A Study to Evaluate the Safety, Pharmacokinetics, and Activity of GDC-0587 as a Monotherapy and in Combination With Giredestrant in Participants With Locally Advanced or Metastatic Estrogen Receptor-Positive and Human Epidermal Growth Factor Receptor 2-Negative (ER+/HER2-) Breast Cancer

Inclusion Criteria: * Agreement to adhere to the contraception requirements * For females of childbearing potential ≤60 years of age and males: treatment with luteinizing hormone-releasing hormone (LHRH) agonist therapy beginning at least 2 weeks prior to Cycle 1, Day 1 and agreement to continue LHRH agonist therapy for the duration of the study * Histologically or cytologically confirmed adenocarcinoma of the breast that is locally advanced or metastatic * Previously documented ER+ and HER2- tumor according to American Society of Clinical Oncology (ASCO)/ College of American Pathologists (CAP) or European Society of Medical Oncology (ESMO) guidelines or any national guidelines with criteria conforming to ASCO/CAP or ESMO guidelines * Disease progression during or following treatment with an approved CDK 4/6 inhibitor, with or without endocrine therapy, in the locally advanced or metastatic setting * Measurable, or non-measurable but evaluable, disease per RECIST v1.1 * Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1 * Life expectancy ≥6 months * Creatinine clearance ≥60 milliliter per minute (mL/min) (calculated through use of the Cockcroft-Gault formula) Exclusion Criteria: * Pregnant or breastfeeding, or intention of becoming pregnant during the study * Advanced, symptomatic, visceral spread that is at risk of life-threatening complications in the short term appropriate for treatment with cytotoxic chemotherapy at time of entry into the study, as …