This invention is directed to 6-[4-METHYL-1-(PYRlMIDIN-2-YLMETHYL)PYRROLIDIN-3-YL]-3-TETRAHYDROPYRAN-4-YL-7H-IMIDAZO[1,5-A]PYRAZIN-8-ONE and pharmaceutically acceptable acid addition salts thereof, which are PDE9 enzyme inhibitors. The invention provides a pharmaceutical composition comprising a therapeutically effective amount of a compound of the invention and a pharmaceutically acceptable carrier. The present invention also provides processes for the preparation of the compounds of invention. The present invention further provides a method of treating a subject suffering from a neurodegenerative disorder comprising administering to the subject a therapeutically effective amount of a compound of the invention. The present invention further provides a method of treating a subject suffering from a psychiatric disorder comprising administering to the subject a therapeutically effective amount of a compound of formula (I).
