No QT interval prolongation effect of sepiapterin: a concentration-QTc analysis of pooled data from phase 1 and phase 3 studies in healthy volunteers and patients with phenylketonuria

Sepiapterin is an exogenously synthesized new chemical entity that is structurally equivalent to endogenous sepiapterin, a biological precursor of tetrahydrobiopterin (BH₄), which is a cofactor for phenylalanine hydroxylase. Sepiapterin is being developed for the treatment of hyperphenylalaninemia in pediatric and adult patients with phenylketonuria (PKU). This study employed concentration-QT interval analysis to assess QT prolongation risk following sepiapterin treatment. Data from three phase 1 studies and one phase 3 study were pooled for this analysis. Pediatric and adult PKU patients ≥ 2 years received multiple doses at 60 mg/kg and adult healthy volunteers received a single or multiple doses at 20 or 60 mg/kg. Time-matched triplicate ECG measurements and plasma samples for pharmacokinetic analysis were collected. Prespecified linear mixed models relating ΔQTcF to concentrations of sepiapterin and the major active circulating metabolite BH₄ were developed for the analysis. The analysis demonstrated that there is no QTcF prolongation risk in patients with PKU following sepiapterin dosing at the highest therapeutic dose, 60 mg/kg/day. The final model showed a marginal but negligible QTcF reduction with increasing sepiapterin and BH₄ concentrations. The effect on ΔQTcF was estimated to -2.72 [-3.72, -1.71] and - 1.25 [-2.75, 0.25] ms at mean baseline adjusted BH₄ C_max of 332 ng/mL (therapeutic exposure) and 675 ng/mL (supratherapeutic exposure) at dose 60 mg/kg, respectively, in PKU patients with food and in healthy volunteers with a high fat diet. Various covariates, such as clinical study ID, age, sex, food effect, race, body weight, and disease status, on QTcF interval were investigated and were found insignificant, except for food effect and age. This study concludes that there is no QTcF prolongation risk in patients with PKU following sepiapterin dosing up to 60 mg/kg/day, and BH₄ and sepiapterin concentrations minimally affect ΔQTcF after adjustment for time, sex, and meal.