Purity >98% by NMR, TLC. 680C91 is a potent and selective inhibitor of tryptophan 2,3-dioxygenase (TDO) Ki=42 nM) Displays no activity against indoleamine 2,3-dioxygenase, MAO-A and B and does not effect serotonin uptake. Elevates CSF tryptophan by up to 260% and CSF serotonin by 170% of basal. TDO inhibition by 680C91 in glioma cells blocked the release of kynurenine, an endogenous tumor promoting AHR ligand.