Palbociclib and FES PET

Inclusion Criteria:

In order to be eligible to participate in this study, a subject must meet all of the following criteria:

1. Patients with ER positive (i.e. \>1% staining), HER2 negative metastatic breast cancer (preferably assessment on fresh metastasis biopsy, alternatively archival metastasis biopsy)
2. Post-menopausal status defined as:

   * Age ≥60 years
   * Previous bilateral oophorectomy
   * Age \<60 years and amenorrhea for \>12 months in the absence of interfering hormonal therapies (such as LH-RH agonists and ER-antagonists
   * Age \<60 years using ER antagonists should have amenorrhea for \>12 months and FSH \>24U/L and LH\>14U/L
3. Adequate bone marrow and organ function defined 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 \<3.0 x upper limit of normal (ULN) or \<5 xULN in case of known liver metastases.
   * Alkaline phosphatase \<2.5 x ULN
   * Total serum bilirubin \< ULN or total bilirubin \<3.0 x ULN with direct bilirubin within normal range in patients with Gilbert's Syndrome
   * Creatinine clearance \>50mL/min
   * Lipase/amylase \<1/5 x ULN
   * Prothrombin time, partial thromboplastin time and INR \<1.5 x ULN
4. ECOG performance 0-2
5. Signed written informed consent
6. Able to comply with the protocol
7. Age ≥18 years

Exclusion Criteria:

A potential subject who meets any of the following criteria will be excluded from participation in this study:

1. Life expectancy \< 3 months
2. Evidence of central nervous system metastases
3. Presence of life-threatening visceral metastases
4. Prior use of CDK4/6 inhibitor
5. Use of estrogen receptor ligands including estrogens, fulvestrant or tamoxifen \<6 weeks before study entry.
6. Use of other anticancer therapy \< 2 weeks prior to start with palbociclib
7. Concurrent malignancy
8. Active cardiac disease or a history of cardiac dysfunction
9. Patient is currently receiving any of the following medications and cannot be discontinued 7 days prior to the start of treatment that are known strong inducers or inhibitors of CYP3A4/5, known risk to prolong the QT interval or induce Torsades de Pointes, or narrow therapeutic window and are predominantly metabolized through CYP3A4/5.