Nanomaterials for Ultrasound Imaging- Guided Sonodynamic Therapy

Abstract

Ultrasound examination is becoming the most popular medical imaging modality because of its low cost and high safety profile. Ultrasound contrast agents enhance the scattering of sound waves, which can improve the clarity and resolution of images. Nanoparticle Ultrasound contrast agents have the characteristics of a large specific surface area and a modifiable surface, which can increase drug loading capacity, prolong circulation time, and enable drug enrichment in specific organs or tissues. This leads to improved therapeutic effects and reducing toxic and side effects. Compared with traditional ultrasound contrast agents, Nano-ultrasound contrast agents overcome the limitation of imaging solely within blood vessels and facilitate imaging within tumor tissues, thereby extending the duration of enhanced imaging. Sonodynamic therapy is an emerging treatment method that has been developed rapidly in recent years, which has the advantages of noninvasive, high spatial and temporal resolution, and low toxicity and side effects. Sonodynamic therapy utilizes a sonosensitizer that, when excited by ultrasound at the tumor site, produces toxic reactive oxygen species, inducing apoptosis or necrosis in tumor cells. Ultrasound-guided sonodynamic therapy allows for real-time observation of lesions, is convenient and flexible, and is free of radiation exposure. With the use of nanomaterials as carriers, ultrasound-guided sonodynamic therapy has made significant strides. This study categorizes and summarizes the current research on acoustic sensitizer carrier materials, including carbon-based, silicon-based, peptide-based, iron-based, metal-organic frameworks, polymers, and liposomes. It concludes by highlighting the current challenges in the integration of ultrasound imaging with sonodynamic therapy and suggests future directions for clinical application development.

Introduction

Ultrasonography is emerging as the most popular medical imaging examination because of its low cost and high safety. With the development of nanomaterials, ultrasound has a broader application prospect. ¹ The evolution of nanomaterials has broadened the scope of ultrasound applications, particularly with the advent of Nanoparticle Ultrasound Contrast Agents (Nano-USCA), which have garnered significant interest. These agents are especially promising in the realms of vascular imaging and targeted drug delivery for cancer treatment. They have the potential to enhance the efficacy of sonodynamic therapy (SDT) while minimizing systemic side effects. Figure 1 illustrates the schematic diagram of the classification and application of various new nanomaterials, including the ability of SDT imaging. The advent of USCA has expedited the use of ultrasound in disease diagnosis and treatmen. ² Figure 2A to C demonstrates that nanoparticles (NPs) can inhibit the growth of tumor cells. Microbubble (MB) contrast agents provide high contrast by scattering ultrasound waves, but their large size restricts their application. They cannot penetrate the interstitial spaces of vascular endothelial cells, being visible only within blood vessels if larger than 700 nanometers. ³ The development of nanomaterials may pave new avenues for ultrasound imaging. These materials offer several advantages, including smaller volumes, larger surface areas, and improved targeting capabilities. Consequently, they can provide greater permeability and enhance the effect known as Enhanced Permeability and Retention (EPR). ⁴ The mechanical effects of ultrasound, such as cavitation, can enhance cell membrane permeability, widen endothelial cell gaps, and thus potentially improve therapeutic outcomes. ⁵ Under low-intensity ultrasound, acoustic sensitizers are activated to generate reactive oxygen species (ROS), a process that can lead to cytotoxic effects and is recognized as SDT. The prevailing understanding of the mechanism behind acoustic dynamics is centered on the effects of ultrasound cavitation. This includes phenomena such as sonoluminescence, the production of ROS, and mechanical damage, all of which are areas that have been evolving with the development of nanomaterials. Sonodynamic therapy is widely used in clinical conditions such as cancer, bacterial infections, and vascular diseases.^6–8 Figure 2D and E depicts the characterization of mesoporous silica (MSN), MSN-NH₂, M@P, and M@P-Fe. We provide a review of the nanomaterials associated with ultrasound imaging and acoustic sensitizer binding.

Figure 1.

Schematic diagram of the classification and application of various new nanomaterials, including the ability of SDT imaging. US, ultrasonic; PFH, perfluorohexane; PLGA, poly(lactic-co-glycolic acid); SDT, sonodynamic therapy.

Figure 2.

Demonstration of the synthesis, preparation and cellular uptake of some new nanomaterials. (A) In vitro and in vivo US imaging effects of nanoparticles. Reproduced with permission. ² (B) Cell viability assessment of 4T1 cells after various treatments after 24 h by CCK-8 assay. Reproduced with permission. ² (C) HE, TUNEL, and Ki67 immunohistochemical staining of resected tumors after the end of treatment. Reproduced with permission. ² (D) Hydrodynamic size distributions of MSN, MSN-NH₂, M@P, and M@P-Fe. Reproduced with permission. ⁸ (E) Fluorescence intensity of DCFH-DA in the presence of different materials upon US irradiation. Reproduced with permission. ⁸ MSN, mesoporous silica.

With the development of nanomedicine and ultrasound diagnosis and therapy, researchers have turned their attention to an integrated nano platform capable of simultaneous tumor ultrasound diagnosis and SDT. The nanoplatforms encompass diagnostic diagnostic molecules, which are often various gas molecules or gas-producing substances used for ultrasound imaging of tumors, and therapeutic molecules, which are typically acoustic sensitizers used for SDT of tumors. A variety of nanomaterials hold significant potential in both ultrasound imaging and SDT applications. Table 1 offers a detailed description of the different types of nanomaterials used in ultrasound-guided SDT.

Table 1.

Several New Types of Nanomaterials and Their Composition, Size, Imaging Methods, and Cell Killing Methods.

System	Acoustic sensitizer	Imaging methods	Therapy	Diseases type	Nanoparticle size	Targeting receptor	Ref
Zr-MOF@AIPH	Zr-MOF	PA, FL, US	SDT	Prostate cancer	138nm	Produce ROS	9
DHMS	DHMS	MRI, US	SDT	Breast cancer	160nm	Produce ROS	10
OLI_NPs	ICG	PA, US	PDT, SDT	Cervical cancer	195.24nm	TLR4	11
Col-H-TiO2	H-TiO2	US	SDT	Pancreatic cancer	100nm	Produce ROS	12
PD-L1 mAb/DOX-NBs	DOX	US	Immune therapy, Chemotherapy, SDT	Hepatocellular carcinoma	457.1nm	PD-L1 antibody	13
α-Fe2O3@Pt	α-Fe2O3	US	SDT	Breast cancer	85nm	Release ROF	14
CPDP NPs	Ce 6	US	Chemotherapy, SDT	Breast cancer	317.3nm	—	15
RB-MB	ARB	FL, US	SDT	Adenocarcinoma of the colon	100nm	Release ROS	16
PFP@tLyP-1-LIP-H(Gd)	H(Gd)	MRI,PA, US, NIRF	SDT	Breast cancer	260.9nm	—	17
PHPMR NPs	HMME	MRI, PA, US	SDT	Plaque neovascularization	295nm	—	18

Abbreviations: Zr-MOF, zirconium-based metal-organic framework; AIPH, 2,2'-Azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride; PA, photoacoustic; FL, fluorescent; DHMS, double-layer hollow manganese silicate; MRI, magnetic resonance imaging; OLI_NPs, oxygen-carried and lipopolysaccharide / indocyanine green-loaded nanoparticles; ICG, indocyanine green; PDT, photo dynamic therapy; TLR4, Toll-like receptor 4; DOX, doxorubicin; PD-L1mAb, programmed death ligand 1 monoclonal antibody; PFP, perfluoropentane; NIRF, near-infrared fluorescence; HMME, hematoporphyrin monomethyl ether; PHPMR, PFP-HMME@PLGA/MnFe₂O₄-ramucirumab; Pt, platinum; RB, Rose bengal; MB, microbubbles; ARB, amphiphilic rose Bengal; SDT, sonodynamic therapy; ROS, reactive oxygen species.

Application of Different Nanomaterials as Carriers in Ultrasound Imaging-Guided SDT

Sonodynamic therapy offers several advantages, including being noninvasive, real-time, and safe. Acoustic sensitizers, with their high stability and superior physicochemical properties, have a wide range of promising SDT applications, and attended widespreadly in the diagnosis and treatment of diseases.

Carbon-Based Nanomaterials as a Carrier

Among the array of nanomaterials, carbon-based nanomaterials are among the most appealing options for cancer therapeutic diagnostics due to their distinctive physicochemical properties. ¹⁹ Figure 3A depicts a schematic diagram outlining the synthesis of fluorinated graphene oxide-poly(ethylene glycol)-folic acid (FGO-PEG-FA), which is employed for synergistic cancer therapy and for the targeted delivery of DOX and carbon nanotubes to cells. Such carbon-based nanomaterials, including carbon nanotubes (CNTs), carbon dots, quartz, and quartz oxides, are utilized for the delivery of various contrast and imaging agents for cell tracking. This is attributed to their strong near-infrared light absorption capabilities, extensive surface area, and favorable modifiability. ²⁰ Carbon nanotubes are among the most extensively studied carbon-based nanomaterials and are instrumental in cancer therapy and diagnosis. This is due to their superior optical properties and an ultra-high surface area that facilitates drug loading. ²¹ Furthermore, the chemical and physical properties of CNTs potentially enable them to exfiltrate and reach the tumor region. ²² In comparison to most current USCA, CNTs are highly effective for imaging superficial tissues. ²³ Additionally, the carbon-based NPs encapsulated within CNTs can significantly amplify the ultrasound signal and extend echo duration, leading to enhanced imaging results. ²⁴ In addition, CNTs have also demonstrated promising applications in SDT. Daneshvar et al have designed a nanocomposite consisting of CNTs and polypyrrole (PPy) to serve as a nanosensitizer for melanoma cancer therapy. Experimental results indicate that after a 10-day treatment period in animals, four-step mesotherapy irradiation of PPy-encapsulated CNTs at 1-h intervals resulted in 75% necrosis of tumor tissues, a 50% reduction in tumor volume, and a significant increase in ROS within the tumor. ²⁵ Functionalized multi-walled CNTs (MWCNTs) can be utilized as a highly echogenic material. Studies have shown that functionalized MWCNTs not only produce a higher ultrasound signal compared to graphene oxide, pristine MWCNTs, and functionalized single-walled CNTs, which are extensively used for ultrasound imaging, but they also offer improved visualization and precision. Moreover, they can target prostate cancer cells more effectively. ²⁶ In addition, MWCNTs have been found to act as carriers when coupled with FA, which aids in targeted delivery to tumor sites and enhances bioavailability. ²⁷ Figure 3B illustrates cellular uptake at varying concentrations, as detected by flow cytometry and confocal laser scanning microscopy.

Figure 3.

Preparation of multifunctional nanomaterial FGO-PEG-FA and demonstration of uptake effect of nanomaterial CLSM. (A) Schematic representation of the synthesis of FGO-PEG-FA for targeting and delivery of DOX and CPT in cancer synergistic therapy. Reprinted with permission. ¹⁹ (B) Flow cytometry of cellular uptake of different concentrations and CLSM of cellular uptake. Reprinted with permission. ²⁸ CLSM, confocal laser scanning microscopy.

In the realm of oncological treatment, the amalgamation of nanotechnology with perfluorocarbons and sonosensitizers signifies a revolutionary step forward in SDT. This integration not only mitigates the limitations of sonosensitizers, such as poor tumor specificity, poor water solubility, and light sensitivity to a certain degree, but it also enables ultrasound-guided SDT.

Metal-Based Nanomaterials as Carriers

Metallic nanomaterials are extensively utilized in SDT and ultrasound imaging. Certain transition metal oxides, such as those of iron and titanium, can serve as acoustic sensitizers for SDT. When modified with a gas-producing substance, these nanomaterials can be employed for ultrasound-guided SDT. ²⁹ Zhang et al synthesized α-Fe₂O₃ NPs via a one-pot hydrothermal method. They then mixed these with a pre-prepared H₂PtCl₆ solution to grow Pt nanocrystals in situ on the α-Fe₂O₃ surface. The particles were subsequently modified with PEG-SH to produce α-Fe₂O₃@Pt heterostructured particles. The unique heterostructure of these particles enables the induction of a significant amount of singlet oxygen (¹O₂) under ultrasonic irradiation, as it suppresses the recombination of acoustic electrons and holes. Furthermore, due to their peroxidase-like activity, Pt nanocrystals can provide sufficient oxygen to facilitate the production of ROS under hypoxic conditions. The generated bubbles also enhance ultrasound imaging. ¹⁴ Titanium dioxide nanomaterials, with their hollow structure that can contain gases, are suitable for use as USCA for detection and therapy. Moreover, TiO₂ can act as an acoustic sensitizer in SDT, generating ROS to inhibit tumor cell growth. ³⁰ Figure 4A and B depicts the characterization of Covalent organic framework-TiO₂, while Figure 4C presents the tumor growth curves of different groups of 4T1 tumor-bearing mice. Thus, titanium dioxide can be utilized for monitoring tumors through ultrasound imaging and SDT, achieving a dual therapeutic effect. Luo et al prepared silica NPs using the Stoëber method, added titanium isopropoxide to obtain a TiO₂ coating, and etched the resulting NPs with sodium hydroxide to create hollow TiO₂ NPs. By adding a free collagenase solution, they produced Col-H-TiO₂ NPs. The accumulation of NPs in Col-H-TiO₂ can be assessed through contrast-enhanced ultrasound due to their hollow structure's gas-retaining capability, which improves the intratumor ultrasound signal and matches the functionality of contrast-enhanced ultrasound imaging. Additionally, Col-H-TiO₂ NPs can generate a substantial amount of ROS, enhancing the SDT effect and causing oxidative damage to cancer cells. This provides an efficient ultrasound imaging-guided SDT for pancreatic cancer. ¹² Figure 4E shows the Calcein-AM/PI staining of BxPC-3 cells after various treatments.

Figure 4.

Structural diagram of COF-TiO₂ and its interaction mechanism with intracellular gold nanomaterial microbubbles, alongside growth morphology and staining of tumor cells. (A) TEM image of COF. Reprinted with permission. ³⁰ (B) TEM image of COF-TiO_2. Reprinted with permission. ³⁰ (C) Tumor growth curves of different groups of 4T1 tumor-bearing mice. Reprinted with permission. ³⁰ Process of SAuNP release from SAuMBs and SAuNP delivery into cancer cells following US treatment. Reprinted with permission. ³⁴ (E) Calcein-AM/PI staining of BxPC-3 cells after various treatments. Reprinted with permission. ¹² TEM, transmission electron microscopy; COF, covalent organic framework; SAuNP, smart gold nanoparticles; SAuMB, smart gold NPs microbubbles.

Inorganic–organic hybrid porous materials, composed of metal ions and organic ligands, have a broad spectrum of biomedical applications in bioimaging, drug delivery, and gas storage. For instance, Zr-MOFs are known for their photophysical properties, low toxicity, and structural stability. Zhang et al internally loaded Zr-MOF with AIPH to synthesize Zr-MOF@AIPH, creating a multifunctional dual acoustic sensitizer nanoplatform. This platform combines singlet oxygen production from Zr-MOF under ultrasound stimulation with oxygen-independent alkyl radical production based on AIPH decomposition.

In addition, the nitrogen released from AIPH lowers the cavitation threshold and enhances the acoustic cavitation effect. This not only promotes NP penetration at the tumor site but also offers excellent ultrasound imaging capabilities. The nanoplatform enhances SDT efficiency for prostate cancer treatment and allows for ultrasound-guided SDT. ⁹ Hence, Zr-MOF can be used not only as an ultrasound sensitizer but also for high drug delivery efficiency.

Pan et al developed metal-organic framework (MOF)-derived multilayered DHMS with oxygen generation and multimodal imaging capabilities. These NPs are designed for multimodal imaging-guided SDT with efficient ROS yield under ultrasound irradiation. ¹⁰ The presence of manganese (Mn) in DHMS enhances ROS production efficiency, as it can be oxidized by holes to improve electron-hole separation. Furthermore, DHMS can generate oxygen within the tumor microenvironment (TME), helping to overcome hypoxia in solid tumors and thus improving therapeutic efficiency. In vivo experiments have demonstrated that DHMS-mediated SDT has potent tumor-suppressive effects under the guidance of ultrasound and MRI. This MOF-derived NP, with its acoustic sensitivity and oxygen capabilities, presents a promising strategy for addressing hypoxia in SDT tumors.

Precious metals such as gold can also serve as sensitizers in SDT. Gold nanomaterials are relatively inert, biocompatible, and easily modified on their surface. Different morphologies of gold nanomaterials are well-established in enhancing echo intensity for ultrasound imaging. For example, gold nanoshells provide excellent echo intensity in both B-mode and pulsed inverse harmonic ultrasound imaging. ³¹ The integration of gold with materials such as perfluorocaprolactone and perfluorocarbon, which have unique light absorption properties, can be converted into heat and MB production under laser or high-intensity focused ultrasound. This process facilitates ultrasound contrast enhancement. ³² Modified gold can enable advanced therapeutic approaches. For example, Xu et al prepared ultra-small gold nanodots loaded onto PEG-modified perfluorinated carbon nanodroplets for multimodal imaging, which serve as therapeutic enhancers. ³³ Yoon et al developed smart gold NPs (SAuNPs) with stable MBs encapsulated within their shells. These MBs can aggregate in tumors and be utilized for ultrasound-mediated cancer therapy and diagnostics. ³⁴ Figure 4D illustrates the process of SAuNP release from Smart gold NPs MBs and the subsequent delivery of SAuNP into cancer cells following ultrasound treatment.

Silicon-Based Nanomaterials as Carriers

Silica-based nanomaterials are extensively utilized in ultrasound imaging due to their exceptional biocompatibility, distinctive pore structure, controlled particle size, and other advantageous properties. ³⁵ In their solid form, these NPs can achieve suitable echoes, allowing for greater impedance differences. ³⁶ These silicon-based nanosystems have shown remarkable performance in contrast-enhanced diagnostic imaging, acoustic sensitizer-enhanced SDT, and High-Intensity Focused Ultrasound ablation that is synergistically enhanced. Wei et al developed core-shell structured CDs@SiO₂@TiO₂ nanoplatforms, which exhibit excellent photothermal effects and fluorescence resonance energy transfer-induced photodynamic properties. These characteristics enable the platforms to achieve photothermal synergy and photodynamic therapy. ³⁷ Mesoporous silica NPs are highly promising for cancer diagnosis and treatment. It has been reported that doping MSN NPs with polycaprolactone not only enhances the biocompatibility of the nanocarriers but also improves their elasticity, thereby increasing their acoustic responsiveness for ultrasound imaging applications. ³⁸ Huang et al encapsulated MSN with an ICG-poly(dopamine) (PDA) layer and polyethylene glycol-folic acid-coated PFP to create MSN-PFH@PDA-ICG-PEG-FA nanocarriers. These are designed for tumor US/near-infrared fluorescence imaging and photothermal therapy/photodynamic therapy. Mesoporous silicas-PFH@PDA-ICG-PEG-FA exhibits good monodispersity with high ICG loading, significantly enhances ICG photostability, and greatly improves cellular uptake.^39,40 Mesoporous silica can be readily integrated with other functional molecules for SDT-based synergistic therapy. Moreover, MSN-based nanosystems provide a substantial reservoir for the encapsulation and delivery of conventional organosonic sensitizers, akin to the delivery of anticancer drugs. Building on this, Chen et al synthesized multifunctional reduction of graphene oxide(rGO)@MSN- iron oxide NPs (ION)-PEG-RB composite nanosystems by coating MSN onto the surface of rGO, which was then conjugated with RB-PEG-modified IONs. The conjugated RB serves as a well-established acoustic sensitizer. The presence of reduction of graphene oxide, MSN, and IONs enhances the cytotoxic effect of RB (as an acoustic sensitizer) in SKBr3 cancer cells after exposure to focused ultrasound. This enhancement occurs because the heating effect induced by ultrasound on rGO and IONs aids the SDT process that generates ROS. In vivo results have demonstrated that this composite nanosystem, when combined with acoustic kinetic/thermal therapy and magnetic guidance, can significantly inhibit tumor growth. ⁴¹

PLGA as Carrier

IR 780 Iodide, a cyanine dye with a peak light absorption wavelength of 780 nm, offers several advantages including strong fluorescence intensity, high photothermal conversion efficiency, US response efficiency, good inherent biosafety, and low long-term toxicity. ⁴² However, its poor solubility in biological organisms, rapid clearance, acute toxicity at high doses, and low tumor uptake limit its direct application in cancer therapy and/or diagnosis. To address these challenges, Li et al synthesized tumor-targeting multifunctional composite NPs, termed DVDMS@IR780@PFP@PLGA, or DIPP-NPs. These were created by encapsulating DVDMS, IR780, and PFP within a PLGA shell to achieve ultrasound-guided SDT loaded with IR780 for precise identification and targeting of tumor cells. HMME, one of the most common photosensitizers, exhibits a satisfactory EPR effect. Additionally, HMME can be activated by ultrasound and has demonstrated significant antitumor effects both in vitro and in vivo. ⁴³ When HMME is subjected to low-frequency ultrasound, it generates ROS, which then activate the caspase-3 and caspase-9 pathways. This leads to DNA breaks and an increase in the expression of proapoptotic cells. ¹⁷ Yao et al synthesized PFP-HMME@PLGA/MnFe₂O₄-Ram by encapsulating manganese ferrite (MnFe₂O₄), HMME, and PFP within a PLGA shell and coupling it with an anti-VEGFR-2 antibody. These NPs, termed HPMR NPs, possess multimodal (MRI/photoacoustic/ultrasound) imaging capabilities, allowing for real-time observation of their distribution within plaques. Furthermore, they accumulate in the mitochondria of rabbit aortic endothelial cells. HPMR NP-mediated SDT promotes mitochondrial cysteine aspartase apoptosis and inhibits rabbit aortic endothelial cell proliferation, migration, and tubulogenesis by generating ROS. It also induces neovascular endothelial cell apoptosis, ameliorates hypoxia in rabbit late-stage plaques, reduces neovascular density, subsequently inhibits plaque hemorrhage and inflammation, and ultimately stabilizes the plaque. ¹⁸

Liposomes as Carriers

ICG is utilized as a photosensitizer for photodynamic therapy due to its excellent photothermal conversion efficiency. ICG can also serve as a sound sensitizer, absorbing energy after US irradiation and transferring it to surrounding oxygen molecules. This process induces apoptosis by oxidizing surrounding substrates to produce ROS, such as singlet oxygen ¹O₂ and free radicals, which trigger ICG. However, ICG faces challenges such as a short half-life, rapid self-quenching, and poor hydrolytic stability. ⁴⁴ To address these issues, Zhao et al constructed NPs, referred to as OLI_NPs, for use in photoacoustic power therapy. These were prepared using PLGA, PFP, and lipopolysaccharide/ICG. ¹¹ PFP undergoes a liquid-gas phase change under low-frequency ultrasound, which is used for contrast-enhanced ultrasound imaging and ultrasound cavitation. ⁴⁵ Lipopolysaccharide, a natural ligand for TLR4, has a targeting effect. It was demonstrated that OLI_NPs, when combined with photoacoustic therapy, could induce antitumor immunity and an E7-specific immune response in TC-1 graft tumor models. This provides an integrated solution for cervical cancer treatment, with specific cellular immune induction and dual-mode (ultrasound and photoacoustic) imaging to guide tumor diagnosis and treatment. Li et al employed an alternative strategy, designing Lip-ICG-PFP-cRGD NPs by encapsulating ICG and PFP within cRGD peptidylated nanoliposomes. ⁴⁶ Upon exposure to low-frequency ultrasound, these Lip-ICG-PFP-cRGD NPs effectively promote the release of damage-related molecular patterns through burst-mediated cell membrane disassembly. The encapsulated PFP and ICG enhance antitumor and immune effects, while PFP generates nanobubbles that fuse into MBs upon low-intensity focused ultrasound triggering. This results in enhanced US imaging and ultimately achieves ultrasound-guided SDT.

PFP plays a role in enhanced ultrasound imaging. ⁴⁷ Rose Bengal, when used as an acoustic sensitizer, can encapsulate gases. For example, Hou et al developed a novel delivery mechanism for acoustic sensitizers using MBs. These were prepared by conjugating Rose Bengal with hexadecylamine via stable amine bonds to encapsulate fluorinated gases. This amphiphilic Rose Bengal acts not only as an ultrasound sensitizer for ex vivo fluorescence imaging and SDT but also enables the MBs to be used in contrast-enhanced ultrasound imaging. This facilitates real-time in vivo monitoring of drug distribution, thereby enhancing both diagnostic and therapeutic capabilities. ¹⁶ Guo et al constructed NPs, TL@HPN, with a PFP core, an acoustic sensitizer HMME, a chemotherapeutic agent (paclitaxel), and a tumor homing penetrating peptide (tLyP-1). This construction achieved dual-mode imaging with ultrasound and photoacoustics while targeting tumors. ⁸ Sun et al encapsulated perfluorocarbons and temoporfin into cRGD peptide-modified multilayered liposomes (C-ML/HPT/O₂), which were then loaded into live neutrophils to create Acouscyte/O₂ nanoplatforms. Acouscyte/O₂ enables realistic tumor monitoring and selective accumulation in tumors via perfluorocarbons MB-enhanced ultrasound imaging. It also effectively triggers an acoustic kinetic response to ultrasound stimulation, providing an integrated platform for both diagnostic and therapeutic tumor ultrasound applications. ⁴⁸

Discussion

Compared with traditional ultrasound examinations, the advent of Nano-USCA represents a significant advancement in ultrasound-guided SDT, offering key advantages in several aspects. Firstly, MB contrast agents are confined to the vascular system due to their large size. In contrast, Nano-USCA, with its nanoscale dimensions, can leverage the EPR effect to penetrate more deeply into tumor tissues, extending its utility beyond that of traditional USCA. ⁴⁹ The enhanced imaging capabilities provide more detailed visual information about blood vessels and tumor tissues, enabling a more precise and comprehensive assessment of tumor morphology and location. This, in turn, facilitates more effective and localized treatment strategies. Secondly, nanoscale formulations possess robust loading capacities and surface modifiability, allowing for efficient drug loading and targeted delivery of sonosensitizers to tumor sites. ⁵⁰ This targeted delivery capability enables precise treatment, potentially reducing systemic toxicity. Moreover, the combination of SDT with other therapeutic strategies has demonstrated superior efficacy in enhancing anticancer activity compared to monotherapy alone. ⁵¹ Overall, Nano-USCA's ability to facilitate ultrasound-guided SDT signifies a major step forward in the field of precision medicine. It offers a more personalized and potent approach to cancer diagnosis and treatment, tailoring therapies to individual patient needs and tumor characteristics.

The application of nanomaterials in ultrasound imaging-guided SDT holds great promise, yet several challenges remain to be addressed. Firstly, the selection of an acoustic sensitizer poses a significant challenge. To date, the most widely studied acoustic sensitizers have been photosensitizers. However, these can cause phototoxicity, leading to severe adverse side effects. ⁵² Additionally, some acoustic sensitizers, such as Adriamycin and LVEX, may produce side effects, despite showing potential in clinical applications. Secondly, regarding the characterization of nanomaterials, the size of NPs significantly impacts SDT. Nanoparticle size affects their distribution within biological systems and their interaction with cells. On one hand, larger NPs, due to surface loading and modification, may struggle to cross the vascular endothelium to reach tumor tissues, particularly when employing enhanced permeation or the EPR effect strategies. Moreover, oversized NPs may diminish the focusing effect of ultrasound, thereby affecting treatment precision. ⁵³ On the other hand, NPs that are too small may gain easier access to normal tissues, potentially causing adverse reactions. Furthermore, NPs that are excessively small may be more likely to cross biological barriers, such as the blood–brain barrier, which could be advantageous for certain applications, like brain tumor treatments. ⁵⁴ Overall, selecting the appropriate NP size is crucial and should be tailored to the specific therapeutic target and condition. The optimal size can balance in vivo delivery, tumor targeting, and side effects, thereby maximizing therapeutic efficacy. The stability of NPs during storage is also vital, as instability can lead to aggregation, affecting performance and safety. Furthermore, without specifically designed carriers for targeted delivery, nanomaterials may accumulate nonspecifically in healthy tissues or organs, leading to toxicity. Additionally, some nanomaterials may induce adverse immune responses or unforeseen side effects when used in vivo, necessitating careful, long-term evaluation of potential immune responses or organ accumulation. Thus, there is a need for optimization of nanomaterials through more comprehensive ex vivo and in vivo studies of their physicochemical properties, biological effects, and potential toxicity. In addition, the acoustic sensitivity of Nanomaterials may vary depending on the ultrasound parameter settings. Specific parameters such as ultrasound frequency, intensity, and irradiation time can have a significant impact on the therapeutic effect, and these parameters need to be tailored to the specific type of tumor. ⁵⁵ In this process, an optimal ultrasound parameter setting scheme needs to be developed considering the characteristics of the tumor as well as the unique interactions between ultrasound waves and the Nanomaterials acoustic sensitizers in order to maximize therapeutic efficacy and minimize adverse effects. In addition to considerations such as acoustic sensitizer selection, nanomaterial characterization, targeted delivery vehicle design, and ultrasound parameter settings, other key factors include the potential for cancer cells to develop resistance to acoustic kinetic therapy. Therefore, therapeutic resistance should be minimized through combinations with other treatments. Finally, the clinical translation of nanomaterials into ultrasound imaging-guided acoustic kinetic therapy presents challenges. The large-scale production of uniformly sized, functional nanomaterials is technically demanding and costly. Ensuring consistency in size, shape, surface properties, and synthesis reproducibility is critical when designing nanomaterials. ⁵⁶

The combined diagnostic platform of NPs and molecular ultrasound represents an important direction for future research. Ideal NPs should exhibit good biocompatibility, sufficient drug loading capacity, and the ability to target specific tumor cells. Thus, interdisciplinary research across fields such as nanomaterials science, medicinal chemistry, and biology is essential to address these limitations. In addition, Nanomaterials have shown great promise in immunotherapy. ⁵⁷ The TME plays a key role in tumor survival, progression, and metastasis and can be considered as a potential target for molecular imaging of cancer. Therefore, the combination of nanomaterial-targeted TME therapy and SDT is also one of the future research directions. ⁵⁸ At the same time, with the aid of computed tomography diagnosis, SDT will also play a greater role. ⁵⁹ Optimizing these NPs to improve their specificity and selectivity, enhance the efficacy of SDT, and reduce side effects is crucial for the successful transition of nanomaterials from the laboratory to clinical practice in ultrasound imaging–guided acoustic kinetic therapy. With the diversification of nanomaterial types and the potential for property control, constructing acoustically responsive nanomaterial diagnostic and therapeutic platforms capable of real-time monitoring of tumor treatment changes offers significant developmental advantages.

Conclusions and Future Perspectives

The integration of nanomaterials with ultrasound imaging-guided SDT has marked significant advances in the medical diagnostics and therapeutics field over the past decade. This paper summarizes the application of NPs, such as carbon-based, metal-based, silicon-based, and liposomal nanomaterials, as multifunctional carriers of USCA and acoustic sensitizers for ultrasound-guided SDT. Ultrasound-guided SDT offers the advantages of being noninvasive, real-time, and safe for targeted drug delivery and real-time monitoring of therapeutic response. This integration not only improves imaging clarity but also enhances therapeutic efficacy, making SDT a promising noninvasive treatment modality for various diseases, including cancer.

Looking ahead, the potential of nanomaterials in ultrasound-guided SDT is substantial. Future research directions should focus on validating the safety and efficacy of nanomaterial-based SDT in diverse patient populations. With ongoing advances in nanotechnology, coupled with more in-depth studies of tumor biology and ultrasound physics, further research into novel ultrasound sensitizers and nanocarriers will improve disease treatment efficacy and help monitor treatment responses in real time. This will lead to more effective and personalized therapies and, ultimately, improved clinical outcomes.

Abbreviations

CNT

carbon nanotubes

EPR

Enhanced Permeability and Retention

MB

microbubbles

MOF

metal-organic framework

MSN

mesoporous silica

MWCNT

multi-walled carbon nanotubes

Nano-USCA

Nanoparticle Ultrasound Contrast Agents

NP

nanoparticles

ROS

reactive oxygen species

SDT

sonodynamic therapy

TME

tumor microenvironment

USCA

Ultrasound Contrast Agents