Cell-permeable. A potent and selective inhibitor of tryptophan 2,3-dioxygenase (TDO, Ki = 51 nM.) Tryptophan is metabolized to the endogenous AHR ligand by TDO which may be inhibited by 680C91. Active in vivo.http://www.creative-enzymes.com/product/680C91_2644.html