Microbubble volume: A definitive dose parameter in blood-brain barrier opening by focused ultrasound

Abstract

Microbubble-assisted focused ultrasound (MB+FUS) is currently being developed to improve drug delivery through the blood-brain-barrier (BBB). However, microbubble dosing has posed challenges for treatment efficiency and safety due to differences in size distribution, as well as the prevalent practice of re-purposing FDA-approved ultrasound contrast agents (UCAs) for therapeutic use. Here, we explore a novel method of establishing microbubble dose, even retroactively, in MB+FUS BBB disruption (BBBD) studies. Specifically, we controlled microbubble size and concentration using a previously established centrifugal size-isolation technique, and adjusted the microbubble volume dose (MVD; 1-40 µL/kg) of two discrete monodisperse formulations (26-µm and 6-µm diameter) for MB+FUS BBBD in adult male Sprague-Dawley rats. Serial multi-site treatment was explored in the right and then left striata to determine the effects of microbubble pharmacokinetics. Near-infrared fluorescence microscopy of extravasated Evans Blue dye, our permeabilization indicator, was summed across brain slices after treatment to establish relative BBB opening. A linear trend between MVD and dye extravasation was observed for both treatment sites. Our results indicated that MVD, not microbubble size, determined the extent of MB+FUS BBBD. This result collapses previous microbubble dosing parameters of size distribution and concentration to one parameter: microbubble volume dose, which facilitates comparison of previous studies, as well as the planning of future MB+FUS BBBD studies. Additionally, a preliminary study of MB+FUS-facilitated gene delivery, expression, and immune response in neural tissue was conducted with dsAAV1-CMV-GFP.