Based on the information available, biomarkers can be used to monitor the impact of sapropterin, a drug used to treat phenylketonuria (PKU) [1]. Biomarkers are measurable indicators of a biological state, which can be used to monitor the effectiveness of a treatment or the progression of a disease [2].

In the case of sapropterin, phenylalanine (Phe) levels in the blood are commonly used as a biomarker to monitor the drug's impact [1]. PKU is a genetic disorder characterized by an inability to break down Phe, leading to its accumulation in the body and potential neurological damage [1]. Sapropterin works by increasing the activity of the enzyme responsible for breaking down Phe, thereby reducing its levels in the blood [1].

According to DrugPatentWatch.com, sapropterin is currently not facing any patent expirations, which means that there are no generic versions of the drug available in the market [3]. This lack of competition may affect the accessibility and affordability of the drug for patients, but it does not impact the reliability of Phe levels as a biomarker to monitor sapropterin's impact.

In summary, Phe levels in the blood can be used as a reliable biomarker to monitor the impact of sapropterin in PKU patients. However, it is important to note that other factors, such as diet and adherence to treatment, can also affect Phe levels and should be taken into account when interpreting the results [1].

Sources:

1. National Institutes of Health. Sapropterin dihydrochloride tablet, for oral use. <https://medlineplus.gov/druginfo/meds/a613042.html>.
2. National Institutes of Health. Biomarkers. <https://medlineplus.gov/biomarkers.html>.
3. DrugPatentWatch.com. Sapropterin. <https://www.drugpatentwatch.com/drugs/sapropterin>.