Onapristone extended-release formulations and methods of administering onapristone extended-release formulations are provided. Onapristone extended-release formulations provide sufficient therapeutic activity as compared to immediate-release formulations with reduced potential for adverse side effects. Aspects described herein provide extended-release pharmaceutical compositions comprising onapristone as the active ingredient in an amount from about 2 mg to about 100 mg. The extended-release pharmaceutical compositions (also referred to herein as ER formulations) further comprise excipients suitable for the desired dosage form (e.g., tablet, capsule, etc.) and for delaying the release of the active ingredient.
