An EC 1.1.1.* (oxidoreductase acting on donor CH-OH group, NAD(+) or NADP(+) acceptor) inhibitor that interferes with the activity of sepiapterin reductase (L-erythro-7,8-dihydrobiopterin forming), EC 1.1.1.153, which plays an important part in the biosynthesis of tetrahydrobiopterin.