Pembrolizumab +/- Bevacizumab for Recurrent GBM

Inclusion Criteria: * Have histologically confirmed World Health Organization Grade IV malignant glioma (glioblastoma or gliosarcoma). Participants will be eligible if the original histology was low-grade glioma and a subsequent histological diagnosis of glioblastoma or variants is made. * Previous first line therapy with at least radiotherapy and temozolomide * Be at first or second relapse. * Participants must have shown unequivocal evidence for tumor progression by MRI or CT scan. * CT or MRI within 14 days prior to start of study drug. * An interval of at least 4 weeks (to start of study agent) between prior surgical resection or one week for stereotactic biopsy. * An interval of at least 12 weeks from the completion of radiation therapy to start of study drug unless there is a new area of enhancement consistent with recurrent tumor outside the radiation field or there is unequivocal histologic confirmation of tumor progression * Participants must have recovered to grade 0 or 1 or pre-treatment baseline from clinically significant toxic effects of prior therapy (including but not limited to exceptions of alopecia, laboratory values listed per inclusion criteria, and lymphopenia which is common after therapy with temozolomide). * From the projected start of scheduled study treatment, the following time periods must have elapsed: 5 half-lives from any investigational agent, 4 weeks from cytotoxic therapy (except 23 days for temozolomide and 6 weeks from nitrosoureas), 6 weeks from antibodies, or 4 weeks (or 5 half-lives, whichever is shorter) from other anti-tumor therapies. Exclusion Criteria: * Current or planned participation in a study of an investigational agent or using an investigational device. * Has a diagnosis of immunodeficiency. * Has tumor primarily localized to the brainstem or spinal cord. * Has presence of diffuse leptomeningeal disease or extracranial disease. * Has received systemic immunosuppressive treatments within 6 months of start of study drug * Requires treatment with high dose systemic corticosteroids defined as dexamethasone \> 4 mg/day or bioequivalent for at least 3 consecutive days within 2 weeks of start of study drug. * Has received prior interstitial brachytherapy, implanted chemotherapy, stereotactic radiosurgery or therapeutics delivered by local injection or convection enhanced delivery. * Requires therapeutic anticoagulation with warfarin at baseline; patients must be off warfarin or warfarin-derivative anti-coagulants for at least 7 days prior to starting study drug; however, therapeutic or prophylactic therapy with low-molecular weight heparin is allowed. * Has history of known coagulopathy that increases risk of bleeding or a history of clinically significant hemorrhage within 12 months of start of study drug * Has evidence of intratumoral or peritumoral hemorrhage on baseline MRI scan other than those that are grade ≤ 1 and either post-operative or stable on at least 2 consecutive MRI scans. * Has gastrointestinal bleeding or any other hemorrhage/bleeding event CTCAE Grade \> 3 within 6 months of start of study drug. * Has a known additional malignancy that is progressing or requires active treatment within 3 years of start of study drug. Exceptions include basal cell carcinoma of the skin, squamous cell carcinoma of the skin, or in situ cervical cancer that has undergone potentially curative therapy. * Has an active autoimmune disease requiring systemic treatment within the past 3 months or a documented history of clinically severe autoimmune disease, or a syndrome that requires systemic steroids or immunosuppressive agents. * Has evidence of interstitial lung disease or active, non-infectious pneumonitis. * Has an active infection requiring systemic therapy. * Has a history or current evidence of any condition, therapy, or laboratory abnormality that might confound the results of the trial, interfere with the subject's participation for the full duration of the trial, or is not in the best interest of the subject to participate, in the opinion of the treating investigator. * Has known psychiatric or substance abuse disorders that would interfere with cooperation with the requirements of the trial. * Is pregnant or breastfeeding, or expecting to conceive within the projected duration of the trial * Has a known history of HIV * Has known active Hepatitis B or Hepatitis C * Has received a live vaccine within 30 days prior to the first dose of study drug. * Has a known hypersensitivity to any of the study therapy products. * Has received anti-angiogenic or anti-VEGF targeted agents (e.g. bevacizumab, cediranib, aflibercept, vandetanib, XL-184, sunitinib, etc) * Has a history of non-healing wounds or ulcers, or bone refractures within 3 months of fracture * Has a history of arterial thromboembolism within 12 months of start of study drug. * Has inadequately controlled hypertension * Has a history of hypertensive crisis or hypertensive encephalopathy * Has had clinically significant cardiovascular disease within 12 months of start of study drug * Has a history of abdominal fistula, gastrointestinal perforation, or intra-abdominal abscess within 6 months prior to start of study drug.