An Open-Label, Randomized, Multicenter Study Evaluating the Activity of Lasofoxifene Relative to Fulvestrant for the Treatment of Postmenopausal Women With Locally Advanced or Metastatic ER+/HER2− Breast Cancer With an ESR1 Mutation

• Postmenopausal women with locally advanced or metastatic breast cancer
• Progression on an aromatase inhibitor (AI) in combination with a CDK4/6 inhibitor
• Demonstrated prior sensitivity to endocrine therapy (recurrence or progression after at least 12 months of treatment in metastatic setting)
• Measurable or non-measurable disease allowed
• One of the following ESR point mutations in cell-free DNA:
  • Y537S, Y537C, D538G, E380Q, S463P, V534E, P535H, L536H, L536P, L536R, L536Q, or Y537N
• May have received up to one chemothearpy regimen in adjuvant or neoadjuvant setting
• ECOG PS 0-1
• No prior use of any SERM with following exception
  • OK if tamoxifen previously used in adjuvant setting with relapse at least 1 year after stopping tamoxifen
• No prior everolimus or PI3K inhibitors
• No presence of CNS disease
• No immediate need for chemotherapy
• No HIV, HBV, HCV
• No prior malignancy in last 5 years except non-melanoma skin cancer or early stage cervical cancer
• No uncontrolled HTN
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• Primary- PFS
• Secondary- CBR, ORR, Safety