Selective tryptophan 2, 3-dioxygenase (TDO) inhibitor (IC50 values are 0.62 and 2 μM for human and mouse TDO, respectively). Exhibits selectivity for TDO over IDO, MAO-A, MAO-B, and a panel of receptors and transporters. Reduces growth of TDO-expressing P815 mastocytoma tumors in mice. Orally bioavailable.