Convection-enhanced distribution of large molecules in gray matter during interstitial drug infusion

Abstract

✓ Many novel experimental therapeutic agents, such as neurotrophic factors, enzymes, biological modifiers, and genetic vectors, do not readily cross the blood-brain barrier. An effective strategy to deliver these compounds to the central nervous system is required for their application in vivo. Under normal physiological conditions, brain interstitial fluid moves by both bulk flow (convection) and diffusion. It has recently been shown that interstitial infusion into the white matter can be used to increase bulk flow, produce interstitial convection, and efficiently and homogeneously deliver drugs to large regions of brain without significant functional or structural damage. In theory, even more uniform distribution is likely in gray matter. In the current study, four experiments were performed to examine if convection-enhanced delivery could be used to achieve regional distribution of large molecules in gray matter. First, the volume and consistency of anatomical distribution of 20 µl of phaseolus vulgaris-leukoagglutinin (PHA-L; molecular weight (MW) 126 kD) after continuous high-flow microinfusion into the striatum of five rats over 200 minutes were determined using immunocytochemistry and quantified with image analysis. Second, the concentration profile of 14C-albumin (MW 69 kD) infused under identical conditions was determined in four hemispheres using quantitative autoradiography. Third, the volume of distribution after convection-enhanced infusion of 250 or 500 µl biotinylated dextran (b-dextran, MW 10 kD), delivered over 310 minutes into the caudate and putamen of a rhesus monkey from one (250 µl) or two (500 µl) cannulas, was determined using immunocytochemistry and quantified with image analysis. Finally, the ability to target all dopaminergic neurons of the nigrostriatal tract via perfusion of the striatum with subsequent retrograde transport was assessed in three experiments by immunohistochemical analysis of the mesencephalon following a 300-minute infusion of 27 µl horseradish peroxidase-labeled wheat germ agglutinin (WGA-HRP) into the striatum. Convection-enhanced delivery reproducibly distributed the large-compound PHA-L throughout the rat striatum (the percent volume of the striatum perfused, Vs, was 86% ± 5%; mean ± standard deviation) and produced a homogeneous tissue concentration in the perfused region (concentration of 14C-albumin relative to infusate concentration 30% ± 5%). In the monkey, the infusion widely distributed b-dextran within the striatum using one cannula (caudate and putamen Vs = 76% and 76%) or two cannulas (Vs = 90% and 71%). Perfusion of the rat striatum with WGA-HRP effectively targeted neurons throughout the pars compacta of the substantia nigra via their efferent connections in the nigrostriatal pathway. Convection-enhanced infusion into gray matter distributes large molecules extensively at a relatively homogeneous concentration. This technique for effective acute delivery of large molecules into the gray matter has several advantages over diffusion alone and has a wide spectrum of potential applications in laboratory and clinical neuroscience.