Sapropterin is a synthetic form of a natural substance that activates an enzyme needed to break down phenylalanine in the body. Phenylketonuria (PKU) is a disorder in which the body cannot break down phenylalanine. Elevated phenylalanine levels can cause problems with thinking and behavior. Sapropterin is used to lower blood levels of phenylalanine in people with a certain type of PKU.