A Phase Ia/Ib Dose-Escalation and Expansion Study Evaluating the Safety, Pharmacokinetics, and Activity of GDC-0587 as a Monotherapy and in Combination With Giredestrant in Patients With Locally Advanced Or Metastatic ER-Positive, HER2-Negative Breast Cancer Who Have Previously Progressed During or After CDK4/6 Inhibitor Therapy

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 per national or local treatment guidelines * Five or more prior lines of systemic therapy in the locally advanced or metastatic setting * Treatment with anti-cancer therapies, including investigational therapies, within 28 days or 5 drug elimination half-lives, whichever is shorter, prior to initiation of study drug * Treatment with an approved oral endocrine therapy within 7 days prior to initiation of study drug or treatment with fulvestrant or an approved/investigational CDK inhibitor within 21 days prior to initiation of study drug * History of Grade ≥3 adverse event attributed to prior CDK inhibitor therapy that resulted in permanent discontinuation of prior CDK inhibitor therapy * Poor peripheral venous access * Malabsorption condition or other gastrointestinal (GI) conditions/surgeries that the investigator assesses may significantly interfere with enteral absorption * Major surgical procedure within 28 days prior to initiation of study drug * Untreated, active CNS metastases * Infection requiring systemic (i.e., oral, IV, or intramuscular) antibiotics, chronic infection requiring treatment within 1 year prior to screening, or any evidence of current infection * History of malignancy within 3 years prior to screening, except for cancer under investigation in this study * Known history of a clinically significant abnormal ECG