LX1606 is an orally bioavailable, small-molecule, tryptophan hydroxylase (TPH) inhibitor with potential antiserotonergic activity.

4-Ethynyl-L-phenylalanine hydrochloride is a selective, reversible, potent and competitive inhibitor of tryptophan hydroxylase (TPH). 4-Ethynyl-L-phenylalanine hydrochloride is a competitive inhibitor with regard to the substrate tryptophan, with a Ki of 32.6 μM. 4-Ethynyl-L-phenylalanine hydrochloride selectively and reversibly inhibits the biosynthesis of serotonin.