Ultrasound contrast agents for ultrasound molecular imaging

Exploring the potential of contrast agents in breast cancer echography: current state and future directions

Breast cancer stands as the most frequent malignancy and leading cause of death among women. Early and accurate detection of this pathology represents a crucial factor in enhancing both incidence and mortality rates. Ultrasound (US) examination has been extensively adopted in clinical practice due to its non-invasiveness, affordability, ease of implementation, and wide accessibility, thus representing a valuable first-line diagnostic tool for the study of the mammary gland. In this scenario, recent developments in nanomedicine are paving the way for new interpretations and applications of US diagnostics, which are becoming increasingly personalized based on the molecular phenotype of each tumor, allowing for more precise and accurate evaluations. This review highlights the current state-of-the-art of US diagnosis of breast cancer, as well as the recent advancements related to the application of US contrast agents to the field of molecular diagnostics, still under preclinical study.

An Ultrasound-Responsive Theranostic Cyclodextrin-Loaded Nanoparticle for Multimodal Imaging and Therapy for Atherosclerosis

Atherosclerosis is a major cause of mortality and morbidity worldwide. Left undiagnosed and untreated, atherosclerotic plaques can rupture and cause cardiovascular complications such as myocardial infarction and stroke. Atherosclerotic plaques are composed of lipids, including oxidized low-density lipoproteins and cholesterol crystals, and immune cells, including macrophages. 2-Hydroxypropyl-beta-cyclodextrin (CD) is FDA-approved for capturing, solubilizing, and delivering lipophilic drugs in humans. It is also known to dissolve cholesterol crystals and decrease atherosclerotic plaque size. However, its low retention time necessitates high dosages for successful therapy. This study reports CD delivery via air-trapped polybutylcyanoacrylate nanoparticles (with diameters of 388 ± 34 nm) loaded with CD (CDNPs). The multimodal contrast ability of these nanoparticles after being loaded with IR780 dye in mice is demonstrated using ultrasound and near-infrared imaging. It is shown that CDNPs enhance the cellular uptake of CD in murine cells. In an ApoE^-/- mouse model of atherosclerosis, treatment with CDNPs significantly improves the anti-atherosclerotic efficacy of CD. Ultrasound triggering further improves CD uptake, highlighting that CDNPs can be used for ultrasound imaging and ultrasound-responsive CD delivery. Thus, CDNPs represent a theranostic nanocarrier for potential application in patients with atherosclerosis.

"PFH/AGM-CBA/HSV-TK/LIPOSOME-Affibody": Novel Targeted Nano Ultrasound Contrast Agents for Ultrasound Imaging and Inhibited the Growth of ErbB2-Overexpressing Gastric Cancer Cells

Objective

Gastric cancer is one of the most lethal malignancies in the world. However, the current research on the diagnosis and treatment of nano-ultrasound contrast agents in the field of tumor is mostly focused on breast cancer, ovarian cancer, prostate cancer, liver cancer, etc. Due to the interference of gas in the stomach, there is no report on the treatment of gastric cancer. Herpes simplex virus thymidine kinase/ganciclovir (HSV-TK/GCV) therapy system is the most mature tumor suicide gene in cancer treatment. At the same time, in order to improve its safety and efficiency, we designed a gastric tumor targeted ultrasound-triggered phase-transition nano ultrasound contrast agent PFH/AGM-CBA/HSV-TK/Liposome (PAHL)-Affibody complex.

Methods

In our study, guanidinylated SS-PAAs polymer poly(agmatine/N, N′-cystamine-bis-acrylamide) (AGM-CBA) was used as a nuclear localization vector of suicide gene to form a polyplex, perfluorohexane (PFH) was used as ultrasound contrast agent, liposomes were used to encapsulate perfluorohexane droplets and the polyplexes of AGM-CBA/HSV-TK, and affibody molecules were conjugated to the prepared PAHL in order to obtain a specific targeting affinity to human epidermal growth factor receptor type 2 (ErbB2) at gastric cancer cells. With the aid of ultrasound targeted microbubble destruction technology and the nuclear localization effect of AGM-CBA vector, the transfection efficiency of the suicide gene in gastric cancer cells was significantly increased, leading to significant apoptosis of gastric cancer cells.

Results

It was shown that PAHL-Affibody complex was nearly spherical with an average diameter of 560 ± 28.9 nm, having higher and specific affinity to ErbB2 (+) gastric cells. In vitro experiments further confirmed that PAHL could target gastric cancer cells expressing ErbB2. In a contrast-enhanced ultrasound scanning study, the prepared ultrasound-triggered phase-change nano-ultrasound contrast agent, PAHL, showed improved ultrasound enhancement effects. With the application of the low-frequency ultrasound, the gene transfection efficiency of PAHL was significantly improved,  thereby inducing significant apoptosis in gastric cancer cells.

Conclusion

This study constructs PFH/AGM-CBA/HSV-TK/Liposome-Affibody nano ultrasound contrast agent, which provides new ideas for the treatment strategy of ErbB2-positive gastric cancer and provides some preliminary experimental basis for its inhibitory effect.

Three Decades of Ultrasound Contrast Agents: A Review of the Past, Present and Future Improvements

Initial reports from the 1960s describing the observations of ultrasound contrast enhancement by tiny gaseous bubbles during echocardiographic examinations prompted the development of the first ultrasound contrast agent in the 1980s. Current commercial contrast agents for echography, such as Definity, Optison, Sonazoid and SonoVue, have proven to be successful in a variety of on- and off-label clinical indications. Whereas contrast-specific technology has seen dramatic progress after the introduction of the first approved agents in the 1990s, successful clinical translation of new developments has been limited during the same period, while understanding of microbubble physical, chemical and biologic behavior has improved substantially. It is expected that for a successful development of future opportunities, such as ultrasound molecular imaging and therapeutic applications using microbubbles, new creative developments in microbubble engineering and production dedicated to further optimizing microbubble performance are required, and that they cannot rely on bubble technology developed more than 3 decades ago.

In vivo Biodistribution of Radiolabeled Acoustic Protein Nanostructures

PURPOSE

Contrast-enhanced ultrasound plays an expanding role in oncology, but its applicability to molecular imaging is hindered by a lack of nanoscale contrast agents that can reach targets outside the vasculature. Gas vesicles (GVs)-a unique class of gas-filled protein nanostructures-have recently been introduced as a promising new class of ultrasound contrast agents that can potentially access the extravascular space and be modified for molecular targeting. The purpose of the present study is to determine the quantitative biodistribution of GVs, which is critical for their development as imaging agents.

PROCEDURES

We use a novel bioorthogonal radiolabeling strategy to prepare technetium-99m-radiolabeled ([^99mTc])GVs in high radiochemical purity. We use single photon emission computed tomography (SPECT) and tissue counting to quantitatively assess GV biodistribution in mice.

RESULTS

Twenty minutes following administration to mice, the SPECT biodistribution shows that 84 % of [^99mTc]GVs are taken up by the reticuloendothelial system (RES) and 13 % are found in the gall bladder and duodenum. Quantitative tissue counting shows that the uptake (mean ± SEM % of injected dose/organ) is 0.6 ± 0.2 for the gall bladder, 46.2 ± 3.1 for the liver, 1.91 ± 0.16 for the lungs, and 1.3 ± 0.3 for the spleen. Fluorescence imaging confirmed the presence of GVs in RES.

CONCLUSIONS

These results provide essential information for the development of GVs as targeted nanoscale imaging agents for ultrasound.

Advances in Diagnostic and Intraoperative Molecular Imaging of Pancreatic Cancer

Pancreatic ductal adenocarcinoma (PDAC) has a dismal prognosis. To improve outcomes, there is a critical need for improved tools for detection, accurate staging, and resectability assessment. This could improve patient stratification for the most optimal primary treatment modality. Molecular imaging, used in combination with tumor-specific imaging agents, can improve established imaging methods for PDAC. These novel, tumor-specific imaging agents developed to target specific biomarkers have the potential to specifically differentiate between malignant and benign diseases, such as pancreatitis. When these agents are coupled to various types of labels, this type of molecular imaging can provide integrated diagnostic, noninvasive imaging of PDAC as well as image-guided pancreatic surgery. This review provides a detailed overview of the current clinical imaging applications, upcoming molecular imaging strategies for PDAC, and potential targets for imaging, with an emphasis on intraoperative imaging applications.

Ultrasound Molecular Imaging of the Breast Cancer Neovasculature using Engineered Fibronectin Scaffold Ligands: A Novel Class of Targeted Contrast Ultrasound Agent

Molecularly-targeted microbubbles (MBs) are increasingly being recognized as promising contrast agents for oncological molecular imaging with ultrasound. With the detection and validation of new molecular imaging targets, novel binding ligands are needed that bind to molecular imaging targets with high affinity and specificity. In this study we assessed a novel class of potentially clinically translatable MBs using an engineered 10^th type III domain of human-fibronectin (MB-FN3_VEGFR2) scaffold-ligand to image VEGFR2 on the neovasculature of cancer. The in vitro binding of MB-FN3_VEGFR2 to a soluble VEGFR2 was assessed by flow-cytometry (FACS) and binding to VEGFR2-expressing cells was assessed by flow-chamber cell attachment studies under flow shear stress conditions. In vivo binding of MB-FN3_VEGFR2 was tested in a transgenic mouse model (FVB/N Tg(MMTV/PyMT634Mul) of breast cancer and control litter mates with normal mammary glands. In vitro FACS and flow-chamber cell attachment studies showed significantly (P<0.01) higher binding to VEGFR2 using MB-FN3_VEGFR2 than control agents. In vivo ultrasound molecular imaging (USMI) studies using MB-FN3_VEGFR2 demonstrated specific binding to VEGFR2 and was significantly higher (P<0.01) in breast cancer compared to normal breast tissue. Ex vivo immunofluorescence-analysis showed significantly (P<0.01) increased VEGFR2-expression in breast cancer compared to normal mammary tissue. Our results suggest that MBs coupled to FN3-scaffolds can be designed and used for USMI of breast cancer neoangiogenesis. Due to their small size, stability, solubility, the lack of glycosylation and disulfide bonds, FN3-scaffolds can be recombinantly produced with the advantage of generating small, high affinity ligands in a cost efficient way for USMI.

Ultrasound molecular imaging: Moving toward clinical translation

Ultrasound is a widely available, cost-effective, real-time, non-invasive and safe imaging modality widely used in the clinic for anatomical and functional imaging. With the introduction of novel molecularly-targeted ultrasound contrast agents, another dimension of ultrasound has become a reality: diagnosing and monitoring pathological processes at the molecular level. Most commonly used ultrasound molecular imaging contrast agents are micron sized, gas-containing microbubbles functionalized to recognize and attach to molecules expressed on inflamed or angiogenic vascular endothelial cells. There are several potential clinical applications currently being explored including earlier detection, molecular profiling, and monitoring of cancer, as well as visualization of ischemic memory in transient myocardial ischemia, monitoring of disease activity in inflammatory bowel disease, and assessment of arteriosclerosis. Recently, a first clinical grade ultrasound contrast agent (BR55), targeted at a molecule expressed in neoangiogenesis (vascular endothelial growth factor receptor type 2; VEGFR2) has been introduced and safety and feasibility of VEGFR2-targeted ultrasound imaging is being explored in first inhuman clinical trials in various cancer types. This review describes the design of ultrasound molecular imaging contrast agents, imaging techniques, and potential future clinical applications of ultrasound molecular imaging.