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 prior to first study drug administration that did not resolve to tolerable toxicity, or prior irradiation to >25% of bone marrow. Prior palliative radiotherapy to metastatic lesion(s) is permitted, provided it has been completed 7 days prior to study enrollment and no clinically significant toxicities are expected (e.g., mucositis, esophagitis). Major surgery within 21 days prior to the first study drug administration (exception: participants may enroll if fully recovered or without intolerable or clinically significant adverse effects but at least 14 days must have elapsed between major surgery and first study drug administration) Known symptomatic brain metastases requiring the use of systemic corticosteroids ≥10 mg/day prednisone or equivalents. Asymptomatic and treated, or asymptomatic untreated brain metastases are allowed as long as participants are clinically stable. Stable doses of anticonvulsants are allowed. Any condition that impairs a participant's ability to swallow whole pills. Impairment of gastrointestinal function (GI) or GI disease or other condition at baseline that will interfere significantly with the absorption, distribution, or metabolism of AC699.