Niraparib in High-grade Endometrial Cancer Trial

Inclusion Criteria: 1. Patients must be female ≥18 years of age, able to understand the study procedures and agree to participate in the study by providing written informed consent 2. Histological and staging criteria: Patients must have histologically diagnosed 1. Grade 3 endometrioid, serous or clear cell endometrial carcinoma, carcinosarcoma, undifferentiated carcinoma in Stage I-III according to International Federation of Gynecology and Obstetrics (FIGO) classification. 2. Grade 2 endometrioid carcinoma with abnormal TP53 by immunohistochemistry. 3. Surgical criteria: patients with operable disease are eligible 4. Patients of childbearing potential must have a negative serum pregnancy test (beta human chorionic gonadotropin \[hCG\]) within 7 days prior to receiving the first dose of niraparib. 5. Patients must be postmenopausal, free from menses for \>1 year, surgically sterilized, or willing to use adequate contraception to prevent pregnancy or must agree to abstain from activities that could result in pregnancy throughout the study, starting with enrollment through 3 months after the last dose of niraparib. 6. Patients must have an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 7. Patients must have adequate organ function, defined as follows: 1. Absolute neutrophil count ≥ 1,500/µL 2. Platelets ≥ 100,000/µL 3. Hemoglobin ≥ 10 g/dL 4. Serum creatinine ≤ 1.5 x upper limit of normal (ULN) or calculated creatinine clearance ≥ 60 mL/min using the Cockcroft-Gault equation 5. Total bilirubin ≤ 1.5 x ULN 6. Aspartate aminotransferase and alanine aminotransferase ≤ 2.0 x ULN 8. Patients must be able to take oral medications Exclusion Criteria: 1. Histology: 1. Grade 1 endometrioid carcinoma OR 2. Grade 2 endometrioid carcinoma with wild type TP53 OR 3. Grade 2 endometrioid carcinoma with an unknown TP53 status 2. Patient did not consent for the study biopsy and one of the following: * the original endometrial biopsy tissue block could not be assessed by the study site pathologist * the original endometrial biopsy tissue block does not contain sufficient tumor tissue 3. Patient is pregnant, breastfeeding, or expecting to conceive children, while receiving study treatment and for 3 months after the last dose of study treatment; 4. Patient has a known hypersensitivity to the components of niraparib or its excipients; 5. Patient is simultaneously enrolled in any clinical trial of niraparib or any other investigational therapy; 6. Patient has had any known ≥Grade 3 anemia, neutropenia or thrombocytopenia due to any prior medication that persisted \>4 weeks; 7. Patient has any known history or current diagnosis of myelodysplastic syndrome (MDS) or acute myelocytic anemia (AML); 8. Patient has undergone major surgery (per investigator judgment) within 3 weeks of starting the study or patient has not recovered from any effects of any major surgery; 9. Patient has a condition (such as transfusion dependent anemia or thrombocytopenia), therapy, or laboratory abnormality that might confound the study results or interfere with the patient's participation for the full duration of the study treatment, including: 1. Patient received a transfusion (platelets or red blood cells) within 2 weeks of the first dose of study treatment; 2. Patient received colony-stimulating factors (eg, granulocyte colony stimulating factor \[G-CSF\], granulocyte macrophage colony-stimulating factor \[GM-CSF\] or recombinant erythropoietin) within 2 weeks prior to the first dose of study treatment. 10. Another concurrent invasive neoplastic disease (including ovarian), diagnosis of cancer in the last 5 years (except for non-melanoma skin cancer), patient previously had cancer (\> 5 years) but she is not considered cured or still treated. 11. Patient is considered a poor medical risk due to a serious, uncontrolled medical disorder, non-malignant systemic disease, or active, uncontrolled infection; \- Examples include, but are not limited to, uncontrolled ventricular arrhythmia, recent (within 90 days) myocardial infarction, uncontrolled major seizure disorder, unstable spinal cord compression, superior vena cava syndrome, or any psychiatric disorder that prohibits obtaining informed consent. 12. Patient is immunocompromised (patients with splenectomy are allowed). 13. Patient has known, active hepatic disease (ie, hepatitis B or C). 14. Patient has a corrected QT interval (QTc) prolongation \> 470 milliseconds at screening; - If a patient has a prolonged QTc interval and the prolongation is deemed to be due to a pacemaker upon investigator evaluation (ie, the patient otherwise has no cardiac abnormalities), then the patient may be eligible to participate in the study following approval of a cardiology specialist.