Clearance of two organic nanoparticles from the brain via the paravascular pathway

Highlights

• rHDL and PEG-PLA nanoparticles were cleared relatively fast from the brain.

• Paravascular glymphatic pathway is the major route for brain elimination of the nanoparticles.

• Microglia facilitates nanoparticles transport to the paravascular route.

• Brain clearance of the nanoparticles was significantly declined under Alzheimer condition.

Abstract

The elaboration of nanotechnology offers valuable therapeutic options to overcome the blood-brain barrier and enable the treatment of brain diseases. However, to date, limit work has been done to reveal the fate of nanoparticles within the brain, which largely hinders their safe and effective applications. Here we demonstrated that the commonly-used organic nanoparticles reconstituted high density lipoprotein and poly(ethylene glycol)-b-poly(lactic acid) nanoparticles were cleared relatively fast from the brain (half-life <5 h). Notably, through various transgenic mice and pharmacological inhibition approaches, we revealed that the paravascular glymphatic pathway plays a key role (about 80%) in the brain clearance of the nanoparticles, and disclosed that microglia-mediated transportation is essential for facilitating nanoparticles elimination through the paravascular route. In addition, we witnessed a significant decline in the brain clearance of both of the nanoparticles in Alzheimer's model mice where the glymphatic system is impaired. These findings provide insightful data on the fate of nanoparticles in the brain, which would shed new light into the rational design and safe application of nanoparticles for brain drug delivery.

Introduction

Nanotechnology has tremendous potential to profoundly change the ways we diagnose and treat diseases. Specifically, nanomaterials can be engineered as nanocarriers to deliver therapeutic or imaging agents, improve pharmacokinetics and biodistribution and enable targeted delivery to specific tissues, cells, or subcellular compartments [1]. The application of nanomaterials also provides new approaches to overcome the blood-brain barrier (BBB), and to improve our understanding and intervention of the brain diseases [2]. However, to date, the ability to control how engineered nanoparticles behave within the body especially in the brain remains largely elusive [1], which largely hinders their applications in biomedicine.

Due to the extremely small size, high surface volume ratio and the complex composition of engineered components, the unique physical and chemical properties of nanoparticles may also lead to unique biological reactivity [3]. As headquarter of the body, the brain exerts centralized control over the other organs. Designed or undesigned accumulation of nanoparticles in the brain may induce neurotoxic effects and even lead to severe pathological changes to the whole body [[4], [5], [6]]. Therefore, safety is the primary concern for nanomedicine, especially for nanomaterial-based theranostics for brain applications. There are several factors that can impact upon a drug's toxicological profile, among which clearance is one of the most important aspects that need to be considered [7,8]. Even for biocompatible and biodegradable nanoparticles with acceptable safety, understanding the kinetics and mechanism of their clearance is also important for optimizing the design of nanomedicines by regulating their retention at the target sites and the amount of drugs that can be released. Therefore, understanding how nanoparticles are cleared from the brain is crucial for the rational development and safe application of nanomedicines. However, to date, the biofate of nanoparticles in the brain is still poorly understood [9]. Limited researches have shown that certain inorganic nanoparticles such as iron oxide nanoparticles and silver nanoparticles tend to retain long in the brain [10,11]. In contrast, the intracerebral fate of those organic nanoparticles which are more commonly used for brain drug delivery remains largely unknown.

High-density lipoprotein (HDL), natural nanocarrier, is highly suitable as a platform for delivering imaging and therapeutic agents due to their ultra-small size and favorable surface properties [12,13]. By mimicking the endogenous shape and structure of HDL, reconstituted high-density lipoprotein (rHDL), well tolerated in clinical trials [14,15], has recently been constructed as an nanoplatform with high BBB permeability for the therapy of Alzheimer's disease (AD) and glioblastoma [16,17]. Nanoparticles prepared with poly(ethylene glycol)-b-poly(lactic acid) (PEG–PLA) copolymers, widely studied nanoparticulate drug delivery systems with suitable safety, clinical translationability and systemic pharmacokinetic profiles [[18], [19], [20]], have also been widely applied for brain drug delivery [[21], [22], [23]]. In light of the broad interest and enormous potential of the above nanoparticles in biomedicine especially for the management of brain diseases, it's therefore particularly interesting to learn about their fate in the brain.

Here we investigated the kinetics as well as the mechanism involved for the brain clearance of the nanoparticles. Intraparenchymal injection, which does not contain an absorption phase, was applied for administration to simplify data interpretation. Interestingly, we found that both rHDL and PEG-PLA were cleared relatively fast from the brain (half-life <5 h). More importantly, through transgenic mice and various pharmacological inhibition experiments, we identified the paravascular glymphatic pathway, a waste clearance system formed by astrocytes to promote efficient elimination of soluble proteins and metabolites from the brain through the exchange between cerebrospinal fluid (CSF) and interstitial fluid (ISF), facilitated by astrocytic aquaporin-4 (AQP4) water channels [[24], [25], [26]], as the major route for their brain elimination, and revealed that microglia-mediated transportation plays an important role in facilitating their clearance through the paravascular route. In addition, we witnessed a significant decline in the brain clearance of the nanoparticles in Alzheimer's model mice compared to wild type (WT) mice, highlighting the influence of pathological factors on the in vivo behavior of nanoparticles. The findings provide insightful data on the fate of nanoparticles in the brain, a critical but largely unexplored area.