First medicine approved for phenylketonuria

The US Food and Drug Administration (FDA) has approved a medicine containing sapropterin dihydrochloride to treat phenylketonuria (PKU).

PKU is an inherited metabolic disorder that can cause learning disability and various nerve problems. It is caused by a missing or faulty enzyme, phenylalanine hydroxylase, which normally breaks down phenylalanine and so keeps its level in the blood down. When the enzyme doesn't work, phenylalanine builds up.

Sapropterin lowers blood phenylalanine. It is a synthetic form of tetrahydrobiopterin, the cofactor the enzyme needs. It helps an estimated 20–50% of people with PKU, because some types of PKU mutation don't respond. About 30,000 people in the United States have PKU.

BioMarin Pharmaceutical markets it as Kuvan. It is the first medicine for PKU and adds to the existing treatment, a diet low in phenylalanine. Treatment starts at 10 mg/kg of body weight a day and was tested in patients aged 4 and over. The doctor should check blood phenylalanine regularly during the first month; if it hasn't fallen, the dose can be raised to 20 mg/kg a day, and if it still hasn't fallen after a month at that dose, the medicine should be stopped.