Mifepristone-induced vaginal bleeding is associated with increased immunostaining for cyclooxygenase-2 and decrease in prostaglandin dehydrogenase in luteal phase endometrium.

The mechanism of mifepristone-induced vaginal bleeding and endometrial shedding was investigated in 13 women who took 200 mg mifepristone in the midluteal phase on d 8 after the onset of the urinary LH surge (LH+8). Endometrial biopsies were collected, 6-24 h after mifepristone (group 1, n = 7) or 36-48 h after mifepristone (group 2, n = 6), and compared with those from a control group in the midluteal phase (n = 7). All women reported vaginal bleeding commencing 36-48 h after taking mifepristone. Treatment with mifepristone significantly reduced serum progesterone levels in all women, when compared with the controls (13.2 nM vs. 34.8 nM, P = 0.001). After mifepristone, a significant increase in cyclooxygenase-2 immunoreactivity was apparent at 36-48 h (P = 0.0018), whereas prostaglandin 15 dehydrogenase enzyme-positive immunostaining declined, to be virtually absent by 36-48 h in both glands and in stroma (P < 0.05). There was no change in intensity or distribution of staining for steroid receptors after mifepristone. The changes in immunostaining for cyclooxygenase-2 and prostaglandin 15 dehydrogenase strongly support the hypothesis that an increase in the local concentration of prostaglandins in the endometrium is involved in the mechanism of bleeding induced by mifepristone in the luteal phase.