Potent and selective inhibitor of tryptophan 2,3-dioxygenase (TDO) (Ki = 51 nM). Exhibits no activity against indoleamine 2,3-dioxygenase, monoamine oxidase A and B, 5-HT uptake or 5-HT1A, 1D, 2A and 2C receptors. Produces large increases in brain tryptophan and serotonin in vitro and in vivo in the rat.