A Study to Evaluate the Effect of GDC-4198 Alone and in Combination With Giredestrant Versus Abemaciclib and Giredestrant in Participants With Locally Advanced or Metastatic Estrogen Receptor-Positive (ER+), Human Epidermal Growth Factor Receptor-Negative (HER2-) Breast Cancer

Inclusion Criteria:

• Histologically and/or cytologically confirmed adenocarcinoma of the breast that islocally advanced or metastatic.

• Previously documented ER+ and HER2- tumor according to American Society of ClinicalOncology (ASCO)/ College of American Pathologists (CAP) or European Society ofMedical Oncology (ESMO) guidelines or any national guidelines with criteriaconforming to ASCO/CAP or ESMO guidelines.

• Disease progression during or after treatment with an approved cyclin-dependentkinase 4/6 (CDK4/6) inhibitor and approved endocrine therapy (ET) in the locallyadvanced or metastatic setting.

• Measurable or non-measurable evaluable, disease per Response Evaluation Criteria inSolid Tumors version 1.1 (RECIST v1.1).

• Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1.

• Life expectancy >= 6 months.

Exclusion Criteria:

• Advanced, symptomatic, visceral spread that is at risk of life-threateningcomplications in the short term appropriate for treatment with cytotoxicchemotherapy at time of entry into the study, as per national or local treatmentguidelines.

• Have received more than one-line of therapy for locally advanced or metastaticdisease.

• Have received prior chemotherapy for metastatic breast cancer.

• Treatment with an approved oral ET within 7 days prior to initiation of study drug;treatment with fulvestrant or an approved CDK4/6 inhibitor within 21 days prior toinitiation of study drug.

• Malabsorption condition or other gastrointestinal (GI) conditions/surgeries that theinvestigator assesses may significantly interfere with enteral absorption

• History of malignancy within 3 years prior to screening, except for cancer underinvestigation in this study and malignancies with a negligible risk of metastasis ordeath.

• Known allergy or hypersensitivity to any component of the study treatments.