Study was prematurely discontinued due to GSK's change in prioritization for the portfolio and is not due to any safety concerns or regulatory interaction.

The primary objective of the study is to assess the efficacy, safety, and tolerability of epelsiban compared with placebo in treatment of women with adenomyosis.

This is a 12-week, randomized, double-blind, placebo-controlled, parallel group study with an interim futility analysis. Subjects will be randomized 1:1:1 to receive 75 milligrams (mg) of epelsiban three times daily (TID), 200 mg of epelsiban TID, or placebo TID. The study will be composed of three periods: screening, treatment, and follow-up and the total time a subject will be in the study will be approximately 6 months.

Inclusion Criteria:

• Eighteen to 55 years of age, inclusive
• Pre-menopausal with a history of regular menstrual cycles every 21-35 days and without intermenstrual bleeding heavier than spotting and staining.
• Females with adenomyosis confirmed on magnetic resonance imaging (MRI), - Females with heavy menstrual bleeding .
• Willing and able to collect all menstrual cycle by-products for each cycle from screening to follow up.
• Not pregnant as confirmed by a negative serum human chorionic gonadotropin

Exclusion Criteria:

• A female subject will not be eligible for inclusion in this study if any of the following criteria apply:
• Abnormal gynecological examination other than adenomyosis and/or breast examination requiring intervention within six months of study start
• Abnormal endometrial biopsy within six months of starting study treatment.
• History of an endometrial ablation within 12 months of starting study treatment.
• Uterine artery embolization within six months of starting study treatment.
• Prior major uterine procedures or any other significant uterine abnormalities on MRI (previous caesarean section, dilation and curettage, and diagnostic hysteroscopy are permitted).
• Confirmed rectovaginal endometriosis in women who have undergone a prior laparoscopy.
• Active pelvic infection or current use of an intrauterine device within three months of screening.
• Women with a history of transfusion for heavy menstrual bleeding within the past 2 years or history of postpartum hemorrhage.
• Any uterine dimension >20 centimeter (cm).
• Other major causes of heavy menstrual bleeding -
• Use within 3 months or anticipated use of medications that modify reproductive function
• Use or anticipated use of the following drugs: anticoagulants aminocaproic acid ,or any other medications that affect menstrual bleeding such as tranexamic acid.
• Use of daily opioid pain medications other than with menses.
• Hemoglobin <8 grams (g)/deciliter.
• History of bleeding disorder or known presence of acquired or inherited thrombophilia, (sickle cell trait individuals are not excluded).