Microbubble-Size Dependence of Focused Ultrasound-Induced Blood–Brain Barrier Opening in Mice In Vivo

The therapeutic efficacy of neurological agents is severely limited, because large compounds do not cross the blood-brain barrier (BBB). Focused ultrasound (FUS) sonication in the presence of microbubbles has been shown to temporarily open the BBB, allowing systemically administered agents into the brain. Until now, polydispersed microbubbles (1-10 ¿m in diameter) were used, and, therefore, the bubble sizes better suited for inducing the opening remain unknown. Here, the FUS-induced BBB opening dependence on microbubble size is investigated. Bubbles at 1-2 and 4-5 ¿m in diameter were separately size-isolated using differential centrifugation before being systemically administered in mice (n = 28). The BBB opening pressure threshold was identified by varying the peak-rarefactional pressure amplitude. BBB opening was determined by fluorescence enhancement due to systemically administered, fluorescent-tagged, 3-kDa dextran. The identified threshold fell between 0.30 and 0.46 MPa in the case of 1-2 ¿m bubbles and between 0.15 and 0.30 MPa in the 4-5 ¿m case. At every pressure studied, the fluorescence was greater with the 4-5 ¿m than with the 1-2 ¿m bubbles. At 0.61 MPa, in the 1-2 ¿m bubble case, the fluorescence amount and area were greater in the thalamus than in the hippocampus. In conclusion, it was determined that the FUS-induced BBB opening was dependent on both the size distribution in the injected microbubble volume and the brain region targeted.

I. Introduction

The EXCHANGE of molecules across the cerebral microvasculature is regulated by a unique interface known as the blood–brain barrier (BBB). Its major functions are to prevent toxins from entering the parenchyma and to maintain molecular environments necessary for proper neuronal firing [1], [2]. Through a combination of passive, transport, and metabolic barriers, nearly all systemically administered compounds larger than 400 Da are excluded from the brain's extracellular space, rendering thus many neurologically potent compounds ineffective [3]. As a result, potential therapeutic agents, such as inhibitors to enzymes (1 kDa) and antibodies (30–300 kDa), will not reach their intended targets if administered systemically. The treatment of central nervous system (CNS) disorders will remain severely impaired until a method to deliver such large agents in the brain at a sufficient dose is shown to be effective [3].