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FDA Approves Drug for Phenylketonuria

The FDA approved on Dec. 13 the first-ever drug for the treatment of phenylketonuria (PKU), a rare genetic disorder that can cause neurological problems in infants, children, and adults. One in 12,000 to 15,000 infants in the US is born with PKU, according to the FDA.

People with PKU are deficient in the enzyme phenylalanine hydroxylase and are therefore unable to metabolize phenylalanine (Phe), an amino acid found in meat, dairy, and other protein-rich foods. In infants and children, high blood Phe levels can cause mental retardation, smaller brain size, delayed speech, and other neurological problems. In adolescents and adults, it can cause problems with concentration and attention, according to the FDA. People with PKU are currently treated with a Phe-restricted diet.

The new drug, sapropterin dihydrochloride (Kuvan) is a synthetic form of tetrahydrobiopterin (BH4). The FDA based its approval on four clinical trials that found that, in some people, BH4 increases activity of phenylalanine hydroxylase.

One study, published last year in the Journal of Inherited Metabolic Disease, tested the drug in 489 people with PKU aged 8 to 48. The patients, who were not on Phe-restricted diets, were given 10 mg a day of Kuvan for eight days. Researchers defined response to treatment as a decrease in blood Phe level of 30 percent or more from a baseline Phe blood level of 450 umol/L or greater. At day eight, only 20 percent had responded to the treatment.

Harvey Levy, MD, senior associate in medicine at Children's Hospital Boston and professor of pediatrics at Harvard Medical School, said Kuvan probably works better in people with lower Phe blood levels. People who respond well to the drug will probably be able to follow a less strict diet because they will be able to metabolize more natural foods, he said. Dr. Levy is also co-chair of the PKU advisory board for Biomarin, the company that manufactures Kuvan.

To determine if the drug is working, the FDA said physicians should frequently check the blood levels of Phe.

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Reference

• Burton BK, Grange DK, Milanowski A, et al. The response of patients with phenylketonuria and elevated serum phenylalanine to treatment with oral sapropterin dihydrochloride (6R-tetrahydrobiopterin): a phase II, multicentre, open-label, screening study. J Inherit Metab Dis 2007;30:700–707.