The invention relates to polymorphic forms of the drug substance (S)-3-(3-(1-methyl-2-oxo-5-(pyrazolo[1,5a]pyridine-3-yl)-1H-imidazo(4,5-b]pyridine-3(2H)-yl)piperidin-1-yl)-3-oxopropanenitrile of the formula (I) and their use as therapeutic agents for the treatment of respiratory diseases.