Border control thwarts particles entering the brain

Abstract

Delivering therapeutic agents to the brain is far from straightforward because the blood–brain barrier (BBB) often blocks their entry. Now, US researchers have uncovered a second barrier beyond the BBB that might prevent some BBB-permeant agents from reaching their target cells in the brain. Finding ways to breach this second barrier could add yet another layer of chemical bureaucracy to the design of therapeutic agents that target the brain.Neuroscientist Leslie Muldoon and neurosurgeon Edward Neuwelt (Oregon Health Sciences University and the Portland Veterans Affairs Medical Center, Portland, OR, USA) have spent years studying the BBB, which they manipulate to improve drug delivery to patients with brain tumours. Injecting patients with a hypertonic solution (generally mannitol or arabinose) shrinks the endothelial cells by osmosis, thereby disrupting the tight junctions that form the BBB, and temporarily allowing entry of even viral-sized particles into the brain. Neuwelt pioneered this technique for chemotherapy in brain tumour patients, allowing access to even deep tumours without brain surgery. Muldoon has advanced the procedure to study the delivery of biocomplexes such as viral vectors to the brain.Their latest research in rats1xA physiological barrier distal to the anatomic bloodbrain barrier in a model of transvascular delivery. Muldoon, L. et al. Am. J. Neuroradiol. 1999; 20: 217–222PubMedSee all References indicates that the barrier might be even more impenetrable to some agents than previously thought. They hope to find out how this border control manages to pick and choose which particles it lets through, in the hope that it will help drug designers to target the brain more effectively.Muldoon and her colleagues, while studying virus-sized particles of iron oxide in rat brain, found that their magnetic resonance images (MRI) did not reflect the distribution of iron oxide particles seen at brain autopsy. The particles had crossed the BBB, as revealed by the MRI, but were not distributed evenly among the neurons; instead they seemed stuck up against another barrier – the basement membrane on the basal surface of the vascular endothelial cells.To mimic viral vectors, the researchers used two types of iron oxide particles of similar size, coated with different amounts of dextran, a polyanionic sugar polymer that allows the iron oxide particles to bind to proteins and membranes. The particles coated to a greater extent with dextran cross the second barrier, and at the microscopic level were well distributed within the brain. However, less coverage meant the particles got stuck at the basement membrane (Fig. 1Fig. 1).Fig. 1Transmission electron micrograph showing partially-coated iron oxide particles that have crossed the blood–brain barrier but are prevented from penetrating the brain by the basement membrane. Abbreviations: BM, basement membrane; EC, capillary endothelial cell; L, capillary lumen; P, particles; TJ, tight junction.View Large Image | Download PowerPoint SlideNeuwelt explains that the basement membrane is not a physical barrier but probably an electrically charged barrier similar to that in the kidney, which might explain the selective passage of particles depending on their charge. The next step is to learn more about the barrier’s properties that allow stealth agents to cross and so provide a passport for therapeutic vectors to enter the brain. ‘Our discussions with the company that makes [the iron oxide] agents has led us to think that opsonization or charge somehow alters how the particles pass through the barrier’, explains Muldoon. ‘As far as we know, this second barrier, inside the BBB, does not pose a barrier to drug delivery to the brain but raises questions about delivery of biological complexes.’ To see whether viruses can penetrate the basement membrane, the researchers plan to use fluorescently tagged virus particles that can be visualized microscopically, in collaboration with Thomas McCown (University of North Carolina, NC, USA).Meanwhile, the question remains whether the basement membrane acts as a barrier in human brain and brain tumours. Neuwelt has already begun to test whether the partially dextran-coated iron oxide particles enter brain tumours.