Theory of porous catheters and their applications in intraparenchymal infusions

In-vitro and in-vivo performance studies of a porous infusion catheter designed for intraparenchymal delivery of therapeutic agents of varying size

BACKGROUND

Limitations have previously existed for the use of brain infusion catheters with extended delivery port designs to achieve larger distribution volumes using convection-enhanced delivery (CED), due to poor transmittance of materials and uncontrolled backflow. The goal of this study was to evaluate a novel brain catheter that has been designed to allow for extended delivery and larger distribution volumes with limited backflow of fluid. It was characterized using a broad range of therapeutic pore sizes both for transmittance across the membranes to address possible occlusion and for distribution in short term infusion studies, both in-vitro in gels and in-vivo in canines.

METHODS

Brain catheters with pore sizes of 10, 12, 15, 20 and 30 µm were evaluated using three infusates prepared in 0.9% sterile saline with diameters approximating 2, 5, and 30 nm, respectively. Magnevist™ was chosen as the small molecule infusate to mimic low-molecular weight therapeutics. Galbumin™ served as a surrogate for an assortment of proteins used for brain cancer and Parkinson's disease. Gadoluminate™ was used to assess the distribution of large therapeutics, such as adeno-associated viral particles and synthetic nanoparticles. The transmittance of the medium and large tracer particles through catheters of different pore size (15, 20 and 30 µm) was measured by MRI and compared with the measured concentration of the control. Infusions into 0.2% agarose gels were performed in order to evaluate differences in transmittance and distribution of the small, medium, and large tracer particles through catheters with different pore sizes (10, 12, 15, 20 and 30 µm). In-vivo infusions were performed in the canine in order to evaluate the ability of the catheter to infuse the small, medium, and large tracer particles into brain parenchyma at high flow rates through catheters with different pore sizes (10, 15, and 20 µm). Two catheters were stereotactically inserted into the brain for infusion, one per hemisphere, in each animal (N = 6).

RESULTS

The transmittance of Galbumin and Gadoluminate across the catheter membrane surface was 100% to within the accuracy of the measurements. There was no evidence of any blockage or retardation of any of the infusates. Catheter pore size did not appear to significantly affect transmittance or distribution in gels of any of the molecule sizes in the range of catheter pore sizes tested. There were differences in the distributions between the different tracer molecules: Magnevist produced relatively large distributions, followed by Gadoluminate and Galbumin. We observed no instances of uncontrolled backflow in a total of 12 in-vivo infusions. In addition, several of the infusions resulted in substantial amounts remaining in tissue. We expect the in-tissue distributions to be substantially improved in the larger human brain.

COMPARISON WITH EXISTING METHODS

The new porous brain catheter performed well in terms of both backflow and intraparenchymal infusion of molecules of varying size in the canine brain under CED flow conditions.

CONCLUSIONS

Overall, the data presented in this report support that the novel porous brain catheter can deliver therapeutics of varying sizes at high infusion rates in the brain parenchyma, and resist backflow that can compromise the efficacy of CED therapy. Additional work is needed to further characterize the brain catheter, including animal toxicity studies of chronically implanted brain catheters to lay the foundation for its use in the clinic.

Delivery strategies for cell-based therapies in the brain: overcoming multiple barriers

Cell-based therapies to the brain are promising for the treatment of multiple brain disorders including neurodegeneration and cancers. In order to access the brain parenchyma, there are multiple physiological barriers that must be overcome depending on the route of delivery. Specifically, the blood-brain barrier has been a major difficulty in drug delivery for decades, and it still presents a challenge for the delivery of therapeutic cells. Other barriers, including the blood-cerebrospinal fluid barrier and lymphatic-brain barrier, are less explored, but may offer specific challenges or opportunities for therapeutic delivery. Here we discuss the barriers to the brain and the strategies currently in place to deliver cell-based therapies, including engineered T cells, dendritic cells, and stem cells, to treat diseases. With a particular focus on cancers, we also highlight the current ongoing clinical trials that use cell-based therapies to treat disease, many of which show promise at treating some of the deadliest illnesses.

Infusion Mechanisms in Brain White Matter and Their Dependence on Microstructure: An Experimental Study of Hydraulic Permeability

OBJECTIVE

Hydraulic permeability is a topic of deep interest in biological materials because of its important role in a range of drug delivery-based therapies. The strong dependence of permeability on the geometry and topology of pore structure and the lack of detailed knowledge of these parameters in the case of brain tissue makes the study more challenging. Although theoretical models have been developed for hydraulic permeability, there is limited consensus on the validity of existing experimental evidence to complement these models. In the present study, we measure the permeability of white matter (WM) of fresh ovine brain tissue considering the localised heterogeneities in the medium using an infusion-based experimental set up, iPerfusion. We measure the flow across different parts of the WM in response to applied pressures for a sample of specific dimensions and calculate the permeability from directly measured parameters. Furthermore, we directly probe the effect of anisotropy of the tissue on permeability by considering the directionality of tissue on the obtained values. Additionally, we investigate whether WM hydraulic permeability changes with post-mortem time. To our knowledge, this is the first report of experimental measurements of the localised WM permeability, also demonstrating the effect of axon directionality on permeability. This work provides a significant contribution to the successful development of intra-tumoural infusion-based technologies, such as convection-enhanced delivery (CED), which are based on the delivery of drugs directly by injection under positive pressure into the brain.

Determinants of Intraparenchymal Infusion Distributions: Modeling and Analyses of Human Glioblastoma Trials

Intra-parenchymal injection and delivery of therapeutic agents have been used in clinical trials for brain cancer and other neurodegenerative diseases. The complexity of transport pathways in tissue makes it difficult to envision therapeutic agent distribution from clinical MR images. Computer-assisted planning has been proposed to mitigate risk for inadequate delivery through quantitative understanding of infusion characteristics. We present results from human studies and simulations of intratumoral infusions of immunotoxins in glioblastoma patients. Gd-DTPA and ¹²⁴I-labeled human serum albumin (¹²⁴I-HSA) were co-infused with the therapeutic, and their distributions measured in MRI and PET. Simulations were created by modeling tissue fluid mechanics and physiology and suggested that reduced distribution of tracer molecules within tumor is primarily related to elevated loss rates computed from DCE. PET-tracer on the other hand shows that the larger albumin molecule had longer but heterogeneous residence times within the tumor. We found over two orders of magnitude variation in distribution volumes for the same infusion volumes, with relative error ~20%, allowing understanding of even anomalous infusions. Modeling and measurement revealed that key determinants of flow include infusion-induced expansion and loss through compromised BBB. Opportunities are described to improve computer-assisted CED through iterative feedback between simulations and imaging.