Described herein is the bromodomain inhibitor 4-[2-(cyclopropylmethoxy)-5-methylsulfonylphenyl]-2-methylisoquinolin-1-one, including crystalline forms, amorphous forms, solvates, and hydrates thereof, as well as pharmaceutical compositions that include this bromodomain inhibitor. In some embodiments, the pharmaceutical composition comprises 4-[2-(cyclopropylmethoxy)-5-methylsulfonylphenyl]-2-methylisoquinolin-1-one that has been processed by micronization or spray dried dispersion. In some embodiments, the pharmaceutical composition further comprises at least one polymer. In some embodiments, the pharmaceutical composition comprises a solid polymer matrix comprising 4-[2-(cyclopropylmethoxy)-5-methylsulfonylphenyl]-2-methylisoquinolin-1-one and at least one polymer. Pharmaceutical compositions comprising 4-[2-(cyclopropylmethoxy)-5-methylsulfonylphenyl]-2-methylisoquinolin-1-one are useful for the treatment of cancer or neoplastic disease.