Effect of Low-dose 500 mg Abiraterone Acetate in Treatment of Metastatic Prostate Cancer Patients

Inclusion Criteria: 1. Histologically confirmed adenocarcinoma prostate 2. Age \>21 years 3. Diagnosis of metastatic castration-resistant prostate cancer (mCRPC) (chemotherapy naïve and chemotherapy pre-treated patients) or metastatic hormone-sensitive prostate cancer (mHSPC) 4. For mCRPC patients, evidence of castration resistance is defined as disease progression despite a testosterone level \<50ng/dL (or surgical castration) 5. Progressive disease was defined as either 1. PSA progression according to Prostate Cancer Working Group (PCWG2) criteria15: PSA evidence for progressive prostate cancer consists of a minimum PSA level of at least 2 ng/ml, which has subsequently risen on at least 2 successive occasions, at least 1 week apart 2. Radiographic progression according to RECIST 1.1 guidelines or 3. 2 or more new lesions on bone scan 6. Newly initiated on abiraterone acetate therapy 7. Eastern Cooperative Oncology Group (ECOG) performance status grade of 0, 1 or 2. 8. Adequate hematologic, hepatic, and renal function would include: * hemoglobin ≥9.0 g/dL independent of transfusions * neutrophils ≥1.5 x 109/L * platelets ≥100 x 109/L * total bilirubin ≤1.5× upper limit of normal (ULN) \[except for subjects with documented Gilbert's disease in which case total bilirubin not to exceed 10× ULN\] * alanine (ALT) and aspartate (AST) aminotransferase ≤2.5X ULN * serum creatinine \<1.5× ULN or calculated creatinine clearance ≥30 mL/min * serum potassium ≥3.5 mmol/L 9. Ability to provide informed consent Exclusion Criteria: 1. Patients with prior use of enzalutamide or other potent androgen pathway targeted therapies 2. Concurrent therapy with strong inhibitors or inducers of CYP3A4 due to concerning possible drug-drug interactions with abiraterone. 3. Concurrent therapy with strong inhibitors or inducers of OATP transporters (e.g., rifampicin, cyclosporine) due to concerning possible effects on CP-I and CP-III. 4. New York Heart Association (NYHA) class II, NYHA class III, or IV congestive heart failure (any symptomatic heart failure) 5. Uncontrolled hypertension (systolic blood pressure ≥160 mmHg or diastolic BP ≥95 mmHg). Subjects with a history of hypertension are allowed provided blood pressure is controlled by anti-hypertensive treatment 6. Patients who do not voluntarily consent to participate in the study