Bubble Therapy: a New Paradigm for Targeted Drug Delivery by Ultrasound

High amplitude ultrasound waves propagating through tissue have been recently reported to induce a range of potentially beneficial phenomena, such as rapid tissue heating, increased permeability of cells to large drug molecules (sonoporation) or enhanced activity of drugs. These bioeffects are heavily correlated with the ultrasound-induced nucleation and subsequent excitation of micron-sized bubbles, yielding two types of acoustic cavitation activity: (1) inertial cavitation, which dramatically increases the energy transfer to tissue and can cause rapid heating and mechanical damage, and (2) stable cavitation, whereby bubbles act as micropumps that dramatically enhance the local mixing and transport length scales of drug molecules. In cancer treatment, local heating combined with chemotherpay will render cancer cells more sensitive to treatment, whilst local micropumping of the drug can help overcome delivery problems arising from the highly complex tumour structure. In the context of breaking down blood clots for stroke therapy, cavitation-enhanced mixing will promote delivery of the drug to a site of low blood flow and greatly increase the diffusion of the thombolyic drug across the clot surface.However, the nucleation of cavitating microbubbles and subsequent interaction with cells in biologically relevant media remain poorly understood. The objectives of the proposed research therefore are (i) to investigate the potential of cell- and site-specific cavitation nucleation using commercially available targeted nanoparticles currently being developed for molecular imaging; (ii) to understand and optimize the mechanism by which ultrasound and cavitation can enhance local drug delivery and drug activity across inaccessible interfaces such as tumours or blood clots; (iii) to develop clinically relevant means of monitoring cavitation activity and exploit them for real-time monitoring of drug delivery and (iv) to test the optimized drug delivery and treatment monitoring protocols in a clinically relevant organ model.It is hoped that the proposed resarch will pave the road for widespread clinical uptake of cavitaiton-enhanced targeted drug delivery by ultrasound. Particular advantages of this technique will include the ability to locally enhance drug activity, thus reducing the necessary drug dosages and their side effects, and to monitor therapy in real time. The outcomes of the proposed research are expected to be directly transferable to many other novel therapeutic ultrasound applications, such as non-invasive tissue ablation by High-Intensity Focussed Ultrasound (HIFU), acoustic haemostasis and ultrasound-induced opening of the blood-brain barrier for transcranial drug delivery.