Real-World Administration Practices of Sapropterin in Paediatric and Adults with Phenylketonuria: Results from a United Kingdom Cross-Sectional Survey

Background/Objectives: Sapropterin dihydrochloride is an established treatment option for individuals with phenylketonuria (PKU) who demonstrate responsiveness, but uncertainty persists regarding dosing frequency, timing relative to meals, the influence of dietary composition, and efficacy of different formulations. Despite widespread use in the UK, real-world administration behaviours have not previously been characterised. This study aimed to characterise sapropterin administration behaviours among people with PKU in the UK. Methods: A 31-item questionnaire was developed and disseminated via the National Society for Phenylketonuria website and social media channels. The survey captured demographic information, dosing schedules, formulation use, administration techniques, co-ingestion with food, and changes in natural protein tolerance following initiation of generic sapropterin. Results: 124 current sapropterin users completed the survey. Most respondents were caregivers of children or adolescents (68.5% aged 0–18 years). Once-daily dosing was most common (66.1%, n = 82), typically administered at breakfast, followed by twice-daily (32.3%, n = 40) and three-times-daily (1.6%, n = 2). Tablets were the predominant formulation (92.7%, n = 115); 50.4% (n = 58/115) swallowed tablets whole, while the remaining (49.6%, n = 57/115) crushed or dissolved them in water or juice. Nine respondents (7.3%, n = 9/124) used powder sachets. Most participants (75%, n = 93/124) took sapropterin with food, with both low-fat (36.6%, n = 34/93) and high-fat (26.9%, n = 24/93) meals reported. Over a third of participants (33.9%, n = 42/124) tolerated a natural protein intake >30 g/day when this was measured, and a further 15.3% (n = 19) were able to maintain a fully unrestricted protein intake without protein substitute supplementation. The magnitude of protein intake improvement was significantly greater among adults (p < 0.001), those with higher baseline natural protein intake (≥30 exchanges/day) (p < 0.001), and individuals who swallowed sapropterin tablets whole (p = 0.038). Although 71.8% (n = 89/124) were pleased with their increased natural protein allowance, many expressed a desire for further improvement. Conclusions: Substantial heterogeneity in dosing schedules, formulation handling, and co-ingestion practices highlights the absence of standardised guidance. These findings emphasise the need for clearer clinical recommendations to optimise treatment effectiveness and support consistent, equitable care.