P1-17-05: Preliminary Results of a Randomized Phase 2 Study of PD 0332991, a Cyclin-Dependent Kinase (CDK) 4/6 Inhibitor, in Combination with Letrozole for First-Line Treatment of Patients (pts) with Post-Menopausal, ER+, HER2−Negative (HER2–) Advanced Breast Cancer.