This phase I trial is testing a new oral drug in patients with ER+, HER2- breast cancerA Phase 1 Open-Label, Multicenter Study to Evaluate Biomarkers for ZN-c5 in Subjects With Estrogen Receptor-Positive, Human Epidermal Growth Factor Receptor 2-Negative Breast Cancer

Inclusion

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