Ablation of folliculogenesis in women by a single dose of gonadotropin-releasing hormone agonist: significance of time in cycle.

Effects of single subcutaneous doses (1, 5, 20, and 100 micrograms) of nafarelin, a potent gonadotropin-releasing hormone agonist, on the physiologic events of the human menstrual cycle were studied in 28 normal women. Nafarelin entered the circulation rapidly after injection. Peak concentrations were observed within 1 hour, and the plasma half-life was 4 to 5 hours. Maximal concentrations of luteinizing hormone and follicle-stimulating hormone were reached 3 to 4 hours after nafarelin administration. The magnitude of the gonadotropin responses depended both on the phase of the menstrual cycle (smallest responses during the early follicular phase) and the dose of nafarelin. Nafarelin administration during the early follicular phase delayed ovulation by 4.6 +/- 1.7 (standard deviation) days and prolonged the duration of the menstrual cycle from a pretreatment length of 29.2 +/- 2.1 days to 33.4 +/- 4.0 days (P less than 0.001). When nafarelin was administered shortly before or after ovulation, cycle length was not altered consistently. Administration 5 to 10 days after ovulation resulted in a truncated luteal phase. These observations suggest that the hormonal events triggered by nafarelin during the early follicular phase temporarily arrest the process of selection of the dominant follicle. Repeated intermittent administration of nafarelin or other gonadotropin-releasing hormone agonists in the early follicular phase may prevent follicular maturation and ovulation and may be a practical approach to contraceptive development.