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cPMP was identified in 1993 as Precursor Z, the first and most stable intermediate in MoCo synthesis.4<\/span>\u00a0When it was confirmed that Precursor Z exists in the cyclo-pyrano form, the alternative name cPMP was proposed.5<\/span>\u00a0cPMP is a tricyclic pteridine molecule that is synthesised in the mitochondrial matrix directly from guanosine-5\u2019-triphosphate (GTP) by two enzymes, molybdenum cofactor synthesis (MOCS)1A and MOCS1B, which are alternative products of the\u00a0MOCS1<\/span>\u00a0gene.6<\/span>\u00a0cPMP is exported into the cytosol where two sulphur atoms are incorporated, producing molybdopterin (MPT). This step is facilitated by the MPT synthase complex, comprising MOCS2A and MOCS2B, alternative products of the\u00a0MOCS2<\/span>\u00a0gene.7<\/span>\u00a0MOCS3, encoded by the\u00a0MOCS3<\/span>\u00a0gene, is required to regenerate MPT synthase.8<\/span>\u00a0Gephyrin, encoded by the GPHN gene, facilitates the adenylation of MPT and subsequent insertion of molybdate with cleavage of adenylate to form the mature MoCo.9<\/span><\/p>\n cPMP, MPT and MoCo are all sensitive to oxidative inactivation. MPT and MoCo have a very short biological half-life in oxidative environments.10<\/span>\u00a0MoCo is protected against oxidation to Form B and enzymatic degradation to urothione by binding to specific binding proteins11<\/span>\u00a0or by incorporation into one of the apoproteins of the four known human molybdoenzymes.12,13<\/span>\u00a0The mature MoCo is directly incorporated as a prosthetic group into the nascent apoproteins of sulphite oxidase and of the mitochondrial amidoxime reducing component. The incorporation of MoCo is effectively required for mitochondrial import and retention of sulphite oxidase (SO).12<\/span>\u00a0In contrast, MoCo must be modified by MoCo sulphurase before it can bind as cofactor to the apoproteins of xanthine oxidoreductase (XOR) or aldehyde oxidase (AO).14<\/span><\/p>\n Clinical presentation of molybdenum cofactor deficiency<\/p>\n Defects in MoCo synthesis lead to deficiencies of all molybdoenzymes, except for defects in MoCo sulphurase, which only affect XOR and AO (Figure 2<\/span><\/em>). By far, the most relevant consequence of MoCD is the resulting lack of SO activity. Isolated sulphite oxidase deficiency (ISOD), due to genetic variants in the\u00a0SUOX<\/span>\u00a0gene, causes a distinctive infantile neurological syndrome that was first described in 1967.15,16<\/span>\u00a0MoCD was first described in 1978.17<\/span>\u00a0Clinically, ISOD and MoCD present with the same symptoms and complications caused by sulphite accumulation, except those patients with MoCD can, in addition, develop nephrolithiasis due to impaired XOR activity.18<\/span><\/p>\n MoCD and ISOD are ultra-rare diseases, with published reports of fewer than 200 patients with MoCD19\u201321<\/span>\u00a0and fewer than 100 with ISOD.22<\/span>\u00a0Biallelic pathogenic variants in\u00a0MOCS1<\/span>\u00a0impair the production of cPMP and are classified as MoCD type A (MoCD-A), which is the most commonly diagnosed subtype of MoCD. Most other cases are MoCD type B (MoCD-B), caused by\u00a0MOCS2<\/span>\u00a0variants23<\/span>\u00a0or, rarely,\u00a0MOCS3<\/span>\u00a0mutations.24,25<\/span>\u00a0Only a few cases of MoCD type C (MoCD-C) (caused by\u00a0GPHN<\/span>\u00a0mutations) have been found.26<\/span>\u00a0There are no published data on the incidence of MoCD available. From the author\u2019s own unpublished observation, the prevalence at birth is at least 1 in 200,000 newborns in the UK.<\/p>\n Pre-natal brain abnormalities have been reported in a small number of infants with MoCD, including subcortical cysts, dysgenesis of the corpus callosum,27<\/span>\u00a0as well as enlarged lateral ventricles and, more commonly, an enlarged cisterna magna and cerebellar hypoplasia.28,29<\/span>\u00a0These abnormalities indicate that early brain injury occurs in some cases from mid-gestation onwards.29<\/span>\u00a0Mild brain oedema has been identified in some foetuses from 36 weeks of gestation,28<\/span>\u00a0leading to consideration of early delivery for affected infants.28,30<\/span>\u00a0Protection from accumulating sulphite through trans-placental maternal clearance30,31<\/span>\u00a0ceases at birth, resulting in fulminant encephalopathy in infants with severe MoCD, which manifests within hours to days after delivery. Clinical signs are feeding difficulties, irritability, exaggerated startle reactions, decreased consciousness, apnoea, seizures, axial hypotonia and limb hypertonia.18<\/span>\u00a0Newborns can manifest with moderate metabolic acidosis and increased lactate, as well as with hypoglycaemia. Other routinely performed biochemical investigations are typically normal. Brain magnetic resonance imaging (MRI) at this early stage of disease reveals generalized oedema and widespread restricted diffusion, indicating neuronal damage.20<\/span>\u00a0The presentation is similar to that of hypoxic brain injury, for which it may be mistaken.<\/p>\n After the neonatal period, surviving infants develop a relatively uniform phenotype. They continue to be irritable, have myoclonic and generalized seizures, and develop severe spastic cerebral palsy with intermittent dystonic crises. They typically have profound developmental delay and their brain is atrophic with acquired porencephaly and microcephaly. Children are at high risk of secondary complications from seizures, aspiration, and lower respiratory tract infections. Older children display marfanoid features, including lens dislocation, and a few cases of urolithiasis due to xanthinuria have been reported.18,32<\/span>\u00a0Their reported median age at death is between 2.4 years21\u00a0<\/span>and 3.0 years,19<\/span>\u00a0and is dependent on the severity of illness but also on the extent of medical care provided.<\/p>\n Attenuated disease manifestations are being increasingly reported and likely represent a milder biochemical defect. Symptom onset may occur during infancy or childhood and can be of insidious or sudden onset, with dystonia, spasticity, and varying degrees of developmental delay.33,34<\/span>\u00a0There is a genotype-phenotype correlation; a few\u00a0MOCS1<\/span>\u00a0and\u00a0MOCS2<\/span>\u00a0variants are associated with later disease onset and milder clinical manifestations,21,35<\/span>\u00a0including less pronounced biochemical abnormalities. There is, however, no overlap between healthy individuals and those with untreated MoCD.21<\/span><\/p>\n Biochemical derangement in molybdenum cofactor deficiency and clinically relevant biomarkers<\/p>\n Most known cases of MoCD are of neonatal onset and are associated with severe biochemical disturbance and disease, including early acute encephalopathy and seizures. MoCD-A and MoCD-B largely overlap in their clinical and biochemical features and can only be distinguished by genetic assessment or measuring urinary cPMP concentrations in a research laboratory.26, 36<\/span><\/p>\n SO converts highly reactive sulphite into inert sulphate (Figure 2<\/span><\/em>).15<\/span>\u00a0SO deficiency leads to sulphite accumulating, resulting in a multitude of secondary biochemical abnormalities. As a small molecule, sulphite readily moves between body compartments and is found in increased concentrations in all body fluids.18<\/span>\u00a0Increased sulphite in urine can be easily detected using sulphite test strips; however, these are not licensed for medical use and can yield false positive and false negative results.18,26<\/span>\u00a0In mitochondria, sulphite can be converted to thiosulphate, which is excreted in urine. Sulphite readily reacts with disulphides, including with free L-cystine, yielding S-sulphocysteine (SSC), which is also renally excreted.37<\/span>\u00a0Thiosulphate and SSC are reliable and stable markers of sulphite accumulation that rise very quickly after birth and can be measured in blood or urine in specialized laboratories. SSC concentrations show great interindividual but little intraindividual variability over time.21<\/span><\/p>\n XOR is involved in removing excess purine bases by converting hypoxanthine to xanthine and eventually to urate (Figure 2<\/span><\/em>). The lack of XOR activity causes xanthine and hypoxanthine to accumulate, as well as greatly reducing urate production. Plasma urate is in equilibrium with the maternal circulation during pregnancy, and plasma concentrations in neonates with MoCD can remain within the normal range during the first few days of life until renal clearance has removed all maternal urate from their circulation.18<\/span>\u00a0Plasma urate is normal in ISOD and can be decreased but present in milder MoCD.21<\/span>\u00a0Plasma or urinary purine metabolites are a more sensitive marker of compromised XOR activity and can be measured in specialized metabolic laboratories.18<\/span><\/p>\n Current hypotheses of pathomechanisms in molybdenum cofactor deficiency<\/p>\n MoCD is a primarily neurological disease affecting neurons in the central and peripheral nervous system. The disease course suggests acute neuronal energetic failure and research has shown that mitochondrial energy metabolism is directly impaired by accumulating sulphite.38<\/span>\u00a0Sulphite increases oxidative stress and reduces adenosine triphosphate (ATP) synthesis by directly inhibiting glutamate hydrogenase in mitochondria that respire on glutamate hydrogenase.39<\/span>\u00a0Sulphite can disrupt mitochondrial integrity and function,40<\/span>\u00a0and mitochondrial respiration was found to be impaired in a\u00a0MOCS1<\/span>\u00a0defective cell line41<\/span>\u00a0and in cells of SO-deficient mice.42<\/span>\u00a0In addition, a specific pathomechanism has been proposed that can explain the apparent peracute neuronal failure after birth: SSC is a strong N-methyl-D-aspartate (NMDA) receptor agonist,43,44<\/span>\u00a0and leads to excitotoxic neuronal cell death and seizures.45\u201347<\/span>\u00a0It is therefore likely that, in MoCD, the encephalopathic crisis after birth is caused by an excitotoxic storm that leads to widespread neuronal necrosis.<\/p>\n Cleavage of protein disulphide bridges by sulphite alters the tertiary structure and function of enzymes and structural proteins.37<\/span>\u00a0Cleavage of such structural proteins disrupts connective tissue, causing marfanoid features and lens dislocation.18<\/span><\/p>\n Direct inhibition of the enzyme alpha-amino adipic semialdehyde (AASA) by sulphite has been demonstrated and leads to a secondary deficiency of bioavailable pyridoxal 5\u2032<\/span>-phosphate, which may contribute to hyperexcitability and seizure activity.48, 49<\/span><\/p>\n Development of cyclic pyranopterin monophosphate treatment for molybdenum cofactor deficiency type A<\/p>\n Preclinical work<\/p>\n Until recently, there was no causal treatment for MoCD. Affected children have benefitted from symptomatic and supportive treatment. Sulphite accumulation can be reduced with dietary restriction of sulphur-containing amino acids,50<\/span>\u00a0but any attempts to modify the course of disease in severe MoCD, for example with administration of ammonium molybdate, sulphate, thiamine, D-penicillamine, mesna and tetrahydrobiopterin, were unsuccessful. Supplementation with pyridoxine has been suggested in milder cases of MoCD.51<\/span><\/p>\n Replacing the lacking MoCo has been tried for decades. Directly replacing isolated MoCo or MPT is hampered by their lability in aerobic environments. However, co-cultivation of cells from different MoCD patients could restore the concentration of MPT and activity of molybdoenzymes in some cell lines, suggesting the presence of a diffusible intermediate in cofactor synthesis and the existence of a stable precursor to MPT.52<\/span>\u00a0Although this precursor was identified in 1993,4<\/span>\u00a0it took another 10 years to establish its exact structure.5<\/span>\u00a0Creating a transgenic\u00a0MOCS1<\/span>\u00a0defective mouse model,53<\/span>\u00a0and the recombinant overexpression of cPMP-producing enzymes in Escherichia coli eventually allowed testing the previously proposed substrate replacement with cPMP\u00a0in vivo<\/span>. E. coli-derived and purified cPMP (rcPMP) given to\u00a0MOCS1<\/span>\u00a0-\/- mice within 5 days after birth and then every 3 days per transabdominal intrahepatic injection ensured survival and normalisation of xanthine and SSC. Liver MPT levels reconstituted to maximally 16% of wild type on the day of injection. SO activity reached a maximum of 26% and XOR reached around 40% of wild-type activity on day 1 after injection. Stopping the treatment led to behavioural deterioration after 6 days of withdrawal.54<\/span><\/p>\n First-in-human use and administration of cyclic pyranopterin monophosphate on named patient basis<\/p>\n The administration of rcPMP to humans with MoCD started in 2008 on a named patient basis as an experimental treatment and according to a strict prospective observational protocol designed by Orphatec Pharmaceuticals GmbH (later renamed Colbourne Pharmaceuticals GmbH), a spin-off from the original research group.55<\/span>\u00a0The first child with MoCD-A was 36 days old when treatment started, and responded within a few days with normalized biomarkers and clinical improvement. However, the sequelae of post-natal neuronal injury could not be averted, and the child later showed typical signs of severe cerebral palsy.36<\/span>\u00a0The second infant with MoCD-A was treated in 2009 under similar circumstances but treatment was started earlier, at the age of 7 days, and prior to developing seizures. This child showed normalized biomarkers and normal early development.56<\/span>\u00a0A third child was treated in 2010 from the age of 5 days. This child was already severely encephalopathic at the start of treatment and, while biomarkers improved as expected, the child developed typical sequelae of brain injury and severe cerebral palsy.56,57<\/span>\u00a0A further two neonates were treated in 201058<\/span>\u00a0and 2011, within hours after birth, and both showed moderate developmental delay in childhood.28<\/span>\u00a0No adverse reactions to treatment were observed under continued cPMP administration for these and a few other children.59<\/span><\/p>\n In September 2010, prompted by these encouraging results, the European Commission granted orphan designation for cPMP for the treatment of MoCD-A to Orphatec Pharmaceuticals GmbH.60<\/span>\u00a0In 2011, Alexion Pharma International Sarl acquired the cPMP programme and started chemically synthesizing cPMP. The synthetically derived cPMP, fosdenopterin, has identical properties to the naturally occurring rcPMP.61<\/span>\u00a0In March 2013, the European Medicines Agency transferred sponsorship to Alexion Europe SAS, who also received breakthrough therapy designation from the US Food and Drug Administration (FDA) in the same year. Origin Biosciences Inc acquired the late stage cPMP therapy programme in 2018 and, in February 2021, the FDA approved fosdenopterin for the treatment of MoCD-A in the USA.62<\/span><\/p>\n Clinical trials and treatment experience with cyclic pyranopterin monophosphate<\/p>\n The circumstances and outcomes of the first cohort of 16 infants treated with recombinant cPMP on a named patient basis and following a prospective monitoring protocol were published in 2015.59<\/span>\u00a0Five of those infants were siblings of previous index cases, including one child with MoCD-B. Eleven others were diagnosed based on their symptoms after birth and, of those, seven were affected with MoCD-A and four with MoCD-B. Treatment in all five infants with MoCD-B was terminated after lack of response to cPMP substitution. Treatment was also discontinued in five of the 11 infants with MoCD-A who showed a biochemical response but whose disease had progressed to an extent that cPMP substitution was deemed futile by the treating physicians and parents.<\/p>\n From 2011, Alexion Pharma International Sarl had designed and sponsored a series of international clinical trial protocols to develop cPMP therapy (Table 1<\/span><\/em>),63-66\u00a0<\/span>which were later continued by Origin Biosciences Inc. Studies ALX-MCD-501 (NCT01640717)63<\/span>\u00a0and ALXN1101-MCD-101 (NCT01894165)64<\/span>\u00a0have been completed. The trials ORGN001-MCD-201 (NCT02047461, EudraCT 2013-002701-56)65<\/span>\u00a0and ORGN001-MCD-202 (NCT02629393, EudraCT 2013-002702-30)66<\/span>\u00a0are ongoing. Data from these trials have not yet been made publicly available, apart from excerpts and summaries contained in the prescribing information for fosdenopterin hydrobromide dihydrate (Nulibry\u2122, Origin Biosciences, Boston, MA, USA)67<\/span>\u00a0and a preliminary poster presentation at a scientific meeting.68<\/span><\/p>\n Six patients from the original cohort and two additional patients who were treated later remained on rcPMP treatment until they were switched to fosdenopterin. This switch was monitored under clinical trial protocol ORGN001-MCD-201, which also included a dose escalation exercise with pharmacokinetic and pharmacodynamic assessments.65<\/span>\u00a0Another patient was started on fosdenopterin under the trial protocol ORGN001-MCD-202,66<\/span>\u00a0bringing the total of long-term treated patients to nine. Survival of all infants treated with cPMP was improved compared to matched untreated controls.67<\/span><\/p>\n Dosage of cyclic pyranopterin monophosphate<\/p>\n <\/a>Intravenously administered cPMP has an elimination half-life of 1.2\u20131.7 hours, with a proportion of renal clearance of around 40%.67<\/span>\u00a0A large proportion of cPMP is oxidized non-enzymatically to compound Z, and a smaller proportion is converted to MPT and eventually to MoCo. Some mature MoCo is incorporated into molybdoenzymes and some degrades to Form B or urothione, which are lost in urine. Estimated daily molybdenum requirements for healthy adults are 25 \u00b5g.69<\/span>\u00a0The European Society for Paediatric Gastroenterology Hepatology and Nutrition and The European Society for Clinical Nutrition and Metabolism recommend 0.25 \u00b5g\/kg\/day of molybdenum in term infants and children receiving long-term parenteral nutrition.70<\/span>\u00a0Assuming this is sufficient to produce adequate amounts of mature MoCo, equivalent cPMP requirements would be around 100 \u00b5g\/day in adults and 1\u20132 \u00b5g\/kg\/day in infants. Pharmacological dosing requirements of the precursor cPMP are dependent on mode and frequency of administration and are likely higher when cPMP is given as intermittent intravenous boluses. Schwarz et al. suggested an intravenous treatment dose for cPMP of 100 \u00b5g\/kg twice weekly, based on their original mouse experimentation and the observed biological half-life of holo-molybdoenzymes,54<\/span>\u00a0which is a highly relevant parameter to consider when establishing dosing intervals.<\/p>\n![\"\"](\"https:\/\/touchneurology.com\/wp-content\/uploads\/sites\/3\/2021\/12\/Table-1.png\")
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