Paul F. Morrison

Focal delivery during direct infusion to brain: role of flow rate, catheter diameter, and tissue mechanics

Direct interstitial infusion is a technique capable of delivering agents over both small and large dimensions of brain tissue. However, at a sufficiently high volumetric inflow rate, backflow along the catheter shaft may occur and compromise delivery. A scaling relationship for the finite backflow distance along this catheter in pure gray matter (x(m)) has been determined from a mathematical model based on Stokes flow, Darcy flow in porous media, and elastic deformation of the brain tissue: x(m) = constant Q(o)(3)R(4)r(c)(4)G(-3)mu(-1) 1/5 [corrected] = volumetric inflow rate, R = tissue hydraulic resistance, r(c) = catheter radius, G = shear modulus, and mu = viscosity). This implies that backflow is minimized by the use of small diameter catheters and that a fixed (minimal) backflow distance may be maintained by offsetting an increase in flow rate with a similar decrease in catheter radius. Generally, backflow is avoided in rat gray matter with a 32-gauge catheter operating below 0.5 microliter/min. An extension of the scaling relationship to include brain size in the resistance term leads to the finding that absolute backflow distance obtained with a given catheter and inflow rate is weakly affected by the depth of catheter tip placement and, thus, brain size. Finally, an extension of the model to describe catheter passage through a white matter layer before terminating in the gray has been shown to account for observed percentages of albumin in the corpus callosum after a 4-microliter infusion of the compound to rat striatum over a range of volumetric inflow rates.

Heparin coinfusion during convection-enhanced delivery (CED) increases the distribution of the glial-derived neurotrophic factor (GDNF) ligand family in rat striatum and enhances the pharmacological activity of neurturin.

Convection-enhanced delivery (CED) distributes macromolecules in the brain in a homogeneous, targeted fashion in clinically useful volumes. However, the binding of growth factors to heparin-binding sites in the extracellular matrix may limit the volume of distribution (V(d)). To overcome this limitation, we examined the effects of heparin coinfusion on V(d) of glial-derived neurotrophic factor (GDNF), neurturin (NTN), artemin, and a nonspecifically bound protein, albumin. Heparin coinfusion significantly enhanced the V(d) of GDNF and GDNF-homologous trophic factors, probably by binding and blocking heparin-binding sites in the extracellular matrix. Furthermore, coinfusion of heparin with NTN enhanced striatal dopamine metabolism, compared to trophic factor administered alone. The negligible benefit of GDNF in recent clinical trials of Parkinsons disease may result from limited tissue distribution. Heparin coinfusion during CED targeting the striatum may alleviate this important limitation. This study demonstrates the influence of receptor binding on the distribution of trophic factors in the CNS.

Quantitative Examination of Tissue Concentration Profiles Associated with Microdialysis

Abstract: Spatial solute concentration profiles resulting from in vivo microdialysis were measured in rat caudate‐putamen by quantitative autoradiography. Radiolabeled sucrose was included in the dialysate, and the tissue concentration profile measured after infusions of 14 min and 61.5 min in an acute preparation. In addition, the changes in sucrose extraction fraction over time were followed in vivo and in a simple in vitro system consisting of 0.5% agarose. These experimental results were then compared with mathematical simulations of microdialysis in vitro and in vivo. Simulations of in vitro microdialysis agreed well with experimental results. In vivo, the autoradiograms of the tissue concentration profiles showed clear evidence of substantial differences between 14 and 61.5 min, even though the change in extraction fraction was relatively small over that period. Comparison with simulated results showed that the model substantially underpre‐dicted the observed extraction fraction and overall amount of sucrose in the tissue. A sensitivity analysis of the various model parameters suggested a tissue extracellular volume fraction of approximately 40% following probe implantation. We conclude that the injury from probe insertion initially causes disruption of the blood‐brain barrier in the vicinity of the probe, and this disruption leads to an influx of water and plasma constituents, causing a vasogenic edema.

Effects of Different Semipermeable Membranes on In Vitro and In Vivo Performance of Microdialysis Probes

The in vitro and in vivo performance of three different semipermeable microdialysis membranes was compared: a proprietary polycarbonate‐ether membrane made by Carnegie Medecin; cuprophan, a regenerated cellulose membrane; and polyacrylonitrile. When microdialysis probes were tested in a stirred in vitro solution, large and statistically significant differences among the three membranes in extraction of acid metabolites (3,4‐dihydroxyphenylacetic acid, 5‐hydroxyindoleacetic acid, and homovanillic acid) and acetaminophen were found. Polyacrylonitrile had the highest extractions in vitro. In contrast, when microdialysis probes were implanted in vivo (in rat striatum), extraction of acid metabolites and acetaminophen did not differ significantly among the different membranes. These results are consistent with predictions made by a mathematical model of microdialysis and can be explained by the fact that in vitro the main factor limiting extraction is membrane resistance to diffusion, whereas tissue resistance to diffusion plays a more dominant role in vivo. These findings suggest that (aside from differences in surface area), the choice of semipermeable membrane will generally have little effect on in vivo microdialysis results. Furthermore, in vitro measurements of microdialysis probe extractions are not a reliable way of calibrating in vivo performance.

Quantitative Microdialysis: Analysis of Transients and Application to Pharmacokinetics in Brain

Abstract: The behavior of a microdialysis probe in vivo is mathematically described. A diffusion‐reaction model is developed that not only accounts for transport of substances through tissues and probe membranes but also accounts for transport across the microvasculature and metabolism. Time‐dependent equations are presented both for the effluent microdialysate concentration and for concentration profiles about the probe. The analysis applies either to measuring the tissue pharmacokinetics of drugs administered systemically, or for sampling of endogenously produced substances from tissue. In addition, an expression is developed for the transient concentration about the probe when it is used as an infusion device. All mathematical expressions are found to be a sum of an algebraic and an integral term. Theoretical prediction of time‐dependent probe behavior in brain has been compared with experimental data for acetaminophen administered at 15 mg/kg to rats by intravenous bolus. Plasma and whole striatal tissue samples were used to describe plasma kinetics and to estimate a capillary permeability‐area product of 0.07 min‐1. Theoretical prediction of transient effluent dialysate concentrations exhibited close agreement with experimental data over 60 min. Terminal decline of the dialysate effluent concentration was slightly overestimated but theoretical concentrations still lay within the 95% confidence interval of the experimental data at 112 min. Microvasculature transport and metabolism play major roles in determining microdialysate transient responses. Extraction fraction (recovery) has been shown to be a declining function in time for five probe operating conditions. High rates of metabolism and/or capillary transport affect the time required to approach steady‐state extraction, shortening the time as the rates increase. Conversely, for substances characterized by low permeabilities and negligible metabolism, experimental situations exist that are predicted to have very slow approaches to microdialysis steady state.

Real-time imaging of convection-enhanced delivery of viruses and virus-sized particles

OBJECT Despite recent evidence showing that convection-enhanced delivery (CED) of viruses and virus-sized particles to the central nervous system (CNS) is possible, little is known about the factors influencing distribution of these vectors with convection. To better define the delivery of viruses and virus-sized particles in the CNS, and to determine optimal parameters for infusion, the authors coinfused adeno-associated virus ([AAV], 24-nm diameter) and/or ferumoxtran-10 (24 nm) by using CED during real-time magnetic resonance (MR) imaging. METHODS Sixteen rats underwent intrastriatal convective coinfusion with 4 microl of 35S-AAV capsids (0.5-1.0 x 10(14) viral particles/ml) and increasing concentrations (0.1, 0.5, 1, and 5 mg/ml) of a similar sized iron oxide MR imaging agent (ferumoxtran-10). Five nonhuman primates underwent either convective coinfusion of 35S-AAV capsids and 1 mg/ml ferumoxtran-10 (striatum, one animal) or infusion of 1 mg/ml ferumoxtran-10 alone (striatum in two animals; frontal white matter in two). Clinical effects, MR imaging studies, quantitative autoradiography, and histological data were analyzed. RESULTS Real-time, T2-weighted MR imaging of ferumoxtran-10 during infusion revealed a clearly defined hypointense region of perfusion. Quantitative autoradiography confirmed that MR imaging of ferumoxtran-10 at a concentration of 1 mg/ml accurately tracked viral capsid distribution in the rat and primate brain (the mean difference in volume of distribution [Vd] was 7 and 15% in rats and primates, respectively). The Vd increased linearly with increasing volume of infusion (Vi) (R2 = 0.98). The mean Vd/Vi ratio was 4.1 +/- 0.2 (mean +/- standard error of the mean) in gray and 2.3 +/- 0.1 in white matter (p < 0.01). The distribution of infusate was homogeneous. Postinfusion MR imaging revealed leakback along the cannula track at infusion rates greater than 1.5 microl/minute in primate gray and white matter. No animal had clinical or histological evidence of toxicity. CONCLUSIONS The CED method can be used to deliver AAV capsids and similar sized particles to the CNS safely and effectively over clinically relevant volumes. Moreover, real-time MR imaging of ferumoxtran-10 during infusion reveals that AAV capsids and similar sized particles have different convective delivery properties than smaller proteins and other compounds.

Convective delivery of glial cell line-derived neurotrophic factor in the human putamen

OBJECT The authors conducted an analysis of the distribution of glial cell line-derived neurotrophic factor in the human striatum following convection-enhanced delivery. METHODS Computational examinations of the effects of differing catheters, infusion rates, infusate concentrations, and target placement on distribution were completed based on the protocols of three recent clinical trials. RESULTS Similar drug distributions around on-target end-hole catheters were predicted in two of the trials (AmgenUT study and Bristol study), although there was slightly deeper penetration for one of the trials (Bristol) due to a higher infusate concentration. However, when positioning uncertainly located catheter tips close to gray-white matter interfaces, backflow could diminish delivery, shunting infusate across the interfaces. For delivery via a multiport catheter at a constant base infusion rate plus a periodic bolus inflow rate (Kentucky study), base inflow alone generated a somewhat smaller distribution volume relative to those in the other trials, was positioned more anteriorly in the putamen, and was somewhat elongated axially; the bolus component extended this putaminal distribution to a larger relative volume but may have been reduced by backflow loss. CONCLUSIONS Results of these computations indicated that for catheters placed exactly on the intended target, ideal drug distributions were similar for two of the trials (AmgenUT and Bristol) and different in terms of location and extent in the third study (Kentucky); yet the pattern of trial outcomes did not reflect these same groupings. This finding suggests that other factors are at play, widely varying statistical power and the possible effects of not excluding data from patients who experienced large drug losses across gray tissue boundaries due to variation in catheter placement.

Effects of systemic and central nervous system localized inflammation on the contributions of metabolic precursors to the L-kynurenine and quinolinic acid pools in brain

l‐Kynurenine and quinolinic acid are neuroactive l‐tryptophan‐kynurenine pathway metabolites of potential importance in pathogenesis and treatment of neurologic disease. To identify precursors of these metabolites in brain, [2H3]‐l‐kynurenine was infused subcutaneously by osmotic pump into three groups of gerbils: controls, CNS‐localized immune‐activated, and systemically immune‐activated. The specific activity of l‐kynurenine and quinolinate in blood, brain and systemic tissues at equilibrium was then quantified by mass spectrometry and the results applied to a model of metabolism to differentiate the relative contributions of various metabolic precursors. In control gerbils, 22% of l‐kynurenine in brain was derived via local synthesis from l‐tryptophan/formylkynurenine versus 78% from l‐kynurenine from blood. Quinolinate in brain was derived from several sources, including: local tissue l‐tryptophan/formylkynurenine (10%), blood l‐kynurenine (35%), blood 3‐hydroxykynurenine/3‐hydroxyanthranilate (7%), and blood quinolinate (48%). After systemic immune‐activation, however, l‐kynurenine in brain was derived exclusively from blood, whereas quinolinate in brain was derived from three sources: blood l‐kynurenine (52%), blood 3‐hydroxykynurenine or 3‐hydroxyanthranilate (8%), and blood quinolinate (40%). During CNS‐localized immune activation, > 98% of both l‐kynurenine and quinolinate were derived via local synthesis in brain. Thus, immune activation and its site determine the sources from which l‐kynurenine and quinolinate are synthesized in brain. Successful therapeutic modulation of their concentrations must take into account the metabolic and compartment sources.

Real-time, Image-Guided, Convection-Enhanced Delivery of Interleukin 13 Bound to Pseudomonas Exotoxin

Purpose: To determine if the tumor-targeted cytotoxin interleukin 13 bound to Pseudomonas exotoxin (IL13-PE) could be delivered to the brainstem safely at therapeutic doses while monitoring its distribution in real-time using a surrogate magnetic resonance imaging tracer, we used convection-enhanced delivery to perfuse rat and primate brainstems with IL13-PE and gadolinium-bound albumin (Gd-albumin). Experimental Design: Thirty rats underwent convective brainstem perfusion of IL13-PE (0.25, 0.5, or 10 μg/mL) or vehicle. Twelve primates underwent convective brainstem perfusion of either IL13-PE (0.25, 0.5, or 10 μg/mL; n = 8), co-infusion of 125I-IL13-PE and Gd-albumin (n = 2), or co-infusion of IL13-PE (0.5 μg/mL) and Gd-albumin (n = 2). The animals were permitted to survive for up to 28 days before sacrifice and histologic assessment. Results: Rats showed no evidence of toxicity at all doses. Primates showed no toxicity at 0.25 or 0.5 μg/mL but showed clinical and histologic toxicity at 10 μg/mL. Quantitative autoradiography confirmed that Gd-albumin precisely tracked IL13-PE anatomic distribution and accurately showed the volume of distribution. Conclusions: IL13-PE can be delivered safely and effectively to the primate brainstem at therapeutic concentrations and over clinically relevant volumes using convection-enhanced delivery. Moreover, the distribution of IL13-PE can be accurately tracked by co-infusion of Gd-albumin using real-time magnetic resonance imaging.

A Computational Model of Direct Interstitial Infusion of Macromolecules into the Spinal Cord

AbstractConvection-enhanced interstitial infusion can deliver macromolecular drugs to large tissue volumes of the central nervous system. To characterize infusion into the spinal cord, an image-based three-dimensional finite element model of the rat spinal cord was developed. The model incorporated convection and diffusion through white and gray matter, including anisotropic transport due to alignment of white matter tracts. Spatial and temporal distribution of the marker substance albumin within the interstitial space was determined. Consistent with previous experiments, predicted distribution was highly anisotropic. Infusing into the dorsal column, albumin was primarily confined to white matter with limited penetration into adjacent gray matter. Distribution was determined primarily by the ratio of fiber-parallel to fiber-perpendicular hydraulic conductivity tensor components (kwm-z/kwm-x), the ratio of transverse white and gray matter hydraulic conductivity (kwm-x/kgm), and tissue porosity. Fits to previous experimental measures of axial and transverse spread, distribution volume, and protein recovery yielded an optimum kwm-z/kwm-x of approximately 20 at 0.1 μl/min. kwm-x/kgm of 100 was sufficient to match experimental transverse distribution data. Best fits to data at 0.1 μl/min were achieved by porosities characteristic of moderate edema (e.g., 0.26). Distribution also varied with catheter placement with more medial placement resulting in greater distribution volumes.