ERBB2 and metastatic neoplasm: Palbociclib is a well-tolerated drug with significant progression-free survival (PFS) benefit in patients, in combination with letrozole, for the treatment of postmenopausal women with ER-positive, HER2-negative advanced breast cancer as initial therapy for their metastatic disease.24 Patients with STS have been tested for palbociclib as well.