A fast-dissolving pharmaceutical composition comprising micronized (R)-2-(4-bromo-2-fluorobenzyl)-1,2,3,4-tetrahydropyrrolo[1,2-a]pyrazine-4- -spiro-3'-pyrrolidine-1,2',3,5'-tetrone (hereinafter, referred to as AS-3201). The present pharmaceutical composition has improved dissolution characteristics as well as a good bioavailability.