Maintenance With Selinexor/Placebo After Combination Chemotherapy in Participants With Endometrial Cancer [SIENDO]

Inclusion Criteria: * Female, at least 18 years of age at the time of informed consent. * Histological confirmed endometrial cancer of the endometrioid, serous, or undifferentiated type. Carcinosarcoma of the uterus is also allowed. * Completed a single line of at least 12 weeks of taxane-platinum combination therapy (not including adjuvant or neoadjuvant therapy), and achieved partial remission (PR) or complete remission (CR) according to RECIST version 1.1 for: * Primary Stage IV disease, defined as: * had a primary or later debulking surgery during first-line taxane-platinum therapy with R0 resection (R0 resection indicates a macroscopic complete resection of all visible tumor) and achieved CR after at least 12 weeks taxane-platinum chemotherapy, OR * had a primary or later debulking surgery during first-line taxane-platinum therapy with R1 resection (R1 resection indicates incomplete removal of all macroscopic disease,) and achieved PR or CR after at least 12 weeks taxane-platinum chemotherapy, OR * had no surgery and achieved PR or CR after at least 12 weeks taxane-platinum chemotherapy. OR * At first relapse (i.e., relapse after primary therapy including surgery and/or chemotherapy therapy for Stage I-IV disease), defined as: * had Stage I-III disease at diagnosis and received at initial diagnosis adjuvant chemotherapy and relapsed later. Participants should have PR or CR after at least 12 weeks of taxane-platinum chemotherapy compared with the start of this chemothe…