The present invention provides Di potassium (S)-2-(4-(2-(2-amino-4-oxo-4,7-dihydro-3H-pyrrolo[2,3-d]pyrimidin-5-yl)ethyl)benzamido) pentanedioate (I) and its hydrates useful as active pharmaceutical ingredient in pharmaceutical compositions for the treatment cancer. The present invention also provides process for preparation thereof.