A Study of ZN-c5 in Participants With Breast Cancer

Inclusion Criteria: 1. Signed and dated ICF 2. Age ≥ 18 years of age, either gender 3. Females must be postmenopausal as defined by at least one of the following: 1. Age ≥ 60 years; 2. Age \< 60 years and cessation of regular menses for at least 12 consecutive months with no alternative pathological or physiological cause; and serum estradiol and follicle stimulating hormone levels within the local laboratory's normal reference range for postmenopausal females; 3. Documented bilateral oophorectomy 4. Histologically or cytologically confirmed diagnosis of adenocarcinoma of the breast 5. Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1 6. Adequate organ function defined as follows: 1. Hematologic: Platelets ≥ 100 × 109/L; Hemoglobin ≥ 9.0 g/dL; Absolute neutrophil count ≥ 1.5 × 109/L (without platelet transfusion or any hematopoietic growth factors within previous 7 days of the hematologic laboratory values obtained at the Screening Visit) 2. Hepatic: Aspartate aminotransferase (AST) / Alanine aminotransferase (ALT) ≤ 2.5 × upper limit of normal (ULN); Total or conjugated bilirubin ≤ 1.5 × ULN 3. Renal: Serum creatinine ≤ 1.5 × ULN or creatinine clearance (CrCl) ≥ 60 mL/min Exclusion Criteria: 1. Major surgery, defined as any surgical procedure that involves general anesthesia and a significant incision (i.e., larger than what is required for placement of central venous access, percutaneous feeding tube, or biopsy) within 28 days of the first administration of study drug 2. Treatment with another investigational drug or other intervention within 28 days before the first administration of study drug 3. Impairment of gastrointestinal (GI) function or GI disease that may significantly alter the absorption of oral medication, including any unresolved nausea, vomiting, or diarrhea that is CTCAE v.5.0 Grade \> 1 4. Myocardial infarction, symptomatic congestive heart failure (New York Heart Association Classification \> Class II), unstable angina, or serious uncontrolled cardiac arrhythmia within the last 6 months of study Day 1 5. Uncontrolled inter-current illness 6. History or evidence of clinically significant disorder, condition, or disease that, in the opinion of the Investigator would pose a risk to subject safety or interfere with the study evaluations, procedures, or completion