Inclusion Criteria:

          -  Histologically confirmed diagnosis at MSK of either (1) granulosa cell ovarian cancer,
             (2) low grade serous ovarian/ primary peritoneal cancer, or (3) endometrioid
             endometrial cancer; with 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 MSK.

          -  Measurable disease as defined by RECIST 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 ≥10mm when measured by CT or MRI. Lymph nodes must be ≥15mm in short axis when
             measured by CT or MRI

          -  Patients must have had one prior chemotherapy regimen for management of disease. Note:
             additional lines of chemotherapy, biologic or immunotherapy are allowed.

          -  Recovery from effects of recent surgery, radiotherapy, or chemotherapy

          -  At least 4 weeks out from their last dose of radiation therapy

          -  At least 4 weeks post-op from any major surgical procedure

          -  At least 3 weeks out from their last dose of chemotherapy and/or biologic/targeted
             therapy

          -  Must be ≥ 18 years of age

          -  Karnofsky Performance Status (KPS) of ≥ 70%

          -  Women of child-bearing potential must have a negative pregnancy test within 14 days
             prior to commencement of study treatment

          -  Women of child bearing potential must use an effective form of contraception during
             study and for at least 6 months after completion of study treatment

          -  Laboratory Test Findings performed within 14 days prior to initiation of study drug
             showing:

          -  Bone marrow function:

               -  Absolute neutrophil count (ANC) ≥ 1,000/mcL

               -  Platelets ≥ 75,000/mcL

               -  Hemoglobin ≥ 8 g/dL

          -  Renal function:

             °Creatinine ≤ 1.5 x ULN

          -  Hepatic function:

               -  Bilirubin ≤ 1.5 x ULN

               -  AST and ALT ≤ 2.5 x ULN

          -  Resolution of all acute toxic effects of prior therapy to NCI 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

        Exclusion Criteria:

          -  History of another invasive malignancy (other than non-melanoma skin cancer or
             curatively treated in situ carcinoma) with evidence of disease within the past 3 years

          -  History of prior hormonal therapy (i.e., megesterol acetate, tamoxifen or aromatase
             inhibitors) for treatment of cancer within 28 days before starting study drug

          -  Any psychological, familial, sociological or geographic condition that would
             potentially hamper compliance with the study protocol

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

          -  Patient has received an oral or IV corticosteroid within the prior 28 days and
             requires chronic corticosteroid therapy (excludes use of steroid premeds for CT
             allergy)

          -  Uncontrolled hypertension (systolic BP ≥ 160 mmHg or diastolic BP ≥ 95mmHg) despite
             medical treatment. Patients with a history of hypertension are allowed provided blood
             pressure is controlled by anti-hypertensive treatment.

          -  Clinically significant heart disease as evidenced by myocardial infarction or arterial
             thrombotic event within the past 6 months, severe or unstable angina, or New York
             Heart Association (NYHA) Class II-IV heart disease or cardiac ejection fraction
             measurement of < 50% at baseline

          -  Refractory nausea and vomiting, requirement for parenteral hydration and/or nutrition,
             drainage gastrostomy tube, malabsorption, external biliary shunt, or significant small
             bowel resection that would preclude adequate study drug absorption

          -  Anticipated or ongoing administration of anti-cancer therapies other than those
             administered in this study

          -  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. Investigators should consult the following table of clinically-relevant
             products http://medicine.iupui.edu/CLINPHARM/ddis/clinical-table.