Abstract
Movement disorders are common symptoms in patients with inborn errors of metabolism (IEMs), and have a serious impact on their quality of life. Treatment of the movement disorder is usually causal, i.e. aimed at the underlying condition. More and more therapeutic strategies are emerging for many IEMs, varying from dietary restriction/supplementation, enzyme cofactor/vitamin supplementation, enzyme-replacement, substrate inhibition, substrate reduction, bone marrow or hematopoietic stem-cell transplantation (HSCT), gene therapy, or newer symptomatic treatment modalities. Dietary manipulations typically aim at substrate reduction, i.e. decreasing the intake of toxic precursors or providing the deficient product. Vitamins and cofactors are used because in some disorders the synthesis of these is affected, while in others these catalyze residual enzyme activity and stability, or act as chaperones.
Introduction
Movement disorders are common symptoms in patients with inborn errors of metabolism (IEMs), and have a serious impact on their quality of life. Treatment of the movement disorder is usually causal, i.e. aimed at the underlying condition. More and more therapeutic strategies are emerging for many IEMs, varying from dietary restriction/supplementation, enzyme cofactor/vitamin supplementation, enzyme-replacement, substrate inhibition, substrate reduction, bone marrow or hematopoietic stem-cell transplantation (HSCT), gene therapy, or newer symptomatic treatment modalities. Dietary manipulations typically aim at substrate reduction, i.e. decreasing the intake of toxic precursors or providing the deficient product. Vitamins and cofactors are used because in some disorders the synthesis of these is affected, while in others these catalyze residual enzyme activity and stability, or act as chaperones.
Enzyme-replacement therapy (ERT) has been used mainly in lysosomal storage disorders and is based on the ability of most cells to take up the deficient enzyme. Over the last few years, efforts have been made to target the brain better with intrathecal ERT. Substrate reduction therapy is also applied for the treatment of lysosomal storage disorders and aims to reduce the synthesis of the substrate of the mutant enzyme or its precursor.
Organ transplantation, specifically liver transplants, has been used mainly in small molecule disorders, especially urea cycle defects. The goal of this treatment is to replace the missing enzyme by replacing the whole organ. HSCT has been used for some lysosomal storage disorders and X-linked adrenoleukodystrophy (X-ALD). Some novel therapeutic approaches include chaperone molecules and proteostasis regulators, read-through drugs, cell therapies, ex vivo and in vivo gene therapy, RNA targeting, and genome editing (CRISPR/Cas9).
In some cases of IEMs, the movement disorder occurs after an acute metabolic decompensation, such as in glutaric aciduria type 1, and prompt management can prevent this life-altering complication. Since the introduction of expanded newborn screening in many countries, early diagnosis and treatment has dramatically reduced the disease burden in patients with IEMs.
The growing interest from the pharmaceutical industry in the field of rare disorders is also contributing to major breakthroughs in the development of clinical trials in IEMs.
Here, we will review novel therapies for movement disorders in IEMs that have emerged over the past decade, some of them proven to be effective, others failed after clinical trials, and some in ongoing trials. This may not be a comprehensive review of all the novel treatment modalities due to the large number of ongoing clinical trials, but it is intended to represent the rapidly expanding field of new treatments in rare diseases [1–11].
Treatment Modalities for IEMs
Natural Treatments
Natural treatments with antioxidants, natural compounds, and vitamins for IEMs began in the late twentieth century. These included vitamin E, selenium, curcumin, antioxidants, endoplasmic reticulum modifiers such as trimethylamine-N-oxide and N-tert-butyl-hydroxylamine derivative (NtBuHA), among others. Unfortunately these approaches address only the secondary consequences of the disease and not the underlying cause [12, 13].
Novel Approaches Using Available Drugs
Sodium benzoate, phenylacetate, and sodium phenylbutyrate are used for treatment in urea cycle disorders, but their side effect of decreasing branched-chain amino acid levels have been investigated in the treatment of maple syrup urine disease patients. A double-blinded, randomized, placebo-controlled trial of phenylbutyrate in the treatment of maple syrup urine disease (ClinicalTrials.gov registration number: NCT01529060) has been completed, with results pending.
In a particular murine model of Leigh syndrome, specifically in Ndufs4 knockout mice, it was found that mTOR was activated in the brain. Administration of an mTOR inhibitor, rapamycin conferred neuroprotection in these mice [14].
In argininosuccinic aciduria (ASA) there is nitric oxide (NO) deficiency, and patients develop long term hypertension and neurocognitive deficits. Supplementation of NO in patients with ASA controlled hypertension and improved cardiac hypertrophy [15, 16].
Anti-Inflammatories
Some lysosomal storage disorders have an abnormal inflammatory response in their pathobiology. The use of anti-inflammatories has been addressed in cases of the neuronal ceroid lipofuscinoses (NCLs) (ClinicalTrials.gov NCT01399047). Ten children participated in a double-blinded, randomized, 22-week crossover study of mycophenolate mofetil vs. placebo; no serious side effects were found but outcomes have not yet been reported.
Cofactor Administration
Molybdenum Cofactor Deficiency Type A
Molybdenum cofactor deficiency (MoCD) is caused by a deficiency in one of the molybdenum-dependent enzymes, sulfite oxidase, xanthine oxidase, and aldehyde dehydrogenase. Mutations in MOCS1 cause impaired sulfite oxidase enzyme activity leading to a deficiency in the first intermediate in the biosynthetic pathway, the cyclic pyranopterin monophosphate (cPMP), and it is classified as MoCD type A. It is characterized by sulfite accumulation. Defects in MOCS2 cause accumulation of cPMP and it is classified as MoCD type B. The very rare type C form is associated with mutations in GPHN. The natural history of the disease is that of neurodegeneration leading to severe epileptic encephalopathy, brain atrophy, severe disability, and death. Later-onset cases have been also described, presenting with parkinsonism and dystonia, and even, in rare cases, infantile onset with status dystonicus [17, 18]. Treatment had been confined to supportive and palliative care until recently when cyclic pyranopterin monophosphate (cPMP) substitution was shown to be effective in improving the neurodevelopmental outcome of patients. In 2002, Lee et al. produced a molybdenum cofactor-deficient mouse model that resembles the human phenotype [19]. The same group later rescued the mouse phenotype with a biosynthetic precursor developed from Escherichia coli [20]. This substance was cPMP. The first few case reports demonstrated a favorable outcome in newborns treated early with daily intravenous injections of cPMP, reporting improved alertness, seizure control, and biochemical parameters [21, 22]. In 2015, there was a report on the efficacy and safety of cPMP in 16 neonates diagnosed with MoCD types A and B who received intravenous cPMP following a standardized protocol. They were enrolled in an observational prospective cohort study and the drug was provided based on a compassionate-use program. There were no drug-related serious adverse events reported and the disease biomarkers, urinary S-sulfocysteine, xanthine, and urate almost normalized. The longest follow up was 5 years; patients with type A had significant improvement in clinical outcome, and three patients who started treatment early were completely seizure-free and had near-normal development. Patients with type B had no improvements in biochemical or clinical parameters. cPMP remains an effective treatment option in early diagnosed MoCD type A patients; however, the daily intravenous administration and cost limit its feasibility and accessibility at this time [23].
Chelation
Neurodegeneration with Brain Iron Accumulation
Neurodegeneration with brain iron accumulation (NBIA) syndromes are a heterogeneous group of neurological disorders caused by excessive iron deposition in the brain (PKAN, PLAN, MPAN, BPAN, CoPAN, FAHN, Kufor–Rakeb syndrome, aceruloplasminemia, neuroferritinopathy; see Chapter 16). Because these disorders are individually very rare, controlled clinical trials are challenging.
Use of chelation therapy in the brain iron overload diseases has been tried since the 1960s but systemic iron chelation limited its use. The role of iron in the neurodegenerative process, however, is still not completely understood and the therapeutic potential of chelation is not well documented. Several case reports and safety and efficacy trials with the chelator deferiprone have shown a decrease in brain iron levels; however, a clinical benefit was not evident in most cases [24–27].
Findings in the Drosophila PKAN model of bypassing the defective enzyme PANK2 with the supplement pantethine, an active form of pantothenic acid (also known as vitamin B5), for coenzyme A (CoA) synthesis led to trials in humans, but because of its instability in serum and difficulties in passing the blood–brain barrier, its use is limited [28–30]. Newer derivatives of pantethine may be explored in the future. Fosmetpantothenate (also known as RE-024) has shown some promising results in animal studies [31].
Trials with small molecule product replacement and gene transfer therapy in mouse models of NBIA are underway (NCT03570931 using a compound RT001, which is deuterated linoleic acid, or using DHA (docosahexanoic acid) [32, 33]. Although evidence-based guidelines are limited, in 2017 an international expert panel published a consensus clinical management guideline for PKAN [34]. There are a few clinical trials in progress (ClinicalTrials.gov), including double-blinded, placebo-controlled studies of fosmetpantothenate, a phosphopantothenate replacement, and deferiprone in PKAN. These trials include open-label extensions.
Dietary Manipulations
GLUT1 Deficiency
Glucose is the main metabolic substrate for the brain. It provides energy for neurons via glycolysis and the tricarboxylic acid (TCA) cycle. In glucose transporter type 1 (GLUT1) deficiency there is decreased glucose transport through the blood–brain barrier and less glucose is available as a substrate for the various biochemical transformations, including anapleurosis, which is the act of replacing the TCA intermediates used for biosynthesis. The main treatment for GLUT1 deficiency has been the ketogenic diet, which offers an alternative energy source for brain metabolism in the form of ketone bodies. However, its efficacy is only about 75% in patients with severe epilepsy and patients may remain with neurological deficits, especially movement disorders. Triheptanoin, a seven-carbon fatty acid triglyceride that can yield three molecules of heptanoate, can refill TCA intermediates via anapleurosis. One clinical trial enrolled 14 patients with GLUT1 deficiency (children and adults) and gave open-label supplementation with food-grade triheptanoin for 3 months [35]. The primary outcome of the study was a decrease in seizures on EEG and improved neurocognitive performance and brain metabolic rate. There were no reported serious adverse events and patients tolerated the oil well, manifesting only some minor or transient digestive discomfort or diarrhea. There was marked subjective motor and cognitive improvement in all participants as well as EEG seizure reduction and favorable neuropsychological results in receptive and expressive vocabulary.
Another open-label study performed in patients with GLUT1 deficiency presenting with non-epileptic paroxysmal movement disorder enrolled eight patients (children and adults) treated with triheptanoin 1 g/kg per day [36]. They performed three phases of two months each: baseline, treatment, and withdrawal, and used patient diaries to record motor and non-motor paroxysmal events. Treatment with triheptanoin resulted in a 90% improvement in non-epileptic paroxysmal manifestations. Larger, controlled studies need to be done to be able to offer an alternative therapy to the ketogenic diet, which is not always feasible in some patients.
SPG5
Spastic paraplegia type 5 (SPG5) is an autosomal-recessive disorder due to mutations in the CYP7B1 gene causing neurodegeneration resulting from alterations in lipid homeostasis. The protein product of CYP7B1 is a cytochrome P450 7α-hydroxylase, implicated in cholesterol metabolism. A recent therapeutic trial in 12 patients with SPG5 showed an improved bile acid profile and plasma oxysterol levels using chenodeoxycholic acid and atorvastatin. The trial was a randomized, placebo-controlled, double-blinded study consisting of a three-period, three-treatment crossover study and the six different sequences of three treatments were randomized. The study concluded that atorvastatin decreased plasma 27-hydroxycholesterol but did not change the 27-hydroxycholesterol to total cholesterol ratio or 25-hydroxycholesterol levels. The bile acid profile normalized on treatment [37].
Another publication described a long-term benefit in one patient treated with a statin (simvastatin) and ezetimibe (which lowers cholesterol by inhibiting its absorption in the small intestine) and one patient with ezetimibe alone, followed for 12 months and 24 months, respectively. The long-term administration of cholesterol-lowering drugs reduced serum 27-hydroxycholesterol by about 50%. They found intolerable side effects when adding chenodeoxycholic acid (diarrhea and elevation of liver enzymes). Although further long-term studies are needed, the investigators concluded that cholesterol-lowering drugs should be considered in SPG5 [38].
Ketogenic Diet
The ketogenic diet has been used widely in children with epilepsy since the 1990s. In GLUT1 deficiency and pyruvate dehydrogenase complex deficiency the ketogenic diet is the therapy of choice. Other inherited metabolic disorders have been the target of trials with the ketogenic diet, such as some of the mitochondrial disorders. Ketone bodies have several salutary properties: they reduce the proportion of mutated mitochondrial DNA (mtDNA) and improve respiratory chain function in cultured cells; they decrease cytochrome C oxidase-negative muscle fibers and induce mitochondrial biogenesis in an animal model for late-onset mitochondrial myopathy; they decrease production of reactive oxygen species in rats; and they increase citrate synthase along with complex I and catalase activity in neuronal cell lines.
There are 135 current clinical trials with the ketogenic diet listed on the ClinicalTrials.gov website, mostly for the treatment of epileptic seizures. One trial is listed as a synergistic intervention in gangliosidoses using a combination of ketogenic diet and miglustat, a substrate reduction therapy (SRT). The authors hypothesize that the ketogenic diet will minimize or prevent the gastrointestinal side effects of miglustat, and will also improve the central nervous system response to miglustat therapy as well as improving seizure control (NCT02030015).
The ketogenic diet has been suggested as a method of treatment in other IEMs, such as mitochondrial disorders with or without seizures, and targeting the epilepsy in other IEMs such as non-ketotic hyperglycinemia, argininosuccinate lyase deficiency, adenosylsuccinate lyase deficiency, and succinic semialdehyde dehydrogenase (SSADH) deficiency [39].
Neuromodulation
SSADH Deficiency
The deterioration of the SSADH-deficient mouse model after weaning lead to a trial of 18 patients with the amino acid taurine in 18 patients. Taurine has numerous neuromodulatory roles, including protection against free radical damage. The study was an open-label study that lasted for 12 months. Outcome was measured with the Adaptive Behavior Assessment Scale and the results showed no significant change in the adaptive behavior scores after taurine treatment [40].
A double-blinded, crossover, phase 2 clinical trial has completed recruitment using the experimental compound SGS-742, a gamma-aminobutyric acid (GABA) B receptor antagonist that has been shown to be safe and well tolerated in clinical trials in adults with cognitive impairment (NCT02019667). The SSADH knockout mouse model has shown benefits from it. The primary outcome measure will be a change in the Auditory Comprehension subtest of the Neuropsychological Assessment Battery Language Module score; the secondary outcome measure will be a change in cortical excitation and inhibition measured by transcranial magnetic stimulation. Additional evaluations include neurological and neuropsychological examinations and CSF collection to measure GABA levels.
Chaperone Therapy
Chaperone therapy is a molecular therapeutic approach to treat protein misfolding diseases. Major targets for researching these compounds are the various lysosomal storage disorders. Lysosomal storage disorders are often caused by mutations that destabilize native folding and impair the trafficking of enzymes, leading to premature endoplasmic reticulum-associated degradation, deficiency of hydrolytic functions, and storage of material in lysosomes. Chemical or pharmaceutical chaperones are low-weight molecular compounds that stabilize the mutant protein and induce the expression of its biological activity in the cell, hence restoring trafficking and increasing enzyme activity and substrate production. They can reach the brain, crossing the blood–brain barrier and are often used in disorders with neurological manifestations [41].
Another class of molecules able to influence the fate of misfolded proteins are the proteostasis regulators. These facilitate protein folding by increasing the function of molecular chaperones and/or by activating the protein quality-control system. Examples of these are: the antitumor antibiotic geldanamycin, antiseizure drugs (i.e. carbamazepine), histone deacetylase inhibitors, and the proteasome inhibitor bortezomib [42, 43].
Gaucher Disease (Neuronopathic)
The efficacy of enzyme-replacement and substrate reduction therapies in treating the neurological manifestations of neuronopathic Gaucher disease is negligible; hence, the development of pharmaceutical chaperone therapies is an alternative approach.
Ambroxol is such a compound, which is commonly used as an expectorant. Results of an open-label pilot study using high-dose ambroxol in combination with enzyme-replacement therapy in five patients showed that ambroxol had good safety and tolerability, significantly increased lymphocyte glucocerebrosidase activity, crossed the blood–brain barrier, and decreased the glucosylsphingosine levels in the CSF. Clinically, patients had improvement in myoclonus, seizures, and the pupillary light reflex. These results are promising but further clinical trials are needed [44].
There are several studies using high-throughput screening for small molecule therapy in Gaucher disease, using either the wild-type enzyme or patient tissue as the source of mutant glucocerebrosidase [45, 46].
Other small molecules have been used in trials in neuronopathic Gaucher disease, either in Gaucher cell lines [47, 48] or patients (molecule AT2101 – isofagomine tartrate).
Unmodified iminosugars are also considered to be good pharmacological chaperones, behaving as competitive inhibitors of the target lysosomal enzyme. Isofagomine (IFG) is one compound that has shown promising results in ex vivo and in vivo experiments, increasing enzyme activity. However, dosing experiments in a mouse model have shown that it gets transported to the endoplasmic reticulum (ER) inefficiently. Iminosugars with alkyl chains of varying lengths, i.e N-butyl- and N-nonyl-deoxynojirimycin, which are forms of the iminosugar miglustat, an SRT, have better ER permeability and bind to the enzyme better. The search for suitable pharmaceutical chaperones has used the concept to conjugate an iminosugar with a lipophilic moiety. Diverse technologies using large libraries of glycomimetics, or using delta-lactams, have been implemented by several research groups. Other compounds researched are C-alkylated iminosugars, sp2-iminosugars, aminocyclitols, and aminosugars [5].
Phase II trials with arimoclomol, a heat shock protein inducer, are planned.
GM1 Gangliosidosis
GM1 gangliosidosis is caused by mutations in the GLB1 gene, encoding lysosomal beta-galactosidase. Only three types of beta-D-galactosidase inhibitors have been investigated as potential pharmacological chaperones in this disorder: carbasugar N-octyl-epi-valienamine (NOEV), selected unbranched analogs, and iminosugars with various structural modifications. Suzuki and coworkers explored chaperone therapy in GM1 gangliosidosis with the galacto-configured iminosugar 1-deoxy-galactonojirimycin (DGJ) and its N-butyl derivative (NB-DGJ) [49]. They showed a significant increase in enzyme activity in mouse fibroblasts expressing human mutations and in patients’ fibroblasts; however, relatively high doses were needed, limiting its use in clinical trials. Further work led to the identification of other compounds that significantly increased residual beta-galactosidase activity: DLHex-DGJ, NN-DGJ, and C-glycoside iminosugar derivatives [5]. The same group also pioneered the work on NOEV that represented a hallmark in the field of pharmaceutical chaperone therapy [50]. Mice expressing human mutations showed an increase in enzyme activity and a decrease in GM1 ganglioside accumulation in the brainstem and cerebral cortex. NOEV treatment starting in the early stage of the disease arrested the neurological progression of GM1 within a few months and significantly prolonged survival. Other research groups modified the structure of NOEV to improve its chaperone properties, which led to an increase in the enzyme activity in mice harboring certain human mutations [5].
Another new family of experimental pharmacological chaperones has been described, the aminocyclopentane-based carbasugars [51]. These are powerful inhibitors of beta-D-galactosidase with increase of enzyme activity several fold in human mutant cells.
Clinical trials of these chaperone therapies are pending.
GM2 Gangliosidosis
Pyrimethamine was found to be an effective chaperone molecule in cells from patients with late-onset GM2 gangliosidosis. Many mutations in either the alpha or beta subunit of hexosaminidase A were partially rescued. An open-label clinical trial with pyrimethamine in eight patients with late-onset GM2 gangliosidosis examined the effect of escalating doses. The activity of the hexosaminidase A enzyme was increased four-fold at doses of 50 mg per day. Doses of 75 mg or above caused significant side effects. Expanded studies have been withdrawn due to lack of funding [52].
Peroxisomal Disorders
There are no curative therapies for peroxisomal disorders. Several compounds have been tried in a small number of patients, such as the plasmalogen precursor batyl alcoho or oral bile acid supplementation (chenodeoxycholic acid, ursodeoxycholic acid, cholic acid), and although the biochemical parameters improved, there was no improvement in clinical outcome [53]. Several other pharmacological therapies are under development, such as plasmalogen precursor therapy other than alkylglycerol (PPI-1011) [54]. Proliferation of peroxisomes in Zellweger disease fibroblasts using 4-phenylbutyrate and related compounds has been achieved in cell lines from patients who have an intermediate or mild phenotype. This resulted in improved peroxisome enzyme function [55, 56]. Geneticin (G418), a nonsense suppressor aminoglycoside also has been shown to promote peroxisome recovery in fibroblast cell lines from patients with PEX2 and PEX12 mutations [57]. Chemical chaperone drugs, such as arginine and betaine, improved peroxisome biogenesis and functions in cell lines with PEX1, PEX6, and PEX12 mutations. Trials are ongoing utilizing betaine (trimethylglycine) in patients with PEX1 mutations. The misfolded protein in the PEX1 Gly843Asp mutation has residual function and treatment with betaine recovered peroxisome function in cultured cells. Clinical trial outcomes have included the measurements of biochemical markers, the C26/C22 ratio, and developmental outcomes. Thus far, clinical improvement has not been demonstrated, and reasons could include the small numbers of patients, the testing of plasma biomarkers possibly being less sensitive than direct measurement in cells, or a high variability of patients’ phenotypes.
Enzyme-Replacement Therapy
New investigational enzyme-replacement therapy is ongoing in several lysosomal disorders. A number of preclinical studies with recombinant enzyme-replacement therapy have been performed in infantile neuronal ceroid lipofuscinosis (INCL) and late-infantile neuronal ceroid lipofuscinosis (LINCL). Intravenous, intrathecal, and intraventricular methods of delivery have been used. Some of these studies led to an ERT clinical trial for the treatment of LINCL (NCT01907087). Twenty-four patients were enrolled between the ages of 3 years and 16 years, and received intraventricular infusion of cerliponase alfa every 2 weeks. Treatment was limited to 300 mg/dose. The results have been published and concluded that intraventricular infusion of cerliponase alfa in patients with CLN2 disease resulted in less decline in motor and language function compared to historical controls. Serious adverse events included failure of the intraventricular device and device-related infections [58].
The challenging obstacle in the treatment of lysosomal storage disorders has been penetrating the blood–brain barrier. Peptide modification is one approach that has been used to overcome this. Some studies have used modified TPP1 protein by altering its glycosylation profile or combining its peptide sequence with a specific region of the apolipoprotein E receptor, both resulting in increased blood–brain barrier penetrance [59, 60]. This method is called the “Trojan horse” approach and utilizes natural cellular pathways to deliver proteins across the blood–brain barrier [61, 62]. Another way to deliver drugs across the blood–brain barrier is by using nanocarriers, including liposomes [63]. One advantage of nanocarriers is targeted cell delivery, using antibodies to different receptors expressed on endothelial cells of the blood–brain barrier.
Substrate Reduction Therapy
The conventional therapy for lysosomal storage disorders is to use enzyme-replacement therapy but since Radin introduced the concept of SRT in 1996, this approach has been expanded and many SRT drugs have been approved: miglustat for Gaucher disease and Niemann–Pick disease type C (NPC), eliglustat tartrate for Gaucher disease, and genistein for mucopolysaccharidoses. To overcome some of the side effects of these drugs, second-generation compounds are under evaluation.
An open-label multicentre clinical trial is underway for studying the tolerability, pharmacokinetics, pharmacodynamics, and efficacy of a compound venglustat in combination with ERT (imiglucerase) in adult patients with Gaucher disease type 3.
Trials are underway to explore synergistic therapy with miglustat and the ketogenic diet for the treatment of childhood-onset gangliosidosis, with a proposed study duration of 5 years and with the primary outcome of duration of survival of each participant and the rate of change in neurocognitive functioning. (NCT02030015).
Gene suppression technologies as tools for substrate reduction have been introduced in the last decade. One of the most promising techniques seems to be RNA interference, and phase 3 clinical trials are underway for various neurological and non-neurological conditions. Antisense oligonucleotides for gene knockdown have been used and are under clinical evaluation. Small interfering RNAs have been used to reduce glycosaminoglycan synthesis in mucopolysaccharidosis type III (MPS III) mice and patient fibroblasts [64].
Vorinostat, a histone deacetylase inhibitor has been shown to increase mutant NPC1 protein levels in vivo and to reverse the cellular accumulation of unesterified cholesterol. An open-label phase 1/2 study of vorinostat in NPC has been completed and posted on the ClinicalTrials.gov website. Twelve adult participants were enrolled and 11 finished the study. Trial participants were treated with vorinostat, utilizing a 3 days on/4 days off regimen to limit toxicity. The primary outcome measure was tolerability and secondary outcomes were measures of biochemical efficacy (NCT02124083).
Another approach to treating lysosomal storage disorders is with recombinant heat-shock protein 70 (rHSP70). Exposure to rHSP70 led to a significant reductions in cellular lysosome enlargement, suggesting that rHSP70 aids in diminishing the build-up of metabolites within lysosomes. Subsequently, rHSP70 treatment was tested in genetic mouse models of NPC leading to diminished lipid metabolite storage in the central nervous system. This reduction in lipid accretion was accompanied by improvements in ataxic gait and general physical activity [65]. In separate experiments, NPC mice treated with the small molecule inducer of HSP70 arimoclomol also demonstrated a reduction in lysosomal enlargement and an improvement in neurological and behavioral symptoms. A double-blind, randomized, placebo-controlled study in pediatric patients with NPC is in progress (NCT02612129).
Another type of inhibition of substrate accumulation is using 2‐hydroxypropyl‐beta‐cyclodextrin (HPβCD) in NPC. In mice, administration of this compound led to a delayed clinical onset, extended lifespan, and reduced unesterified cholesterol and glycolipid accumulation within the central nervous system and other organs [66].
A phase 2b/3 prospective, randomized, double-blinded, sham-controlled three-part trial of VTS-270 (2-hydroxypropyl-beta-cyclodextrin) intrathecal injections in subjects with neurological manifestations of NPC is also in progress (NCT 02534844).
Umbilical Cord Blood Cell Transplantation and Hematopoietic Stem-Cell Therapy
X-Linked Adrenoleukodystrophy
Allogenic HSCT is an established long-term treatment method for boys with childhood cerebral X-ALD. The mechanism of action is replacing the defective microglia by bone marrow-derived long-lived macrophages of the allogeneic donor. Recently, lentivirus-based ex vivo gene therapy has been introduced as a treatment option. Both HSCT and gene therapy are effective when performed early in the course of childhood cerebral X-ALD [67, 68].
Mucopolysaccharidosis II
Although there is enzyme-replacement therapy for MPS II, it does not cross the blood–brain barrier, limiting treatment of the neurological complications of the disease. HSCT is the treatment of choice in MPS I, but has not been recommended for MPS II. The clinical experience with HSCT in MPS II is limited and no systematic or well-designed clinical trials have been conducted. However, the overall outcome suggests that the neurological deterioration can be halted or slowed in the few cases who had HSCT. Well-designed clinical trials are needed to confirm this assumption [69].
Niemann–Pick Disease Type C
Evidence is limited to one case report on presymptomatic cord blood transplantation in a child with NPC whose older brother died of complications of the disease at the age of 3 years. The patient was transplanted at the age of 5 months; there was some neurological deterioration at the age of 4 years but by age 8 years he was severely affected, with loss of ambulation, ataxia, and generalized myoclonus [70].
Neuronal Ceroid Lipofuscinoses
A number of NCL mouse model and patient studies have been used to explore the benefits of HSCT, with limited success [71–73]. Other studies have used a combination with gene therapy in Ppt-/- mice and this showed significant improvement. Neural stem-cell therapy has also been proven to be effective and the results of a clinical trial with human central nervous system stem cells in six children tested in an open-label dose escalation phase 1 trial showed that the procedure is well tolerated. There were no results on efficacy since the children enrolled were all in advanced stages of the disease [74].
Leukodystrophies
Currently, presymptomatic HSCT is the only therapeutic modality that alleviates Krabbe disease-induced central nervous system injury. However, all HSCT-treated patients exhibit severe deterioration in peripheral nervous system function, and patients still present with major motor and expressive language dysfunction. Studies in mice studies, using an aggressive busulfan conditioning regimen, have shown an extended life span after HSCT [75]. The same authors have also demonstrated that combining this protocol with lentiviral vector-based gene therapy did not significantly change the outcome.
A review of the last 20 years’ experience on cord blood transplantation in patients with leukodystrophies, such as metachromatic leukodystrophy (MLD), globoid-cell leukodystrophy (Krabbe disease), and X-ALD found that factors associated with higher overall survival included presymptomatic status, well-matched cord blood units, and a higher baseline performance status (PS). Long-term survival was best in patients who were presymptomatic and had a performance status of >80. Of these patients, 50% remained stable, 20% declined to PS 60–80, and 30% to <60 [6].
A review on late mortality in patients transplanted for IEMs, including 264 patients (104 with X-ALD, 96 with MPS I, 28 with MLD, and 36 with other diseases), found that the 10-year overall survival exceeded 85% [76].
Gene Therapy
Gene therapy is a direct approach to the treatment of genetic diseases regardless of how well the underlying pathophysiology is understood, as it delivers a normal gene to a diseased cell or organ, resulting in the expression of the normal protein. The severe adverse reactions in the early stages of gene therapy halted the progression of these types of intervention, but recently, newer techniques and vectors for delivery have been proven to be safer. Encouraging short-term safety and efficacy studies have been published in X-ALD and aromatic L-amino acid decarboxylase (AADC) deficiency. Delivery of the gene can be done by non-viral transfer or by viral gene delivery, such as using simple retrovirus, lentivirus, adenovirus, and adeno-associated virus (AAV) vectors. Vectors capable of targeted integration and DNA editing exist and have shown promising results in some in vitro and preclinical studies. Improvements in immunosuppression regimens will also improve safety and efficacy of using viral vectors in the future [7].
Twenty-eight clinical trials have been reported in 2017 based on the local delivery of AAV vectors for lysosomal storage disorders [2]. Most of them use intraparenchymal injections; one was based on intrathecal (Batten disease) and one on intravenous injection (MPSIIIA). Unfortunately, some of the gene therapies for severe neurodegenerative disorders, such as Canavan disease or LINCL, although successfully expressing the normal protein in the brain, have shown little clinical improvements. These trials have demonstrated the safety of recombinant AAV (rAAV)-mediated gene delivery through direct brain injections but failed to achieve a substantial rescue (NCT00151216, NCT01161576).
Ex vivo gene therapy uses reprogrammed isolated patient cells for functional protein synthesis. For example, in an X-ALD gene therapy trial, the bone marrow cells were genetically reprogrammed using a lentiviral vector carrying the missing ABCD1 gene and reinjected into the patients. Follow-up studies have shown that demyelination was halted 14–16 months after treatment, and, 24–30 months later, ABCD1 protein expression was still present in several cell types [77, 78].
A combination of HSCT and gene therapy is recruiting in a phase 1/2 trial aiming at the assessment of the safety and efficacy of arylsulfatase A (ARSA)/adenosine-triphosphate-binding cassette, subfamily D (ABCD1) gene transfer into hematopoietic stem/progenitor cells for the treatment of MLD and X-ALD, respectively, after an Italian group conducted a gene therapy clinical trial based on autologous HSCT and advanced generation lentiviral vectors for patients affected by the most severe, early-onset forms of the disease (HCT01560182). The safety and efficacy of this gene therapy approach in MLD patients was evaluated. During 3 years of follow-up, they reported multilineage ARSA expression and an ability to prevent and correct neurological disease manifestations. In a newer study, the investigators will recruit symptomatic patients for transduced cluster of differentiation 34 positive (CD34+) HSCT treatment. In the treated patients, the short-term and long-term safety of the administration of the autologous transduced hematopoietic stem cells, their long-term engraftment, the expression of vector-derived ARSA or ABCD1, and the ability of the transduced cells to provide a clinical benefit to the patients will be studied. The treated patients will be followed for 3 years and thereafter monitored for the safety of gene therapy for an additional 5 years. If successful, this study will provide key results on the safety and efficacy of gene therapy for MLD and X-ALD patients (NCT02559830). A phase 1/2, open-labelled, monocentric study of direct intracranial administration of a replication deficient AAV gene transfer vector expressing the human ARSA complementary DNA (cDNA) to five children with MLD has been conducted, but results are not yet available (NCT01801709).
Another phase 2/3 trial assesses the efficacy and safety of autologous CD34+ hematopoietic stem cells, transduced ex-vivo with the Lenti-D lentiviral vector, for the treatment of cerebral ADL. A subject’s blood stem cells were collected and modified (transduced) using the Lenti-D lentiviral vector encoding human ADL protein. After modification (transduction) with the Lenti-D lentiviral vector, the cells were transplanted back into the subject following myeloablative conditioning. This interventional open-label trial started in 2013 with the goal of recruiting 30 participants (NCT01896102) and was recently published [79], with positive results.
A single-stage, adaptive, open-label, dose escalation safety and efficacy study of gene therapy of AADC deficiency was first posted on the ClinicalTrials.gov website in 2016. This trial is a single treatment arm interventional open-label trial with the goal of recruitment of six participants between the ages of 5 years and 18 years, using the AAV2 vector. The AAV2-hAADC dose is infused via MRI-guided infusion in both the left and right substantia nigra pars compacta and ventral tegmental areas (NCT02852213).
A similar study was previously done with some promising results (NCT01395641). Ten patients were enrolled and received bilateral intraputaminal injections of AAV2-hAADC through stereotactic brain surgery. Primary efficacy outcomes were an increase in the Peabody Developmental Motor Scale, 2nd edition (PDMS-2) score of greater than 10 points, and an increase in homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA) concentrations in the CSF, 12 months after the gene therapy. All patients met the primary efficacy endpoint: 12 months after gene therapy the PDMS-2 scores were increased by a median of 62 points and HVA concentrations by a median of 25 nmol/L. There were 101 adverse effects reported, the most common being pyrexia (16%) and orofacial dyskinesia (10%). Twelve serious side effects occurred, including one death (treatment-unrelated influenza-B encephalitis) [80]. An expansion of this study is offering this clinical trial to patients who are not enrolled in the phase 1/2 trial with a slightly increased dosage (NCT02926066).
LYS-SAF302 has been also approved for a phase 2/3 single arm clinical trial in children with MPS III type A. This is an open-label, single arm, interventional study using intracerebral administration of AAV serotype rh.10 carrying human N-sulfoglucosamine sulfohydrolase cDNA. In addition, a phase 1/2 clinical trial of scAAV9.U1a.hSGSH for MPS III type A is administering the vector in a peripheral vein is also in progress (NCT02716246).
Another phase 1/2, dose escalation interventional trial for MPS III type B uses AAV serotype 9 carrying the human alpha-N-acetylglucosaminidase (NAGLU) gene under the control of a cytomegalovirus (CMV) enhancer/promoter (rAAV9.CMV.hNAGLU) (NCT03315182).
Specific gene therapies for IEMs with particular mutations have been also tried in cell lines. An example is a particular mutation in ALDH7A1, causing pyridoxine-dependent epilepsy. Perez et al. used antisense therapy in a patient’s lymphoblast cell line harbouring the c.75 C>T mutation, a novel splicing mutation, creating a new donor site inside exon 1, and this was successful [81].
Genome editing with site-specific endonucleases, such as zinc-finger nucleases and the CRISPR/Cas9 system, in combination with delivery vectors engineered to target disease tissue, have been successful in correcting mutations in murine models, in a number of IEMs, including tyrosinemia type I, ornithine transcarbamylase deficiency, and lysosomal storage disorders [82–87].
An important safety issue for genome editing is the accurate assessment of the off-target cleavage by endonucleases and mitigating the effects of non-specific activity. Overall, the better understanding of the CRISPR/Cas9 mechanisms have led to improved technology. However, considerable work is needed before genome editing becomes a common therapeutic avenue in IEMs [8].
Read-Through Drugs
These drugs allow the ribosome to selectively read through nonsense mutations to generate functional proteins by preventing messenger RNA degradation by nonsense-mediated decay. Ataluren is such a drug and has been proposed for treatment in several IEMs: lysosomal storage diseases [88], the NCLs [89], and phenylalanine hydroxylase deficiency [90], among others.
Lysosomal Modulators
Accumulation of lysosomal material can be cleared by modulating the lysosome. TFEB, a transcription factor, can achieve this by altering the expression of different lysosomal genes. Studies have identified a number of TFEB activators that reduce storage accumulation in patients’ fibroblasts [91–93]. Other compounds that have been shown to modulate the lysosomes of lysosomal storage disorders include delta-tocopherol [94].
Small Molecules and Alternatively Targeted Pathways
One small molecule, the collapsing response mediator protein-2, has been recently identified to be associated with neurodegenerative diseases, including the NCLs. Targeting this molecule with various compounds (i.e. lanthionine ketimine, lacosamide) may be a therapeutic option [95–98]. Another small molecule, N-tert-(butyl)hydroxylamine, was used in INCL cell lines and a mouse model with promising results [99]. Other small molecules, cysteamine bitartrate and N-acetylcysteine, have been also used in models of INCL.
Mitochondrial Disease
Treatment in mitochondrial disorders is most challenging due to the extreme clinical heterogeneity, multiple organ involvement, and the unique genetic make-up of the mitochondria.
Primary mitochondrial disorders can be attributed to mutations in both mitochondrial and nuclear genomes. For many decades patients with mitochondrial disorders have been treated with vitamins, cofactors, and nutritional supplements, with no proven benefit.
Therapies to improve mitochondrial dysfunction, to decrease the amount of reactive oxygen and nitrogen species that result in redox imbalance and glutathione deficiency, have been used in clinical trials in recent years, and the results published in a recent update by Enns et al., 2017 [9]. The therapies with EPI-743 (alpha-tocotrienol quinone) and RP103 (cysteamine bitartrate) have the theoretical potential to improve redox balance by increasing intracellular glutathione. There have been five clinical trials with EPI-743 (four open-label and one randomized, double-blinded, placebo-controlled) completed in primary mitochondrial disorders and the outcomes show clinical improvement, reversal or arrested disease progression, and/or decreased rates of hospitalization. There has been a cysteamine open-label clinical trial (NCT02023866) for short-term safety and efficacy in children with mitochondrial disease, including Leigh syndrome, with continuation to a long-term extension study (NCT02473445), but unfortunately this has been prematurely terminated due to lack of efficiency in the baseline study.
New therapeutic approaches have emerged with some benefit, some in preclinical animal models and others in clinical trials [10, 11]. The therapeutic strategies can be categorized into non-specific and disease-specific strategies.
The non-specific, general treatment strategies include new protein delivery, stimulation of mitochondrial biogenesis, regulating execution pathways, improving mitochondrial dynamics, or bypassing respiratory chain defects.
These general treatment strategies have a wide applicability and address common disease mechanisms, could be potentially cost-effective, but often present with challenges such as off-target effects.
In the new protein delivery category there is systemic protein delivery enhancing nucleotide metabolism by transfusing platelets or erythrocyte-encapsulated thymidine phosphorylase, for treatment of mitochondrial neuro-gastro-intestinal encephalopathy (MNGIE); however, neither of these approaches have proven to have sustained clinical benefits [100–102]. Direct enzyme-replacement therapy would be an approach to improve long-term outcomes, which has been done in animal models, but the reported antibody generation means that these protocols require modification [103]. Another approach in animal models (Tymp-/- and Tk2-/-) is to rescue nucleotide imbalance and mtDNA instability with supplementation of deoxycytidine or tetrahydrouridine (inhibitor of cytidine deaminase in the case of the Tymp-/- mouse model), or with deoxycytidine monophosphate and deoxythimidine monophosphate (in the case of Tk2-/- mouse model) [104, 105].
The stimulation of mitochondrial biogenesis is achieved with several approaches, such as aminoimidazolecarboxamide ribotide (AICAR), nicotinic acid, bezafibrates, and resveratrol. Mitochondrial biogenesis is regulated mainly by the transcription co-activator peroxisome proliferator-activated receptor-γ1 (PGC1), which interacts with several transcription factors, which in turn control the expression of genes involved in oxidative phosphorylation. PGC1 is activated with either deacetylation by the protein sirtuin 1 (Sirt1) or phosphorylation by AMP-dependent kinase (AMPK). AICAR, an adenosine monophosphate analogue, activates AMPK. Nicotinamide riboside increases NAD+ levels, which activates Sirt1. Administration of AICAR and nicotinic acid in mouse models of mitochondrial myopathy has been shown to induce mitochondrial biogenesis and ameliorate clinical phenotypes [106–108].
Bezafibrate is a pan-agonist for the PGC1, upregulating its gene expression. Its use has generated controversial reports, with improvement observed in the muscle-specific Cox10 knock-out mouse, which later was not reproducible in other mouse models [107, 109].
Resveratrol increases NAD+ levels enhancing Sirt1 activation. It has been shown to improve mitochondrial function in fibroblasts [110] and in Friedreich ataxia (NCT01339884) [111]. Inhibition of the NAD+-consuming enzyme poly-ADP polymerase 1 (PARP1) increases NAD+ availability, Sirt1 activity, and oxidative metabolism. Experiments in animal models have shown efficacy [106, 112].
Tripeptides that penetrate cells and accumulate in mitochondria, binding to cardiolipin, a lipid component of the inner mitochondrial membrane, can change the shape of the mitochondrial cristae. The mechanism is linked to modulating Opa1 activity, which is a dynamin-like GTPase of the inner membrane [113].
Single-peptide enzymes derived from yeast, called xenogenes, have been used to bypass the block of respiratory chain due to specific complex deficiencies, for example NADH reductase [114, 115].
Inhibiting autophagy is another general strategy, which is achieved by using the mammalian target of rapamycin (mTOR) inhibitor, rapamycin, although long-term side effects may limit its use in mitochondrial disease [116]. Lithium chloride has also been used as autophagy modulator [117].
Disease-specific therapies have been designed for several nuclear and mtDNA mutations in the form of stem-cell therapies and other strategies directed at manipulating DNA. There are quite a few case reports and small studies on treatments with allogeneic hematopoietic stem-cell transplantations in MNGIE and a clinical trial in progress (NCT02427178). Endogenous stem cells that contain less mutated mtDNA than mature post-mitotic skeletal muscle cells have been transfused for mtDNA mutations; however, there were no clinical benefits. More recently, in vitro experiments have used a somatic-cell nuclear transfer approach to correct the metabolic disturbance in these disorders, using induced pluripotent stem cells [118].
The other forms of targeted treatment approaches are based on manipulating DNA. This can be achieved by the restriction endonuclease approach, which would recognize specific DNA sequences, produce double-stranded DNA breaks, and initiate target molecule degradation. In theory, a single treatment would suffice to correct the biochemical defect in mutated cells. Unfortunately, there are not many human pathogenic mutations that create restriction sites enabling the use of this approach more widely. To circumvent this problem, special custom-designed restriction endonucleases have been used in some preclinical trials especially for eliminating the m8993 T>G mutation (causing Leigh syndrome or neuropathy ataxia retinitis pigmentosa, [NARP]). One of the examples of such endonucleases is the zinc finger nuclease (ZFN) consisting of tandem repeat zinc fingers. Unfortunately, early experiments found that ZNFs cause significant cytotoxicity and hence are not suitable for human trials, but continuing efforts have been made to improve their design [119].
Other approaches are with target specific DNA nucleases (TALENs), which are more potent than ZFNs, but larger in size, limiting their use with AAV vectors [120] and with Cas9 nuclease. This latter is targeted, using a shorter RNA sequence CRISPR [121, 122], and is not hindered by context-specific binding characteristics for ZFNs and TALENs. MitoTALENs have been used in oocytes in recent years to reduce germline transmission of mutated mtDNA responsible for Leber hereditary optic neuropathy (LHON) and NARP [123].
Manipulating transfer RNA (tRNA) with tRNA synthases has been used to partially rescue the mitochondrial dysfunction in mitochondrial tRNA defects, but human trials are needed [124, 125].
Gene therapy for nuclear mitochondrial diseases has been tried in several murine models, such as ethylmalonic aciduria (caused by mutations in the ETHE1 gene), MNGIE, and mutations in MPV17, using AAV vector approaches [126–128]. However, several challenges have not been addressed mainly because of tissue specificity and the fact that the vector was targeted to liver and did not reach skeletal muscle or brain.
Delivering gene therapy for mtDNA disorders presents an even greater challenge. The mitochondrial membrane is relatively impermeable and there are thousands of affected mitochondria in each individual cell. Alternative approaches of adding an engineered targeting peptide presequence to the AAV vector have emerged. This method has been used in preclinical experiments in LHON caused by the mtDNA m.11778 G>A mutation. This has been expanded to murine trials using intraocular injections of the human nuclear ND4 gene constructs expressed by AAV [129]. However, it has not yet been demonstrated that the allotopically expressed proteins can integrate in the respiratory chain. Human clinical trials have been either recently completed or are currently recruiting LHON patients for AAV-vector gene therapy. A safety and efficacy study (NCT01267422) with a single intravitreal injection of recombinant AAV-NADH dehydrogenase, subunit 4 (complex I) (rAAV2-ND4) has been completed in nine patients with LHON. The visual acuity of the injected eyes of six patients improved by at least 0.3 on the LogMAR chart after 9 months of follow-up. In these six patients, the visual field was enlarged but the retinal nerve fibre layer remained relatively stable. No other outcome measure was significantly changed. None of the nine patients had local or systemic adverse events related to the vector during the 9-month follow-up period. The study was terminated early because of a limited number of participants [130]. Several similar studies are underway (NCT03153293, NCT02064569).
Neurotransmitter Therapy
Movement disorders can be a hallmark of the monoamine neurotransmitter disorders. This is a heterogeneous group of neurological disorders characterized by primary and secondary defects in the biosynthesis, degradation, and transport of dopamine, norepinephrine, epinephrine, and serotonin. The deficiency of dopamine and serotonin will cause characteristic neurological features, including pyramidal and extrapyramidal movement disorders. Treatment strategies include the replacement of the monoamines with their precursors and the inhibition of their degradation. The primary neurotransmitter disorders have been addressed elsewhere in this book; there are also a host of secondary neurotransmitter deficiencies. Many IEMs have low levels of CSF HVA and 5-HIAA, including untreated phenylketonuria, Lesch–Nyhan disease, mitochondrial disorders, and different leukodystrophies. Many of these patients will have low levels of HVA in the CSF and will present with dyskinesia, tremor, dystonia, and eye movement disorders, similar to what is seen in primary monoamine disorders. The possibility of symptomatic treatment with levodopa and/or 5-hydroxytryptophan should be considered to improve brain maturation and neurological outcome [131].
Surgical Treatment
Surgical management of severe movement disorders, specifically deep brain stimulation has been discussed in detail in the previous chapter. Disorders associated with benefit using this novel therapeutic approach include PKAN, Lesch–Nyhan disease, glutaric aciduria type 1 after severe encephalopathic episodes, and methylmalonic acidemia.
Conclusions
The treatment of rare diseases is a rapidly expanding field in medicine. Clinical trials are numerous and in various stages, and can be searched online at the ClinicalTrials.gov website (www.clinicaltrials.gov). A wide range of disease-specific and non-specific approaches have become available with a variety of approaches, including substrate reduction therapy, enzyme-replacement therapy, and gene therapy.
Key Points and Clinical Pearls
Emerging therapeutic strategies include dietary restriction/supplementation, enzyme cofactor/vitamin supplementation, enzyme replacement, substrate inhibition, substrate reduction, bone marrow or hematopoietic stem-cell transplantation, gene therapy, or newer symptomatic treatment modalities.
Enzyme-replacement and substrate-reduction therapies have focused especially on lysosomal storage disorders and are based on the ability of most cells to take up the deficient enzyme.
Organ transplantation, specifically liver transplantation, has been used mainly in small molecule disorders, especially urea cycle defects. The goal of this treatment is to replace the missing enzyme by replacing the whole organ.
Hematopoietic stem-cell transplantation has been used for some lysosomal storage disorders and X-linked adrenoleukodystrophy.
Novel therapeutic approaches are using chaperone molecules and proteostasis regulators, read-through drugs, cell therapies, ex vivo and in vivo gene therapy, RNA targeting, and genome editing (CRISPR/Cas9).
Mitochondrial disorders are especially challenging due to clinical heterogeneity, multiple organ involvement, and the unique genetic make-up of the mitochondria.
Directions for Future Research
Preclinical studies of non-specific and disease-specific interventions are expected to rapidly evolve into clinical trials.
Selection of measurable and meaningful clinical endpoints are a challenge in clinical trial development.
Safer approaches to viral-mediated gene therapy and gene editing will invariably lead to increasing clinical trials and applicability of novel therapeutic strategies to inborn errors of metabolism.
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Findit@CUHK Library | Google Scholar | PubMedRelated posts:
Chapter 1 – Treatable Metabolic Movement Disorders: The Top 10
Chapter 10 – A Phenomenology-Based Approach to Inborn Errors of Metabolism with Spasticity
Chapter 7 – A Phenomenology-Based Approach to Inborn Errors of Metabolism with Ataxia
Chapter 16 – Syndromes of Neurodegeneration with Brain Iron Accumulation
Chapter 4 – Imaging in Metabolic Movement Disorders
Chapter 28 – Disorders of Creatine Metabolism: Creatine Deficiency Syndromes and Movement Disorders
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