Pilot Study to Evaluate Melatonin Secretion as a Marker of Decreased Serotonin in Individuals With PKU: Evaluation of the CNS Effects of Tetrahydrobiopterin
Description
Brief Summary
This study examines the effect of tetrahydrobiopterin (Kuvan) and Large Neutral Amino Acid
(LNAA) therapy on melatonin and dopamine levels in individuals with Phenylketonuria (PKU).
The investigators hypothesize that Kuvan therapy will improve melatonin secretion and urine
dopamine levels to some extent. However significantly greater responses in melatonin and
dopamine secretions may be observed with combined treatment with Kuvan and supplementation of
LNAA.
Detailed Description
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.
Phase
OtherInclusion and Exclusion Criteria
- Adult English-speaking patients who have confirmed PKU, and have tolerated Kuvan therapy at 20 mg/kg/day in the past, and are currently on the LNAA supplements will be considered as candidates for the study.
- Subjects must be able to stop LNAA therapy for 4 weeks.
- This will be determined by the subjects themselves, based on their past personal experiences.
- Individuals who have never taken Kuvan, who have never been on LNAA therapy, who are under the age of 18, or who do not speak English will be excluded.
- Subjects who cannot stop LNAA therapy for 4 weeks will be excluded.
Sites
-
California
- Keck School of Medicine, Clinical Trials Unit at Keck Medical Center of USC, Los Angeles, California, 90033