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

This study is testing a new drug combination — GDC-0587 with or without giredestrant — for people with advanced estrogen receptor-positive, HER2-negative (ER+/HER2-) breast cancer that has progressed after CDK 4/6 inhibitor treatment (a common class of targeted therapy). **You may be eligible if...** - You have advanced or metastatic ER+/HER2- breast cancer confirmed by biopsy - Your cancer progressed during or after treatment with a CDK 4/6 inhibitor - You are reasonably functional (ECOG 0 or 1) with at least 6 months life expectancy - Your kidney function is adequate (creatinine clearance at least 60 mL/min) **You may NOT be eligible if...** - You are pregnant or breastfeeding - You have had 5 or more prior treatment regimens for advanced/metastatic disease - You have untreated active brain metastases - You have had major surgery, serious infections, or certain prior cancer treatments within the past 4 weeks - You have a history of grade 3 or higher side effects from prior CDK inhibitor therapy that caused permanent discontinuation Talk to your doctor to see if this trial is right for you.