This is an exploratory Phase 2 multicenter, randomized, open-label study with a randomization allocation ratio of 1:1 [abiraterone acetate + prednisone + LHRH-therapy (Arm A) versus abiraterone acetate + prednisone (Arm B)]. For both groups patients will receive a dose of 1000 mg abiraterone acetate and 10mg prednisone daily (QD). Study drug will be administered as 4 x 250-mg abiraterone acetate tablets and prednisone will be administered as 5 mg orally twice a day (BID). Patients randomized to the LHRH-therapy group will receive the same LHRH-therapy they received prior to entering the trial. 70 medically castrated male patients with metastatic CRPC who have shown tumor progression and are non- or mildly-symptomatic will be enrolled from approximately 12 German study sites.

Inclusion Criteria:

1. Willing and able to provide written informed consent

2. Written Data Protection Consent has been obtained

3. Male aged 18 years and above

4. Histologically or cytologically confirmed adenocarcinoma of the prostate

5. Metastatic disease documented by positive CT/MRI and/or bone scan (both must be performed). If lymph node metastasis is the only evidence of metastasis, it must be ≥2 cm in diameter

6. Prostate cancer progression documented by PSA according to PCWG2 or radiographic progression according to modified RECIST criteria

7. Asymptomatic or mildly symptomatic from prostate cancer. A score of 0-1 for the question of worst pain within last 24 hours (Appendix 8) will be considered asymptomatic, and a score of 2-3 will be considered mildly symptomatic.

8. Medically castrated, with testosterone levels of <20-50 ng/dl (< 2.0 nM).

9. Combined androgen blockade is permitted, but not required. If patients received combined androgen blockade with an anti-androgen they must have shown PSA progression after discontinuing the anti-androgen prior to enrollment (≥4 weeks since last flutamide, ≥6 weeks since last bicalutamide or nilutamide).

10. Eastern Cooperative Oncology Group (ECOG) Performance Status of ≤2 (Appendix 6)

11. Hemoglobin ≥9.0 g/dL independent of transfusion

12. Platelet count ≥100,000 /μl

13. Serum albumin ≥3.0 g/dl

14. Serum creatinine < 1.5 x ULN or a calculated creatinine clearance ≥60 ml/min (Appendix 7)

15. Serum potassium ≥3.5 mmol/l

16. Liver function:

    1. Serum bilirubin <1.5 x ULN (except for patients with documented Gilbert's disease)
    2. AST or ALT <2.5 x ULN

17. Able to swallow the study drug whole as a tablet

18. Life expectancy of at least 6 months

19. Patients who have partners of childbearing potential must be willing to use a method of birth control with adequate barrier protection as determined to be acceptable by the principal investigator and sponsor during the study and for 1 week after last study drug administration.

Exclusion Criteria:

1. Surgical castration (i.e. orchiectomy).
2. Application of any LHRH-therapy (LHRH-analogue or LHRH-antagonist) within 3 months (for patients receiving a 3-months formulation) or 1 months (for patients receiving a 1-month formulation) prior to Cycle 1 day 1.
3. Patients receiving a 6- or 12-months formulation of LHRH-therapy
4. Active infection or other medical condition that would make prednisone/prednisolone (corticosteroid) use contraindicated
5. Any chronic medical condition requiring a higher dose of corticosteroid than 5mg prednisone/prednisolone bid.
6. Pathological finding consistent with small cell carcinoma of the prostate
7. Liver or visceral organ metastasis
8. Known brain metastasis
9. Use of opiate analgesics for cancer-related pain, including codeine, tramadol, tilidin and others (see Appendix 9), currently or anytime within 4 weeks of Cycle 1 Day 1.
10. Prior cytotoxic chemotherapy or biologic therapy for the treatment of CRPC
11. Radiation therapy for treatment of the primary tumour within 6 weeks of Cycle 1, Day 1
12. Radiation or radionuclide therapy for treatment of metastatic CRPC
13. Prior treatment with Abiraterone acetate or other CYP17 inhibitors (ketoconazole, TAK700, TOK001) ), Enzalutamide (Xtandi) or investigational agents targeting the androgen receptor for prostate cancer for more than 7 days
14. Prior systemic treatment with an azole drug (e.g. fluconazole, itraconazole) within 4 weeks of Cycle 1, Day 1
15. Prior flutamide (Eulexin) treatment within 4 weeks of Cycle 1, Day 1 (patients whose PSA did not decline for three or more months in response to antiandrogen given as a second line or later intervention will require only a two week washout prior to Cycle 1, Day 1)
16. Bicalutamide (Casodex), nilutamide (Nilandron) within 6 weeks of Cycle 1 Day 1 (patients whose PSA did not decline for three or more months in response to antiandrogen given as a second line or later intervention will require only a two week washout prior to Cycle 1, Day1)
17. Uncontrolled hypertension (systolic BP ≥160 mmHg or diastolic BP ≥95 mmHg). Patients with a history of hypertension are allowed provided that blood pressure is controlled by anti- hypertensive treatment
18. Active or symptomatic viral hepatitis or chronic liver disease
19. History of pituitary or adrenal dysfunction
20. Clinically significant heart disease as evidenced by myocardial infarction, or arterial thrombotic events in the past 6 months, severe or unstable angina, or New York Heart Association (NYHA) Class II-IV heart disease or cardiac ejection fraction measurement of <50 % at baseline
21. Any condition that requires treatment with Digoxin, digitoxin, and other digitalis drugs
22. Atrial Fibrillation, or other cardiac arrhythmia requiring therapy
23. Other malignancy with a ≥30 % probability of recurrence within 24 months, except non- melanoma skin cancer.
24. Administration of an investigational therapy within 30 days of Cycle 1, Day 1
25. Any condition, which, in the opinion of the investigator, would preclude participation in this trial.