ABSTRACT
Since the isolation of progesterone in 1934,1,2 the primary natural ligand of the progesterone receptor (PR), many steroidal and non-steroidal compounds have been synthesized, which exhibit a high binding affinity to PR and specific effects in progesterone target tissues. Three classes of compounds belong to the PR ligand family: PR agonist (progestins), PR antagonists (PRAs, antiprogestins, also known as progesterone antagonists), and selective progesterone receptor modulators (SPRMs). In this review, to avoid confusion with the PR-A isoform (PR-A), we use the abbreviation PA to mean PR antagonists.
