Inclusion Criteria:
* Histologically confirmed and newly diagnosed metastatic hormone-sensitive adenocarcinoma of the prostate without small cell carcinoma or small cell components.
* Metastatic adenocarcinoma of the prostate proved by imaging (CT/MRI and/or bone scan).
* Patient must give written informed consent before registration and prior to any trial related investigations.
* Age ≥18 years.
* Serum potassium ≥3.5mmol/ L.
* ECOG performance status 0-2
* Ongoing androgen deprivation therapy with drugs or bilateral orchiectomy, and continuous abiraterone plus prednisone.
* Patient agrees not to father a child during participation in the trial and during 3 months thereafter.
* Patient agrees not to participate other interventional trials.
* Patients are able to swallow study drug as whole tablet.
Exclusion Criteria:
* Diagnosed diabetes or fasting blood-glucose ≥ 6.1mmol/L, or glycosylated hemoglobin ≥ 5.6%.
* Previous malignancy within 2 years prior to randomization, with the exception of localized non-melanoma skin cancer and Ta bladder cancer.
* Prior treatment for prostate cancer with drugs, radiotherapy and surgery, with the exception below:
* ADT within 3 months before randomization, and no evidence of progression.
* Palliative radiotherapy or surgery for metastases due to symptoms, at least 4 weeks before randomization.
* Major surgery within 4 weeks prior to randomization.
* Treatments with 5a-reductase inhibitors, estrogen, cyproterone acetate, and androgen within 4 weeks prior to randomization.
* Known or suspected Central nervous system CNS metastases or active leptomeningeal disease.
* Equivalent dosage of \>5mg/day prednisone of glucocorticoids for the treatment of prostate cancer within 4 weeks prior to randomization, or treatment with glucocorticoids for other reasons.
* Prior treatment for prostate cancer with flutamide, bicalutamide, ketoconazole, abiraterone, enzalutamide, apalutamide, docetaxel chemotherapy, or other interventional drugs for prostate cancer.
* Neutrophils \< 1.5 x 109/L, platelets \< 75 x 109/L, hemoglobin \< 100 g/L.
* ALT and AST ≥ 2.5 x ULN, bilirubin ≥ 1.5 x ULN.
* eGFR\<45 ml/min/1.73m2.
* Allergic to metformin or any ingredients of this tablet.
* Acute or chronic metabolic acidosis, including diabetic ketoacidosis.
* Albumin\< 30 g/L.
* Clinically significant cardiovascular disease including:
* Myocardial infarction within 6 months prior to randomization.
* Uncontrolled angina within 3 months prior to registration.
* Congestive heart failure NYHA class III or IV.
* History of clinically significant ventricular arrhythmias (e.g. ventricular tachycardia, ventricular fibrillation, torsades de pointes).
* History of Mobitz II second or third degree heart block without a permanent pacemaker in place.
* Systolic pressure\< 86 mmHg.
* Bradycardia, heart rate\<45/min.
* Uncontrolled hypertension as indicated by systolic blood pressure \> 170 mmHg OR diastolic blood pressure \> 105 mmHg.
* ADT with or without anti-androgens more than 3 months prior to randomization.
* Prior treatment with metformin after diagnosis of prostate cancer.
* Allergic to metformin or any drugs used in this trial.
* Serious underlying medical condition (at the judgment of the investigator) which could impair the ability of the patient to participate in the trial (e.g. uncontrolled or acute severe infection, uncontrolled diabetes).
* Active or symptomatic viral hepatitis or chronic liver disease.
* History of pituitary or adrenal dysfunction.
* Gastrointestinal disorder affecting absorption.
