Inclusion Criteria:

          -  Age greater than or equal to 18 years old

          -  Histologically confirmed diagnosis of endometrial cancer with ER and/or PR expression
             >= 1% by IHC on archival tissue taken within the prior 3 years or new biopsy if no
             archival tissue is available. IHC results do not have to be from Thomas Jefferson
             University

          -  Patients who have failed front line therapy with carboplatin/paclitaxel

          -  Measurable disease as defined by Response Evaluation Criteria in Solid Tumors (RECIST)
             version (v.)1.1. Measurable disease is defined as at least one lesion that can be
             accurately measured in at least one dimension. Each lesion must be >= 10 mm when
             measured by computed tomography (CT) or magnetic resonance imaging (MRI). Lymph nodes
             must be >= 10 mm in short axis when measured by CT or MRI

          -  Patients with the following histologic epithelial cell types are eligible:

               -  Endometrioid adenocarcinoma

               -  Serous adenocarcinoma

               -  Undifferentiated carcinoma

               -  Clear cell adenocarcinoma

               -  Mixed epithelial carcinoma

               -  Adenocarcinoma not otherwise specified (NOS)

               -  Please note: patients with carcinosarcoma are ineligible for this trial

          -  Patients must have had one prior treatment with a platinum/taxane chemotherapy regimen
             for management of disease

          -  They cannot receive chemotherapy, immunotherapy or other endocrine therapy
             concurrently

          -  Eastern Cooperative Oncology Group (ECOG) performance status 0-1

          -  Must have a life expectancy of at least 12 weeks as judged by the treating physician

          -  Postmenopausal females are only eligible for this study. This is defined as being
             status post (s/p) hysterectomy or patients who are in menopause is defined clinically
             as 12 months of amenorrhea in a woman over 45 in the absence of other biological or
             physiological causes. In addition, women under the age of 55 must have a documented
             serum follicle stimulating hormone (FSH) level less than 40 mIU/mL

          -  Body weight > 30 kg

          -  Absolute neutrophil count 1500/ul or more

          -  Platelets 100,000/ul or more

          -  Hemoglobin 9 g/dl or more

          -  Bilirubin less than or equal to 1.5 x the upper limit of normal (except subjects with
             Gilbert syndrome, who can have total bilirubin < 3 mg/dl)

          -  Endocrine and targeted therapy protocols usually enroll patients with aspartate
             aminotransferase (AST)/alanine aminotransferase (ALT) < 2.5 x upper limit of normal
             (ULN) in patients without underlying liver metastasis and < 5.0 x ULN in patients with
             underlying liver metastasis

          -  Glomerular filtration rate (GFR) greater than or equal to 40 ml/min using the
             Cockcroft-Gault formula or measured creatinine clearance using 24 hours urine
             collection

          -  International normalized ratio (INR) OR prothrombin time (PT) and activated partial
             thromboplastin time (aPTT) =< 1.5 x ULN unless participant is receiving anticoagulant
             therapy as long as PT or aPTT is within therapeutic range of intended use of
             anticoagulants

          -  All subjects must be able to comprehend and sign a written informed consent document

          -  Resolution of all acute toxic effects of prior therapy to National Cancer Institute
             (NCI) Common Terminology Criteria for Adverse Events (CTCAE) (version 5.0) grade =< 1,
             with the exception of unresolved grade 2 neuropathy and grade 2 alopecia, which are
             allowed

          -  Patient has recovered from any prior radiotherapy

          -  Patients must be able to swallow tablets whole, without crushing

          -  Be able to read and speak English

        Exclusion Criteria:

          -  Concurrent or recent chemotherapy, radiotherapy, immunotherapy, or general
             anesthesia/major surgery within 3 weeks

          -  History of prior hormonal therapy (i.e., megestrol acetate, tamoxifen or aromatase
             inhibitors) for treatment cancer within the past 2 months. Other concurrent hormonal
             therapy will not be allowed on this trial

          -  Patients with concurrent second malignancy (other than non-melanoma skin cancer or
             curatively treated in situ carcinoma)

          -  Patients must have recovered from all known or expected toxicities from previous
             treatment and passed a treatment-free "washout" period of 3 weeks before starting this
             program. However, grade 1 or 2 neuropathy and alopecia are acceptable

          -  If participant received major surgery, they must have recovered adequately from the
             toxicity and/or complications from the intervention prior to starting study treatment

          -  Has received prior systemic anti-cancer therapy including investigational agents
             within 3 weeks prior to randomization

          -  Participants must have recovered from all adverse events (AEs) due to previous
             therapies to =< grade 1 or baseline. Participants with =< grade 2 neuropathy may be
             eligible

          -  Known brain metastasis which have not been treated or showed stability for >= 6 months

          -  Proteinuria > 1+ on urinalysis or > 1 gm/24 hours (hr)

          -  Known history of New York Heart Association stage 3 or 4 cardiac disease

          -  A pleural or pericardial effusion of moderate severity or worse

          -  Women who are pregnant or nursing

          -  Women who are pre-menopausal

          -  Has an active infection requiring systemic therapy

          -  Use of any prescription medication during the prior 28 days of first onapristone
             dosing that the investigator judges is likely to interfere with onapristone activity;
             specifically strong inhibitors or inducers, or sensitive substrates of cytochrome P450
             CYP3A4

          -  Patients may not be on a concurrent clinical trial, unless approved by investigator