Effective suppression of luteinizing hormone and testosterone by single doses of the new gonadotropin-releasing hormone antagonist cetrorelix (SB-75) in normal men

In rats and nonhuman primates the new GnRH antagonist cetrorelix (SB-75; [Ac-D-Nal(2)1,D-Phe(4Cl)2,D-Pal(3)3,D-Cit6,D-Ala10]GnR H) has been shown to suppress testosterone secretion effectively and persistently. A clinical phase I study was performed to assess the hormonal effects of this highly potent antagonist in normal men. After 2 control examinations 30 young male volunteers were randomly assigned to 6 treatment groups (n = 5/group), and single doses of 0 (placebo), 0.25, 0.5, 1.0, 2.0, and 5.0 mg cetrorelix were administered sc. Blood samples were obtained over the course of 7 days postinjection. After maximal cetrorelix serum levels were achieved 1 h postinjection, the GnRH antagonist serum levels decreased with a terminal t1/2 of 29.8 +/- 4.2 h (mean +/- SE). LH secretion was suppressed dose- and time-dependently; maximal suppression occurred 4-6 h postinjection. Suppression of FSH did not reach statistical significance. Doses of 1.0, 2.0, and 5.0 mg cetrorelix significantly suppressed testosteron...