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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
Neurotherapeutics. Author manuscript; available in PMC 2011 July 1.
Published in final edited form as:
PMCID: PMC2908599

Tetrahydrobiopterin as a novel therapeutic intervention for autism

Richard E. Frye, M.D., Ph.D.,* Lynne C. Huffman, M.D., and Glen R. Elliott, Ph.D., M.D.δ


Tetrahydrobiopterin (BH4) is an essential cofactor for several critical metabolic pathways that have been reported to be abnormal in autism spectrum disorder (ASD). In addition, the cerebrospinal fluid concentration of BH4is reported to be depressed in children with ASD. Over the past 25 years, several clinical trials have suggested that treatment with BH4improves ASD symptomatology in some individuals. Two ongoing clinical protocols may help further define the efficacy of BH4 treatment in children with ASD. First, children with ASD who had low concentrations of cerebrospinal fluid or urine pterins were treated in an open-label manner with 20mg/kg/day of BH4. The majority (63%) of children responded positively to treatment with minimal adverse effects. Second, a double-blind placebo-controlled study examining the efficacy of 20mg/kg/day of BH4 treatment in children with ASD is currently underway. Safety studies from the commercially available forms of BH4 document the low incidence of adverse effects, particularly serious adverse effects. Studies have also documented the ability of BH4 to cross the blood brain barrier. Given the importance of BH4 in neurodevelopmental metabolic pathways, the safety of BH4 treatment, and the evidence for a therapeutic benefit of BH4 treatment in children with ASD, we believe that BH4 represents a novel therapy for ASD that may gain wider use after further clinical studies have established efficacy and treatment guidelines.

Keywords: Tetrahydrobiopterin, autism spectrum disorder, monoamine neurotransmitters, nitric oxide, treatment

1. Introduction

Although many studies have clearly linked autism spectrum disorder (ASD) to central nervous system (CNS) and systemic biological abnormalities, most autism treatment research has focused on behavioral interventions.1 Behavioral treatments are costly, labor-intensive, and may progress slowly.1 In addition, the biologically based treatments that have been evaluated do not appear to alter the course of the disorder.2, 3 Most studies of biological interventions have focused on symptom reduction, targeting behaviors that are associated with ASD—e.g., self-injurious behavior, aggression, hyperactivity, and obsessive-compulsive behavior—as opposed to core features of the disorder.3, 4

Tetrahydrobiopterin or BH4 is a naturally occurring substance that is an essential cofactor for several critical metabolic pathways, including those responsible for the production of monoamine neurotransmitters, the breakdown of phenylalanine, and the production of nitric oxide. Abnormalities in several of these metabolic pathways or the products of these pathways for which BH4 is essential have been noted in individuals with ASD. In addition, abnormalities in the CNS concentrations of BH4 have been reported in individuals with ASD. Over the past 25 years, a limited number of clinical trials have reported encouraging results with BH4 treatment for children with ASD. In this article, we review the evidence supports the idea that children with ASD may have a BH4 deficiency and that treatment with BH4 may be beneficial in children with ASD. In addition, we review the evidence that systemically administered BH4 actually enters the CNS and discuss the safety of the commercially available BH4 products that have regulatory approval.

2. General role of tetrahydrobiopterin in metabolic systems

BH4 is part of the pteridine family – naturally occurring compounds with a heterocyclic pteridine structure. Pterins are pteridine derivatives with a 2-amino-4oxo base. The pterins include BH4, biopterin and neopterin, but only BH4 is biologically active. BH4 is synthesized de novo from guanosine-5'-triphosphate (GTP), a purine nucleotide, by three enzymatic reactions (guanosine-5'-triphosphate cyclohydrolase, 6-pyruvoyltetrahydropterin synthase, and sepiapterin reductase; Figure 1).5 Once BH4 is synthesized, it is used as a cofactor by three aromatic amino acid hydroxylases -- phenylalanine-4-hydroxylase, tyrosine-3-hydroxylase and tryptophan-5-hydroxylase -- to catalyze the conversion of phenyalanine, tyrosine and tryptophan to tyrosine, L-dopa and 5-hydroxytryptophan, respectively (Figure 1). In this process BH4 is converted into 7,8-dihydrobiopterin, which can be recycled to remake BH4 through a salvage pathway catalyzed by dihydropteridine reductase . Some of these products of the hydroxylase enzymes are further converted into neurotransmitters: L-dopa is converted into dopamine, which is further converted into norepinephrine; 5-hydroxytryptophan is converted into serotonin. Additionally, BH4 is essential in the production of nitric oxide, an important second messenger molecule used primarily for communication in vascular and neural tissues. In this reaction, nitric oxide synthase converts L-arginine and oxygen to L-citrulline and nitric oxide (Figure 1).

Figure 1
Simplified pathway for the production and recycling of tetrahydrobiopterin (BH4). Major enzymes, intermediates and metabolites in this pathway are included. Dashed lines represent the enzymatic reactions that use BH4 to produce neurotransmitters and nitric ...

A deficiency in BH4 synthesis or recycling can result in neurological disorders. Hyperphenylalaninemia, specifically phenylketonuria (PKU) type IV, presents at birth with elevated phenylalanine in the blood and represents approximately 1–2% of PKU cases. Severe mental retardation develops if phenylalanine levels are not controlled early in life. BH4 deficiency can also result in dopamine-responsive dystonia, a neurological disorder that demonstrates a wide range of symptoms and ages of presentations. Patients with dopamine-responsive dystonia invariably demonstrate dystonia with diurnal variation.6

3. Evidence for tetrahydrobiopterin depletion in autism spectrum disorder

The role of BH4 in neurodevelopmental disorders, such as ASD, is not well understood. BH4 cerebrospinal fluid (CSF) concentration has been reported to be 42% lower in children with ASD than in neurotypical children.7 Further analysis of the ASD children found younger and older subgroups. When the subgroups were compared to the control group, only the ASD children younger than 7 years old demonstrated significantly lower BH4 concentrations. This younger group also demonstrated a reduction in total pterins and biopterin, essentially indicating that the deficit in BH4 was not due to a synthesis or recycling deficit but a problem with the general availability of pterins.

Although the reason for a reduction of CSF BH4 in ASD has not been explained, there is evidence that metabolic pathways that consume and recycle BH4 are dysfunctional in children with ASD. Several lines of evidence suggest that children with ASD manifest excessive inflammation and overactivation of the immune system.810 As noted earlier, nitric oxide, a key mediator of inflammation and immune response, is produced by a BH4-dependent reaction. Thus, it is possible that BH4 in ASD could be depleted by the overactivation of the immune system and inflammatory processes during an excessive production of nitric oxide. Children with ASD might also have an underlying reduction in BH4 recycling. Folate is necessary for an alternative BH4 salvage pathway the uses the enzyme dihydrofolate reductase. Available folate might be reduced in children with ASD, as folate supports oxidative stress pathways and the oxidative stress pathways seem to be overactive in children with ASD.1114 Lastly, since BH4 can act as a superoxide radical scavenger, it may be depleted in the setting of high oxidative stress states.15, 16

Children with ASD have symptoms not inconsistent with a CNS BH4 deficiency. BH4 deficiency can result in low production of monoamine neurotransmitters, including serotonin, dopamine, and norepinephrine. Many children with ASD have clinical symptoms, such as obsessive-compulsive disorder and anxiety, that are seen in other disorders where serotonin deficiency has been implicated,17 and dysfunction in the serotonin system in ASD has been documented by investigators.13 However, response of children with ASD to selective-serotonin reuptake inhibitors is mixed across studies18, 19, 20 and research suggests that at least a subgroup of children with ASD have elevations in serotonin levels.21 Children with ASD have a high rate of executive function problems, inattention, and hyperactivity, all symptoms suggesting dopamine and norepinephrine deficits.2224 In some children with ASD, such behaviors are mitigated by psychopharmaceuticals designed to increase levels of dopamine and norepinephrine.25, 26 This provides indirect evidence that some children with ASD may have deficits in the production or utilization of monoamine neurotransmitters. However, since ASD is a very heterogeneous disorder, further research will be needed to clarify the potential role of monoamine neurotransmitter deficits in ASD.

4. Previous clinical trials of tetrahydrobiopterin for autism spectrum disorder

In 1984, Japanese researchers reported the first case studies of BH4 treatment in children with ASD.27 Two boys with ASD, one 7- and one 5-year-old, were treated with 1–2.5 mg/kg/day of BH4.28 The 7-year-old showed marked improvement in hyperactivity, mood lability, and stereotypic behaviors; both boys showed moderate improvement in general autistic symptoms. This initial evidence led to further clinical studies on BH4 as a treatment for autistic symptomatology, the majority of which were conducted in Japan. Over a six-year period, from 1985 to 1990, four Japanese researchers studied over 300 mildly to severely affected autistic children in five open-label studies and one double-blind placebo controlled study.2735 Response was measured as the percentage of children who demonstrated moderate or marked global improvement. Treatment with oral BH4 at a dose of 1–3mg/kg/day over 4–24 weeks resulted in a response rate of 41% to 64% (Table 1). Two additional clinical studies, one double-blind placebo controlled crossover and one open-label, were conducted by Swedish researchers using an oral BH4 dose of 3 to 6 mg/kg/day.36, 37 These studies demonstrated limited general improvement in autistic symptoms with BH4 treatment.

Table 1
Clinical Trials Evaluating Tetrahydrobiopterin in Autism

Several studies have pointed to improvement in specific autistic symptoms with BH4 treatment. Nakane et al.34 noted that within the autistic symptoms cluster of the rating scale for abnormal behavior in children (RSABC), the greatest improvement was found in communication, cognitive ability, adaptability, and verbal expression. Fernell et al.37 found specific improvements in social responsiveness (primarily in eye contact and social interactions), communication, and cognitive abilities using the Parental Satisfaction Survey (PASS). Danfors et al.36 demonstrated specific improvement in social interactions using the Childhood Autism Rating Scale (CARS).

Several studies have suggested that specific patient characteristics may be associated with better response to treatment with BH4. In an open-label study, Naruse et al.30 noted that younger children (i.e., under the age of 5 years of age) with ASD were more likely to respond to BH4 treatment than older children (i.e., over the age of 10 years of age) with ASD. In a double-blind placebo-controlled study, Naruse et al.32 noted that patients under 5 years of age, as compared to those over the 5 years of age, had a more robust response to BH4 as compared to placebo. In a double-blind placebo controlled crossover trial, Danfors et al.36 noted that improvement in social interactions on the CARS was positively correlated with the intellectual quotient as measured by the Griffith Developmental Scales.

While the aforementioned Japanese studies treated children without knowledge of CSF BH4 concentration, both Swedish studies measured CSF BH4 concentration. Fernell et al.37 and Danfors et al.36 only treated children with CSF BH4 concentrations of less than 12 and 30 nM/L, respectively. Danfors et al.36 found a borderline significant correlation (r2 = 0.25, P = 0.10) between CSF BH4 concentration before treatment and improvement in social interactions on the CARS with BH4 treatment. While Fernell et al.37 demonstrated that CSF BH4 concentration increased by an average of 63% with treatment, the study did not correlate initial or final CSF BH4 concentration with behavioral variables.

Only Fernell et al.37 examined functional CNS changes with treatment. Using positron emission tomography, BH4 treatment was shown to decrease the baseline elevation in D2 receptor binding found in children with ASD. The authors suggested that BH4 treatment resulted in a normalization of dopamine metabolism in children with ASD.

While the clinical trials performed to date suggest some therapeutic effect of BH4 treatment on autism symptoms, these studies are limited in several respects. First, most of the studies are open-label, leading to potential biases from the non-blind clinicians and parents. Only two double-blind placebo controlled studies have been performed with one study limited in sample size. Clearly, larger double-blind placebo controlled studies are needed to assess efficacy. Second, most studies have only examined behavioral markers of ASD. This may be due, in part, to the fact that the mechanism of action of BH4 in the treatment of ASD is poorly understood. Clearly, it will be important to understand and measure the biological effects of BH4 treatment and understand how these effects correlate with autism symptoms. The development of such biological markers will allow clinical studies to more objectively measure the therapeutic effects of BH4 and can potentially help improve the understanding of the biological underpinnings of ASD. Lastly, the dose of BH4 is variable across studies, potentially affecting the CNS availability of BH4 across studies. A standard treatment protocol needs to be defined in order to compare and combine information across trials.

5. Current treatment of autism spectrum disorder with tetrahydrobiopterin

Currently, there are only two protocols available for the treatment of ASD with BH4. Both protocols provide BH4 in a single daily dose of 20mg/kg/day. One of the authors (R.E.F.) is targeting children with both ASD and evidence of pterin deficiency while the other authors (G.R.E., L.H.) are conducting a double-blind placebo-controlled trial examining treatment efficacy.

5.1 Open-labeled treatment in children with autism spectrum disorder and pterin deficiency

Given the importance of BH4 in critical metabolic pathways and the potential therapeutic role of BH4 in autism, patients seen in the medically-based autism clinic at University of Texas Health Science Center at Houston who were diagnosed with pervasive developmental or autism disorder were screened for abnormally low pterin levels. Screening occurred after medical-disorders that might result in neurodevelopmental disabilities, including mitochondrial, metabolic, epileptiform and genetic disorders, were ruled-out. CSF BH4 levels were examined if available; otherwise, a fasting urine neopterin level was measured.

Patients received BH4 as Kuvan®. Eight patients met criteria for treatment, agreed to Kuvan® treatment after the benefits and risks were explained, had no contraindications for treatment with Kuvan® and provided informed consent for the examination of their clinical information for research purposes. All patients were treated with 20mg/kg/day of Kuvan® to be taken with a meal once per day. Response to Kuvan® was assessed at a clinical follow-up visit that occurred, on average, three months after starting Kuvan®. The severity, improvement and efficacy subscales of the Clinical Global Impression Scale (CGI) were completed by the treating physician at follow-up. Severity was rated on a seven-point scale ranging from normal (1) to the most severely ill patients (7). Improvement was rated on a seven--point scale ranging from very much improved (1) to very much worse (7). The efficacy index represented clinical response vs. side effects and ranges from 0.25 (unchanged or worsening of disease with side effects that outweigh the therapeutic effect) to 4.00 (marked improvement with no side effects).

At the start treatment, CGI severity scale scores indicated that patients were markedly ill. Overall, five of the eight patients (63%) responded positively to Kuvan® with improvement in social interactions and either verbal or non-verbal communication. Figure 1 depicts the mean and standard error across all patients for the three CGI scales, as measured after Kuvan®. At follow-up, on average, the patients were minimally improved to much improved with efficacy index scores representing minimal to moderate improvement with mild to no side effects.

The parents and teacher of one patient completed several behavioral questionnaires before and after BH4 supplementation. This patient demonstrated improvements in many cognitive and behavioral areas, including improvements in social awareness, communication, cognition, motivation, and autistic mannerisms on the parent and teacher Social Responsiveness Scale, lethargy and stereotypy on the parent and teacher Aberrant Behavior Checklist and adaptive skills, depression, somatization, attention, social skills, functional communication and activities of daily living on the parent and teacher Behavioral Assessment System for Children scale.

Five of the eight patients (63%) demonstrated some adverse effects (AEs). Parents of four of the eight (50%) patients reported mild irritability. Two of these four patients responded positively to Kuvan®. In these patients, the irritability was not significant enough to discontinue or decrease the dose of Kuvan®. The other two patients with irritability did not respond positively Kuvan®. Kuvan® was discontinuation in these patients. Interestingly, these latter two patients where older than most of the other patients (6 and 8 years of age). One child demonstrated sleep disturbance. Decrease of the Kuvan® dose to 10mg/kg/day resulted in resolution of the sleep disturbance with continuation of the previously noted positive effects.

5.2 Double-blind placebo controlled trial with open-label extension

At Children’s Health Council in Palo Alto, California, a Phase II randomized double-blind, placebo-controlled (RCT) study of sapropterin began in early 2009. Study participants are ages three- to six-years-old, with a confirmed diagnosis of Autistic Disorder and without significant cognitive delay. The primary objective of this study is to evaluate the efficacy of sapropterin (20 mg/kg/day) vs. placebo to demonstrate measureable improvements on the core symptoms of Autism measured by the CGI after 16 weeks of treatment. The secondary objectives of the study are to evaluate the efficacy of sapropterin on changes in core behavioral domains and co-occurring problems such as hyperactivity, attention, and impulsiveness as indicated by direct and observational measures. In addition, the safety of sapropterin at the proposed dose in this population is being assessed, with monitoring of treatment-emergent side effects.

At the end of the 16-week randomized double-blind study, families are offered the choice of entering an open-label protocol. In this protocol, an open-label extension option is provided to subjects who completed that randomized study; those subjects are eligible for commercial sapropterin (Kuvan®) at 20 mg/kg/day and they are assessed for efficacy and safety for 16 additional weeks of treatment. As in the RCT, the primary outcome measure is the CGI, collected before, during, and after 16 weeks of treatment. Since half of the subjects will have been exposed to placebo in the RCT study, exposing all subjects to active drug in the extension study creates the potential to see improvement in children not previously exposed to active treatment. Although this part of the study is open-label, finding improvement primarily in children not previously exposed to sapropterin will add evidence of its benefits. Since the study is ongoing at this point, the participant assignment (treatment vs. placebo) remains unknown to the investigators and the effect of treatment cannot be assessed at this time.

6. Tetrahydrobiopterin: Ability to enter the central nervous system

Several animal studies have documented the ability of BH4 to cross the blood-brain barrier into the CNS. Five Rhesus monkeys were administered 15–20 mg/kg of a BH4 diastereoisomers intravenously. The CSF BH4 concentration increased 20–700-fold above baseline within 90 minutes of administration with peak CSF concentration occurring at 90–180 min following BH4 administration; the BH4 concentration gradually returning to baseline over 15 hours.38 Intraperitoneal administration of approximately 24 mg/kg of racemic BH4 in twelve rats increased CNS BH4 concentration 2-fold after 90 minutes. 39 Mice administered 100 µmol/kg of BH4 subcutaneously demonstrated significant increases in CNS BH4 within 1–2 hours with a significant increase in CNS BH4 sustained for up to 8 hours.40, 41 Oral administration of BH4 in mice has also been shown to resulted in an increased in the CNS activity of tyrosine hydroxylase, a BH4-dependent enzyme.42

Several case studies have examined CSF concentrations of BH4 or BH2 with oral administration of BH4 in humans. CSF BH4 concentrations increased 16- to 21-fold after 6 doses of racemic BH4 given at 10mg/kg/dose every 12 hours in a 10 year-old boy with hyperphenylalaninemia.43 CSF BH2 concentration increased following 20mg/kg of BH4 in two patients with 6-pyruvoyltetrahydropterin synthase deficiency.44, 45 Fernell et al.37 demonstrated that CSF BH4 concentration increased by an average of 63% 24 hours after the last administration of 3 months of oral treatment with 3mg/kg/day of BH4 divided into two daily doses. These studies provide evidence that BH4 readily crosses into the CNS in humans.

7. Synthetic Tetrahydrobiopterin

Daiichi Asubio Pharma (DAP) was the first to study the clinical use of BH4. Biopten Granules 2.5% (sapropterin), a synthetic form of BH4 developed by DAP, was approved in Japan to treat BH4 deficiency in 1992. Kuvan® (sapropterin dihydrochloride), is a synthetic preparation of the dihydrochloride salt of naturally occurring tetrahydrobiopterin (6R-BH4 or BH4) manufactured by BioMarin. Kuvan® has the same active ingredient as Biopten. Kuvan® is approved by the Federal Drug Administration of the United States of America for BH4-responsive PKU.

7. 1. Clinical Safety of Synthetic Tetrahydrobiopterin

More than a decade of market experience with Biopten in Japan has established that it is generally safe.46 Clinical trials conducted by DAP and BioMarin exposed more than 1100 human participants to with sapropterin formulations at various dosages. These studies indicate that sapropterin has a favorable safety profile.46 BioMarin has studied sapropterin with a dose ranging from 5–20 mg/kg/day. The clinical safety of Kuvan® has been evaluated in several BioMarin-sponsored studies. A total of 72 healthy volunteers participated in two Phase 1 pharmacokinetic studies, 579 subjects with PKU were treated with Kuvan® in four Phase 2 or 3 studies, and 116 poorly controlled systemic hypertension patients participated in a placebo-controlled Phase 2 study.46

An overall analysis of the available safety data indicates that Kuvan® is well tolerated with a favorable safety profile, in healthy volunteers, in subjects with PKU, and in subjects with hypertension. No deaths were reported in any of the BioMarin-sponsored studies. AEs reported in all studies were generally graded mild; severe AEs were infrequent. AE incidence and frequency had little or no correlation with study drug dose. In placebo-controlled studies, the incidence and frequency of AEs reported for participants treated with placebo were generally comparable to those reported for subjects treated with Kuvan®. The most frequently reported (>4%) AEs were headache, diarrhea, abdominal pain, upper respiratory tract infection, pharyngolaryngeal pain, vomiting, and nausea.

In ongoing studies in children with PKU or BH4 deficiency, there was one serious AE, gastroesophageal reflux, assessed by an investigator as drug-related. The participant was concomitantly taking ibuprofen, a medication well known for its association with gastrointestinal disorders; ibuprofen was reported as a second suspect medication. The physiochemical properties of 6R-BH4 include low pH and high solubility; combined, these properties could potentially increase the likelihood of developing gastrointestinal disorders, especially with the concurrent use of ibuprofen. After the event, Kuvan® was reintroduced without further gastrointestinal symptoms.

In a 10-year post-marketing safety surveillance program of sapropterin, three patients with underlying neurologic disorders experienced convulsions, exacerbation of convulsions, over-stimulation or irritability during co-administration of levodopa and sapropterin.

Forty-three nonclinical toxicology studies have been conducted for sapropterin.46 Most of these studies were conducted in mice, rats, dogs, and marmosets at doses ranging from 20 to 4000 mg/kg using intravenous (IV) and oral administration routes. In a 52-week marmoset toxicology study, the ‘no observable AE’ level was 320 mg/kg/day. The only observed AEs were salivation and vomiting, both possibly caused by the low pH of the dosing solution; no laboratory or histopathologic abnormalities were found. In a similar 52–week rat toxicology study, the no observable AE level was 40 mg/kg/day; animals treated with 400 mg/kg/day for 52 weeks demonstrated a low incidence of mild basophilic infiltrates in the renal collecting tubules; this was not associated with changes in clinical chemistry or urinalysis results. No renal tubular changes were found in a subsequent rat carcinogenicity study in which rats were exposed to doses up to 250 mg/kg/day of sapropterin for 104-weeks. The safety of Kuvan® is support by these data and other nonclinical pharmacology, pharmacokinetic, and toxicology studies.

7.2 Clinical Safety of Synthetic Tetrahydrobiopterin Safety in Autism

DAP conducted seven studies in which 451 patients with autism were treated for 5–105 weeks with sapropterin at doses of 1–5 mg/kg/day. Ninety-seven (21.5%) of the 451 treated patients experienced 149 AEs for which a causal relationship with study drug could not be excluded. The most frequently reported AEs were behavioral (i.e., sleep disorders, excitement, hyperkinesias), urinary symptoms (i.e., increased frequency, enuresis, polyuria) and loose stools. There were no significant differences in the incidence of AEs between patients treated with sapropterin in all seven studies and the 115 patients treated with placebo in the two placebo-treated studies. No deaths were reported. Four participants developed AEs severe enough to discontinue sapropterin. These AEs included hyperkinesias, panic and tics; transient mild syncope attacks were noted in one patient with a prior history of epilepsy. For some patients, symptoms persisted for several weeks after discontinuation of sapropterin, but AEs eventually resolved in all participants. AEs were also reported in the Swedish clinical trials. Fernell et al.37 reported sleep disruption and increased anger and aggression in three of the six patients studied but these symptoms were not severe enough to discontinue treatment with BH4. Ten of the 12 patients studied by Danfors et al.36 developed AEs of sleep disruption and/or increased aggression but these AEs were just as frequent during the treatment and placebo arm of the trial. As described above, AEs of sleep disruption and aggression were also observed in the recent series of children treated with Kuvan® at University of Texas Health Science Center at Houston, but did not necessitate discontinuation of Kuvan® in the children who gained benefit from the treatment.

8. Potential for Tetrahydrobiopterin as a novel therapy in autism

Clinical studies provide both direct and indirect evidence that the concentration of CNS BH4 is depressed in children with ASD. Clinical trials suggest that treatment with BH4 results in improvement in autism symptomatology in some children with ASD. These clinical studies, however, are mostly open-label and use potentially subjective behavioral dependent measures. Two double-blind studies that have been completed, but the number of participants is limited in these studies. Clearly, further double-blind placebo controlled studies will be needed to document further the efficacy of BH4 treatment in children with ASD. Currently one such clinical trial underway should shortly provide additional information regarding the efficacy of BH4 treatment in children with ASD.

One particularly important positive characteristic of BH4 is its safety profile. It has a low incidence of serious AEs and appears generally safe. In clinical trials, it has been well tolerated and very few patients who benefit from BH4 treatment have discontinued treatment due to AEs. It is not known whether AEs are dose dependent. The treatment dose has varied widely throughout clinical studies. Further investigations may provide better insight into the optimal therapeutic dose that maximizes development and minimizes the incidence of AEs.

The importance of BH4 in ASD and other neurodevelopmental disorders should not be understated. Low CNS BH4 levels can result in devastating neurodevelopmental consequences. First, BH4 is necessary for the production of nitric oxide, a soluble molecule that is important for signaling cell proliferation, neuronal motility, and synaptic maturation during development 47 and communication between neurons and both neuronal and non-neuronal cells.48 Second, BH4 has been associated with growth factors, including nerve growth factors, in animal models.49 Third, BH4 has been shown to be a superoxide radical scavenger.15, 16 Fourth, reduced levels of monoamine neurotransmitters could result in dysfunction of important neural pathways, leading to underdevelopment of such important pathways. Fifth, BH4 is an enhancer of the synaptic release of a wide range of neurotransmitters including the catecholamines, serotonin, acetylcholine, glutamate and gamma aminobutyric acid.50, 51 Clearly BH4 is involved in several metabolic and neurotransmitter pathways critical for CNS function and development. Low BH4 levels during development could have devastating consequences to the CNS, leading to or potentiating the neuropathology underlying ASD. The central role of BH4 in a wide variety of CNS and non-CNS metabolic pathways that are known to be dysfunctional in ASD makes BH4 a potentially important factor that may have a therapeutic role in ASD.

Figure 2
The average Clinical Global Impression subscale values for eight patients with autism spectrum disorder after three months of oral 20mg/kg/day tetrahydrobiopterin therapy. On average, patients were minimally improved to much improved and had efficacy ...


This study was supported by NS046565 to Dr. Richard E. Frye, M.D., Ph.D. and Drs. Elliott and Huffman also have partial salary support from an investigator-initiated clinical intervention project funded by BioMarin.


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This study has not been published previously and is not under consideration for publication elsewhere.


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