Selinexor in Maintenance Therapy After Systemic Therapy for Participants With p53 Wild-Type, Advanced or Recurrent Endometrial Carcinoma

Inclusion Criteria:

• At least 18 years of age at the time of signing informed consent.

• Histologically confirmed EC including: endometrioid, serous, undifferentiated, andcarcinosarcoma.

• TP53 wt assessed by next generation sequencing (NGS), evaluated by a central vendor.

• Completed a single line, at least 12 weeks of platinum-based therapy (not includingadjuvant or neoadjuvant therapy for Stage I-III disease) and achieved confirmedpartial or complete response (PR or CR) by imaging, according to RECIST version 1.1.The participants should have received treatment for:

Primary Stage IV disease, defined as:

• had a primary or later debulking surgery during first-line platinum-based therapywith R0 resection (R0 resection indicates a macroscopic complete resection of allvisible tumor) and achieved CR after at least 12 weeks platinum-based therapy, OR

• had a primary or later debulking surgery during first-line platinum-based therapywith R1 resection (R1 resection indicates incomplete removal of all macroscopicdisease) and achieved PR or CR after at least 12 weeks platinum-based chemotherapy,OR

• had no surgery and achieved PR or CR after at least 12 weeks platinum-basedchemotherapy

OR

At first relapse (i.e., relapse after primary therapy including surgery and/or chemotherapy and/or immunotherapy for Stage I-IV disease), defined as:

• had Stage I - III disease at diagnosis and received, at initial diagnosis, adjuvantchemotherapy and relapsed later. Participants should have PR or CR after at least 12weeks of platinum-based chemotherapy compared with the start of this chemotherapy atthe time of relapse,

• had Stage I-III disease at diagnosis and did not receive adjuvant chemotherapy atinitial diagnosis and relapsed later. Participants should have PR or CR after atleast 12 weeks of platinum-based chemotherapy compared with the start of thischemotherapy at the time of relapse, OR

• had Stage IV disease at diagnosis and received initially chemotherapy with orwithout surgery and relapsed later. At the time of relapse, participants should havePR or CR after at least 12 weeks of platinum-based chemotherapy compared with thestart of this chemotherapy at the time of relapse.

• Previous treatment with anti-programmed cell death protein 1(PD-1) oranti-programmed death-ligand 1(PD-L1) monoclonal antibody and concomitantbiologic agents (e.g., bevacizumab, trastuzumab) is allowed.

• Must be able to initiate study drug 3 to 8 weeks after completion of theirfinal dose of chemotherapy.

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

• Participants must have adequate bone marrow function and organ function within 2 weeks before starting study drug as defined by the following laboratorycriteria:

• Hepatic function: total bilirubin up to less than (<) 3upper limit of normal (ULN); alanine aminotransferase (ALT) and aspartate aminotransferase (AST) less thanor equal to (<=) 2.5ULN in participants without liver metastasis. Forparticipants with known liver involvement of their tumor: AST and ALT (<=) 5*ULN

• Hematopoietic function within 1 week: Absolute neutrophil count (ANC) greater thanor equal to (>=) 1.510^9/liter (L); platelet count >= 10010^9/L; hemoglobin >= 9.0 gram per deciliter (g/dL) per local laboratory results

• Renal function: estimated creatinine clearance (CrCl) of >= 20 milliliter perminute (mL/min), calculated using the standard local formula, as applicable

• In the opinion of the Investigator, the participant must:

• Have a life expectancy of at least 12 weeks, and

• Be fit to receive investigational therapy

• Premenopausal females of childbearing potential must have a negative pregnancytest (serum β-human chorionic gonadotropin test) prior to the first dose ofstudy drug. Female participants of childbearing potential must agree to usehighly effective methods of contraception throughout the study and for 90 daysfollowing the last dose of study drug.

• Written informed consent signed in accordance with federal, local, andinstitutional guidelines prior to the first screening procedure.

Exclusion Criteria:

• Participants meeting any of the following exclusion criteria are not eligible toenroll in this study:

• Has any uterine sarcomas (carcinosarcomas - not excluded), clear cell or small cellcarcinoma with neuroendocrine differentiation

• Received a blood or platelet transfusion during the 2 weeks prior to Cycle 1 Day 1 (C1D1). Participants' hemoglobin must be assessed within 2 weeks of screening and atleast 1 week post transfusion

• Concurrent systemic steroid therapy higher than physiologic dose (> 10 milligramper day [mg/day] of prednisone or equivalent). Systemic steroid therapy aspre-medication for taxane is allowed

• Insufficient time since or not recovered from procedures or anti-cancer therapy,defined as:

• Not recovered from major surgery <= 28 days prior to Day 1 dosing. Minorprocedures, such as biopsies, dental work, or placement of a port orintravenous (IV) line for infusion are permitted

• Having ongoing clinically significant anti-cancer therapy-related toxicities CTCAEGrade > 1, with the exception of alopecia. In specific cases, participants whosetoxicity has stabilized or with Grade 2 non-hematologic toxicities can be allowedfollowing documented approval by the Sponsor's Medical Monitor

• Palliative radiotherapy within 14 days of the intended C1D1. Palliative radiotherapymay be permitted for symptomatic control of pain from bone metastases, provided thatthe radiotherapy does not involve target lesions, and the reason for theradiotherapy does not reflect evidence of disease progression.

• Any gastrointestinal dysfunctions that could interfere with the absorption ofselinexor (e.g., bowel obstruction, inability to swallow tablets, malabsorptionsyndrome, unresolved nausea, vomiting, diarrhea CTCAE v 5.0 > grade 1).

• Participants unable to tolerate two forms of antiemetics for at least 2 cycles willnot be eligible for the trial.

• Active, ongoing or uncontrolled active infection requiring parenteral antibiotics,antivirals, or antifungals within 1 week of screening.

• Serious psychiatric or medical condition that could interfere with participation inthe study or in the opinion of the Investigator would make study involvementunreasonably hazardous.

• Previous treatment with an XPO1 inhibitor.

• Stable disease or PD on the post-chemotherapy scan or clinical evidence ofprogression prior to randomization.

• Participants who received any systemic anticancer therapy including investigationalagents <= 3 weeks (or <= 5 half-lives of the drug [whichever is shorter])prior to C1D1.

• Major injuries or surgery within 14 days prior to C1D1 and/or planned major surgeryduring the on-treatment study period.

• Other malignant disease with disease-free <= 3 years except: curatively treatedcarcinoma in situ of the cervix, basal cell carcinoma of the skin, or ductalcarcinoma in situ (DCIS) of the breast.

• History of allergic reactions attributed to compounds of similar chemical orbiologic composition to selinexor, or other agents used in the study.

• Active brain metastases (e.g., stable for < 8 weeks, no adequate previoustreatment with radiotherapy and/or surgery, symptomatic, requiring treatment withanti-convulsant therapy. Corticoid therapy is allowed if administered as stable dosefor at least 1 month before randomization).

• Females who are pregnant or lactating.

• Any other life-threatening illness, active medical condition, organ systemdysfunction, or serious active psychiatric issue which, in the Investigator'sopinion, could compromise the participant's safety or the participant's ability toremain compliant with study procedures.