Phase II Investigation of Pembrolizumab in Combination With Bevacizumab and Oral Cyclophosphamide in Patients With High Grade Ovarian Cancer and Surgically Documented Minimal Residual Disease After Frontline Therapy

Inclusion Criteria: 1. Female participants who are at least 18 years of age on the day of signing informed consent with histologically confirmed diagnosis of high grade non-mucinous epithelial ovarian cancer will be enrolled in this study. 2. A female participant is eligible to participate if she is not pregnant, not breastfeeding, and at least one of the following conditions applies: 1. Not a woman of childbearing potential (WOCBP) as defined in Appendix 2 OR 2. A WOCBP who agrees to follow the contraceptive guidance in Appendix 2 during the treatment period and for at least 120 days after the last dose of study medication. 3. The participant provides written informed consent for the trial. 4. Have an Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1. Evaluation of ECOG is to be performed within 7 days prior to the first dose of study intervention. 5. Have received standard of care frontline surgical and chemotherapy treatment (at least six cycles of platinum and taxane (or equivalent, with or without bevacizumab/biosimilar therapy). Patients who received neoadjuvant therapy are included. Omission of one or more doses of chemo for toxicity or hypersensitivity reaction is allowed. Have minimal residual disease (MRD) after a complete clinical response to frontline therapy (defined as: having a normalization of CA-125 and no obvious evidence of disease on exam or imaging unless the patient has an elevated CA-125 thought to be due to other confounding causes by the treating physician). Imaging with subtle or indeterminate findings will not disqualify participation. 6. Having MRD is defined as either of the following conditions (must occur within 3 months on the last dose of frontline platinum-taxane chemotherapy): 1. Second-look surgery that may be performed either as standard of care or in the context of a non-therapeutic clinical trial with positive tissue or cytology. OR 2. Detectable ctDNA by the Signatera assay. Assay results that are "positive below analytical range" qualify as detectable. 7. Participants who have undergone second look surgery, must have histologically confirmed residual ovarian cancer at time of second-look surgery. Patients with cytological evidence of malignant cells in washings obtained as part of the second look procedure are eligible even if biopsies are negative. 8. Be willing to allow use of archival tissue from second-look surgery (if performed) and primary surgery or biopsy for use in this study. 9. Have adequate organ function as determined by the following laboratory values: 1. ANC ≥ 1,000 /mcL 2. Platelets ≥ 100,000 / mcL 3. Hgb ≥8 g/dL (transfusion allowed) 4. Creatinine Clearance ≥30 mL/min (measured or calculated per local practice) 5. Total Bilirubin ≤ 1.5 × ULN or ≤ 3 × ULN in the case of suspected/documented Gilbert's Syndrome 6. AST (SGOT) and ALT (SGPT) ≤ 2.5 X ULN 10. Have adequately recovered from second look surgery to be able to start the investigational regimen no sooner than 28 days from the date of second look surgery, and within 7 weeks of this procedure. 11. Negative serum pregnancy test within 72 hours prior to receiving the first dose of study medication (unless surgically sterile or postmenopausal for greater than one year). Exclusion Criteria: 1. Has received prior therapy with an anti-PD-1, anti-PD-L1, or anti PD L2 agent or with an agent directed to another stimulatory or co-inhibitory T-cell receptor (eg, CTLA-4, OX 40, CD137). 2. Has received prior radiotherapy within 2 weeks of start of study intervention. Participants must have recovered from all radiation-related toxicities, not require corticosteroids, and not have had radiation pneumonitis. A 1-week washout is permitted for palliative radiation (≤2 weeks of radiotherapy) to non-CNS disease. 3. Has received a live vaccine or live-attenuated vaccine within 30 days prior to the first dose of study drug. Administration of killed vaccines is allowed. 4. Is currently receiving or has received an investigational agent within 4 weeks prior to the first dose of study intervention. 5. Has a diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy (in dosing exceeding 10 mg daily of prednisone equivalent) or any other form of immunosuppressive therapy within 7 days prior to the first dose of study drug. Steroids to reduce risk of radiologic contrast allergy is permitted. 6. Has a known additional malignancy that is progressing or has required active treatment within the past 3 years. Participants with basal cell carcinoma of the skin, squamous cell carcinoma of the skin or carcinoma in situ (e.g., breast carcinoma, cervical cancer in situ) excluding carcinoma in situ of the bladder, that have undergone potentially curative therapy are not excluded. 7. Has known active CNS metastases and/or carcinomatous meningitis. Participants with previously treated brain metastases may participate provided they are radiologically stable, i.e. without evidence of progression for at least 4 weeks by repeat imaging (note that the repeat imaging should be performed during study screening), clinically stable and without requirement of steroid treatment for at least 14 days prior to first dose of study intervention. 8. Has active autoimmune disease that has required systemic treatment in the past 2 years (i.e. with use of disease modifying agents, corticosteroids or immunosuppressive drugs). Replacement therapy (e.g., thyroxine, insulin, or physiologic corticosteroid replacement therapy for adrenal or pituitary insufficiency, type I diabetes mellitus, etc.) is not considered a form of systemic treatment and is allowed. 9. Has a history of (non-infectious) pneumonitis/interstitial lung disease that required steroids or has current pneumonitis/interstitial lung disease. 10. Serious active infection that in the opinion of treating physician would preclude participation. 11. Has a known history of uncontrolled Human Immunodef