Primary Condition Treated with Sapropterin
Sapropterin dihydrochloride (Kuvan) treats phenylketonuria (PKU), a genetic disorder caused by mutations in the PAH gene that impair phenylalanine hydroxylase enzyme activity. This leads to toxic buildup of phenylalanine (Phe) in the blood, risking brain damage and intellectual disability if untreated. Sapropterin acts as a cofactor for residual enzyme function, lowering Phe levels in responsive patients.[1][2]
How Sapropterin Works in PKU
It mimics tetrahydrobiopterin (BH4), the natural cofactor for PAH. About 20-30% of PKU patients respond, confirmed via a Phe reduction test after short-term dosing. Responders maintain lower Phe on 10-20 mg/kg/day alongside a Phe-restricted diet.[1]
Who Qualifies for Treatment
Tetrahydrobiopterin (BH4)-responsive PKU patients, typically milder forms like PAH variants with partial activity. Non-responders rely on diet alone. Testing starts in infancy via newborn screening, with sapropterin approved for ages 1 month and older.[2]
Related Conditions and Off-Label Use
Primarily FDA-approved for PKU hyperphenylalaninemia. Rarely considered for BH4 deficiencies (e.g., PTPS or DHPR mutations), but those require different synthetic BH4 forms due to synthesis defects, not cofactor need.[1][3]
Testing Responsiveness
A 24-48 hour trial measures Phe drop >30% post-dose. Genetic testing (PAH sequencing) predicts response in some cases, but trial confirms.[2]
Treatment Timeline and Monitoring
Lifelong for responders, with blood Phe checks weekly initially, then monthly. Diet relaxes as Phe control improves.[1]
Sources
[1]: FDA Label for Kuvan
[2]: BioMarin Kuvan Prescribing Information
[3]: NORD: Phenylketonuria