Described herein are amorphous and crystalline forms of pharmaceutically acceptable salts of the lysine specific demethylase-1 inhibitor 4-[2-(4-aminopiperidin-1-yl)-5-(3-fluoro-4-methoxy-phenyl)-1-methyl-6-oxo-1,6-dihydropyrimidin-4-yl]-2-fluorobenzonitrile. Also described are pharmaceutical compositions suitable for administration to a mammal that include the lysine specific demethylase-1 inhibitor, and methods of using the lysine specific demethylase-1 inhibitor for treating diseases or conditions that are associated with lysine specific demethylase-1 activity.