PTC Therapeutics has reported that the Phase III APHENITY clinical trial of sepiapterin to treat adult and paediatric patients with phenylketonuria (PKU), an inherited metabolic disease, achieved primary endpoint.

PKU diagnosis usually takes place during newborn screening programmes.

The international double-blind, placebo-controlled, registration-directed trial of the oral formulation of synthetic sepiapterin enrolled 156 children and adults with PKU.

They were randomised to receive sepiapterin or placebo for a period of six weeks. Reduction in blood phenylalanine levels was the study’s primary endpoint.

Part 1 of the trial was a run-in phase, where all subjects who were screened received sepiapterin for two weeks.

In Part 2, only those participants who demonstrated a reduction in phenylalanine levels of 15% or more from baseline in the initial part, were randomised to receive either sepiapterin or placebo.

Subjects who reported greater than 30% reduction in phenylalanine levels from baseline during Part 1 were part of the primary analysis population.

The primary outcome measure of the trial is blood phenylalanine levels reduction from baseline against weeks five and six in patients from Part 2.

All patients can enrol in an open-label long term clinical trial, which will further assess sepiapterin’s long-term safety and durable effect.

PTC Therapeutics CEO Matthew Klein said: “The positive results from the APHENITY trial bring us one step closer to providing a therapy that could deliver meaningful benefit to PKU patients.

“The Phe reductions observed in the placebo-controlled portion of the study are consistent with, and, in some cases, exceed the magnitude of Phe reductions recorded in the open label portion of the study. We look forward to meeting with regulatory authorities to discuss the path to approval.”

The trial’s placebo-controlled portion had 98 patients in the primary analysis population.

The mean percent Phe reduction in patients who received sepiapterin was found to be 63% and in the subset of classical PKU patients, it was 69%. Additionally, Sepiapterin was generally well tolerated and no serious adverse events were observed.