A PHASE 1B STUDY OF ZN-C5 IN CHINESE SUBJECTS

Inclusion Criteria: * Male or female * Age ≥ 18 years * Menopausal Status \[Female subjects\] * Histologically or cytologically confirmed diagnosis of advanced adenocarcinoma of the breast, not amenable to any potential curative intervention * Estrogen Receptor (ER) positive disease * Human Epidermal Growth Factor Receptor 2 (HER2) negative disease * Refractory to or intolerant of established therapy(ies) known to provide clinical benefit for their malignancy * Prior Hormonal Therapy: * Documented prior response to endocrine therapy for advanced or metastatic disease (SD, PR, or CR) lasting \> 6 months24 weeks or disease recurrence after at least 24 months of adjuvant endocrine treatment. * Prior Chemotherapy: Up to 2 prior lines of chemotherapy for the treatment of advanced breast cancer * Prior treatment with a CDK4/6 inhibitor is allowed * Evaluable or measurable disease per RECIST v1.1. * Eastern Cooperative Oncology Group (ECOG) Performance Status of ≤ 1 * All acute toxic effects of any prior anti-tumor therapy resolved to Grade ≤ 1 or baseline (with the exception of alopecia \[any grade permitted\]) * Adequate organ function * \[Premenopausal and perimenopausal female subjects\]: Negative serum pregnancy test * Male and female subjects of childbearing potential or partners of subjects who engage in intercourse must agree to use protocol specified method(s) of contraception. Exclusion Criteria: * Any of the following within the specified window prior to the first dose of study drug * Prior hematopoietic stem cell or bone marrow transplantation * Prior radiotherapy to \> 25% of bone marrow * Brain metastases that require immediate treatment or are clinically or radiologically unstable (i.e., have been stable for \< 1 month). If receiving steroids, subjects must be receiving a stable to decreasing corticosteroid dose during at least 1 week before enrollment. * Leptomeningeal disease that requires or is anticipated to require immediate treatment. * Presence of life-threatening metastatic visceral disease or symptomatic pulmonary lymphangitic spread * Other known active cancer(s) likely to require treatment in the next year that would impact the assessment of any study endpoints * \[Female subjects\]: Pregnant or breast-feeding * Unexplained symptomatic endometrial disorders (including, but not limited to endometrial hyperplasia, dysfunctional uterine bleeding, or cysts) * Impairment of gastrointestinal (GI) absorption for oral medications * Nausea, vomiting, or diarrhea \> Grade 1 * Myocardial infarction, symptomatic congestive heart failure (NYHA \> Class II), unstable angina, or serious uncontrolled cardiac arrhythmia within the last 6 months * QTc interval \> 480 msec (based on the mean value of the triplicate ECGs), family or personal history of long or short QT syndrome, Brugada syndrome or history of Torsade de Pointes * Concurrent use of food or drugs known to be moderate or strong CYP3A or CYP2C9 inducers and moderate or strong CYP3A4 or CYP2C9 inhibitors. * Positive serum virological tests (HBsAg, HCV-AB, HIV-AB, TP-AB) at screening stage will be excluded. * Any clinically significant disorder, condition, or disease that, in the opinion of the Investigator or Medical Monitor would pose a risk to subject safety or interfere with the study evaluations, procedures, or completion