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.