Carcinoma of the breast is the most common cancer in women. Currently, over 180,000 woman are diagnosed with breast cancer in the US alone. In 1996 over 44,000 women in the US died of breast cancer.
As an anti-progestin, mifepristone blocks the action of progesterone, and therefore may be effective in treating progesterone-dependant breast cancer. The molecular mechanisms by which anti-progestins inhibit human breast cancer cell growth are unknown. It is believed that initial interaction with the progesterone receptor is an essential first step in initiating the as yet ill defined cascade of events leading to growth inhibition.
Animal studies performed in the Netherlands have shown that mifepristone reduces breast cancer tumours as well as tamoxifen ( an anti-oestrogenic drug which is currently being used to treat breast cancer). The trials showed that mifepristone and tamoxifen administered together were more effective in reducing tumour size than either drug alone.
Mifepristone has been tested as a second-line treatment following first-line treatment with tamoxifen and over half the patients in one study showed stable disease of 3 to 8 months duration.