A Single-Arm, Global, Phase 4 Study of Abemaciclib, a CDK 4 & 6 Inhibitor, in Combination With Nonsteroidal Aromatase Inhibitors (Anastrozole or Letrozole) in Participants With Hormone Receptor Positive, Human Epidermal Growth Factor Receptor 2 Negative Advanced Breast Cancer With Poor Prognostic Factors

The reason for this study is to see if the drug abemaciclib in combination with nonsteroidal aromatase inhibitors (anastrozole or letrozole) is effective in participants with Hormone Receptor Positive (HR+), Human Epidermal Growth Factor Receptor 2 Negative (HER2-) advanced breast cancer that have certain disease characteristics.