Study of AC699 in Patients With Estrogen Receptor Positive/Human Epidermal Growth Factor Receptor 2 Negative (ER+/HER2-) Locally Advanced or Metastatic Breast Cancer

Inclusion Criteria: * Signed written informed consent (ICF) * Adult male and female participants, at least 18 years-of-age at the time of signature of the ICF * Female participants must be postmenopausal * Confirmed diagnosis of advanced, unresectable, and/or metastatic breast cancer following disease progression on standard treatment, or for whom no therapy of proven efficacy exists, or who are not amenable to standard therapies * Histologically and/or cytologically confirmed diagnosis of estrogen-receptor positive (ER+) human epidermal growth factor 2 negative (HER2-) breast cancer * Must have received at least 2 prior endocrine or at least 1 prior line of endocrine therapy if combined with CDK4/6 inhibitor * Prior chemotherapy is not required, but up to 3 prior regimens of cytotoxic chemotherapy will be allowed in the locally advanced/ metastatic setting * At least 1 measurable lesion according to Response Evaluation Criteria in Solid Tumors (RECIST) Version 1.1 (Appendix B) or at least 1 predominantly lytic bone lesion in the absence of measurable disease * Acceptable organ and hematologic function at baseline * Life expectancy ≥12 weeks after the start of the treatment Exclusion Criteria: * Treatment with any of the following: * Any cytotoxic chemotherapy, investigational agents or other anti-cancer drugs for the treatment of locally advanced or metastatic breast cancer within 14 days prior to the first administration of AC699 * Radiation therapy within 14 days p…