Lonaprisan (INN, USAN) (developmental code names ZK-230211, BAY 86-5044, ZK-PRA) is a synthetic, steroidal antiprogestogen which was under development by Bayer HealthCare Pharmaceuticals for the treatment of endometriosis, dysmenorrhea, and breast cancer but was discontinued.

[1][2][3] It is a potent and highly selective silent antagonist of the progesterone receptor (PR).

[2][3][4] The drug reached phase II clinical trials prior to its discontinuation.

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