Sapropterin's effectiveness was measured in patients through a series of clinical trials and studies. According to the National Center for Biotechnology Information (NCBI) [2], sapropterin's efficacy was evaluated by measuring phenylalanine (Phe) levels in patients with phenylketonuria (PKU) before and after treatment. PKU is a genetic disorder characterized by an inability to break down Phe, an essential amino acid found in proteins. High levels of Phe can lead to brain damage.

In a randomized, double-blind, placebo-controlled study [3], sapropterin's effectiveness was assessed by measuring changes in blood Phe concentrations in PKU patients. The study found that sapropterin treatment led to a significant decrease in blood Phe concentrations compared to placebo.

Moreover, the Drug Patent Watch [1] states that sapropterin's effectiveness was also evaluated based on the patients' ability to maintain their Phe levels within the desired range, as well as their dietary freedom and adherence.

In summary, sapropterin's effectiveness in patients was measured primarily by evaluating changes in blood Phe concentrations, the patients' ability to maintain Phe levels within the desired range, and their dietary freedom and adherence.

Sources:

1. <https://www.drugpatentwatch.com/p/tradename/SAPROPTERIN>
2. <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9013411/>
3. <https://pubmed.ncbi.nlm.nih.gov/28274234/>