The blood–brain barrier (BBB) is conservative, standing athwart the brain’s blood supply yelling “stop” at harmful chemicals and invaders the rest of the body is inclined to welcome. Although the BBB performs an essential gatekeeping function, accepting innocuous nutrients and rejecting potentially dangerous substances, it can try the patience of drug developers, who struggle to design blood-borne neuroactive therapeutics that can actually reach the brain.

To keep the BBB from frustrating progress against diseases of the central nervous system, researchers based at Brigham and Women’s Hospital (BWH) have looked beyond traditional approaches to the screening of brain-penetrating drugs. These approaches include animal models and cell cultures. Unfortunately, animal models are expensive and laborious and can only be used to test a limited number of compounds at a time. And cell cultures often fail to replicate the conditions found in the human body. For example, in typical cultures, cells are grown on flat plastic surfaces and isolated by cell type. They may become less and less like the unique cells found in the brain over time.

Brain tissue section showing accumulation of a blood–brain barrier-penetrating peptide (in white) in the brain tissue (outside of blood vessels). A green dye is used to highlight the blood vessels within the animal, and cell nuclei are represented in blue. [Choi-Fong Cho/BWH]

Demonstrating an openness to new approaches, the BWH team has developed an innovative but easily implemented approach that uses “spheroids” to mimic the BBB more accurately. These spheroids appear to overcome several challenges for discovering and advancing new drugs for treating brain conditions.

Details appeared June 6 in the journal Nature Communications, in an article entitled “Blood-Brain-Barrier Spheroids as an In Vitro Screening Platform for Brain-Penetrating Agents.” This article describes how different kinds of brain cells—endothelial cells, pericytes, and astrocytes—can be grown together so that they spontaneously form multicellular spheroids. Astrocytes form the core of the spheroids, whereas endothelial cells and pericytes encase the surface, acting as a barrier that regulates transport of molecules.

“The spheroid surface exhibits high expression of tight junction proteins, VEGF [vascular endothelial growth factor]-dependent permeability, efflux pump activity, and receptor-mediated transcytosis of angiopep-2,” report the article’s authors. “In contrast, the transwell co-culture system displays comparatively low levels of BBB regulatory proteins, and is unable to discriminate between the transport of angiopep-2 and a control peptide.”

These self-assembled structures closely resemble the BBB organization and can be used to predict drug penetration capabilities—molecules that can penetrate the surface of the spheres and accumulate inside are more likely to be able to penetrate the BBB and enter the brain in a living organism.

“Our model takes a new approach to mimic the BBB outside of a living system. These miniature spheroids are relatively straightforward to culture, and yet it is able to reproduce many of the key BBB properties and functions,” said lead author Choi-Fong Cho, Ph.D. “Our hope is that these findings will further advance neuroscience research and expedite the discovery and design of brain-penetrant drugs to treat diseases of the brain and central nervous system.”

The research team performed several tests on these spheroids to establish some of the key properties of the BBB that allow it to restrict the influx of foreign molecules. The spheroid model scored much better on many of these properties than the standard model in use today. The team also used the spheroids to identify new brain-penetrant molecules, which could hold high potential for delivering therapeutics across the BBB.

“We plan to use this model going forward in our own research to identify new therapeutics for glioblastoma,” said senior author Sean Lawler, Ph.D. “This is a very versatile model and should allow our group and others to test not only molecules but also viruses, cells and more that may be able to cross the BBB”

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