Lasofoxifene or Fulvestrant Combined With Abemaciclib in Locally Advanced or Metastatic ER+/​HER2- Breast Cancer With an ESR1 Mutation

To find out if lasofoxifene plus abemaciclib works better than fulvestrant plus abemaciclib in treating people with ER+/HER2− breast cancer that has an ESR1 mutation and has progressed after prior CDK4/6 inhibitor therapy.

• Adults 18+ years of age
• Diagnosed with estrogen receptor positive (ER+), HER2-negative breast cancer with an ESR1 mutation that has spread beyond the breast (locally advanced or metastatic)
• Previously received treatment with ribociclib or palbociclib

• Treatment with either lasofoxifene plus abemaciclib, or fulvestrant plus abemaciclib
• CT/MRI scans to check response to treatment
• Blood and urine tests to monitor your health and treatment effects
• Electrocardiogram (ECG) to monitor heart function
• Questionnaires