A Study to Evaluate Serum Testosterone Levels in Patients With Metastatic Castration-Resistant Prostate Cancer

Inclusion Criteria: 1. Written informed consent obtained prior to any study-related procedure being performed 2. Male subjects at least 18 years of age or older at time of consent 3. Pathologically confirmed adenocarcinoma of the prostate 4. Ongoing therapy with a GnRH agonist or antagonist AND serum testosterone level \<50 ng/dL at screening 5. Metastatic disease documented by computed tomography (CT)/ magnetic resonance imaging (MRI) or bone scan. Imaging obtained within 42 days prior to the start of study medication will be accepted. 6. Meeting disease progression according to the recommendations of the prostate cancer working group 2 by one of the following criteria: * Two rises of PSA (taken a minimum of 1 week apart) from a baseline measurement of at least 2 ng/mL, * Imaging progression (CT/MRI) by RECIST criteria * Nuclear scan progression by new lesion. 7. Discontinuation of flutamide or nilutamide, and other anti-androgens at least 4 weeks prior to the start of study medication; discontinuation of bicalutamide at least 6 weeks prior to start of study medication. 8. Discontinuation of Radiotherapy \> 4 weeks prior to start of study medication. 9. ECOG performance status of 0-1 at screening 10. Screening blood counts of the following: * Absolute neutrophil count \> 1500/µL * Platelets \> 100,000/µL * Hemoglobin \> 9 g/dL 11. Screening chemistry values of the following: * ALT and AST \< 2.5 x ULN * Total bilirubin \< 1.5 x ULN * Creatinine\< 1.5 x ULN * Albumin \> 3.0 g/dL 12. Potassium \> 3.5 mmol/L 13. Life expectancy of at least 6 months at screening 14. Subject is willing and able to comply with all protocol requirements assessments 15. Agrees to protocol-defined use of effective contraception. Exclusion Criteria: 1. History of impaired pituitary or adrenal gland function 2. Prior therapy with abiraterone acetate, orteronel, ketoconazole or any other CYP17 inhibitor 3. Prior therapy with enzalutamide 4. Prior use of experimental androgen receptor antagonist 5. Previous exposure to Ra-223:Xofigo 6. Previous chemotherapy 7. Initiation of bisphosphonate or denosumab therapy within 30 days prior to the start of study medication. Patients who are on a stable dose of these medications for at least 30 days at the time of starting study drug are eligible. 8. Therapy with estrogen within 30 days prior to the start of study medication 9. Use of systemic glucocorticoids equivalent to \> 10 mg of prednisone daily; patients who have discontinued or have reduced dose to \< 10 mg prednisone within 14 days prior to the start of study medication will be eligible 10. Prior use of any herbal products that may decrease PSA levels (eg., saw palmetto) within 30 days of start of study medication 11. Known metastases to the brain or CNS involvement 12. History of other malignancy within the previous 2 years 13. Major surgery within 30 days prior to the start of study medication 14. Blood transfusion within 30 days of screening 15. Serious, persistent infection within 14 days of the start of study medication 16. Persistent pain that requires the use of a narcotic analgesic 17. Known gastrointestinal disease or condition that may impair absorption 18. Treatment with any investigational drug within 4 weeks prior to Day -1 of the study. 19. Known history of human immunodeficiency virus (HIV) or seropositive test for hepatitis C virus or hepatitis B virus 20. Have poorly controlled diabetes. 21. Uncontrolled hypertension 22. History of New York Heart Association (NYHA) class III or IV heart failure 23. Serious concurrent illness, including psychiatric illness, that would interfere with study participation 24. Inability to swallow tablets whole 25. Known hypersensitivity to any excipients in study medications 26. Moderate to severe hepatic impairment (Child-Pugh Classes B and C)