Ultrasound Molecular Imaging with Targeted Microbubbles for Cancer Diagnostics: From Bench to Bedside

Ultrasound sensitive O2 microbubbles radiosensitize murine breast cancer but lead to higher metastatic spread

The inadequate level of oxygenation in tumors has been shown to correlate not only with greater invasiveness of cancer cells, but also with a reduction in their sensitivity to anticancer therapies. Over the years, many attempts have been made to increase the oxygenation level of cancer, but most of them have been ineffective. We investigated the heterogeneous response of tumor tissue to phospholipid-coated oxygen microbubbles (OMB) in murine tumors in vivo using oxygen and hemoglobin saturation mapping and the influence of OMB treatment on microvasculature, perfusion, and radiotherapy effectiveness. Intravenous administration of OMB followed by ultrasound pulse leads to increased oxygenation of a tumor, found mainly in the vicinity of tumor vessels, while intratumoral delivery resulted in areas of increased pO₂ more evenly distributed within the tumor. Furthermore, hemoglobin contributes little to the increase in tumor oxygenation caused by oxygen microbubbles. Extensive vasculature disruption was observed in the groups treated with both oxygen/nitrogen microbubbles and ultrasound pulse. This therapy also led to a reduction in the coverage of the vessels by pericytes, while the density of the microvessels was unchanged. Radiotherapy with a single dose of 12Gy reduced tumor growth by 50% in all treated groups. Unfortunately, at the same time, the number of macroscopic metastases in the lungs increased significantly after intravenous administration of oxygen/nitrogen microbubbles and the application of an ultrasound pulse. In conclusion, ultrasound-sensitive oxygen microbubbles are effective in delivering oxygen to tumor tissue, thus increasing the effectiveness of radiotherapy. However, cavitation effects and destruction of the integrity of tumor vessels result in greater spread of cancer cells in the host organism.

Tumour associated vasculature-on-a-chip for the evaluation of microbubble-mediated delivery of targeted liposomes

The vascular system is the primary route for the delivery of therapeutic drugs throughout the body and is an important barrier at the region of disease interest, such as a solid tumour. The development of complex 3D tumour cultures has progressed significantly in recent years however, the generation of perfusable vascularised tumour models still presents many challenges. This study presents a microfluidic-based vasculature system that can be induced to display properties of tumour-associated blood vessels without direct incorporation of tumour cells. Conditioning healthy endothelial-fibroblast cell vasculature co-cultures with media taken from tumour cell cultures was found to result in the formation of disorganised, tortuous networks which display characteristics consistent with those of tumour-associated vasculature. Integrin αvβ3, a cell adhesion receptor associated with angiogenesis, was found to be upregulated in vasculature co-cultures conditioned with tumour cell media (TCM) - consistent with the reported αvβ3 expression pattern in angiogenic tumour vasculature in vivo. Increased accumulation of liposomes (LSs) conjugated to antibodies against αvβ3 was observed in TCM networks compared to non-conditioned networks, indicating αvβ3 may be a potential target for the delivery of drugs specifically to tumour vasculature. Furthermore, the use of microbubbles (MBs) and ultrasound (US) to further enhance the delivery of LSs to TCM-conditioned vasculature was investigated. Quantification of fluorescent LS accumulation post-perfusion of the vascular network showed 3-fold increased accumulation with the use of MBs and US, suggesting that targeted LS delivery could be further improved with the use of locally administered MBs and US.

Increasing oxygen tension in tumor tissue using ultrasound sensitive O2 microbubbles

Low tissue oxygenation significantly impairs the effectiveness of cancer therapy and promotes a more aggressive phenotype. Many strategies to improve tissue oxygenation have been proposed throughout the years, but only a few showed significant effects in clinical settings. We investigated stability and ultrasound pulse (UP) triggered oxygen release from phospholipid coated oxygen microbubbles (OMB) in vitro and in murine tumors in vivo using EPR oximetry. In solution, the investigated microbubbles are stable and responsive to ultrasound pulse. The addition of the OMB solution alone resulted in an increase in pO₂ of approximately 70 mmHg which was further increased for an additional 80 mmHg after the application of UP. The in vivo kinetic study revealed a substantial, up to 120 mmHg, increase in tumor pO₂ after UP application and then pO₂ was decreasing for 20 min for intravenous injection and 15 min for intratumoral injection. A significant increase was also observed in groups that received microbubbles filled with nitrogen and ultrasound pulse and OMB without UP, but the effect was much lower. Oxygen microbubbles lead to a decrease in HIF-1a and VEGF-A both at the level of mRNA and protein. Toxicity analysis showed that intravenous injection of OMB does not cause oxidative damage to the heart, liver, or kidneys. However, elevated levels of oxidative damage to lipids and proteins were observed short-term in tumor tissue. In conclusion, we have demonstrated the feasibility of oxygen microbubbles in delivering oxygen effectively and safely to the tumor in living animals. Such treatment might enhance the effectiveness of other anticancer therapies.

Ultrasound-Assisted Extraction and the Encapsulation of Bioactive Components for Food Applications

Various potential sources of bioactive components exist in nature which are fairly underutilized due to the lack of a scientific approach that can be sustainable as well as practically feasible. The recovery of bioactive compounds is a big challenge and its use in food industry to develop functional foods is a promising area of research. Various techniques are available for the extraction of these bioactives but due to their thermolabile nature, there is demand for nonthermal or green technologies which can lower the cost of operation and decrease operational time and energy consumption as compared to conventional methods. Ultrasound-assisted extraction (UAE) is gaining popularity due to its relative advantages over solvent extraction. Thereafter, ultrasonication as an encapsulating tool helps in protecting the core components against adverse food environmental conditions during processing and storage. The review mainly aims to discuss ultrasound technology, its applications, the fundamental principles of ultrasonic-assisted extraction and encapsulation, the parameters affecting them, and applications of ultrasound-assisted extraction and encapsulation in food systems. Additionally, future research areas are highlighted with an emphasis on the energy sustainability of the whole process.

Therapeutic oxygen delivery by perfluorocarbon-based colloids

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

Magnetic Resonance Detection of Gas Microbubbles via HyperCEST: A Path Toward Dual Modality Contrast Agent

Gas microbubbles are an established clinical ultrasound contrast agent. They could also become a powerful magnetic resonance (MR) intravascular contrast agent, but their low susceptibility-induced contrast requires high circulating concentrations or the addition of exogenous paramagnetic nanoparticles for MR detection. In order to detect clinical in vivo concentrations of raw microbubbles via MR, an alternative detection scheme must be used. HyperCEST is an NMR technique capable of indirectly detecting signals from very dilute molecules (concentrations well below the NMR detection threshold) that exchange hyperpolarized 129 Xe. Here, we use quantitative hyperCEST to show that microbubbles are very efficient hyperCEST agents. They can accommodate and saturate millions of 129 Xe atoms at a time, allowing for their indirect detection at concentrations as low as 10 femtomolar. The increased MR sensitivity to microbubbles achieved via hyperCEST can bridge the gap for microbubbles to become a dual modality contrast agent.

Lipid Shell Retention and Selective Binding Capability Following Repeated Transient Acoustic Microdroplet Vaporization

Targeted therapy and molecular imaging using ultrasound have been widely explored using microbubble contrast agents, and more recently, activatable droplet contrast agents that vaporize when exposed to focused ultrasound have been explored. These droplets are coated with a stabilizing, functionalizable shell, typically comprised of fully saturated phospholipids. While the shedding of the lipid shell under ultrasound exposure is a well-studied phenomenon in microbubbles, it has not been fully explored in droplet-based contrast agents, particularly in those that undergo a reversible phase change and recondense following vaporization. Here, we investigate the retention of the lipid shell following repeated transient vaporization events. Two separate fluorescent markers were used to track individual lipid subpopulations: PEGylated lipids, to which targeting ligands are typically bound, and non-PEGylated lipids, which primarily contribute to droplet stability. Following confirmation of the homogeneous surface distribution of each subpopulation of shell lipids using confocal microscopy, high-speed optical imaging provided visual evidence of the ability to repeatedly induce vaporization and recondensation in micron-scale droplets using 5.208 MHz, 3.17 MPa focused ultrasound pulses transmitted from an imaging transducer. Flow cytometry analysis indicated that while PEGylated lipids were fully retained following repeated transient phase change events, 20% of the bulk lipids were shed. While this likely contributed to an observed significant reduction in the average droplet diameter, the selective binding capabilities of droplets functionalized with an RGD peptide, targeted to the integrin α_vβ₃, were not affected. These results indicate that repeated droplet activation may promote shifts in the droplet size distribution but will not influence the accuracy of targeting for therapy or molecular imaging.

Integration of microbubbles with biomaterials in tissue engineering for pharmaceutical purposes

Tissue engineering with the aid of biomaterials is a novel and promising knowledge aiming at improving human life expectancy. Besides, microbubbles are increasingly employed in biomedical applications due to their capability as a reservoir of therapeutic agents and oxygen molecules. In the present study, Microbubbles as the backbone of the research are produced as one of the potent devices in tissue engineering approaches, including drug delivery, wound healing, 3D printing, and scaffolding. It was shown that microbubbles are capable of promoting oxygen penetration and boosting the wound healing process by supplying adequate oxygen. Microbubbles also demonstrated their strength and potency in advancing drug delivery systems by reinforcing mass transfer phenomena. Furthermore, microbubbles developed the mechanical and biological characteristics of engineered scaffolds by manipulating the pores. Increasing cell survival, the biological activity of cells, angiogenesis, cell migration, and also nutrient diffusion into the inner layers of the scaffold were other achievements by microbubbles. In conclusion, the interest of biomedical communities in simultaneous usage of microbubbles and biomaterials under tissue engineering approaches experiences remarkable growth in Pharmaceutical studies.

Core–shell particles for drug-delivery, bioimaging, sensing, and tissue engineering

Core–shell particles offer significant advantages in their use for bioimaging and biosensors.

Nanobubble Liposome Complexes for Diagnostic Imaging and Ultrasound-Triggered Drug Delivery in Cancers: A Theranostic Approach

The ability of ultrasound contrast agents to enhance the cell membrane permeability in response to an ultrasound pulse has unveiled avenues to facilitate the delivery of a higher intracellular payload at target sites. In light of the above, we report the development of submicron-sized (528.7 ± 31.7 nm) nanobubble-paclitaxel liposome (NB-PTXLp) complexes for ultrasound imaging and ultrasound responsive drug delivery in cancer cells. With a paclitaxel entrapment efficiency of 85.4 ± 4.39%, the 200 nm-sized liposomes tethered efficiently (conjugation efficiency ∼98.7 ± 0.14%) with the nanobubbles to form conjugates. Sonoporation of MiaPaCa-2 cells upon treatment with nanobubbles and ultrasound enhanced cellular permeability, resulting in 2.5-fold higher uptake of liposomes in comparison to only liposome treatment. This manifested into more than 300-fold higher anticancer activity of NB-PTXLps in the presence of ultrasound in MiaPaCa-2, Panc-1, MDA-MB-231, and AW-8507 cell lines, compared to commercial formulation ABRAXANE. Also, the NB-PTXLp conjugates were found to exhibit echogenicity comparable to the commercial ultrasound contrast agent SonoVue. In addition, the developed nanobubbles were found to exhibit more than 1 week echogenic stability as opposed to 6 h stability of the commercially available ultrasound contrast agent SonoVue. Hence, the NB-PTXLps developed herein could prove to be a promising and minimally invasive theranostic platform for cancer treatments in the future.

Developing Protein-Based Nanoparticles as Versatile Delivery Systems for Cancer Therapy and Imaging

In recent years, it has become apparent that cancer nanomedicine's reliance on synthetic nanoparticles as drug delivery systems has resulted in limited clinical outcomes. This is mostly due to a poor understanding of their "bio-nano" interactions. Protein-based nanoparticles (PNPs) are rapidly emerging as versatile vehicles for the delivery of therapeutic and diagnostic agents, offering a potential alternative to synthetic nanoparticles. PNPs are abundant in nature, genetically and chemically modifiable, monodisperse, biocompatible, and biodegradable. To harness their full clinical potential, it is important for PNPs to be accurately designed and engineered. In this review, we outline the recent advancements and applications of PNPs in cancer nanomedicine. We also discuss the future directions for PNP research and what challenges must be overcome to ensure their translation into the clinic.