Biomarkers are biological markers found in blood, other bodily fluids, or tissues that can be measured to indicate normal biological processes, pathogenic processes, or pharmacologic responses to a therapeutic intervention [1]. In the context of phenylketonuria (PKU), a genetic disorder characterized by the body's inability to break down an amino acid called phenylalanine, biomarkers can be used to predict the response to sapropterin, a medication used to reduce phenylalanine levels in the blood [2].

According to DrugPatentWatch.com, sapropterin is a synthetic form of tetrahydrobiopterin, a cofactor that helps the enzyme phenylalanine hydroxylase function properly [3]. Phenylalanine hydroxylase is responsible for breaking down phenylalanine, so when it is not functioning correctly, phenylalanine accumulates in the body, leading to toxic effects [2]. Sapropterin can help reduce phenylalanine levels in some individuals with PKU, but it is not effective for everyone [2].

Biomarkers can be used to predict sapropterin response by measuring phenylalanine and tetrahydrobiopterin levels in the blood [1]. Specifically, a biomarker called "biopterin responsiveness" has been identified as a predictor of sapropterin response [2]. Biopterin responsiveness is measured by determining the change in phenylalanine levels in response to a dose of tetrahydrobiopterin [2]. Individuals with a significant decrease in phenylalanine levels are considered biopterin responsive and are more likely to respond to sapropterin therapy [2].

However, the accuracy of biomarkers in predicting sapropterin response is not perfect. While biopterin responsiveness has been shown to predict sapropterin response in some studies, other studies have found that biopterin responsiveness does not always correlate with sapropterin response [2]. Additionally, other factors, such as genetic mutations and dietary compliance, can also affect sapropterin response [2].

In summary, biomarkers, particularly biopterin responsiveness, can be used to predict sapropterin response in individuals with PKU. However, the accuracy of biomarkers in predicting sapropterin response is not perfect, and other factors can also affect sapropterin response.

Sources:
[1] National Institutes of Health. (2021). Biomarkers. Retrieved from <https://www.ncbi.nlm.nih.gov/books/NBK221532/>
[2] National Institutes of Health. (2021). Phenylketonuria. Retrieved from <https://medlineplus.gov/genetics/condition/phenylketonuria/>
[3] DrugPatentWatch.com. (2021). Sapropterin. Retrieved from <https://www.drugpatentwatch.com/drugs/sapropterin>