SWITCH: Study of the Prednisone to Dexamethasone Change in mCRPC Patients Treated With Abiraterone

INCLUSION CRITERIA 1. Provision of signed informed consent 2. Patients must be 18 years old or older 3. Patients must have an acceptable performance status at study entry ECOG \<2, without prior deterioration due to disease clinical progression on abiraterone plus prednisone 4. Patients must have prior histological confirmation of prostate cancer diagnosis prior to study entry 5. Maintained castration status to LHRH analogs/antagonist or surgical castration with Testosterone blood levels \<0.5ng/mL should have been documented before the initiation of prior abiraterone plus prednisone treatment and confirmed again at study entry. Patients on LHRHa must be able to continue on them through the duration of the study. 6. Biochemical progression to abiraterone plus prednisone is required before study entry. This progression will be documented by a rising PSA value with an increase ≥25% and \>2ng/dL over nadir, and must be confirmed by a second determination at least 2 weeks later should be documented before study entry. 7. Candidates must be able to swallow pills and to continue with abiraterone acetate dose of 1000mg/24h and must not have any contraindication for dexamethasone use at 0.5 mg/24h. 8. Patients must be asymptomatic or do not have any symptomatic deterioration attributable to prostate cancer progression at study entry 9. Absence of significant radiological progression to abiraterone plus prednisone at study entry. Only those cases with limited progression will be eligible if: a) they have not developed any new visceral, nodal or other soft tissue metastases; b) their measurable target lesions on abiraterone plus prednisone according to RECIST 1.1 should have not increased more than 40% from baseline or from their best response on treatment measurements; and c) they must have \< 3 new bone metastasis on bone-scan from baseline according to PCWG2 10. Acceptable hematological, hepatic and renal functions, without contraindications for the administration of abiraterone: a) WBC count \>2000/mm\^3; b) Haemoglobin level \>10 g/dL; c) Platelets \>75000/mm\^3; AST/ALT \<2.5 times the upper normal limit; Total bilirubin \<1.5 times the upper normal limit; Creatinine value \<1.5 times the upper normal limit or creatinine clearance \>50 ml/min EXCLUSION CRITERIA 1. Any medical contraindication to continue on abiraterone acetate or to receive continuous daily low-dose of dexamethasone (0.5 mg/24h) 2. Any event which is considered clinical progression to abiraterone acetate by the attending physicians in the investigators team. 3. Any skeletal symptomatic event related to prostate cancer progression on abiraterone-acetate, except the administration of external beam radiotherapy due to bone-metastasis related-pain in a single area and which have resulted in a adequate symptom control for at least 4-weeks before study entry. 4. Radiological progression: a) New nodal, visceral or other soft tissue metastasis during the treatment with Abiraterone acetate and prednisone; b) increase of any target lesion \>40% according to RECIST v1.1 criteria; c) any known visceral, nodal or soft tissue metastasis localisation causing symptomatic progression, and d) ≥ 3 new bone metastasis on bone-scan during treatment with abiraterone plus prednisone. 5. Previous cancer diagnosis, except those patients who had a localized malignant tumour and who are five years cancer-free, as well as subjects with a history of skin cancers (of non-melanoma type) or excised in situ carcinomas. 6. Any prior medical history, be they psychiatric or of any other character, which, according to the judgement of the investigator, might interfere with the subject's granting of informed consent or the safe execution of the procedures required in the study.