Selinexor in Treating Patients With Abiraterone Acetate and/or Enzalutamide Refractory Metastatic Castration-Resistant Prostate Cancer

Inclusion Criteria: * Histologically confirmed adenocarcinoma of the prostate * Patients must have castrate levels of testosterone (\< 50 ng/dL) on gonadotropin-releasing hormone (GnRH) analogues or have had prior orchiectomy; GnRH analogues must be continued while on study * Tumor tissue submitted for molecular and genetic analysis through the companion Stand-up 2 Cancer (SU2C) radiologically guided biopsy of abiraterone and/or enzalutamide refractory mCRPC protocol * Patients who consent to participate in the companion biopsy protocol and are subsequently determined to be ineligible for biopsy are eligible to participate in the current protocol * Progressive disease as demonstrated by a rising PSA (at least two determinations) prior to study entry, and/or radiographic evidence of tumor progression in soft tissue according to modified Response Evaluation Criteria In Solid Tumors (RECIST) criteria or identification of new lesions by bone scan (i.e., \>= 2 new lesions) * Primary resistance or acquired resistance (i.e., acquired resistance will be defined as disease progression following a period of response defined as \>= 50% decline in PSA within 12 weeks of starting therapy and not otherwise meeting criteria for primary resistance) to any of the following agents/combinations of therapy: * Abiraterone acetate; primary resistance to abiraterone will be defined as: * No PSA decline * PSA decline less than 50% after 12 weeks of abiraterone therapy * PSA progression wi…