Microbubble-Enhanced Heating: Exploring the Effect of Microbubble Concentration and Pressure Amplitude on High-Intensity Focused Ultrasound Treatments

Current Research in Drug-Free Cancer Therapies

Cancer treatment has historically depended on conventional methods like chemotherapy, radiation, and surgery; however, these strategies frequently present considerable limitations, including toxicity, resistance, and negative impacts on healthy tissues. In addressing these challenges, drug-free cancer therapies have developed as viable alternatives, utilizing advanced physical and biological methods to specifically target tumor cells while reducing damage to normal tissues. This review examines several drug-free cancer treatment strategies, such as high-intensity focused energy beams, nanosecond pulsed electric fields, and photothermal therapy as well as the use of inorganic nanoparticles to promote selective apoptosis. We also investigate the significance of targeting the tumor microenvironment, precision medicine, and immunotherapy in the progression of personalized cancer therapies. Although these approaches demonstrate significant promise, challenges including scalability, safety, and regulatory obstacles must be resolved for clinical application. This paper presents an overview of current research in drug-free cancer therapies, emphasizing recent advancements, underlying scientific principles, and the steps required for clinical implementation.

Delivery of Cavitation Therapy With a Modified Clinical Scanner: In Vitro Evaluation

In this study, we design and implement pulses [1.67 MHz, 20-1000 cycles, 0.8-2.5 MPa, and 5-100 ms pulse repetition time (PRT)] suitable for microbubble cavitation treatments with a phased array of a clinical ultrasound scanner. A range of acoustic parameters was evaluated in a tissue-mimicking phantom with suspended Sonazoid microbubbles. Hydrophone measurements were used to optimize the transmit beamforming. A passive cavitation detection (PCD) system was designed to measure the microbubble scattered signals over a 1 s exposure. Postprocessing of the scattered signals evaluated frequency content to extract broadband energy and calculate the inertial cavitation dose (ICD). ICD was maximized at 1000 cycles (maximum pulse length), 5 ms (fastest firing rate), and 2.5 MPa peak negative pressure (PNP) (maximum pressure). Inertial cavitation was only sustained for about three pulses (out of hundreds fired) occurring within the first 100 ms of treatment. Temporal analysis of the first 1000-cycle pulse revealed that broadband energy is sustained for the entire pulse. We also demonstrate that while inertial cavitation is possible with clinically available pulse wave Doppler settings, ICD can be significantly increased using the new conditions suggested in this work. We have delivered successful image-guided cavitation treatment after modifying a clinical scanner and monitored the cavitation dose with a PCD system on a gel phantom with suspended microbubbles. We plan to apply this technique in vivo in animal tumor models next. This work demonstrates the first implementation of long, high-pressure pulses on a clinical scanner that users can optimize for cavitation treatments.

Biofluid-Permeable and Erosion-Resistant Wireless Neural-Electronic Interfaces for Neurohomeostasis Modulation

Neural-electronic interfaces through delivering electroceuticals to lesions and modulating pathological endogenous electrical environments offer exciting opportunities to treat drug-refractory neurological disorders. Such an interface should ideally be compatible with the neural tissue and aggressive biofluid environment. Unfortunately, no interface specifically designed for the biofluid environments is available so far; instead, simply stacking an encapsulation layer on silicon-based substrates makes them susceptible to biofluid leakage, device malfunction, and foreign-body reactions. Here, we developed a biofluid-permeable and erosion-resistant wireless neural-electronic interface (BNEI) that is composed of a flexible 3D interconnected poly(l-lactide) fibrous network with a dense and axially aligned piezoelectrical molecular chain arrangement architecture. The organized molecular chain structure enhances the tortuous pathway and longitudinal piezoelectric coefficient of poly(l-lactide) fibers, improves their water barrier properties, and enables efficient conversion of low-intensity acoustic vibrations transmitted in biofluids into electrical signals, achieving long-term stable and wireless neuromodulation. A 3-month clinical trial demonstrated that the BNEI can effectively accelerate the pathological cascade in peripheral neuropathy for nerve regeneration and transcranially modulate cerebellar-cerebral circuit dynamics, suppressing seizures in temporal lobe epilepsy. The BNEI can be a clinically scalable approach for wireless neuromodulation that is broadly applicable to the modulation of neurohomeostasis in both the peripheral and central nervous systems.

Ultrasound Molecular Imaging Enhances High-Intensity Focused Ultrasound Ablation on Liver Cancer With B7-H3-Targeted Microbubbles

BACKGROUND

High-intensity focused ultrasound (HIFU) is a promising minimally invasive treatment for liver cancer; however, its efficacy is often limited by the attenuation of ultrasonic energy. This study investigates the effectiveness of B7-H3-targeted microbubbles (T-MBs) in enhancing HIFU ablation of liver cancer and explores their potential for clinical translation.

METHODS

T-MBs and isotype control microbubbles (I-MBs) were synthesized through the conjugation of biotinylated anti-B7-H3 antibody and isotype control antibody to the microbubble surface, respectively. Contrast-enhanced ultrasound imaging was performed to compare the accumulation of T-MBs and I-MBs in liver cancer at various time points. The efficacy of T-MBs in enhancing HIFU treatment was evaluated by measuring the immediate tumor ablation rate and long-term tumor growth suppression. Additionally, the induced antitumor immune response was assessed through cytokine quantification in serum and tumor tissue, along with immunofluorescence staining conducted on days 1, 3, and 7 post-treatment.

RESULTS

T-MBs demonstrated superior liver cancer-specific accumulation, characterized by higher concentrations and prolonged retention compared to I-MBs. The combination of T-MBs with HIFU resulted in significantly enhanced tumor ablation rates and superior tumor growth suppression. Post-treatment analysis revealed a gradual uptick in cytokine levels within the tumor microenvironment, along with progressive infiltration of antitumor immune cells.

CONCLUSION

T-MBs effectively enhance the therapeutic efficacy of HIFU for liver cancer treatment while simultaneously promoting an antitumor immune response. These findings provide a strong experimental foundation for the clinical translation of ultrasound molecular imaging combined with HIFU as a novel approach for tumor therapy.

Microbubble-enhanced transcranial MR-guided focused ultrasound brain hyperthermia: heating mechanism investigation using finite element method

Recently, our group developed a synergistic brain drug delivery method to achieve simultaneous transcranial hyperthermia and localized blood-brain barrier opening via MR-guided focused ultrasound (MRgFUS). In a rodent model, we demonstrated that the ultrasound power required for transcranial MRgFUS hyperthermia was significantly reduced by injecting microbubbles (MBs). However, the specific mechanisms underlying the power reduction caused by MBs remain unclear. The present study aims to elucidate the mechanisms of MB-enhanced transcranial MRgFUS hyperthermia through numerical studies using the finite element method. The microbubble acoustic emission (MAE) and the viscous dissipation (VD) were hypothesized to be the specific mechanisms. Acoustic wave propagation was used to model the FUS propagation in the brain tissue, and a bubble dynamics equation for describing the dynamics of MBs with small shell thickness was used to model the MB oscillation under FUS exposures. A modified bioheat transfer equation was used to model the temperature in the rodent brain with different heat sources. A theoretical model was used to estimate the bubble shell's surface tension, elasticity, and viscosity losses. The simulation reveals that MAE and VD caused a 40.5% and 52.3% additional temperature rise, respectively. Compared with FUS only, MBs caused a 64.0% temperature increase, which is consistent with our previous animal experiments. Our investigation showed that MAE and VD are the main mechanisms of MB-enhanced transcranial MRgFUS hyperthermia.

Interaction Between Microbubbles and Microwave Ablation: A Phantom and Rabbit Model

OBJECTIVES

This study aimed to explore the interactions between microbubbles and microwave ablation (MWA).

METHODS

The study employed custom-made phantoms (in vitro) and white New Zealand rabbits (in vivo). MWA was performed with or without microbubbles in the phantoms (2 × 105 particles mL-1) and rabbit livers (intravenous injection of 0.05 mL/kg SonoVue). During the MWA, K-type thermocouple probes were used to monitor the MWA-induced temperature increase. Contrast-enhanced ultrasound imaging (CEUS) was used to monitor and analyze the microbubbles signal intensity. After MWA, the ablation-zone volumes were evaluated and compared between the groups with and without microbubbles.

RESULTS

In both the phantom models and rabbits, microbubbles showed no significant influence on MWA, including the ablation range and MWA-induced temperature increase. In phantoms and rabbit livers filled with microbubbles, MWA caused the formation of a gradually expanding microbubble-defect region over the ablation time. An increase in the temperature caused microbubble destruction.

CONCLUSIONS

Microbubbles had no significant influence on MWA. However, MWA induced the destruction of microbubbles in a temperature-dependent manner. Thus, the poor thermotolerance of microbubbles is a non-negligible barrier when using CEUS to monitor the ablation range during MWA in real-time.

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.

Sono-activated materials for enhancing focused ultrasound ablation: Design and application in biomedicine

The ablation effect of focused ultrasound (FUS) has played an increasingly important role in the biomedical field over the past decades, and its non-invasive features have great advantages, especially for clinical diseases where surgical treatment is not available or appropriate. Recently, rapid advances in the adjustable morphology, enzyme-mimetic activity, and biostability of sono-activated materials have significantly promoted the medical application of FUS ablation. However, a systematic review of sono-activated materials based on FUS ablation is not yet available. This progress review focuses on the recent design, fundamental principles, and applications of sono-activated materials in the FUS ablation biomedical field. First, the different ablation mechanisms and the key factors affecting ablation are carefully determined. Then, the design of sono-activated materials with high FUS ablation efficiencies is comprehensively discussed. Subsequently, the representative biological applications are summarized in detail. Finally, the primary challenges and future perspectives are also outlined. We believe this timely review will provide key information and insights for further exploration of focused ultrasound ablation and new inspiration for designing future sono-activated materials. STATEMENT OF SIGNIFICANCE: The ablation effect of focused ultrasound (FUS) has played an increasingly important role in the biomedical field over the past decades. However, there are also some challenges of FUS ablation, such as skin burns, tumour recurrence after thermal ablation, and difficulty in controlling cavitation ablation. The rapid advance in adjustable morphology, enzyme-mimetic activity, and biostability of sono-activated materials has significantly promoted the medical application of FUS ablation. However, the systematic review of sono-activated materials based on FUS ablation is not yet available. This progress review focuses on the recent design, fundamental principles, and applications in the FUS ablation biomedical field of sono-activated materials. We believe this timely review will provide key information and insights for further exploration of FUS ablation.

Safety of Sonazoid in Assisting High-Intensity Focused Ultrasound Ablation Therapy for Advanced Liver Malignant Lesions: A Single-Arm Clinical Study

OBJECTIVE

The aim of the study described here was to evaluate the safety of Sonazoid-assisted high-intensity focused ultrasound (HIFU) in the treatment of advanced malignant liver lesions.

METHODS

A single-arm study was designed to enroll participants who were diagnosed with advanced primary liver cancer or liver metastases and proposed to receive Sonazoid assistance during HIFU treatment. Serological examination was conducted within 1 wk, and side effects in each patient were monitored for 1 mo. To evaluate therapeutic efficacy, the contrast-enhanced magnetic resonance imaging was performed 1 mo after treatment, and short-term follow-up was conducted a year later.

RESULTS

A total of 17 participants (12 male, 5 female) with an average age of 58 y (range: 46-73 y) were enrolled, including 11 patients with hepatocellular carcinoma, 2 patients with hepatic metastasis and 4 patients with cholangiocarcinoma. The total volume of tumor mass was 111.82 (11.01-272.30) cm3. The average total ablation time for a patient was 2021 ± 1030 s, and the energy efficiency factor was 5979.7 (3108.0, 45634.5) J/cm3. Immediately after HIFU treatment, 1 patient (5.9%) achieved complete response (CR), 4 patients (23.5%) had a moderate response, 8 patients (47.1%) had partial reperfusion and 4 patients (23.5%) had stable disease (SD). The average ablation rate for all the tumors was 51.5 ± 26.7%. The level of glutamic-pyruvic transaminase (ALT) was mildly increased in 71.6% (12/17) of patients after HIFU therapy. Mean ALT values before and after treatment were 22 (14, 35) U/L and 36 (25, 41) U/L, respectively (Z = 1.947, p = 0.051). Mild or obvious edema in skin and subcutaneous soft tissues were observed in 76.5% of patients, but no serious side effects were found. Twelve months after treatment, the follow-up results revealed that 1 patient (5.8%) achieved a CR, 8 patients (47.1%) had SD and 8 patients (47.1%) had progressive disease. The estimated median time to progression was 11 mo after treatment, with a 95% confidence interval of 6, 11 for all involved patients.

CONCLUSION

Use of Sonazoid is safe and feasible for improving HIFU ablation efficiency during the treatment of advanced malignant liver lesions. The therapeutic efficacy of Sonazoid-assisted HIFU needs to be explored in additional controlled clinical investigations.

Recent advances towards overcoming the blood-brain barrier

The blood-brain barrier (BBB) is a protective element of the neurovascular unit (NVU) surrounded by astrocytes, pericytes, extracellular matrix, and the tight junctional complex, which play a fundamental role in brain homeostasis. Due to its impeccable structural architecture, the BBB is referred to as the brain's gatekeeper, a near-impenetrable barrier to therapeutics. This review summarises the significant strides that have been made in the last 5 years towards circumventing the BBB and developing efficient drug delivery systems. Challenges associated with several CNS disorders related to BBB failure and exploitation of this unique NVU component for targeted treatment of brain-related disorders are also discussed.

Controlled Hyperthermia With High-Intensity Focused Ultrasound and Ultrasound Contrast Agent Microbubbles in Porcine Liver

OBJECTIVE

The objective of this work was to study microbubble-enhanced temperature elevation with high-intensity focused ultrasound (HIFU) at different acoustic pressures and under image guidance. The microbubbles were administered with either local or vascular injections (that mimic systemic injections) in perfused and non-perfused ex vivo porcine liver under ultrasound image guidance.

METHODS

Porcine liver was insonified for 30 s with a single-element HIFU transducer (0.9 MHz, 0.413 ms, 82% duty cycle, focal pressures of 0.6-3.5 MPa). Contrast microbubbles were injected either locally or through the vasculature. A needle thermocouple at the focus measured temperature elevation. Diagnostic ultrasound (Philips iU22, C5-1 probe) guided placement of the thermocouple and delivery of microbubbles and monitored the procedure in real time.

RESULTS

At lower acoustic pressures (0.6 and 1.2 MPa) in non-perfused liver, inertial cavitation of the injected microbubbles led to greater temperatures at the focus compared with HIFU-only treatments. At higher pressures (2.4 and 3.5 MPa) native inertial cavitation in the tissue (without injecting microbubbles) resulted in temperature elevations similar to those after injecting microbubbles. The heated area was larger when using microbubbles at all pressures. In the presence of perfusion, only local injections provided a sufficiently high concentration of microbubbles necessary for significant temperature enhancement.

CONCLUSION

Local injections of microbubbles provide a higher concentration of microbubbles in a smaller area, avoiding acoustic shadowing, and can lead to higher temperature elevation at lower pressures and increase the size of the heated area at all pressures.

Magnetic Nanoparticle-Mediated Heating for Biomedical Applications

Magnetic nanoparticles, especially superparamagnetic nanoparticles (SPIONs), have attracted tremendous attention for various biomedical applications. Facile synthesis and functionalization together with easy control of the size and shape of SPIONS to customize their unique properties, have made it possible to develop different types of SPIONs tailored for diverse functions/applications. More recently, considerable attention has been paid to the thermal effect of SPIONs for the treatment of diseases like cancer and for nanowarming of cryopreserved/banked cells, tissues, and organs. In this mini-review, recent advances on the magnetic heating effect of SPIONs for magnetothermal therapy and enhancement of cryopreservation of cells, tissues, and organs, are discussed, together with the non-magnetic heating effect (i.e., high Intensity focused ultrasound or HIFU-activated heating) of SPIONs for cancer therapy. Furthermore, challenges facing the use of magnetic nanoparticles in these biomedical applications are presented.

Enhanced HIFU Theranostics with Dual-Frequency-Ring Focused Ultrasound and Activatable Perfluoropentane-Loaded Polymer Nanoparticles

High-intensity focused ultrasound (HIFU) has been widely used in tumor ablation in clinical settings. Meanwhile, there is great potential to increase the therapeutic efficiency of temporary cavitation due to enhanced thermal effects and combined mechanical effects from nonlinear vibration and collapse of the microbubbles. In this study, dual-frequency (1.1 and 5 MHz) HIFU was used to produce acoustic droplet vaporization (ADV) microbubbles from activatable perfluoropentane-loaded polymer nanoparticles (PFP@Polymer NPs), which increased the therapeutic outcome of the HIFU and helped realize tumor theranostics with ultrasound contrast imaging. Combined with PFP@Polymer NPs, dual-frequency HIFU changed the shape of the damage lesion and reduced the acoustic intensity threshold of thermal damage significantly, from 216.86 to 62.38 W/cm². It produced a nearly 20 °C temperature increase in half the irradiation time and exhibited a higher tumor inhibition rate (84.5% ± 3.4%) at a low acoustic intensity (1.1 MHz: 23.77 W/cm²; 5 MHz: 0.35 W/cm²) in vitro than the single-frequency HIFU (60.2% ± 11.9%). Moreover, compared with the traditional PFP@BSA NDs, PFP@Polymer NPs showed higher anti-tumor efficacy (81.13% vs. 69.34%; * p < 0.05) and better contrast-enhanced ultrasound (CEUS) imaging ability (gray value of 57.53 vs. 30.67; **** p < 0.0001), probably benefitting from its uniform and stable structure. It showed potential as a highly efficient tumor theranostics approach based on dual-frequency HIFU and activatable PFP@Polymer NPs.

The Role of Ultrasound in Modulating Interstitial Fluid Pressure in Solid Tumors for Improved Drug Delivery

The unique microenvironment of solid tumors, including desmoplasia within the extracellular matrix, enhanced vascular permeability, and poor lymphatic drainage, leads to an elevated interstitial fluid pressure which is a major barrier to drug delivery. Reducing tumor interstitial fluid pressure is one proposed method of increasing drug delivery to the tumor. The goal of this topical review is to describe recent work using focused ultrasound with or without microbubbles to modulate tumor interstitial fluid pressure, through either thermal or mechanical effects on the extracellular matrix and the vasculature. Furthermore, we provide a review on techniques in which ultrasound imaging may be used to diagnose elevated interstitial fluid pressure within solid tumors. Ultrasound-based techniques show high promise in diagnosing and treating elevated interstitial pressure to enhance drug delivery.