This phase I trial is trying to understand how a new oral drug works and whether it is safe for the treatment of stage I-III, ER+ breast cancer that is able to be surgically removedA Phase I, Multicenter, Open-Label Preoperative, Short-Term Window Study of GDC-9545 in Postmenopausal Women With Stage I-III Operable, Estrogen Receptor-Positive Breast Cancer

Exclusion

• Diagnosis of inflammatory breast cancer
• Diagnosis of bilateral breast cancer
• Concurrent use of hormone replacement therapies
• Previous systemic or local treatment for the primary breast cancer currently under investigation
• Concurrent treatment with other experimental drugs or participation in another clinical trial with any investigational drug within 30 days prior to study entry
• Current treatment with any systemic anti-cancer therapies
• Major surgery within 4 weeks prior to enrollment
• Radiation therapy within 2 weeks prior to enrollment
• Diagnosis of any secondary malignancy within 3 years prior to enrollment, except for appropriately treated carcinoma in situ of the cervix, non-melanoma skin carcinoma, or Stage I uterine cancer
• Active inflammatory bowel disease or chronic diarrhea, short bowel syndrome, or upper gastrointestinal surgery including gastric resection
• Known HIV infection
• Known clinically significant history of liver disease consistent with Child-Pugh Class B or C, including active viral or other hepatitis, current alcohol abuse, or cirrhosis
• Other severe acute or chronic medical or psychiatric condition or laboratory abnormality that may increase the risk associated with study participation or investigational product administration or may interfere with the interpretation of study results and, in the judgment of the investigator, would make the patient inappropriate for entry into this study
• History of allergy to giredestrant or any of its excipients
• Any condition requiring anti-coagulants, such as warfarin, heparin, or thrombolytic drugs
• History of documented hemorrhagic diathesis or coagulopathy
• History or presence of symptomatic bradycardia or sick sinus syndrome
• Baseline heart rate ≤55 beats per minute (bpm) prior to enrollment
• History or presence of an abnormal electrocardiogram (ECG) that is clinically significant in the investigator's opinion, including complete left bundle branch block, second- or third-degree heart block, or evidence of prior myocardial infarction
• QT interval corrected through use of Fridericia's formula (QTcF) >470 milliseconds demonstrated by at least two ECGs >30 minutes apart
• History of ventricular dysrhythmias or risk factors for ventricular dysrhythmias such as structural heart disease, coronary heart disease, clinically significant electrolyte abnormalities, or family history of sudden unexplained death or long QT syndrome
• Current treatment with medications that are well known to prolong the QT interval
• History or presence of uncontrolled hypothyroidism
• Any other disease, metabolic dysfunction, physical examination finding, or clinical laboratory finding that, in the investigator's opinion, gives reasonable suspicion of a disease or condition that contraindicates the use of an investigational drug or that may affect the interpretation of the results or render the patient at high risk from treatment complications