Enzyme replacement therapy (ERT) has paved the way for treating the somatic symptoms of lysosomal storage diseases (LSDs), but the inability of intravenously administered enzymes to cross the blood–brain barrier (BBB) has left the central nervous system (CNS)-related symptoms of LSDs largely impervious to the therapeutic benefits of ERT, although ERT via intrathecal and intracerebroventricular routes can be used for some neuronopathic LSDs (in particular, mucopolysaccharidoses). However, the considerable practical issues involved make these routes unsuitable for long-term treatment. Efforts have been made to modify enzymes (e.g., by fusing them with antibodies against innate receptors on the cerebrovascular endothelium) so that they can cross the BBB via receptor-mediated transcytosis (RMT) and address neuronopathy in the CNS. This review summarizes the various scientific and technological challenges of applying RMT to the development of safe and effective enzyme therapeutics for neuronopathic mucopolysaccharidoses; it then discusses the translational and methodological issues surrounding preclinical and clinical evaluation to establish RMT-applied ERT.

Mucopolysaccharidosis I (MPS I), a lysosomal storage disease caused by dysfunction of α-L-iduronidase (IDUA), is characterized by the deposition of dermatan sulfate (DS) and heparan sulfate (HS) throughout the body, which causes several somatic and central nervous symptoms. Although enzyme-replacement therapy (ERT) is currently available to treat MPS I, it does not alleviate central nervous disorders, as it cannot penetrate the blood-brain barrier. Here we evaluate the brain delivery, efficacy, and safety of JR-171, a fusion protein comprising humanized anti-human transferrin receptor antibody Fab and IDUA, using monkeys and MPS I mice. Intravenously administered JR-171 was distributed in major organs, including the brain, and reduced DS and HS concentrations in the central nervous system and peripheral tissues. JR-171 exerted similar effects on peripheral disorders similar to conventional ERT and further reversed brain pathology in MPS I mice. We found that JR-171 improved spatial learning ability, which was seen to deteriorate in the vehicle-treated mice. Further, no safety concerns were noted in repeat-dose toxicity studies in monkeys. This study provides nonclinical evidence that JR-171 might potentially prevent and even improve disease conditions in patients with neuronopathic MPS I without serious safety concerns.

Heparan sulfate (HS) is an essential glycosaminoglycan (GAG) as a component of proteoglycans, which are present on the cell surface and in the extracellular matrix. HS-containing proteoglycans not only function as structural constituents of the basal lamina but also play versatile roles in various physiological processes, including cell signaling and organ development. Thus, inherited mutations of genes associated with the biosynthesis or degradation of HS can cause various diseases, particularly those involving the bones and central nervous system (CNS). Mucopolysaccharidoses (MPSs) are a group of lysosomal storage disorders involving GAG accumulation throughout the body caused by a deficiency of GAG-degrading enzymes. GAGs are stored differently in different types of MPSs. Particularly, HS deposition is observed in patients with MPS types I, II, III, and VII, all which involve progressive neuropathy with multiple CNS system symptoms. While therapies are available for certain symptoms in some types of MPSs, significant unmet medical needs remain, such as neurocognitive impairment. This review presents recent knowledge on the pathophysiological roles of HS focusing on the pathogenesis of MPSs. We also discuss the possible use and significance of HS as a biomarker for disease severity and therapeutic response in MPSs.

Pabinafusp alfa, which is an anti-mucopolysaccharidosis II drug, consists of iduronate-2-sulfatase (IDS) genetically fused with an anti-transferrin receptor (TfR) antibody. While IDS is known to enter cells via mannose-6-phosphate receptor (M6PR)-mediated endocytosis, the anti-TfR antibody moiety of pabinafusp alfa is supposed to trigger the TfR-mediated transcytosis involved in its blood-brain barrier (BBB) penetration to deliver IDS into the brain, which thus makes it effective for treatment of brain symptoms of the disease. However, since these uptake processes remain unexamined in vitro, this study aims at elucidating how human brain cells manipulate these receptors to facilitate pabinafusp alfa uptake. The results of pabinafusp alfa uptake assays showed that the TfR played an primary role in its uptake by brain microvascular endothelial cells. The TfR contribution was also found in neuronal cells at levels comparable to M6PR. Interestingly, the predominant roles of TfR over M6PR in pabinafusp alfa uptake were also observed in astrocytes and pericytes. To summarize, our results support the TfR-targeting strategy of pabinafusp alfa for facilitating its BBB penetration while simultaneously identifying previously unnoticed TfR roles in its uptake into human neuronal and non-neuronal brain cells. These findings are certain to provide important insights into the mechanisms behind clinical actions of pabinafusp alfa.

GM1 gangliosidosis is a lysosomal storage disorder (LSD) caused by genetic defects in lysosomal β-galactosidase (β-gal). The primary substrate of β-gal is GM1 ganglioside (GM1), a sialylated glycosphingolipid abundant in the central nervous system (CNS). Deficiency in β-gal causes GM1 to accumulate in neural cells, leading to a rapid decline in psychomotor functions, seizures, and premature death. There is currently no therapy available. Although enzyme replacement therapy has been approved for other LSDs, its effects on the CNS are limited owing to the blood-brain barrier (BBB). Here, we assessed the therapeutic efficacy of a systemic infusion of an adeno-associated virus vector carrying a gene expressing a BBB-penetrable enzyme under the control of a liver-specific promoter in GM1 gangliosidosis model mice. The BBB-penetrable enzyme consisted of the variable region of the anti–transferrin receptor antibody fused with β-gal. The BBB-penetrable enzyme was only produced in the liver and secreted into the blood, which was efficiently distributed to various organs, including the brain. GM1 accumulation in the CNS was completely normalized, with improved neurological functions and animal survival. This therapeutic approach is expected to be applied for the treatment of several hereditary neurological diseases with CNS involvement.

Under clinical development for patients with growth hormone deficiency, JR-142 is a long-acting growth hormone with a half-life extended by fusion with modified serum albumin. We conducted a Phase 1 study to investigate the safety, tolerability, and pharmacokinetic (PK) and pharmacodynamic (PD) profiles of once-weekly subcutaneous administrations of JR-142. The study consisted of two parts: an open-label single ascending dosing study (Part 1), and a randomized, placebo-controlled, assessor-blinded multiple ascending dosing study (Part 2). A total of 31 healthy Japanese male participants were enrolled. In Part 1, seven of them received a single subcutaneous injection of JR-142 each at dosages of 0.15 mg/kg (n = 1), 0.25 mg/kg (n = 2), 0.5 mg/kg (n = 2), or 1.0 mg/kg (n = 2). In Part 2, one weekly subcutaneous injection of JR-142 at 0.25 mg/kg, 0.5 mg/kg, 1.0 mg/kg or a placebo were given for four weeks to each of the other 24 participants (six in each group). Plasma JR-142 and serum insulin-like growth factor-1 (IGF-1) concentrations were measured for PK and PD assessments. Safety was evaluated on the basis of adverse events (AEs), laboratory tests, and other measures. JR-142 induced dose-dependent increases in the maximum plasma JR-142 concentration (C_max) and the area under the plasma concentration-time curve from time 0 to τ (AUC_0-τ). A similar dose-response relationship was observed in serum IGF-1 concentrations. All trough IGF-1 levels were well sustained one week after the final administrations of JR-142 at the three dosages, while the peak concentrations of IGF-1 remained mildly elevated. No serious AEs were observed, and laboratory tests, including assessment of anti-drug antibodies, uncovered no significant safety issues. Once-weekly subcutaneous injections of JR-142 produced positive dose-dependent PK and PD profiles over the dosage range. Drug accumulation was observed after the four-week administration period but did not raise safety concerns, indicating that JR-142 is well-tolerated in healthy participants. The PD profiles observed in terms of IGF-1 concentrations were also positive, and we believe the encouraging results of this study warrant substantiation in further clinical trials in patients with GHD. This clinical study was conducted at one investigational site in Osaka, Japan, where the clinical study and the non-clinical data of JR-142 were reviewed and approved by its Institutional Review Board on 9th May 2019. The study was conducted in compliance with the approved study protocol, the Declaration of Helsinki, 1964, as revised in 2013, and Good Clinical Practice.