Encyclopedia
Please note this is an old version of this entry, which may differ significantly from the current revision.
Subjects:
Engineering, Biomedical
Ultrasound-mediated cavitation shows great promise for improving targeted drug delivery across a range of clinical applications. Cavitation nuclei—sound-sensitive constructs that enhance cavitation activity at lower pressures—have become a powerful adjuvant to ultrasound-based treatments, and emerged as a drug delivery vehicle in their own right. The unique combination of physical, biological, and chemical effects that occur around these structures, as well as their varied compositions and morphologies, make cavitation nuclei an attractive platform for creating delivery systems tuned to particular therapeutics.
- drug delivery
- ultrasound
- cavitation
- sonosensitive
- nanoparticles
- cavitation nuclei
1. Drug Delivery to Solid Tumors
Ninety percent of drugs under development fail after advancing to the early phases of clinical trials, with over half of them failing due to lack of efficacy [158]. Almost all the anti-cancer agents reaching the human trial phase would have had favorable preclinical data, reflecting the complex barriers to drug efficacy encountered in clinical translation [159]. Whilst a lack of efficacy in early-phase oncology trials could be multifactorial, there is strong evidence to suggest that suboptimal drug delivery and poor penetration of drugs in solid tumors contributes significantly to the reduced efficacy of anti-cancer agents [160,161]. Attempts to optimize drug delivery to the tumor by means of increasing the systemic dose of cytotoxic agents would invariably result in dose-limiting toxicities in the patient due to such agents targeting both healthy cells as well as target tumor cells indiscriminately.
The last two decades have seen an exponential growth in the discovery of targeted drugs, designed to target the cancer cells with high potency whilst sparing normal cells [162]. Targeted therapies could be broadly classified into two categories based on their size: small molecules (mass <1 kDa, effective size <5 nm) and larger macromolecules such as monoclonal antibodies (mAbs) (100–150 kDa, ~10 nm) [11,163]. Many small molecules in targeted therapy are kinase inhibitors (in particular, tyrosine kinase inhibitors—TKIs) which block signalling pathways dysregulated during tumor formation.
mABs specifically act on extracellular proteins as they are typically too large to enter the cells and so they inhibit tumor growth by preventing the interactions between receptors and ligands and triggering events such as antibody-directed cell cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC) [163]. Whilst the advances in targeted therapy have produced a paradigm shift in the survival outcomes of a subset of cancer patient populations, such as patients with non-small-cell lung cancer and targetable mutations [164], for some others the benefits have been modest at best [165]. A recent analysis has disappointingly highlighted that of the 207 cancer drugs approved by the FDA between the years 2016 and 2021, only 28 (14%) managed to displace the existing first-line therapies [166]. Failure of these highly specific agents to alter the landscape of current therapy emphasizes the importance and prescience of addressing the issue of suboptimal drug delivery and penetrance within the tumor.
Clinical Progress
Despite the breadth of preclinical data on ultrasound-enhanced drug delivery, clinical trials on ultrasound-enhanced drug delivery for extracranial solid tumors with published reported outcomes are limited. Several studies have looked at combining thermosensitive liposomes with ultrasound, as reviewed in Chaudhry et al. [167]. This includes TARDOX, a first-in-human clinical study looking at ultrasound-triggered targeted drug delivery of doxorubicin from thermosensitive liposomes in liver tumors. The study concluded that the approach is a safe and feasible delivery system and is also able to achieve single-cycle chemo-ablative response in liver tumors which are refractory to standard therapy [168]. Another clinical study utilized microbubbles and ultrasound for the enhancement of the delivery of chemotherapy to pancreatic tumors. Ten patients with inoperable pancreatic cancer were treated with gemcitabine chemotherapy followed by immediate intermittent bolus injections of SonoVue microbubbles and sonoporation using a diagnostic ultrasound machine. The authors reported an improvement in median survival of 8.7 months. However, it should be noted that the study did not have a control group and the results were compared to a historical control, requiring cautious interpretation [122]. SONCHIMIO, a randomized early-phase study in patients with metastatic colorectal cancer, is examining standard chemotherapy versus chemotherapy with sonoporation using microbubbles [169]. The trial has now completed recruitment after enrolling seven participants, but the results have not been published yet. Table 1 shows a summary of clinical trials on ultrasound-enhanced drug delivery in extra-cranial solid tumors, recruiting as of 12 January 2023. The table highlights that whilst consensus is emerging around the most appropriate target indications, with liver resident and pancreatic tumors commonly chosen, there is no agreement yet on the need for or type of CN.
Table 1. Overview of clinical trials involving ultrasound-enhanced drug delivery to extra-cranial solid tumors.
| Tumor Site | Study Design | n | Cavitation Agent | Anti-Cancer Agent | US Modality | ClinicalTrials.Gov or ISCRTN ID |
|---|---|---|---|---|---|---|
| Colorectal liver metastases | Phase 1–2 | 45 | SonoTran® Particles (OxsSonics) (nanocups) | FOLFIRI and Cetuximab | Focused ultrasound | ISRCTN17598292 [170] |
| Solid tumours with liver metastases (colon and pancreas) | Phase 1–2 | 37–43 | Microbubble–microdroplet clusters (PS101) | FOLFOX/FOLFIRI for colorectal cancer; Gemcitabine and nab-paclitaxel for pancreatic cancer | Dual-frequency ultrasound | NCT04021277 [171] |
| Breast | Window-of-opportunity study, randomized | 48 | No cavitation agent | Gemcitabine | Focused ultrasound | NCT04796220 [172] |
| Liver metastases from breast and colon/rectum | 2 liver metastases in each patient; 1 target lesion and the other control lesion | 60 | SonoVue® (Bracco) Microbubbles |
Chemotherapy not otherwise specified | Focused ultrasound | NCT03477019 [173] |
| Metastatic colorectal cancer | Early phase, single arm | 10 | No cavitation agent | Toripalimab and regorafenib | HIFU | NCT04819516 [174] |
| Locally advanced pancreatic cancer | Early-phase, single-arm, exploratory clinical trial | 60 | No cavitation agent | FOLFIRINOX | HIFU | NCT05262452 [175] |
| Pancreas | Phase I/II, randomized | 120 | Sonazoid microbubbles | FOLFIRINOX | Focused ultrasound | NCT04821284 [176] |
| Inoperable pancreatic ductal adenocarcinoma | Early phase, randomized | 30 | SonoVue Microbubbles |
Chemotherapy, not otherwise specified | Focused ultrasound | NCT04146441 [177] |
| Pediatric refractory solid tumor | Phase 1, non-randomized | 34 | No cavitation agent | ThermoDox | MR-HIFU | NCT02536183 [178] |
2. Transdermal Vaccine Delivery
Needle-free administration of drugs through the skin has long been desired in clinical medicine, as it offers the potential for painless, non-invasive delivery which may avoid first-pass metabolism, and in the context of vaccination could provide greater activity per dose [179]. The primary obstacle to transcutaneous or transdermal drug delivery is the highly effective barrier of the stratum corneum (“SC”, the skin’s outermost layer), which excludes all but very small (<500 Da), lipophilic (log P = 1–3) therapeutic molecules from diffusing through to the body [180]. Ultrasound has been studied with increasing interest over the past 30 years as a method of overcoming the SC and delivering drugs through the skin, a process termed “sonophoresis” [181].
Of the many bioeffects of ultrasound, inertial cavitation has been repeatedly indicated as the primary mechanism behind sonophoresis [182,183,184,185]. The violent collapse of bubbles aids transport of drugs through the skin in two main ways: increasing the permeability of the SC and providing a convective force to “pump” drugs through it. Cavitation permeabilizes the skin primarily through high-velocity fluid microjets caused by asymmetric bubble collapse [183,185], which mechanically disrupt the SC and cause the reversible formation of channels through it. Microstreaming flows generated around cavitating bubbles can also help to push drug molecules through the skin, further improving delivery [186]. CN therefore hold significant potential to improve sonophoretic transcutaneous and transdermal vaccine and drug administration, as they allow much greater cavitation activity at much lower pressures than is possible with endogenous tissue nuclei alone.
Another promising application of CN in transdermal delivery is to address the phenomenon of Localized Transport Regions (LTRs). Many sonophoresis studies have demonstrated a few, seemingly random, discrete patches of skin (LTRs) experiencing far more permeabilization, and therefore receiving far more drug, than the rest of the sonicated area [182,187,188,189]. LTRs are thought to occur at locations of greater cavitation activity [187,188], and increasing LTR coverage of the skin is an important topic in the interest of maximizing delivered dose. Pre-seeding the skin’s surface with CN could allow much more uniform, repeatable cavitation activity and thereby LTR formation.
3. Wound Healing
Oxygen is essential for wound healing in tissues. More than just a nutrient, it is required for oxidative metabolism and the regulation of many signal transduction pathways and immune cell activity [190]. Hypoxia can lead to chronic ischemic wounds [191]. Wound oxygenation is thus a key determinant of healing outcomes and often used as a metric for treatment plans, including whether amputation is required [192]. CN have the potential to be a very useful vehicle in delivering oxygen to wound sites.
Hyperbaric oxygen therapy (HBOT) and topical oxygen therapy (TOT) are the most common clinical treatments for mitigating hypoxia in wounds. HBOT, in which a patient breathes pure oxygen at pressures greater than atmospheric pressure to induce hyperoxygenation, carries a risk of oxygen organ toxicity due to oxidative stress and genotoxicity because it is not targeted [193]. TOT, in contrast, applies oxygen gas at 100% saturation directly to the wound bed, and is possible with lightweight wearable systems that utilize compression-stacked electrochemical oxygen generators [194]. While it has been demonstrated to increase oxygen partial pressure levels at the wound base centre and decrease wound size and healing time compared to patients who did not undergo the therapy [193], diffusion of oxygen is limited. Oxygen gas on its own can penetrate through the epidermis into the dermis, but at deeper dermis layers there is no change in oxygen concentration [195].
Perfluorocarbon (PFC) emulsions developed to absorb large amounts of oxygen have recently emerged as an alternative treatment option. When applied to a hypoxic area, the oxygen will diffuse down the concentration gradient, providing oxygen to the needed area. These emulsions have been found to reduce complications after skin procedures, decrease tissue hypoxia in phlegmons, increase the speed of local wound healing, and enhance the rate of epithelialization [196,197,198].
Researchers hypothesize that techniques used to deliver oxygen via CN could also improve wound healing applications. In 2015, Eisenbrey proposed that his technique of exposing oxygen-loaded microbubbles to ultrasound to increase oxygen content in breast tumors could benefit wound healing in the future [199]. In 2023, Ho et al. found that cavitation of oxygen-loaded microbubbles increased vasodilation and angiogenesis at ischaemia–perfusion vessels. Their proposed pathway of cavitation induced endothelial nitric oxide synthase (eNOS) activation, vascular endothelial growth factor (VEGF) expression, and a reduction in interstitial hydrogen peroxide, all instrumental in remodelling of the vascular architecture in the wound healing process. They thus hypothesized that this pathway could drive further investigation into wound healing and avoid the side effects induced by hyperbaric oxygen therapy [200].
Though cavitation-mediated oxygen delivery in wound healing has not yet been clinically tested, current applications in hydrogels and other biomaterial scaffolds show promise. Nanodroplets engineered to carry oxygen via haemoglobin encapsulation increased cell viability of cardiomyocytes in a GelMA hydrogel and expedited the repair of infarcted tissues when exposed to low-intensity pulsed ultrasound [201]. Preliminary work exploring the effect of oxygen-loaded microbubbles exposed to ultrasound on human dermal fibroblasts (HDFs) also found that they increased HDF viability [202]. As HDFs are one of the main cell types involved in the wound healing process, increased viability could aid in a timely repair process.
PFCs have also been commonly used in CN [199,203,204,205,206]. PFC oxygen-loaded cavitation nuclei can be used to deliver oxygen to hypoxic regions of wound beds, combining the advantages of high-oxygen PFC emulsions with the added benefits of ultrasound-mediated cavitation to include temporal release, deeper penetration, and specific localization within the wound bed. Some physicians argue that oxygen therapy for wounds should be a multi-faceted approach in which oxygen is specifically dosed as a function of tissue hypoxia [191] and focused at the centre of the wound where hypoxia is typically greatest. Sonosensitive cavitation nuclei as a vehicle for oxygen delivery present a plausible solution to these challenges.
4. Biofilms
Between 65% and 80% of clinical bacterial infections involve biofilms: communities of microbes existing within a matrix of extracellular polymeric substances including proteins, polysaccharides, and extracellular DNA [207]. Biofilms present a major challenge to effective antibiotic treatments; not only do they form a physical barrier to antibiotic drug penetration [208], but they also promote functional changes associated with resistance, including slower growth rates and communal stress responses [209]. Because of this, bacteria residing in biofilms are known to be 10–1000 times more resistant to antibiotics than planktonic bacteria [210]. As emerging pathogens become increasingly resistant to new antibiotics, novel strategies of treating infections by targeting the biofilm matrix have gained momentum, such as through biofilm-degrading enzymes [211] or, more recently, ultrasound in combination with CN [212].
CN have been shown to be effective in treating a wide variety of infections, including in vitro monospecies biofilms of both Gram-positive [213,214] and Gram-negative [215,216] bacterial species as well as more complex models, such as an infected clot model [217], a bladder organoid model [218], and an in vivo infected catheter model [219]. In all cases, the benefit of applying CN to a biofilm is twofold; first, cavitation can disrupt the structural integrity of the biofilm matrix via production of craters [220] and micropores [221], and second, it can enhance the delivery of antibiotic drugs, including gentamicin [216,218,220], vancomycin [213,221], oxacillin [212], and streptomycin [220]. Most studies have found that the synergistic application of cavitation with an antibiotic is more effective than either cavitation or antibiotic administration alone, in part because dispersion changes the physiological state of bacteria [222] and most antibiotic drugs are effective only on cells that are metabolically active [223,224]. That being said, dispersed bacteria may also be more virulent and more likely to reinfect than their planktonic counterparts [222].
The ease of functionalization of CN has also been explored in biofilm applications, primarily the conjugation of drugs and/or targeting ligands to lipid-shelled microbubbles. Vancomycin, in particular, has been explored both for its binding affinity and its cytotoxicity to Gram-positive bacteria [214], as its mode of action involves binding to the D-ala-D-ala moiety on the bacterial cell wall, hindering cell wall synthesis [225]. This theranostic strategy could theoretically be employed with other antibiotics that work by binding to surface receptors. Liposomes containing gentamicin have also been covalently attached to microbubbles, which has shown to result in higher intracellular delivery after ultrasound exposure than liposomal formulations of the drug alone [218]. Finally, “bioactive” gases have been evaluated for their ability to enhance the therapeutic potential of cavitation-mediated biofilm disruption and antibiotic delivery. Nitric oxide (NO) is a gaseous signalling molecule that has been shown to disperse biofilms at low concentrations and kill pathogens at high concentrations [226], but is limited by its short half-life and limited penetration [227]. Encapsulating NO into lipid-shelled microbubbles is therefore able to increase the bioavailability of NO and has shown to result in biofilm removal and clinically relevant log reductions in culturable bacteria when combined with ultrasound and gentamicin [215]. Although lingering questions remain regarding the downstream effects and overall safety of cavitation-mediated biofilm dispersion, the urgency of antibiotic resistance warrants continued research in this area.
5. Blood–Brain Barrier
The blood–brain barrier is a key feature of the vasculature of the brain and consists of a layer of endothelial cells sealed together by specialized cell–cell junctions and supported by other cell types, including pericytes and astrocytes. The cells comprising the BBB have an exceptionally low number of transcytosis vesicles, specialized drug metabolizing enzymes, and efflux pumps to expel potentially toxic substances from brain to blood, culminating in a dynamic physical and metabolic barrier [228,229,230,231]. In tandem, specific transporters and carrier proteins are enriched in brain endothelial cells to facilitate controlled transport of specific essential nutrients and metabolites, such as glucose (GLUT-1), amino acids (LAD1), and transferrin [232]. The tight control of transport in both directions prevents the entry of most substances from the systemic blood supply, including leukocytes, into the brain, which helps maintain the physiological conditions required for neural signalling, and to shield neural tissues from neurotoxins in the blood [233]. Unfortunately, the BBB also acts as the major bottleneck in drug delivery to treat diseases in the central nervous system (CNS) [233]; 98% of small-molecule drugs, and all biologics, cannot pass through the BBB into a non-disrupted brain unaided [234]. This presents a significant barrier to delivering a meaningful concentration of drugs to treat neurological diseases, which were the second leading cause of death between 1990 and 2016, including psychiatric diseases, neurodegenerative diseases, brain cancers, and strokes.
The combination of ultrasound and multiple types of CN has been shown to non-invasively and reversibly open the BBB to achieve the delivery of macromolecular therapeutic drugs such as monoclonal antibodies, genes, and chemotherapies into the brain parenchyma [235,236,237]. The ability to focus the US beam down to the millimetre scale provides much more spatiotemporal precision and control than alternative approaches, such as co-administration of vasodilators or hyperosmotic agents, Trojan horse (MAb conjugation), etc., and hence limits the risk of off-target adverse effects in the brain [238]. US + CN can be combined with real-time stereotactic image guidance or acoustic emission monitoring to provide feedback on efficacy and risk, allowing fine tuning of the US dose during treatment [238,239].
6. Gastrointestinal Drug Delivery
Due to its convenience, non-invasiveness, and ease of use, oral drug delivery is the preferred route of administration for patients [240]. The two major barriers to effective oral delivery of biopharmaceuticals includes their instability in the gastrointestinal tract, and their erratic absorption across the intestinal membrane into systemic circulation. Permeability is limited by two physiological barriers: the mucus layer and the epithelial layer. The mucus layer slows the diffusion rate of large molecules, whilst the epithelium prevents the diffusion of large molecules [241,242].
It is hypothesized that the application of ultrasound to the gastrointestinal tract could provide a means of rapidly delivering small molecules, but also facilitate the delivery of macromolecules across the mucus and epithelial layers of the gastrointestinal tract [243]. Ultrasound-mediated gastrointestinal drug delivery research is largely influenced by the transdermal drug delivery literature, which has highlighted the potential of low-frequency ultrasound to facilitate drug delivery [244,245,246,247,248,249,250,251].
Schoellhammer et al. demonstrated the safety and tolerance of low-frequency ultrasound-mediated drug delivery of small and large molecules in vitro and in vivo, showing successful delivery in both the rectum and buccal cavity without cavitation nuclei [252,253,254,255,256]. Stewart et al. reported on the development of a multimodal diagnostic endoscopic capsule device, with high-frequency quantitative micro-ultrasound complementing video imaging, allowing subsurface visualization and computer-assisted diagnosis. The studies showed that the application of ultrasound to Caco-2 cell monolayers alone and ultrasound combined with Sonovue microbubbles (Sonovue®, phospholipid coating with sulfur hexafluoride gas core; Bracco diagnostics, Inc., Milan, Italy) decreased transepithelial electrical resistance, suggesting permeabilization of the cell layer. The team also demonstrated the ability to direct microbubble streams to the focus of the transducer using acoustic radiation forces [257,258].
Using a modified version of the prototype capsule reported previously, ultrasound-mediated targeted drug delivery of quantum dots has been demonstrated with ex vivo tissue and in vivo. Fluorescent markers chosen as a model drug were used to demonstrate in vivo delivery, using a porcine small intestine with this capsule in vivo. The fluorescent markers combined with microbubbles and focused ultrasound were shown to penetrate the mucus layer of the small intestine. These findings illustrate the potential of this device for ultrasound-mediated gastrointestinal drug delivery, and the challenges (e.g., tethering of capsule, debris lodged into outlets) to be overcome before focused ultrasound and microbubbles could be used with this device for the oral delivery of biopharmaceuticals [259].
Fix et al. evaluated the potential of using ultrasound-stimulated phase-change ultrasound contrast agents (1,2-distearoylsn-glycero-3-phosphocholine and 1,2-distearoyl-snglycero-3-phosphoethanolamine-N-methoxy(polyethyleneglycol)-2000 with octofluoropropane gas) to cause transient disruption of Caco-2 epithelial monolayers cultured on permeable Transwell supports and enhance the permeation of a model macromolecular drug. The team assessed ultrasound-mediated drug delivery through Caco-2 monolayers where ultrasound and phase-change ultrasound contrast agents combined were found to enhance dextran delivery in comparison to the negligible amount delivered in the control samples (phase-change ultrasound contrast agents alone; ultrasound alone) [260].
In summary, ultrasound-mediated gastrointestinal drug delivery is a nascent field with many unexplored research questions, but the flexibility of the various CN described above could be of enormous benefit.
This entry is adapted from the peer-reviewed paper 10.3390/molecules28237733
This entry is offline, you can click here to edit this entry!
