Developing Microbubble-Nanodroplet Composites for Enhanced Hydrophobic Drug Delivery

The modernisation of the pharmaceutical industry together with the sophistication of drug screening protocols have lead to a dramatic increase in the number of therapeutic agents developed annually. As per 2017, there are almost 1500 FDA approved anticancer drugs. Treatments with many of those are impaired by their toxicity, pharmacokinetic
profile, reduced bioavailability or poor water solubility. In an effort to improve patient experience whilst offering the most effective anticancer treatment, numerous drug delivery vehicles have been developed in recent years. These systems often encapsulate the drug to prevent its degradation and help reduce the required doses by increasing the specificity of the treatments. Drug hydrophobicity presents an added challenge for the design of drug delivery systems, as they must be capable of transporting the therapeutic agents whilst preventing their agglomeration. This project has been concerned with the design of a delivery system for hydrophobic drugs. The microbubble-nanodroplets (MB-LONDs) architecture consists of an echogenic MB decorated with LONDs, which attach to the shell of the MB via biotin-NeutrAvidin chemistry. The LONDs are able to encapsulate hydrophobic drugs in their oil core, keeping them away from the aqueous solution and thus preventing drug agglomeration and degradation. The MB acts as a vehicle for the LONDs, and its ultrasound properties can be used for both imaging and controlling the release of the LONDs.

A number of biocompatible oils were chosen and characterised in terms of their light absorption
and emission properties, as well as their ability to solvate a number of hydrophobic drugs and drug mimics. Informed by the results obtained in this study squalane, triacetin and tripropionin were chosen as model oils for subsequent experiments on LOND formation. LONDs were produced in a two-step process, and generally exhibited sizes between 80 - 300 nm with good stability for at least 6 weeks when stored at 4 C. Encapsulation of the hydrophobic drug CA4 in triacetin and tripropionin LONDs was shown to be possible, with an encapsulation effciency of at least 76% in the case of tripropionin LONDs.
To better understand the attachment of LONDs onto the MB shells, the attachment of LONDs to model membranes was investigated. This study explored three different attachment chemistries, namely the biotin-NeutrAvidin, maleimide-thiol and pyridyl-thiol. The results revealed the viability of the LONDs attachment using the different linkers, but also pointed to the existence of a high number of non-specific interactions between the
LONDs and the membranes. The assembly of MB-LONDs was performed by a number of methods and with different LOND types. MB-LONDs prepared in a two-step process on-chip exhibited average sizes around 2 um, good stability over 2 h at 37 C and were found to be US responsive.