Abstract: The present disclosure relates to pharmaceutical composition comprising 1) an amorphous solid dispersion comprising pralsetinib, or a pharmaceutically acceptable salt thereof; and a pharmaceutically acceptable hydrophilic polymer; and 2) an effervescent couple; and crystalline forms of pralsetinib and pralsetinib hydrochloride salt, which are useful as a RET selective inhibitors. The present disclosure also provides pharmaceutically acceptable compositions comprising the crystalline forms and methods of using said compositions in the treatment of various disorders.