CONVECTION-ENHANCED DISTRIBUTION OF LARGE MOLECULES IN GRAY-MATTER DURING INTERSTITIAL DRUG INFUSION

Many novel experimental therapeutic agents, such as neurotrophic facto rs, 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 applica tion in vivo. Under normal physiological conditions, brain interstitia l fluid moves by both bulk flow (convection) and diffusion. It has rec ently been shown that interstitial infusion into the white matter can be used to increase bulk flow, produce interstitial convection, and ef ficiently and homogeneously deliver drugs to large regions of brain wi thout significant functional or structural damage. In theory, even mor e uniform distribution is likely in gray matter. In the current study, four experiments were performed to examine if convection-enhanced del ivery could be used to achieve regional distribution of large molecule s in gray matter. First, the volume and consistency of anatomical dist ribution of 20 mu l of phaseolus valgaris-leukoagglutinin (PHA-L; mole cular weight (MW) 126 kD) after continuous high-flow microinfusion int o the striatum of five rats over 200 minutes were determined using imm unocytochemistry and quantified with image analysis. Second, the conce ntration profile of C-14 albumin (MW 69 kD) infused under identical co nditions was determined in four hemispheres using quantitative autorad iography. Third, the volume of distribution after convection-enhanced infusion of 250 or 500 mu l biotinylated dextran (b-dextran, MW 10 kD) , delivered over 310 minutes into the caudate and putamen of a rhesus monkey from one (250 mu I) or two (500 mu l) cannulas, was determined using immunocytochemistry and quantified with image analysis. Finally, the ability to target all dopaminergic neurons of the nigrostriatal t ract via perfusion of the striatum with subsequent retrograde transpor t was assessed in three experiments by immunohistochemical analysis of the mesencephalon following a 300-minute infusion of 27 mu l horserad ish peroxidase-labeled wheat germ agglutinin (WGA-HRP) into the striat um. Convection-enhanced delivery reproducibly distributed the large-co mpound PHA-L throughout the rat striatum (the percent volume of the st riatum perfused, V-s, was 86% +/- 5%; mean +/- standard deviation) and produced a homogeneous tissue concentration in the perfused region (c oncentration of C-14-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 V-s = 76% and 76% ) or two cannulas (V-s = 90% and 71%). Perfusion of the rat striatum w ith WGA-HRP effectively targeted neurons throughout the pars compacta of the substantia nigra via their efferent connections in the nigrostr iatal pathway. Convection-enhanced infusion into gray matter distribut es large molecules extensively at a relatively homogeneous concentrati on. This technique for effective acute delivery of large molecules int o the gray matter has several advantages over diffusion alone and has a wide spectrum of potential applications in laboratory and clinical n euroscience.