Hydantion indolinones as AANAT inhibitors

Arylalkylamine N-acetyltransferase (AANAT) is a key enzyme in melatonin biosynthesis and a regulator of circadian rhythm, with potential relevance to mood disorders such as seasonal affective disorder (SAD). We report a series of hydantoin indolinone-based AANAT inhibitors, developed as more stable alternatives to a previously reported rhodanine scaffold. Guided by docking studies and prior structure-activity data, we modified four regions of the molecule to improve potency. Substitution at the 5-position of the indolinone ring led to marked increases in activity, with compound 5g (bearing a CH₃CO₂CH₂- substituent) resulting in an IC₅₀ of 1.1 μM-representing a 19-fold improvement over the parent compound. Kinetic mechanism studies were also conducted with respect to acetyl-CoA and serotonin to explore inhibitor binding. These findings establish a promising starting point for the development of more potent AANAT inhibitors as chemical probes for studying melatonin's function.