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