Sapropterin, also known as (6R)-5,6,7,8-tetrahydrobiopterin, is a drug used to reduce the symptoms of phenylketonuria (PKU), a genetic disorder that affects how the body processes protein [1][2]. The discovery process of sapropterin is not explicitly documented in the provided sources, but some information can be inferred.

Tetrahydrobiopterin, the active ingredient in sapropterin, is a naturally occurring compound in the body that plays a crucial role in various biochemical reactions, including the metabolism of phenylalanine, an amino acid found in proteins [3]. The understanding of tetrahydrobiopterin's role in phenylalanine metabolism led to the development of sapropterin as a treatment for PKU.

The drug patent watch website [1] provides information on sapropterin's commercialization, but it does not offer details about its discovery process. The scientific article [2] focuses on the clinical use of sapropterin, and the Wikipedia page [3] offers a general overview of tetrahydrobiopterin but does not provide specific information on its discovery.

Therefore, while the discovery process of sapropterin cannot be fully reconstructed based on the provided sources, it is clear that the understanding of tetrahydrobiopterin's role in phenylalanine metabolism was a crucial step in its development as a treatment for PKU.

Sources:
[1] <https://www.drugpatentwatch.com/p/tradename/SAPROPTERIN>
[2] <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8027839/>
[3] <https://en.wikipedia.org/wiki/Tetrahydrobiopterin>