Study on the Effectiveness and Safety of Giredestrant Compared to Doctor's Choice of Hormone Therapy in Patients with Estrogen Receptor-Positive, HER2-Negative Early Breast Cancer

Inclusion Criteria

• Have breast cancer that is estrogen receptor-positive and HER2-negative, confirmed by lab tests.
• May have multiple tumors in one or both breasts, as long as all tumors meet the criteria for estrogen receptor positivity and HER2 negativity.
• Must have had surgery to remove the primary breast tumor(s) and lymph nodes.
• If chemotherapy was given after surgery, it must be completed before starting the study treatment, with a 21-day gap after the last dose.
• Any side effects from previous cancer treatments or surgeries must be mild or resolved, except for hair loss, mild nerve pain, joint pain, or other non-risky side effects.
• Must not have had any hormone therapy before, unless enrolled within 12 months after surgery.
• Must have a sample of the untreated primary breast tumor available for testing.
• Can have cancer in lymph nodes or not, but must meet additional risk criteria.
• Must be able to take oral medication and have good organ function.

Substudy Inclusion Criteria:

• Must have completed any radiation therapy before joining the substudy, with a 14-day gap after the last session.

Exclusion Criteria

• Cannot be pregnant or breastfeeding, or planning to become pregnant during the study or shortly after.
• Cannot have received any experimental treatments within 28 days before starting the study.
• Cannot be receiving or planning to receive certain cancer drugs called CDK4/6 inhibitors, except for a short course before starting the study.
• Cannot have serious heart disease or a history of heart problems.
• Cannot have Stage IV breast cancer.
• Cannot have had invasive breast cancer or ductal carcinoma in situ (DCIS) before, except for certain treated cases.
• Cannot have had another cancer within the last 3 years, except for certain treated cases.
• Cannot have had certain hormone treatments before, except for a short course.
• Cannot have serious liver disease or active hepatitis.
• Cannot be taking certain medications that affect liver enzymes within 14 days before starting the study.
• Cannot have allergies to the study drugs.
• Cannot have certain blood disorders or require dialysis.
• Cannot have had major surgery unrelated to breast cancer within 28 days before starting the study.
• Cannot have had a serious infection recently.
• Cannot have any medical condition that would make participating in the study unsafe.

Substudy Exclusion Criteria:

• Cannot have participated in the main study before.
• Cannot have received certain cancer drugs called CDK4/6 inhibitors before joining the substudy.
• Cannot be taking certain medications that affect liver enzymes within 14 days before starting the substudy.