Therapeutic microbubbles tailored for blood-brain barrier opening applications

Project reference:  SIE_15_22 
1st supervisor:  Dr Antonios Pouliopoulos

2nd supervisor: Dr Kirsten Christensen-Jeffries

Start date:  February 2023

Aim of the project: The aim of the project is to develop therapeutic microbubbles tailored to blood-brain barrier (BBB) opening applications. Focused ultrasound (FUS) in combination with systemically administered microbubbles can non-invasively and reversibly open the BBB. Most pre-clinical and clinical studies use commercially available microbubbles (e.g., SonoVue or Definity). However, all commercial microbubbles were originally designed and developed as ultrasound imaging contrast agents. Here, we will design and develop microbubbles optimised for therapeutic ultrasound exposure. We will perform numerical and finite-element-modelling simulations to define the microbubble shell formulation that provides the optimal balance between surface tension, viscosity, elasticity, and sufficient expansion ratio under a therapeutic pulse. In collaboration with Bracco, we will manufacture the specified formulation and then test it in vitro and in vivo. We will assess their in vitro and in vivo stability across a range of acoustic parameters, relevant to BBB opening.   

Project description: Focused ultrasound (FUS) in combination with pre-formed circulating microbubbles is the  only method that allows both non-invasive and localized opening of the BBB. The majority of investigations has focused on the ultrasound hardware and software development, rather than on the microbubble formulation. As a consequence, imaging contrast agents are used sub-optimally as therapeutic agents.
Previous work has shown that BBB opening is influenced by the microbubble size and type. Our previous work has shown that the temporal stability of the microbubble population during therapeutic ultrasound application changes over time and affects the BBB opening volume and contrast enhancement. There is a pressing need for purpose-made microbubble formulations with increased stability and improved performance within a therapeutically-relevant acoustic field.

Here, we propose to perform ab initio calculations for the optimal microbubble formulation in therapeutic ultrasound applications. In collaboration with Bracco Suisse S.A., we will manufacture new FUS-tailored microbubbles and test their stability and performance in
vitro and in vivo.  

The primary aims of this project are the following:

1) Perform simulations of microbubble stability across different lipid formulations. We will first implement the Marmottant model for a single microbubble exposed to therapeutic ultrasound. We will estimate the range of shell viscosity and shell elasticity that will allow expansion ratios up to 5, for a 2 μm microbubble. We will perform the same simulations for variable microbubble sizes from 1 to 5 μm. Next, we will perform ab initio molecular simulations of microbubble shell monolayers with different viscosity and elasticity. We will identify the optimal lipid combination that provides a stable molecular structure with desirable viscosity/elasticity and synthesize it.


2) Evaluate microbubble stability in vitro. Tailored microbubbles will be exposed to therapeutic ultrasound pulses while flowing in tissue-mimicking phantoms. We will investigate the effect of flow, tissue density, and vessel diameter on microbubble stability. We will first characterise the microbubble acoustic emissions through passive cavitation detection (PCD). PCD signals will be analysed to estimate the microbubble population lifetime during a therapeutic pulse. Then, we will replace the single element transducer with an imaging array to perform passive acoustic mapping (PAM). PAM will allow for an estimation of the spatial distribution of cavitation activity within the focus during a therapeutic pulse. We will estimate the spatial and temporal uniformity of cavitation activity within the focus. We will compare the stability metrics of the new formulation with established imaging microbubbles, such as SonoVue and Definity.


3) Evaluate microbubble stability in vivo. The final aim of the project will be to characterise the microbubbles in vivo. We will induce BBB opening in the left hippocampus of the murine brain. We will acquire PCD and PAM measurements to identify the microbubble stability within the in vivo vasculature. BBB opening metrics will be correlated with microbubble stability evaluated through PCD and PAM. We hypothesize that enhanced stability will correlate with improved safety, as violent microbubble collapses may lead to red blood cell extravasation and sterile inflammation.