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 ofsignature of 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 ofproven efficacy exists, or who are not amenable to standard therapies

• Histologically and/or cytologically confirmed diagnosis of estrogen-receptorpositive (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 SolidTumors (RECIST) Version 1.1 (Appendix B) or at least 1 predominantly lytic bonelesion 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 drugsfor the treatment of locally advanced or metastatic breast cancer within 14days prior 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 noclinically 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 intolerableor clinically significant adverse effects but at least 14 days must haveelapsed between 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 asymptomaticuntreated brain metastases are allowed as long as participants are clinicallystable. 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 atbaseline that will interfere significantly with the absorption, distribution, ormetabolism of AC699.