Based on the information provided, I cannot directly observe changes in a specific patient on Sapropterin. However, I can summarize the relevant information about Sapropterin from the provided sources.

Sapropterin, under the trade name KUVAN, is a prescription medication used to reduce blood phenylalanine levels in patients with a certain genetic disorder called phenylketonuria (PKU) [1][2]. PKU is an inherited metabolic disorder that results in an inability to break down an amino acid called phenylalanine, which can lead to harmful levels in the body [3].

According to the FDA label, Sapropterin is indicated to decrease blood phenylalanine levels in patients with PKU who have a specific genetic mutation called "tetrahydrobiopterin (BH4) responsive PKU" [2]. The effectiveness of Sapropterin is determined by monitoring blood phenylalanine levels during a Sapropterin trial period [2].

The clinical study mentioned in the third source, [3], supports the use of Sapropterin in BH4-responsive PKU patients. The study found that Sapropterin significantly reduced blood phenylalanine levels in these patients [3].

In summary, Sapropterin is used to lower blood phenylalanine levels in PKU patients with a specific genetic mutation. The effectiveness of Sapropterin is assessed by monitoring blood phenylalanine levels during a trial period.

Sources:
[1] <https://www.drugpatentwatch.com/p/tradename/SAPROPTERIN>
[2] <https://www.accessdata.fda.gov/drugsatfda_docs/label/2014/022181s013lbl.pdf>
[3] <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9305189/>