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)