Orphan drug gets FDA nod for phenylketonuria

TIPS TO REMEMBER Kuvan

There has never been a drug used to treat phenylketonuria (PKU), a genetic disorder caused by a deficiency of the enzyme phenylalanine hydroxylase (PAH), until now. Last month, the Food & Drug Administration gave its approval to sapropterin dihydrochloride (Kuvan, BioMarin), a new molecular entity indicated to reduce blood phenylalanine levels in patients with hyperphenylalaninemia due to tetrahydrobiopterin (BH4)-responsive PKU. The orphan drug will offer some hope to about 30,000 PKU patients in the United States and 50,000 worldwide.

Composition of drug

Sapropterin is a synthetic form of BH4, the cofactor for the enzyme PAH, which is normally responsible for hydroxylating Phe to form tyrosine. But in patients with PKU, PAH activity is absent or deficient. According to BioMarin, treatment with BH4 can activate residual PAH enzyme, improve the normal oxidative metabolism of Phe, and decrease Phe levels in some patients. But not all PKU patients will respond to sapropterin.

Precautions to watch

Once treatment is initiated, blood Phe should be monitored routinely. Sapropterin should be taken with food to increase its absorption and also should be dissolved in four to eight ounces of water or apple juice prior to administration. The mixture should then be consumed within 15 minutes. All patients receiving sapropterin should also be simultaneously maintained on a Phe-restricted diet.

The most common adverse reactions during clinical trials included abdominal pain, diarrhea, headache, upper respiratory infection, vomiting, nausea, and pharyngolaryngeal pain. As part of several postmarketing commitments, the manufacturer will implement a PKU registry program along with several clinical trials, including one in children eight years of age and younger.

According to BioMarin, sapropterin was co-developed with Merck Serono and will be available in nine specialty pharmacies immediately at a price of $29 per 100-mg tablet. The company expects 2008 sales of the drug to reach $35 to $70 million. Kuvan will be available through a specialty pharmacy limited distribution network, including Accredo, Aetna, Caremark, Curascript, Fairview, McKesson, Pharmacare, Precision Rx, and Tel-Drug.