The median progression-free survival was 24.8 months (95% confidence interval, 22.1 to not estimable) in the palbociclib-letrozole group, as compared with 14.5 months (95% CI, 12.9 to 17.1) in the placebo-letrozole group (hazard ratio for disease progression or death, 0.58 95% CI, 0.46 to 0.72 P<0.001).

The primary end point was progression-free survival, as assessed by the investigators secondary end points were overall survival, objective response, clinical benefit response, patient-reported outcomes, pharmacokinetic effects, and safety. In this double-blind study, we randomly assigned, in a 2:1 ratio, 666 postmenopausal women with ER-positive, HER2-negative breast cancer, who had not had prior treatment for advanced disease, to receive palbociclib plus letrozole or placebo plus letrozole. We performed a phase 3 study that was designed to confirm and expand the efficacy and safety data for palbociclib plus letrozole for this indication. Anderson Cancer Center, University of Texas, Houston (S.M.) and Institut Curie, Paris (V.D.).Ī phase 2 study showed that progression-free survival was longer with palbociclib plus letrozole than with letrozole alone in the initial treatment of postmenopausal women with estrogen-receptor (ER)-positive, human epidermal growth factor receptor 2 (HER2)-negative advanced breast cancer. Oncology Research, The Woodlands, TX (S.J.) Cancer Research Institute, Seoul National University Hospital, Seoul National University College of Medicine, Seoul, South Korea (S.-A.I.) British Columbia Cancer Agency, Vancouver, Canada (K.G.) Brustzentrum der Universität München (LMU), Munich, Germany (N.H.) State Budget Medical Institution Republican Clinical Oncology, Ufa, Russia (O.N.L.) All-Ireland Cooperative Oncology Research Group, Dublin (J.M.W.) M.D. 1 From the Department of Medicine, Division of Hematology and Oncology, David Geffen School of Medicine at the University of California, Los Angeles, Santa Monica (R.S.F., D.J.S.), the Helen Diller Family Comprehensive Cancer Center, University of California, San Francisco, San Francisco (H.S.R.), and Pfizer, La Jolla (E.G., D.R.L., S.R.) - all in California Hospital Gregorio Maranon, Universidad Complutense, Madrid (M.M.) U.S.

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