Provided herein are compounds, salts, crystalline forms, and pharmaceutical compositions that are related to IDO inhibitors, in particular, Compound 1,1-(5-((4-chlorophenyl)amino)-6-(ethyl(tetrahydro-2H-pyran-4-yl)amino)pyridin-3-yl)cyclobu tane-1-carboxylic acid. Also provided are methods of treating a disease or disorder such as a cancer or infectious disease that comprises administering to a subject in need thereof one or more of the compounds or compositions of the present disclosure.