The primary objective of this study is to demonstrate the efficacy and safety of linzagolix administered orally once daily for 3 months at a dose of 75 mg alone or of 200 mg in combination with add-back hormone replacement therapy (ABT: estradiol (E2) 1 mg / norethisterone acetate (NETA) 0.5 mg) versus placebo, in the management of moderate to severe endometriosis-associated pain (EAP).

This is a prospective, randomized, double-blind, placebo-controlled study to demonstrate the efficacy and safety of linzagolix administered orally once daily at doses of 75 mg alone and 200 mg in combination with low dose ABT (E2 1 mg/NETA 0.5 mg) versus placebo in the management of moderate to severe EAP in 450 women.

Eligible subjects who have completed the 6-month treatment period may enter a separate extension study for 6 additional months of active treatment (no placebo control). Subjects who do not continue in the extension study will enter a 6 month treatment-free follow-up phase.

Inclusion Criteria:

The subject must have:

• Her most recent surgical and - if available - histological diagnosis of pelvic endometriosis up to 10 years before screening.
• Moderate to severe endometriosis-associated pain during the screening period.
• Regular menstrual cycles.
• BMI ≥ 18 kg/m2 at the screening visit. -

Exclusion Criteria:

The subject will be excluded if she:

• Is pregnant or breast feeding or is planning a pregnancy within the duration of the treatment period of the study.
• Is less than 6 months postpartum or 3 months postabortion/ miscarriage at the time of entry into the screening period.
• Has had a surgical history of any major abdominal surgery within 6 months or any interventional surgery for endometriosis performed within a period of 2 months before screening.
• Did not respond to prior treatment with GnRH agonists or GnRH antagonists for endometriosis.
• Has a history of, or known, osteoporosis or other metabolic bone disease.
• Has chronic pelvic pain that is not caused by endometriosis and requires chronic analgesic or other chronic therapy which would interfere with the assessment of endometriosis-associated pain.

Females Only