Design of Drug Delivery Methods for the Brain and Central Nervous System
2015-07-21T00:00:00Z (GMT) by
Due to the impermeability of the blood-brain barrier (BBB) to macromolecules delivered systemically, drug delivery to the brain and central nervous system (CNS) is quite difficult and has become an area of intense research. Techniques such as convection-enhanced intraparenchymal delivery and intrathecal magnetic drug targeting offer a means of circumventing the blood-brain barrier for targeted delivery of therapeutics. This dissertation focuses on three aspects of drug delivery: pharmacokinetics, convection-enhanced delivery, and intrathecal magnetic drug targeting. Classical pharmacokinetics mainly uses black-box curve fitting techniques without biochemical or biological basis. This dissertation advances the state-of-the-art of pharmacokinetics and pharmacodynamics by incorporating first principles and biochemical/biotransport mechanisms in the prediction of drug fate in vivo. A whole body physiologically-based pharmacokinetics (PBPK) modeling framework is engineered which creates multiscale mathematical models for entire organisms composed of organs, tissues, and a detailed vasculature network to predict drug bioaccumulation and to rigorously determine kinetic parameters. These models can be specialized to account for species, weight, gender, age, and pathology. Systematic individual therapy design using the proposed mechanistic PBPK modeling framework is also a possibility. Biochemical, anatomical, and physiological scaling laws are also developed to accurately project drug kinetics in humans from small animal experiments. Our promising results demonstrate that the whole-body mechanistic PBPK modeling approach not only elucidates drug mechanisms from a biochemical standpoint, but offers better scaling precision. Better models can substantially accelerate the introduction of drug leads to clinical trials and eventually to the market by offering more understanding of the drug mechanisms, aiding in therapy design, and serving as an accurate dosing tool. Convection-enhanced drug delivery (CED) is a technique used to bypass the BBB via direct intracranial injection using a catheter driven by a positive pressure gradient from an infusion pump. Although CED boasts the advantage of achieving larger drug distribution volumes compared to diffusion driven methods, difficulty in predicting drug spread and preventing backflow along the catheter shaft commonly occur. In this dissertation, a method for predicting drug distributions in the brain using diffusion tensor imaging (DTI) data is employed to show how small variations in catheter placement can lead to drastically different volumes of drug distribution in vivo. The impact that microfluid flow has on deformable brain phantom gel is studied in order to elucidate the causes of backflow, and the results are used to develop backflow-free catheters with safe volumetric flow rates up to 10 μl/min. Through implementation of our backflow-free catheter designs, physicians will be able to target specific regions of the brain with improved accuracy, increased drug concentration, and larger drug distribution geometries. Intrathecal (IT) drug delivery involves direct drug infusion into the spinal canal and has become standard practice for treating many CNS diseases. Although IT drug delivery boasts the advantage of reduced systemic toxicity compared to oral and intravenous techniques, current IT delivery protocols lack a means of sufficient drug targeting at specific locations of interest within the CNS. In this dissertation, the method of intrathecal magnetic drug targeting (IT-MDT) is developed to overcome the limited targeting capabilities of standard IT drug delivery protocols. The basic idea behind IT-MDT is to guide intrathecally-injected, drug-functionalized magnetic nanoparticles (MNPs) using an external magnetic field to diseased regions within the spinal canal. Cerebrospinal fluid (CSF) transport phenomena are studied, and in vitro human spine surrogates are built. Experiments are run on the in vitro human spine model to determine the feasibility of IT-MDT and to develop novel treatment therapies. Computer simulations are performed to optimize magnetic field placement and/or implant design for generating high gradient magnetic fields, as well as to study how these fields aid in therapeutic nanoparticle localization. Large collection efficiencies of MNPs were achieved during in vitro IT-MDT and implant-assisted IT-MDT experiments with concentration levels nearly nine times that of the control when no magnetic field was present. Testing different magnetizable implants showed that implant design is a key factor in achieving the largest MNP collection efficiency within the targeting region. Knowledge gained from the in vitro IT-MDT experiments and simulations will be used in the future to develop IT-MDT methods in animals and humans.