Inclusion Criteria:

          -  Have signed an informed consent document indicating that the subject understands the
             purpose of and procedures required for the study and are willing to participate in the
             study

          -  Be willing/able to adhere to the prohibitions and restrictions specified in this
             protocol

          -  Eastern Cooperative Oncology Group (ECOG) performance status =< 2

          -  Documented histologically confirmed adenocarcinoma of the prostate

          -  Patient must have evidence of castration resistant prostate cancer as evidenced by a
             confirmed rising PSA (per Prostate Cancer Working Group 2 [PCWG2] criteria) and a
             castrate serum testosterone level (i.e. =< 50 mg/dL)

          -  Patient must be eligible for treatment with enzalutamide

          -  Patient must have previously progressed on abiraterone (either by PCWG2 criteria or
             Response Evaluation Criteria in Solid Tumors [RECIST] criteria)

          -  Documented metastatic disease on bone scan, computed tomography (CT) scan or magnetic
             resonance imaging (MRI)

        Exclusion Criteria:

          -  Have known allergies, hypersensitivity, or intolerance to enzalutamide or niclosamide
             or their excipients

          -  Ongoing systemic therapy (other than a gonadotropin releasing hormone [GnRH]
             agonist/antagonist) for prostate cancer including, but not limited to:

               -  Cytochrome P450, family 17 (CYP-17) inhibitors (e.g. ketoconazole, abiraterone)

               -  Antiandrogens (e.g. bicalutamide, nilutamide)

               -  Second generation antiandrogens (e.g. ARN-509)

                    -  Note: patients receiving ongoing treatment with enzalutamide will be allowed
                       to join the study

               -  Immunotherapy (e.g. sipuleucel-T, ipilimumab)

               -  Chemotherapy (e.g. docetaxel, cabazitaxel)

               -  Radiopharmaceutical therapy (e.g. radium-223, strontium-89, samarium-153)

          -  Have any condition that, in the opinion of the investigator, would compromise the
             well-being of the subject or the study or prevent the subject from meeting or
             performing study requirements

          -  Any psychological, familial, sociological, or geographical condition that could
             potentially interfere with compliance with the study protocol and follow-up schedule

          -  Severe hepatic impairment (Child-Pugh class C)

          -  Severe renal impairment (creatinine clearance =< 30 ml/min)

          -  History of prior seizures

          -  Central nervous system metastases

          -  Symptomatic patients who, in the opinion of the investigator, may benefit from
             docetaxel-based chemotherapy