Testosterone & Tamoxifen Trial

Inclusion Criteria: 1. Male 2. A history of proven ER+ (\>10% of cells), AR+ (\>10% of cells), and HER2- metastatic BC 3. Tumor progression after at least one line of conventional endocrine therapy (tamoxifen, AI, fulvestrant, CDK4/6, ±LHRH analogue). 4. Age ≥ 18 years 5. Adequate hematological, renal and liver function as follows: * Absolute neutrophil count \> 1.5 x 109/L * Platelet count \>100 x 109/L * White blood cell count \>3 x 109/L * AST and ALT \<2.5 or \<5.0 in case of liver metastases x upper limit of normal (ULN) * Creatinine clearance \>50mL/min * Prothrombin time, partial thromboplastin time and INR \<1.5 x ULN 6. Written informed consent Exclusion Criteria: 1. History of prostate, testicular or liver cancer 2. Patients already using testosterone supplements 3. Patients using medication with anti-androgenic effects (e.g. spironolactone) 4. Elevated PSA (\>4μg/L) or severe urinary tract problems (as defined with a Prostate Symptom Score \>19). Patients with known BRCA mutation and PSA \>3 μg/L will be referred to the urologist for prostate cancer screening, and can participate if they have no signs of prostate cancer. 5. Hematocrit \>50% 6. Patients with uncontrolled hypertension, diabetes mellitus or other significant cardiovascular morbidity. 7. Patients with recent history of coronary artery disease or trombo-embolic events within 6 months prior to screening 8. Severe concurrent disease, infection, co morbid condition that, in the judgment of the investigator would make the patient inappropriate for enrollment 9. Visceral crisis and/or rapid progression necessitating chemotherapy 10. Previous allergic reaction to androgen agonists 11. Contra-indication for PET imaging 12. Tamoxifen or fulvestrant treatment \<5 weeks prior to FES-PET.