On February 3, 2015, the US Food and Drug Administration (FDA) approved palbociclib in combination with letrozole for first-line treatment of postmenopausal women with estrogen receptor (ER)-positive, human epidermal growth factor receptor 2 (HER2)-negative advanced breast cancer. The combination was reviewed and approved 2 months ahead of schedule under the FDA’s breakthrough therapy designation and priority review program. Palbociclib is an oral small molecule inhibitor of cyclin-dependent kinases (CDK) 4 and 6, while letrozole is an aromatase inhibitor (AI), a type of endocrine therapy that reduces circulating estrogen levels. The 2 agents were shown to have potentially synergistic tumor growth inhibitory activity in preclinical models of ER-positive breast cancer.
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