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Published in final edited form as: Ultrasound Med Biol. 2021 Feb 27;47(6):1465–1474. doi: 10.1016/j.ultrasmedbio.2021.01.032

Emerging Applications of Ultrasound Contrast Agents in Radiation Therapy

Quezia Lacerda 1,2, Mohamed Tantawi 2, Dennis B Leeper 3, Margaret A Wheatley 1, John Eisenbrey 2
PMCID: PMC8044052  NIHMSID: NIHMS1669667  PMID: 33653626

Abstract

Radiation therapy (RT) causes DNA damage through ionization, leading to double-strand breaks. In addition, it generates reactive oxygen species (ROS) which are toxic to tumor cells and the vasculature. However, hypoxic regions in the tumor have been shown to not only decrease treatment response but also increase the likelihood of recurrence and metastasis. Ultrasound-sensitive microbubbles are emerging as a useful diagnostic and therapeutic tool within RT. Contrast-enhanced ultrasound (CEUS) has shown great promise in early prediction of tumor response to RT. Ultrasound-triggered microbubble cavitation has also been shown to induce bioeffects that can sensitize angiogenic tumor vessels to RT. Additionally, ultrasound can trigger the release of drugs from microbubble carriers via localized microbubble destruction. This approach has numerous applications in RT including targeted oxygen delivery prior to radiotherapy. Furthermore, microbubbles can be used to locally create ROS without radiation. Sonodynamic therapy uses focused ultrasound and a sonosensitizer to selectively produce ROS in the tumor region, and has been explored as a treatment option for cancer. This review summarizes emerging applications of ultrasound contrast agents in RT and ROS augmentation.

Keywords: Ultrasound, contrast-enhanced ultrasound, radiotherapy, tumor hypoxia, microbubbles

Introduction

Ultrasound contrast agents (UCA) are generally comprised of microbubbles, which are traditionally made of inert high molecular weight gases stabilized by a shell composed of surfactants, phospholipids, polymers, or proteins (Lyshchik 2019). Commercially available ultrasound contrast agents are injected intravenously and their size (1-8 μm) allows them to pass through the pulmonary bed to reach the systemic circulation. These agents have an excellent safety profile with minimal toxicities when used for both diagnosis and therapy (Dimcevski et al. 2016; Eisenbrey et al. 2015a; 2015b; 2020). The gas contained inside the bubbles provides them with different acoustic impedance and compressibility compared with the surrounding tissue, yielding enough ultrasound signal enhancement to visualize the vasculature (Eisenbrey et al. 2010a; Nanda et al. 1997). Thus, contrast-enhanced ultrasound (CEUS) has been investigated as a potential means of early estimation of the tumor response to treatments affecting the blood vessels such as antiangiogenic chemotherapy and radiotherapy.

Microbubbles have become a significant area of investigation in the field of biomedical research due to their wide range of diagnostic and interventional applications. An important property of microbubbles is ultrasound-triggered cavitation, which can take the form of either stable or inertial cavitation. Inertial cavitation describes the transient collapse of a microbubble and subsequent generation of localized energy, while stable cavitation involves oscillation of the microbubble without destruction. Both types of cavitation can generate bioeffects in the surrounding environment, including microstreaming, microjetting, direct impingement, ballistic motion, shock waves, and secondary radiation waves (Kooiman et al. 2020). These effects can cause alterations to the vascular and cellular permeability and endothelial cell apoptosis, also known as enhanced permeation and retention (EPR) effect.

Ultrasound can also trigger the release of drugs from microbubble carriers through mechanical effects of the ultrasound waves (Wang et al. 2014). Thus, microbubbles have been designed as vehicles to deliver drugs in a spatially and temporally controlled fashion (Cochran et al. 2011; Eisenbrey et al. 2010b; Vishal et al. 2019). Different gasses such as xenon, argon, helium, nitrogen, and perfluorocarbon have been loaded into microbubbles for image-guided applications (Chattaraj et al. 2020; Ibsen et al. 2013; Sirlin et al. 1997). More recently, oxygen has been loaded into microbubbles and delivered to tumors to combat the hypoxic tumor environment rendering the tumor more responsive to radiation therapy (RT) (Eisenbrey et al. 2018).

RT is a common therapeutic modality for cancer treatment. It damages the DNA by ionization of atoms leading to double-stranded DNA breaks (Lehnert 2007). Furthermore, RT creates reactive oxygen species (ROS), which in turn contribute to the irreversible DNA damage in the tumor cell (Srinivas et al. 2019). Ultimately, both mechanisms result in apoptosis and necrosis of cancer cells and destruction of tumor vessels. However, tumors are often more resistant than the surrounding normal tissue to radiation effects because of their hypoxic microenvironment. Chronic hypoxia has been identified as the key mechanism of RT resistance in tumors due to the limited production of ROS (Brown and Wilson 2004; Rockwell et al. 2009). Due to the tumor angiogenesis, it is estimated that 10 - 30% of solid tumor mass is made up of hypoxic and/or anoxic areas (pO2 of 0-20 mm Hg) that are heterogeneously distributed within the tumor (Brown and Wilson 2004; Moulder et al. 1984). As these hypoxic regions occur and desensitize tumors to chemotherapy and RT, they also promote the progression of the tumor, leading to poor clinical outcomes (Knowles et al. 2001). As a result, ultrasound-guided delivery of oxygen has gained special attention in recent years since it allows spatially confined delivery into target areas while minimizing systemic toxicity (Mullik et. al 2017). In this review, we discuss the latest applications of ultrasound-sensitive microbubbles in RT as well as its promising role in promoting ROS via sonodynamic therapy. All cited studies with human subjects obtained informed consent from each study participant and the protocol was approved by an ethics committee or institutional review board. Likewise, all preclinical studies involving animal work was performed under IACUC approval.

Contrast-enhanced ultrasound for monitoring radiation response

In addition to the DNA damage, high doses of radiation (>8-10 Gy) destroy tumor blood vessels, leading to secondary death of tumor cells (Korpela et al. 2014; Paris et al. 2001; Peña et al. 2000). These vascular effects have been shown to regulate the tumor response to radiation (Garcia-Barros et al. 2003). Accordingly, monitoring changes in the tumor microvasculature can provide an early indication of the treatment outcome. Vascular shutdown can be an early indicator of radiation effectiveness (García-Figueiras et al. 2016). The current criteria for response assessment depend on the changes in tumor volume, which need months to be detectable on imaging. On the other hand, changes in tumor vascularity can be quantitatively assessed within days after treatment. Minimizing the time between radiation treatment and response evaluation may ultimately allow a timely switch to an alternative therapy in cases where the treatment is ineffective. Contrast-enhanced ultrasound (CEUS) can quantify early vascularization and perfusion changes, and its utility for such purpose has been demonstrated in a variety of other kinds of cancer therapy (Gregory et al. 2020, Tantawi et al. 2020a).

An early study evaluated CEUS-derived perfusion in choroidal melanoma patients receiving RT (Schlottmann et al. 2005). The results showed that high perfusion detected by CEUS could be a sign of early recurrence after RT. Other studies used CEUS to monitor RT treatment response, although primarily in preclinical models. One study evaluated the early tumoral vascular changes detected by CEUS within 48 hours of RT and their correlation with histological changes (Arteaga-Marrero et al. 2018). The study used mice with CWR22 prostate tumor xenografts, which received 7.5 Gy or 10 Gy single radiation treatment, and the response was monitored at either 24 h or 48 h post-treatment. Longitudinal changes in microvascular parameters were detected by CEUS in the group that received radiation and such changes were not observed in the control group. Changes in the contrast elimination from the plasma (kel) were correlated with microvascular density and necrosis percentage detected on histology. Histological changes commonly require a longer period (more than 48h) to be apparent after treatment; however, this study showed that CEUS could detect early microvascular changes in the treated tumors.

Kasoji et al. (2018) also studied the potential of CEUS perfusion imaging in the early evaluation of tumor response after radiation treatment (Kasoji et al. 2018). The study was performed on female Fischer 344 rats with subcutaneously implanted fibrosarcoma. This cancer type is characterized by its highly chaotic and tortuous vascularity that makes it optimal for studying the anti-vascular effects of radiotherapy. The rats were divided into four groups; each received a different radiation dose (0 Gy, 15 Gy, 20 Gy, and 25 Gy). Volumetric vascular density (VVD) was used as an indicator of the tumor microvasculature and was calculated using acoustic angiography, a superharmonic CEUS technique capable of providing high-resolution microvascular imaging. Tumor volume was calculated using the B-mode ultrasound images. For tumors that recurred after treatment, an increase in VVD was observed 4-10 days before the increase in the tumor volume in the three groups that received radiation treatment (15 Gy, 20 Gy, and 25 Gy) (Figure 1). Additionally, VVD and tumor volume were higher in the recurrent tumors compared to the controlled ones. These results suggest VVD as a biomarker for early assessment of tumor response to radiation. While these results are encouraging, further studies in different human-derived tumor models and humans are needed to verify the potential of acoustic angiography in detecting early changes in tumor microvasculature in response to radiotherapy.

Figure 1.

Figure 1.

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A visual comparison of vascular density and tumor volume changes during tumor regrowth after 15 Gy local irradiation. Vascular density (right) noticeably increases from day 7 to day 19, while tumor volume (left) continues to decrease until day 19. The solid yellow line indicates the tumor boundary in the b-mode images. Note that microvascular data are shown as a maximum intensity projection but are a 3-D data set [Reprinted from (Kasoji et al. 2018)].

A different study assessed CEUS backscatter parameters via quantitative ultrasound in relation to tumor response in prostate cancer bearing mice that received RT combined with ultrasound and microbubbles as a novel antivascular therapy (Lee et al. 2012). Backscatter parameters included midband fit and spectral intercept, and results demonstrated an elevation in these parameters in responsive tumors and further elevation when UTMD was combined with RT (Lee et al. 2012). Similarly, as part of an ongoing clinical trial (NCT#03199274) using microbubble cavitation-based bioeffects for augmentation of transarterial radioembolization (TARE) in hepatocellular carcinoma (HCC) patients, our group has shown that CEUS quantification of perfusion and fractional vascularity can be used as predictors for tumor response (Tantawi et al. 2020b). Within 1-2 weeks after TARE, CEUS detected changes in tumor vascularity that correlated with tumor response using modified response evaluation criteria in solid tumors at 4-6 months post-treatment on contrast-enhanced MRI (Tantawi et al. 2020b). These results show the potential advantages of using CEUS as an early indicator of tumor response following RT.

Inertial Microbubble Cavitation Enhanced Radiation Therapy

Ultrasound-triggered microbubble destruction (UTMD) can increase endothelial cell radiosensitivity by inducing cell apoptosis (Czarnota et al. 2015; Kolesnick et al. 2003). This phenomenon has been attributed to increased ceramide production after interactions with the shear stress generated from inertially cavitating microbubbles (Al-Mahrouki et al. 2014; Czarnota et al. 2012; El Kaffas et al. 2018a). While this mechanism does not directly overcome tumor hypoxia, an acid sphingomyelinase-induced ceramide-dependent mechanism has been shown to reduce the required radiation dose (> 8-10 Gy) to kill angiogenic vessels (Briggs et al. 2013; El Kaffas et al. 2013; Nofiele et al. 2013; Sharma et al. 2019; El Kaffas et al. 2018b). In a bladder cancer model, UTMD in combination with radiation resulted in 40-60% endothelial cell death with 2-8 Gy doses of radiation, along with causing tumor regression (Czarnota et. al 2012).

Al-Mahrouki et al. (2015) also investigated cell death signaling pathways that involve ceramide. Human umbilical vein cells were used to further investigate microbubble therapy effects, along with characterizing pathways affected by microbubbles and radiation. Results showed that radiation treatments and microbubble treatments resulted in the production of ceramide, justifying the additive effect in vitro. The same group published a study on breast tumor response to UTMD and RT (Lai et. al 2016) to further examine the combined effect of both treatments in vivo. Results demonstrated that microbubbles are a synergistic treatment modality with radiation and can be used to reduce required RT dosages while maintaining treatment effectiveness (Lai et al. 2016).

Later, the role of 2-hydroxyacylsphingosine 1-beta-galactosyltransferase (UGT8) in modulating cell stress response to UTMD in combination with RT was investigated (Al-Mahrouki et al. 2017). Briefly, the main function of UGT8 is galactosylceramide synthesis through the transfer of UDP galactose ceramide (Al-Mahrouki et al. 2017; Owczarek et al. 2013), which acts as an anti-apoptotic molecule that targets tumorigenic and metastatic properties in cancerous cells (Owczarek et. al 2013). In vivo studies conducted in immune-deficient mice analyzed the UGT8 effects on tumor response to treatment. Tumors were exposed to no treatment, UTMD treatment, radiation treatment, and UTMD + radiation. Focusing on the mechanism of damage transduction linked to the anoxia caused by ceramide signaling, they concluded that the down-regulation of UGT8 reduced the cell survival and enhanced the radiation effect when combined with UTMD (Al-Mahrouki et al. 2017). Later, El Kaffas et al. (2018b) showed that acid sphingomyelinase gene knockout mice are more resistant to radiation treatments than wild-type mice. It was shown that the mechanical forces exerted on endothelial membranes are able to promote cell death via mechanotransduction of ceramide, thereby confirming the role of ceramide and acid sphingomyelinase in microbubble vascular disruption leading to radiation enhancement.

Work has demonstrated the ability of UTMD to induce vascular disruption in preclinical models of HCC (D’Souza et al. 2019). A study by Daecher et al. (2017) investigated the use of UTMD to sensitize an orthotropic model of HCC to radiation in nude rats implanted with human HCC cells in the right lobe of the liver. Analysis of liver function assays showed that serum alanine aminotransferase (ALT) and bilirubin levels were normal (0.31-0.93 mg/dL bilirubin and 13.4–32.5 U/L ALT) in all treatment groups. Thus, it was concluded that UTMD prior to RT is safe and selective for HCC tumor tissue, sparing normal liver tissue (Daecher et al. 2017). Using photoacoustic imaging, no changes in tumor oxygenation were observed pre- and post-UTMD. Additionally, animals receiving UTMD + RT survived 24 days longer than animals receiving radiation alone and 26 days longer than animals receiving UTMD alone. Building on previous findings, Klein et al. (2020) conducted a study to investigate the influence of timing and sequencing of UTMD + RT in a preclinical prostate cancer model. Results from this study showed that UTMD, when administered prior to radiation therapy, showed a greater cytotoxicity effect that was maximized at 6 hours but no sequencing differences of combined therapy (UTMD administered before or after RT) (Klein et al. 2020).

The first clinical trial using UTMD for RT augmentation in humans was recently published (Eisenbrey et al. 2020), demonstrating the translatability of this work. Patients with HCC were randomly assigned to received either transarterial radioembolization (TARE) alone (standard of care) or TARE followed by three sessions of UTMD. Immediately after the treatment, one week after treatment, and two weeks after treatment (Figure 2). Physiological monitoring was performed over the course of UTMD and changes in all participants’ liver function tests were evaluated at 1 month. Additionally, SPECT imaging in a cohort of patients ruled out any displacement of radioactive beads outside the liver post UTMD. The patients’ response was assessed at 4-6 months using the modified response evaluation criteria in solid tumors. The authors hypothesized that UTMD would help the radiation-induced disruption of the microvasculature and consequently cause further tumor necrosis. Besides demonstrating the safety of UTMD in humans, the results showed that patients who received UTMD post TARE had higher tumor response rate compared to patients who received TARE alone (93% vs. 50%, p = 0.02). These results, albeit preliminary, demonstrate the potential effects of UTMD in sensitizing tumors to RT.

Figure 2.

Figure 2.

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Example series from a 54-year-old male participant undergoing hepatocellular carcinoma radioembolization with ultrasound-triggered microbubble destruction (UTMD). The series shows baseline contrast-enhanced MRI in transverse plane (A), Technetium-99m MAA (macroaggregated albumin) SPECT-CT in the transverse plane during treatment planning (B), angiography during yttrium-90 delivery (C), B-mode ultrasound immediately post radioembolization (D), and peak contrast-enhanced ultrasound enhancement during UTMD 2 hours (E), 1 week (F), and 2 weeks (G) post radioembolization [Reprinted from (Eisenbrey et al. 2020)].

Therapeutic application of nanobubbles has also recently been investigated for their potential to enhance RT. Nanobubbles (100-300 nm diameter) are a new generation of contrast agents and are thought to target tumors through the enhanced permeation and retention effect, making them a promising multifunctional theranostics agent (Bing et. al 2018; Hysi et. al 2020). The biophysical changes during nanobubble-augmented RT showed that the lipid shell nanobubbles used in this study had a circulation time six times longer than conventional microbubbles (survive 30 mins after injection). Additionally, nanobubbles with radiation treatments resulted in twice the cell death compared to nanobubbles alone and showed improvement compared to traditional microbubble treatments (Hysi et. al 2020). This is an encouraging approach and may further improve the already substantial treatment advantages of using UTMD in combination with RT.

Microbubble Delivery of Radiosensitizers

Ultrasound-sensitive microbubbles containing oxygen have been proposed as a mechanism of locally overcoming tumor hypoxia prior to radiotherapy (see Figure 3). Oxygen microbubbles with a phospholipid shell have been explored both in vitro and in vivo (Kheir et al. 2012; Kwan et al. 2012; McEwan et al. 2015; Reusser et al. 2020). These agents have been shown to improve radiotherapy tumor control in a rat fibrosarcoma model (Fix et al. 2018). This preliminary study compared oxygen microbubbles and nitrogen microbubbles in vitro and in vivo. In vitro work demonstrated the potential of oxygen microbubbles to increase dissolved oxygen saturation in hypoxic solutions compared to nitrogen microbubbles. In vivo studies used female Fisher 344 rats with subcutaneous fibrosarcoma tumor allografts randomly matched between radiotherapy treatment groups. In initial experiments, tumor oxygenation was measured continuously in real-time during treatment sessions. Results showed that oxygen microbubbles increased tumor oxygenation whereas nitrogen microbubbles did not. In a second series of experiments, oxygen microbubbles yielded a 35% improvement for all tumors larger than the initial volume of 0.5 cm3 when compared to RT administered alone (Fix et al. 2018).

Figure 3.

Figure 3.

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Schematic of enhanced delivery of oxygen-loaded microbubbles. The microbubbles oscillate and rupture under ultrasound irradiation. Therefore, a series of physical forces are generated (including microstreaming, shock waves, and micro-jets), which cause damage to endothelial cells. Simultaneously, the oxygen is released and can be delivered into tumor cells, making them more susceptible to radiation therapy.

Our group demonstrated the feasibility of this approach in mice with breast tumor xenografts using a surfactant-shelled microbubble (SE61O2) and continuous monitoring of tumor partial oxygen pressure using an Oxylite pO2 probe (Eisenbrey et al. 2015a; Oeffinger et al. 2019). Later, the therapeutic efficacy of delivering oxygen prior to RT was demonstrated in female immunodeficient nude mice implanted with the human triple-negative breast cancer cell line MDA-MB-231 (Eisenbrey et al. 2018). The tumor-bearing mice receiving SE61O2 with ultrasound triggering and 5 Gy of radiation showed significant improvement in tumor control compared to controls, along with a delay in tumor growth of 25-35 days. Data from these experiments are shown in Figure 4. More recently, SE61O2 was shown to enhance tumor response to radiotherapy in a murine model of metastatic breast cancer in the brain (Delaney et al. 2019). Mice receiving oxygen microbubbles in combination with ultrasound and radiation (10 Gy) showed significant tumor control (40% growth in tumor volume over 12 days) when compared to groups treated with only SE61O2 and radiation. These results suggest that local delivery of oxygen can successfully sensitize the tumor to radiation in an orthotopic model in the brain.

Figure 4.

Figure 4.

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Regression modeled curves of tumoral response to therapy with 95% confidence bands (dashed lines) showing the influence of ultrasound (US), surfactant-shelled microbubbles (SE61), and RT. Tumor volumes were normalized to the day of treatment. The green plot shows US plus surfactant-shelled microbubble with oxygen core (SE61O2); red plot, SE61O2 plus 5 Gy; blue plot, US plus 5 Gy; orange plot, US plus surfactant-shelled microbubble with nitrogen core (SE61N2) plus 5 Gy; and black plot, US plus SE61O2 plus 5 Gy [Reprinted from (Eisenbrey et al. 2018)].

More recently, the ability of copper oxide nanosuperparticles to generate oxygen under microwave irradiation was explored to upregulate tumor reoxygenation (Chen et al. 2020). Human lung adenocarcinoma A549 cells were injected into the peritoneum of immunodeficient BALB/c nude mice to establish an ascites tumor model. Briefly, nanosuperparticles were fabricated by incorporating copper oxide nanoparticles, radio and microwave sensitizer of Quercetin, microwave sensitizer of 1-butyl-3-methylimidazolium hexafluorophosphate, and surface modifier of monomethoxy polyethylene glycol sulfonyl (mPEG-SH, 5k Da) into a mesoporous sandwich of silicon dioxide and zirconium dioxide (SiO2@ZrO2) nanosuperparticles (Wachsberger et al. 2003). Results showed that the presence of copper oxide nanoparticles increases tumor inhibition by 98.62%, concluding that the nanosuperparticles are able to generate sufficient oxygen to upregulate tumor reoxygenation under microwave irradiation; improving both RT and microwave thermal therapy (Chen et al. 2020).

The ability to increase tumor oxygenation using nano-emulsions of dodecafluoropentane prior to radiation has also been explored. A preliminary study by Johnson et al. (2015) tested NVX-108, an oxygen therapeutic composed of emulsified dodecafluoropentane that has a unique fluorocarbon composition with similar O2 delivery properties to microbubbles. Twenty-nine mice were injected with Hs-766T pancreatic tumor cells, 9 mice were used for tumor pO2 measurements while being treated with different doses of NVX-108. The remaining mice (20 mice) were used to analyze tumor growth rates under the following groups: no treatment, breathing carbogen (while being treated with a 12 Gy radiation dose ) and treated with NVX-108 (0.6 cc per kg, 2% w/vol dodecafluoropentane and breathing carbogen while being treated with a 12 Gy radiation dose). For the pO2 experiments, NVX-108 was highly effective in increasing tumor oxygenation (400% increase in pancreatic tumor xenograft model) lasting up to 2 hours when compared to trans sodium crocetinate (an oxygen diffusion enhancer (Sheehan et al. 2009)). Results confirmed that there were significant differences in the rates of tumor growth between all groups (control, carbogen + RT, and carbogen + RT + NVX-108). The rate of tumor growth was 25x slower for the carbogen + RT + NVX-108 group than the control. Tumor growth was twice as slow in the carbogen + RT + NVX-108 group as in the carbogen + RT group (Johnson et al. 2015). These findings show a promising strategy in using NVX-108 (a nano-emulsion of DDFP) as a radiation sensitizer.

Gold nanoparticles have also been investigated in combination with UTMD and RT in vitro for their ability to enhance the therapeutic effects of RT (Thu Lee Tran et al. 2015). Human adenocarcinoma breast cell line (MDA-MB-231) was exposed to ultrasound in the presence of Definity® (Lantheus Medical Imaging, Inc., North Billerica, MA, USA) microbubbles alone and in combination with gold nanoparticles, followed by RT treatment. Results showed that cell viability decreased by 22 and 11 folds (difference) respectively, with gold nanoparticles with ultrasound microbubbles and RT compared to RT alone and gold nanoparticles with RT alone, demonstrating a significant radiosensitization effect. Furthermore, this approach showed that gold nanoparticles in combination with UTMD and radiation could potentially improve clinical translation by reducing the amount of gold nanoparticles required for therapeutic effect (Thu Lee Tran et al. 2015; Trono et al. 2011).

Microbubble Generation of ROS

Sonodynamic therapy (SDT) is a noninvasive approach that depends on the ultrasound-induced targeted production of toxic ROS using a nontoxic sonosensitizer in the presence of molecular oxygen. It uses the same concept as photodynamic therapy but with an ultrasound stimulus instead of a light. However, SDT achieves deeper penetration into tissues compared with photodynamic therapy (Hoogenboom et al. 2015), which expands the scope of treatable tumors. SDT works through cavitation-dependent mechanisms where microbubble cavitation produces extremely high temperatures (up to 10000 K) and pressures (> 81 MPa) in the surrounding microenvironment, which act as a sonochemical reactor (Misik and Rieze 2000). These cavitation bioeffects activate the sensitizer, which then releases energy converting the surrounding oxygen molecules into ROS (McHale et al. 2016). In addition, cavitating microbubbles emit light upon irradiation with ultrasound, a phenomenon known as sonoluminescence, which also helps in the generation of ROS (Yin et al. 2016). Besides the cytotoxic effects of ROS generation, SDT inhibits tumor angiogenesis (formation of new blood vessels) depriving the tumor of oxygen and nutrients (Gao et al. 2013), which leads to secondary death of cancer cells. Furthermore, SDT can provoke an immune response to cancer cell debris after their destruction, which further aids in tumor reduction (Wang et al. 2014).

The anti-tumor effects of SDT have been demonstrated in numerous types of cancer xenografts. These have included liver (Shi et al. 2011), colon (Yumita and Umemura 2004), gastric (Hachimine et al. 2007), prostate (Narang and Varia 2011), and tongue cancers (Narang and Varia 2011). Nanocarriers have been exploited in targeted delivery of sonosensitizer to the tumor area through enhanced permeation and retention effects as well as other targeting effects. Sonodynamic efficiency has been shown in sensitizer-conjugated gold nanoparticles (Sazgarnia et al. 2013). Microbubble-sensitizer conjugates demonstrated better anticancer effects than sensitizer alone (Nomikou et al. 2012). Recently, oxygen-loaded microbubbles in combination with SDT showed similar success in preclinical models (McEwan et al. 2015).

Sonodynamic therapy has also been combined with other cancer treatment methods rather than used as a single treatment modality. Combinations of SDT with chemotherapy (Nesbitt et al. 2018), photodynamic therapy (Liu et al. 2016), and hyperthermia therapy (Ju et al. 2016) have demonstrated synergistic effects in tumor growth control. Despite the encouraging results in animal experiments, clinical trials in humans are still lacking. Only a handful of studies have reported the clinical use of SDT, and those included SDT as part of a combination treatment with photodynamic therapy (Kenyon et al. 2009; Wang et al. 2009). However, this field continues to progress and is expected to serve as a strong compliment to traditional RT.

Conclusions

Microbubbles have an important and expanding role within the field of RT. Contrast-enhanced ultrasound is a potentially useful tool for the early detection of microvascular changes in tumors after radiation therapy. While this has been demonstrated in multiple animal studies, more studies are needed for clinical adoption. As a therapeutic tool, UTMD has demonstrated substantial radiosensitization through vascular disruption in a variety of preclinical models. This approach (which uses commercially available ultrasound contrast agents) is readily translatable to the clinical setting and initial results in humans have been encouraging. Preclinical studies have demonstrated that oxygen-loaded microbubbles can locally elevate oxygen levels in tumors, thereby improving radiotherapy outcome. Similarly, SDT, which locally generates ROS without the need for ionizing radiation has confirmed significant anti-tumor effects, although these approaches have yet to be verified in clinical trials. Advances in these topics are expected to continue their exponential growth and ultimately improve the treatment of cancer patients.

Acknowledgments

Funding for this work was provided in part by the United States National Institute of Health R01 EB26881 and R01 CA238241.

Footnotes

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Conflict of Interest

None.

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