Investigating the optimum size of nanoparticles for their delivery into the brain assisted by focused ultrasound-induced blood–brain barrier opening

 For when our researchers find a drug they need to get across the blood brain barrier. Assuming that our fucking failures of stroke associations  can remember this when researchers need it.

Investigating the optimum size of nanoparticles for their delivery into the brain assisted by focused ultrasound-induced blood–brain barrier opening

• Seiichi Ohta,
• Emi Kikuchi,
• Ayumu Ishijima,
• Takashi Azuma,
• Ichiro Sakuma &
• Taichi Ito 

Scientific Reports volume 10, Article number: 18220 (2020) Cite this article

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Abstract

The blood–brain barrier (BBB) has hampered the efficiency of nanoparticle delivery into the brain via conventional strategies. The widening of BBB tight junctions via focused ultrasound (FUS) offers a promising approach for enhancing the delivery of nanoparticles into the brain. However, there is currently an insufficient understanding of how nanoparticles pass through the opened BBB gaps. Here we investigated the size-dependence of nanoparticle delivery into the brain assisted by FUS-induced BBB opening, using gold nanoparticles (AuNPs) of 3, 15, and 120 nm diameter. For 3- and 15-nm AuNPs, FUS exposure significantly increased permeation across an in vitro BBB model by up to 9.5 times, and the permeability was higher with smaller diameter. However, in vivo transcranial FUS exposure in mice demonstrated that smaller particles were not necessarily better for delivery into the brain. Medium-sized (15 nm) AuNPs showed the highest delivery efficiency (0.22% ID), compared with 3- and 120-nm particles. A computational model suggested that this optimum size was determined by the competition between their permeation through opened BBB gaps and their excretion from blood. Our results would greatly contribute to designing nanoparticles for their delivery into the brain for the treatment of central nervous system diseases.

Introduction

Nanoparticles have attracted global attention in the biomedical field. It has been revealed that their interaction with cells and/or tissues can be tailored through nanoparticle design, such as their size, shape, and surface chemistry¹. Combined with the advances in nanoparticle functionalization methods, this has opened the way for various biomedical applications of nanoparticles, including drug delivery, imaging, and therapies^2,3. However, despite the promise of nanoparticle-based systems, their translation to clinical use remains a challenge, mainly due to the low efficiency of their delivery to target sites^4,5. Various factors have been proposed as hampering nanoparticle delivery, including uptake by the reticuloendothelial system (RES), restricted diffusion in dense extracellular matrix (ECM), resistance by interstitial pressure, and clearance via the renal system^6,7,8.

The brain is one of the most difficult target organs to deliver nanoparticles to because of the existence of the blood–brain barrier (BBB). The BBB is composed of brain endothelial cells attached to a continuous basement membrane and linked together by tight junctions that prevents foreign substances from entering into the brain⁹. Even small molecular drugs can barely cross the BBB, which is a major limitation for the treatment of central nervous system (CNS) diseases, such as Alzheimer’s disease and Parkinson’s disease; diseases whose prevalence is rapidly increasing as societies around the world are aging. In the case of nanoparticles, the restriction of their permeation across the BBB is even more pronounced because of their relatively large size. Although various delivery methods have been attempted, e.g., using receptor-mediated endocytosis^10,11,12, transcytosis^13,14, or transporters^15,16, the efficiency of delivering nanoparticles into the brain is insufficient to fully exploit their therapeutic and diagnostic potential. For example, using transferrin receptor-targeted nanoparticles is one of the most widely used strategies to get nanoparticles across the BBB, but it typically results in < 0.1% delivery efficiency to the brain¹⁰.

Focused ultrasound (FUS) in combination with the administration of microbubbles (MBs) is an emerging technique being investigated to enhance the permeation of therapeutics across the BBB in a noninvasive, localized, and transient manner¹⁷. FUS induces inertial or stable cavitation with MBs that exerts a mechanical force onto capillary walls, leading to a temporary opening of the BBB via the widening of tight junctions^17,18,19. The enhanced delivery of small molecular drugs^20,21, oligonucleotides^22,23, and antibodies^24,25,26 into the brain via FUS-induced BBB opening has been demonstrated in vivo. In addition, clinical trials are now being conducted into the FUS-assisted delivery of small molecular drugs into gliomas^27,28. This technology could provide a promising strategy for improving the efficiency of nanoparticle delivery to the brain, although there are still only a limited number of reports on its application for nanoparticles^{29,30,31,32,33,34}.

To employ FUS-induced BBB opening for nanoparticle delivery, a question that must be addressed is how the size of nanoparticles can affect the enhanced permeation through opened BBB gaps. It is expected that the optimum nanoparticle design for this delivery mechanism would be different from that usually employed for enhanced permeation and retention (EPR)-based delivery strategies for tumors, in which nanoparticles are extravasated from naturally leaky blood vessels¹. However, although some previous studies investigated the effect of the size of nanoparticles, such as liposomes, on this strategy³⁴, the mechanism still needs to be clarified, especially in single to sub-hundred nanometer range. Here, we explored the effect of nanoparticle size on their delivery into the brain assisted by FUS-induced BBB opening, using polyethylene glycol (PEG)-coated gold nanoparticles (AuNPs) of different sizes, 3 to 120 nm, as a model (Fig. 1). An in vitro BBB model capable of FUS exposure was developed to examine the size-dependent permeation behavior of these particles. The size-dependent delivery of AuNPs into the brain was further investigated in vivo via transcranial FUS exposure in mice. Based on the obtained results, a kinetic model was proposed to estimate the optimum nanoparticle size for delivery into the brain assisted by FUS-induced BBB opening.