The CDK4/6 inhibitor Ibrance (palbociclib), when added to hormone therapy normally given to certain advanced breast cancer patients, considerably benefits these people, researchers at the University of California, Los Angeles (UCLA), reported in an article that detailed their findings in a Phase 3 clinical trial combining this breakthrough drug with Femara (letrozole). Specifically, trial results showed that Ibrance plus Femara significantly delayed cancer progression in postmenopausal women with estrogen receptor-positive (ER+), human epidermal growth factor receptor 2 (HER2)-negative metastatic breast cancer, without the severe side effects associated with Femara monotherapy. The Phase 1 and Phase 2 trials that preceded this study were the basis for the U.S. Food and Drug Administration (FDA) approving Ibrance in combination with Femara for postmenopausal women with ER-positive, HER2-negative advanced breast cancer in February 2015, and for its European approval in that same indication earlier this month. The Phase 3 clinical trial and its study, "Palbociclib and Letrozole in Advanced Breast Cancer," published in The New England Journal of Medicine, seems to place Ibrance on a track for worldwide approval. Ibrance, developed by Pfizer, is designed to prevent cells from dividing by targeting a family of proteins, CDK4/6, that are critical for cell proliferation. The earlier, multi-year trials had shown that combining Ibrance with Femara led to a significant increase in progression-free survival compared to patients receiving Femara alone.