Sapropterin, also known as tetrahydrobiopterin, is a biomarker and medication used to manage phenylketonuria (PKU), a genetic disorder that results in an inability to break down an amino acid called phenylalanine (Phe) [1]. Sapropterin's effectiveness can be measured by monitoring changes in several biomarkers.

Firstly, sapropterin's primary mechanism of action is to increase the activity of the phenylalanine hydroxylase (PAH) enzyme, which is responsible for breaking down Phe [1]. Consequently, a decrease in blood Phe levels is a key biomarker reflecting sapropterin's effectiveness [1][2].

Secondly, biopterin levels can also indicate sapropterin's efficacy. Biopterin is a cofactor for PAH, and sapropterin is a stable form of biopterin [1]. Thus, an increase in biopterin levels in response to sapropterin treatment suggests that the medication is working as intended [1].

Lastly, neurotransmitter levels, such as dopamine and serotonin, can be used as biomarkers for sapropterin's effectiveness [1]. Since PAH is involved in the synthesis of these neurotransmitters, an increase in their levels can indicate that sapropterin is enhancing PAH activity [1].

In summary, sapropterin's effectiveness can be measured by monitoring changes in blood Phe levels, biopterin levels, and neurotransmitter levels.

Sources:
[1] <https://www.drugpatentwatch.com/drugs/sapropterin>
[2] <https://www.ncbi.nlm.nih.gov/books/NBK1168/#:~:text=Sapropterin%20dihydrochloride%20is%20a%20synthetic,the%20activity%20of%20phenylalanine%20hydroxylase>