Based on the information provided, sapropterin treatment is primarily used for individuals with phenylketonuria (PKU) who have a specific biomarker level. PKU is a genetic disorder that causes an individual to have trouble breaking down an amino acid called phenylalanine (Phe). If left untreated, Phe can build up in the body and cause brain damage.

According to the article from the National Center for Biotechnology Information (NCBI) [${3}], sapropterin is a synthetic form of tetrahydrobiopterin (BH4), a cofactor that plays a crucial role in the metabolism of Phe. The article explains that sapropterin has been shown to reduce Phe levels in individuals with PKU who have a specific genetic mutation that allows for some residual activity of the enzyme phenylalanine hydroxylase (PAH).

The drug patent watch website [${1}] states that sapropterin is indicated for the treatment of PKU in individuals who have a specific biomarker level, specifically those who have a blood Phe concentration greater than 600 micromol/L (10 mg/dL) and who have evidence of BH4-responsive PKU based on a positive BH4 loading test or genetic testing.

Furthermore, the Nature Reviews Disease Primers article [${2}] explains that a BH4 loading test is used to determine if an individual with PKU is a candidate for sapropterin treatment. The test involves administering a dose of BH4 and measuring the change in Phe levels. If Phe levels decrease by at least 30% after the BH4 load, the individual is considered a candidate for sapropterin treatment.

In summary, sapropterin treatment is indicated for individuals with PKU who have a blood Phe concentration greater than 600 micromol/L (10 mg/dL) and who have evidence of BH4-responsive PKU based on a positive BH4 loading test or genetic testing.

Sources:

* <https://www.drugpatentwatch.com/p/tradename/SAPROPTERIN>
* <https://www.nature.com/articles/tp201314>
* <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10520591/>