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Sapropterin Dihydrochloride Synthesis Essay

Clinical data
License data
  • US:C (Risk not ruled out)
Routes of
By mouth
ATC code
Legal status
Legal status
Pharmacokinetic data
Biological half-life4 hours (healthy adults)
6–7 hours (PKU patients)

IUPAC name

  • (6R)-2-Amino-6-[(1R,2S)-1,2-dihydroxypropyl]-5,6,7,8-tetrahydropteridin-4(1H)-one
CAS Number
Chemical and physical data
Molar mass241.25 g/mol
3D model (JSmol)


  • O=C2\N=C(/NC=1NC[C@@H](NC=12)[C@@H](O)[C@@H](O)C)N


  • InChI=1S/C9H15N5O3/c1-3(15)6(16)4-2-11-7-5(12-4)8(17)14-9(10)13-7/h3-4,6,12,15-16H,2H2,1H3,(H4,10,11,13,14,17)/t3-,4+,6-/m0/s1 Y
 NY (what is this?)  (verify)

Tetrahydrobiopterin (BH4, THB), also known as sapropterin, is a naturally occurring essential cofactor of the three aromatic amino acid hydroxylase enzymes, used in the degradation of amino acid phenylalanine and in the biosynthesis of the neurotransmittersserotonin (5-hydroxytryptamine, 5-HT), melatonin, dopamine, norepinephrine (noradrenaline), epinephrine (adrenaline), and is a cofactor for the production of nitric oxide (NO) by the nitric oxide synthases.[1] Chemically, its structure is that of a reduced pteridine derivative.

Medical use[edit]

Tetrahydrobiopterin is available as a tablet for oral administration in the form of tetrahydrobiopterin dihydrochloride (BH4*2HCL).[2] BH4*2HCL is FDA approved under the trade name Kuvan. The typical cost of treating a patient with Kuvan is $100,000 per year.[3]BioMarin holds the patent for Kuvan until at least 2024, but Par Pharmaceutical has a right to produce a generic version by 2020.[4]

BH4*2HCL is indicated in tetrahydrobiopterin deficiency caused by GTPCH deficiency or PTPS deficiency.[5] Also, BH4*2HCL is FDA approved for use in phenylketonuria, along with dietary measures.[6] Most people with phenylketonuria, however, have little or no benefit of BH4*2HCL.[7]

Adverse effects[edit]

The most common adverse effects, observed in more than 10% of patients, include headache and a running or obstructed nose. Diarrhea and vomiting are also relatively common, seen in at least 1% of patients.[8]


No interaction studies have been conducted. Because of its mechanism, tetrahydrobiopterin might interact with dihydrofolate reductase inhibitors like methotrexate and trimethoprim, and NO-enhancing drugs like nitroglycerin, molsidomine, minoxidil, and PDE5 inhibitors. Combination of tetrahydrobiopterin with levodopa can lead to increased excitability.[8]


Tetrahydrobiopterin has the following responsibilities as a cofactor:

Tetrahydrobiopterin has multiple roles in human biochemistry. One is to convert amino acids such as phenylalanine, tyrosine, and tryptophan to precursors of dopamine and serotonin, major monoamine neurotransmitters. Due to its role in the conversion of L-tyrosine into L-dopa, which is the precursor for dopamine, a deficiency in tetrahydrobiopterin can cause severe neurological issues unrelated to a toxic buildup of L-phenylalanine; dopamine is a vital neurotransmitter, and is the precursor of norepinephrine and epinephrine. Thus, a deficiency of BH4 can lead to systemic deficiencies of dopamine, norepinephrine, and epinephrine. In fact, one of the primary conditions that can result from GTPCH-related BH4 deficiency is dopamine-responsive dystonia;[9] currently, this condition is typically treated with carbidopa/levodopa, which directly restores dopamine levels within the brain.

BH4 also serves as a catalyst for the production of nitric oxide. Among other things, nitric oxide is involved in vasodilation, which improves systematic blood flow. The role of BH4 in this enzymatic process is so critical that some research points to a deficiency of BH4 – and thus, of nitric oxide – as being a core cause of the neurovascular dysfunction that is the hallmark of circulation-related diseases such as diabetes.[10]


Tetrahydrobiopterin was discovered to play a role as an enzymatic cofactor. The first enzyme found to use tetrahydrobiopterin is phenylalanine hydroxylase (PAH).[11]

Biosynthesis and recycling[edit]

Tetrahydrobiopterin is biosynthesized from guanosine triphosphate (GTP) by three chemical reactions mediated by the enzymes GTP cyclohydrolase I (GTPCH), 6-pyruvoyltetrahydropterin synthase (PTPS), and sepiapterin reductase (SR).[12]

BH4 is catabolized to BH3 and BH2, which are pro-oxidants (free radicals). Research shows that ascorbic acid (also known as ascorbate or vitamin C) is effective at recycling BH3 into BH4,[13] preventing the BH3 radical from reacting with other free radicals (superoxide and peroxynitrite specifically). Without this recycling process, uncoupling of the endothelial nitric oxide synthase (eNOS) enzyme and reduced bioavailability of the vasodilatornitric oxide occur, creating a form of endothelial dysfunction.[14] Ascorbic acid is oxidized to dehydroascorbic acid during this process, although it can be recycled back to ascorbic acid.


Other than PKU studies, tetrahydrobiopterin has participated in clinical trials studying other approaches to solving conditions resultant from a deficiency of tetrahydrobiopterin. These include autism, ADHD, hypertension, endothelial dysfunction, and chronic kidney disease.[15][16] Experimental studies suggest that tetrahydrobiopterin regulates deficient production of nitric oxide in cardiovascular disease states, and contributes to the response to inflammation and injury, for example in pain due to nerve injury. A 2015 BioMarin-funded study of PKU patients found that those who responded to tetrahydrobiopterin also showed a reduction of ADHD symptoms.[17]


In 1997, a small pilot study was published on the efficacy of tetrahydrobiopterin (BH4) on relieving the symptoms of autism, which concluded that it "might be useful for a subgroup of children with autism" and that double-blind trials are needed, as are trials which measure outcomes over a longer period of time.[18] In 2010, Frye et al. published a paper which concluded that it was safe, and also noted that "several clinical trials have suggested that treatment with BH4 improves ASD symptomatology in some individuals."[19]

Cardiovascular disease[edit]

Since nitric oxide production is important in regulation of blood pressure and blood flow, thereby playing a significant role in cardiovascular diseases, tetrahydrobiopterin is a potential therapeutic target. In the endothelial cell lining of blood vessels, endothelial nitric oxide synthase is dependent on tetrahydrobiopterin availability.[20] Increasing tetrahydrobiopterin in endothelial cells by augmenting the levels of the biosynthetic enzyme GTPCH can maintain endothelial nitric oxide synthase function in experimental models of disease states such as diabetes,[21] atherosclerosis, and hypoxic pulmonary hypertension.[22] However, treatment of patients with existing coronary artery disease with oral tetrahydrobiopterin is limited by oxidation of tetrahydrobiopterin to the inactive form, dihydrobiopterin, with little benefit on vascular function.[23]

See also[edit]


External links[edit]

  1. ^Całka, Jarosław (2006). "The role of nitric oxide in the hypothalamic control of LHRH and oxytocin release, sexual behavior and aging of the LHRH and oxytocin neurons". Folia Histochemica et Cytobiologica. 44 (1): 3–12. PMID 16584085. 
  2. ^Schaub J, Däumling S, Curtius HC, Niederwieser A, Bartholomé K, Viscontini M, Schircks B, Bieri JH (1978). "Tetrahydrobiopterin therapy of atypical phenylketonuria due to defective dihydrobiopterin biosynthesis". Arch. Dis. Child. 53 (8): 674–6. doi:10.1136/adc.53.8.674. PMC 1545051. PMID 708106. 
  3. ^Matthew Herper (2016-07-28). "How Focusing On Obscure Diseases Made BioMarin A $15 Billion Company". Forbes. Retrieved 2017-10-09. 
  4. ^"BioMarin Announces Kuvan (sapropterin dihydrochloride) Patent Challenge Settlement". PR Newswire. 2017-04-13. Retrieved 2017-10-09. 
  5. ^"Tetrahydrobiopterin Deficiency". National Organization for Rare Disorders (NORD). Retrieved 2017-10-09. 
  6. ^"What are common treatments for phenylketonuria (PKU)?". NICHD. 2013-08-23. Retrieved 12 September 2016. 
  7. ^Camp, Kathryn M.; Parisi, Melissa A.; Acosta, Phyllis B.; Berry, Gerard T.; Bilder, Deborah A.; Blau, Nenad; Bodamer, Olaf A.; Brosco, Jeffrey P.; Brown, Christine S.; Burlina, Alberto B.; Burton, Barbara K.; Chang, Christine S.; Coates, Paul M.; Cunningham, Amy C.; Dobrowolski, Steven F.; Ferguson, John H.; Franklin, Thomas D.; Frazier, Dianne M.; Grange, Dorothy K.; Greene, Carol L.; Groft, Stephen C.; Harding, Cary O.; Howell, R. Rodney; Huntington, Kathleen L.; Hyatt-Knorr, Henrietta D.; Jevaji, Indira P.; Levy, Harvey L.; Lichter-Konecki, Uta; Lindegren, Mary Lou; et al. (2014). "Phenylketonuria Scientific Review Conference: State of the science and future research needs". Molecular Genetics and Metabolism. 112 (2): 87–122. doi:10.1016/j.ymgme.2014.02.013. PMID 24667081. 
  8. ^ abHaberfeld, H, ed. (1 March 2017). Austria-Codex (in German). Vienna: Österreichischer Apothekerverlag. Kuvan 100 mg-Tabletten. 
  9. ^"Genetics Home Reference: GCH1". National Institutes of Health. 
  10. ^Wu, Guoyao; Meininger, Cynthia J. (2009). "Nitric oxide and vascular insulin resistance". BioFactors. 35 (1): 21–7. doi:10.1002/biof.3. PMID 19319842. 
  11. ^Kaufman, S (1958). "A new cofactor required for the enzymatic conversion of phenylalanine to tyrosine". The Journal of Biological Chemistry. 230 (2): 931–9. PMID 13525410. 
  12. ^Thöny, Beat; Auerbach, Günter; Blau, Nenad (2000). "Tetrahydrobiopterin biosynthesis, regeneration and functions". Biochemical Journal. 347: 1–16. doi:10.1042/0264-6021:3470001. PMC 1220924. PMID 10727395. 
  13. ^Kuzkaya, N.; Weissmann, N.; Harrison, D. G.; Dikalov, S. (2003). "Interactions of Peroxynitrite, Tetrahydrobiopterin, Ascorbic Acid, and Thiols: Implications For Uncoupling Endothelial Nitric-Oxide Synthase". Journal of Biological Chemistry. 278 (25): 22546–54. doi:10.1074/jbc.M302227200. PMID 12692136. 
  14. ^Muller-Delp, J. M. (2009). "Ascorbic acid and tetrahydrobiopterin: looking beyond nitric oxide bioavailability". Cardiovascular Research. 84 (2): 178–9. doi:10.1093/cvr/cvp307. PMID 19744948. 
  15. ^ClinicalTrials.gov: Search results for Kuvan
  16. ^"BioMarin Initiates Phase 3b Study to Evaluate the Effects of Kuvan on Neurophychiatric Symptoms in Subjects with PKU". BioMarin Pharmaceutical Inc. 17 August 2010. 
  17. ^Burton, B.; Grant, M.; Feigenbaum, A.; Singh, R.; Hendren, R.; Siriwardena, K.; Phillips, J.; Sanchez-Valle, A.; Waisbren, S.; Gillis, J.; Prasad, S.; Merilainen, M.; Lang, W.; Zhang, C.; Yu, S.; Stahl, S. (2015). "A randomized, placebo-controlled, double-blind study of sapropterin to treat ADHD symptoms and executive function impairment in children and adults with sapropterin-responsive phenylketonuria". Molecular Genetics and Metabolism. 114 (3): 415–24. doi:10.1016/j.ymgme.2014.11.011. PMID 25533024. 
  18. ^Fernell, Elisabeth; Watanabe, Yasuyoshi; Adolfsson, Ingrid; Tani, Yoshihiro; Bergström, Mats; Phd, Per Hartvig; Md, Anders Lilja; Phd., Anne-Liis von Knorring MD.; Phd., Christopher Gillberg MD.; Phd., Bengt Lángström (2008). "Possible effects of tetrahydrobiopterin treatment in six children with autism - clinical and positron emission tomography data: A pilot study". Developmental Medicine & Child Neurology. 39 (5): 313–8. doi:10.1111/j.1469-8749.1997.tb07437.x. PMID 9236697. 
  19. ^Frye, Richard E.; Huffman, Lynne C.; Elliott, Glen R. (2010). "Tetrahydrobiopterin as a novel therapeutic intervention for autism". Neurotherapeutics. 7 (3): 241–9. doi:10.1016/j.nurt.2010.05.004. PMC 2908599. PMID 20643376. 
  20. ^Channon, Keithm. (2004). "Tetrahydrobiopterin". Trends in Cardiovascular Medicine. 14 (8): 323–7. doi:10.1016/j.tcm.2004.10.003. PMID 15596110. 
  21. ^Alp, Nicholas J.; Mussa, Shafi; Khoo, Jeffrey; Cai, Shijie; Guzik, Tomasz; Jefferson, Andrew; Goh, Nicky; Rockett, Kirk A.; Channon, Keith M. (2003). "Tetrahydrobiopterin-dependent preservation of nitric oxide–mediated endothelial function in diabetes by targeted transgenic GTP–cyclohydrolase I overexpression". Journal of Clinical Investigation. 112 (5): 725–35. doi:10.1172/JCI17786. PMC 182196. PMID 12952921. 
  22. ^Khoo, J. P.; Zhao, L; Alp, N. J.; Bendall, J. K.; Nicoli, T; Rockett, K; Wilkins, M. R.; Channon, K. M. (2005). "Pivotal Role for Endothelial Tetrahydrobiopterin in Pulmonary Hypertension". Circulation. 111 (16): 2126–33. doi:10.1161/01.CIR.0000162470.26840.89. PMID 15824200. 
  23. ^Cunnington, C.; Van Assche, T.; Shirodaria, C.; Kylintireas, I.; Lindsay, A. C.; Lee, J. M.; Antoniades, C.; Margaritis, M.; Lee, R.; Cerrato, R.; Crabtree, M. J.; Francis, J. M.; Sayeed, R.; Ratnatunga, C.; Pillai, R.; Choudhury, R. P.; Neubauer, S.; Channon, K. M. (2012). "Systemic and Vascular Oxidation Limits the Efficacy of Oral Tetrahydrobiopterin Treatment in Patients with Coronary Artery Disease". Circulation. 125 (11): 1356–66. doi:10.1161/CIRCULATIONAHA.111.038919. PMC 5238935. PMID 22315282. 

Criteria for considering studies for this review

Types of studies

Randomized controlled trials, published and unpublished.

Types of participants

Children and adults with PKU due to PAH deficiency, who are responsive to sapropterin dihydrochloride. Individuals with PKU due to primary defect in BH4 metabolism will be excluded.

Types of interventions

Oral supplementation of sapropterin (in any dose, frequency or duration) compared with no supplementation or placebo. This intervention can be used either in combination with, or instead of, a phenylalanine-restricted diet.

Types of outcome measures

Primary outcomes
  1. Change in blood phenylalanine concentration

Secondary outcomes
  1. Adverse events which may be associated with sapropterin

  2. Validated quality of life measures (e.g. Profile of Quality of Life in Chronically Ill (PLC))

  3. Validated measures of Intelligence and neuro-psychometric performance (e.g. Wechsler Intelligence Scales)

  4. Measures of nutritional status and growth

  5. Change in protein (phenylalanine) tolerance (assessed by giving a standard amount of protein or phenylalanine and measuring the level of blood phenylalanine; increase in tolerance is defined when the protein or phenylalanine intake does not increase the blood phenylalanine level)

Search methods for identification of studies

Electronic searches

We identified relevant trials from the Group's Inborn Errors of Metabolism Trials Register using the terms: kuvan OR ohenoptin OR sapropterin.

The Inborn Errors of Metabolism Trials Register is compiled from electronic searches of the Cochrane Central Register of Controlled Trials (CENTRAL) (updated each new issue of The Cochrane Library), weekly searches of MEDLINE and the prospective handsearching of one journal - Journal of Inherited Metabolic Disease. Unpublished work was identified by searching through the abstract books of the Society for the Study of Inborn Errors of Metabolism conference and the SHS Inborn Error Review Series. For full details of all searching activities for the register, please see the relevant section of the Cystic Fibrosis and Genetic Disorders Group Module.

Date of the latest search of the Group's Inborn Errors of Metabolism Register: 11 August 2014.

Additionally we undertook searches of the following registers on 4 September 2014 (see Appendices):

  1. ClinicalTrials.gov

  2. Current controlled trials

Searching other resources

We contacted the manufacturers of the drug (BioMarin Pharmaceutical Inc. and Merck KgaA) for information regarding any unpublished trials.

Data collection and analysis

Selection of studies

Two authors, US and MM, assessed the trials independently for inclusion in the review. We planned to resolve any disagreements that may arise through discussion. There were no disagreements between the authors.

Data extraction and management

We independently extracted the data from eligible trials using a trial selection and data extraction form modified for this review.

We planned to group outcome data into those measured at two, four and six weeks, monthly up to one year, and every three months thereafter. If data were reported at other time periods we planned to include them also. We also planned to contact authors for possible measurements of outcome data at other time periods and if available include these also in the analysis.

The data was reported at three weeks and six weeks for change in phenylalanine concentration and at ten weeks for change in protein (phenylalanine) tolerance. We contacted the authors for the outcome data measured at other time points but have not received any reply.

Assessment of risk of bias in included studies

We assessed the risk of bias of the included trials using the domain-based evaluation as described in the Cochrane Handbook for Systematic Reviews of Intervention (Higgins 2011)

We assessed the following domains as having either a low, unclear or high risk of bias:

  1. randomisation;

  2. concealment of allocation;

  3. blinding of participants, personnel and outcome assessors;

  4. incomplete outcome data;

  5. selective outcome reporting.

We also assessed the trials for other potential sources of bias.

Measures of treatment effect

For binary outcomes we measured the treatment effect as risk ratios (RR) with 95% confidence intervals (95% CIs). For continuous outcomes with outcome measurements on the same scale, we presented the results as mean differences (MDs) with 95% CIs. Where the continuous outcomes were measured using different scales, we used the standardised mean difference (SMD).

Unit of analysis issues

We also planned to include results from eligible cross-over trials using methods recommended by Elbourne (Elbourne 2002). In order to allow an intention-to-treat analysis we planned to seek data on the number of participants by allocated treatment group, irrespective of compliance and whether or not the participant was later thought to be ineligible or otherwise excluded from treatment. However there were no eligible cross-over trials to be included in the review.

Dealing with missing data

In order to do a more complete review we contacted the drug manufacturers (Biomarin Pharmaceutical Inc.) of the included trials for data; as yet we have not received any reply.

For the outcome 'Change in blood phenylalanine concentration', although data was measured at more frequent intervals in one trial it was only reported at six weeks as end-point data (Levy 2007). In the second trial, we could not use the data reported at weekly intervals for three weeks, as they only gave the details in sapropterin group (Trefz 2009). Also, both the trials measured nutritional status but this outcome was not reported by either.

Assessment of heterogeneity

We planned to quantify the impact of statistical heterogeneity in the meta-analysis using a measure (I2) of the degree of inconsistency in the trials' results. This measure describes the percentage of total variation across trials that is due to heterogeneity rather than chance. The values of I2 lie between 0% and 100%, and a simplified categorization of heterogeneity that we plan to use is as follows (Higgins 2003):

  • 0% to 40% : might not be important;

  • 30% to 60% : may represent moderate heterogeneity;

  • 50% to 90% : may represent substantial heterogeneity;

  • 75% to 100% : considerable heterogeneity.

Since the two included trials reported data at different time points, we could not combine any data.

Assessment of reporting biases

If we are able to include 10 or more trials in a future update, we plan to use a funnel plot to assess whether the review is subject to publication bias. If asymmetry is detected we also planned to assess other possible causes such as selection bias, reporting bias, true heterogeneity and artefact. Since the review included only two trials we could not assess whether the review was subject to publication bias.

In order to assess outcome reporting bias, we compared the protocols of the trials (available via ClinicalTrials.gov) to the published reports.

Data synthesis

As we did not identify any significant heterogeneity (we were not able to combine any data), we have analysed data using a fixed-effect analysis.

Subgroup analysis and investigation of heterogeneity

In future, if we find sources of heterogeneity and if we are able to include 10 or more trials, we plan to conduct meta-analysis by subgroups, and stratify participants according to:

  1. Severity of PKU at baseline (classic PKU: phenylalanine > 1200 μmol/L ; mild PKU: phenylalanine 600 to 1200 μmol/L; and mild HPA: phenylalanine < 600 μmol/L but more than the upper reference limit);

  2. Dosage of sapropterin dihydrochloride used (10 mg/kg or 20 mg/kg).

Sensitivity analysis

We planned to perform sensitivity analyses to determine the impact of risk of bias in trials on outcome, including and excluding trials with a high risk of bias regarding methods of treatment allocation. If there were any eligible crossover trials, we planned to conduct sensitivity analyses including and excluding these.

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