In this research study, investigators are testing if a dose-increasing strategy for abemaciclib will have less side effects and be better tolerated than the standard dosage of abemaciclib for participants with early-stage high-risk hormone receptor positive breast cancer. The names of the study drugs involved in this study are: * Abemaciclib (CDK4 and CDK6 inhibitor) * Tamoxifen (Selective estrogen receptor modulator) * Anastrozole/Letrozole (Non-steroidal aromatase inhibitors) * Exemestane (steroidal aromatase inhibitor) * LHRH (Gonadotropin-releasing hormone agonist, or Luteinizing hormone-releasing hormone agonist)
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Composite Adverse Rate at 3 months
Timeframe: 3 months