Tumor necrosis factor receptor-IgG fusion protein for targeted drug delivery across the human blood-brain barrier

A Historical Review of Brain Drug Delivery

The history of brain drug delivery is reviewed beginning with the first demonstration, in 1914, that a drug for syphilis, salvarsan, did not enter the brain, due to the presence of a blood-brain barrier (BBB). Owing to restricted transport across the BBB, FDA-approved drugs for the CNS have been generally limited to lipid-soluble small molecules. Drugs that do not cross the BBB can be re-engineered for transport on endogenous BBB carrier-mediated transport and receptor-mediated transport systems, which were identified during the 1970s-1980s. By the 1990s, a multitude of brain drug delivery technologies emerged, including trans-cranial delivery, CSF delivery, BBB disruption, lipid carriers, prodrugs, stem cells, exosomes, nanoparticles, gene therapy, and biologics. The advantages and limitations of each of these brain drug delivery technologies are critically reviewed.

Blood-Brain Barrier Modulation to Improve Glioma Drug Delivery

The blood-brain barrier (BBB) is formed by brain microvascular endothelial cells that are sealed by tight junctions, making it a significant obstacle for most brain therapeutics. The poor BBB penetration of newly developed therapeutics has therefore played a major role in limiting their clinical success. A particularly challenging therapeutic target is glioma, which is the most frequently occurring malignant brain tumor. Thus, to enhance therapeutic uptake in tumors, researchers have been developing strategies to modulate BBB permeability. However, most conventional BBB opening strategies are difficult to apply in the clinical setting due to their broad, non-specific modulation of the BBB, which can result in damage to normal brain tissue. In this review, we have summarized strategies that could potentially be used to selectively and efficiently modulate the tumor BBB for more effective glioma treatment.

Treating brain diseases using systemic parenterally-administered protein therapeutics: Dysfunction of the brain barriers and potential strategies

The parenteral administration of protein therapeutics is increasingly gaining importance for the treatment of human diseases. However, the presence of practically impermeable blood-brain barriers greatly restricts access of such pharmaceutics to the brain. Treating brain disorders with proteins thus remains a great challenge, and the slow clinical translation of these therapeutics may be largely ascribed to the lack of appropriate brain delivery system. Exploring new approaches to deliver proteins to the brain by circumventing physiological barriers is thus of great interest. Moreover, parallel advances in the molecular neurosciences are important for better characterizing blood-brain interfaces, particularly under different pathological conditions (e.g., stroke, multiple sclerosis, Parkinson's disease, and Alzheimer's disease). This review presents the current state of knowledge of the structure and the function of the main physiological barriers of the brain, the mechanisms of transport across these interfaces, as well as alterations to these concomitant with brain disorders. Further, the different strategies to promote protein delivery into the brain are presented, including the use of molecular Trojan horses, the formulation of nanosystems conjugated/loaded with proteins, protein-engineering technologies, the conjugation of proteins to polymers, and the modulation of intercellular junctions. Additionally, therapeutic approaches for brain diseases that do not involve targeting to the brain are presented (i.e., sink and scavenging mechanisms).

Targeting receptor-mediated transport for delivery of biologics across the blood-brain barrier

Biologics are an emerging class of medicines with substantial promise to treat neurological disorders such as Alzheimer's disease, stroke, and multiple sclerosis. However, the blood-brain barrier (BBB) presents a formidable obstacle that appreciably limits brain uptake and hence the therapeutic potential of biologics following intravenous administration. One promising strategy for overcoming the BBB to deliver biologics is the targeting of endogenous receptor-mediated transport (RMT) systems that employ vesicular trafficking to transport ligands across the BBB endothelium. If a biologic is modified with an appropriate targeting ligand, it can gain improved access to the brain via RMT. Various RMT-targeting strategies have been developed over the past 20 years, and this review explores exciting recent advances, emphasizing studies that show brain targeting in vivo.

Drug delivery systems, CNS protection, and the blood brain barrier

Present review highlights various drug delivery systems used for delivery of pharmaceutical agents mainly antibiotics, antineoplastic agents, neuropeptides, and other therapeutic substances through the endothelial capillaries (BBB) for CNS therapeutics. In addition, the use of ultrasound in delivery of therapeutic agents/biomolecules such as proline rich peptides, prodrugs, radiopharmaceuticals, proteins, immunoglobulins, and chimeric peptides to the target sites in deep tissue locations inside tumor sites of brain has been explained. In addition, therapeutic applications of various types of nanoparticles such as chitosan based nanomers, dendrimers, carbon nanotubes, niosomes, beta cyclodextrin carriers, cholesterol mediated cationic solid lipid nanoparticles, colloidal drug carriers, liposomes, and micelles have been discussed with their recent advancements. Emphasis has been given on the need of physiological and therapeutic optimization of existing drug delivery methods and their carriers to deliver therapeutic amount of drug into the brain for treatment of various neurological diseases and disorders. Further, strong recommendations are being made to develop nanosized drug carriers/vehicles and noninvasive therapeutic alternatives of conventional methods for better therapeutics of CNS related diseases. Hence, there is an urgent need to design nontoxic biocompatible drugs and develop noninvasive delivery methods to check posttreatment clinical fatalities in neuropatients which occur due to existing highly toxic invasive drugs and treatment methods.

Receptor-based targeting of therapeutics

Receptor-based targeting of therapeutics may be a fascinating proposition to improve the therapeutic efficacy of encapsulated drugs. The development of safe and effective nanomedicines is a prerequisite in the current nanotechnological scenario. Currently, the surface engineering of nanocarriers has attracted great attention for targeted therapeutic delivery by selective binding of targeting ligand to the specific receptors present on the surface of cells. In this review, we have discussed the current status of various receptors such as transferrin, lectoferrin, lectin, folate, human EGF receptor, scavenger, nuclear and integrin, which are over-expressed on the surface of cancer cells; along with the relevance of targeted delivery systems such as nanoparticles, polymersomes, dendrimers, liposomes and carbon nanotubes. The review also focuses on the effective utilization of receptor-based targeted delivery systems for the management of cancer in effective ways by minimizing the drug-associated side effects and improving the therapeutic efficacy of developed nano-architectures.

Drug delivery systems for the treatment of ischemic stroke

Stroke is the third leading cause of death in the United States. Reduced cerebral blood flow causes acute damage to the brain due to excitotoxicity, reactive oxygen species (ROS), and ischemia. Currently, the main treatment for stroke is to revive the blood flow by using thrombolytic agents. Reviving blood flow also causes ischemia-reperfusion (I/R) damage. I/R damage results from inflammation and apoptosis and can persist for days to weeks, increasing the infarct size. Drugs can be applied to stroke to intervene in the sub-acute and chronic phases. Chemical, peptide, and genetic therapies have been evaluated to reduce delayed damage to the brain. These drugs have different characteristics, requiring that delivery carriers be developed based on these characteristics. The delivery route is another important factor affecting the efficiency of drug delivery. Various delivery routes have been developed, such as intravenous injection, intranasal administration, and local direct injection to overcome the blood-brain-barrier (BBB). In this review, the delivery carriers and delivery routes for peptide and gene therapies are discussed and examples are provided. Combined with new drugs, drug delivery systems will eventually provide useful treatments for ischemic stroke.

Brain protection from stroke with intravenous TNFα decoy receptor-Trojan horse fusion protein

Tumor necrosis factor (TNF)-α is produced in brain in response to acute cerebral ischemia, and promotes neuronal apoptosis. Biologic TNF inhibitors (TNFIs), such as the etanercept, cannot be developed as new stroke treatments because these large molecule drugs do not cross the blood-brain barrier (BBB). A BBB-penetrating biologic TNFI was engineered by fusion of the type II human TNF receptor (TNFR) to each heavy chain of a genetically engineered chimeric monoclonal antibody (MAb) against the mouse transferrin receptor (TfR), designated as cTfRMAb-TNFR fusion protein. The cTfRMAb domain of the fusion protein acts as a molecular Trojan horse to deliver the fused TNFR across the BBB. Etanercept or the cTfRMAb-TNFR fusion protein (1 mg/kg) was administered intravenously in adult mice subjected to 1-hour reversible middle cerebral artery occlusion up to 90 minutes after the occlusion. Neuroprotection was assessed at 24 hours or 7 days after occlusion. The cTfRMAb-TNFR fusion protein treatment caused a significant 45%, 48%, 42%, and 54% reduction in hemispheric, cortical, and subcortical stroke volumes, and neural deficit, respectively. Intravenous etanercept had no therapeutic effect. Biologic TNFIs can be reengineered for BBB penetration, and the IgG-TNFR fusion protein is therapeutic after delayed intravenous administration in experimental stroke.

Pharmacokinetics of recombinant bifunctional fusion proteins

INTRODUCTION

The development of biotechnology has enabled the creation of various recombinant fusion proteins as a new class of biotherapeutics. The uniqueness of fusion proteins lies in their ability to fuse two or more protein domains, providing vast opportunities to generate novel combinations of functions. Pharmacokinetic (PK) studies, which are critical components in preclinical and clinical drug development, have not been fully explored for fusion proteins. The lack of general PK models and study guidelines has become a bottleneck for translation of fusion proteins from basic research to the clinic.

AREAS COVERED

This article reviews the current status of PK studies for fusion proteins, covering the processes that affect PK. According to their PK properties, a classification of fusion proteins is suggested along with examples from the clinic or under development. Current limitations and future perspectives for PK of fusion proteins are also discussed.

EXPERT OPINION

A PK model for bifunctional fusion proteins is presented to highlight the importance of mechanistic studies for a thorough understanding of the PK properties of fusion proteins. The model suggests investigating the receptor binding and subsequent intracellular disposition of individual domains, which can have dramatic impact on the PK of fusion proteins.

Peripheral anti-inflammatory effects explain the ginsenosides paradox between poor brain distribution and anti-depression efficacy

Background

The effectiveness of ginseng in preventing and treating various central nervous system (CNS) diseases has been widely confirmed. However, ginsenosides, the principal components of ginseng, are characterized by poor accessibility to the brain, and this pharmacokinetic-pharmacological paradox remains poorly explained. Anti-inflammatory approaches are becoming promising therapeutic strategies for depression and other CNS diseases; however, previous studies have focused largely on anti-inflammatory therapies directed at the central nervous system. It is thus of interest to determine whether ginsenosides, characterized by poor brain distribution, are also effective in treating lipopolysaccharide- (LPS) induced depression-like behavior and neuroinflammation.

Methods

In an LPS-induced depression-like behavior model, the antidepressant effects of ginseng total saponins (GTS) were assessed using a forced swimming test, a tail suspension test, and a sucrose preference test. The anti-inflammatory efficacies of GTS in brain, plasma, and LPS-challenged RAW264.7 cells were validated using ELISA and quantitative real-time PCR. Moreover, indoleamine 2,3-dioxygenase (IDO) activity in the periphery and brain were also determined by measuring levels of kynurenine/tryptophan.

Results

GTS significantly attenuated LPS-induced depression-like behavior. Moreover, LPS-induced increases in 5-HT and tryptophane turnover in the brain were significantly reduced by GTS. IDO activities in brain and periphery were also suppressed after pretreatment with GTS. Furthermore, GTS-associated recovery from LPS-induced depression-like behavior was paralleled with reduced mRNA levels for IL-1β, IL-6, TNF-α, and IDO in hippocampus. Poor brain distribution of ginsenosides was confirmed in LPS-challenged mice. GTS treatment significantly decreased production of various proinflammatory cytokines in both LPS-challenged mice and RAW264.7 cells.

Conclusion

This study suggests that the anti-depression efficacy of GTS may be largely attributable to its peripheral anti-inflammatory activity. Our study also strengthens an important notion that peripheral anti-inflammation strategies may be useful in the therapy of inflammation-related depression and possibly other CNS diseases.

Neuroprotection with a brain-penetrating biologic tumor necrosis factor inhibitor

Biologic tumor necrosis factor (TNF)-α inhibitors do not cross the blood-brain barrier (BBB). A BBB-penetrating TNF-α inhibitor was engineered by fusion of the extracellular domain of the type II human TNF receptor (TNFR) to the carboxyl terminus of the heavy chain of a mouse/rat chimeric monoclonal antibody (MAb) against the mouse transferrin receptor (TfR), and this fusion protein is designated cTfRMAb-TNFR. The cTfRMAb-TNFR fusion protein and etanercept bound human TNF-α with high affinity and K(D) values of 374 ± 77 and 280 ± 80 pM, respectively. Neuroprotection in brain in vivo after intravenous administration of the fusion protein was examined in a mouse model of Parkinson's disease. Mice were also treated with saline or a non-BBB-penetrating TNF decoy receptor, etanercept. After intracerebral injection of the nigral-striatal toxin, 6-hydroxydopamine, mice were treated every other day for 3 weeks. Treatment with the cTfRMAb-TNFR fusion protein caused an 83% decrease in apomorphine-induced rotation, a 67% decrease in amphetamine-induced rotation, a 82% increase in vibrissae-elicited forelimb placing, and a 130% increase in striatal tyrosine hydroxylase (TH) enzyme activity. In contrast, chronic treatment with etanercept, which does not cross the BBB, had no effect on neurobehavior or striatal TH enzyme activity. A bridging enzyme-linked immunosorbent assay specific for the cTfRMAb-TNFR fusion protein showed that the immune response generated in the mice was low titer. In conclusion, a biologic TNF inhibitor is neuroprotective after intravenous administration in a mouse model of neurodegeneration, providing that the TNF decoy receptor is reengineered to cross the BBB.

Strategies for enhancing antibody delivery to the brain

Antibodies and antibody conjugates have emerged as important tools for cancer therapy. However, a major therapeutic challenge for the use of antibodies is their inability to cross the blood-brain barrier (BBB) to reach tumors localized in the central nervous system (CNS). Multiple methods have been developed to enhance antibody delivery to the CNS, including direct injection, mechanical or biochemical disruption of the BBB, conjugation to a 'molecular Trojan horse', cationization, encapsulation in nanoparticles and liposomes, and more recently, stem cell-mediated antibody delivery. In this review, we discuss each of these approaches, highlighting their successes and the obstacles that remain to be overcome.

Brain-penetrating tumor necrosis factor decoy receptor in the mouse

Biologic tumor necrosis factor inhibitors (TNFIs) include TNF decoy receptors (TNFRs). TNFα plays a pathologic role in both acute and chronic brain disease. However, biologic TNFIs cannot be developed as brain therapeutics because these large molecule drugs do not cross the blood-brain barrier (BBB). To enable penetration of the brain via receptor-mediated transport, the human TNFR type II was re-engineered as an IgG fusion protein, where the IgG part is a chimeric monoclonal antibody (MAb) against the mouse transferrin receptor (TfR), and this fusion protein is designated cTfRMAb-TNFR. The cTfRMAb part of the fusion protein acts as a molecular Trojan horse to ferry the TNFR across the BBB via transport on the endogenous BBB TfR. cTfRMAb-TNFR was expressed by stably transfected Chinese hamster ovary cells and purified by affinity chromatography to homogeneity on electrophoretic gels. The fusion protein reacted with antibodies to both mouse IgG and the human TNFR and bound TNFα with high affinity (K(d) = 96 ± 34 pM). cTfRMAb-TNFR was rapidly transported into mouse brain in vivo after intravenous administration, and the brain uptake of the fusion protein was 2.8 ± 0.5% of injected dose per gram of brain, which is >45-fold higher than the brain uptake of an IgG that does not recognize the mouse TfR. This new IgG-TNFR fusion protein can be tested in mouse models of brain diseases in which TNFα plays a pathologic role.

Biopharmaceutical drug targeting to the brain

Biopharmaceuticals are large molecule drugs that do not cross the blood-brain barrier (BBB). The limiting factor in the drug development of biopharmaceuticals as new drugs for the human brain is the engineering of effective brain drug targeting technology platforms. Recombinant proteins, enzymes, and monoclonal antibodies can be re-engineered for transport across the human BBB with the molecular Trojan horse technology. The most active BBB molecular Trojan horse is a monoclonal antibody to the human insulin receptor. The genetic engineering of IgG fusion proteins has been demonstrated for neurotrophic factors, decoy receptors, therapeutic enzymes, single chain Fv antibodies, and avidin. The IgG fusion proteins are not toxic on repeated administration in high doses to primates and do not interfere with glycemic control in plasma or brain. IgG fusion proteins contain amino acid sequences that induce immune tolerance, and show low immunogenicity in primates. The IgG fusion proteins are new bifunctional biopharmaceuticals that are both targeted to brain via transport on endogenous BBB receptors, and exert pharmacological effects in brain at the cognate receptor, ligand, or enzyme substrate.

Biologic TNFα-inhibitors that cross the human blood-brain barrier

Tumor necrosis factor (TNF)α inhibitors (TNFI) are a major class of biologic therapeutics, and include decoy receptor and monoclonal antibody (MAb) therapeutics that block TNFα action. TNFα is a pro-inflammatory cytokine in brain disease, such as stroke, brain or spinal cord injury, or Alzheimer disease. However, the biologic TNFIs cannot be developed for the brain, because these large molecules do not cross the blood-brain barrier (BBB). Brain penetrating forms of TNFα decoy receptors or anti-TNFα antibody therapeutics can be re-engineered as IgG fusion proteins with a BBB molecular Trojan horse, such as the mAb against the human insulin receptor (HIR). The HIRMAb undergoes receptor-mediated transport across the BBB via the endogenous insulin receptor, and carries into brain the fused biologic TNFI. A fusion protein of the HIRMAb and the type II TNF receptor (TNFR) extracellular domain, designated the HIRMAb-TNFR fusion protein, has been engineered and expressed in stably transfected Chinese hamster ovary (CHO) cells. The HIRMAb-TNFR fusion protein binds both the HIR and TNFα with low nM affinity. The HIRMAb cross reacts with the Rhesus monkey insulin receptor, and the HIRMAb-TNFR is rapidly, and selectively, taken up by primate brain at concentrations that inhibit TNFα. In addition, a fusion protein of the HIRMAb and a therapeutic single chain Fv (ScFv) antibody has been engineered and also expressed in stably transfected CHO cells. The BBB molecular Trojan horse platform technology allows for the engineering of brain-penetrating recombinant proteins as new biologic therapeutics for the human brain.

IgG-single chain Fv fusion protein therapeutic for Alzheimer's disease: Expression in CHO cells and pharmacokinetics and brain delivery in the rhesus monkey

Monoclonal antibodies (MAb) directed against the Abeta amyloid peptide of Alzheimer's disease (AD) are potential new therapies for AD, since these antibodies disaggregate brain amyloid plaque. However, the MAb is not transported across the blood-brain barrier (BBB). To enable BBB transport, a single chain Fv (ScFv) antibody against the Abeta peptide of AD was re-engineered as a fusion protein with the MAb against the human insulin receptor (HIR). The HIRMAb acts as a molecular Trojan horse to ferry the ScFv therapeutic antibody across the BBB. Chinese hamster ovary (CHO) cells were stably transfected with a tandem vector encoding the heavy and light chains of the HIRMAb-ScFv fusion protein. A high secreting line was isolated following methotrexate amplification and dilutional cloning. The HIRMAb-ScFv fusion protein in conditioned serum-free medium was purified by protein A affinity chromatography. The fusion protein was stable as a liquid formulation, and retained high-affinity binding of both the HIR and the Abeta amyloid peptide. The HIRMAb-ScFv fusion protein was radiolabeled with the (125)I-Bolton-Hunter reagent, followed by measurement of the pharmacokinetics of plasma clearance and brain uptake in the adult Rhesus monkey. The HIRMAb-ScFv fusion protein was rapidly cleared from plasma and was transported across the primate BBB in vivo. In conclusion, the HIRMAb-ScFv fusion protein is a new class of antibody-based therapeutic for AD that has been specifically engineered to cross the human BBB.

Selective targeting of a TNFR decoy receptor pharmaceutical to the primate brain as a receptor-specific IgG fusion protein

Decoy receptors, such as the human tumor necrosis factor receptor (TNFR), are potential new therapies for brain disorders. However, decoy receptors are large molecule drugs that are not transported across the blood-brain barrier (BBB). To enable BBB transport of a TNFR decoy receptor, the human TNFR-II extracellular domain was re-engineered as a fusion protein with a chimeric monoclonal antibody (MAb) against the human insulin receptor (HIR). The HIRMAb acts as a molecular Trojan horse to ferry the TNFR therapeutic decoy receptor across the BBB. The HIRMAb-TNFR fusion protein was expressed in stably transfected CHO cells, and was analyzed with electrophoresis, Western blotting, size exclusion chromatography, and binding assays for the HIR and TNFalpha. The HIRMAb-TNFR fusion protein was radio-labeled by trititation, in parallel with the radio-iodination of recombinant TNFR:Fc fusion protein, and the proteins were co-injected in the adult Rhesus monkey. The TNFR:Fc fusion protein did not cross the primate BBB in vivo, but the uptake of the HIRMAb-TNFR fusion protein was high and 3% of the injected dose was taken up by the primate brain. The TNFR was selectively targeted to brain, relative to peripheral organs, following fusion to the HIRMAb. This study demonstrates that decoy receptors may be re-engineered as IgG fusion proteins with a BBB molecular Trojan horse that selectively targets the brain, and enables penetration of the BBB in vivo. IgG-decoy receptor fusion proteins represent a new class of human neurotherapeutics.