Strategies for Neoglycan conjugation to human acid α-glucosidase

Cipaglucosidase alfa plus miglustat: linking mechanism of action to clinical outcomes in late-onset Pompe disease

Enzyme replacement therapy (ERT) is the only approved disease-modifying treatment modality for Pompe disease, a rare, inherited metabolic disorder caused by a deficiency in the acid α-glucosidase (GAA) enzyme that catabolizes lysosomal glycogen. First-generation recombinant human GAA (rhGAA) ERT (alglucosidase alfa) can slow the progressive muscle degeneration characteristic of the disease. Still, most patients experience diminished efficacy over time, possibly because of poor uptake into target tissues. Next-generation ERTs aim to address this problem by increasing bis-phosphorylated high mannose (bis-M6P) N-glycans on rhGAA as these moieties have sufficiently high receptor binding affinity at the resultant low interstitial enzyme concentrations after dosing to drive uptake by the cation-independent mannose 6-phosphate receptor on target cells. However, some approaches introduce bis-M6P onto rhGAA via non-natural linkages that cannot be hydrolyzed by natural human enzymes and thus inhibit the endolysosomal glycan trimming necessary for complete enzyme activation after cell uptake. Furthermore, all rhGAA ERTs face potential inactivation during intravenous delivery (and subsequent non-productive clearance) as GAA is an acid hydrolase that is rapidly denatured in the near-neutral pH of the blood. One new therapy, cipaglucosidase alfa plus miglustat, is hypothesized to address these challenges by combining an enzyme enriched with naturally occurring bis-M6P N-glycans with a small-molecule stabilizer. Here, we investigate this hypothesis by analyzing published and new data related to the mechanism of action of the enzyme and stabilizer molecule. Based on an extensive collection of in vitro, preclinical, and clinical data, we conclude that cipaglucosidase alfa plus miglustat successfully addresses each of these challenges to offer meaningful advantages in terms of pharmacokinetic exposure, target-cell uptake, endolysosomal processing, and clinical benefit.

Pompe disease: Unmet needs and emerging therapies

Pompe disease is a debilitating and life-threatening disease caused by aberrant accumulation of glycogen resulting from reduced acid alpha-glucosidase activity. The first treatment for Pompe disease, the enzyme replacement therapy, Myozyme® (recombinant human acid alpha-glucosidase, alglucosidase alfa), is a lifesaving treatment for the most severe form of the disease and provided clinically meaningful benefits to patients with milder phenotypes. Nonetheless, many patients display suboptimal responses or clinical decline following years of alglucosidase alfa treatment. The approval of avalglucosidase alfa (Nexviazyme®) and cipaglucosidase alfa (Pombiliti®) with miglustat (Opfolda®) represents a new generation of enzyme replacement therapies seeking to further improve patient outcomes beyond alglucosidase alfa. However, the emergence of a complicated new phenotype with central nervous system involvement following long-term treatment, coupled with known and anticipated unmet needs of patients receiving enzyme replacement therapy, has prompted development of innovative new treatments. This review provides an overview of the challenges of existing treatments and a summary of emerging therapies currently in preclinical or clinical development for Pompe disease and related lysosomal storage disorders. Key treatments include tissue-targeted enzyme replacement therapy, which seeks to enhance enzyme concentration in target tissues such as the central nervous system; substrate reduction therapy, which reduces intracellular glycogen concentrations via novel mechanisms; and gene therapy, which may restore endogenous production of deficient acid alpha-glucosidase. Each of these proposed treatments shows promise as a future therapeutic option to improve quality of life in Pompe disease by more efficiently treating the underlying cause of disease progression: glycogen accumulation.

Clinical insight meets scientific innovation to develop a next generation ERT for Pompe disease

Years of research into the structure, processing, and function of acid alpha-glucosidase led to the development and 2006 approval of alglucosidase alfa (recombinant human acid alpha-glucosidase, Myozyme®/Lumizyme®), an enzyme replacement therapy and the first approved treatment for Pompe disease. Alglucosidase alfa has been a lifesaving treatment for patients with infantile-onset Pompe disease and radically improved daily life for patients with late-onset Pompe disease; however, long-term experience with alglucosidase alfa unraveled key unmet needs in these populations. Despite treatment, Pompe disease continues to progress, especially from a skeletal muscle perspective, resulting in a multitude of functional limitations. Strong collaboration between the scientific and patient communities led to increased awareness of Pompe disease, a better understanding of disease pathophysiology, knowledge of the clinical course of the disease as patients surpassed the first decade of life, and the strengths and limitations of enzyme replacement therapy. Taken together, these advancements spurred the need for development of a next generation of enzyme replacement therapy and provided a framework for progress toward other novel treatments. This review provides an overview of the development of avalglucosidase alfa as a model to highlight the interaction between clinical experience with existing treatments, the role of the clinician scientist, translational research at both system and cellular levels, and the iterative and collaborative process that optimizes the development of therapeutics.

Targeted protein degradation through site-specific antibody conjugation with mannose 6-phosphate glycan

Recent developments in targeted protein degradation have provided great opportunities to eliminating extracellular protein targets using potential therapies with unique mechanisms of action and pharmacology. Among them, Lysosome-Targeting Chimeras (LYTACs) acting through mannose 6-phosphate receptor (M6PR) have been shown to facilitate degradation of several soluble and membrane-associated proteins in lysosomes with high efficiency. Herein we have developed a novel site-specific antibody conjugation approach to generate antibody mannose 6-phosphate (M6P) conjugates. The method uses a high affinity synthetic M6P glycan, bisM6P, that is coupled to an Fc-engineered antibody NNAS. This mutant without any effector function was generated by switching the native glycosylation site from position 297 to 298 converting non-sialylated structures to highly sialylated N-glycans. The sialic acid of the glycans attached to Asn298 in the engineered antibody was selectively conjugated to bisM6P without chemoenzymatic modification, which is often used for site-specific antibody conjugation through glycans. The conjugate is mainly homogeneous by analysis using mass spectrometry, typically with one or two glycans coupled. The M6P-conjugated antibody against a protein of interest (POI) efficiently internalized targeted soluble proteins, such as human tumor necrosis factor (TNF), in both cancer cell lines and human immune cells, through the endo-lysosomal pathway as demonstrated by confocal microscopy and flow cytometry. TNF in cell culture media was significantly depleted after the cells were incubated with the M6P-conjugated antibody. TNF internalization is mediated through M6PR, and it is correlated well with cell surface expression of cation-independent M6PR (CI-MPR) in immune cells. A significant amount of CI-MPR remains on the cell surface, while internalized TNF is degraded in lysosomes. Thus, the antibody-M6P conjugate is highly efficient in inducing internalization and subsequent lysosome-mediated protein degradation. Our platform provides a unique method for producing biologics-based degraders that may be used to treat diseases through event-driven pharmacology, thereby addressing unmet medical needs.

A universal GlycoDesign for lysosomal replacement enzymes to improve circulation time and biodistribution

Currently available enzyme replacement therapies for lysosomal storage diseases are limited in their effectiveness due in part to short circulation times and suboptimal biodistribution of the therapeutic enzymes. We previously engineered Chinese hamster ovary (CHO) cells to produce α-galactosidase A (GLA) with various N-glycan structures and demonstrated that elimination of mannose-6-phosphate (M6P) and conversion to homogeneous sialylated N-glycans prolonged circulation time and improved biodistribution of the enzyme following a single-dose infusion into Fabry mice. Here, we confirmed these findings using repeated infusions of the glycoengineered GLA into Fabry mice and further tested whether this glycoengineering approach, Long-Acting-GlycoDesign (LAGD), could be implemented on other lysosomal enzymes. LAGD-engineered CHO cells stably expressing a panel of lysosomal enzymes [aspartylglucosamine (AGA), beta-glucuronidase (GUSB), cathepsin D (CTSD), tripeptidyl peptidase (TPP1), alpha-glucosidase (GAA) or iduronate 2-sulfatase (IDS)] successfully converted all M6P-containing N-glycans to complex sialylated N-glycans. The resulting homogenous glycodesigns enabled glycoprotein profiling by native mass spectrometry. Notably, LAGD extended the plasma half-life of all three enzymes tested (GLA, GUSB, AGA) in wildtype mice. LAGD may be widely applicable to lysosomal replacement enzymes to improve their circulatory stability and therapeutic efficacy.

Chemoenzymatic glycan-selective remodeling of a therapeutic lysosomal enzyme with high-affinity M6P-glycan ligands. Enzyme substrate specificity is the name of the game

Functionalization of therapeutic lysosomal enzymes with mannose-6-phosphate (M6P) glycan ligands represents a major strategy for enhancing the cation-independent M6P receptor (CI-MPR)-mediated cellular uptake, thus improving the overall therapeutic efficacy of the enzymes. However, the minimal high-affinity M6P-containing N-glycan ligands remain to be identified and their efficient and site-selective conjugation to therapeutic lysosomal enzymes is a challenging task. We report here the chemical synthesis of truncated M6P-glycan oxazolines and their use for enzymatic glycan remodeling of recombinant human acid α-glucosidase (rhGAA), an enzyme used for treatment of Pompe disease which is a disorder caused by a deficiency of the glycogen-degrading lysosomal enzyme. Structure-activity relationship studies identified M6P tetrasaccharide oxazoline as the minimal substrate for enzymatic transglycosylation yielding high-affinity M6P glycan ligands for the CI-MPR. Taking advantage of the substrate specificity of endoglycosidases Endo-A and Endo-F3, we found that Endo-A and Endo-F3 could efficiently deglycosylate the respective high-mannose and complex type N-glycans in rhGAA and site-selectively transfer the synthetic M6P N-glycan to the deglycosylated rhGAA without product hydrolysis. This discovery enabled a highly efficient one-pot deglycosylation/transglycosylation strategy for site-selective M6P-glycan remodeling of rhGAA to obtain a more homogeneous product. The Endo-A and Endo-F3 remodeled rhGAAs maintained full enzyme activity and demonstrated 6- and 20-fold enhanced binding affinities for CI-MPR receptor, respectively. Using an in vitro cell model system for Pompe disease, we demonstrated that the M6P-glycan remodeled rhGAA greatly outperformed the commercial rhGAA (Lumizyme) and resulted in the reversal of cellular pathology. This study provides a general and efficient method for site-selective M6P-glycan remodeling of recombinant lysosomal enzymes to achieve enhanced M6P receptor binding and cellular uptake, which could lead to improved overall therapeutic efficacy of enzyme replacement therapy.

Endolysosomal N-glycan processing is critical to attain the most active form of the enzyme acid alpha-glucosidase

Acid alpha-glucosidase (GAA) is a lysosomal glycogen-catabolizing enzyme, the deficiency of which leads to Pompe disease. Pompe disease can be treated with systemic recombinant human GAA (rhGAA) enzyme replacement therapy (ERT), but the current standard of care exhibits poor uptake in skeletal muscles, limiting its clinical efficacy. Furthermore, it is unclear how the specific cellular processing steps of GAA after delivery to lysosomes impact its efficacy. GAA undergoes both proteolytic cleavage and glycan trimming within the endolysosomal pathway, yielding an enzyme that is more efficient in hydrolyzing its natural substrate, glycogen. Here, we developed a tool kit of modified rhGAAs that allowed us to dissect the individual contributions of glycan trimming and proteolysis on maturation-associated increases in glycogen hydrolysis using in vitro and in cellulo enzyme processing, glycopeptide analysis by MS, and high-pH anion-exchange chromatography with pulsed amperometric detection for enzyme kinetics. Chemical modifications of terminal sialic acids on N-glycans blocked sialidase activity in vitro and in cellulo, thereby preventing downstream glycan trimming without affecting proteolysis. This sialidase-resistant rhGAA displayed only partial activation after endolysosomal processing, as evidenced by reduced catalytic efficiency. We also generated enzymatically deglycosylated rhGAA that was shown to be partially activated despite not undergoing proteolytic processing. Taken together, these data suggest that an optimal rhGAA ERT would require both N-glycan and proteolytic processing to attain the most efficient enzyme for glycogen hydrolysis and treatment of Pompe disease. Future studies should examine the amenability of next-generation ERTs to both types of cellular processing.

In Vitro N-Glycan Mannosyl-Phosphorylation of a Therapeutic Enzyme by Using Recombinant Mnn14 Produced from Pichia pastoris

Enzyme replacement therapy for lysosomal storage diseases usually requires recombinant enzymes containing mannose-6-phosphate (M6P) glycans for cellular uptake and lysosomal targeting. For the first time, a strategy is established here for the in vitro mannosyl-phosphorylation of high-mannose type N-glycans that utilizes a recombinant Mnn14 protein derived from Saccharomyces cerevisiae. Among a series of N-terminal- or C-terminal-deleted recombinant Mnn14 proteins expressed in Pichia pastoris, rMnn14_77-935 with deletion of N-terminal 76 amino acids spanning the transmembrane domain (46 amino acids) and part of the stem region (30 amino acids), showed the highest level of mannosyl-phosphorylation activity. The optimum reaction conditions for rMnn14_77-935 were determined through enzyme assays with a high-mannose type N-glycan (Man₈GlcNAc₂) as a substrate. In addition, rMnn14_77-935 was shown to mannosyl-phosphorylate high-mannose type Nglycans (Man_7-9GlcNAc₂) on recombinant human lysosomal alpha-glucosidase (rhGAA) with remarkably high efficiency. Moreover, the majority of the resulting mannosyl-phosphorylated glycans were bis-form which can be converted to bis-phosphorylated M6P glycans having a superior lysosomal targeting capability. An in vitro N-glycan mannosyl-phosphorylation reaction using rMnn14_77-935 will provide a flexible and straightforward method to increase the M6P glycan content for the generation of "Biobetter" therapeutic enzymes.

Surface plasmon resonance analysis of complex formation of therapeutic recombinant lysosomal enzymes with domain 9 of human cation-independent mannose 6-phosphate receptor

The efficacy of enzyme replacement therapy (ERT) for lysosomal storage diseases (LSDs) possibly depends on the cellular uptake of recombinant lysosomal enzymes (LEs), and it is known that cation-independent mannose 6-phosphate receptor (CI-M6PR) on the cell membrane is predominantly involved in the endocytosis of many LEs. To examine the biomolecular interaction between therapeutic LEs and CI-M6PR, we biophysically analyzed the complex formation of four LEs available with domain 9 of human CI-M6PR, a binding site of the receptor, by means of surface plasmon resonance (SPR) biosensor assays. The results revealed that the affinity of the LEs for domain 9 of the receptor increased in the following order: laronidase, agalsidase beta, idursulfase, and alglucosidase alfa; and the high affinity of laronidase for domain 9 of CI-M6PR was due to fast complex formation rather than slow dissociation of the complex. The affinity of the enzymes for domain 9 of CI-M6PR almost coincided with their cellular uptake. The SPR biosensor assay is sensitive and provides important information for the development of effective therapeutic LEs for LSDs.

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The biomolecular interaction between LEs and domain 9 of human CI-M6PR was examined by means of SPR biosensor assays.

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The binding of LEs with the receptor increased in the order: laronidase, agalsidase beta, idursulfase, and agalsidase alfa.

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The strong binding of laronidase with the receptor was due to fast complex formation rather than slow dissociation of the complex.

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The affinity of the LEs for domain 9 of CI-M6PR almost coincided with the cellular uptake of the enzymes.

Splice modulating antisense oligonucleotides restore some acid-alpha-glucosidase activity in cells derived from patients with late-onset Pompe disease

Pompe disease is caused by mutations in the GAA gene, resulting in deficient lysosomal acid-α-glucosidase activity in patients, and a progressive decline in mobility and respiratory function. Enzyme replacement therapy is one therapeutic option, but since not all patients respond to this treatment, alternative interventions should be considered. One GAA mutation, c.-32-13T > G, impacts upon normal exon 2 splicing and is found in two-thirds of late-onset cases. We and others have explored a therapeutic strategy using splice modulating phosphorodiamidate morpholino oligomers to enhance GAA exon 2 inclusion in the mature mRNA of patients with one c.-32-13T > G allele. We designed 20 oligomers and treated fibroblasts derived from five patients to identify an oligomer sequence that maximally increased enzyme activity in all fibroblasts. The most effective splice correcting oligomer was chosen to treat forced-myogenic cells, derived from fibroblasts from nine patients carrying the c.-32-13T > G mutation. After transfection, we show increased levels of the full-length GAA transcript, acid-α-glucosidase protein, and enzyme activity in all patients’ myogenic cells, regardless of the nature of the mutation in the other allele. This data encourages the initiation of clinical trials to assess the therapeutic efficacy of this oligomer for those patients carrying the c.-32-13T > G mutation.

Single enzyme nanoparticle, an effective tool for enzyme replacement therapy

The term "single enzyme nanoparticle" (SEN) refers to a chemically or biologically engineered single enzyme molecule. SENs are distinguished from conventional protein nanoparticles in that they can maintain their individual structure and enzymatic activity following modification. Furthermore, SENs exhibit enhanced properties as biopharmaceuticals, such as reduced antigenicity, and increased stability and targetability, which are attributed to the introduction of specific moieties, such as poly(ethylene glycol), carbohydrates, and antibodies. Enzyme replacement therapy (ERT) is a crucial therapeutic option for controlling enzyme-deficiency-related disorders. However, the unfavorable properties of enzymes, including immunogenicity, lack of targetability, and instability, can undermine the clinical significance of ERT. As shown in the cases of Adagen®, Revcovi®, Palynziq®, and Strensiq®, SEN can be an effective technology for overcoming these obstacles. Based on these four licensed products, we expect that additional SENs will be introduced for ERT in the near future. In this article, we review the concepts and features of SENs, as well as their preparation methods. Additionally, we summarize different types of enzyme deficiency disorders and the corresponding therapeutic enzymes. Finally, we focus on the current status of SENs in ERT by reviewing FDA-approved products.

Chemoenzymatic Methods for the Synthesis of Glycoproteins

Glycosylation is one of the most prevalent posttranslational modifications that profoundly affects the structure and functions of proteins in a wide variety of biological recognition events. However, the structural complexity and heterogeneity of glycoproteins, usually resulting from the variations of glycan components and/or the sites of glycosylation, often complicates detailed structure-function relationship studies and hampers the therapeutic applications of glycoproteins. To address these challenges, various chemical and biological strategies have been developed for producing glycan-defined homogeneous glycoproteins. This review highlights recent advances in the development of chemoenzymatic methods for synthesizing homogeneous glycoproteins, including the generation of various glycosynthases for synthetic purposes, endoglycosidase-catalyzed glycoprotein synthesis and glycan remodeling, and direct enzymatic glycosylation of polypeptides and proteins. The scope, limitation, and future directions of each method are discussed.

Lysosomal Targeting Enhancement by Conjugation of Glycopeptides Containing Mannose-6-phosphate Glycans Derived from Glyco-engineered Yeast

Many therapeutic enzymes for lysosomal storage diseases require a high content of mannose-6-phosphate (M6P) glycan, which is important for cellular uptake and lysosomal targeting. We constructed glyco-engineered yeast harboring a high content of mannosylphosphorylated glycans, which can be converted to M6P glycans by uncapping of the outer mannose residue. In this study, the cell wall of this yeast was employed as a natural M6P glycan source for conjugation to therapeutic enzymes. The extracted cell wall mannoproteins were digested by pronase to generate short glycopeptides, which were further elaborated by uncapping and α(1,2)-mannosidase digestion steps. The resulting glycopeptides containing M6P glycans (M6PgPs) showed proper cellular uptake and lysosome targeting. The purified M6PgPs were successfully conjugated to a recombinant acid α-glucosidase (rGAA), used for the treatment of Pompe disease, by two-step reactions using two hetero-bifunctional crosslinkers. First, rGAA and M6PgPs were modified with crosslinkers containing azide and dibenzocyclooctyne, respectively. In the second reaction using copper-free click chemistry, the azide-functionalized rGAA was conjugated with dibenzocyclooctyne-functionalized M6PgPs without the loss of enzyme activity. The M6PgP-conjugated rGAA had a 16-fold higher content of M6P glycan than rGAA, which resulted in greatly increased cellular uptake and efficient digestion of glycogen accumulated in Pompe disease patient fibroblasts.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2011-2012

This review is the seventh update of the original article published in 1999 on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2012. General aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, and fragmentation are covered in the first part of the review and applications to various structural types constitute the remainder. The main groups of compound are oligo- and poly-saccharides, glycoproteins, glycolipids, glycosides, and biopharmaceuticals. Much of this material is presented in tabular form. Also discussed are medical and industrial applications of the technique, studies of enzyme reactions, and applications to chemical synthesis. © 2015 Wiley Periodicals, Inc. Mass Spec Rev 36:255-422, 2017.

Enzyme replacement therapy and beyond-in memoriam Roscoe O. Brady, M.D. (1923-2016)

Lysosomal storage disorders are strong candidates for the development of specific innovative therapies. The discovery of enzyme deficiencies is an important milestone in understanding the underlying cause of disease. Being able to replace the first missing enzyme in a lysosomal storage required three decades of dedicated research. Successful drug development for lysosomal storage disorders was fostered by the U.S. Orphan Drug Act. Various optimization strategies have the potential to overcome the current limitations of enzyme replacement therapies. In addition, substrate reduction therapies are an alternative approach to treat lysosomal storage disorders, chemical chaperones enhance residual enzyme activity, and small molecules can facilitate substrate transport through subcellular compartments. Bone-marrow derived multipotent stem cells and gene therapies have received FDA orphan drug designation status. The science of small clinical trials played an essential role: non-neurological endpoints, biomarker, and regulatory alignment are key factors in successful drug development for lysosomal storage disorders. Being able to treat brain disease is the next frontier. This review is dedicated to the memory of Roscoe O. Brady, an early pioneer in the research of lysosomal storage diseases.

Chemoenzymatic Synthesis and Receptor Binding of Mannose-6-Phosphate (M6P)-Containing Glycoprotein Ligands Reveal Unusual Structural Requirements for M6P Receptor Recognition

Mannose-6-phosphate (M6P)-terminated oligosaccharides are important signals for M6P-receptor-mediated targeting of newly synthesized hydrolases from Golgi to lysosomes, but the precise structural requirement for the M6P ligand-receptor recognition has not been fully understood due to the difficulties in obtaining homogeneous M6P-containing glycoproteins. We describe here a chemoenzymatic synthesis of homogeneous phosphoglycoproteins carrying natural M6P-containing N-glycans. The method includes the chemical synthesis of glycan oxazolines with varied number and location of the M6P moieties and their transfer to the GlcNAc-protein by an endoglycosynthase to provide homogeneous M6P-containing glycoproteins. Simultaneous attachment of two M6P-oligosaccahrides to a cyclic polypeptide was also accomplished to yield bivalent M6P-glycopeptides. Surface plasmon resonance binding studies reveal that a single M6P moiety located at the low α-1,3-branch of the oligomannose context is sufficient for a high-affinity binding to receptor CI-MPR, while the presence of a M6P moiety at the α-1,6-branch is dispensable. In addition, a binding study with the bivalent cyclic and linear polypeptides reveals that a close proximity of two M6P-oligosaccharide ligands is critical to achieve high affinity for the CI-MPR receptor. Taken together, the present study indicates that the location and valency of the M6P moieties and the right oligosaccharide context are all critical for high-affinity binding with the major M6P receptor. The chemoenzymatic method described here provides a new avenue for glycosylation remodeling of recombinant enzymes to enhance the uptake and delivery of enzymes to lysosomes in enzyme replacement therapy for the treatment of lysosomal storage diseases.

Glyco-engineering strategies for the development of therapeutic enzymes with improved efficacy for the treatment of lysosomal storage diseases

Lysosomal storage diseases (LSDs) are a group of inherent diseases characterized by massive accumulation of undigested compounds in lysosomes, which is caused by genetic defects resulting in the deficiency of a lysosomal hydrolase. Currently, enzyme replacement therapy has been successfully used for treatment of 7 LSDs with 10 approved therapeutic enzymes whereas new approaches such as pharmacological chaperones and gene therapy still await evaluation in clinical trials. While therapeutic enzymes for Gaucher disease have N-glycans with terminal mannose residues for targeting to macrophages, the others require N-glycans containing mannose-6-phosphates that are recognized by mannose-6-phosphate receptors on the plasma membrane for cellular uptake and targeting to lysosomes. Due to the fact that efficient lysosomal delivery of therapeutic enzymes is essential for the clearance of accumulated compounds, the suitable glycan structure and its high content are key factors for efficient therapeutic efficacy. Therefore, glycan remodeling strategies to improve lysosomal targeting and tissue distribution have been highlighted. This review describes the glycan structures that are important for lysosomal targeting and provides information on recent glyco-engineering technologies for the development of therapeutic enzymes with improved efficacy. [BMB Reports 2015; 48(8): 438-444]

Lysosomal storage diseases: from pathophysiology to therapy

Lysosomal storage diseases are a group of rare, inborn, metabolic errors characterized by deficiencies in normal lysosomal function and by intralysosomal accumulation of undegraded substrates. The past 25 years have been characterized by remarkable progress in the treatment of these diseases and by the development of multiple therapeutic approaches. These approaches include strategies aimed at increasing the residual activity of a missing enzyme (enzyme replacement therapy, hematopoietic stem cell transplantation, pharmacological chaperone therapy and gene therapy) and approaches based on reducing the flux of substrates to lysosomes. As knowledge has improved about the pathophysiology of lysosomal storage diseases, novel targets for therapy have been identified, and innovative treatment approaches are being developed.

Comparative study on mannose 6-phosphate residue contents of recombinant lysosomal enzymes

As most recombinant lysosomal enzymes are incorporated into cells via mannose 6-phosphate (M6P) receptors, the M6P content is important for effective enzyme replacement therapy (ERT) for lysosomal diseases. However, there have been no comprehensive reports of the M6P contents of lysosomal enzymes. We developed an M6P assay method comprising three steps, i.e., acid hydrolysis of glycoproteins, derivatization of M6P, and high-performance liquid chromatography, and determined the M6P contents of six recombinant lysosomal enzymes now available for ERT and one in the process of development. The assay is easy, specific, and reproducible. The results of the comparative study revealed that the M6P contents of agalsidase alfa, agalsidase beta, modified α-N-acetylgalactosaminidase, alglucosidase alfa, laronidase, idursulfase, and imiglucerase are 2.1, 2.9, 5.9, 0.7, 2.5, 3.2, and <0.3 mol/mol enzyme, respectively. The results were correlated with those of the biochemical analyses previously performed and that of the binding assay of exposed M6P of the enzymes with the domain 9 of the cation-independent M6P receptor. This assay method is useful for comparison of the M6P contents of recombinant lysosomal enzymes for ERT.

Site-specific antibody-drug conjugation through glycoengineering

Antibody-drug conjugates (ADCs) have been proven clinically to be more effective anti-cancer agents than native antibodies. However, the classical conjugation chemistries to prepare ADCs by targeting primary amines or hinge disulfides have a number of shortcomings including heterogeneous product profiles and linkage instability. We have developed a novel site-specific conjugation method by targeting the native glycosylation site on antibodies as an approach to address these limitations. The native glycans on Asn-297 of antibodies were enzymatically remodeled in vitro using galactosyl and sialyltransferases to introduce terminal sialic acids. Periodate oxidation of these sialic acids yielded aldehyde groups which were subsequently used to conjugate aminooxy functionalized cytotoxic agents via oxime ligation. The process has been successfully demonstrated with three antibodies including trastuzumab and two cytotoxic agents. Hydrophobic interaction chromatography and LC-MS analyses revealed the incorporation of ~1.6 cytotoxic agents per antibody molecule, approximating the number of sialic acid residues. These glyco-conjugated ADCs exhibited target-dependent antiproliferative activity toward antigen-positive tumor cells and significantly greater antitumor efficacy than naked antibody in a Her2-positive tumor xenograft model. These findings suggest that enzymatic remodeling combined with oxime ligation of the native glycans of antibodies offers an attractive approach to generate ADCs with well-defined product profiles. The site-specific conjugation approach presented here provides a viable alternative to other methods, which involve a need to either re-engineer the antibody sequence or develop a highly controlled chemical process to ensure reproducible drug loading.

Oxidation of sialic acid using hydrogen peroxide as a new method to tune the reducing activity

Functionalized sialic acids are useful intermediates to prepare a wide range of biological products. As they often occur at a non-reducing terminal of oligosaccharides, the most used technique to activate them is by periodate-mediated oxidation of their glycerol side chain. Here, we describe an alternative, non toxic, and environmentally-friendly method to activate the sialic acid residues by hydrogen peroxide oxidation. Four oxidative systems involving H2O2, EDTA, iron chloride, and UV light were studied and the products obtained were analyzed by LC-MS and NMR, before and after a derivatization reaction. At first, we observed, for each system, an irreversible decarbonylation reaction at the reducing end. Then, the decarbonylated sialic acid (DSA) was oxidized and fragmented into a mix of carbonyls and carboxyl acids, more or less fast according to the experimental conditions. Analysis of the reaction indicated an apparent radical mechanism and heterolytic alpha-hydroxy-hydroperoxide cleavages. The modest reducing activity was mainly explained as a consequence of over-oxidation reactions.

A bispecific protein capable of engaging CTLA-4 and MHCII protects non-obese diabetic mice from autoimmune diabetes

Crosslinking ligand-engaged cytotoxic T lymphocyte antigen-4 (CTLA-4) to the T cell receptor (TCR) with a bispecific fusion protein (BsB) comprised of a mutant mouse CD80 and lymphocyte activation antigen-3 (LAG-3) has been shown to attenuate TCR signaling and to direct T-cell differentiation toward Foxp3⁺ regulatory T cells (Tregs) in an allogenic mixed lymphocyte reaction (MLR). Here, we show that antigen-specific Tregs can also be induced in an antigen-specific setting in vitro. Treatment of non-obese diabetic (NOD) female mice between 9–12 weeks of age with a short course of BsB elicited a transient increase of Tregs in the blood and moderately delayed the onset of autoimmune type 1 diabetes (T1D). However, a longer course of treatment (10 weeks) of 4–13 weeks-old female NOD animals with BsB significantly delayed the onset of disease or protected animals from developing diabetes, with only 13% of treated animals developing diabetes by 35 weeks of age compared to 80% of the animals in the control group. Histopathological analysis of the pancreata of the BsB-treated mice that remained non-diabetic revealed the preservation of insulin-producing β-cells despite the presence of different degrees of insulitis. Thus, a bifunctional protein capable of engaging CTLA-4 and MHCII and indirectly co-ligating CTLA-4 to the TCR protected NOD mice from developing T1D.

Carbohydrate-mediated polyethylene glycol conjugation of TSH improves its pharmacological properties

Thyrogen (thyrotropin alfa for injection), recombinant human TSH (rhTSH), has been successfully used to enhance diagnostic radioiodine scanning and thyroglobulin testing in the follow-up of patients with thyroid cancer and as an adjunctive treatment for radioiodine thyroid remnant ablation. However, the short half-life of rhTSH in the circulation requires a multidose regimen. We developed novel sialic acid-mediated and galactose-mediated conjugation chemistries for targeting polyethylene glycol (PEG) to the three N-linked glycosylation sites on the protein, to prolong plasma half-life by eliminating kidney filtration and potential carbohydrate-mediated clearance. Conjugates of different PEG sizes and copy numbers were screened for reaction yield, TSH receptor binding, and murine phamacokinetics/pharmacodynamics studies. The best performing of these products, a 40-kDa mono-PEGylated sialic acid-mediated conjugate, exhibited a 3.5-fold longer duration of action than rhTSH in rats, as a 5-fold lower affinity was more than compensated by a 23-fold extension of circulation half-life. Biochemical characterization confirmed conjugation through the sialic acids. Correlation of PEG distribution on the three N-linked glycosylation sites and the PEG effect on receptor binding supported the previously reported structure-function relationship of rhTSH glycosylation. This long-acting rhTSH has the potential to significantly improve patient convenience and provider flexibility while reducing potential side effects associated with a sudden elevation of serum TSH.

Site-specific PEGylation of human thyroid stimulating hormone to prolong duration of action

Recombinant human thyroid stimulating hormone (rhTSH or Thyrogen) has been approved for thyroid cancer diagnostics and treatment under a multidose regimen due to its short circulating half-life. To reduce dosing frequency, PEGylation strategies were explored to increase the duration of action of rhTSH. Lysine and N-terminal PEGylation resulted in heterogeneous product profiles with 40% or lower reaction yields of monoPEGylated products. Eleven cysteine mutants were designed based on a structure model of the TSH-TSH receptor (TSHR) complex to create unique conjugation sites on both α and β subunits for site-specific conjugation. Sequential screening of mutant expression level, oligomerization tendency, and conjugation efficiency resulted in the identification of the αG22C rhTSH mutant for stable expression and scale-up PEGylation. The introduced cysteine in the αG22C rhTSH mutant was partially blocked when isolated from conditioned media and could only be effectively PEGylated after mild reduction with cysteine. This produced a higher reaction yield, ~85%, for the monoPEGylated product. Although the mutation had no effect on receptor binding, PEGylation of αG22C rhTSH led to a PEG size-dependent decrease in receptor binding. Nevertheless, the 40 kDa PEG αG22C rhTSH showed a prolonged duration of action compared to rhTSH in a rat pharmacodynamics model. Reverse-phase HPLC and N-terminal sequencing experiments confirmed site-specific modification at the engineered Cys 22 position on the α-subunit. This work is another demonstration of successful PEGylation of a cysteine-knot protein by an engineered cysteine mutation.

Enzyme replacement therapy for lysosomal diseases: lessons from 20 years of experience and remaining challenges

In 1964, Christian de Duve first suggested that enzyme replacement might prove therapeutic for lysosomal storage diseases (LSDs). Early efforts identified the major obstacles, including the inability to produce large quantities of the normal enzymes, the lack of animal models for proof-of-concept studies, and the potentially harmful immune responses to the "foreign" normal enzymes. Subsequently, the identification of receptor-mediated targeting of lysosomal enzymes, the cloning and overexpression of human lysosomal genes, and the generation of murine models markedly facilitated the development of enzyme replacement therapy (ERT). However, ERT did not become a reality until the early 1990s, when its safety and effectiveness were demonstrated for the treatment of type 1 Gaucher disease. Today, ERT is approved for six LSDs, and clinical trials with recombinant human enzymes are ongoing in several others. Here, we review the lessons learned from 20 years of experience, with an emphasis on the general principles for effective ERT and the remaining challenges.

Efficient uptake of recombinant α-galactosidase A produced with a gene-manipulated yeast by Fabry mice kidneys

To economically produce recombinant human α-galactosidase A (GLA) with a cell culture system that does not require bovine serum, we chose methylotrophic yeast cells with the OCH1 gene, which encodes α-1,6-mannosyltransferase, deleted and over-expressing the Mnn4p (MNN4) gene, which encodes a positive regulator of mannosylphosphate transferase, as a host cell line. The enzyme (yr-hGLA) produced with the gene-manipulated yeast cells has almost the same enzymological parameters as those of the recombinant human GLA produced with cultured human fibroblasts (agalsidase alfa), which is currently used for enzyme replacement therapy for Fabry disease. However, the basic structures of their sugar chains are quite different. yr-hGLA has a high content of phosphorylated N-glycans and is well incorporated into the kidneys, the main target organ in Fabry disease, where it cleaves the accumulated glycosphingolipids. A glycoprotein production system involving this gene-manipulated yeast cell line will be useful for the development of a new enzyme replacement therapy for Fabry disease.

Enzyme replacement therapy for lysosomal storage diseases

PURPOSE OF REVIEW

Enzyme replacement therapy (ERT) for type 1 Gaucher has been highly successful. ERT is now available for other lysosomal storage disorders (LSDs) but none of these highly expensive treatments has had the same efficacy. This review explores why these newer treatments have failed to live up to expectations and how future products might be made more effective.

RECENT FINDINGS

In Gaucher, the target cells for ERT are macrophages, which are efficiently accessed by intravenously injected recombinant enzyme. The target tissues in other LSDs receive much lower doses of enzyme and intravenous ERT does not enter the brain at all. Uptake of recombinant enzyme is via the mannose-6-phosphate receptor (M6PR). Recent work has looked at improving the efficiency of enzyme delivery to tissues by altering both the ligand on the infused enzyme and the expression of the M6PR on cells. For delivery to the central nervous system, intrathecal routes of administration have been explored.

SUMMARY

Work in tissue culture and in animal models has shown increased efficiency of enzyme delivery and clinical trials of second-generation products and novel delivery systems are now underway.