The present invention relates to the mono-sulfate salt, crystalline forms A and B of the mono-sulfate salt, an amorphous form of the mono-sulfate salt, hemi-sulfate salts, and a crystalline form A of the hemi-sulfate salt of the compound 2-chloro-4-[1-(4-fluoro-phenyl)-2,5-dimethyl-1H-imidazol-4-ylethynyl]-pyridine.