The patient populations primarily focused on in sapropterin studies are those with phenylketonuria (PKU) [1]. PKU is a genetic disorder characterized by a deficiency of the enzyme phenylalanine hydroxylase, leading to an accumulation of phenylalanine in the body, which can cause brain damage if left untreated [2].

Sapropterin, also known as tetrahydrobiopterin, is a cofactor for phenylalanine hydroxylase, and sapropterin studies have investigated its potential to lower phenylalanine levels in PKU patients [1]. These studies have included both pediatric and adult PKU patients, with some focusing on patients with mild to moderate PKU and others including patients with severe PKU [1].

According to DrugPatentWatch.com, sapropterin is approved for use in patients with PKU in several countries, including the United States, Canada, and several European countries [3]. The approval of sapropterin for PKU treatment has provided a new treatment option for patients with this disorder, offering the potential to improve their quality of life and reduce the risk of long-term complications [2].

In summary, sapropterin studies have primarily focused on PKU patient populations, investigating the potential of sapropterin to lower phenylalanine levels and improve outcomes for these patients.

Sources:

1. [ClinicalTrials.gov](https://clinicaltrials.gov/). (2023). Sapropterin Dihydrochloride - Drugs. Retrieved from <https://clinicaltrials.gov/ct2/results?term=sapropterin&Search=Search>
2. National Institutes of Health. (2021). Phenylketonuria. Genetics Home Reference. Retrieved from <https://medlineplus.gov/genetics/condition/phenylketonuria/>
3. DrugPatentWatch.com. (2023). Sapropterin. Retrieved from <https://www.drugpatentwatch.com/drugs/sapropterin>