Onapristone is a full progesterone receptor antagonist that was originally developed as an oral contraceptive and shown to have efficacy in breast cancer and other malignancies.

Liver enzyme elevations led to a halt in its original development program.

A review of antiprogestin pharmacology and pharmacokinetic data suggests that liver enzyme elevations observed in clinical trials with onapristone might be related to off-target effects associated with serum maximum plasma concentrations, which are mitigated by the extended-release formulation.