Nested-Nanobubbles for Ultrasound Triggered Drug Release

Due to their size (1-10 μm) microbubble-based drug delivery agents suffer from confinement to the vasculature, limiting tumour penetration and potentially reducing drug efficacy. Nanobubbles (NBs) have emerged as promising candidates for ultrasound triggered drug delivery, due to 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-NBs, by encapsulation of NBs (~ 100 nm) within drug loaded liposomes. This method combines the efficient and well-established drug loading capabilities of liposomes, whilst 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 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 peak negative pressures (2.01 – 3.90 MPa), likely due to a synergistic effect of mechanical and increased thermal stimuli. Due to 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.