature-sensitive doxorubicin and ultrasound hyperthermia. We hypothesize that the local release of drug into the tumor vasculature and resulting high drug concentration can alter vascular transport rate constants along with having direct tumoricidal effects. Dynamic contrast enhanced MRI (DCE-MRI) coupled with a pharmacokinetic model can detect and quantify changes in such vascular transport rate constants. Here, we set out to determine whether changes in rate constants resulting from intravascular drug release were detectable by MRI. We found that the accumulation of gadoteridol was enhanced in tumors treated with temperature-sensitive liposomal doxorubicin and ultrasound hyperthermia. While the initial uptake rate of the small molecule tracer was slower (k₁=0.0478±0.011s^-1 versus 0.116±0.047s^-1) in treated compared to untreated tumors, the tracer was retained after treatment due to a larger reduction in the rate of clearance (k₂=0.291±0.030s^-1 versus 0.747±0.24s^-1). While DCE-MRI assesses a combination of blood flow and permeability, ultrasound imaging of microvascular flow rate is sensitive only to changes in vascular flow rate; based on this technique, blood flow was not significantly altered 30min after treatment. In summary, DCE-MRI provides a means to detect changes that are associated with treatment by thermally-activated particles and such changes can be exploited to enhance local delivery.

The effects of percutaneous ethanol injection followed by 20-kHz ultrasound and microbubbles on rabbit hepatic tumors

OBJECTIVE

Low-frequency ultrasound (US) in combination with microbubbles (MBs) is able to inhibit the growth of VX2 rabbit liver tumors. In this study, we investigated the feasibility of using percutaneous ethanol injection (PEI) followed by low-frequency ultrasound and microbubbles (USMB) to inhibit VX2 tumor growth.

METHODS

Eighteen New Zealand rabbits with hepatic VX2 tumors were divided into three groups: PEI, low-frequency ultrasound and MBs followed by PEI (USMB + PEI), and PEI followed by USMB (PEI + USMB). PEI was performed by ultrasound-guided injection of 95% anhydrous alcohol into internal liver tumors in rabbits twice a week for 2 weeks. The US parameters were 20 kHz, 2 W/cm(2), 40% duty cycle, 5 min, and once every other day for 2 weeks. Magnetic resonance imaging (MRI) was used to observe tumors before and after treatment, to examine changes in the tumors, and to measure the diameters of the tumors. All animals were followed up for 180 days after tumor implantation. Autopsy was performed at the end of the scheduled follow-up or immediately after death. Anatomically observed metastatic sites included the liver, lung, abdomen, and pelvic cavity. The survival time of all rabbits was recorded.

RESULTS

After 4 weeks of treatment, on MRI, the tumor diameters in the PEI, USMB + PEI, and PEI + USMB groups were 8.33 ± 1.83, 19 ± 2.61, and 4.5 ± 1.22 mm, respectively. There was a significant difference in tumor size indicated by MRI in the three groups. Tumor size was smaller in the PEI + USMB group than in the PEI and USMB + PEI groups, with t = 4.54, p = 0.0062, and t = 16.38, p < .0001, respectively. The PEI + USMB group showed the fewest metastasis sites (χ(2) = 11.7333, p = 0.0194) and the longest survival period (χ(2) = 7.448, p = 0.0241).

CONCLUSION

Percutaneous ethanol injection followed by low-frequency ultrasound and microbubbles can be effective in inhibiting rabbit liver tumors and prolonging survival time.

A review of low-intensity ultrasound for cancer therapy

The literature describing the use of low-intensity ultrasound in four major areas of cancer therapy-sonodynamic therapy, ultrasound-mediated chemotherapy, ultrasound-mediated gene delivery and anti-vascular ultrasound therapy-was reviewed. Each technique consistently resulted in the death of cancer cells, and the bio-effects of ultrasound were attributed primarily to thermal actions and inertial cavitation. In each therapeutic modality, theranostic contrast agents composed of microbubbles played a role in both therapy and vascular imaging. The development of these agents is important as it establishes a therapeutic-diagnostic platform that can monitor the success of anti-cancer therapy. Little attention, however, has been given either to the direct assessment of the mechanisms underlying the observed bio-effects or to the viability of these therapies in naturally occurring cancers in larger mammals; if such investigations provided encouraging data, there could be prompt application of a therapy technique in the treatment of cancer patients.