The progesterone receptor antagonist mifepristone does not lower serum progesterone induced blocking factor (PIBF) in the presence of progesterone

PURPOSE: To determine if mifepristone can lower serum levels of a progesterone (P) induced immunomodulatory protein believed to be needed for the fetus to escape immune surveillance. MATERIALS AND METHODS: A female volunteer had her serum P induced blocking factor (PIBF) increased by ingestion of oral micronized P. While remaining on P mifepristone, 200 mg/day was given for six days when another serum PIBF level was obtained. RESULTS: The serum PIBF was 273 ng/ml after five days of oral micronized P. It increased further to 737 ng/ml despite taking six days of 200 mg mifepristone. CONCLUSIONS: The mechanism for inducing abortion by mifepristone does not seem to be related to decreasing serum levels of PIBF. This does not eliminate the possibility that the mechanism involves reducing the intracytoplasmic PIBF levels.