Abstract
Because of their size (1–10 μm), microbubble-based drug delivery agents suffer from confinement to the vasculature, limiting tumor penetration and potentially reducing the drug efficacy. Nanobubbles (NBs) have emerged as promising candidates for ultrasound-triggered drug delivery because of their small size, allowing drug delivery complexes to take advantage of the enhanced permeability and retention effect. In this study, we describe a simple method for production of nested-nanobubbles (Nested-NBs) by encapsulation of NBs (∼100 nm) within drug-loaded liposomes. This method combines the efficient and well-established drug-loading capabilities of liposomes while utilizing NBs as an acoustic trigger for drug release. Encapsulation was characterized using transmission electron microscopy with an encapsulation efficiency of 22 ± 2%. Nested-NBs demonstrated echogenicity using diagnostic B-mode imaging, and acoustic emissions were monitored during high-intensity focused ultrasound (HIFU) in addition to monitoring of model drug release. Results showed that although the encapsulated NBs were destroyed by pulsed HIFU [peak negative pressure (PNP) 1.54–4.83 MPa], signified by loss of echogenicity and detection of inertial cavitation, no model drug release was observed. Changing modality to continuous wave (CW) HIFU produced release across a range of PNPs (2.01–3.90 MPa), likely because of a synergistic effect of mechanical and increased thermal stimuli. Because of this, we predict that our NBs contain a mixed population of both gaseous and liquid core particles, which upon CW HIFU undergo rapid phase conversion, triggering liposomal drug release. This hypothesis was investigated using previously described models to predict the existence of droplets and their phase change potential and the ability of this phase change to induce liposomal drug release.
Highlights
Chemotherapy, in combination with surgery or radiotherapy, is one of the primary treatment methods for malignant tumors and can significantly increase patient survival rates
The negatively buoyant population likely consists of a combination of lipid particles that were not converted into bubbles as well as potentially containing PFB droplets, which due to their small size have condensed from a gas into liquid PFB droplets
A small proportion of
Summary
Chemotherapy, in combination with surgery or radiotherapy, is one of the primary treatment methods for malignant tumors and can significantly increase patient survival rates. Drug-loaded liposomes, such as Doxil and Onivyde, reduce the exposure of healthy tissues to drug and are currently approved for clinical use. MB stability is enhanced by using high-molecular-weight, low-solubility gases such as perfluorocarbons[5] or sulfur hexafluoride (SF6),[6] as well as a coating, typically a phospholipid monolayer, a protein, or a polymer.[7−9] Recently, research has focused on the potential use of MBs as theranostic agents.[10,11] MBs driven by an US field can enhance sonoporation in cell membranes, which has been shown to increase drug uptake.[9,12−14] Therapeutics can be incorporated with MBs in multiple ways including therapeutic gas,[15] direct attachment of drugs to the lipid shell,[16] and attachment of drug-filled liposomes,[17−19] which can be released by increasing the US intensity.
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