The goal of this clinical trial is to assess the efficacy, safety and tolerability of the combination of lasofoxifene and abemaciclib compared to fulvestrant and abemaciclib for the treatment of pre- and postmenopausal women and men who have previously received ribociclib or palbociclib-based treatment and have locally advanced or metastatic estrogen receptor positive (ER+)/human epidermal growth factor 2 negative (HER2-) breast cancer with an estrogen receptor 1 (ESR1) mutation.
The main question the study aims to answer is:
• To compare the efficacy of the combination of lasofoxifene and abemaciclib with that of fulvestrant and abemaciclib Participants will receive either receive 5 mg/d of oral lasofoxifene plus oral abemaciclib 150 mg twice a day or the combination of fulvestrant 500 mg intramuscular (IM) on Days 1, 15, and 29 and then once monthly thereafter plus oral abemaciclib 150 mg twice a day.
Pre- or postmenopausal women or men.
Locally advanced and/or metastatic ER+ breast cancer with radiological or clinical evidence of progression on an AI in combination with either palbociclib or ribociclib as their first hormonal treatment for metastatic disease.
Histological or cytological confirmation of ER+/HER2 - disease
No evidence of progression for at least 6 months on an AI/CDKi combination for advanced breast cancer.
Lymphangitic carcinomatosis involving the lung.
History of Grade 3 or Grade 4 interstitial lung disease (ILD) on previous therapy.
Visceral crisis in need of cytotoxic chemotherapy as assessed by the investigator.
Prior progression of disease on abemaciclib, fulvestrant, or other selective estrogen receptor degrader (SERD) therapy.