A Phase 1/2 Trial of ARV-471 Alone and in Combination With Palbociclib (IBRANCE®) in Patients With ER+/HER2- Locally Advanced or Metastatic Breast Cancer

Inclusion Criteria: Part A, Part B, and Part C: * Patients at least 18 years of age at the time of signing the informed consent. * Patients must have histologically or cytologically confirmed ER+ and HER2- advanced breast cancer for which standard curative therapy is no longer effective or does not exist. * Patients must have measurable or non-measurable disease by RECIST criteria (version1.1), with radiologic tumor assessments performed within 28 days of the first dose of therapy. * Patients must be willing to undergo a core biopsy of accessible tumor within 4 weeks prior to the initiation of study treatment and a follow-up biopsy on treatment for ER immunohistochemistry (IHC) testing and pharmacodynamics (PD) studies. (Patients without accessible tumor tissue may be eligible after discussion with the Medical Monitor.) * Women must be postmenopausal due to surgical or natural menopause. Part A: \- Patients must have received at least 2 prior endocrine regimens in any setting (neoadjuvant, adjuvant or advanced/metastatic) a CDK4/6 inhibitor and up to 3 prior regimens of cytotoxic chemotherapy in the locally advanced or metastatic setting. Part B: * Patients must have received at least 1 prior endocrine regimen for a minimum of 6 months in the locally advanced or metastatic setting; if more than 1 prior endocrine regimen has been administered, only one of the regimens must have been administered for a minimum of 6 months in the locally advanced or metastatic setting * Pa…