A Study of ZN-c5 and Abemaciclib in Participants With Breast Cancer

Inclusion criteria

• Age ≥ 18 years of age
• Women can be peri- or postmenopausal, as defined by at least one of the following:

• Age ≥ 60 years;
• 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 (FSH) level within the laboratory's reference range for postmenopausal females;
• Documented bilateral oophorectomy;
• Must receive a gonadotrophin-releasing hormone agonist beginning at least 4 weeks prior to the first dose of study medication
• Histologically or cytologically confirmed diagnosis of advanced adenocarcinoma of the breast
• Estrogen receptor positive disease
• Human Epidermal Growth Factor Receptor 2 negative disease
• Measurable disease per Response Evaluation Criteria in Solid Tumors v1.1

Exclusion criteria

• Prior therapy within the following windows:

• Tamoxifen, aromatase inhibitor, fulvestrant, or other anti-cancer endocrine therapy < 14 days;
• Any investigational drug therapy < 28 days or 5 half-lives (whichever is shorter)
• Any prior systemic chemotherapy regardless of the stop date, but the subject must have recovered to eligibility levels from prior toxicity
• Prior treatment with CDK4/6 inhibitors
• Unexplained symptomatic endometrial disorders (including, but not limited to, endometrial hyperplasia, dysfunctional uterine bleeding, or cysts)