The invention provides Saxagliptin Schiff bases, polymorphs of Saxagliptin and (1S,3S,5S)-2-[(2S)-2-propan-2-ylideneamino-2-(3-hydroxy-1-adamantyl)acetyl]-2-azabicyclo[3.1.0]hexane-3-carbonitrile, processes for preparing Saxagliptin hydrates, and pharmaceutical compositions thereof.