- Part 2, Cohort 3: Histologically and/or cytologically confirmed diagnosis of breast cancer with hormone receptor-positive status (ER and/or PgR positive) and HER2-negative status with prior exposure to tamoxifen and/or an aromatase inhibitor and/or an aromatase inhibitor plus palbociclib. Prior treatment with tamoxifen in the neoadjuvant setting is allowed but must have been discontinued for at least 1 year prior to the first dose.