Drug Targeting to the Brain

The goal of brain drug targeting technology is the delivery of therapeutics across the blood–brain barrier (BBB), including the human BBB. This is accomplished by re-engineering pharmaceuticals to cross the BBB via specific endogenous transporters localized within the brain capillary endothelium. Certain endogenous peptides, such as insulin or transferrin, undergo receptor-mediated transport (RMT) across the BBB in vivo. In addition, peptidomimetic monoclonal antibodies (MAb) may also cross the BBB via RMT on the endogenous transporters. The MAb may be used as a molecular Trojan horse to ferry across the BBB large molecule pharmaceuticals, including recombinant proteins, antibodies, RNA interference drugs, or non-viral gene medicines. Fusion proteins of the molecular Trojan horse and either neurotrophins or single chain Fv antibodies have been genetically engineered. The fusion proteins retain bi-functional properties, and both bind the BBB receptor, to trigger transport into brain, and bind the cognate receptor inside brain to induce the pharmacologic effect. Trojan horse liposome technology enables the brain targeting of non-viral plasmid DNA. Molecular Trojan horses may be formulated with fusion protein technology, avidin–biotin technology, or Trojan horse liposomes to target to brain virtually any large molecule pharmaceutical.     

Lipopolysaccharide Impairs Blood–Brain Barrier P-glycoprotein Function in Mice Through Prostaglandin- and Nitric Oxide-Independent Pathways

P-glycoprotein (P-gp) is a brain-to-blood efflux system that controls the ability of many drugs and endogenous substances to access the brain. In vitro work has shown that inflammatory states mediated through lipopolysaccharide (LPS) and tumor necrosis factor-alpha first impair and then stimulate P-gp activity. Here, we determined whether LPS can affect P-gp function in vivo. Mice treated with a single intraperitoneal injection of LPS (3 mg/kg) showed an inhibition of P-gp function. As assessed by brain perfusion, inhibition began 18 h after LPS administration and lasted until 36 h after administration. P-gp protein was increased by 44%, consistent with P-gp inhibition occurring through post-translational mechanisms. Unlike other effects of LPS on blood–brain barrier function, neither nitric oxide nor prostaglandin inhibition had an effect. We conclude that induction of proinflammatory states as exemplified by LPS treatment can inhibit P-gp function in vivo at the blood–brain barrier. Support: VA merit review; R01 NS051334; R01 NS050547; R01 AG029839. This work has not been previously presented at a meeting, but was presented in part as a poster at the June 2008 Gordon Conference on blood–brain barrier.     

Contribution of Carrier-Mediated Transport Systems to the Blood–Brain Barrier as a Supporting and Protecting Interface for the Brain; Importance for CNS Drug Discovery and Development

The blood–brain barrier (BBB) forms an interface between the circulating blood and the brain and possesses various carrier-mediated transport systems for small molecules to support and protect CNS function. For example, the blood-to-brain influx transport systems supply nutrients, such as glucose and amino acids. Consequently, xenobiotic drugs recognized by influx transporters are expected to have high permeability across the BBB. On the other hand, efflux transporters, including ATP-binding cassette transporters such as P-glycoprotein located at the luminal membrane of endothelial cells, function as clearance systems for metabolites and neurotoxic compounds produced in the brain. Drugs recognized by these transporters are expected to show low BBB permeability and low distribution to the brain. Despite recent progress, the transport mechanisms at the BBB have not been fully clarified yet, especially in humans. However, an understanding of the human BBB transport system is critical, because species differences mean that it can be difficult to extrapolate data obtained in experimental animals during drug development to humans. Recent progress in methodologies is allowing us to address this issue. Positron emission tomography can be used to evaluate the activity of human BBB transport systems in vivo. Proteomic studies may also provide important insights into human BBB function. Construction of a human BBB transporter atlas would be a most important advance from the viewpoint of CNS drug discovery and drug delivery to the brain.     

Intravenous siRNA of Brain Cancer with Receptor Targeting and Avidin–Biotin Technology

Purpose The effective delivery of short interfering RNA (siRNA) to brain following intravenous administration requires the development of a delivery system for transport of the siRNA across the brain capillary endothelial wall, which forms the blood–brain barrier in vivo. Methods siRNA was delivered to brain in vivo with the combined use of a receptor-specific monoclonal antibody delivery system, and avidin–biotin technology. The siRNA was mono-biotinylated on either terminus of the sense strand, in parallel with the production of a conjugate of the targeting MAb and streptavidin. Results Rat glial cells (C6 or RG-2) were permanently transfected with the luciferase gene, and implanted in the brain of adult rats. Following the formation of intra-cranial tumors, the rats were treated with a single intravenous injection of 270 μg/kg of biotinylated siRNA attached to a transferrin receptor antibody via a biotin–streptavidin linker. The intravenous administration of the siRNA caused a 69–81% decrease in luciferase gene expression in the intracranial brain cancer in vivo. Conclusions Brain delivery of siRNA following intravenous administration is possible with siRNAs that are targeted to brain with the combined use of receptor specific antibody delivery systems and avidin–biotin technology.     

Translational Research Models and Novel Adjunctive Therapies for NeuroAIDS

The goal of this component of the meeting was to discuss novel therapeutics that attenuate inflammatory pathways, enhance drug delivery into the brain and/or bypass the blood–brain barrier (BBB). Presentations included discussions of (i) antiretroviral drugs packaged into nanoparticles by Howard Gendelman, (ii) how to engage the nasal epithelium and its closely associated neural fibers as a route for drug entry to the brain, by William H. Frey and Lynn Pulliam, and (iii) antioxidant gene delivery to neurons using vector systems that can protect neurons against HIV-associated toxicity, by David Strayer. The session was capped by active discourse among leaders in the field of adjunctive therapies in a broad range of scientific disciplines. From the proceedings of the National Institute of Mental Health HIV Preclinical–Clinical Therapeutics Research Meeting, May 16–17, 2006.     

Pharmacokinetic Consequences of Active Drug Efflux at the Blood–Brain Barrier

Purpose The objective of this simulation study was to investigate how the nature, location, and capacity of the efflux processes in relation to the permeability properties influence brain concentrations. Methods Reduced brain concentrations can be due to either influx hindrance, a gatekeeper function in the luminal membrane, which has been suggested for ABCB1 (P-glycoprotein), or efflux enhancement by transporters that pick up molecules on one side of the luminal or abluminal membrane and release them on the other side. Pharmacokinetic models including passive transport, influx hindrance, and efflux enhancement were built using the computer program MATLAB. The simulations were based on experimentally obtained parameters for morphine, morphine-3-glucuronide, morphine-6-glucuronide, and gabapentin. Results The influx hindrance process is the more effective for keeping brain concentrations low. Efflux enhancement decreases the half-life of the drug in the brain, whereas with influx hindrance the half-life is similar to that seen with passive transport. The relationship between the influx and efflux of the drug across the blood–brain barrier determines the steady-state ratio of brain to plasma concentrations of unbound drug, K_p,uu. Conclusions Both poorly and highly permeable drugs can reach the same steady-state ratio, although the time to reach steady state will differ. The volume of distribution of unbound drug in the brain does not influence K_p,uu, but does influence the total brain-to-blood ratio K_p and the time to reach steady state in the brain.     

Strategies for Intranasal Delivery of Therapeutics for the Prevention and Treatment of NeuroAIDS

Intranasal drug administration is a noninvasive method of bypassing the blood–brain barrier (BBB) to deliver neurotrophins and other therapeutic agents to the brain and spinal cord. This method allows drugs that do not cross the BBB to be delivered to the central nervous system (CNS) and eliminates the need for systemic delivery, thereby reducing unwanted systemic side effects. Delivery from the nose to the CNS occurs within minutes along both the olfactory and trigeminal neural pathways. Intranasal delivery occurs by an extracellular route and does not require that drugs bind to any receptor or undergo axonal transport. Intranasal delivery also targets the nasal associated lymphatic tissues (NALT) and deep cervical lymph nodes. In addition, intranasally administered therapeutics are observed at high levels in the blood vessel walls and perivascular spaces of the cerebrovasculature. Using this intranasal method in animal models, researchers have successfully reduced stroke damage, reversed Alzheimer’s neurodegeneration, reduced anxiety, improved memory, stimulated cerebral neurogenesis, and treated brain tumors. In humans, intranasal insulin has been shown to improve memory in normal adults and patients with Alzheimer’s disease. Intranasal delivery strategies that can be employed to treat and prevent NeuroAIDS include: (1) target antiretrovirals to reach HIV that harbors in the CNS; (2) target therapeutics to protect neurons in the CNS; (3) modulate the neuroimmune function of moncyte/macrophages by targeting the lymphatics, perivascular spaces of the cerebrovasculature, and the CNS; and (4) improve memory and cognitive function by targeting therapeutics to the CNS. Presented at an NIMH workshop “HIV Preclinical–Clinical Therapeutics Research Meeting,” May 5–16, 2006.     

Blood–brain Barrier: Structural Components and Function Under Physiologic and Pathologic Conditions

The blood–brain barrier (BBB) is the specialized system of brain microvascular endothelial cells (BMVEC) that shields the brain from toxic substances in the blood, supplies brain tissues with nutrients, and filters harmful compounds from the brain back to the bloodstream. The close interaction between BMVEC and other components of the neurovascular unit (astrocytes, pericytes, neurons, and basement membrane) ensures proper function of the central nervous system (CNS). Transport across the BBB is strictly limited through both physical (tight junctions) and metabolic barriers (enzymes, diverse transport systems). A functional polarity exists between the luminal and abluminal membrane surfaces of the BMVEC. As a result of restricted permeability, the BBB is a limiting factor for the delivery of therapeutic agents into the CNS. BBB breakdown or alterations in transport systems play an important role in the pathogenesis of many CNS diseases (HIV-1 encephalitis, Alzheimer's disease, ischemia, tumors, multiple sclerosis, and Parkinson's disease). Proinflammatory substances and specific disease-associated proteins often mediate such BBB dysfunction. Despite seemingly diverse underlying causes of BBB dysfunction, common intracellular pathways emerge for the regulation of the BBB structural and functional integrity. Better understanding of tight junction regulation and factors affecting transport systems will allow the development of therapeutics to improve the BBB function in health and disease.     

A Novel Tool to Characterize Paracellular Transport: The APTS–Dextran Ladder

Purpose The aim of this work was to develop an easy, manageable, and precise analytic tool to describe the tightness of cell layers by a molecular weight ladder. Methods Dextrans were labeled by reductive amination with fluorescent 8-aminopyrene-1,3,6-trisulfonate (APTS). This mixture, including the internal standard diazepam, was used for transport studies in Transwell models using Caco-2, ECV304, and PBMEC/C1–2 cell lines. Samples were analyzed by fluorimetry, capillary electrophoresis, and reverse-phase high-performance liquid chromatography. Results Following this approach, a logarithm correlation of R² = 0.8958 between transepithelial electrical resistance (TEER) and APTS–dextran permeability was shown. In addition, a TEER-dependent permeability pattern could be observed including each single fraction from free APTS, APTS–glucose up to APTS–dextran consisting of 35 glucose units. The TEER-independent permeability coefficients of diazepam and confocal laser scanning microscopy images confirmed the paracellular transport of APTS–dextran. Conclusions All in all, the developed APTS–dextran ladder is a useful tool to characterize cell layer tightness and especially to describe paracellular transport ways and the extent of leakiness of cell layers (for blood–brain barrier or intestinal studies) over time—applying a wide array from smaller to larger molecules at the same time to refine TEER, sucrose, or Evans blue measurements.