Effects of Kuvan on Melatonin Secretion

Background: Phenylketonuria (PKU) is a genetic condition due to missing one of the key enzymes of phenylalanine (Phe) degradation. The missing enzyme is phenylalanine hydroxylase (PAH). Because of the deficiency of the enzyme, plasma Phe is highly elevated compared with other plasma amino acids. This abnormal ratio i.e., Phe vs. each of the other large neutral amino acids (LNAA) which are transported into the brain as precursors of neurotransmitters through the common transporter LAT1, is likely to contribute to neurotransmitter deficiencies in individuals with PKU. Patients who do not restrict Phe intake develop mental retardation and neuropsychological disorders. Treatment of PKU has been historically limited to restriction of Phe intake, meaning protein restriction.

The FDA recently approved a new medication, Kuvan, a synthetic form of tetrahydrobiopterin (BH4) manufactured by BioMarin Pharmaceutical Inc., for patients with PKU. Kuvan is a cofactor for PAH, tyrosine hydroxylase, and tryptophan hydroxylase. Patients with PKU typically undergo a month-long trial of Kuvan to determine if they are responders to the drug, meaning blood phenylalanine drops by more than 30 percent. Those who are not responders do not usually continue Kuvan, however our recent study (HS 08-00147) showed that classical PKU patients who are not Kuvan responders demonstrated improvement in their maladaptive behaviors, suggesting some effects of Kuvan in the CNS system without reducing blood phenylalanine concentrations (Moseley et al, manuscript under preparation). Seven subjects who did not respond to Kuvan with lower blood Phe levels had significant increases in blood tyrosine after 6 months (p=0.015), along with improvement in the Vineland Internalizing and Overall Maladaptive Behavior Indexes at 12 months (p=0.032 and 0.049, respectively). Kaufman reported that Kuvan at dose of 20 mg/kg/day increased CSF BH4 levels (1). Based on these observations, it is likely that Kuvan has some effects on neurotransmitter metabolism.

Supplementation of large neutral amino acids (LNAA) as a medical food has been used in Europe for the past 25 years and it is believed to be effective in treating patients with PKU. In the article by Shindeler et al. (2), 16 individuals (7 males and 9 females; age 11y to 45 y) participated in the study which consisted 4 weeks wash-out and 4 two-weeks stages (on phenylalanine restricted diet and with/without LNAA, and on no phenylalanine restricted diet and with/without LNAA). No specific adverse effects are reported in the article. Large neutral amino acid therapy for the treatment of PKU has been done in the past (3). LNAA is used primarily for adolescents and adults who cannot adhere to the standard phe-restricted diet. LNAA treatment has been approved for PKU patients just like other special formulas. Our previous studies have demonstrated deficiency of serotonin in the brain, as evidenced by low nocturnal melatonin secretion and reduced urine dopamine compared to controls, and that supplementation with LNAA improves, but does not normalize these markers.

Rationale: Patients with PKU are thought to be deficient in tyrosine and tryptophan in the brain tissue level because of the abnormal phe/tyrosine and phe/tryptophan ratios. Previous studies showed low serotonin levels in cerebrospinal fluid in PKU individuals and some improvement was observed after tryptophan supplementation (4). Our prior study suggested deficiency of tryptophan in the brain tissue was demonstrated by low nocturnal melatonin secretion and 6-sulfatoxymelatonin, which is a stable metabolite of melatonin (5). LNAA supplementation resulted in some improvement in melatonin secretion, however, melatonin secretion in patients with PKU on LNAA was still significantly lower than the control group. Supplementation of Kuvan may activate the brain tyrosine and tryptophan hydroxylases (6), resulting in improvement of nocturnal melatonin secretion and urine melatonin and dopamine concentrations. By supplementing LNAA increased amount of neurotransmitter precursor amino acids will be transported into the brain and activated brain tyrosine and tryptophan hydroxylases by Kuvan will act on these precursors to produce neurotransmitters, i.e., serotonin and dopamine. These synergistic effects may increase melatonin and dopamine levels to closer to the control levels.

Aims of the Study: To show scientific evidence of improvement in metabolism of neurotransmitters including serotonin and dopamine in patients with PKU treated with LNAA only, Kuvan only, and Kuvan and LNAA.

Objectives: This investigator initiated study has three objectives: (1) To evaluate if tetrahydrobiopterin (Kuvan) supplementation has beneficial effects on melatonin secretion and urine dopamine levels in individuals with PKU when they are and are not treated with large neutral amino acids; (2) To demonstrate synergistic effects of LNAA supplementation and Kuvan therapy in improvement of neurotransmitter metabolism in individuals with PKU; and (3) To gather pilot information necessary to design a larger, multicenter trial of these interventions, should results of these pilot studies indicate further study is warranted.

Study Methodology: This study will be conducted in adult patients with PKU in four 4-week phases: phase I (large neutral amino acid/LNAA), phase II (Washout), phase III (Kuvan only) and phase IV (Kuvan and LNAA). Kuvan supplementation will be adjusted to provide 20 mg/kg/day. LNAA therapy is adjusted to the manufacturer's instruction (total tablets of LNAA is 1/2 X BW kg). On the last day of each phase, subjects will stay approximately 14 to 18 hours overnight at the CTU at USC University Hospital. Blood specimens will be collected every 2 hours from 7:00 pm to 7:00 am. First void urine will be collected on the day of discharge (patients will be asked to void urine at 11:00 pm and the first void urine will be collected).

Blood will be collected in a red top tube (5 ml) through an IV line which is saline locked during the study period. Avoid hemolysis by collecting blood through an IV line into a reasonably large peripheral vein. Blood will be left to clot for 45 minutes at room temperature (18-28 C) and protected from light (a dim flash light or a yellow light < 100 lux). Serum will be collected and stored at - 20 C until they are subjected to analysis for plasma melatonin and plasma amino acids. Urine 20 ml first void sample will be collected into two plain tubes (10 ml per each tube) and stored in a deep freezer (-20 C) until subjected to be analyzed for dopamine and 6-sulfatoxymelatonin.

This is an open-labeled study. During all four 4-week phases, subjects will remain on their protein-restricted diet. LNAA tablets are not considered a medicine, but are under the category of medical foods; examples include low protein foods. LNAA tablets will be provided by the LNAA manufacture (Applied Nutrition). Kuvan will be taken per orally with breakfast and LNAA tablets will be taken with meals three times per day.