Garrett W. Astary

Quantitative assessment of macromolecular concentration during direct infusion into an agarose hydrogel phantom using contrast-enhanced MRI

Convection-enhanced delivery (CED), that is, direct tissue infusion, has emerged as a promising local drug delivery method for treating diseases of the nervous system. Determination of the spatial distribution of therapeutic agents after infusion is important in evaluating the efficacy of treatment, optimizing infusion protocols and improving the understanding of drug pharmacokinetics. In this study, we provide a methodology to determine the concentration distribution of Gd-labeled tracers during infusion using contrast-enhanced magnetic resonance imaging (MRI). To the best of our knowledge, MR studies that quantify concentration profiles for CED have not been previously reported. The methodology utilizes intrinsic material properties (T(1) and R(1)) and reduces the effect of instrumental factors (e.g., inhomogeneity of MR detection field). As a methodology investigation, this study used an agarose hydrogel phantom as a tissue substitute for infusion. An 11.1-T magnet system was used to image infusion of Gd-DTPA-labeled albumin (Gd-albumin) into the hydrogel. By using data from preliminary scans, Gd-albumin distribution was determined from the signal intensity of the MR images. As a validation test, MR-derived concentration profiles were found comparable to both results measured directly using quantitative optical imaging and results from a computational transport model in porous media. In future studies, the developed methodology will be used to quantitatively monitor the distribution of Gd tracer following infusion directly into tissues.

Regional Convection-Enhanced Delivery of Gadolinium-labeled Albumin in the Rat Hippocampus In Vivo

Convection-enhanced delivery (CED) has emerged as a promising method of targeted drug delivery for treating central nervous system (CNS) disorders, but the influence of brain structure on infusate distribution is unclear. We have utilized this approach to study extracellular transport and distribution of a contrast agent in the hippocampus, a complex structure susceptible to CNS disorders. The magnetic resonance (MR) contrast agent diethylene triamene penta-acetic acid chelated gadolinium-labeled albumin (Gd-albumin), tagged with Evans blue dye, was directly infused (V(i)=5 microl) into the dorsal and ventral hippocampus of seven male Sprague-Dawley rats. The final distribution profile of the contrast agent, a product of CED and limited diffusion, was observed in vivo using high-resolution T1-weighted MR imaging at 11.1T. Dense cell layers, such as the granule cell layer of the dentate gyrus and the pyramidal cell layer of CA1, appeared to be barriers to transport of the tracer. Three-dimensional distribution shape and volume (V(d)) differences, between the dorsal and ventral hippocampus infusions, were determined from the MR images using a semi-automatic segmentation routine (dorsal V(d)=23.4+/-1.8 microl, ventral V(d)=36.4+/-5.1 microl). Finer structural detail of the hippocampus was obtained using a combination of histological analysis and fluorescence imaging. This study demonstrates that CED has the potential to target all regions of the hippocampus and that tracer distribution is influenced by infusion site, underlying structure and circuitry, and extent of backflow. Therefore, CED, combined with high-resolution MR imaging, may be a useful strategy for delivering therapeutics for the treatment of CNS disorders affecting the hippocampus.

Comparison of Particle Image Velocimetry and Phase Contrast MRI in a Patient-Specific Extracardiac Total Cavopulmonary Connection

Particle image velocimetry (PIV) and phase contrast magnetic resonance imaging (PC-MRI) have not been compared in complex biofluid environments. Such analysis is particularly useful to investigate flow structures in the correction of single ventricle congenital heart defects, where fluid dynamic efficiency is essential. A stereolithographic replica of an extracardiac total cavopulmonary connection (TCPC) is studied using PIV and PC-MRI in a steady flow loop. Volumetric two-component PIV is compared to volumetric three-component PC-MRI at various flow conditions. Similar flow structures are observed in both PIV and PC-MRI, where smooth flow dominates the extracardiac TCPC, and superior vena cava flow is preferential to the right pulmonary artery, while inferior vena cava flow is preferential to the left pulmonary artery. Where three-component velocity is available in PC-MRI studies, some helical flow in the extracardiac TCPC is observed. Vessel cross sections provide an effective means of validation for both experiments, and velocity magnitudes are of the same order. The results highlight similarities to validate flow in a complex patient-specific extracardiac TCPC. Additional information obtained by velocity in three components further describes the complexity of the flow in anatomic structures.

Voxelized model of interstitial transport in the rat spinal cord following direct infusion into white matter.

Direct tissue infusion, e.g., convection-enhanced delivery (CED), is a promising local delivery technique for treating diseases of the central nervous system. Predictive models of spatial drug distribution during and following direct tissue infusion are necessary for treatment optimization and planning of surgery. In this study, a 3D interstitial transport modeling approach in which tissue properties and anatomical boundaries are assigned on a voxel-by-voxel basis using tissue alignment data from diffusion tensor imaging (DTI) is presented. The modeling approach is semi-automatic and utilizes porous media transport theory to estimate interstitial transport in isotropic and anisotropic tissue regions. Rat spinal cord studies compared predicted distributions of albumin tracer (for varying DTI resolution) following infusion into the dorsal horn with tracer distributions measured by Wood et al. in a previous study. Tissue distribution volumes compared favorably for small infusion volumes (<4 microl). The presented DTI-based methodology provides a rapid means of estimating interstitial flows and tracer distributions following CED into the spinal cord. Quantification of these transport fields provides an important step toward development of drug-specific transport models of infusion.

A Preclinical Assessment of Neural Stem Cells as Delivery Vehicles for Anti-Amyloid Therapeutics

Transplantation of neural stems cells (NSCs) could be a useful means to deliver biologic therapeutics for late-stage Alzheimers disease (AD). In this study, we conducted a small preclinical investigation of whether NSCs could be modified to express metalloproteinase 9 (MMP9), a secreted protease reported to degrade aggregated Aβ peptides that are the major constituents of the senile plaques. Our findings illuminated three issues with using NSCs as delivery vehicles for this particular application. First, transplanted NSCs generally failed to migrate to amyloid plaques, instead tending to colonize white matter tracts. Second, the final destination of these cells was highly influenced by how they were delivered. We found that our injection methods led to cells largely distributing to white matter tracts, which are anisotropic conduits for fluids that facilitate rapid distribution within the CNS. Third, with regard to MMP9 as a therapeutic to remove senile plaques, we observed high concentrations of endogenous metalloproteinases around amyloid plaques in the mouse models used for these preclinical tests with no evidence that the NSC-delivered enzymes elevated these activities or had any impact. Interestingly, MMP9-expressing NSCs formed substantially larger grafts. Overall, we observed long-term survival of NSCs in the brains of mice with high amyloid burden. Therefore, we conclude that such cells may have potential in therapeutic applications in AD but improved targeting of these cells to disease-specific lesions may be required to enhance efficacy.

Dynamic contrast-enhanced MRI of Gd-albumin delivery to the rat hippocampus in vivo by convection-enhanced delivery

Convection-enhanced delivery (CED) shows promise in treating neurological diseases due to its ability to circumvent the blood-brain barrier (BBB) and deliver therapeutics directly to the parenchyma of the central nervous system (CNS). Such a drug delivery method may be useful in treating CNS disorders involving the hippocampus such as temporal lobe epilepsy and gliomas; however, the influence of anatomical structures on infusate distribution is not fully understood. As a surrogate for therapeutic agents, we used gadolinium-labeled-albumin (Gd-albumin) tagged with Evans Blue dye to observe the time dependence of CED infusate distributions into the rat dorsal and ventral hippocampus in vivo with dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI). For finer anatomical detail, final distribution volumes (V(d)) of the infusate were observed with high-resolution T(1)-weighted MR imaging and light microscopy of fixed brain sections. Dynamic images demonstrated that Gd-albumin preferentially distributed within the hippocampus along neuroanatomical structures with less fluid resistance and less penetration was observed in dense cell layers. Furthermore, significant leakage into adjacent cerebrospinal fluid (CSF) spaces such as the hippocampal fissure, velum interpositum and midbrain cistern occurred toward the end of infusion. V(d) increased linearly with infusion volume (V(i)) at a mean V(d)/V(i) ratio of 5.51 ± 0.55 for the dorsal hippocampus infusion and 5.30 ± 0.83 for the ventral hippocampus infusion. This study demonstrated the significant effects of tissue structure and CSF space boundaries on infusate distribution during CED.

Role of convection and diffusion on DCE-MRI parameters in low leakiness KHT sarcomas

The solid tumor is an abnormal environment that is resistant to systemically delivered drugs. Increased plasma leakiness and extracellular matrix density along with poor lymphatic function can result in interstitial flow that attenuates the effectiveness of therapeutics. This study expands upon a previously presented magnetic resonance (MR) imaging-based porous media model by investigating low permeability tumors, where interstitial flow may have increased effect on systemically delivered solutes. The solute transport of the porous media model is compared to that of experiment and the two-compartment model. Small non-necrotic tumors (n=3) were MR-imaged, serially, for 90 min after a bolus injection of Gd-based contrast agent (CA). These data provided for the calculation of experimental CA concentration over 90 min, while only early time points (15 min) were used to create vascular permeability, K(trans), maps for the porous media model. A K(trans) scale factor (range=1.3-2.5) in the porous media model was found to account for the reduction of permeability (measured by two-compartment model) due to interstitial flow. The optimized porous media simulations showed: 1) better dynamic CA behavior agreement with the experimental data than the two-compartment model (>33% reduction of RMS error); 2) similar spatial CA distribution trends across tumor with increased uptake at the tumor boundary.

MR measurement of alloy magnetic susceptibility: towards developing tissue-susceptibility matched metals.

Magnetic resonance imaging (MRI) can be used to relate structure to function mapped with high-temporal resolution electrophysiological recordings using metal electrodes. Additionally, MRI may be used to guide the placement of electrodes or conductive cannula in the brain. However, the magnetic susceptibility mismatch between implanted metals and surrounding brain tissue can severely distort MR images and spectra, particularly in high magnetic fields. In this study, we present a modified MR method of characterizing the magnetic susceptibility of materials that can be used to develop biocompatible, metal alloys that match the susceptibility of host tissue in order to eliminate MR distortions proximal to the implant. This method was applied at 4.7T and 11.1T to measure the susceptibility of a model solid-solution alloy of Cu and Sn, which is inexpensive but not biocompatible. MR-derived relative susceptibility values of four different compositions of Cu-Sn alloy deviated by less than 3.1% from SQUID magnetometry absolute susceptibility measurements performed up to 7T. These results demonstrate that the magnetic susceptibility varies linearly with atomic percentage in these solid-solution alloys, but are not simply the weighted average of Cu and Sn magnetic susceptibilities. Therefore susceptibility measurements are necessary when developing susceptibility-matched, solid-solution alloys for the elimination of susceptibility artifacts in MR. This MR method does not require any specialized equipment and is free of geometrical constraints, such as sample shape requirements associated with SQUID magnetometry, so the method can be used at all stages of fabrication to guide the development of a susceptibility matched, biocompatible device.

Absolute magnetic susceptibility of rat brain tissue

This study was performed to test the commonly held hypothesis that the absolute magnetic susceptibility of brain tissue is close to that of water since water accounts for over 50% of the tissue composition. In addition, the absolute value of susceptibility of brain tissue is needed for the development of materials that are implanted into or in close proximity to tissue.

Voxelized Model of Brain Infusion That Accounts for Small Feature Fissures: Comparison With Magnetic Resonance Tracer Studies

Convection enhanced delivery (CED) is a promising novel technology to treat neural diseases, as it can transport macromolecular therapeutic agents greater distances through tissue by direct infusion. To minimize off-target delivery, our group has developed 3D computational transport models to predict infusion flow fields and tracer distributions based on magnetic resonance (MR) diffusion tensor imaging data sets. To improve the accuracy of our voxelized models, generalized anisotropy (GA), a scalar measure of a higher order diffusion tensor obtained from high angular resolution diffusion imaging (HARDI) was used to improve tissue segmentation within complex tissue regions of the hippocampus by capturing small feature fissures. Simulations were conducted to reveal the effect of these fissures and cerebrospinal fluid (CSF) boundaries on CED tracer diversion and mistargeting. Sensitivity analysis was also conducted to determine the effect of dorsal and ventral hippocampal infusion sites and tissue transport properties on drug delivery. Predicted CED tissue concentrations from this model are then compared with experimentally measured MR concentration profiles. This allowed for more quantitative comparison between model predictions and MR measurement. Simulations were able to capture infusate diversion into fissures and other CSF spaces which is a major source of CED mistargeting. Such knowledge is important for proper surgical planning.