Additional Study Information: The purpose of this research study is to see if the combination of medications approved for the treatment of glioblastoma, together with the study drug, selinexor (also known as XPOVIO), has any effects on your cancer. Selinexor works by trapping tumor suppressor proteins within the cell nucleus. This is expected to cause the cancer cells to die or stop growing. Selinexor is approved by the U.S. Food and Drug Administration (FDA) for the use in certain patients with multiple myeloma but has not yet been approved by the FDA for the treatment of your type of cancer.