Transient inhibition of RU 486 antiglucocorticoid action by dexamethasone

RU 486 antagonizes both progesterone and glucocorticoids at the receptor level. To study the duration of RU 486 antiglucocorticoid activity on corticotropic function and to establish the means of overcoming it with dexamethasone, plasma corticolipotropic hormones and cortisol were measured in 10 healthy male patients during the 3 days after intake, at 2200 h, of a single 400-mg dose of RU 486, alone or combined with a single dexamethasone dose (1, 2, or 4 mg) given at 2400 h. On the first day after RU 486 alone, ACTH, lipotropin, and cortisol plasma levels were significantly higher than basal values. The 1-mg dose of dexamethasone totally abolished the stimulatory effect of RU 486, and higher doses of dexamethasone (2 and 4 mg) further depressed hormone levels. During the succeeding days, antiglucocorticoid activity of RU 486 alone was still present 34 h after administration, while on the third day all hormone levels returned to normal. After the combined administration, the RU 486 effect reappeared as early as the first day with the 1-mg dose of dexamethasone, while it was delayed until the third day with the higher doses. These results showed that a single 400-mg dose of RU 486 induced a response that lasted at least 34 h. Thus, a dose-dependent competition between RU 486 and dexamethasone was demonstrated. However, the suppressive effect of dexamethasone was only transient, after which the antiglucocorticoid activity of RU 486 reappeared.