The "perivascular pump" driven by arterial pulsation is a powerful mechanism for the distribution of therapeutic molecules within the brain

Abstract

We investigated the movement of interstitially infused macromolecules within the central nervous system (CNS) in rats with high and low blood pressure (BP)/heart rate and in rats euthanized immediately before infusion (no heart action). Adeno-associated virus 2 (AAV2), fluorescent liposomes, or bovine serum albumin was infused into rat striatum (six hemispheres per group) by convection-enhanced delivery (CED). After infusion, distribution volumes were evaluated. The rats with high BP/heart rate displayed a significantly larger distribution of the infused molecules within the injected site and more extensive transport of those molecules to the globus pallidus. This difference was particularly apparent for AAV2, for which a 16.5-fold greater distribution of viral capsids was observed in the rats with high BP/heart rate than in the rats with no heartbeat. Similar results were observed for liposomes, despite their larger diameter. The distribution of all infused molecules in all rats that had low or no blood flow was confined to the space around brain blood vessels. These findings show that fluid circulation within the CNS through the perivascular space is the primary mechanism by which viral particles and other therapeutic agents administered by CED are spread within the brain and that cardiac contractions power this process.

FIG. 1

Volumes of distribution (V_d) of three substances infused into rat striatum (top) and distributed from there into the globus pallidus (bottom). Dark-shaded columns represent values for group H (high blood pressure/heart rate). Light-shaded columns represent values from group L (low blood pressure/heart rate). White columns represent values for group N (rats with no heart action). Standard deviation bars are shown on each column. Statistically significant differences in both the striatum and the globus pallidus among rats infused with the different molecules are shown as relative differences with *P < 0.009 and **P < 0.00005. Of interest is that the striatal V_d of liposomes in group H was almost identical to that of bovine serum albumin. AAV2 distribution was significantly less due to entrapment of viral capsids by heparan sulfate proteoglycan receptors widely present on neurons [20].

FIG. 2

Photographs of brain sections stained for AAV2 capsids (antibody A20) in rats after intrastriatal infusion of AAV2 by convection-enhanced delivery. (A and B) Striatal sections from two different rats from group H (high blood pressure/heart rate); (C and D) brain sections from a more caudal part of the same brains showing positive signal in globus pallidus. For all rats from group H, the fluorescent signal was spread evenly within the hemisphere, forming a mostly uninterrupted distribution. (E and F) Brain sections from two rats with low blood pressure/heart rate when AAV2 was infused. The signal is restricted to the vicinity of infusion and the perivascular area surrounding nearby blood vessels. (G and H) Higher magnification views of E and F (original magnification ×20). Size bars: 500 (A through F) and 125 µm (G and H).

FIG. 3

Sections from rat brains infused with fluorescent liposomes. (A through D) Note the robust and widespread liposomal distribution within the striatum (A and B) and globus pallidus (C and D) of rats from group H (high blood pressure/heart rate). This signal pattern forms a fairly continuous area of fluorescence. (E and F) Striatal sections from rats from group L (low blood pressure/heart rate). Note the arborized pattern of spread suggesting perivascular pathways. (G) Brain section from a more caudal part of the brain from a rat from group L showing signal confined in the globus pallidus. (H) Higher magnification view of F (original magnification ×20). Size bars: 500 (A–G) and 125 µm (H).