This is a Phase 1b, open-label, multicenter, dose-escalation study to evaluate the safety, tolerability, pharmacokinetics (PK), and preliminary efficacy of ZN-c5 administered orally in combination with abemaciclib (VERZENIO®) in participants with advanced estrogen-receptor positive, human epidermal growth factor receptor-2 negative (ER+/HER2-) 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)