Does timing matter when initiating elagolix in a natural menstrual cycle?

OBJECTIVE: To investigate the efficacy of elagolix when administered at different time points in a menstrual cycle. DESIGN: Clinical case series. SETTING: Academic reproductive endocrinology center. PATIENTS: Ovulatory women not desiring pregnancy. INTERVENTION(S): Six doses of elagolix 200 mg were administered over 4 days, starting at 3 different points in a menstrual cycle: early follicular; late follicular; and midluteal. Serum follicle-stimulating hormone (FSH), luteinizing hormone (LH), estradiol (E2), and progesterone (P) concentrations were measured at baseline, during elagolix administration, and 1 day after the last dose. Transvaginal ultrasounds were performed to monitor follicle sizes. MAIN OUTCOME MEASURE(S): Serum FSH, LH, E2, and P. RESULT(S): Twelve women, four per group, completed the study. Subjects were 23–42 years of age. Demographics and ovarian reserve parameters were similar among participants. Elagolix suppressed FSH, LH, E2, and P when administered in the early follicular and midluteal phases but had mixed results when administered in the late follicular phase. Two participants demonstrated suppression of all four hormones. One participant ovulated, indicated by an increase in P concentration and development of a corpus luteum. A second participant did not ovulate yet demonstrated an increase in E2 concentration with growth of a dominant follicle. There were no significant differences in median percent change of hormone concentrations across study groups. CONCLUSION(S): The results of this study suggest that elagolix can suppress the hypothalamic–pituitary–ovarian axis when initiated at different points in a menstrual cycle. Optimal dosing and treatment window for consistent hormone suppression have yet to be determined. CLINICAL REGISTRATION NUMBER: NCT04060992