(291b) Surfactant Effects on Nanotherapeutic Fate within the Brain

Background: Drug delivery to the brain is limited by poor transport of pharmaceutical agents across the blood-brain barrier (BBB), extracellular space (ECS), and into specific cells of interest. Nanoparticle drug carriers, due to their small size and highly-tailorable surfaces, have emerged as one strategy to overcome these barriers. In particular, polymeric nanoparticles made of poly(lactic-co-glycolic) acid (PLGA) can cross the BBB and, with a dense surface layer of poly(ethylene glycol) (PEG), can diffuse through the brain parenchyma. However, the formulation of PLGA-PEG nanoparticles often incorporates additional stabilizing molecules, called surfactants, whose effects have been understudied. Previous studies of surfactant influence on nanoparticle behavior have been limited to a single biological barrier. For example, cellular uptake in some immune and tumor cells is promoted by incorporation of the surfactant Pluronic F127 (F127) compared to poly(vinyl alcohol) (PVA). Polysorbate 80 (P80), another surfactant, can induce transcytosis across the BBB. Since the in vivo brain microenvironment exhibits many barriers in combination, the goal of this study was to determine the effect of surfactant choice on nanoparticle transport at multiple length and time scales.

Methods: AlexaFluor 647-labeled PLGA-PEG nanoparticles were formulated with the surfactants F127 (PLGA-PEG/F127), PVA (PLGA-PEG/PVA), or P80 (PLGA-PEG/P80). Control nanoparticles were formulated without surfactant (PLGA-PEG) and without PEG (PLGA). Nanoparticles were characterized by dynamic light scattering for size, surface charge, and polydispersity. To evaluate in vivo nanoparticle transport, we administered nanoparticles intravenously via tail vein to postnatal (P) day 9 female Sprague-Dawley (SD) rats and extracted all major organs four hours later. Uptake within neurons and microglia was observed in fixed, sectioned brains using immunohistochemistry and confocal microscopy. For all other organs, nanoparticle accumulation was measured by fluorescence spectrometry after tissue homogenization. Accumulation in the brain was also measured by fluorescence, but in order to more precisely capture nanoparticle extravasation across the BBB, we first used a capillary depletion technique to separate brain microvessels from the brain parenchyma. Finally, nanoparticle diffusion through the ECS was measured using multiple particle tracking. Freshly extracted brains from P9 female SD rats were sliced into 300 mm sections and 0.5 nL of nanoparticles was injected after a brief rest period. Videos of nanoparticles were acquired at 100 frames per second for 6.5 seconds and nanoparticle trajectories were extracted with TrackMate (ImageJ). Trajectory analysis was completed with a lab-developed Python package available on GitHub.

Results: The nanoprecipitation formulation technique resulted in nanoparticles of small size (59-65 nm in diameter), low polydispersity (0.06-0.2), and near-neutral surface charge (-11.4 to -9.86 mV) independent of surfactant choice. After intravenous administration, all formulations except PLGA-PEG/P80 were strongly associated with vascular structures, demonstrating the significant challenge posed by the BBB. However, the P80 formulation was able to demonstrate low endothelial association as well as internalization in microglia and neurons. These results were supported by quantification of fluorescent nanoparticles in brain microvessels versus the parenchyma: the average ratio of nanoparticle concentration in the parenchyma to capillaries was 8.5 for PLGA-PEG/P80 compared to 4.3, 1.9, and 0.2 for PLGA-PEG/F127, PLGA-PEG/PVA, and PLGA-PEG, respectively. Surfactant differences were also apparent at the microscopic scale of diffusion. Inclusion of a surfactant significantly decreased the diffusive ability of PLGA-PEG nanoparticles, which had an ensemble-averaged effective diffusion coefficient (D_eff) of 12.9 µm²/s. The PLGA-PEG/PVA, PLGA-PEG/F127, and PLGA-PEG/P80 nanoparticles had diffusion coefficients of 8.48, 7.50, and 4.72 µm²/s, respectively. The PLGA formulation had the lowest diffusion coefficient (D_eff=1.02 µm²/s), likely due to the absence of a PEG layer to reduce interactions with cellular and protein components of the tissue environment. Analysis of trajectory features confirmed a comparatively large portion of PLGA nanoparticle trajectories as “bounded” by circular boundaries, consistent with cellular internalization.

Conclusions: In this study, we evaluated the effect of surfactant choice on nanoparticle transport behavior in the brain at multiple length and time scales. Characterization across transport scales is important because effective drug carriers must be able to both diffuse through a congested brain ECS in milliseconds and uptake into endothelial and target cells over hours. By more fully understanding the role of surfactants in determining nanoparticle fate, we can engineer nanoparticles that leverage biological interactions for enhanced therapeutic delivery to the brain.