PTC923 (formerly CNSA-001), an oral formulation of sepiapterin, a natural precursor of intracellular tetrahydrobiopterin (BH₄), has been shown in humans to induce larger increases in circulating BH₄ vs. sapropterin dihydrochloride. Sapropterin reduces blood phenylalanine (Phe) by ≥20-30% in a minority of subjects with PKU. This was a Phase 2 randomized, multicenter, three-period crossover, open-label, active controlled, all-comers [regardless of phenylalanine hydroxylase (PAH) variants] comparison of PTC923 60 mg/kg, PTC923 20 mg/kg and sapropterin 20 mg/kg in 24 adults with phenylketonuria (PKU) and hyperphenylalaninemia. Eligible subjects were adult men or women (18-60 y) with PKU. Subjects enrolled received 7 days of once-daily oral treatment with PTC923 20 mg/kg/day, PTC923 60 mg/kg/day and sapropterin dihydrochloride 20 mg/kg/day each in a random order. Treatments were separated by a 7-day washout. Subjects maintained their usual pre-study diet, including consumption of amino acid mixtures. Blood Phe was measured on Day 1 (predose baseline), Day 3, Day 5, and Day 7 of each treatment period. Least squares mean changes (SE) from baseline in blood Phe were: -206.4 (41.8) μmol/L for PTC923 60 mg/kg (p < 0.0001); -146.9 (41.8) μmol/L for PTC923 20 mg/kg (p = 0.0010); and - 91.5 (41.7) μmol/L for sapropterin (p = 0.0339). Effects of PTC923 60 mg/kg on blood Phe vs. sapropterin were significantly larger (p = 0.0098) and faster in onset with a significantly larger mean reduction in blood Phe at day 3 of treatment, p = 0.0135 (20 mg/kg) and p = 0.0007 (60 mg/kg). Only PTC923 60 mg/kg reduced blood Phe in classical PKU subjects (n = 11, p = 0.0287). The mean blood Phe reduction (PTC923 60 mg/kg) in a cofactor responder analysis (n = 8; baseline Phe ≥300 μmol/L and blood Phe reduction ≥30%) was -463.3 μmol/L (SE 51.5) from baseline. Adverse events were mostly mild to moderate, transient, and similar across treatment groups with no serious adverse events or discontinuations. The substantially significantly better effect of PTC923 60 mg/kg on blood Phe reduction vs. sapropterin supports further clinical development of PTC923 for PKU; ANZCTR number, ACTRN12618001031257.

Frequent blood phenylalanine (Phe) measurement is required for phenylketonuria (PKU) patients for diagnosis and disease status monitoring. Though various methods are available for blood Phe measurement, there is a lack of validated quantitative methods for measuring Phe with less than 15% variability. A method to allow at home blood sample collection for the PKU community is in high demand. A volumetric absorptive microsampling (VAMS) dried blood collection high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) method was developed and fully validated for blood Phe measurement in compliance with regulatory guidances. The method accuracy, precision, stability, selectivity, matrix and hematocrit effects were assessed. A venous plasma collection HPLC-MS/MS method was developed and validated as a reference method. 311 matching VAMS and plasma samples were collected from 24 PKU subjects in a Phase 2 clinical study. Phe measurements using the two methods were compared. Both VAMS and the plasma sample collection methods met the acceptance criteria for Good Laboratory Practice (GLP) bioanalytical analysis. Comparisons showed a high Pearson's correlation of 0.9813. The Passing-Bablok analysis showed that the difference was estimated to be less than 5% and Bland Altman analysis indicated that the difference was proportional with Phe concentration and for the majority of samples (88.85%) the measurement was within ±20% difference. Following 7 days treatment with 60 or 20 mg/kg/day PTC923 (Sepiapterin) or 20 mg/kg/day sapropterin, PKU patients exhibited respectively -206.4, -146.9, and -91.5 µmol/L reductions of blood Phe as measured by the VAMS method. Concordant results were obtained using VAMS and plasma methods, which demonstrated that VAMS is a reliable method for clinical applications to monitor blood Phe for PKU patients.

Background: Sepiapterin is a natural precursor of tetrahydrobiopterin (BH₄), a key cofactor for phenylalanine hydroxylase. It is being developed for the treatment of patients with phenylketonuria. In this study, the ethnic differences in pharmacokinetics and safety of sepiapterin in Japanese and non-Japanese participants and food effects were evaluated. Methods: Healthy participants (n = 60) received a single oral dose of sepiapterin at either 20, 40, or 60 mg/kg with a low-fat diet. The Japanese participants received two doses at 40 mg/kg, either under fasted conditions or with a low-fat diet with a 3-day washout period in between. Results: Sepiapterin was well tolerated in all participants, with no serious adverse events. Sepiapterin was quickly absorbed (T_max 1.4–4.5 h) and rapidly and extensively converted to BH₄ (T_max ~4 h). Exposures to sepiapterin were A mutation in the Japanese population.

To report interim results from the ongoing, open-label, phase 3 APHENITY Extension Study (NCT05166161), evaluating long-term treatment with sepiapterin in patients with phenylketonuria. Participants received an age-based dose of oral sepiapterin daily; those with mean blood phenylalanine (Phe) levels <360 μmol/L (<5.95 mg/dL) after 2 weeks underwent a 26-week dietary Phe tolerance assessment, wherein dietary Phe intake was adjusted and blood Phe levels monitored. Other participants continued treatment with optional diet liberalization. Primary endpoints included change from baseline to week 26 in dietary Phe intake and treatment-emergent adverse events (TEAEs). As of September 2, 2024, 169 participants received sepiapterin (median [minimum, maximum] age: 14.0 [0.2, 55.0] years, median exposure: 72.9 weeks); 102 participants underwent dietary Phe tolerance assessments. Mean (SD) dietary Phe intake increased from 27.6 (18.0) mg/kg/day at baseline to 62.5 (41.5) mg/kg/day at week 26 (least-squares mean change [SE]: 36.4 [2.8] mg/kg/day from baseline) (P < .0001 from post hoc analysis). The incidence of treatment-related TEAEs was 29.0%; 3 participants (1.8%) discontinued treatment owing to treatment-related TEAEs. There were no treatment-related serious TEAEs or deaths. Interim results support the long-term safety of sepiapterin and demonstrate the potential for diet liberalization in adults and children with phenylketonuria. NCT05166161 (https://www. gov/study/NCT05166161; date of registration, December 8, 2021).

Sepiapterin and its major metabolite 6R ‐L‐erythro‐5,6,7,8‐tetrahydrobiopterin (BH ₄ ) bind to distinct variants of phenylalanine hydroxylase (PAH), which converts excess phenylalanine to tyrosine, thereby stabilizing, enhancing, and prolonging PAH activity. Sepiapterin was recently approved in Europe and the USA for the treatment of hyperphenylalaninemia patients with phenylketonuria, an inherent metabolic disease caused by PAH deficiency. A thorough QT study of sepiapterin in healthy volunteers at therapeutic (60 mg/kg) and supratherapeutic (120 mg/kg) doses was conducted to assess potential cardiovascular risks. Thirty‐two participants were randomized into one of 12 sequences and received single doses of sepiapterin (60 or 120 mg/kg), moxifloxacin 400 mg, or placebo in separate periods. Sepiapterin had no effect on heart rate or cardiac conduction (PR/QRS interval). Saturable sepiapterin absorption was observed, which resulted in less than dose‐proportional increase of sepiapterin and BH ₄ and limited the maximum plasma concentrations clinically achievable. Using concentration‐QT analysis, the placebo‐corrected change from baseline in QT interval corrected using Fridericia's formula (ΔΔQTcF) was −2.11 (90% CI: −3.44, −0.79) ms at geometric mean baseline‐corrected BH ₄ C _max (728 ng/mL) and −1.9 (−3.25, −0.56) ms at sepiapterin C _max (2.08 ng/mL) at the supratherapeutic dose of 120 mg/kg. An effect on ΔΔQTcF exceeding 10 ms was excluded within the observed concentration range of baseline‐corrected BH ₄ up to 1088 ng/mL and sepiapterin up to 5.77 ng/mL. The consistency of results from this study and the previous concentration‐QTc analysis based on pooled data from multiple clinical studies demonstrated the reliability of using concentration‐QTc for assessing cardiovascular risks in early clinical development.

Oral sepiapterin (PTC923), a precursor of tetrahydrobiopterin (BH₄), was evaluated in patients with 6-pyruvoyl-tetrahydropterin synthase (PTPS) deficiency in a phase 1/2, multicenter, open-label, randomized, intra-individual dose-escalation study. Patients stopped sapropterin (used to control hyperphenylalaninemia) for 2-4-days before being randomized 1:1 into 2 cohorts receiving sepiapterin at 2 doses (Cohort 1: 2.5 and 10 mg/kg/day; Cohort 2: 5 and 20 mg/kg/day) each for 7 days, separated by a 2-4-day washout period. Eight participants (5 male, 3 female, aged 2.2-20.0 years) completed the study (4 in each cohort). Overall, 7/8 participants (87.5%) experienced a total of 35 treatment-emergent adverse events; none were severe, serious, or led to discontinuation. Mean phenylalanine levels increased (up to 800-1200 μmol/L) after discontinuation of sepiapterin or sapropterin, and normalized (<130 μmol/L) with sepiapterin at all tested doses by Day 2. Blood BH₄ concentrations increased with increasing single doses of sepiapterin, reaching a peak at approximately 3-4 hours, which was 2-3 hours after the sepiapterin peak. These results show that in patients with PTPS deficiency, sepiapterin is rapidly converted to BH₄ and normalizes blood phenylalanine concentrations, with no dose-limiting toxicity or dose-related adverse events.