The present application provides novel inhibitors of indoleamine 2,3-dioxygenase-1 and/or indoleamine 2,3-dioxygenase-2 and/or tryptophan 2,3-dioxygenase, metabolites thereof, and pharmaceutically acceptable salts or prodrugs thereof. Also provided are methods for preparing these compounds. A therapeutically effective amount of one or more of the compounds of formula (I) is useful in treating diseases resulting from dysregulation of the kynurenine pathway. Compounds of formula (I) act by inhibiting the enzymatic activity or expression of indoleamine 2,3-dioxygenase-1 and/or indoleamine 2,3-dioxygenase-2 and/or tryptophan 2,3-dioxygenase.