Ki67 is determined by FNA or core biopsy before start and after 2 weeks of treatment. After the initial 4-week period, patients with signs of response in terms of decrease of Ki67 by ≥20% are randomized to endocrine treatment either alone or in combination with the cdk 4/6 inhibitor palbociclib (arm A and B). Patients with tumors with stable disease, defined as <20% decrease or increase of Ki67 and without radiological indication of tumor progression at the 4-week evaluation are offered continuous endocrine treatment with the addition of palbociclib (arm C).