A physiological barrier distal to the anatomic blood-brain barrier in a model of transvascular delivery

Abstract

Background and purpose: Osmotic disruption of the blood-brain barrier (BBB) provides a method for transvascular delivery of therapeutic agents to the brain. The apparent global delivery of viral-sized iron oxide particles to the rat brain after BBB opening as seen on MR images was compared with the cellular and subcellular location and distribution of the particles.

Methods: Two dextran-coated superparamagnetic monocrystalline iron oxide nanoparticle contrast agents, MION and Feridex, were administered intraarterially in rats at 10 mg Fe/kg immediately after osmotic opening of the BBB with hyperosmolar mannitol. After 2 to 24 hours, iron distribution in the brain was evaluated first with MR imaging then by histochemical analysis and electron microscopy to assess perivascular and intracellular distribution.

Results: After BBB opening, MR images showed enhancement throughout the disrupted hemisphere for both Feridex and MION. Feridex histochemical staining was found in capillaries of the disrupted hemisphere. Electron microscopy showed that the Feridex particles passed the capillary endothelial cells but did not cross beyond the basement membrane. In contrast, after MION delivery, iron histochemistry was detected within cell bodies in the disrupted hemisphere, and the electron-dense MION core was detected intracellularly and extracellularly in the neuropil.

Conclusion: MR images showing homogeneous delivery to the brain at the macroscopic level did not indicate delivery at the microscopic level. These data support the presence of a physiological barrier at the basal lamina, analogous to the podocyte in the kidney, distal to the anatomic (tight junction) BBB, which may limit the distribution of some proteins and viral particles after transvascular delivery to the brain.

fig 1.

MR imaging of transvascular delivery of Feridex and MION in the rat brain. A, T1-weighted image obtained 24 hours after osmotic BBB disruption and intracarotid administration of Feridex (10 mg Fe/kg) shows enhancement in the right cerebral hemisphere. B, T1-weighted image 24 hours after administration of MION (10 mg Fe/kg) with BBB disruption. The disrupted right cerebral hemisphere (R) enhances (white) with iron, while the left hemisphere (L) shows the noncontrast-enhanced rat brain.

fig 2.

Light microscopy of transvascular delivery of iron oxide particles in the rat brain. A, Histochemical staining for iron after delivery of Feridex (10 mg Fe/kg) with BBB disruption. Iron staining is increased in the disrupted right cerebral hemisphere compared with the contralateral nondisrupted left hemisphere, or saline controls. V = ventricle, LC = left corpus callosum and ventricle, RC = right corpus callosum (original magnification ×20). B, Capillary staining for iron in right cerebral hemisphere after Feridex delivery with BBB disruption is indicated by arrowheads. Arrows indicate staining of red blood cells (original magnification ×50). C, Histochemical staining for iron in rat brain 2 hours after delivery of MION (10 mg Fe/kg) with BBB disruption shows an increase in iron staining in the disrupted right cerebral hemisphere (RH) compared with the nondisrupted left hemisphere (LH). V indicates the ventricle (original magnification ×20). D, Cellular staining for iron (arrows) is widespread in the right cerebral hemisphere 24 hours after delivery of 10 mg/kg MION with BBB disruption. Arrowheads indicate capillary staining (original magnification ×100).

fig 3.

A–E, Electron microscopy of transvascular delivery of iron particles. Rat brain micrographs after BBB disruption-enhanced delivery of Feridex are shown in A (original magnification ×19,000), B (original magnification ×48,000), and C (original magnification ×87,000). Electron-dense particles (arrows) are located around capillaries, adjacent to the basement membrane (BM) and pericapillary pericyte (P). L = capillary lumen, MA = myelinated axon, EC = capillary endothelial cell. Location of MION after BBB disruption is shown in D (original magnification ×65,000) and E (original magnification ×36,700). Electron-dense MION particles (D, arrows) were found near synapses (S) and mitochondria (Mt). MION particles were detected in pericapillary pericytes (P) and in what appear to be pericyte processes (E, arrow). EC = capillary endothelial cell, RBC = red blood cell, BM = basement membrane