This study will include two groups of patients: Cohort 1 and Cohort 2.
Cohort 1: will help researchers learn how a medicine called imlunestrant (LY3484356) affects a specific type of breast cancer. Some patients will take both imlunestrant and another treatment to suppress their ovarian function. Some will take it without ovarian suppression. Researchers will compare the effects in breast cancer cells to those of another medicine called tamoxifen. All patients in this group will be premenopausal women who have a type of early breast cancer called estrogen receptor-positive, HER2-negative. The treatment in this group will last for up to 29 days.
Cohort 2: will help researchers understand how imlunestrant affects the ovaries when it is taken without ovarian suppression. Researchers will compare the effects to those of another medicine called tamoxifen. This group will also include premenopausal women with the same type of breast cancer. The treatment in this group will last for up to 6 months.
Have histologically confirmed Stage I to III Estrogen Receptor positive (ER+), human epidermal growth factor receptor 2 negative (HER2-) invasive breast carcinoma with Ki-67 at least 10%
Be willing and able to provide pre- and on-treatment tumor samples.
Have an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1.
Be able to swallow capsules or tablets.
Have bilateral invasive metastatic, occult primary, or inflammatory breast cancer.
Have had prior bilateral oophorectomy or ovarian ablation.
Have a serious medical condition
Had major surgery within 28 days prior to randomization.