Nanobubbles for Ultrasound Mediated Drug Delivery

Systemic delivery of therapeutic agents for treating a wide range of diseases can be
challenging, where treatment effectiveness is determined by the total dose of drug delivered
to the disease site. Thus, new platforms for the non-invasive delivery and triggered release of
therapeutics are of significant interest, especially in an aging population where the rate of
disease is likely to increase.
Lipid-shelled microbubbles (1-10 μm, MBs) are already in widespread clinical use as contrast
agents for echocardiography. In combination with ultrasound (US), MBs can locally increase
intra-cellular drug uptake via a process called sonoporation. MBs can be functionalised to act
as biomarkers for molecular imaging of disease vasculature and provide localised triggered
release of a therapeutic payload. However, they are confined to the vasculature, which can
result in poor uptake in the targeted region. Nanobubbles (NBs, < 1 μm) have emerged as
promising candidates for US-triggered drug delivery because of their small size, which allows
them to passively extravasate and accumulate within tumour tissue.
A new type of therapeutic NB was developed, Nested-NBs, by encapsulation of NBs within
drug-loaded liposomes, combining the efficient and well-established drug-loading capabilities
of liposomes and utilizing NBs as an acoustic trigger for drug release. Although the
encapsulated NBs were destroyed by pulsed HIFU, determined by cavitation detection, no
model drug release was observed. Changing modality to continuous wave (CW) HIFU
produced release across a range of pressures, likely due to a synergistic effect of mechanical
and thermal stimuli. In combination with theoretical models of droplet vaporisation, we
predict that NBs contain a mixed population of both gaseous and liquid core particles, which
upon CW HIFU undergo rapid phase conversion, triggering liposomal drug release.
Accurate characterisation of NB size and concentration is challenging, due to their sub-micron
nature and mixed populations, containing both bubbles and liposomes. A novel method of
using a commercially available Nanoparticle Tracking Analysis system was developed, able
to distinguish between NBs and liposomes owing to their differing optical properties. This
technique was then used to assess the in vitro sonoporation performance on-chip of different
sized NBs. However, sonoporation efficiency did not depend exclusively on NB size and
concentration. It is hypothesized that both the total lipid and liposome concentration, as well
as inter-bubble distance plays an important role in NB stability, consistent with previously
proposed theories and simulations. Future work could consist of further optimisation of the
development of Nested-NBs, whilst also investigating more fundamental questions such as
the mechanisms behind NB stability.