“The potential effectiveness of sepiapterin was evaluated in three in vitro studies using COS-7 cells expressing different PAH protein variants and an in vivo single dose non-GLP study with a Pah-R261Q mouse model of human PKU disease.”
Not reported in the public reviews. Reviewers summarise selectively, so this usually means it wasn't discussed publicly, not that it wasn't studied.
Reviewer's view of the efficacy data
FDA: studies have limitations; in vitro non-GLP data insufficient to define the dual mechanism
“These studies have limitations in supporting sepiapterin effectiveness and mechanism of action. The in vitro non-GLP studies are not sufficient to clearly define a dual mechanism for sepiapterin.”