Reversal of lysosomal storage in brain of adult MPS-I mice with intravenous Trojan horse-iduronidase fusion protein

Receptor-mediated drug delivery of bispecific therapeutic antibodies through the blood-brain barrier

Therapeutic antibody drug development is a rapidly growing sector of the pharmaceutical industry. However, antibody drug development for the brain is a technical challenge, and therapeutic antibodies for the central nervous system account for ~3% of all such agents. The principal obstacle to antibody drug development for brain or spinal cord is the lack of transport of large molecule biologics across the blood-brain barrier (BBB). Therapeutic antibodies can be made transportable through the blood-brain barrier by the re-engineering of the therapeutic antibody as a BBB-penetrating bispecific antibody (BSA). One arm of the BSA is the therapeutic antibody and the other arm of the BSA is a transporting antibody. The transporting antibody targets an exofacial epitope on a BBB receptor, and this enables receptor-mediated transcytosis (RMT) of the BSA across the BBB. Following BBB transport, the therapeutic antibody then engages the target receptor in brain. RMT systems at the BBB that are potential conduits to the brain include the insulin receptor (IR), the transferrin receptor (TfR), the insulin-like growth factor receptor (IGFR) and the leptin receptor. Therapeutic antibodies have been re-engineered as BSAs that target the insulin receptor, TfR, or IGFR RMT systems at the BBB for the treatment of Alzheimer's disease and Parkinson's disease.

Introducing or removing heparan sulfate binding sites does not alter brain uptake of the blood-brain barrier shuttle scFv8D3

The blood-brain barrier (BBB) greatly limits the delivery of protein-based drugs into the brain and is a major obstacle for the treatment of brain disorders. Targeting the transferrin receptor (TfR) is a strategy for transporting protein-based drugs into the brain, which can be utilized by using TfR-binding BBB transporters, such as the TfR-binding antibody 8D3. In this current study, we investigated if binding to heparan sulfate (HS) contributes to the brain uptake of a single chain fragment variable of 8D3 (scFv8D3). We designed and produced a scFv8D3 mutant, engineered with additional HS binding sites, HS(+)scFv8D3, to assess whether increased HS binding would improve brain uptake. Additionally, a mutant with a reduced number of HS binding sites, HS(-)scFv8D3, was also engineered to see if reducing the HS binding sites could also affect brain uptake. Heparin column chromatography showed that only the HS(+)scFv8D3 mutant bound HS in the experimental conditions. Ex vivo results showed that the brain uptake was unaffected by the introduction or removal of HS binding sites, which indicates that scFv8D3 is not dependent on the HS binding sites for brain uptake. Conversely, introducing HS binding sites to scFv8D3 decreased its renal excretion while removing them had the opposite effect.

Engineering antibody and protein therapeutics to cross the blood-brain barrier

Diseases in the central nervous system (CNS) are often difficult to treat. Antibody- and protein-based therapeutics hold huge promises in CNS disease treatment. However, proteins are restricted from entering the CNS by the blood-brain barrier (BBB). To achieve enhanced BBB crossing, antibody-based carriers have been developed by utilizing the endogenous macromolecule transportation pathway, known as receptor-mediated transcytosis. In this report, we first provided an overall review on key CNS diseases and the most promising antibody- or protein-based therapeutics approved or in clinical trials. We then reviewed the platforms that are being explored to increase the macromolecule brain entry to combat CNS diseases. Finally, we have analyzed the lessons learned from past experiences and have provided a perspective on the future engineering of novel delivery vehicles for antibody- and protein-based therapies for CNS diseases.

MPSI Manifestations and Treatment Outcome: Skeletal Focus

Mucopolysaccharidosis type I (MPSI) (OMIM #252800) is an autosomal recessive disorder caused by pathogenic variants in the IDUA gene encoding for the lysosomal alpha-L-iduronidase enzyme. The deficiency of this enzyme causes systemic accumulation of glycosaminoglycans (GAGs). Although disease manifestations are typically not apparent at birth, they can present early in life, are progressive, and include a wide spectrum of phenotypic findings. Among these, the storage of GAGs within the lysosomes disrupts cell function and metabolism in the cartilage, thus impairing normal bone development and ossification. Skeletal manifestations of MPSI are often refractory to treatment and severely affect patients’ quality of life. This review discusses the pathological and molecular processes leading to impaired endochondral ossification in MPSI patients and the limitations of current therapeutic approaches. Understanding the underlying mechanisms responsible for the skeletal phenotype in MPSI patients is crucial, as it could lead to the development of new therapeutic strategies targeting the skeletal abnormalities of MPSI in the early stages of the disease.

Mucopolysaccharidoses and the blood-brain barrier

Mucopolysaccharidoses comprise a set of genetic diseases marked by an enzymatic dysfunction in the degradation of glycosaminoglycans in lysosomes. There are eight clinically distinct types of mucopolysaccharidosis, some with various subtypes, based on which lysosomal enzyme is deficient and symptom severity. Patients with mucopolysaccharidosis can present with a variety of symptoms, including cognitive dysfunction, hepatosplenomegaly, skeletal abnormalities, and cardiopulmonary issues. Additionally, the onset and severity of symptoms can vary depending on the specific disorder, with symptoms typically arising during early childhood. While there is currently no cure for mucopolysaccharidosis, there are clinically approved therapies for the management of clinical symptoms, such as enzyme replacement therapy. Enzyme replacement therapy is typically administered intravenously, which allows for the systemic delivery of the deficient enzymes to peripheral organ sites. However, crossing the blood-brain barrier (BBB) to ameliorate the neurological symptoms of mucopolysaccharidosis continues to remain a challenge for these large macromolecules. In this review, we discuss the transport mechanisms for the delivery of lysosomal enzymes across the BBB. Additionally, we discuss the several therapeutic approaches, both preclinical and clinical, for the treatment of mucopolysaccharidoses.

IgG Fusion Proteins for Brain Delivery of Biologics via Blood-Brain Barrier Receptor-Mediated Transport

The treatment of neurological disorders with large-molecule biotherapeutics requires that the therapeutic drug be transported across the blood–brain barrier (BBB). However, recombinant biotherapeutics, such as neurotrophins, enzymes, decoy receptors, and monoclonal antibodies (MAb), do not cross the BBB. These biotherapeutics can be re-engineered as brain-penetrating bifunctional IgG fusion proteins. These recombinant proteins comprise two domains, the transport domain and the therapeutic domain, respectively. The transport domain is an MAb that acts as a molecular Trojan horse by targeting a BBB-specific endogenous receptor that induces receptor-mediated transcytosis into the brain, such as the human insulin receptor (HIR) or the transferrin receptor (TfR). The therapeutic domain of the IgG fusion protein exerts its pharmacological effect in the brain once across the BBB. A generation of bifunctional IgG fusion proteins has been engineered using genetically engineered MAbs directed to either the BBB HIR or TfR as the transport domain. These IgG fusion proteins were validated in animal models of lysosomal storage disorders; acute brain conditions, such as stroke; or chronic neurodegeneration, such as Parkinson’s disease and Alzheimer’s disease. Human phase I–III clinical trials were also completed for Hurler MPSI and Hunter MPSII using brain-penetrating IgG-iduronidase and -iduronate-2-sulfatase fusion protein, respectively.

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.

Liver-directed gene therapy corrects neurologic disease in a murine model of mucopolysaccharidosis type I-Hurler

Mucopolysaccharidosis type I-Hurler (MPS I-H) is a neurodegenerative lysosomal storage disorder (LSD) caused by inherited defects of the α-L-iduronidase (IDUA) gene. Current treatments are ineffective for treating central nervous system (CNS) manifestations because lysosomal enzymes do not effectively cross the blood-brain barrier (BBB). To enable BBB transport of the enzyme, we engineered a modified IDUA protein by adding a brain-targeting peptide from melanotransferrin. We demonstrated that fusion of melanotransferrin peptide (MTfp) at the N terminus of human IDUA (hIDUA) was enzymatically active and could efficiently cross the BBB in vitro. Then, liver-directed gene therapy using the adeno-associated virus 8 (AAV8) vector, which encoded the modified hIDUA cDNA driven by a liver-specific expression cassette was evaluated in an adult MPS I-H mouse model. The results showed that intravenous (i.v.) infusion of AAV8 resulted in sustained supraphysiological levels of IDUA activity and normalized glycosaminoglycan (GAG) accumulation in peripheral tissues. Addition of MTfp to the hIDUA N terminus allowed efficient BBB transcytosis and IDUA activity restoration in the brain, resulting in significant improvements in brain pathology and neurobehavioral deficits. Our results provide a novel strategy to develop minimally invasive therapies for treatment of MPS I-H and other neurodegenerative LSDs.

Blood-brain barrier delivery for lysosomal storage disorders with IgG-lysosomal enzyme fusion proteins

The majority of lysosomal storage diseases affect the brain. Treatment of the brain with intravenous enzyme replacement therapy is not successful, because the recombinant lysosomal enzymes do not cross the blood-brain barrier (BBB). Biologic drugs, including lysosomal enzymes, can be re-engineered for BBB delivery as IgG-enzyme fusion proteins. The IgG domain of the fusion protein is a monoclonal antibody directed against an endogenous receptor-mediated transporter at the BBB, such as the insulin receptor or the transferrin receptor. This receptor transports the IgG across the BBB, in parallel with the endogenous receptor ligand, and the IgG acts as a molecular Trojan horse to ferry into brain the lysosomal enzyme genetically fused to the IgG. The IgG-enzyme fusion protein is bi-functional and retains both high affinity binding for the BBB receptor, and high lysosomal enzyme activity. IgG-lysosomal enzymes are presently in clinical trials for treatment of the brain in Mucopolysaccharidosis.

Delivering gene therapy for mucopolysaccharide diseases

Mucopolysaccharide diseases are a group of paediatric inherited lysosomal storage diseases that are caused by enzyme deficiencies, leading to a build-up of glycosaminoglycans (GAGs) throughout the body. Patients have severely shortened lifespans with a wide range of symptoms including inflammation, bone and joint, cardiac, respiratory and neurological disease. Current treatment approaches for MPS disorders revolve around two main strategies. Enzyme replacement therapy (ERT) is efficacious in treating somatic symptoms but its effect is limited for neurological functions. Haematopoietic stem cell transplant (HSCT) has the potential to cross the BBB through monocyte trafficking, however delivered enzyme doses limit its use almost exclusively to MPSI Hurler. Gene therapy is an emerging therapeutic strategy for the treatment of MPS disease. In this review, we will discuss the various vectors that are being utilised for gene therapy in MPS as well as some of the most recent gene-editing approaches undergoing pre-clinical and clinical development.

Therapeutic Approaches in Lysosomal Storage Diseases

Lysosomal Storage Diseases are multisystemic disorders determined by genetic variants, which affect the proteins involved in lysosomal function and cellular metabolism. Different therapeutic approaches, which are based on the physiologic mechanisms that regulate lysosomal function, have been proposed for these diseases. Currently, enzyme replacement therapy, gene therapy, or small molecules have been approved or are under clinical development to treat lysosomal storage disorders. The present article reviews the main therapeutic strategies that have been proposed so far, highlighting possible limitations and future perspectives.

Mathematical Models of Blood-Brain Barrier Transport of Monoclonal Antibodies Targeting the Transferrin Receptor and the Insulin Receptor

We develop and analyze mathematical models for receptor-mediated transcytosis of monoclonal antibodies (MAb) targeting the transferrin receptor (TfR) or the insulin receptor (IR), which are expressed at the blood-brain barrier (BBB). The mass-action kinetic model for both the TfR and IR antibodies were solved numerically to generate predictions for the concentrations of all species in all compartments considered. Using these models, we estimated the rates of MAb endocytosis into brain capillary endothelium, which forms the BBB in vivo, the rates of MAb exocytosis from the intra-endothelial compartment into brain extracellular space, and the rates of receptor recycling from the endothelial space back to the luminal endothelial plasma membrane. Our analysis highlights the optimal rates of MAb association with the targeted receptor. An important role of the endogenous ligand, transferrin (Tf) or insulin, in receptor-mediated-transport (RMT) of the associated MAb was found and was attributed to the five order magnitude difference between plasma concentrations of Tf (25,000 nM) and insulin (0.3 nM). Our modeling shows that the very high plasma concentration of Tf leads to only 5% of the endothelial TfR expressed on the luminal endothelial membrane.

Examination of a blood-brain barrier targeting β-galactosidase-monoclonal antibody fusion protein in a murine model of GM1-gangliosidosis

GM1-gangliosidosis is a lysosomal disease resulting from a deficiency in the hydrolase β-galactosidase (β-gal) and subsequent accumulation of gangliosides, primarily in neuronal tissue, leading to progressive neurological deterioration and eventually early death. Lysosomal diseases with neurological involvement have limited non-invasive therapies due to the inability of lysosomal enzymes to cross the blood-brain barrier (BBB). A novel fusion enzyme, labeled mTfR-GLB1, was designed to act as a ferry across the BBB by fusing β-gal to the mouse monoclonal antibody against the mouse transferrin receptor and tested in a murine model of GM1-gangliosidosis (β-gal^-/-). Twelve hours following a single intravenous dose of mTfR-GLB1 (5.0 mg/kg) into adult β-gal^-/- mice showed clearance of enzyme activity in the plasma and an increase in β-gal enzyme activity in the liver and spleen. Long-term efficacy of mTfR-GLB1 was assessed by treating β-gal^-/- mice intravenously twice a week with a low (2.5 mg/kg) or high (5.0 mg/kg) dose of mTfR-GLB1 for 17 weeks. Long-term studies showed high dose mice gained weight normally compared to vehicle-treated β-gal^-/- mice, which are significantly heavier than heterozygous controls. Behavioral assessment at six months of age using the pole test showed β-gal^-/- mice treated with mTfR-GLB1 had improved motor function. Biochemical analysis showed an increase in β-gal enzyme activity in the high dose group from negligible levels to 20% and 11% of heterozygous levels in the liver and spleen, respectively. Together, these data show that mTfR-GLB1 is a catalytically active β-gal fusion enzyme in vivo that is readily taken up into tissues. Despite these indications of bioactivity, behavior tests other than the pole test, including the Barnes maze, inverted screen, and accelerating rotarod, showed limited or no improvement of treated mice compared to β-gal^-/- mice receiving vehicle only. Further, administration of mTfR-GLB1 was insufficient to create measurable increases in β-gal enzyme activity in the brain or reduce ganglioside content (biochemically and morphologically).

Mucopolysaccharidosis Type I: Current Treatments, Limitations, and Prospects for Improvement

Mucopolysaccharidosis type I (MPS I) is a lysosomal disease, caused by a deficiency of the enzyme alpha-L-iduronidase (IDUA). IDUA catalyzes the degradation of the glycosaminoglycans dermatan and heparan sulfate (DS and HS, respectively). Lack of the enzyme leads to pathologic accumulation of undegraded HS and DS with subsequent disease manifestations in multiple organs. The disease can be divided into severe (Hurler syndrome) and attenuated (Hurler-Scheie, Scheie) forms. Currently approved treatments consist of enzyme replacement therapy (ERT) and/or hematopoietic stem cell transplantation (HSCT). Patients with attenuated disease are often treated with ERT alone, while the recommended therapy for patients with Hurler syndrome consists of HSCT. While these treatments significantly improve disease manifestations and prolong life, a considerable burden of disease remains. Notably, treatment can partially prevent, but not significantly improve, clinical manifestations, necessitating early diagnosis of disease and commencement of treatment. This review discusses these standard therapies and their impact on common disease manifestations in patients with MPS I. Where relevant, results of animal models of MPS I will be included. Finally, we highlight alternative and emerging treatments for the most common disease manifestations.

Brain Delivery of Nanomedicines: Trojan Horse Liposomes for Plasmid DNA Gene Therapy of the Brain

Non-viral gene therapy of the brain is enabled by the development of plasmid DNA brain delivery technology, which requires the engineering and manufacturing of nanomedicines that cross the blood-brain barrier (BBB). The development of such nanomedicines is a multi-faceted problem that requires progress at multiple levels. First, the type of nanocontainer, e.g., nanoparticle or liposome, which encapsulates the plasmid DNA, must be developed. Second, the type of molecular Trojan horse, e.g., peptide or receptor-specific monoclonal antibody (MAb), must be selected for incorporation on the surface of the nanomedicine, as this Trojan horse engages specific receptors expressed on the BBB, and the brain cell membrane, to trigger transport of the nanomedicine from blood into brain cells beyond the BBB. Third, the plasmid DNA must be engineered without bacterial elements, such as antibiotic resistance genes, to enable administration to humans; the plasmid DNA must also be engineered with tissue-specific gene promoters upstream of the therapeutic gene, to insure gene expression in the target organ with minimal off-target expression. Fourth, upstream manufacturing of the nanomedicine must be developed and scalable so as to meet market demand for the target disease, e.g., annual long-term treatment of 1,000 patients with an orphan disease, short term treatment of 10,000 patients with malignant glioma, or 100,000 patients with new onset Parkinson's disease. Fifth, downstream manufacturing problems, such as nanomedicine lyophilization, must be solved to ensure the nanomedicine has a commercially viable shelf-life for treatment of CNS disease in humans.

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).

Mucopolysaccharidosis Type I

Mucopolysaccharidosis type I (MPS I) is caused by the deficiency of α-l-iduronidase, leading to the storage of dermatan and heparan sulfate. There is a broad phenotypical spectrum with the presence or absence of neurological impairment. The classical form is known as Hurler syndrome, the intermediate form as Hurler-Scheie, and the most attenuated form is known as Scheie syndrome. Phenotype seems to be largely influenced by genotype. Patients usually develop several somatic symptoms such as abdominal hernias, extensive dermal melanocytosis, thoracolumbar kyphosis odontoid dysplasia, arthropathy, coxa valga and genu valgum, coarse facial features, respiratory and cardiac impairment. The diagnosis is based on the quantification of α-l-iduronidase coupled with glycosaminoglycan analysis and gene sequencing. Guidelines for treatment recommend hematopoietic stem cell transplantation for young Hurler patients (usually at less than 30 months of age). Intravenous enzyme replacement is approved and is the standard of care for attenuated-Hurler-Scheie and Scheie-forms (without cognitive impairment) and for the late-diagnosed severe-Hurler-cases. Intrathecal enzyme replacement therapy is under evaluation, but it seems to be safe and effective. Other therapeutic approaches such as gene therapy, gene editing, stop codon read through, and therapy with small molecules are under development. Newborn screening is now allowing the early identification of MPS I patients, who can then be treated within their first days of life, potentially leading to a dramatic change in the disease's progression. Supportive care is very important to improve quality of life and might include several surgeries throughout the life course.

Targeting Macromolecules to CNS and Other Hard-to-Treat Organs Using Lectin-Mediated Delivery

The greatest challenges for therapeutic efficacy of many macromolecular drugs that act on intracellular are delivery to key organs and tissues and delivery into cells and subcellular compartments. Transport of drugs into critical cells associated with disease, including those in organs protected by restrictive biological barriers such as central nervous system (CNS), bone, and eye remains a significant hurdle to drug efficacy and impacts commercial risk and incentives for drug development for many diseases. These limitations expose a significant need for the development of novel strategies for macromolecule delivery. RTB lectin is the non-toxic carbohydrate-binding subunit B of ricin toxin with high affinity for galactose/galactosamine-containing glycolipids and glycoproteins common on human cell surfaces. RTB mediates endocytic uptake into mammalian cells by multiple routes exploiting both adsorptive-mediated and receptor-mediated mechanisms. In vivo biodistribution studies in lysosomal storage disease models provide evidence for the theory that the RTB-lectin transports corrective doses of enzymes across the blood–brain barrier to treat CNS pathologies. These results encompass significant implications for protein-based therapeutic approaches to address lysosomal and other diseases having strong CNS involvement.

Bi-functional IgG-lysosomal enzyme fusion proteins for brain drug delivery

Most lysosomal storage disorders affect the central nervous system. However, lysosomal enzymes do not cross the blood-brain barrier (BBB), and intravenous enzyme infusion is not effective for the brain. Lysosomal enzymes can be re-engineered for BBB transport as IgG-enzyme fusion proteins, where the IgG domain is a monoclonal antibody (MAb) against an endogenous BBB receptor/transporter, and which acts as a molecular Trojan horse to deliver the enzyme to brain. However, the problem is retention of high enzyme activity following enzyme fusion to the IgG. The present investigation shows this is possible with a versatile approach that employs fusion of the enzyme to either the IgG heavy chain or light chain using a long flexible linker. The model IgG is a chimeric monoclonal antibody (MAb) against the human insulin receptor (HIR). The enzyme activity of the HIRMAb-enzyme fusion protein is preserved for hexosaminidase A, which is mutated in Tay Sachs disease, for protein palmitoylthioesterase-1, which is mutated in Batten disease type 1, acid sphingomyelinase, which is mutated in Niemann Pick disease type A, and beta galactosidase-1, which is mutated in GM1 gangliosidosis.

First-in-human trial of blood-brain barrier opening in amyotrophic lateral sclerosis using MR-guided focused ultrasound

MR-guided focused ultrasound (MRgFUS) is an emerging technology that can accurately and transiently permeabilize the blood-brain barrier (BBB) for targeted drug delivery to the central nervous system. We conducted a single-arm, first-in-human trial to investigate the safety and feasibility of MRgFUS-induced BBB opening in eloquent primary motor cortex in four volunteers with amyotrophic lateral sclerosis (ALS). Here, we show successful BBB opening using MRgFUS as demonstrated by gadolinium leakage at the target site immediately after sonication in all subjects, which normalized 24 hours later. The procedure was well-tolerated with no serious clinical, radiologic or electroencephalographic adverse events. This study demonstrates that non-invasive BBB permeabilization over the motor cortex using MRgFUS is safe, feasible, and reversible in ALS subjects. In future, MRgFUS can be coupled with promising therapeutics providing a targeted delivery platform in ALS.

Safety and efficacy of focused ultrasound induced blood-brain barrier opening, an integrative review of animal and human studies

The blood-brain barrier, while fundamental in maintaining homeostasis in the central nervous system, is a bottleneck to achieving efficacy for numerous therapeutics. Improved brain penetration is also desirable for reduced dose, cost, and systemic side effects. Transient disruption of the blood-brain barrier with focused ultrasound (FUS) can facilitate drug delivery noninvasively with precise spatial and temporal specificity. FUS technology is transcranial and effective without further drug modifications, key advantages that will accelerate adoption and translation of existing therapeutic pipelines. In this review, we performed a comprehensive literature search to build a database and provide a synthesis of ultrasound parameters and drug characteristics that influence the safety and efficacy profile of FUS to enhance drug delivery.

δ-Tocopherol Effect on Endocytosis and Its Combination with Enzyme Replacement Therapy for Lysosomal Disorders: A New Type of Drug Interaction?

Induction of lysosomal exocytosis alleviates lysosomal storage of undigested metabolites in cell models of lysosomal disorders (LDs). However, whether this strategy affects other vesicular compartments, e.g., those involved in endocytosis, is unknown. This is important both to predict side effects and to use this strategy in combination with therapies that require endocytosis for intracellular delivery, such as lysosomal enzyme replacement therapy (ERT). We investigated this using δ-tocopherol as a model previously shown to induce lysosomal exocytosis and cell models of type A Niemann-Pick disease, a LD characterized by acid sphingomyelinase (ASM) deficiency and sphingomyelin storage. δ-Tocopherol and derivative CF3-T reduced net accumulation of fluid phase, ligands, and polymer particles via phagocytic, caveolae-, clathrin-, and cell adhesion molecule (CAM)-mediated pathways, yet the latter route was less affected due to receptor overexpression. In agreement, δ-tocopherol lowered uptake of recombinant ASM by deficient cells (known to occur via the clathrin pathway) and via targeting intercellular adhesion molecule-1 (associated to the CAM pathway). However, the net enzyme activity delivered and lysosomal storage attenuation were greater via the latter route. Data suggest stimulation of exocytosis by tocopherols is not specific of lysosomes and affects endocytic cargo. However, this effect was transient and became unnoticeable several hours after tocopherol removal. Therefore, induction of exocytosis in combination with therapies requiring endocytic uptake, such as ERT, may represent a new type of drug interaction, yet this strategy could be valuable if properly timed for minimal interference.

Enzyme enhancement therapeutics for lysosomal storage diseases: Current status and perspective

Small-molecule- enzyme enhancement therapeutics (EETs) have emerged as attractive agents for the treatment of lysosomal storage diseases (LSDs), a broad group of genetic diseases caused by mutations in genes encoding lysosomal enzymes, or proteins required for lysosomal function. The underlying enzyme deficiencies characterizing LSDs cause a block in the stepwise degradation of complex macromolecules (e.g. glycosaminoglycans, glycolipids and others), such that undegraded or partially degraded substrates progressively accumulate in lysosomal and non-lysosomal compartments, a process leading to multisystem pathology via primary and secondary mechanisms. Missense mutations underlie many of the LSDs; the resultant mutant variant enzyme hydrolase is often impaired in its folding and maturation making it subject to rapid disposal by endoplasmic reticulum (ER)-associated degradation (ERAD). Enzyme deficiency in the lysosome is the result, even though the mutant enzyme may retain significant catalytic functioning. Small molecule modulators - pharmacological chaperones (PCs), or proteostasis regulators (PRs) are being identified through library screens and computational tools, as they may offer a less costly approach than enzyme replacement therapy (ERT) for LSDs, and potentially treat neuronal forms of the diseases. PCs, capable of directly stabilizing the mutant protein, and PRs, which act on other cellular elements to enhance protein maturation, both allow a proportion of the synthesized variant protein to reach the lysosome and function. Proof-of-principle for PCs and PRs as therapeutic agents has been demonstrated for several LSDs, yet definitive data of their efficacy in disease models and/or in downstream clinical studies in many cases has yet to be achieved. Basic research to understand the cellular consequences of protein misfolding such as perturbed organellar crosstalk, redox status, and calcium balance is needed. Likewise, an elucidation of the early in cellulo pathogenic events underlying LSDs is vital and may lead to the discovery of new small molecule modulators and/or to other therapeutic approaches for driving proteostasis toward protein rescue.

Identification of Blood-Brain Barrier-Permeable Proteins Derived from a Peripheral Organ: In Vivo and in Vitro Evidence of Blood-to-Brain Transport of Creatine Kinase

Certain proteins, such as inflammatory cytokines, that are released from injured or diseased organs are transported from the circulating blood through the blood-brain barrier (BBB) into the brain and contribute to the pathogenesis of related central nervous system dysfunctions. However, little is known about the protein transport mechanisms involved in the central nervous system dysfunctions. The aims of the present study were to identify BBB-permeable protein(s) derived from liver and to clarify their transport characteristics at the BBB. After administration of biotin-labeled liver cytosolic protein fraction to mice in vivo, we identified 9 biotin-labeled proteins in the brain. Among them, we focused here on creatine kinase (CK). In vitro uptake studies with human brain microvessel endothelial cells (hCMEC/D3 cells) showed preferential uptake of muscle-type CK (CK-MM) compared with brain-type CK (CK-BB) at the BBB. Integration plot analysis revealed that CK-MM readily penetrated into brain parenchyma from the circulating blood across the BBB. The uptake of CK-MM by hCMEC/D3 cells was decreased at 4 °C and in the presence of clathrin- and caveolin-dependent endocytosis inhibitors. These results indicate that entry of CK into the brain is mediated by a transport system(s) at the BBB.

Expanding the Boundaries of Biotherapeutics with Bispecific Antibodies

Bispecific antibodies have moved from being an academic curiosity with therapeutic promise to reality, with two molecules being currently commercialized (Hemlibra® and Blincyto®) and many more in clinical trials. The success of bispecific antibodies is mainly due to the continuously growing number of mechanisms of actions (MOA) they enable that are not accessible to monoclonal antibodies. One of the earliest MOA of bispecific antibodies and currently the one with the largest number of clinical trials is the redirecting of the cytotoxic activity of T-cells for oncology applications, now extending its use in infective diseases. The use of bispecific antibodies for crossing the blood-brain barrier is another important application because of its potential to advance the therapeutic options for neurological diseases. Another noteworthy application due to its growing trend is enabling a more tissue-specific delivery or activity of antibodies. The different molecular solutions to the initial hurdles that limited the development of bispecific antibodies have led to the current diverse set of bispecific or multispecific antibody formats that can be grouped into three main categories: IgG-like formats, antibody fragment-based formats, or appended IgG formats. The expanded applications of bispecific antibodies come at the price of additional challenges for clinical development. The rising complexity in their structure may increase the risk of immunogenicity and the multiple antigen specificity complicates the selection of relevant species for safety assessment.

Blood-Brain Barrier Transport, Plasma Pharmacokinetics, and Neuropathology Following Chronic Treatment of the Rhesus Monkey with a Brain Penetrating Humanized Monoclonal Antibody Against the Human Transferrin Receptor

A monoclonal antibody (mAb) against the blood-brain barrier (BBB) transferrin receptor (TfR) is a potential agent for delivery of biologic drugs to the brain across the BBB. However, to date, no TfRMAb has been tested with chronic dosing in a primate model. A humanized TfRMAb against the human (h) TfR1, which cross reacts with the primate TfR, was genetically engineered with high affinity (ED50 = 0.18 ± 0.04 nM) for the human TfR type 1 (TfR1). For acute dosing, the hTfRMAb was tritiated and injected intravenously (IV) in the Rhesus monkey, which confirmed rapid delivery of the humanized hTfRMAb into both brain parenchyma, via transport across the BBB, and into cerebrospinal fluid (CSF), via transport across the choroid plexus. For chronic dosing, a total of 8 adult Rhesus monkeys (4 males, 4 females) were treated twice weekly for 4 weeks with 0, 3, 10, or 30 mg/kg of the humanized hTfRMAb via a 60 min IV infusion for a total of 8 doses prior to euthanasia and microscopic examination of brain and peripheral organs. A pharmacokinetics analysis showed the plasma clearance of the hTfRMAb in the primate was nonlinear, and plasma clearance was increased over 20-fold with chronic treatment of the low dose, 3 mg/kg, of the antibody. Chronic treatment of the primates with the 30 mg/kg dose caused anemia associated with suppressed blood reticulocytes. Immunohistochemistry of terminal brain tissue showed microglia activation, based on enhanced IBA1 immuno-staining, in conjunction with astrogliosis, based on increased GFAP immuno-staining. Moderate axonal/myelin degeneration was observed in the sciatic nerve. Further studies need to be conducted to determine if this neuropathology is induced by the antibody effector function, or is an intrinsic property of targeting the TfR in brain. The results indicate that chronic treatment of Rhesus monkeys with a humanized hTfRMAb may have a narrow therapeutic index, with associated toxicity related to microglial activation and astrogliosis of the brain.

Neurocognitive and somatic stabilization in pediatric patients with severe Mucopolysaccharidosis Type I after 52 weeks of intravenous brain-penetrating insulin receptor antibody-iduronidase fusion protein (valanafusp alpha): an open label phase 1-2 trial

BACKGROUND

Mucopolysaccharidosis (MPS) Type I (MPSI) is caused by mutations in the gene encoding the lysosomal enzyme, α-L-iduronidase (IDUA), and a majority of patients present with severe neurodegeneration and cognitive impairment. Recombinant IDUA does not cross the blood-brain barrier (BBB). To enable BBB transport, IDUA was re-engineered as an IgG-IDUA fusion protein, valanafusp alpha, where the IgG domain targets the BBB human insulin receptor to enable transport of the enzyme into the brain. We report the results of a 52-week clinical trial on the safety and efficacy of valanafusp alpha in pediatric MPSI patients with cognitive impairment. In the phase I trial, 6 adults with attenuated MPSI were administered 0.3, 1, and 3 mg/kg doses of valanafusp alpha by intravenous (IV) infusion. In the phase II trial, 11 pediatric subjects, 2-15 years of age, were treated for 52 weeks with weekly IV infusions of valanafusp alpha at 1, 3, or 6 mg/kg. Assessments of adverse events, cognitive stabilization, and somatic stabilization were made. Outcomes at 52 weeks were compared to baseline.

RESULTS

Drug related adverse events included infusion related reactions, with an incidence of 1.7%, and transient hypoglycemia, with an incidence of 6.4%. The pediatric subjects had CNS involvement with a mean enrollment Development Quotient (DQ) of 36.1±7.1. The DQ, and the cortical grey matter volume of brain, were stabilized by valanafusp alpha treatment. Somatic manifestations were stabilized, or improved, based on urinary glycosaminoglycan levels, hepatic and spleen volumes, and shoulder range of motion.

CONCLUSION

Clinical evidence of the cognitive and somatic stabilization indicates that valanafusp alpha is transported into both the CNS and into peripheral organs due to its dual targeting mechanism via the insulin receptor and the mannose 6-phosphate receptor. This novel fusion protein offers a pharmacologic approach to the stabilization of cognitive function in MPSI.

TRIAL REGISTRATION

Clinical Trials.Gov, NCT03053089 . Retrospectively registered 9 February, 2017; Clinical Trials.Gov, NCT03071341 . Registered 6 March, 2017.

RTB lectin-mediated delivery of lysosomal α-l-iduronidase mitigates disease manifestations systemically including the central nervous system

Mucopolysaccharidosis type I (MPS I) is a lysosomal disease resulting from deficiency in the α-L-iduronidase (IDUA) hydrolase and subsequent accumulation of glycosaminoglycan (GAG). Clinically, enzyme replacement therapy (ERT) with IDUA achieves negligible neurological benefits presumably due to blood-brain-barrier (BBB) limitations. To investigate the plant lectin ricin B chain (RTB) as a novel carrier for enzyme delivery to the brain, an IDUA:RTB fusion protein (IDUAL), produced in N. benthamiana leaves, was tested in a murine model of Hurler syndrome (MPS I). Affect mice (n=3 for each group) were intravenously injected with a single dose of IDUAL (0.58, 2 or 5.8mgIDUAequivalents/kg) and analyzed after 24h. IDUA activities in liver, kidney and spleen increased significantly, and liver GAG levels were significantly reduced in all three groups. Plasma IDUA levels for all treated groups were high at 1h after injection and decreased by 95% at 4h, indicating efficient distribution into tissues. For long-term evaluations, IDUAL (0.58 or 2mg/kg, 8 weekly injections) was intravenously injected into MPS I mice (n=12 for each group). Thirteen days after the 8th injection, significant IDUA activity was detected in the liver and spleen. GAG levels in tissues including the brain cortex and cerebellum were significantly reduced in treated animals. Treated MPS I mice also showed significant improvement in neurocognitive testing. ELISA results showed that while there was a significant antibody response against IDUAL and plant-derived IDUA, there was no significant antibody response to RTB. No major toxicity or adverse events were observed. Together, these results showed that infusion of IDUAL allowed for significant IDUA levels and GAG reduction in the brain and subsequent neurological benefits. This RTB-mediated delivery may have significant implications for therapeutic protein delivery impacting a broad spectrum of lysosomal, and potentially neurological diseases.

Guanidinylated Neomycin Conjugation Enhances Intranasal Enzyme Replacement in the Brain

Iduronidase (IDUA)-deficient mice accumulate glycosaminoglycans in cells and tissues and exhibit many of the same neuropathological symptoms of patients suffering from Mucopolysaccharidosis I. Intravenous enzyme-replacement therapy for Mucopolysaccharidosis I ameliorates glycosaminoglycan storage and many of the somatic aspects of the disease but fails to treat neurological symptoms due to poor transport across the blood-brain barrier. In this study, we examined the delivery of IDUA conjugated to guanidinoneomycin (GNeo), a molecular transporter. GNeo-IDUA and IDUA injected intravenously resulted in reduced hepatic glycosaminoglycan accumulation but had no effect in the brain due to fast clearance from the circulation. In contrast, intranasally administered GNeo-IDUA entered the brain rapidly. Repetitive intranasal treatment with GNeo-IDUA reduced glycosaminoglycan storage, lysosome size and number, and neurodegenerative astrogliosis in the olfactory bulb and primary somatosensory cortex, whereas IDUA was less effective. The enhanced efficacy of GNeo-IDUA was not the result of increased nose-to-brain delivery or enzyme stability, but rather due to more efficient uptake into neurons and astrocytes. GNeo conjugation also enhanced glycosaminoglycan clearance by intranasally delivered sulfamidase to the brain of sulfamidase-deficient mice, a model of Mucopolysaccharidosis IIIA. These findings suggest the general utility of the guanidinoglycoside-based delivery system for restoring missing lysosomal enzymes in the brain.

Treatment of brain disease in the mucopolysaccharidoses

The mucopolysaccharidosis (MPS) disorders are a group of lysosomal storage diseases caused by lysosomal enzyme deficits that lead to glycosaminoglycan accumulation, affecting various tissues throughout the body based on the specific enzyme deficiency. These disorders are characterized by their progressive nature and a variety of somatic manifestations and neurological symptoms. There are established treatments for some MPS disorders, but these mostly alleviate somatic and non-neurological symptoms and do not cure the disease. Patients with MPS I, II, III, and VII can present with neurological manifestations such as neurocognitive decline and behavioral problems. Treatment of these neurological manifestations remains challenging due to the blood-brain barrier (BBB) that limits delivery of therapeutic agents to the central nervous system (CNS). New therapies that circumvent this barrier and target brain disease in MPS are currently under development. They primarily focus on facilitating penetration of drugs through the BBB, delivery of recombinant enzyme to the brain by gene therapy, or direct CNS administration. This review summarizes existing and potential future treatment approaches that target brain disease in MPS. The information in this review is based on current literature and presentations and discussions during a closed meeting by an international group of experts with extensive experience in managing and treating MPS.

Moving towards effective therapeutic strategies for Neuronal Ceroid Lipofuscinosis

The Neuronal Ceroid Lipofuscinoses (NCLs) are a family of autosomal recessive neurodegenerative disorders that annually affect 1:100,000 live births worldwide. This family of diseases results from mutations in one of 14 different genes that share common clinical and pathological etiologies. Clinically, the diseases are subcategorized into infantile, late-infantile, juvenile and adult forms based on their age of onset. Though the disease phenotypes may vary in their age and order of presentation, all typically include progressive visual deterioration and blindness, cognitive impairment, motor deficits and seizures. Pathological hallmarks of NCLs include the accumulation of storage material or ceroid in the lysosome, progressive neuronal degeneration and massive glial activation. Advances have been made in genetic diagnosis and counseling for families. However, comprehensive treatment programs that delay or halt disease progression have been elusive. Current disease management is primarily targeted at controlling the symptoms rather than "curing" the disease. Recognizing the growing need for transparency and synergistic efforts to move the field forward, this review will provide an overview of the therapeutic approaches currently being pursued in preclinical and clinical trials to treat different forms of NCL as well as provide insight to novel therapeutic approaches in development for the NCLs.

Insulin Receptor Antibody-α-N-Acetylglucosaminidase Fusion Protein Penetrates the Primate Blood-Brain Barrier and Reduces Glycosoaminoglycans in Sanfilippo Type B Fibroblasts

Mucopolysaccharidosis Type IIIB (MPSIIIB) is caused by mutations in the gene encoding the lysosomal enzyme, α-N-acetylglucosaminidase (NAGLU). MPSIIIB presents with severe disease of the central nervous system, but intravenous NAGLU enzyme replacement therapy has not been developed because the NAGLU enzyme does not cross the blood-brain barrier (BBB). A BBB-penetrating form of the enzyme was produced by re-engineering NAGLU as an IgG-enzyme fusion protein, where the IgG domain is a monoclonal antibody (mAb) against the human insulin receptor (HIR). The HIRMAb traverses the BBB via transport on the endogenous insulin receptor and acts as a molecular Trojan horse to ferry the fused NAGLU across the BBB from blood. The NAGLU was fused to the carboxyl terminus of each heavy chain of the HIRMAb via an extended 31-amino acid linker, and the fusion protein is designated HIRMAb-LL-NAGLU. The fusion protein retains high affinity binding to the HIR, and on a molar basis has an enzyme activity equal to that of recombinant human NAGLU. Treatment of MPSIIIB fibroblasts with the fusion protein normalizes intracellular NAGLU enzyme activity and reduces sulfate incorporation into intracellular glycosoaminoglycan. The fusion protein is targeted to the lysosomal compartment of the cells as shown by confocal microscopy. The fusion protein was radiolabeled with the [(125)I]-Bolton-Hunter reagent and injected intravenously in the adult Rhesus monkey. The fusion protein was rapidly cleared from plasma by all major peripheral organs. The high brain uptake of the fusion protein, 1% injected dose/brain, enables normalization of brain NAGLU enzyme activity with a therapeutic dose of 1 mg/kg. The HIRMAb-LL-NAGLU fusion protein is a new treatment of the brain in MPSIIIB, which can be administered by noninvasive intravenous infusion.

Lysosomale Speicherkrankheiten: Therapeutische Optionen

Für einige lysosomale Speicherkrankheiten steht eine Enzymersatz-Therapie zur Verfügung oder wird derzeit entwickelt. Um zu erreichen, dass intravenös applizierte Enzyme das Zentralnervensystem erreichen, werden Methoden entwickelt, durch Modifizierung der Enzyme oder durch Anwendung von Nanopartikeln die Bluthirnschranke zu überwinden. Eine andere Therapie-Option besteht in der Anwendung von Substrathemmern, die für den M. Gaucher und den M. Niemann-Pick Typ C eingesetzt werden. Derzeit werden Chaperone für verschiedene lysosomale Speicherkrankheiten entwickelt, die jedoch den Nachteil haben, dass sie nur bei bestimmten Mutationen eingesetzt werden können. „Read-Through“ Substanzen werden lediglich bei Vorliegen einer Nonsense-Mutation wirksam sein können. Auf dem Gebiet der lysosomalen Speicherkrankheiten wird eine Gen-Therapie derzeit nur im Rahmen klinischer Studien durchgeführt. Bevor dieses Behandlungs-Prinzip breite Anwendung finden kann, sollten jedoch noch Fragen zum Beispiel bezüglich der Langzeit-Sicherheit, der möglichen Immun-Reaktion und der Organ-Spezifität des für die Insertion verwendeten Vektors beantwortet werden. Um eine Behandlung einleiten zu können, bevor irreversible Organschäden auftreten, ist in vielen Ländern ein Neugeborenen-Screening für lysosomale Speicherkrankheiten eingeführt worden. Da jedoch damit mehr Mutationsträger diagnostiziert wurden als auf Grund epidemiologischer Untersuchungen zu erwarten war, muß angenommen werden, dass auch sehr leicht betroffene Patienten damit erfaßt werden. Eine sichere Aussage über den zu erwartenden Schweregrad kann jedoch auch durch eine Gen-Analyse nicht gemacht werden, so dass eine Therapie-Entscheidung im Einzelfall eventuell sehr schwierig ist. Für dieses Dilemma ist bisher noch keine Lösung gefunden worden.

Pharmacological chaperone therapy for lysosomal storage diseases

Pharmacological chaperone therapy is an emerging approach to treat lysosomal storage diseases. Small-molecule chaperones interact with mutant enzymes, favor their correct conformation and enhance their stability. This approach shows significant advantages when compared with existing therapies, particularly in terms of the bioavailability of drugs, oral administration and positive impact on the quality of patients' lives. On the other hand, future research in this field must confront important challenges. The identification of novel chaperones is indispensable to expanding the number of patients amenable to this treatment and to optimize therapeutic efficacy. It is important to develop new allosteric drugs, to address the risk of inhibiting target enzymes. Future research must also be directed towards the exploitation of synergies between chaperone treatment and other therapeutic approaches.

Gene therapy for neurologic manifestations of mucopolysaccharidoses

INTRODUCTION

Mucopolysaccharidoses (MPS) are a family of lysosomal disorders caused by mutations in genes that encode enzymes involved in the catabolism of glycoaminoglycans. These mutations affect multiple organ systems and can be particularly deleterious to the nervous system. At the present time, enzyme replacement therapy and hematopoietic stem-cell therapy are used to treat patients with different forms of these disorders. However, to a great extent, the nervous system is not adequately responsive to current therapeutic approaches.

AREAS COVERED

Recent advances in gene therapy show great promise for treating MPS. This article reviews the current state of the art for routes of delivery in developing genetic therapies for treating the neurologic manifestations of MPS.

EXPERT OPINION

Gene therapy for treating neurological manifestations of MPS can be achieved by intraventricular, intrathecal, intranasal and systemic administrations. The intraventricular route of administration appears to provide the most widespread distribution of gene therapy vectors to the brain. The intrathecal route of delivery results in predominant distribution to the caudal areas of the brain. The systemic route of delivery via intravenous infusion can also achieve widespread delivery to the CNS; however, the distribution to the brain is greatly dependent on the vector system. Intravenous delivery using lentiviral vectors appear to be less effective than adeno-associated viral (AAV) vectors. Moreover, some subtypes of AAV vectors are more effective than others in crossing the blood-brain barrier. In summary, the recent advances in gene vector technology and routes of delivery to the CNS will facilitate the clinical translation of gene therapy for the treatment of the neurological manifestations of MPS.

Targeted delivery of protein and gene medicines through the blood-brain barrier

The development of biologic drugs (recombinant proteins, therapeutic antibodies, peptides, nucleic acid therapeutics) as new treatments of brain disorders has been difficult, and a major reason is the lack of transport through the blood-brain barrier (BBB) of these large molecule pharmaceuticals. Biologic drugs can be re-engineered as brain-penetrating neuropharmaceuticals using BBB molecular Trojan horse technology. Certain peptidomimetic monoclonal antibodies that target endogenous receptors on the BBB, such as the insulin or transferrin receptor, enable the re-engineering of biologic drugs that cross the BBB.

Blood-brain barrier drug delivery of IgG fusion proteins with a transferrin receptor monoclonal antibody

INTRODUCTION

Biologic drugs are large molecules that do not cross the blood- brain barrier (BBB). Brain penetration is possible following the re-engineering of the biologic drug as an IgG fusion protein. The IgG domain is a MAb against an endogenous BBB receptor such as the transferrin receptor (TfR). The TfRMAb acts as a molecular Trojan horse to ferry the fused biologic drug into the brain via receptor-mediated transport on the endogenous BBB TfR.

AREAS COVERED

This review discusses TfR isoforms, models of BBB transport of transferrin and TfRMAbs, and the genetic engineering of TfRMAb fusion proteins, including BBB penetrating IgG-neurotrophins, IgG-decoy receptors, IgG-lysosomal enzyme therapeutics and IgG-avidin fusion proteins, as well as BBB transport of bispecific antibodies formed by fusion of a therapeutic antibody to a TfRMAb targeting antibody. Also discussed are quantitative aspects of the plasma pharmacokinetics and brain uptake of TfRMAb fusion proteins, as compared to the brain uptake of small molecules, and therapeutic applications of TfRMAb fusion proteins in mouse models of neural disease, including Parkinson's disease, stroke, Alzheimer's disease and lysosomal storage disorders. The review covers the engineering of TfRMAb-avidin fusion proteins for BBB targeted delivery of biotinylated peptide radiopharmaceuticals, low-affinity TfRMAb Trojan horses and the safety pharmacology of chronic administration of TfRMAb fusion proteins.

EXPERT OPINION

The BBB delivery of biologic drugs is possible following re-engineering as a fusion protein with a molecular Trojan horse such as a TfRMAb. The efficacy of this technology will be determined by the outcome of future clinical trials.

Agile delivery of protein therapeutics to CNS

A variety of therapeutic proteins have shown potential to treat central nervous system (CNS) disorders. Challenge to deliver these protein molecules to the brain is well known. Proteins administered through parenteral routes are often excluded from the brain because of their poor bioavailability and the existence of the blood-brain barrier (BBB). Barriers also exist to proteins administered through non-parenteral routes that bypass the BBB. Several strategies have shown promise in delivering proteins to the brain. This review, first, describes the physiology and pathology of the BBB that underscore the rationale and needs of each strategy to be applied. Second, major classes of protein therapeutics along with some key factors that affect their delivery outcomes are presented. Third, different routes of protein administration (parenteral, central intracerebroventricular and intraparenchymal, intranasal and intrathecal) are discussed along with key barriers to CNS delivery associated with each route. Finally, current delivery strategies involving chemical modification of proteins and use of particle-based carriers are overviewed using examples from literature and our own work. Whereas most of these studies are in the early stage, some provide proof of mechanism of increased protein delivery to the brain in relevant models of CNS diseases, while in few cases proof of concept had been attained in clinical studies. This review will be useful to broad audience of students, academicians and industry professionals who consider critical issues of protein delivery to the brain and aim developing and studying effective brain delivery systems for protein therapeutics.

Current and potential therapeutic strategies for mucopolysaccharidoses

WHAT IS KNOWN AND OBJECTIVE

Mucopolysaccharidoses (MPSs) are a group of rare inherited metabolic diseases caused by genetic defects in the production of lysosomal enzymes. MPSs are clinically heterogeneous and are characterized by progressive deterioration in visceral, skeletal and neurological functions. This article aims to review the classification and pathophysiology of MPSs and discuss current therapies and new targeted agents under development.

METHODS

A Medline search through PubMed was performed for relevant articles and treatment guidelines on MPSs published in English for years 1970 to September of 2013 inclusive. The references listed in the identified articles, prescribing information of the drugs approved for the treatment of MPSs, as well as recent clinical trial information posted on Clinicaltrials.gov website, were reviewed.

RESULTS AND DISCUSSION

Until recently, supportive care was the only option available for the management of MPSs. In the early 2000s, enzyme replacement therapy (ERT) was approved by the United States Food and Drug Administration (FDA) for the treatment of MPS I, II and VI. Clinical trials of ERT showed substantial improvements in patients' somatic symptoms; however, no benefit was found in the neurological symptoms because the enzymes do not readily cross the blood-brain barrier (BBB). Haematopoietic stem cell transplantation (HSCT), another potentially curative treatment, is not routinely advocated in clinical practice due to its high risk profile and lack of evidence for efficacy, except in preserving cognition and prolonging survival in young patients with severe MPS I. In recent years, substrate reduction therapy (SRT) and gene therapy have been rapidly gaining greater recognition as potential therapeutic avenues.

WHAT IS NEW AND CONCLUSION

Enzyme replacement therapy (ERT) is effective for the treatment of many somatic symptoms, particularly walking ability and respiratory function, and remains the mainstay of MPS treatment. The usefulness of HSCT has not been established adequately for most MPSs. Although still under investigation, SRT and gene therapy are promising MPS treatments that may prevent the neurodegeneration not affected by ERT.

Comparative binding, endocytosis, and biodistribution of antibodies and antibody-coated carriers for targeted delivery of lysosomal enzymes to ICAM-1 versus transferrin receptor

Targeting lysosomal enzymes to receptors involved in transport into and across cells holds promise to enhance peripheral and brain delivery of enzyme replacement therapies (ERTs) for lysosomal storage disorders. Receptors being explored include those associated with clathrin-mediated pathways, yet other pathways seem also viable. Well characterized examples are that of transferrin receptor (TfR) and intercellular adhesion molecule 1 (ICAM-1), involved in iron transport and leukocyte extravasation, respectively. TfR and ICAM-1 support ERT delivery via clathrin- vs. cell adhesion molecule-mediated mechanisms, displaying different valency and size restrictions. To comparatively assess this, we used antibodies vs. larger multivalent antibody-coated carriers and evaluated TfR vs. ICAM-1 binding and endocytosis in endothelial cells, as well as in vivo biodistribution and delivery of a model lysosomal enzyme required in peripheral organs and brain: acid sphingomyelinase (ASM), deficient in types A-B Niemann Pick disease. We found similar binding of antibodies to both receptors under control conditions, with enhanced binding to activated endothelium for ICAM-1, yet only anti-TfR induced endocytosis efficiently. Contrarily, antibody-coated carriers showed enhanced binding, engulfment, and endocytosis for ICAM-1. In mice, anti-TfR enhanced brain targeting over anti-ICAM, with an opposite outcome in the lungs, while carriers enhanced ICAM-1 targeting over TfR in both organs. Both targeted carriers enhanced ASM delivery to the brain and lungs vs. free ASM, with greater enhancement for anti-ICAM carriers. Therefore, targeting TfR or ICAM-1 improves lysosomal enzyme delivery. Yet, TfR targeting may be more efficient for smaller conjugates or fusion proteins, while ICAM-1 targeting seems superior for multivalent carrier formulations.