The objective of this study is to demonstrate the efficacy and safety of BG2109 administered orally once daily at a dose of 100 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, while under randomized treatment, in the management of moderate to severe endometriosis-associated pain (EAP) in chinese women with surgically confirmed endometriosis

This is a prospective, randomized, double-blind, placebo-controlled study to demonstrate the efficacy and safety of BG2109 administered orally once daily at doses of 100 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 chinese women with surgically confirmed endometriosis.

The entire study is divided into the core stage and the extension stage, with a total duration of approximately 72 weeks. This includes the prescreening wash-out period (if applicable), screening period (approximately 5-8 weeks), core stage (24 weeks) and extension stage (28 weeks), and the safety follow-up period (approximately 12 weeks)

Key Inclusion Criteria:

1. The subject must be a premenopausal woman aged ≥18 years.

2. The subject must have had her most recent surgical and histological diagnosis of pelvic endometriosis (laparoscopy, laparotomy, vaginal fornix or other biopsy) within 10 years and at least 2 months ago prior to screening.

3. The subject has moderate to severe EAP during the screening period defined as:

   1. At the screening visit, a score of at least 2 for DYS and at least 2 for NMPP for the previous month assessed with the modified Biberoglu & Behrman (mB&B) scale
   2. Subject is confirmed to meet the following criteria during the screening period, within 35 consecutive calendar days prior to the baseline visit:

   i. Mean overall pelvic pain scores on the 0-10 NRS over the 5 days with the highest score ≥ 4; ii. At least two days with "moderate" or "severe" pain on the 0-3 VRS for pelvic pain over the days with uterine bleeding; iii. At least two days with "moderate" or "severe" pain on the 0-3 VRS for pelvic pain over the days without uterine bleeding;

4. The subject has a Body Mass Index (BMI) ≥ 18 kg/m2 at the screening visit

Key Exclusion Criteria:

1. The subject is pregnant or breastfeeding, or plans to become pregnant during the study treatment period.

2. The subject has a surgical history of:

   1. Hysterectomy,
   2. Bilateral oophorectomy,
   3. Surgeries that interfere with gastrointestinal motility, pH value, or absorption (including vagotomy, enterectomy, or gastric surgery),
   4. Any major abdominal surgery (including laparotomy for endometriosis) within 6 months or any interventional surgery for endometriosis (i.e. laparoscopy) performed within a period of 2 months before screening, or the subject is scheduled for a surgical abdominal procedure during the course of the study.

3. The subject may need to take prohibited medications during the study or in the stipulated time before screening

4. The subject has a contra-indication to ABT

5. The subject has chronic pelvic pain that, in the opinion of the Investigator, is not caused by endometriosis and requires chronic analgesic or other chronic therapy which would interfere with the assessment of EAP

6. The subject has conditions that affect bone mass density (BMD) assessment

7. The subject did not respond to prior treatment with GnRH agonists or GnRH antagonists for endometriosis