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 signatureof the ICF
• Female participants must be postmenopausal
• Confirmed diagnosis of advanced, unresectable, and/or metastatic breast cancerfollowing disease progression on standard treatment, or for whom no therapy of provenefficacy 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 endocrinetherapy if combined with CDK4/6 inhibitor
• Prior chemotherapy is not required, but up to 3 prior regimens of cytotoxicchemotherapy 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 theabsence 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 forthe treatment of locally advanced or metastatic breast cancer within 14 daysprior to the first administration of AC699
• Radiation therapy within 14 days prior to first study drug administration thatdid not resolve to tolerable toxicity, or prior irradiation to >25% of bonemarrow. Prior palliative radiotherapy to metastatic lesion(s) is permitted,provided it has been completed 7 days prior to study enrollment and no clinicallysignificant 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 orclinically significant adverse effects but at least 14 days must have elapsedbetween major surgery and first study drug administration)
• Known symptomatic brain metastases requiring the use of systemic corticosteroids ≥10mg/day prednisone or equivalents. Asymptomatic and treated, or asymptomatic untreatedbrain metastases are allowed as long as participants are clinically stable. Stabledoses of anticonvulsants are allowed.
• Any condition that impairs a participant's ability to swallow whole pills. Impairmentof gastrointestinal function (GI) or GI disease or other condition at baseline thatwill interfere significantly with the absorption, distribution, or metabolism ofAC699.