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Phenylketonuria (PKU) is an autosomal recessive disorder characterised by an inborn error of phenylalanine (Phe) metabolism. Such errors are attributed to pathogenic gene variants causing phenylalanine hydroxylase (PAH) deficiency, impairing the hydroxylation of phenylalanine to tyrosine in the Phe metabolic pathway. This defect leads to plasma Phe concentrations above the normal range. If untreated, hyperphenylalaninemia can adversely affect brain function, leading to severe intellectual disability and seizures. Since 1969, the newborn dried blood spot test has remained the main method of early screening and diagnosis for PKU. The primary therapeutic management is a lifelong phenylalanine-restricted diet with the aim of decreasing plasma Phe levels. The recommended diet consists of avoiding high-protein foods such as meat, fish, eggs and nuts, and can be supplemented with high-protein medical formulas which are low in phenylalanine. Pharmacological interventions such as sapropterin, sepiapterin and pegvaliase can also be used as treatment adjuncts in patients with PKU. Currently, small-molecule inhibitors reducing renal phenylalanine reabsorption are being explored as a potential therapeutic intervention. Furthermore, novel gene-editing techniques are under evaluation as potential curative strategies, with preclinical studies showing promising results in correcting pathogenic phenylalanine hydroxylase variants. This non-systematic review synthesises current literature on the management of PKU, with a focus on dietary interventions and recommendations.

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AIM AMPLIPHY is the first Phase 3 study comparing sepiapterin versus sapropterin in children and adults with phenylketonuria (PKU). METHODS AMPLIPHY was an international, Phase 3, two-part, open-label study in participants with PKU aged ≥2 years. Participants responsive to sepiapterin (60 mg/kg/day) in Part 1 (≥20% reduction in blood phenylalanine [Phe]) entered Part 2, a crossover treatment period, and were randomized 1:1 to alternative treatment sequences of sepiapterin (60 mg/kg/day, licensed dosage) and sapropterin (20 mg/kg/day, maximum licensed dosage) for 4 weeks each, with a 14-day washout between treatments. The primary endpoint was mean change in blood Phe from baseline to Weeks 3-4 of each treatment period (Part 2). RESULTS Of 82 participants enrolled, 67 (81.7%) and 62 (75.6%) had reductions in blood Phe ≥20% and ≥ 30%, respectively, in Part 1. Sixty-two participants were randomized in Part 2 (mean [SD] age, 15.8 [10.8] years). In the primary analysis set (≥30% reduction in blood Phe in Part 1, n = 58), mean (SD) baseline blood Phe before sepiapterin and sapropterin treatment was 725.8 (302.1) and 790.4 (370.0) μmol/L, respectively. Least-squares mean (SE) reduction in blood Phe from baseline was -437.0 (28.0) and - 256.6 (28.2) μmol/L, respectively, representing a least-squares mean difference of -180.4 μmol/L (95% CI: -229.5, -131.4; p < 0.0001) and a relative 70% greater reduction with sepiapterin versus sapropterin. Both treatments were well tolerated, with safety profiles consistent with previous reports. CONCLUSIONS Sepiapterin was superior to the highest approved dose of sapropterin in lowering blood Phe. No new safety signals were observed. The trial was registered in the UK Clinical Study Registry, ISRCTN, on January 29, 2024 (ID number, ISRCTN79102999; https://www.isrctn.com/ISRCTN79102999).

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AimsThe aim of the present meta‐analysis was to evaluate the efficacy and safety of sapropterin dihydrochloride in phenylketonuria (PKU) patients.MethodsThe following databases were searched for randomized controlled trials (RCT) regarding PKU patients treated with sapropterin dihydrochloride: PubMed, Embase, Cochrane Library and clinicaltrials. Two authors independently selected studies, assessed the risk of bias and extracted data. The meta‐analysis was performed in RevMan 5.3 provided by the Cochrane Collaboration.ResultsFour studies met the inclusion criteria. In PKU patients with low blood phenylalanine (Phe) concentration, no significant difference was indicated for the decrease of Phe level (weighted mean difference (WMD) = −7.75 μmol L−1; 95% confidence intervals (CI): −82.63 to 67.13, P = 0.84, I2 = 0%), however, the dietary Phe tolerance was significantly improved in the sapropterin group (WMD = 19.89 mg kg−1 d−1; 95% CI: 10.26 to 29.52, P < 0.0001, I2 = 0%). In PKU patients with high blood Phe level, sapropterin showed a significant lowering in blood Phe concentration (WMD = −225.31 μmol L−1; 95% CI: −312.28 to −138.34, P < 0.00001, I2 = 0%). There was no significant difference for adverse events.ConclusionsSapropterin could bring benefit for PKU patients with high or low Phe level, due to Phe reduction in a short time or dietary Phe tolerance improvement respectively. Sapropterin has an acceptable safety profile.

Abstract licence: CC BY-NC 4.0

AbstractBackgroundDuring the initial 26-week SPARK (Safety Paediatric efficAcy phaRmacokinetic with Kuvan®) study, addition of sapropterin dihydrochloride (Kuvan®; a synthetic formulation of the natural cofactor for phenylalanine hydroxylase, tetrahydrobiopterin; BH4), to a phenylalanine (Phe)-restricted diet, led to a significant improvement in Phe tolerance versus a Phe-restricted diet alone in patients aged 0–4 years with BH4-responsive phenylketonuria (PKU) or mild hyperphenylalaninaemia (HPA). Based on these results, the approved indication for sapropterin in Europe was expanded to include patients < 4 years of age. Herein, we present results of the SPARK extension study (NCT01376908), evaluating the long-term safety, dietary Phe tolerance, blood Phe concentrations and neurodevelopmental outcomes in patients < 4 years of age at randomisation, over an additional 36 months of treatment with sapropterin.ResultsAll 51 patients who completed the 26-week SPARK study period entered the extension period. Patients who were previously treated with a Phe-restricted diet only (‘sapropterin extension’ group; n = 26), were initiated on sapropterin at 10 mg/kg/day, which could be increased up to 20 mg/kg/day. Patients previously treated with sapropterin plus Phe-restricted diet, remained on this regimen in the extension period (‘sapropterin continuous’ group; n = 25). Dietary Phe tolerance increased significantly at the end of the study versus baseline (week 0), by 38.7 mg/kg/day in the ‘sapropterin continuous’ group (95% CI 28.9, 48.6;p < 0.0001). In the ‘sapropterin extension’ group, a less pronounced effect was observed, with significant differences versus baseline (week 27) only observed between months 9 and 21; dietary Phe tolerance at the end of study increased by 5.5 mg/kg/day versus baseline (95% CI − 2.8, 13.8;p = 0.1929). Patients in both groups had normal neuromotor development and growth parameters.ConclusionsLong-term treatment with sapropterin plus a Phe-restricted diet in patients who initiated sapropterin at < 4 years of age with BH4-responsive PKU or mild HPA maintained improvements in dietary Phe tolerance over 3.5 years. These results continue to support the favourable risk/benefit profile for sapropterin in paediatric patients (< 4 years of age) with BH4-responsive PKU. Frequent monitoring of blood Phe levels and careful titration of dietary Phe intake to ensure adequate levels of protein intake is necessary to optimise the benefits of sapropterin treatment.Trial registrationClinicalTrials.gov, NCT01376908. Registered 17 June 2011,https://clinicaltrials.gov/ct2/show/NCT01376908.

Abstract licence: CC BY 4.0